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題目(和文)

高難度選択酸化反応用Fe含有ゼオライト触媒に関する研究

Title(English)

Study on Fe-containing zeolite catalysts for extremely difficult selective
oxidation reactions

著者(和文)

XiaoPeipei

Author(English)

PEIPEI XIAO

出典(和文)

学位:博士(工学),学位授与機関:東京工業大学,
報告番号:甲第10979号,
授与年月日:2018年9月20日,
学位の種別:課程博士,
審査員:野村 淳子,鎌田 慶吾,本倉 健,庄子 良晃,横井 俊之

Citation(English)

Degree:Doctor (Engineering),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第10979号,
Conferred date:2018/9/20,
Degree Type:Course doctor,
Examiner:,,,,

学位種別(和文)

博士論文

Type(English)

Doctoral Thesis

Powered by T2R2 (Science Tokyo Research Repository)

Doctor thesis 2018

Study on Fe-containing zeolite catalysts for extremely difficult selective
oxidation reactions(高難度選択酸化反応用 Fe 含有ゼオライト触媒に関する研究)

This thesis was submitted to Tokyo Institute of Technology for the degree of Doctor of Engineering.

Xiao Peipei

Department of Electronic Chemistry
Interdisciplinary Graduate School of Science and Engineering
Tokyo Institute of Technology

Contents
Chapter 1 ..................................................................................................................................................................... 1
Introduction ............................................................................................................................................................... 1
1.1. Zeolites ................................................................................................................................................................. 1
1.1.1 Structure and composition ....................................................................................................................... 1
1.1.2 Fe-containing zeolites ............................................................................................................................... 4
1.2. Conversion of methane to methanol (MTM) .................................................................................................... 6
1.2.1 Two-step method (industrial route) ......................................................................................................... 6
1.2.2 Enzymatic activation of methane ............................................................................................................ 7
1.2.3 Homogeneous catalytic system in liquid phase ...................................................................................... 7
1.2.4 Direct gas phase system ............................................................................................................................ 8
1.2.5 Heterogeneous approaches in the liquid phase ...................................................................................... 8
1.3. Conversion of benzene to phenol (BTP) ......................................................................................................... 11
1.3.1 Three-step method (industrial route) .................................................................................................... 11
1.3.2 Direct gas phase system .......................................................................................................................... 11
1.3.3 Homogeneous catalytic system in liquid phase .................................................................................... 12
1.3.4 Heterogeneous catalytic system in liquid phase ................................................................................... 13
1.4. Relationship between MTM and BTP ............................................................................................................ 15
1.5. Aim of the thesis ................................................................................................................................................ 15
1.6. Scope of the thesis ............................................................................................................................................. 16
Chapter 2 ................................................................................................................................................................... 24
Direct synthesis of phenol by hydroxylation of benzene with hydrogen peroxide over Fe-containing MFI zeolite
catalysts .................................................................................................................................................................... 24
2.1. Introduction ...................................................................................................................................................... 24
2.2. Experiments ...................................................................................................................................................... 26
2.2.1. Materials ................................................................................................................................................. 26
2.2.2. Catalysts preparation ............................................................................................................................ 26
2.2.3. Characterization of catalysts ................................................................................................................ 27
2.2.4. Catalytic tests ......................................................................................................................................... 28
2.3. Results and discussion ...................................................................................................................................... 28
2.3.1 Characterization of catalysts ................................................................................................................. 28
2.3.1.1 Physicochemical properties ......................................................................................................... 28
2.3.1.2 Fe states......................................................................................................................................... 29
2.3.1.3 Acidity ........................................................................................................................................... 31
2.3.2 Benzene to phenol reaction with H2O2 (BTP) ....................................................................................... 32
2.3.2.1 BTP reaction over Fe-containing MFI catalysts ........................................................................ 32
2.3.2.2 The influence of the reaction conditions on the reaction performance ................................... 32
2.3.2.3 Reusability of FMD15 catalyst.................................................................................................... 34
2.3.3 Discussion on the active sites in the BTP reaction................................................................................ 35
2.3.3.1 Fe content and state ..................................................................................................................... 35
2.3.3.2 The importance of isolated and oligomeric Fe species on the extra framework .................... 35
2.3.3.3 The impact of the framework Al ................................................................................................ 36
2.4. Conclusions........................................................................................................................................................ 37

1

Chapter 3 ................................................................................................................................................................... 58
Dramatic impacts of the distribution of Fe species in Fe-silicalite-1 zeolites and solvent on liquid-phasemethane conversion to methanol with H2O2.......................................................................................................... 58
3.1. Introduction ...................................................................................................................................................... 58
3.2. Experiments ...................................................................................................................................................... 60
3.2.1. Materials ................................................................................................................................................. 60
3.2.2. Catalysts preparation ............................................................................................................................ 60
3.2.3. Characterization of catalysts ................................................................................................................ 61
3.2.4. Catalytic tests ......................................................................................................................................... 62
3.3. Results and discussion ...................................................................................................................................... 62
3.3.1 Characterization ..................................................................................................................................... 62
3.3.1.1 Structure and composition .......................................................................................................... 62
3.3.1.2 Fe states......................................................................................................................................... 63
3.3.1.3 Acidity ........................................................................................................................................... 64
3.3.2 Methane to methanol reaction ............................................................................................................... 65
3.3.2.1 Solvent selection ........................................................................................................................... 65
3.3.2.2 The influence of reaction conditions........................................................................................... 66
3.3.2.3 The catalytic performance over FS(T)x and FS(TN)y catalysts in MTM reaction ................ 70
3.4. Conclusions........................................................................................................................................................ 72
Chapter 4 ................................................................................................................................................................... 97
The influence of iron and aluminum location in MFI zeolites on the catalytic performance in hydroxylation ofbenzene to phenol and methane to methanol with H2O2 ...................................................................................... 97
4.1. Introduction ...................................................................................................................................................... 97
4.2. Experiments ...................................................................................................................................................... 99
4.2.1. Materials ................................................................................................................................................. 99
4.2.2. Catalysts preparation ............................................................................................................................ 99
4.2.3. Characterization of catalysts .............................................................................................................. 100
4.2.4 Catalytic tests ........................................................................................................................................ 101
4.2.4.1 Direct hydroxylation of benzene to phenol with H2O2 (BTP) ................................................ 101
4.2.4.2 Direct conversion of methane to methanol with H2O2 (MTM) .............................................. 101
4.3. Results and discussion .................................................................................................................................... 101
4.3.1 Characterization of catalysts ............................................................................................................... 101
4.3.1.1 Physicochemical properties ....................................................................................................... 101
4.3.1.2 Fe states....................................................................................................................................... 102
4.3.1.3 Acidity ......................................................................................................................................... 103
4.3.1.4 The states of Si and Al in Fe-ZSM-5 zeolites ........................................................................... 104
4.3.2 Direct oxidation of benzene to phenol ................................................................................................. 105
4.3.3 Direct oxidation of methane to methanol............................................................................................ 105
4.4. Conclusions...................................................................................................................................................... 106
Chapter 5 ................................................................................................................................................................. 123
Alkaline treatment on as-synthesized and calcined Fe-silicalite-1 and Fe-ZSM-5 zeolites for hydroxylation ofbenzene to phenol with H2O2 ................................................................................................................................ 123
5.1. Introduction .................................................................................................................................................... 123
5.2. Experiments .................................................................................................................................................... 124

2

5.2.1 Materials ................................................................................................................................................ 124
5.2.2 Catalysts preparation ........................................................................................................................... 125
5.2.3 Characterization of catalysts ............................................................................................................... 126
5.2.4 Catalytic tests ........................................................................................................................................ 126
5.3. Results and discussion .................................................................................................................................... 126
5.3.1 Characterization of the catalysts ......................................................................................................... 126
5.3.1.1 Physicochemical properties ....................................................................................................... 127
5.3.1.2 Fe states....................................................................................................................................... 128
5.3.1.3 Acidity ......................................................................................................................................... 129
5.3.2. Direct oxidation of benzene to phenol with H2O2 (BTP)................................................................... 130
5.4. Conclusions...................................................................................................................................................... 131
Chapter 6 ................................................................................................................................................................. 147
Direct synthesis of Fe-containing MWW zeolite for direct hydroxylation of benzene to phenol and methane tomethanol with H2O2 ............................................................................................................................................... 147
6.1. Introduction .................................................................................................................................................... 147
6.2. Experiments .................................................................................................................................................... 149
6.2.1 Materials ................................................................................................................................................ 149
6.2.2 Catalysts preparation ........................................................................................................................... 150
6.2.3 Characterization of catalysts ............................................................................................................... 150
6.2.4 Catalytic tests ........................................................................................................................................ 151
6.2.4.1 Direct hydroxylation of benzene to phenol (BTP)................................................................... 151
6.2.4.2 Direct conversion of methane to methanol (MTM) ................................................................ 152
6.3. Results and discussion .................................................................................................................................... 152
6.3.1 Characterization of catalysts ............................................................................................................... 152
6.3.1.1 Physicochemical properties ....................................................................................................... 152
6.3.1.2 Fe states....................................................................................................................................... 153
6.3.1.3 Acidity ......................................................................................................................................... 155
6.3.1.4 The states of Si and Al in Fe-containing MWM zeolites ......................................................... 156
6.3.2 Direct oxidation of benzene to phenol with H2O2 (BTP) ................................................................... 157
6.3.3 Direct oxidation of methane to methanol with H2O2 (MTM) ........................................................... 159
6.4. Conclusions...................................................................................................................................................... 159
Chapter 7 ................................................................................................................................................................. 179
Iron- and copper-exchanged Beta zeolite catalysts for hydroxylation of benzene to phenol and methane tomethanol with H2O2 ............................................................................................................................................... 179
7.1. Introduction..................................................................................................................................................... 179
7.2. Experiments .................................................................................................................................................... 181
7.2.1 Materials ................................................................................................................................................ 181
7.2.2 Catalyst preparation ............................................................................................................................. 181
7.2.3 Characterization of catalysts ............................................................................................................... 181
7.2.4 Catalytic tests ........................................................................................................................................ 182
7.2.4.1 Direct conversion of benzene to phenol with H2O2 (BTP) ...................................................... 182
7.2.4.2 Direct conversion of methane to methanol with H2O2 (MTM) .............................................. 182
7.3. Results and discussion .................................................................................................................................... 183
7.3.1 Characterization of catalysts ............................................................................................................... 183

3

7.3.1.1 Physicochemical properties ....................................................................................................... 183
7.3.1.2 The states of Fe and Cu species ................................................................................................ 184
7.3.1.3 Acidity ......................................................................................................................................... 185
7.3.2 Direct oxidation of benzene to phenol with H2O2 (BTP) ................................................................... 186
7.3.3 Direct oxidation of methane to methanol with H2O2 (MTM) ........................................................... 189
7.4. Conclusions...................................................................................................................................................... 189
Chapter 8 ................................................................................................................................................................. 207
Summary ................................................................................................................................................................ 207
List of publications ................................................................................................................................................ 210
Acknowledgement ................................................................................................................................................... 212

4

Chapter 1

Chapter 1
Introduction
1.1. Zeolites
1.1.1 Structure and composition
The term zeolite was originally created in 1756 by Swedish mineralogist Axel Fredrik Cronstedt.
They are also called molecular sieves, regarding to the ability to discriminate molecules on the basisof their micropore size (< 2 nm). Zeolites occur naturally but are also produced industrially on a large
scale. The first synthesis of zeolite in laboratory was performed by Saint Clair Deville in the middleof 18th century.
Zeolites consist of the primary building unit tetrahedral [TO4], typically [SiO4] and [AlO4], which
are linked to each other by corner-sharing oxygen bridges to form the secondary building units (SBUs)(Figure 1.1(a)). More complex composite building units (CBUs) can be formed by linking the groupsof SBUs (Figure 1.1(b)). These CBUs units are further combined to form a specific structure in thepresence of channels and cavities of different shapes, which finally determine the framework types ofthe zeolite.

(a)

(b)

3

6

12

5-1

4-1

6-6

d4r

d6r

d8r

sod

doh

aft

Figure 1.1 (a) example of SBUs and (b) CBUs [1].
The introduction of aluminum atom in the silica lattice results in the formation of negative charges
on the framework, i.e. AlO4-, which is compensated by the extra framework cations, such as Na+and
K+, located in the channels [2]. When proton is the compensating cation, zeolites possess strong
Brønsted acid sites (Figure 1.2). The strength of the Brønsted acid site depends significantly on thetype of next nearest neighbor substitutions. The metallosilicates containing trivalent elements in the1Chapter 1

framework other than Al (B, Ga, Fe, In) are of interest in designing the acid strength and active. The
heteroatoms reach weaker acid sites upon framework substitution than Al. The ranking in the acidstrength displays as the following order: B(OH)Si < In(OH)Si < Fe(OH)Si < Ga(OH)Si< Al(OH)Si[3-5].

H
O

O
Al

Si

Si

Figure 1.2 Schematic representation of the Brønsted acid site associated with the presence offramework Al substitution.

Normally, zeolites possess the advantages of favorable textural properties, high surface area, highconnectivity of uniformly-sized micropores, high thermal stability and tunable surface acidity [6-8].
Thus they are commonly used as commercial adsorbents and catalysts in industry such as arsenic
control, water purification, construction, detergents, and conversion of crude oil into gasoline and otherfuels and so on [9-14].
232 unique zeolite frameworks have been identified and over 40 naturally occurring zeolite
frameworks have been known until September 2016. Among them, the most popular ones in industrial
applications are MFI, FAU, MWW, MOR, BEA and CHA-type zeolites.
10 MR
5.6 X 5.3 Å

8.9 Å

10 MR
5.5 X 5.1 Å

Figure 1.3 (Left) Skeletal diagram of the [010] face of MFI zeolite. (Right) Channel structure of MFI.
The structure of MFI zeolite has 10-membered-ring and two types of channel systems with2

Chapter 1

straight channels about 5.3 × 5.6 Å and sinusoidal channels about 5.1 × 5.5 Å (Figure 1.3) [15]. The
two different channels are perpendicular to each other, generating intersections with diameters of 8.9
Å. ZSM-5, Zeolite Socony Mobil-5, is one of the most typical aluminosilicate MFI structure zeolite,
belonging to the pentasil family of zeolites [16]. The crystallographic unit cell of ZSM-5 has 96 T sites(Si or Al), 192 O sites, and a number of compensating cations depending on the Si/Al ratio, whichranges from 12 to infinity [17, 18]. Patented by Mobil Oil Company in 1975, it is widely applied inpetrochemical chemistry [19, 20], environmental protection [21, 22] and adsorption [23] because of
the high surface area, special channel structure, abundant acid sites, and thermal and hydrothermal
stability [24]. Catalytic cracking of hydrocarbon is significant for industrial manufacture, due to theadvantages of high cracking conversion efficiency, high light alkene selectivity and less carbon
deposition compared with thermal cracking [25]. When it was applied in n-hexane cracking under highpressure, good catalytic activity and stability are achieved [26].

12 MR
7.1 X 7.1 X 9.1 Å

10 MR
4.1 X 5.1 Å

Hexagonal
prism

Figure 1.4 (Left) Skeletal diagram of the [001] face of MWW zeolite. (Right) Channel structure of
MWW.
MWW structure zeolite is a kind of layered zeolite with high surface area, complex porosity andcontaining both medium and large pores [27, 28]. The structural complexity derives from the presenceof two independent pore systems accessible through 10-ring openings. One of these systems is definedby two-dimensional sinusoidal channels, maintaining an effective 10-ring diameter throughout thestructure [15]. The second one includes large supercages with 12-ring openings defined by innerdiameter of 7.1 Å and inner length of 18.2 Å (Figure 1.4) [27, 29]. MWW such as MCM-22 zeolite
has been studied extensively as shape selective catalyst for many hydrocarbon conversions such as
3

Chapter 1

isomerization [30, 31], etherification [32], disproportionation [33], and alkylation [34]. Good catalytic
performance for fluid catalytic cracking (FCC) process in petro-chemistry industry using MCM-22 hasbeen achieved [35].
BEA zeolite adopts large 12-membered-ring with three-dimensional interconnection with 6.6 ×
6.7 Å channels running straight along the a (or b) axis and 5.6 × 5.6 Å channels running tortuously
along the c axis (Figure 1.5) [36]. Among several types of natural and artificial synthetic zeolites,
BEA has received special attention from the scientific community regarding its high Si/Al ratio, highsurface area, strong acidity and hydrothermal stability [15]. Thus, BEA, normally Beta zeolite catalysts
are widely applied in catalytic cracking reactions, alkane hydroisomerization, and alkylation andacylation of aromatic hydrocarbons [37].
12 MR
6.6 X 6.7 Å

ca. 13 Å

12 MR
5.6 X 5.6 Å

Figure 1.5 (Left) Skeletal diagram of the [100] face of BEA zeolite. (Right) Channel structure of BEA.
1.1.2 Fe-containing zeolites
Al-containing zeolite catalysts, such as ZSM-5, Beta, MCM-22, SSZ-13, are widely applied in
catalytic cracking reaction. While zeolites containing other heteroatoms, such as Fe, Cu, Ti and Sn,
present excellent performance in oxidation reactions [5]. Fe-containing zeolites have been widely used
and revealed outstanding performance in various catalytic conversions related to environmental
applications, such as direct N2O decomposition [38-40], selective catalytic reduction of N2O and NOx[41, 42], selective oxidation of NH3 to N2 [43] and so on; and in extremely difficult selective oxidation
reactions, for instance hydroxylation of benzene and phenol [44-46], propane oxidative
dehydrogenation [39, 47-49], and hydroxylation of methane to methanol [50-52].
The preparation method of Fe-containing zeolites strongly influences the nature and distribution
4

Chapter 1of the resulting Fe species, which basically determines the catalytic performance. Extensive efforts
have been dedicated to establishing reliable procedures for iron incorporation, including direct
synthesis [41, 53, 54], ion exchange [55], impregnation [56, 57], chemical vapor deposition (CVD)[58, 59] and so on. Incorporation of heteroatoms into zeolites can adjust the surface properties to obtainhighly dispersed active sites and to improve the catalytic activity. Compared to the post-synthesis
approaches, the direct synthetic method is more feasible and has industrial potential [60].

Figure 1.6 Schematic representation of (a) possible mechanisms leading to the Fe migration fromframework to extra framework positions ([61]) and (b) diagrammatic sketch of the Fe species: Fe2O3
particles, FexOy clusters, grafted Fe2+ ions and Fe3+ in framework position [62].

Figure 1.6 displays a general scheme involving migration, grafting and clustering, and gives a
pictorial representation of the structure of the Fe species formed by migration from the framework to
the extra framework positions and successive grafting on the walls of the zeolite channels. As for theanchoring sites, two different structures have been considered: Brønsted sites (structure a) associatedwith residual M3+ in the lattice (M stands for Al or Fe in Fe-ZSM-5, M stands for Fe in Fe-silicalite-1)
5Chapter 1

and silanol nests (structure b) [61, 62]. Generally, the Fe species on the zeolite can be divided into
several categories: framework Fe, isolated Fe cations, oligomeric cationic Fe complexes, and neutral
iron oxide clusters (FexOy) as well as larger Fe-oxide aggregates [44], which play different activity.

1.2. Conversion of methane to methanol (MTM)
Methane is the main component of natural gas, including unconventional sources like coalbed
methane and shale gas, landfill gas, and a by-product of oil refining and chemical processing [63, 64].
It has been suggested that natural gas can be a potential energy source to replace the more polluting
coal and oil until carbon-free energy sources become mature and deployed. However, to store andtransport natural gas from the remote sites undertake great challenge, making it difficult to competitive
with the fossil oil [65]. Therefore methane is often required to convert to other transportable productswith high-density energy and high value-added [66]. Based on that, the conversion of methane into
liquid methanol is much more economical and energy-efficiency [63]. In addition, methanol is a majorcarbon chemical feedstock and important building blocks for a wide range of commodities such as
cosmetics, lubricants, detergents, and polymers [66]. A methanol economy is generally regarded as
one of the most promising alternative energy platforms that can be employed to replace fossil fuels in
the near future [67, 68]. Methane is recognized as one of the most stable hydrocarbon due to its
perfectly symmetrical tetrahedron structure [69]. The dissociation energy of the methane C-H bond is
440 kJ·mol-1, hence its activation requires harsh conditions [69].

1.2.1 Two-step method (industrial route)So far, the industrial route to conversion of methane to methanol is indirect. Firstly, methane
reacts with steam to convert to CO and H2 (synthesis gas) under high temperature using Ni-containing
catalysts (Reaction 1) [70]. The catalysts are usually prepared by impregnation Ni salt solution on
MgO, α-Al2O3 or the mixture of these supports. Typical operating conditions are 1073 K and 15-20
bar with a H2O/CH4 stoichiometric ratio of 3.0-3.5. Usually the excess steam is added to suppress the
carbon formation on the catalyst, and promote the reforming reaction [64, 71]. The second stage formethanol production is carried out by passing synthesis gas on the Cu/ZnO/Al2O3 catalyst at around523 K and high pressure in the range of 50-100 bar (Reaction 2) [52, 72]. In practice CO2 is added to
the feed, which produces advantageous effects, H2/CO/CO2 reaction ratios are usually 80-86/8-10/610. The Cu/ZnO/Al2O3 catalyst produces highly methanol selectively, generally greater than 99% [73].
The stream in the exit of the reactor usually contains methanol with the content of 4-7 vol. %, which
can be removed and recycled the gas stream. The two separate steps method has been applied to6

Chapter 1

produce nearly 100% of methanol worldwide [73].

1.2.2 Enzymatic activation of methane
Even so, the highly selective method to oxidize methane to methanol at low temperature inaqueous phase has already existed in nature. Methanotrophic bacteria can transfer methane to methanol
under aerobic conditions due to the methane monooxygenase (MMO) enzymes contained. There are
two types of MMOs: soluble MMO (sMMO) and particulate MMO (pMMO). sMMO has been foundto be a multi-enzyme complex composed of hydroxylase unit, reductase unit, and regulatory protein.
The oxidation of methane by molecular oxygen occurs at the active site, diiron μ-oxo species in sMMO
(Figure 1.7(a)) and dicopper center in pMMO (Figure 1.7(b)) are found to be the active sites [74].

Figure 1.7 Structure of the active centers in the methane monooxygenase enzymes: (a) the diiron
center in soluble MMO and (b) the dicopper center in particulate MMO [74].
1.2.3 Homogeneous catalytic system in liquid phase
The enzymatic systems inspired researchers to develop homogeneous catalysts for low
temperature partial oxidation of methane. The organometallic approach to methane conversion became
an intense research area [74]. Periana et al. described the oxidation of methane to methanol throughmethyl bisulfate catalyzed by mercuric bisulfate. Methanol was recovered by hydrolysis of methyl
bisulfate. An unprecedented 85% selectivity of methyl bisulfate at 50% methane conversion wasreported [75]. Besides, a subsequent publication showed platinum bipyrimidine complexes to be more
efficient in oleum, resulting in 90% methane conversion and methyl bisulfate at 81% selectivity [76].
7Chapter 1

Note that, the homogeneous liquid phase system usually accompanies by high acid and high pollution,and that methanol is not the direct product. Thus relative research and application are limited.

1.2.4 Direct gas phase system
In the gas phase, generally transition metals (Fe [77, 78], Co [79] and Cu [80-85]) exchanged
zeolites are used as catalysts to form highly active metal-oxygen species with O2 [83, 86-90] or N2O[91-93] (in the case of Fe-containing zeolites). Until now, there are two means for partial oxidation of
methane: high temperature route based on radical gas phase reaction and low temperature routeinvolving heterogeneous catalysis [94]. High temperature partial oxidation reaction works undertemperature higher than 773K, involving fuel-rich feed to minimize combustion, and inert reactor walls
for high selectivity to methanol. Under these conditions the desired oxygenated products are more
active than methane. A few report addressed outstanding yields to methanol and formaldehyde (710%), a large number of studies agreed on the level of CH3OH selectivity of 30-40% with methane
conversion of 5-10% [95]. Nevertheless, the over oxidation still exists and limits the selectivity of thesystem to partial oxygenates. Decreasing the methane partial pressure reduced the concentration of
methyl radicals thus diminished competition with coupling reactions. A number of studies haveadopted this high temperature and low pressure approach, however it was found that methanol was
only selectively produced when methane conversion was <1% [96]. In order to overcome thesedisadvantages, more recent works have focused on low temperature system. The low temperatureapproach (< 523K) requires the high active catalytic system. Isothermal step-wise CH3OH formation
has been reported at 423K and 473K using NO [84] or O2 [97] as the oxidant, but the CH3OH produced
amount of 0.6 mmol CH3OH gzeolite-1 on Cu-ZSM-5 [80] and 5.4 mmol CH3OH gzeolite-1 on Cu-MOR[97] were much lower than those obtained by O2 activation at 723K, 8.1 mmol CH3OH gzeolite-1 on CuZSM-5 [84] and 45.3 mmol CH3OH on Cu-MOR [97].

1.2.5 Heterogeneous approaches in the liquid phase
It is preferable that the direct conversion of methane to methanol takes place on the heterogeneous
catalysts owing to the advantages of separation and reusability when compared with the homogenous
system. With this in mind heterogeneous analogues of the Periana catalyst (Pt-bipyrimidine complex)
were produced by Palkovits et al.[98, 99]. This kinds of catalyst consisted of polymer frameworks
8Chapter 1

coordinated to a Pt centre, where the Pt was coordinated to a covalent triazine-based framework. These
catalysts were reusable and more active than the homogeneous system. However they still required ahighly acidic solvent. Cationic gold was used as the heterogeneous catalyst to selective methane tomethanol in strong acid solvents such as sulfuric acid by Periana and co-workers [100]. In theirresearch gold cations was consider as the uniquely efficient electrophilic catalyst for methane
conversion. The highest methanol yield of 94% and methane conversion of 28% were achieved under
453K.
In addition, transition metal exchanged zeolites are widely used as heterogeneous catalysts inliquid phase. Hammond et al. used iron- and/or copper-containing zeolites for oxidation of methane
selectively to methanol in an aqueous medium with hydrogen peroxide [50-52, 101, 102]. The activeiron site is identified as the extra framework binuclear core, which forms a Fe-OOH intermediate uponactivation with H2O2 while copper acts as a modulator to ensure high methanol selectivity [103].
Besides, supported gold-palladium alloy nanoparticles [104] and titania supported gold palladium
copper catalysts [105] were both applied in direct conversion of methane to methanol with H2O2,
achieving selectivity of 90.3% and 82.7% to methanol, respectively. In the above mention conditions,normally water is used as the solvent, thus the conversion of methane is not satisfactory.
H2O2

CH4

Fe2O22+CH3OOH

CH3OH
× . Cu2+
HO

CO2

HCOOH

Figure 1.8 A potential reaction scheme for the oxidation of methane based on the time-on-line profile.
Methanol is formed through the conversion of the methyl hydrogen peroxide intermediate over the Fesites present in the catalyst. HO· radicals produced during the reaction are later responsible for theover-oxidation of methanol [52].

The reaction pathway that is determined for the oxidation of methane with H2O2 on copperpromoted Fe-ZSM-5 is shown in Figure 1.8 [52]. Methyl hydroperoxide (CH3OOH) was found to be
the primary product of the reaction which underwent subsequent reactions to consecutively form
methanol, formic acid and carbon dioxide. CH3OOH was found to decompose to methanol without thepresence of catalyst. It seems that control of the product distribution could be achieved by the addition
9

Chapter 1of extra framework copper to the zeolite [52, 102]. The Fe-ZSM-5 catalyst achieved a selectivity of
12% to methanol without affecting the overall conversion of the reaction (0.7%). EPR spectroscopy
studies found that Cu2+ did not play a direct role in methane activation but scavenged OH· radicals toplay a role in oxidation of methanol to HCOOH and CO2 [52]. Thus the presence of Cu to Fe-ZSM-5
can improve the selectivity of methanol due to a decrease in concentration of hydroxyl radicals [52].
H
H2O
HO

Fe

O
O

OH2

Fe

OH

H
H2O2

CH3OH

H2O
CH3OOH

HO
HO
HO

H2O2

H
Fe

O
O

OH2

Fe

OH

H
H
HO

O
HO

CH3
Fe

O
O

H
Fe

OH2

HOO

OH

HO

H

HOO
HO

Fe

O
O

O
O

Fe

OH2
OH

H
H2O2

H
CH4

Fe

Fe

O
OH

H

Figure 1.9 Catalytic cycle for the oxidation of methane to CH3OOH using H2O2 catalyzed by abinuclear Fe species in ZSM-5 [52].

Moreover, the ctalytic cycle for the oxidation of methane to CH3OOH using H2O2 catalyzed by abinuclear Fe species is presented in Figure 1.9. The mechanism which is distinguished from Fenton’schemistry, α-oxygen, and a MMO-type rebound mechanism was put forward. Hydrogen peroxide
reacts at the iron centers to produce species capable of the activation of the carbon-hydrogen bond,forming methyl hydroperoxide as the primary product [101].
Besides, Chadwick et al. [106] set forth a slightly different reaction scheme over Fe-containing
zeolites from Hammond [52], as shown in Figure 1.10. Formaldehyde was identified as an
10

Chapter 1

intermediate in the oxidation reaction pathway from MeOOH to formic acid and ultimately CO2. The
relationship between methanol and formaldehyde is competitive, not formaldehyde is oxidated by
H2O2 from methanol.
H2O2

CH4
H2O2

H2

Fe2O22+CH3OOH

CH3OH

HCOOH

HCOH

CO2

HO(CH2O)nH
H2O2

H2O

Figure 1.10 A simplified methane oxidation reaction scheme over Fe-containing catalysts [106].

1.3. Conversion of benzene to phenol (BTP)
Phenol is an important intermediate for synthesis of petrochemicals, agrochemicals, and plastics.
The conversion of benzene to phenol, is one of the most active topics in applied and fundamental
catalytic research [107].

1.3.1 Three-step method (industrial route)
In industry, phenol is obtained by the so-called “three-step cumene process”. Firstly, cumene ismade from benzene and propene using solid pelletised phosphoric acid as catalyst in vapor gas phase or
aluminium chloride as catalyst in liquid phase. Second, cumene is oxidated with oxygen at temperaturesin the range 363-403 K and pressures of 1-10 atm. Thirdly, the hydroperoxide is mixed with dilute
sulphuric acid at 333-343 K to produce both phenol and acetone as products [108, 109]. This process
has drawbacks such as high pollution, high-energy consumption, fussy process and relatively low
selectivity towards phenol.

1.3.2 Direct gas phase system
The researches aimed at studying the possibility of direct converting benzene to phenol using
molecular oxygen as an oxidant have shown low phenol yield. Cu/ZSM-5 zeolites were used as
catalysts for oxidation of benzene to phenol with oxygen in gas phase at 673K, achieving the highest
phenol yield of 5% [110]. Cu-containing zeolite catalysts were also researched by Ichihashi and coworkers in direct conversion of benzene to phenol with oxygen, attaining phenol yield of 4% andselectivity of 88% under a low O2 partial pressure [111]. Iwasawa et al. innovatively applied theRe/zeolites for selective oxidation of benzene with molecular oxygen at 573K in the presence of11Chapter 1

ammonia, achieving the best results benzene conversion of 10% with phenol selectivity of 91% [112,113]. However, the catalytic performance was very poor under the condition of none ammonia.
Besides, the most commonly used oxidant is N2O. Because it contains 36 wt.% oxygen and theby-product is N2 for hydroxylation of benzene to phenol. The disadvantage of using N2O is the highcost. The processes using metal modified zeolite catalysts, such as V2O5/ MoO5/ ZSM-5 and Fe2O3/
MoO3/ ZSM-5, transfer atomic oxygen from the decomposition of the N2O on the catalyst surface tobenzene, which active catalyst appears to be the metal species occupying the pores in the zeolite
structure [44, 46, 114-119].
Panov and co-workers found that Fe-containing zeolites were useful in the oxidation of benzene
to phenol using N2O as oxidant [120-122]. The mechanism of oxidation benzene to phenol with N2O
on Fe-containing zeolites involves two steps [44]. N2O decomposition conducts on the active ironcenters to form a surface oxygen species that is often called “α-oxygen”, which is able to oxidize
benzene to phenol [44].
N2O + ()α(O)α + N2(3)

C6H6 + (O)α()α + C6H6O(4)Many references reported direct oxidation of benzene to phenol with N2O. Hensen et al.
researched on many kinds of hierarchical and nanosheet Fe/ZSM-5 zeolites for conversion of benzene
to phenol with N2O at 523K, the sheet-like and hierarchical structure present slowly deactivation
properties [44, 123-127]. Xiao’s group synthesized the hierarchical Fe-ZSM-5 for conversion ofbenzene to phenol with N2O, achieving more than 20% in benzene conversion [46]. Sheldon and coworkers prepared Fe-ZSM-5 for hydroxylation of benzene with N2O, resulting in high selectivity(>99%) and phenol yields (up to 27%) [114]. Sachtler et al. has been studied a variety of Fe/MFI
catalysts at 673 and 723K for conversion of benzene to phenol with N2O, they identified three typesof Fe species in Fe/MFI catalysts: (1) mononuclear Fe, catalyzing phenol formation, (2) dinuclear Fe,
catalyzing NOx reduction, and (3) iron oxide nanoparticles, catalyzing deep oxidation [128].

1.3.3 Homogeneous catalytic system in liquid phase
The direct oxidation of benzene to phenol using homogeneous catalysts has been less explored.
Homogeneous catalysts based on transition metal complexes with organic ligands offer the advantage
of chemoselectivity. Fukuzumi and co-workers used 3-cyano-1-methylquinolinium (QuCN+) as
photocatalyst for selective oxygenation of benzene to phenol with oxygen and water underhomogeneous and ambient conditions, achieving the highest phenol yield of 26% [129]. The vanadyl(Ⅳ), iron (III) and several first row transition metals acetylacetonate complexes with N2O2 and N4
12

Chapter 1

Schiff base ligands were studied as homogeneous catalysts. The results show that the most selective
and active transition metal complex is the Fe(II) complex with the N4 Schiff base ligand with aselectivity of 98% to phenol and 64% conversion of benzene with a turnover number (TON) of 89 injust 3 hours of reaction [130].

1.3.4 Heterogeneous catalytic system in liquid phase
The homogenous catalysts can provide good catalytic performance, but the disadvantages ofdifficult separation from the reaction media and the non-reusable limit its application in industry. Fromthe sustainable point of view, heterogeneous catalysts are the focus of research. The anchoring oftransition metal complexes with Schiff base ligands can be complicated as it often involves a changeof the ligand structure in order to allow the efficient covalent attachment onto a support, which is themost effective strategy against active phase leaching. Amine-functionalized activated carbon (AC) andhexagonal mesoporous silica (HMS) are widely used as the supports due to the rich hydroxyl on thelarge surface to incorporate vanadyl and iron acetylacetonate complexes [130]. According to Silva etal., [Fe(acac)2]APTES@HMS achieved the highest phenol yield of 68% with hydrogen peroxide at
323K [130]. Mesoporous graphitic carbon nitride (g-CN) was utilized as a new support to immobilize
vanadyl acetylacetonate ([VO(acac)2]), affording a maximum phenol yield of ca. 20% [131].
Another common way is to build a transition metal complex on the porous supports, such asactivated carbon [132-135], carbon nanotubes [136], zeolites [111, 137-139], mesoporous materials[45, 140], metal organic frameworks (MOF) [141, 142], graphene oxide (GO) [143], nanoparticles[144] and so on. There are many investigations using titanium-containing molecular sieves asheterogeneous catalysts for the hydroxylation of benzene to phenol. Pinnavaia et al. obtained a benzene
conversion of 31% with phenol selectivity of 95% in TS-1 catalyzed benzene hydroxylation withhydrogen peroxide [145]. Amorphous titanosilicates showed the selectivity to phenol almost 100% at
a conversion of benzene of 13% [146]. Balducci et al. observed a dramatic improvement of selectivity
by using sulfolane as a cosolvent in the oxidation of benzene with hydrogen peroxide using a NH4HF2
modified TS-1 catalyst (TS-1B), limiting the formation of multiple-oxidation products probably due
to the formation of complexes with phenolic compounds [147].
Molecular sieves such as MCM-48 and MCM-41 take high surface area, rich pores and silanolgroups providing the superiority to graft active species on the surface of mesopores materials or toincorporate the metal ions in the mesoporous system directly by substitution of Si atoms in regulartetrahedral positions. The incorporation of two different metals into the framework of MCM-41 cancreate new properties such as redox activity and acidity. The V-MCM-41 catalysts incorporated withtitanium increased the activity significantly in the oxidation of benzene, achieving the highest
13Chapter 1

conversion of 78% [148]. Cubic KIT-6 and mesoporous hexagonal SBA-15 supported CuO weresynthesized, achieving the phenol yield of 11% with 85% phenol selectivity and phenol yield of 19%
with 92% phenol selectivity, respectively [149]. VOx/SBA-16 with 7.3 wt.% vanadium showed
excellent activity for the hydroxylation of benzene to phenol with hydrogen peroxide, affording phenol
yield of 14% with selectivity of 98% [150]. Fe/SBA-16 was used as catalyst, under the optimized
conditions, 12% phenol yield with 96% selectivity to phenol were obtained [45]. Metal oxides Fe2O3
and Co3O4 were stabilized in the mesopores of MCM-41 molecular sieve, the total conversion ofbenzene could not exceed 11% at 348K [151].

Figure 1.11 Free-radical mechanism of photo-catalytic benzene hydroxylation over MIL-100(Fe)[141].

Figure 1.12 Free-radical and nonradical mechanisms of the catalytic benzene hydroxylation by H2O2
in the presence of Fe/SBA-16[45].
As for the reaction mechanism using Fe-containing catalysts for hydroxylation of benzene to14

Chapter 1

phenol with H2O2, it is still a matter for debate, radical mechanism (Figure 1.11) [141, 152-154] and/or
non-radical mechanism (Figure 1.12) [45, 130, 155] are the mainstreams.
In addition, heterogeneous catalysts are used in the liquid phase, oxygen (gas) can be applied asoxidant [156-158]. Interstitial-N/Re Cluster/Zeolite as catalysts were used in conversion of benzene tophenol, attaining benzene conversion of 10% and phenol selectivity of 91% at 553 K [113]. Besides,
yield of 4% and selectivity of 88% to phenol were achieved over Cu/Ti/HZSM-5 under the O2 partialpressure of 2.5 kPa and 673 K [111]. Note that, under this kind of condition, high reaction temperature
is normally required.

1.4. Relationship between MTM and BTP
Extremely difficult selective oxidation reactions take hydrocarbon containing stable C-H bond asreactant to produce the product with higher value-added, including hydroxylation of methane, selective
oxidation of propane, direct oxidation of aromatic compounds and so on. In this thesis, only MTM and
BTP reaction were involved. Studies have shown that the reaction intermediates and the transition
states in the reaction pathway of benzene to phenol are quite similar to those in the methane to methanol
conversion pathway with respect to essential bonding characters [159]. Benzene and methane,especially methane is recognized as one of the most stable hydrocarbon due to its perfectly symmetricaltetrahedron structure [69]. The dissociation energy of the methane C-H bond is 440 kJ·mol-1 and hence
its activation may require harsh conditions [69]. Typically, the products of direct conversion methane
to methanol are trace and high requirement on the analysis equipment [52]. It seems that the benzene
to phenol process is slightly easier compared to the methane to methanol process, judging from thepublished papers concerning the methane hydroxylation and benzene hydroxylation [160]. Besides,
the products of benzene to phenol process are easier to detect [161]. When Fe-containing zeolites areused as catalysts with H2O2, the processes of hydroxylation of benzene to phenol and methane tomethanol possess the similar mechanism. Hence, benzene to phenol process can be selected as thescreening reaction to investigate the catalytic performance firstly. The catalysts with good performance
are then applied in the conversion of methane to methanol process.

1.5. Aim of the thesis
Selective oxidation of methane in a stirred autoclave and selective oxidation of benzene in a
stirred reflux setup were studied using hydrogen peroxide as oxidant and Fe-containing zeolites,
15

Chapter 1

mainly including MFI, MWW and BEA topological structure, as catalysts. High activity Fecontaining zeolite catalysts were prepared and the reaction parameters based on the catalyst were
optimized for both MTM and BTP reaction in order to obtain an efficient and environmentally friendly
reaction system.

1.6. Scope of the thesis
Chapter 2 explored Fe-containing MFI zeolite catalysts including Fe-silicalite-1 and Fe-ZSM-5
by direct or post-synthesized method and post-modification by acid or alkaline treatment. The reaction
conditions were investigated in detail. The relationship between the states of Fe species and catalytic
performance was established. The formation of isolated and oligomeric extra framework Fe species in
MFI zeolite played a key role for achieving high yield of phenol.
Chapter 3 investigated the Fe-silicalite-1 zeolites by direct method using TPAOH as OSDA withor without Na cations in the synthesis gel. The physicochemical properties, especially the states of Fe
species were characterized. The impact of solvent on the reaction performance was mainly investigated.
The use of aqueous sulfolane with an appropriate proportion led to an extremely high methanol
production with a high selectivity. The influence of distribution of Fe species for the Fe-silicalite-1
zeolites on the catalytic performance in direct conversion of methane to methanol with H2O2 have beenstudied.
Chapter 4 discussed the Fe-ZSM-5 zeolites by direct method using TPAOH as OSDA with orwithout Na cations. The physicochemical properties, especially the states of Fe species werecharacterized. Fe-ZSM-5 zeolites synthesized without Na cations displayed more uniform distribution
for each kind of Fe species and higher proportion of framework Fe than those synthesized with Na
cations. Thus synthesized Fe-ZSM-5 zeolites were applied in direct oxidation of benzene to phenol
and methane to methanol with H2O2. Meanwhile, the catalytic performance in the two reactions werecompared with the Fe-silicalite-1 zeolites to explore the function of aluminum.
Chapter 5 researched the influence of desilication on as-synthesized and calcined Fe-ZSM-5 andFe-silicate-1 zeolites. The textural properties and the states of Fe species were thoroughly discussed.
The function of OSDA and metals, including Fe and Al, in the process of desilication were researched.
Finally, the thus prepared zeolites were used in hydroxylation of benzene to phenol with H2O2.
16

Chapter 1

Chapter 6 discovered Fe-containing MWW zeolites, including Fe-MWW and Fe,Al-MWW, bydirect method. Extensively characterization, especially the states of Fe species and the acidity ofcatalysts were researched. The effects of calcination temperature in different steps on the catalytic
performance have been studied in detail. The influence of aluminum in the Fe-containing MWW
zeolites on the formation of Fe species and catalytic performance for both hydroxylation of benzene
to phenol and methane to methanol with H2O2 have been researched.
Chapter 7 included Fe and/ or Cu exchanged Beta catalysts with varying metal contents forhydroxylation of benzene to phenol and methane to methanol with H2O2. The states of Fe and Cu
species were characterized by UV-vis, NO adsorbed FT-IR and NH3-TPD in detail. The influence ofthe reaction conditions in BTP reaction on Fe and/ or Cu-containing Beta zeolite catalysts have been
investigated. The reusability of the Fe and/ or Cu-containing Beta zeolite catalysts and the effects ofdifferent metallic states of the bimetallic Beta zeolite catalysts on the catalytic performance in BTP
reaction were studied. The catalysts with noble performance in each serial were applied in conversionof methane to methanol with H2O2.

17

Chapter 1

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23

Chapter 2

Chapter 2
Direct synthesis of phenol by hydroxylation of benzene with hydrogen peroxideover Fe-containing MFI zeolite catalysts
Abstract
Fe-containing MFI zeolite catalysts, including Fe-silicalite-1 and Fe-ZSM-5, with different states ofFe species were prepared by the direct and post synthesis method. The states of Fe species and acidic
property were characterized by UV-vis, NO adsorbed FT-IR, and NH3-TPD in details. The preparedcatalysts have been investigated in the hydroxylation of benzene to phenol with H2O2. The effects ofvarious reaction parameters on the catalytic performance and the reusability of the catalyst have beenstudied. The phenol yield of the directly synthesized Fe-silicalite-1 zeolites were increased with the Fecontent due to the increasing of isolated and oligomeric Fe species on the extra framework, while forthe post synthesized Fe-silicalite-1 and Fe-ZSM-5 zeolites, phenol yield were not proportional to the
Fe content. Under optimized reaction conditions, directly synthesized Fe-silicalite-1 showed thehighest phenol yield of 5.7 %, which was higher than the post-synthesized Fe-silicalite-1 and Fe-ZSM5. Furthermore, the phenol yield can be improved to 7.6 % by the post-treatment under alkalineconditions. Combination of the catalytic performance and the characterization results, we haveconsidered that the isolated and oligomeric Fe species on the extra framework play an essential role indirect hydroxylation of benzene to phenol with H2O2.

2.1. Introduction
The conversion of benzene to oxygen-containing aromatic compounds, such as phenol, is one of
the most active topics in applied and fundamental catalytic research. In industry, phenol is obtained by
the “three-step cumene process” [1-3]. This process has drawbacks such as high pollution, high-energy
consumption, fussy process and relatively low selectivity towards phenol. Thus, much effort has been
devoted to the one-step hydroxylation of benzene to phenol (hereinafter called “BTP”) using different
oxidants such as N2O [4], O2 [5], air [6], H2/O2 [7], and H2O2 [8]. However, the direct introduction ofhydroxyl functionality into benzene is challenging. Almost all the developed catalytic systems in thegas phase require an elevated temperature and suffer from low conversion because of the notoriously
low reactivity of aromatic C-H bonds [9,10]. Moreover, phenol is easily over-oxidized to produce byproducts such as catechol (CL), hydroquinone (HQ), benzoquinones (p-BQ), and tar in the liquid phase,
which makes the selective oxidation of benzene to phenol very difficult [11].
24Chapter 2

Fe-containing MFI zeolite has been recognized as a potential catalyst in the BTP reaction, butalmost all the related reports were about acting in concert with N2O in the gas phase under 500-700 K[4,12,13]. Round 20-30% initial benzene conversion were achieved by Fe-MFI zeolites under the hightemperature [14]. However, the common problem was the rapid deactivation of the catalysts due to thecoke formation [4,15]. In general, the activity can be reduced by 50% in the first three hours [15].
Meanwhile, more than one literatures reported that the catalytic activity required the participation ofaluminum besides iron, but aluminum on the extra framework could result in a faster catalyst
deactivation [14,16,17], which was difficult to get a good balance.
Among the oxidants for the BTP reaction, H2O2 has a clear advantage from the viewpoint of anenvironmentally benign green process and economical efficiency because of relatively mild reaction
conditions and environmental friendly product [18]. Although few reports were about Fe-MFI zeolites
with H2O2 to directly convert of benzene to phenol. Other metal-based heterogeneous catalysts
including amorphous microporous mixed oxides [19], mesoporous silica and silica-alumina [20,21],
activated carbon [22,23], graphitic carbon nitride [24] and metal-organic framework [9] were often
reported with H2O2 in liquid phase [22,25]. Bianchi et al. used titanium silicalite (TS-1) as catalyst,
4.5% of benzene conversion with 43% of phenol selectivity in acetonitrile was obtained;26 when they
applied TS-1B, which modified with NH4HF2 and H2O2, the benzene conversion and phenol selectivity
were up to 8.6% and 94% in sulfolane [27]. CuAPO-5 zeolite was applied in the BTP reaction, 5.9%
benzene conversion with 68% phenol selectivity was achieved [28,29]. Besides, the BTP reaction was
performed by Fe/TS-1 mixture, a moderate phenol yield of 7.6% with selectivity of 15% was obtained[30]. Hu and co-workers reported the stability of different vanadium species existing on vanadiumsilicalite-1 (VS-1) zeolite catalysts during the hydroxylation conditions, vanadium incorporation in theframework showed a stable phenol yield of 11% with above 90% selectivity [31].
It is well known that the reaction mechanism in hydroxylation of benzene was related to theoxidants and catalysts. The published literatures showed that the activation of H2O2 to radicals (HO·)
via a Fenton-like route over Fe-containing catalysts can be conceived for the oxidation of benzene[9,20,32]. Moreover, it was normally accepted that the isolated or oligomeric Fe species on the Fecontaining zeolite catalysts are active for the BTP reaction [33,34]. But the differences in activity of
the Fe species have not been fully investigated in BTP reaction with H2O2. The preparation method of
Fe-containing zeolites strongly influences the nature and states of the resulting Fe species, whichbasically determine the catalyst performance. Extensive efforts have been dedicated to establishingreliable procedures for iron incorporation, including isomorphous substitution [35], ion exchange [36],impregnation [37], chemical vapor deposition (CVD) [38] and so on. Incorporation of heteroatoms
25

Chapter 2

into zeolites can modify the surface properties to obtain highly dispersed Fe species so as to improvethe catalytic activity. Compared to the post-synthetic modification, the direct synthetic method is morefeasible and has industrial potential [18].
To our best knowledge, no open literature has been reported the direct synthesis of phenol frombenzene using Fe-containing MFI zeolite catalysts with H2O2 to date. In this study, Fe-containing
MFI-type zeolite catalysts were prepared by different methods, modified with acid or alkaline
treatment, and applied in the BTP reaction. The effects of solvent and the reaction conditions on the
BTP reaction and the reuse of the catalyst were investigated. In addition, the catalytic performancewas considered based on the states and species of iron in the catalysts.

2.2. Experiments
2.2.1. Materials
Tetraethyl orthosilicate (TEOS), tetrapropylammonium hydroxide (TPAOH) (20-25% in water)
and tert-butanol were purchased from Tokyo Chemical Industry. NaOH, Fe(NO3)3·9H2O, α-Fe2O3,
NH4NO3, H2O2 (30%), benzene, acetone, acetonitrile, sulfolane and anisole were purchased from
Wako. All of the reagents were used as received, without further purification.

2.2.2. Catalysts preparation
Fe-containing MFI-type zeolites were synthesized by directly hydrothermal or post-synthesis
method. In the direct synthesis method, TEOS was added to the solution containing water, TPAOH,
NaOH and Fe(NO3)3·9H2O. The gels with molar composition of 1 Si: (0.01-0.067) Fe: 0.25 TPA: 0.25
Na: 30 H2O were crystallized at 443 K for 24 h (40 rpm) after aging 24 h at 353 K. The solid products
were filtered, washed, and dried to get the as-synthesized ones. The as-made samples were calcined at
823 K for 10 h to remove TPA+, and then ion-exchanged with 1 M NH4NO3 aqueous solution at 353
K for 3 h twice to obtain the NH4+-type ones. H-type samples were made from the NH4+-ones by
calcination at 823 K for 5 h. Thus obtained products were denoted as “FMDx”, where x was the Si/Fe
molar ratio in the mother gel. In the post-synthesis method, silicalite-1, which has a siliceous
framework with the MFI topology, was synthesized by the similar procedure to that of FMDx.
Fe-silicalite-1 catalysts were also prepared by incipient wetness impregnation. A certain amountof Fe(NO3)3 aqueous solution was added to the silicalite-1, and then the mixture was dried at 373 K
overnight and calcined at 823 K for 5 h in air. The obtained products were denoted as “FMPy”, where
y was the inputting Fe content (wt.%) (based on the zeolite).
The as-synthesized FMD15 sample was representatively treated in acidic or alkaline aqueous
26

Chapter 2

solution using 6 M HNO3 at 373 K refluxed for 20 h or 0.2 M NaOH at 353 K for 2 h, respectively.
The resultant samples were NH4+ exchanged and calcined to obtain the H-type ones. The final products
were designated as “ACT-FMD15” and “ALT-FMD15”, respectively.
In order to study the influence of aluminum in the post-synthesized Fe-containing MFI zeolites,
Fe-ZSM-5 zeolites were prepared by incipient wetness impregnation using NH4-type ZSM-5(Si/Al=40, Zeolyst) as parent. The preparation procedure was similar to that of FMPy. The final
products were designated as “FZPz”, where z was the inputting Fe content (wt.%) (based on the zeolite).

2.2.3. Characterization of catalysts
XRD patterns were collected on a Rint-Ultima III (Rigaku) using a Cu Kα X-ray source (40 kV,
20 mA). Field-emission scanning electron microscopic (FE-SEM) images of the powder samples wereobtained on S-5200 (Hitachi) microscope operating at 1 kV. Elemental analyses of the samples were
performed on an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu
ICPE-9000). Nitrogen adsorption and desorption measurements to obtain information on the microand meso-porosities were conducted at 77 K on a Belsorp-mini II (MicrotracBEL).
UV-vis diffuse reflectance spectra were recorded on a V-650DS spectrometer (JASCO). The
diffuse reflectance spectra were converted into the absorption spectra using the Kubelka-Munkfunction. Fourier Transform Infrared (FT-IR) spectra were obtained by a JASCO FT-IR 4100
spectrometer equipped with a triglycine sulfate (TGS) detector. For FT-IR observation, the sample waspressed into a self-supporting disk (20 mm diameter, ca. 30 mg) and placed in an IR cell attached to aclosed gas-circulation system. The sample was pretreated by evacuation at 773 K for 2 h, then adsorbed
5-1000 Pa NO at ambient temperature. The IR spectra resulting from the subtraction of the background
spectra from those with NO adsorbed are shown unless otherwise noted.
Temperature-programmed ammonia desorption (NH3-TPD) profiles were noted down on a
Multitrack TPD equipment (Japan BEL). Normally, 25 mg catalyst was pretreated at 773 K for 1 h ina He flow of 50 mL min-1 and then cooled down to 423 K. The sample was evacuated at 423 K for 1
h prior to the adsorption of NH3. Approximately 2500 Pa of NH3 contacted with the sample at 423 K
for 10 min. Subsequently, the sample was evacuated to remove the weakly adsorbed NH3 at the same
temperature for 30 min. Finally, the sample was heated from 423 to 873 K at a ramping rate of 10 K
min-1 in a He flow of 50 mL min-1.
27

Chapter 2

2.2.4. Catalytic tests
The BTP reaction was carried out in a 25 mL round-bottom flask equipped with a reflux condenser
and a magnetic stirrer. In a typical run, the mixture containing 50 mg catalyst, 10 mL acetonitrile, 5mmol benzene, and 10 mmol H2O2 was stirred at 333 K for 6 h. After the mixture was cooled down, acertain amount of anisole as internal standard was added. Then the catalyst was removed and the
products were fixed by exhaustive acetylation with excess (CH3CO)2O-K2CO3, the derivative products
were analyzed by GC [39]. The amount of unconverted H2O2 was quantified by standard titration
method with 0.1 mol/L Ce(SO4)2 solution. The phenol yield was based on the amount of phenol
produced per the initial amount of benzene. The product selectivity was based on phenol,hydroquinone and catechol. As for the reusability tests, the catalyst was recovered by filtering andwashing with water and ethanol, and drying at 373 K overnight.

2.3. Results and discussion
2.3.1 Characterization of catalysts
2.3.1.1 Physicochemical properties
Figure 2.1 shows the XRD patterns of the samples with the typical MFI structure. The diffraction
peaks ascribed to large bulk iron oxide were not observed, suggesting that the Fe ions were welldispersed. The FMD15 showed a relatively lower crystallinity compared to other FMDx samples,
probably due to the formation of amorphous silica and/or Fe species under the high Fe content. Afteracid or alkaline modification (ACT-FMD15 and ALT-FMD15), the crystallinity was slightly increased
owing to the removing of the amorphous silica and/or Fe species on the surface. In addition, the MFI
structure and crystallinity remained, and the peaks ascribed to large bulk iron oxide were not observed
for the post-synthesized FMPy and FZPz. The post-synthesized samples showed similar morphology
and crystal size to the parent silicalite-1 and ZSM-5, respectively.
The FE-SEM images of the samples are shown in Figure 2.2. The FMDx catalysts possess atypical coffin-shaped crystal with the crystal size increasing from ca. 0.5 to 2.4 µm (Figs. 2(a-d)).
Some small particles, probably silica and/or Fe species were observed on the surface of FMD15, which
was consistent with the results of XRD (Figure 2.1). After acid treatment, the morphology was
unchanged, and the floccule on the surface of ACL-FMD15 still exist, may be the amorphous silica(Figure 2.2(e)). However, most of the floccule species disappeared and lots of tiny holes appeared on
the surface of ALT-FMD15 (Figure 2.2(f)). Combination the characters of demetalization by acidtreatment and desiliconization by alkaline treatment, the floccule can be inferred to contain most of
amorphous silica and small amount of amorphous Fe species. The morphology and particle size of28

Chapter 2

silicalite-1 was similar to FMD100 (Figure 2.2(g)). The parent zeolite of FZPz, i.e. NH4+-ZMS-5,
showed the irregular shape with tiny particle size but reunion (Figure 2.2(h)).
BET surface area (SBET) and external surface area (SEXT) of FMDx zeolites determined by N2
adsorption isotherms were deceased from 378 m2g−1 to 234 m2g−1 and 103 m2g−1 to 81 m2g−1,
respectively, along with the Fe content increase (Table 2.1 and Figure 2.3). The possible reasons were
the increased particle size and the increased Fe species on the surface and channels. The micropore
volume of FMDx zeolites maintained at 0.17-0.13 cm3g−1 and the total pore volume were 0.42-0.49cm3g−1. The SBET of ACT-FMD15 and ALT-FMD15 were drastically increased from 234 m2g−1 to 491
m2g−1 and 452 m2g−1, respectively. For ACT-FMD15, the formation of considerable amountmicropores (0.30 cm3g-1) contributed to the increase of SEXT from 81 to 121 m2g−1. In contrast, thesignificantly incremental SEXT of ALT-FMD15 from 81 to 245 m2g−1 was owing to the formation of
considerable amount mesopores (0.58 cm3g-1) [40]. The SBET of FMPy and FZPz were decreased
comparison with the parents. The compositions of the catalysts are listed in Table 2.1. The Si/Fe ratios
in the FMDx were higher than those in the synthesis gels, meaning that only a part of Fe species wasintroduced into the zeolite. After acid treatment of FMD15, the Si/Fe ratio was drastically increased
from 22 to 184. Contrarily, the Si/Fe ratio was slightly decreased from 22 to 20 after the alkaline
treatment.

2.3.1.2 Fe states
According to the references [41,42], different types of Fe species exist in Fe-containing zeolites.
The possible Fe species are illustrated in Scheme 1. UV-vis spectroscopy is a facile technique tocharacterize the Fe species in Fe-containing zeolites. The oxygen-to-metal charge transfer bands cangive useful information on the coordination states and aggregation extent of Fe species [36]. Generally,
the bands below 250 nm are assigned to framework Fe species, the bands between 250-350 nm are
ascribed to isolated and oligomeric Fe species on the extra framework within the zeolite channels (e.g.,
dimeric, trimeric, and tetrameric Fe species), the bands between 350-450 nm are ascribed to iron oxide
clusters, and the bands above 450 nm are ascribed to bulk iron oxide particles smaller or larger than 2
nm [43].
Figure 2.4 shows the UV-vis spectra of the Fe-containing MFI zeolite catalysts, which were
deconvoluted into several bands by applying Gauss functions. The amount and proportion of different
Fe species based on the relative peak areas are listed in Table 2.2. For FMD100, ca. 68% of Fe ionsin the zeolite were in the framework. The extra framework Fe species were only in the form of isolated
and oligomeric types. When the Fe content was increased (FMD50), the proportion of framework Fe
29

Chapter 2

species was decreased, and the iron oxide clusters were newly formed. Further increasing Fe content(FMD25), the proportion of isolated and oligomeric extra framework Fe species were further increased.
Moreover, iron oxide species including clusters and bulk particles were also formed. The distribution
of Fe species in FMD15 was similar to that of FMD25, while obtained much higher content in eachkind of Fe species due to the high total Fe content. For ACT-FMD15, only ca. 14% of Fe species werein the form of isolated and oligomeric Fe species on the extra framework. The fact meant that most of
Fe species including in the framework and extra framework were removed by the acid treatment. On
the other hand, the alkaline treatment caused an increase in the proportions of iron oxide clusters andbulk iron oxide particles (ALT-FMD15), while those of framework Fe species were greatly decreased
from 45 to 30%, the isolated and oligomeric Fe species were slightly decreased from 39 to 36%.
Because acid treatment was demetallization and alkali treatment was desiliconization. Unlike thedirectly synthesized FMDx samples, the post-synthesized FMPy and FZPz catalysts showed broad
bands at 250-450 nm. Around 80% Fe species were on the extra framework, and most of them were in
the forms of iron oxide species including clusters and bulk particles (Table 2.2). The UV-vis spectraof FMPy zeolites showed the shoulders on high wavenumbers (550 nm), while FZPz samples displayed
the corresponding on the low wavenumbers (210 nm). The FZPz samples showed similar amount ofisolated and oligomeric Fe species to the FMPy, but less iron oxide under the similar Fe content.
Even though there was no references to report the fitting and classification based on the bands
position for catalysts with high Fe content. It was not suitable to deconvolute for FMPy and FZPz
zeolites. Because no matter there were tetra and hexa-coordinated Fe species, or isolated andoligomeric Fe species, the intensity of bands from 190 to 350 nm were higher than zero, which also
has been proved by the spectrum of α-Fe2O3. Besides, it was hard to distinguish the FMPy and FZPz
zeolites based on the spectra of UV-vis. Thus the in-situ FT-IR spectroscopy using NO as probe
molecule was required.
According to the references [44-46], NO adsorbed FT-IR technology would characterize thecoordination unsaturated Fe species on the extra framework. Figure 2.5 shows the FT-IR spectracollected from the annealed Fe-containing MFI zeolites during stepwise adsorbed NO at 298 K. For
FMDx zeolites, the main bands centered at 1867 cm−1 have been assigned to Fe2+(NO) complexes
formed on oligomeric Fe species [45,47]. Further increasing NO pressure, new bands at 1915 and 1809
cm−1 assigned to Fe2+(NO)3 complexes, 1840 and 1765 cm−1 assigned to Fe2+(NO)2 complexes on theisolated Fe species appeared [45]. These bands were gradually increased along with the NO pressure
until saturated. For ACT-FMD15, the intensities of all the bands were decreased compared to theparent FMD15, indicating that both the isolated and oligomeric Fe cations were removed by the acid30

Chapter 2

treatment. In contrast, for ALT-FMD15, the intensities were slightly higher than those of FMD15,
suggesting the possibility of higher amount of isolated and oligomeric Fe species after desilication.
No remarkable bands were observed for FMP2.5, FMP5.0, the mixture of α-Fe2O3 and silicalite1, indicating that iron oxide species including aggregated clusters and bulk particles were dominant in
the FMP2.5 and FMP5.0, which were silent in NO adsorbed FT-IR [45,48]. While for FZP2.5 and
FZP5.0, both the bands at 1810 cm-1 assigned to Fe2+(NO)3 complexes and the shoulder at 1850 cm-1
attributed to the Fe2+(NO)2 complexes on the isolated Fe species were weak, indicating the low content
of isolated Fe species [44,49]. The main bands at 1880 cm-1 assigned to Fe2+(NO) complexes formedon the surface of the iron oxide clusters or Fe2+-O-Al particles [50]. The Fe content of FZP5.0 was
twice that of FZP2.5, but the intensity of NO adsorbed FT-IR was similar, suggesting that the content
of coordination unsaturated Fe species was approximative.

2.3.1.3 Acidity
Figure 2.6 shows the NH3-TPD profiles of the Fe-containing MFI zeolite catalysts. All the
profiles can be deconvoluted into three peaks at low, medium, and high temperatures (designated as
LT, MT, and HT). The amounts of NH3 adsorbed and the temperature of the maximum peak (Tmax) arelisted in Table 2.3. According to the references [51,52], the LT peak, which corresponds to NH3
adsorbed on the non-acidic OH groups and NH4+ by hydrogen bonding, was not related to the true acidsite and excluded in the discussion. The MT and HT peaks were corresponded to NH3 adsorbed on theextra framework Fe species (Lewis acid site), and on the bridged Si-O(H)-Fe species (Brønsted acidsite), respectively [53].
When x was decreased from 100 to 25, the amounts of NH3 adsorbed for both the MT and the HT
peaks were increased, meaning that both the extra framework and framework Fe species wereincreased. Meanwhile, the MT peak was shifted to lower temperature, suggesting the decrease in theacid strength. It may be related to the different fractions of extra framework Fe species in these samples.
Further decrease the Si/Fe ratio, FMD15 showed slightly low acid amounts for both the MT and the
HT peaks due to the severely aggregated Fe species. After acid treatment, ACT-FMD15 showed alower amount of NH3 adsorbed for the HT peak, meaning that the framework Fe was easily removed
during the acid treatment. However, the amount of NH3 adsorbed for the MT peak was increased andthe peak was shifted to high temperature. On the other hand, after alkaline treatment, ALT-FMD15
showed a high amount of NH3 adsorbed for MT peak. Of particular note is that the MT for ALTFMD15 was lower than that for ACT-FMD15. These facts suggested that the extra framework Fe
species were different in the two samples; more isolated Fe species existed in ACT-FMD15 and more
31

Chapter 2clustered Fe species were formed in ALT-FMD15. In addition, NH3 was hardly adsorbed on the postsynthesized FMPy samples, so as to the pure α-Fe2O3, suggesting that iron oxide species including
severe aggregated clusters and bulk particles were dominant in the FMPy samples once more. Note
that, although the total Fe content of ACL-FMD15 and FMD100 were only 0.5 wt.%, the acid amountat both MT and HT were much higher than FMP2.5. It implied that it was the states of Fe species, notthe content, that affected the acidity. For the H-ZSM-5, the HT was higher than that of the FMD
samples, meaning the acidity of H-ZSM-5 zeolite was stronger than Fe-silicalite-1 [54]. Compared to
H-ZSM-5, FZP2.5 and FZP5.0 showed an increased NH3 adsorbed amount on the MT due to theintroduction of Fe species on the extra framework (Table 2.3). Meanwhile, the HT for FZP2.5 and
FZP5.0 were reduced to 614 and 619 K from 752 K, but the acid amount were maintained, probably
due to the affected states of aluminum by the incorporation of Fe species [52].
2.3.2 Benzene to phenol reaction with H2O2 (BTP)
2.3.2.1 BTP reaction over Fe-containing MFI catalysts
The catalytic results in BTP reaction of the thus prepared catalysts are summarized in Table 2.4.
When x was decreased from 100 to 15, the phenol yield was increased from 1.5 to 5.7%. Meanwhile,the H2O2 conversion was also increased along with Fe content. The selectivity of phenol was sustained
at 95-97%. The phenol yield for ACT-FMD15 was decreased to 1.3% due to the removed iron. In
contrast, the phenol yield for ALT-FMD15 was improved to 7.6% owing to the increased Fe content,
surface area and pore size. Compared to FMDx, the post-synthesized FMPy catalysts exhibited
extremely low phenol yields regardless of the Fe content, so as to the silicalite-1. In contrast, theFZP2.5 achieved 3.8% of phenol yield, which was comparable to FMD25. Further increasing the Fecontent, FZP5.0 resulted in a decrease to 3.0% in the phenol yield. It was noteworthy that the catalytic
performance of FMDx zeolites were increased with Fe content, while high Fe content of the postsynthetic zeolites were not beneficial to improve the reaction performance.

2.3.2.2 The influence of the reaction conditions on the reaction performance
The influences of reaction temperature, the molar ratio of H2O2/benzene, reaction time andcatalyst amount on the BTP performance were investigated by using FMD15 as catalyst. As shown in
Figure 2.7(a), the phenol yield was firstly increased with temperature increasing from 313 to 333 K,
and then decreased with the temperature increasing from 333 to 373 K. Meanwhile, the H2O2
conversion was gradually increased from 43 to 93% along with the temperature. The increased H2O2
conversion was partly to produce HO· and HO2·, and another part was consumed due to the self32

Chapter 2

decomposition.55 Although the phenol selectivity was almost remained above 98%, the darker colorsof the liquid phase after reaction indicated the production of undetectable by-products with reaction
temperature increasing.
As for the influence of the molar ratio of H2O2/benzene on the catalytic performance, the results
are showed in Figure 2.7(b). When the molar ratio of H2O2/benzene was increased from 0.5 to 8, thephenol yield was firstly increased from 1.9 to 6.9% and then slightly decreased to 5.9% with theselectivity of phenol decreasing from 100 to 93%. Although the H2O2 conversion was decreased from90 to 50, the consumed molar amount of H2O2 was increased. Theoretically, more H2O2 can provide
more HO· and HO2· when the catalyst activity was high enough [20], thus more products including
the undetectable ones were produced. The increased benzene conversion from 6.6 to 57% wasadvantageous to prove. On the other hand, the increased H2O2 amount diluted the reaction system ona certain degree, this may be a reason to reduce the phenol yield when the molar ratio of H2O2/benzene
was up to 8.
Thirdly, the effects of reaction time on the catalytic performance were investigated (Figure
2.7(c)). When the reaction time was prolonged from 2 to 6 h, both of the benzene and H2O2 conversions
were increased, leading to the increase of the phenol yield from 2.6 to 5.7% with the phenol selectivity
stabled above 98%. The H2O2 conversion reached above 72% at 6 h; most of the H2O2 was consumed
by the self-decomposition as well as the reaction. Further prolonging the reaction time from 6 to 10 h,
the benzene conversion and phenol selectivity were slightly increased or maintained, resulting in theslight increase in the phenol yield to 5.8%. It was possible explained by the fact that no more active
HO· was provided by H2O2 [20].
The effect of catalyst dosage was evaluated on the direct production of phenol from benzene in
the range of 10 to 100 mg (Figure 2.7(d)). The phenol yield was increased from 1.5% to 7.2% withthe catalyst amount increasing due to the presence of more catalytic active sites. Both the conversionof H2O2 and benzene were rapidly growth from 50 to 80% and 15 to 25%, respectively, with the catalystamount increasing from 10 to 50 mg. The conversion of H2O2 and conversion were slowly increased
from 80 to 88% and 25 to 29%, respectively, with further increasing the catalyst dosage from 50 to
100 mg, possibly due to the limitation of other reactants. The phenol selectivity was maintained above
95% with catalyst amount increasing.
Besides, solvent is vitally important for the BTP reaction [19]. Several solvents such as acetone,
acetonitrile, sulfolane and tert-butanol were investigated in the study (Figure 2.8). When acetonitrile
was used as solvent, the highest phenol yield, high phenol selectivity and H2O2 conversion wereachieved. In the open literature, sulfolane has been used as a decent solvent in BTP reaction with TS33

Chapter 2

1 as catalyst due to its good solubility, giving high phenol selectivity [26]. However, in our case, thephenol yield was very low in the cases of acetone, sulfolane and tert-butanol, which was consistent
with the report by Bianchi et al.[55]. Because the compatibility of metal active centers and reaction
solvents are critical factor to influence the reaction performance. Basically, TS-1 catalysts achieved
better performance in sulfolane [25,26], while Fe- and Cu- containing catalysts were suitable inacetonitrile [9,28].

2.3.2.3 Reusability of FMD15 catalyst
The reusability of FMD15 in the BTP reaction with H2O2 was investigated. Figure 2.9 shows thechanges in the phenol yield, phenol selectivity, and H2O2 conversion in the 6 times reactions, indicatingthat the phenol selectivity was fairly constant above 95% and that the phenol yield and H2O2
conversion were gradually decreased by 10-15%. The fresh catalyst showed 5.7% of phenol yield and
88% of H2O2 conversion (the first-run). The used catalyst was simply washed with water and ethanoland dried at 373 K overnight after each run. Thus regenerated catalyst gave 5.1% of phenol yield and
69% of H2O2 conversion after the 5th run. The reduced phenol yield and H2O2 conversion would be
partly related to the leaching of the Fe content, which decreased from 4.0 to 3.3 wt.% after the 5th run(Table 2.5). Moreover we found that the phenol yield was restored to the level of slightly less than the
fresh catalyst by calcination at 823 K for 5 h in air of the 5th run catalyst. The fact suggested that theformation of heavy carbonaceous by-products, which were difficult to remove by washing, resulting
the decrease of the activity. In addition, calcination can not only remove the coke but also rearrange
the Fe species so as to improve the activity, even though the total Fe content was reduced [56].
Besides, the leaching Fe species during the BTP reaction was curtly researched. Firstly, the liquid
phase after BTP reaction (noted as LA) was separated with the catalyst and re-reacted in the same
condition, the results are listed in Table 2.6. 0.5% phenol yield and 0.4% HQ yield were achieved,
indicating that the lost Fe species contained a part of active Fe species. Considering 72% H2O2 hasbeen already consumed in the first run, 1 g additional H2O2 was added to eliminate the effect of
insufficient H2O2 (noted as LA+H2O2). Surprisingly, 3.8% phenol yield and 0.3% CL yield wereachieved in the system of LA+H2O2, with extra 16% H2O2 conversion. The results once again
confirmed the presence of active Fe species in the leaching ones. The catalytic performance of α-Fe2O3
and Fe(NO3)3·9H2O were investigated in the typical reaction conditions to explore the leaching Fe
species, results showed that Fe(NO3)3· 9H2O, i.e. Fe3+ was active while α-Fe2O3 was not active in the
BTP reaction. 50 mg Fe(NO3)3· 9H2O contained about 6.9 mg Fe3+, achieved 104 µmol phenol (1.7%
phenol yield) and 101 µmol CL (1.7% CL yield). The produced phenol amount of Fe(NO3)3· 9H2O
34

Chapter 2

was equivalent to that in the system of LA+H2O2, but the system of LA+H2O2 only contained 0.15 mg
Fe species, which was calculated based on the loss of Fe content after the first run. It implied that theleaching Fe species included active Fe3+ or more active Fe species.

2.3.3 Discussion on the active sites in the BTP reaction
In this section, we will discuss the active sites for the BTP reaction with H2O2 as oxidant in liquid
phase, by correlating the catalytic performance with the characterization results.

2.3.3.1 Fe content and state
Figure 2.10 summaries the phenol yield as a function of Fe content for all the catalysts used inthis study. Fe-containing MFI zeolites achieved 0.1-7.6% of phenol yield dependent on the Fe content.
Considering that silicalite-1 did not show any activity in the BTP reaction, the introduction of Fe
species was indispensable for the reaction. It was observed that the phenol yield was dependent on the
Fe content for the directly synthesized FMDx zeolites, which was increased from 1.5 to 5.7% with Fecontent increasing from 0.6 to 4.0 wt.%. ACT-FMD15 showed 1.3% phenol yield, which was similar
to FMD100 with comparable low Fe content (0.5-0.6 wt.%). Contrarily, ALT-FMD15 displayed 7.6%
phenol yield, achieving the highest value among the catalysts investigated in the research. On one hand,the desilication made the increase of Fe content from 4.0 to 4.5 wt.%, thus increased in the isolated
and oligomeric Fe species. On the other hand, the increased pore size and surface area reduced themass transfer resistance [14]. The catalytic performance of the post-synthesized FMPy and FZPz
illustrated the importance of the Fe states besides the Fe content in the BTP reaction.
As mentioned above, the states and the extent of aggregation of the Fe species on the extra
framework were affected by several factors, such as preparation method, post-treatment, and theabsence or presence of Al in the parent zeolite. According to the reference [57], the extra framework
Fe species played an essential role in the BTP reaction with N2O in gas phase, though the exact
structure of active Fe centers (α-sites) remained unclear. In our case, iron oxide species (α-Fe2O3) were
inactive for the BTP reaction. The post-synthesized FMPy samples, which mainly contained iron oxide
clusters and bulk iron oxides, showed very low phenol yield (0.1-0.2%). Thus, the isolated andoligomeric Fe species on the extra framework were discussed in the next part.

2.3.3.2 The importance of isolated and oligomeric Fe species on the extra framework
Figure 2.11(a) shows the change in the content of isolated and oligomeric extra framework Fe
species as a function of the total Fe content for the FMDx samples, indicating that the content of35

Chapter 2

isolated and oligomeric Fe species was increased along with the total Fe content both at low Fe content(< 1.5 wt.%) and high Fe content (> 3.1 wt.%). Actually, the degree of aggregation of Fe species wasincreased with Fe content. It was reasonable that the fraction of oligomeric Fe species in the FMDx
with high Fe content was higher than that in the samples with low Fe content. Although the band
ascribed to the isolated or oligomeric Fe species cannot be separated accurately in the UV-vis spectrum,
it can be confirmed that the proportion of coordination unsaturated oligomeric Fe species wasincreased at high Fe content from the NO adsorbed FT-IR results. For ACT-FMD15, the content ofisolated and oligomeric Fe species was lower than FMD100 although the Fe content was similar, which
was in consistent with the NO adsorbed FT-IR result. Contrarily, for ALT-FMD15, the content ofisolated and oligomeric Fe species was almost similar to FMD15, although the Fe content wasincreased by ca. 10%. On one hand, the degree of aggregation of Fe species was increased after alkali
treatment, leading to the formation of mesopores, large Fe clusters and iron oxide particles. On theother hand, the content of highly coordination unsaturated oligomeric Fe species were increased whilethat of isolated Fe species was decreased, which can be proved by the NO adsorbed FT-IR and NH3TPD results (Figs. 4 and 5).
The change in the phenol yield as a function of the isolated and oligomeric extra framework Fe
species in the FMDx catalysts is shown in Figure 2.11(b), clearly indicating that the phenol yield wascorrelated with the content of isolated and oligomeric extra framework Fe species. Thus, the formationof isolated and oligomeric extra framework Fe species in the MFI zeolite is vital for achieving a highyield of phenol. FMD50 showed slightly higher phenol yield than FMD100, although it had more than
two times isolated and oligomeric Fe species, which may be due to the difference in the extent of
increased isolated or oligomeric Fe species. With further increasing Fe content, FMD25 showed thephenol yield about two times higher than FMD50, due to the significant increase in the isolated andoligomeric Fe species from 30 to 40%. Further increasing the Fe content, FMD15 showed the phenol
yield about 1.5 times higher than FMD25, although the content of isolated and oligomeric Fe species
was increased by ca. 1.3 times. It was also suggests the essential role of the oligomeric Fe species.
After alkali treatment, ALT-FMD15 showed the phenol yield about 1.3 times higher than FMD15,
although they had similar isolated and oligomeric Fe content. Considering that ALT-FMD15 contained
more oligomeric Fe species rather than isolated ones, it can be concluded that oligomeric Fe species
would be more active than isolated ones.

2.3.3.3 The impact of the framework Al
Finally, the impact of aluminum in the zeolites on the catalytic performance was considered. FZPz
36

Chapter 2

attained higher phenol yield than FMPy, though the content of isolated and oligomeric Fe species weresimilar in the UV-vis spectra. Significant differences were displayed in the NO adsorbed FT-IR spectra
and NH3-TPD profiles between FMPy and FZPz. FZPz exhibited stronger NO adsorptions and moreacid amount, indicating the abounding coordination unsaturated Fe species. We can estimate that theoligomeric Fe species with low nuclearity (such as dimeric Fe) was formed in FZPz, while those witha high nuclearity (such as trimeric and tetrameric Fe) existed in FMPy. The function of aluminum wasto provide location site and disperse the Fe species, which has been reported in the reference [43].
Therefore, completely different catalytic performances were displayed when silicalite-1 and ZSM-5
were used as parents. It was concluded that the isolated and oligomeric Fe species with low nuclearity
were the active sites in the BTP reaction with H2O2.

2.4. Conclusions
Fe-containing MFI-type zeolite catalysts including Fe-silicalite-1 and Fe-ZSM-5 were preparedby direct or post-synthesized method and post-modification by acid or alkaline treatment. In the BTP
reaction with H2O2, direct-synthesized Fe-silicalite-1 achieved much higher phenol yield than the postsynthesized ones at similar Fe content. Based on the specific characterizations, we successfully
clarified that the formation of isolated and oligomeric Fe species on the extra framework in MFI
zeolites were critical for achieving high phenol yield. Among isolated and oligomeric extra framework
Fe species, low nuclear oligomeric Fe species would be more active. Moreover, alkaline treatment was
found to be advantageous to the formation of oligomeric Fe species, and as a result, 7.6% of phenol
yield with 92% selectivity was attained. Our findings would contribute to the improvement of catalytic
activity of Fe-containing zeolite catalysts, and also the development of the catalytic process for direct
production of phenol from benzene.

37

Chapter 2

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[54] J. Luo, F. Gao, K. Kamasamudram, N. Currier, C. H. F. Peden, A. Yezerets, J. Catal., 348 (2017)291.
[55] L. Balducci, D. Bianchi, R. Bortolo, R. D'Aloisio, M. Ricci, R. Tassinari, R. Ungarelli, Angew.
Chem. Int. Ed., 115 (2003) 5087.
[56] E. A. Pidko, E. J. M. Hensen, R. A. Van Santen, In: Proc. R. Soc. A., 468 (2012) 2070.
[57] M. Schwidder, M. Kumar, K. Klementiev, M. Pohl, A. Bruckner, W. Grunert, J. Catal., 231 (2005)
314.

39

Chapter 2(f)Intensity (cps.)

Intensity (cps.)

(f)
(e)
(d)
(c)

5

10

15

20

25

30

35

(e)
(d)
(c)

(b)

(b)

(a)

(a)

40

5

10

15

20

25

30

35

40

2 Theta (Degree)

2 Theta (Degree)

Figure 2.1 XRD patterns of left: (a) FMD100, (b) FMD50, (c) FMD25, (d) FMD15, (e) ACT-FMD15
and (f) ALT-FMD15; and right: (a) FMP2.5, (b) FMP5.0, (c) silicalite-1, (d) H-ZSM-5, (e) FZP2.5 and(f) FZP5.0.

40

Chapter 2

Figure 2.2 FE-SEM images of (a) FMD100, (b) FMD50, (c) FMD25, (d) FMD15, (e) ACT-FMD15
and (f) ALT-FMD15, (g) silicalite-1 and (h) H-ZSM-5 catalysts.

41

Chapter 2
1800

(h)
1500

Va (cm3(STP) g-1)

(g)
1200

(f)
(e)

900

(d)
600

300

(c)
(b)
(a)

0
0.0

0.2

0.4

0.6

0.8

1.0

p/p0

Figure 2.3 N2 adsorption desorption isotherms of (a) FMD100, (b) FMD50, (c) FMD25, (d) FMD15,(e) ACT-FMD15, (f) ALT-FMD15, (g) FMP2.5 and (h) FZP2.5.

42

Chapter 2

27.0

27.0

(f)
22.5

22.5

(e)

(e)
18.0

(d)

K/M

K/M

18.0

(d)

13.5

13.5

(c)

(c)
9.0

9.0

(b)

4.5

(a)

0.0
200

300

400

500

600

Wavelength (nm)

(b)
4.5

(a)
0.0
200

300

400

500

600

Wavelength (nm)Figure 2.4 UV-vis spectra of left: (a) FMD100, (b) FMD50, (c) FMD25, (d) FMD15, (e) ACT-FMD15
and (f) ALT-FMD15; and right: (a) FMP2.5, (b) FMP5.0, (c) FZP2.5, (d) FZP5.0 and (e) α-Fe2O3.

43

Chapter 2
0.5

1.6
1.4

1915

Absorbance

1.2

1880

1850

1810

1809
1765

(e)

1.0
0.8

(d)
0.6

(e)

0.4

(f)

Absorbance

1867
1840

(d)
0.3

(c)
0.2

(c)
0.4

(b)

0.1
(b)

0.2
0.0
1950

(a)

(a)
1900

1850

1800

1750

Wavenumbers (cm-1)

0.01700 1950

1900

1850

1800

1750

Wavenumbers (cm-1)

1700

Figure 2.5 FT-IR spectra of NO adsorbed on left: (a) FMD100, (b) FMD50, (c) FMD25, (d) FMD15,(e) ACT-FMD15, and (f) ALT-FMD15 and right: (a) FMP2.5, (b) FMP5.0, (c) mixture of silicalite-1
and α-Fe2O3, (d) FZP2.5, (e) FZP5.0.

44

Chapter 2
3000

4200

2700

3600
2400

(f)

(g)
3000

TCD signal

(e)

1800
1500
(d)

1200
900

TCD signal

2100

2400
(f)
1800

(c)

1200

(b)

600

(e)

600

(d)
(c)
(b)
(a)

300
0

(a)
400

500

600

700

800

900

Temperature (K)

0

400

500

600

700

800

900

Temperature (K)Figure 2.6 NH3-TPD profiles of left: (a) FMD100, (b) FMD50, (c) FMD25, (d) FMD15, (e) ACTFMD15 and (f) ALT-FMD15; right: (a) silicalite-1, (b) FMP2.5, (c) FMP5.0, (d) α-Fe2O3, (e) H-ZSM5, (f) FZP2.5, (g)FZP5.0.

45

Chapter 2
Phenol Selectivity

100

(a)

(b)

90

Phenol Yield (%)

7

80

6

70
5

60

4

50

3

40
30

2

20
1
0
310

10
320

330

340

350

360

370

380 0

1

2

Temperature (K)

3

4

5

6

7

9

Molar ratio of H2O2/Benzene

8

(d)

(c)

0
100
90

7

Phenol Yield (%)

8

80

6

70
5

60

4

50

3

40
30

2

20
1
0

10
2

4

6

8

10

0

Reaction Time (h)

10

20

30

40

50

60

70

80

Conversion and Conversion(%)

8

Benzene Conversion

H2O2 Conversion

Conversion and Conversion(%)

Phenol Yield

0
90 100 110

Catalyst Amount (mg)Figure 2.7 Changes of phenol yield, selectivity, and H2O2 conversion over FMD15 catalyst as afunction of (a) temperature, (b) molar ratio of H2O2/benzene, (c) reaction time and (d) catalyst amount.
Reaction conditions: (a) 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, H2O2/Benzene=2, 6 h. (b)50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 333 K, 6 h. (c) 50 mg catalyst, 10 ml CH3CN, 5 mmol
Benzene, H2O2/Benzene=2, 333 K. (d) 10 ml CH3CN, 5 mmol Benzene, H2O2/Benzene=2, 333 K, 6 h.

46

Chapter 2
Phenol Yield

Phenol Selectivity

H2O2 Conversion
100

Phenol Yield (%)

5

80

4
60
3
40
2
20

1

0

Phenol Selectivity or H2O2 Conversion(%)

60

Acetonitrile

Acetone

Sulfolanetert-butanol

Solvent

Figure 2.8 Changes of phenol yield, selectivity, and H2O2 conversion over the FMD15 catalyst withdifferent solvents. Reaction conditions: 50 mg catalyst, 10 ml solvent, 5 mmol Benzene,
H2O2/Benzene=2, 333 K, 6 h.

47

Chapter 2
Phenol Selectivity

H2O2 Conversion

wash and dry

further
calcine6

Phenol Yield (%)

5

100

80

4
60
3
40
2
20

1

0

Phenol Selectivity or H2O2 Conversion(%)

Phenol Yield

0
1

2

3

4

5

6

CyclesFigure 2.9 Reuse and regeneration of the FMD15 catalyst. Reaction conditions: 50 mg catalyst, 10 ml
CH3CN, 5 mmol Benzene, H2O2/Benzene=2, 333 K, 6 h.

48

Chapter 2
8

ALT-FMD15

7

Phenol Yield (%)

6

FMD15

5

FZP2.54

FMD25

FZP5.0

3
2

FMD1001

ACT-FMD15
S-1

0

0

1

FMD50
FMP5.0FMP2.5
2

3

4

5

6

Total Fe content (wt. %)Figure 2.10 Change of phenol yield as a function of Fe content for different catalysts. Reaction
conditions: 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, H2O2/Benzene=2, 333 K, 6 h.

49

Chapter 2
2.0

8

(b)

ALT-FMD15

7

ALT-FMD15
1.6

Phenol Yield (%)

The content of isolated andoligomeric Fe species(wt.%)

(a)

FMD15
1.2

FMD25

0.8

0.4

0.0

FMD15
5
4

FMD25

3
2

FMD50
FMD100

FMD50

FMD1001

ACT-FMD15
0

6

1

2

3

Total Fe content (wt. %)4

ACT-FMD15

0
0.0

5

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

The content of isolated and oligomeric Fe species (wt. %)Figure 2.11 (a) change of isolated and oligomeric extra framework Fe species content as a function of
Fe content for FMD catalysts, and (b) change in the phenol yield as a function of isolated andoligomeric extra framework Fe species content for the FMDx catalysts. Reaction conditions: 50 mg
catalyst, 10 ml CH3CN, 5 mmol Benzene, H2O2/Benzene=2, 333 K, 6 h.

50

Chapter 2

Fe2O3> 2 nmIsolatedFe

Fe2O3< 2 nmFe cluster
Framework
Fe
Dimeric Fe

Fe
Si

O
H
Scheme 2.1 Possible Fe species in the Fe-silicalite-1 zeolite.

51

Chapter 2

Table 2.1 Composition and textural property of the Fe-containing MFI zeolites catalysts.
Si/Fe
Si/Fe
Fe content
SBET
SEXT
(product) a(wt.%) a(m2 g−1) b (m2 g−1) c(gel)
FMD100
100
167
0.6
378
103
FMD50
50
60
1.5
314
98
FMD25
25
29
3.1
278
94
FMD15
15
22
4.0
234
81
ACT-FMD15
15
184
0.5
491
121
ALT-FMD15
15
20
4.5
452
245
silicalite-1
415
146
FMP2.52.5
407
235
FMP5.0
5.0
403
203
H-ZSM-5
485
87
FZP2.52.5
410
118
FZP5.0
5.0
399
115
a
Si/Fe was determined by ICP-AES analysis, Fe content was calculated by Si/Fe.
Catalystb

VTotal(cm3 g-1) d
0.42
0.46
0.43
0.49
0.55
0.74
0.35
0.50
0.47
0.43
0.46
0.44

VMicro(cm3 g-1) e
0.17
0.17
0.13
0.15
0.30
0.16
0.13
0.12
0.12
0.23
0.18
0.15

Specific surface areas of the catalysts were calculated using the Brunauer-Emmett-Teller (BET) equation on the N2
adsorption isotherms.
c External surface area (SEXT) of the catalysts were calculated by the t-plot method based on the adsorption isotherms.
dTotal pore volumes of the catalysts were calculated based on BET equation on the N2 adsorption isotherms.
eMicropore volumes of the catalysts were calculated by the t-plot method based on the adsorption isotherms.

52

Chapter 2

Table 2.2 Percentage of sub-band areas (I1:λ< 250 nm, I2: 250 <λ< 350 nm, I3: 350 <λ< 450 nm, andI4:λ>450 nm) based on the UV-vis spectra and contents of corresponding Fe species.
Catalyst
FMD100
FMD50
FMD25
FMD15
ACT-FMD15
ALT-FMD15
FMP2.5
FMP5.0FZP2.5FZP5.0

Framework Fe
I1(%)
wt.%
68.0
0.4
60.5
0.9
46.3
1.4
45.4
1.8
86.0
0.4
29.7
1.3
20.2
0.5
17.0
0.9
30.3
0.8
24.2
1.2

Isolated and oligomeric Fe
I2(%)
wt.(%)
32.0
0.2
30.2
0.5
39.6
1.2
39.3
1.6
14.0
0.1
36.0
1.6
19.8
0.5
18.7
0.9
19.6
0.5
20.2
1.0

53

Iron oxide cluster
I3(%)
wt.%
9.3
0.1
8.8
0.3
9.4
0.4
20.2
0.9
49.1
1.2
50.4
2.5
43.0
1.1
45.3
23

Bulk iron oxide
I4(%)
wt.%
5.3
0.2
5.9
0.2
14.1
0.6
10.9
0.3
13.8
0.7
7.0
0.2
10.3
0.5

Chapter 2

Table 2.3 NH3 adsorbed amount on the samples used in this study.
Catalyst
FMD100
FMD50
FMD25
FMD15
ACT-FMD15
ALT-FMD15
silicalite-1
FMP2.5
FMP5.0α-Fe2O3H-ZSM-5
FZP2.5FZP5.0NH3 adsorbed amount (mmol/g) / Tmax (K)LTMTHT
0.04/ 436
0.08/ 541
0.05/ 578
0.04/ 423
0.10/ 482
0.06/ 574
0.06/ 433
0.14/ 486
0.10/ 602
0.05/ 434
0.10/ 5120.08/ 607
0.05/ 433
0.13/ 545
0.01/ 701
0.06/ 435
0.15/ 487
0.11/ 589
0
0
0
0
0
0
0
0
0.21/ 448
0.32/ 752
00.17/ 440
0.13/ 481
0.39/ 614
0.14/ 445
0.13/ 486
0.34/ 619

54

Chapter 2

Table 2.4 Catalytic results of the FMDx, modified ALT-FMD15 and ALT-FMD15, post-synthesized
FMPy and FZPz samples in hydroxylation of benzene to phenol with H2O2.
Product Selectivity (%) b
H2O2
Conv. (%)c
Phenol
HQ
CL
FMD100
1.5
97
2
1
46
FMD50
2.0
97
1
2
59
FMD25
3.7
97
2
1
76
FMD15
5.7
95
4
1
72
ACT-FMD15
1.3
99
1
0
49
ALT-FMD15
7.6
92
7
1
82
silicalite-1
0.0
0.2
FMP2.5
0.1
100
0
0
8
FMP5.0
0.2
100
0
0
2
H-ZSM-5
0.1
100
0
0
1
FZP2.5
3.8
99
1
0
47
FZP5.0
3.0
99
1
0
44
Reaction conditions: 333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
aPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
b
Product selectivity = (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL).
c H O conversion = (moles of H O consumed)*100/(initial moles of H O ).
2 2
2 2
2 2
Catalyst

PhenolYield (%) a55

Chapter 2

Table 2.5 Chemical composition of FMD15 catalyst after each run of benzene to phenol reaction.
Run
0
1
2
3
4
a
b

Si/Fe a
22
24
25
26
27

Si/Fe was determined by ICP
Fe content was calculated according to Si/Fe ratio.

56

Fe (wt.%) b
4.0
3.7
3.6
3.4
3.3

Chapter 2

Table 2.6 Reaction results of hydroxylation of benzene to phenol.
Yield (%)/ (µmol) d
Selectivity (%) e
H2O2
Conv.
(%) f
PhenolHQ
CL
Phenol
HQ
CL
FMD15 a5.7/ 340
0.2/ 130.1/ 4
95
4
1
72
LA b
0.5/ 170.4/ 13
0/ 0
56
44
0
4
c
LA+H2O2
3.8/ 1110/ 00.3/ 9930
716
α-Fe2O3 a
0.1/ 7
0/ 0
0/ 0
100
0
0
2
a
Fe(NO3)3·9H2O
0.6/ 35
1.7/ 101
43
15
42
77
1.7/ 104a
333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
bLiquid phase after BTP reaction separated with catalyst.
c Liquid phase after BTP reaction separated with catalyst was mixed with 1 g H O (30 wt.%).
2 2dPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
e
Product selectivity = (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL).
f H O conversion = (moles of H O consumed)*100/(initial moles of H O ).
2 2
2 2
2 2
Sample57

Chapter 3

Chapter 3
Dramatic impacts of the distribution of Fe species in Fe-silicalite-1 zeolites andsolvent on liquid-phase methane conversion to methanol with H2O2
Abstract
The conversion of methane to methanol over Fe-containing MFI zeolites under mild conditions has
been investigated. Fe-silicalite-1 zeolites were directly synthesized using TPAOH as OSDA with andwithout Na cations. The physicochemical properties, especially the states of Fe species wascharacterized. The impact of solvent on the performance was investigated. We firstly found that
sulfolane was the best one among the solvents used in terms of the improvement in the production of
methanol and its stability during the reaction. Finally, the effects of various reaction parameters, suchas reaction temperature, CH4 pressure, reaction time, catalyst amount and the amount of H2O2 were
optimized. Furthermore, the use of mixture of sulfolane-water solvent with an appropriate proportion
led to an extremely high CH3OH production with a high selectivity.

3.1. Introduction
Methane, as the main component of natural gas, is a highly abundant and inexpensive source offuel and chemicals [1, 2]. So far, two approaches to conversion of CH4 to CH3OH have been
implemented. The first route is CH4 reaction with steam conversion to synthesis gas under 723-823 K[3] and then the synthesis gas is converted into CH3OH under high temperate and high pressure [4, 5].
The two separate steps method has been applied to produce nearly 100% of methanol worldwide [6].
Another approach is a direct conversion, including gas-phase using O2 [7, 8], N2O [9, 10] or H2O [6]as oxidant. For example, Bokhoven and co-workers introduced a direct stepwise method for convertingof CH4 into CH3OH over Cu-MOR with water under 473 K and 7 bars of CH4 [6]. Krisnandi et al.
achieved 79% methane yield from CH4 with O2 over Co-modified mesoporous H-ZSM-5 catalyst
under 423 K [11]. However, gas-phase oxidation of CH4 require high temperatures (473 to 773 K) to
activate the oxidant and desorb products [2, 6, 12-14]. The liquid-phase oxidation of CH4 overhomogenous and heterogeneous catalysts with H2O2 have been researched [2, 15-17]. Nevertheless thereaction results have not been satisfactory. Recently, Hutchings and co-workers have made great
58

Chapter 3

achievements in the field of direct catalytic conversion of CH4 to CH3OH in an aqueous medium with
H2O2 and various kinds of catalysts, including Cu and/or Fe-ZSM-5 [1, 4, 18, 19], Au-Pd/TiO2 [20],AuPdCu/TiO2 [21] and Au-Pd colloids [22]. Among of these catalysts, they achieved the highest 92%
of selectivity to CH3OH at 0.5% of CH4 conversion [18]. It is still a challenging issue in terms of
conversion and selectivity, and the development of novel catalyst and catalytic process have strongly
been desired.
As described above, Fe-containing MFI-type zeolites have been found to be valid in the liquidphase oxidation of CH4 to CH3OH (MTM) reaction with H2O2; a part of the extra framework Fe species
are the active component for this reaction [1, 4, 19]. The extra framework Fe specie consist of isolated
Fe cations, oligomeric Fe complexes, and neutral iron oxide clusters (FexOy) as well as larger ironoxide aggregates [23, 24]. Among various states of Fe species, isolated and oligomeric Fe species are
involved in the MTM reaction over Fe/ZSM-5 catalyst [25]. Fe-containing MFI-type zeolites can beprepared by several methods. In the post-synthesis method, ion-exchange and impregnation techniques
have been applied but accordingly prepared catalysts exhibited poor activity [24, 26]. On the other
hand, a method for one-pot construction of Fe-containing MFI-type zeolite catalyst has been
established by adding Fe source into the synthesis gel for the MFI-type zeolite, the isolated andoligomer Fe species are formed by calcination in air [24, 27]. It is expected that the fine-tuning of thestate of Fe species must be a key factor for improving the catalytic activity.
Meanwhile, the distribution of hetero atoms in zeolite framework has been recognized as animportant factor for catalytic activity and selectivity. In aluminosilicate zeolites, to balance the charge,
Al3+ atoms in silica framework, AlO4-, are located near cations including inorganic cations, such as
Na+ and K+, and organic ones. We have reported the control of the Al distribution in the MFIframework based on the rational choice of inorganic and/or organic cations [28, 29], and the use ofalcohols as pore-filling agent [30]. However, the distribution and state of Fe species in ferrosilicate
zeolites has not been achieved, and their impact on the catalytic activity has also been reported to date.
In addition to the inertia C-H bond of CH4 molecule [2, 31-33], the difficulty in conversion of
CH4 under the liquid-phase system is related to the solubility of CH4, which is a kind of non-polar
molecule with very symmetrical tetrahedron structure and is extremely difficult to dissolve in H 2O
[34]. According to Weare and co-worker’s results, only 1.9 mg of CH4 dissolves in 100 g water at 303
59

Chapter 3

K and 0.1 Mpa [34]. Besides, the solubility of CH4 is dependent on the temperature and pressure of
CH4. When the pressure increase to 5 Mpa, the solubility of CH4 is increased to 87.5 mg / 100 g waterat the same temperature [34]. In addition, ionic liquid has been employed as solvent in the MTM
reaction over nano-Au/SiO2 catalyst system, 25 % of CH4 conversion with 72 % of CH3OH selectivity
was achieved [35]. However, the use of ionic liquid hinder a facile operation because it contains otherkinds of chemicals like potassium persulfate, trifluoroacetic acid and trifluoroacetic anhydride.
In this work, Fe-silicatelite-1 zeolites were synthesized directly using TPAOH as OSDA with orwithout Na cations in the synthesis gel. Different organic solvents were used in the MTM reaction toenhance the solubility and the conversion of methane. Finally, the effects of various reaction
parameters, such as reaction temperature, CH4 pressure, reaction time, catalyst amount, H2O2 amount
were optimized. Besides, we reported the control of the distribution of the Fe species in the MFI-typezeolite, and its impact on the activity for liquid-phase MTM reaction with H2O2.

3.2. Experiments
3.2.1. Materials
Colloid silica (HS-40) and tetrapropylammonium hydroxide (TPAOH) (20-25% in water) werepurchased from Tokyo Chemical Industry. NaOH, Fe(NO3)3·9H2O, NH4NO3, H2O2 (30 wt.%), ethanol,
acetonitrile,sulfolane,

mesitylene,

1,4-dioxane,

tetramethylsilane(TMS),

2,2,3,3-d(4)-3-(Trimethylsilyl)propionic acid sodium (TMSP-D4), CD3CN and D2O were purchased from Wako.
Methane (99.99%) gas was provided by Taiyo Nippon Sanso Co., Ltd.. All of the reagents were used
as received, without further purification.

3.2.2. Catalysts preparation
The Fe-silicalite-1 zeolites were directly synthesized with TPAOH as OSDA in the absence ofNa cation. First, colloid silica was added to the solution containing water, TPAOH and Fe (NO3)3·9H2O.
The gel with the molar composition of 1 Si: 0.01-0.067 Fe: 0.25 TPA: 20 H2O was crystallized at 443
K for 7 days with 40 rpm before aged at 353 K for 24 h. Then, the solid product was collected after
filtering, rinsing and drying. The as-synthesized material was calcined at 823 K for 10 h to remove
TPA+ species. The calcined samples were treated with 1 M NH4NO3 aq. at 353 K for 3 h twice, and60

Chapter 3

filtered, washed, dry and calcined at 823 K for 5 h to obtain the H-type samples. Thus obtained catalyst
was designated as “FS(T)x”, where x is the atomic ratio of Si/Fe in the gel ranging from 15 to 100.
The Fe-silicalite-1 zeolites were also directly prepared with TPA cation in the presence of Na
cation. The gel was aged at 353 K for 24 h and then crystallized at 443 K for 3 days. The final molar
composition was 1 Si: 0.01-0.067 Fe: 0.25 TPA: 0.25 Na: 20 H2O. The calcined Na-type samples weretreated with 1 M NH4NO3 aq. at 353 K for 3 h twice, and filtered, washed, dry and calcined at 823 K
for 5 h to obtain the H-type samples. Thus obtained catalyst was designated as “FS(TN)y ”, where y is
the atomic ratio of Si/Fe in the gel ranging from 15 to 100.

3.2.3. Characterization of catalysts
XRD patterns were collected on a Rint-Ultima III (Rigaku) using a Cu Kα X-ray source (40 kV,
20 mA). Field-emission scanning electron microscopic (FE-SEM) images of the powder samples wereobtained on S-5200 (Hitachi) microscope operating at 1 kV. Elemental analyses of the samples were
performed on an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu
ICPE-9000). Nitrogen adsorption and desorption measurements to obtain information on the microand meso-porosities were conducted at 77 K on a Belsorp-mini II (MicrotracBEL).
UV-vis diffuse reflectance spectra were recorded on a V-650DS spectrometer (JASCO). The
diffuse reflectance spectra were converted into the absorption spectra using the Kubelka-Munkfunction. Fourier Transform Infrared (FT-IR) spectra were obtained by a JASCO FT-IR 4100
spectrometer equipped with a triglycine sulfate (TGS) detector. For FT-IR observation, the sample waspressed into a self-supporting disk (20 mm diameter, ca. 30 mg) and placed in an IR cell attached to aclosed gas-circulation system. After the sample was pretreated by evacuation at 773 K for 2 h, thenadsorbed 5-1000 Pa NO at ambient temperature. The IR spectra resulting from the subtraction of the
background spectra from those with NO adsorbed are shown unless otherwise noted.
Temperature-programmed ammonia desorption (NH3-TPD) profiles were noted down on a
Multitrack TPD equipment (Japan BEL). Normally, 25 mg catalyst was pretreated at 773 K for 1 h ina He flow of 50 mL min-1 and then cooled down to 423 K. The sample was evacuated at 423 K for 1
h prior to the adsorption of NH3. Approximately 2500 Pa of NH3 contacted with the sample at 423 K
for 10 min. Subsequently, the sample was evacuated to remove the weakly adsorbed NH3 at the same
61

Chapter 3

temperature for 30 min. Finally, the sample was heated from 423 to 873 K at a ramping rate of 10 K
min-1 in a He flow of 50 mL min-1.

3.2.4. Catalytic tests
The liquid-phase MTM reactions were carried out in a 100 ml PTFE autoclave. The CH4pressure was controlled by the pressure gage ranging from 0.5 to 3 MPa. The reactants were stirred
vigorously by an agitator blade. Aqueous hydrogen peroxide (30 wt.%) was used as oxidant. In atypical standard CH4 oxidation experiment, deionized water (10 ml) or sulfolane (10 ml) was exploited
as solvent, 50 mg catalyst and 27 mmol H2O2 were added to the autoclave and heated to 323 K. The
sealed reactor was then purged with CH4 to achieve the desired pressure, typically 3 Mpa. After thedesired reaction time, usually 2 h, the autoclave was cooled rapidly to 278 K in an ice bath to minimize
any further chemical reaction and reduce loss of volatile products.
Liquid-phase products were analyzed by 1H NMR spectroscopy on JEOL ECA-600 spectrometer(14.1 T) equipped with an additional 1 kW power ampliﬁer. Mesitylene and 1.4-dioxane were took as
the internal standard when the reaction solvent were sulfolane and water, respectively. TMS/CD 3CN
and TMSP-D4/D2O were took as the chemical shift calibrator when sulfolane and water using as
solvent, respectively. The detectable products in the liquid-phase were CH3OH, HCOH and HCOOH.
The amount of unconverted H2O2 was quantified by standard titration method with 0.1 mol/L Ce(SO4)2
solution.

3.3. Results and discussion
3.3.1 Characterization
3.3.1.1 Structure and composition
Figure 3.1 shows the XRD patterns of the as-synthesized FS(T)x and FS(TN)y catalysts. All the
directly synthesized Fe-MFI samples had the typical MFI structure, and the crystallinity weredecreased with the Fe content increasing for both the FS(T)x and FS(TN)y samples. No features
belonging to large iron oxide particles were observed, which was indicative for the high Fe dispersion
in the as-synthesized zeolites.
Figure 3.2 shows the FE-SEM images of FS(T)x and FS(TN)y samples. Figure 3.2(a-d) reveal
the spherical morphology for FS(T)x samples with the particle size varying from 0.5 to 1.2 µm. The
62

Chapter 3

surface of FS(T)x zeolites changed from smooth to coarse with the Si/Fe ratio varying from 100 to 15.
However, the particle size of FS(TN)y catalysts with irregular shape, about 100-200 nm, were similarfrom each other and smaller than the FS(T)x ones, displayed in Figure 3.2(e-h).
The chemical composition and textural properties of the catalysts are listed in Table 3.1. For both
FS(T)x and FS(TN)y zeolites, the actual Si/Fe ratios in the products were higher than those in thesynthesis gels, meaning that only part of iron was introduced into the zeolite. Meanwhile, the actual
Si/Fe ratios of FS(T)x samples were higher than those of FS(TN)y samples, possibly due to the lessamount of positive charge provided by TPA+ for FS(T)x than that provided by TPA+ and Na+ for
FS(TN)y.
The corresponding textural properties of these zeolites are also listed in Table 3.1. The BET
surface area (SBET), external surface area (SEXT), the total pore volume (Vtotal), and the micropore
volume (Vmicro) of the zeolites were similar to each other for FS(T)x and FS(TN)y samples at different
Si/Fe ratios. In addition, the SBET of all the catalysts were higher than 400 cm2g-1 due to the small
particle size, which was verified by SEM images in Figure 3.2.

3.3.1.2 Fe states
According to the references [36, 37], the coordination state and extent aggregation of Fe in the
Fe-containing MFI zeolites were investigated by UV-vis spectroscopy. Figure 3.3 show the UV-vis
spectra of FS(T)x and FS(TN)y catalysts, respectively, which were deconvoluted into sub-bands byapplying Gauss functions. The amount and proportion of different Fe species based on the relative
peak areas are listed in Table 3.2. Generally, the bands below 250 nm were assigned to framework Fe
species, the bands between 250-350 nm were ascribed to isolated and oligomeric Fe species on theextra framework (e.g., dimeric, trimeric, and tetrameric Fe species), the bands between 350-450 nm
were ascribed to iron oxide clusters, and the bands above 450 nm were ascribed to bulk iron oxide
particles less or larger than 2 nm [38]. One of the most obvious difference between FS(T)x andFS(TN)y was the distribution of different Fe species. For FS(T)x, no matter the samples with high or
low Fe content, the Fe species were evenly distributed as framework Fe, isolate and oligomer Fe, ironoxide clusters, and bulk iron oxide, only the content of each kind of Fe species changed. However,
FS(TN)100 and FS(TN)50 samples contained higher Fe content than FS(T)100 and FS(T)50,
respectively, but only framework Fe, isolate and oligomer Fe species were detected. When the Si/Fe
ratio was decreased to 25 and 15, the Fe species of iron oxide clusters and bulk iron oxide for FS(TN)25
and FS(TN)15 appeared and the corresponding amount were directly increased to a high value. As forthe reason, it may be related to the charge position and amount. As mentioned before, only TPA+ can
63

Chapter 3

balance the negative charge in FS(T)x samples which produced by coordinated Fe species, like FeO4-,
and the location was concentrated at the intersection, as shown in Scheme 3.1. Influenced by the charge
amount and position, negative coordinated Fe species preferentially were distributed in the intersection
at the low Fe content. Increase Fe content, no more TPA+ balanced the Fe species, which only overlay
on the formed ones. As a result, Fe species for FS(T)x samples showed no significant change at both
low and high Fe content. While for FS(TN)y, TPA+ coupled with Na+, more positive charge can beprovided and the location expand to the channels due to the tiny particle size of Na+ (Scheme 3.1).
Consequently, iron in FS(TN)y existed as isolated species at low Fe content, like FS(TN)100 andFS(TN)50. And iron oxide clusters and bulk iron oxide species appeared when the Fe content reacheda certain high degree.
In addition to UV-vis spectroscopy, in-situ FT-IR spectroscopy using NO as probe molecule wasalso applied to investigate the state of Fe species [39, 40]. Figure 3.4 shows the FT-IR spectra collectedfrom the annealed FS(T)x and FS(TN)y zeolites during stepwise NO adsorption at 298 K. The mainband centered at 1867 cm−1 assigned to Fe2+(NO) with a shoulder at 1843 cm−1 attributed to Fe2+(NO)2complexes formed on Fe clusters [40-42]. The intensities of these bands were increased along withNO pressure increasing until saturated. Further increasing NO pressure, new bands at 1810 cm-1
assigned to Fe2+(NO)3 appeared, which was related to the highly coordination unsaturated Fe species[43]. The weak bands at 1915 and 1902 cm-1 were also attributed to Fe2+(NO)3 on isolated Fe species[40, 41]. It was clear to see that the intensity of bands at 1810 cm-1 for FS(T)x zeolites were increased
with Fe content and basically higher than FS(TN)y zeolites. But the intensity of bands at 1810 cm-1 forFS(TN)y zeolites showed no obvious regularity with Fe content changing. The intensity of the mainband at 1867 cm-1 for FS(TN)y were higher than FS(T)x, possibly due to the higher Fe amount for theformer ones. The differences in the proportion of isolated Fe and Fe clusters between FS(T)x andFS(TN)y samples specified the influence of the present of Na cation in the synthesis gel.

3.3.1.3 Acidity
The situation of the ammonia adsorbed amount for the Fe-silicalite-1 zeolites measured by NH3TPD revealed the Fe environment to a certain degree. Figure 3.5 show the NH3-TPD profiles of the
FS(T)x and FS(TN)y samples, respectively. Most of the profiles can be deconvoluted into three kindsof peaks at low (373-473 K), medium (473-573 k), and high temperature (573-773 K) (designated as
LT, MT and HT). The amounts of NH3 adsorbed and the temperature of the maximum peak (Tmax) arelisted in Table 3.3. According to the references [44, 45], the LT peak corresponds to NH3 adsorbed on
the non-acidic-OH groups and NH4+ by hydrogen bonding, which was not related to the true acid site
64

Chapter 3

and excluded in the discussion. The MT peak corresponds to NH3 adsorbed on the extra framework Fe
species (Lewis acid site), and the HT peak corresponds to NH3 adsorbed on the bridged Si-O(H)-Feand Fe-O(H)-Fe species (Brønsted acid site) [45].
For FS(T)x, the amounts of NH3 adsorbed on both the MT and HT peaks were increased alongwith the Si/Fe ratios, indicating the evenly distribution of each kinds of Fe species, which wasconsistent with the results of UV-vis. Similarly, the amounts of NH3 adsorbed on both the MT and HT
peaks of FS(TN)y samples were increased along with the Si/Fe ratio decreasing under the condition of
low Fe content, including FS(TN)100 and FS(TN)50. However, for FS(TN)15 and FS(TN)25, a partof adsorbed NH3 amount for the MT peak shift to the LT peak, leading to the decrease of thecounterpart for the MT. The difference involved more bridged hydroxyl for the FS(TN) samples withhigh Fe content. Moreover, the amount of each kind of Fe species is not proportional to the total Fe
content of FS(TN)y catalysts and the ability of adsorbed NH3 for each kind of coordinate iron isdifferent, leading to the different NH3-adsorbed amounts for the MT peaks between FS(TN)50 andFS(TN)25, and the HT peaks between FS(TN)15 and FS(TN)25. At the same time, the NH3-adsorbed
amount of the FS(T)x samples were lower than those of the FS(TN)y ones under the same Si/Fe ratio
in the synthesis gel, which was similar to the ZSM-5 synthesized using TPAOH as OSDA with orwithout Na cations [28,29]. One of main reasons is the lower Fe content for the FS(T)x products thanFS(TN)y under the same x value. Another reason is the different coordinated and aggregated state for
the Fe species. These results validate the difference Fe state between FS(T)x and FS(TN)y samples.

3.3.2 Methane to methanol reaction
3.3.2.1 Solvent selection
H2O has been widely applied as solvent in the MTM reaction [38, 46]. However, the solubility of
CH4 in H2O is extremely low, wherefore it limits the reaction performance. Taking this into account,some kinds of organic solvents with higher solubility of CH4 such as acetonitrile, ethanol and sulfolane
were employed. Firstly, the stability of the solvents should be checked. Here, FS(T)15 wasrepresentatively took as catalyst and treated in the solvents with H2O2 at 323 K in the absence of CH4
for 2 h, named blank test. After separation the catalyst, the resultant solutions were analyzed by liquidphase 1H NMR technique.
Although acetonitrile has been took as the solvent in the MTM reaction by homogeneous catalysis[47, 48], the stability has not been checked. First, the blank test for acetonitrile was checked (Figure3.6), several peaks were observed at around 4.9 and 8.1 ppm, which were assigned to the hydrogen of65

Chapter 3

HCOH and hydroxyl hydrogen of HCOOH, revealing that these products were resultant from thereaction of acetonitrile with H2O2. As a result, acetonitrile is not stable in MTM reaction with H2O2 in
the fundamental research. According to the reference [49], acetonitrile is easily reacted with H2O2
thorough chemiluminescence reaction, revealing the unstable characteristics.
Similarly, methyl hydrogen of MeOH (3.4), proton of HCOH (4.9 ppm) and hydroxyl hydrogenof HCOOH (8.1 ppm) were also observed in the blank test using ethanol as solvent (Figure 3.7). Thus,
ethanol is also not suitable applied as solvent in MTM reaction with H2O2.
However, when sulfolane was employed as solvent, no peaks were newly observed after the blanktest (Figure 3.8), indicating that sulfolane was stable under this reaction condition. According to thereference [50], sulfolane is a dipolar aprotic, very stable and water-soluble industrial solvent. To thebest of our knowledge, there has been still no open report on its application in MTM reaction with
H2O2. The 1H-NMR spectra of liquid phase solution after MTM reaction with H2O2 over FS(T)15 arepresented in Figure 3.9. These signals ascribed to hydrogen of CH3OH, HCOH and HCOOH proved
the reaction between CH4 and H2O2 in both sulfolane and H2O.

3.3.2.2 The influence of reaction conditions
In the section the effects of reaction conditions such as reaction temperature, methane pressure,reaction time, H2O2 amount and catalyst amount on the reaction performance were investigated using
sulfolane as solvent and FS(T)15 as catalyst.

3.3.2.1.1 Reaction temperature
The influence of reaction temperature on the performance of MTM reaction was investigated byvarying temperature from 303 to 353 K, under the reaction conditions: 10 ml sulfolane, 3 Mpa CH4,27 mmol H2O2, 50 mg FS(T)15 as catalyst,1000 rpm, 2 h.
As shown in Figure 3.10(a), the total liquid products productivity and HCOOH amount wereincreased by rising the reaction temperature from 303 to 353 K, indicating that more methane
converted to the products. The yield amounts of MeOH and HCOH were firstly increased when thereaction temperature was enhanced from 303 to 323 K. However, part of MeOH and HCOH converted
to HCOOH by increasing the temperature from 323 to 353 K, thus the amount of MeOH and HCOH
were decreased with the temperature increasing from 323 to 353 K. Similar trends for the selectivity
66

Chapter 3of liquid products were demonstrated in Figure 3.11(a). Meanwhile the H2O2 conversion wasincreased along with reaction temperature.
Temperature is a “double-edged sword”. On one hand, the solubility of methane in sulfolane will
be decreased by increasing temperature. According to Mather’s research results, the solubility ofmethane in sulfolane was slightly decreased from 1229 mg/ 100 g sulfolane at 298 K and 3.36 Mpa to1137 mg/ 100 g sulfolane at 343 K and 3.04 Mpa [51]. On the other hand, high temperature could
improve the reaction activity for both catalysts and reactants due to the activation energy change [52],which has been verified by the increased conversions of H2O2 and methane. Actually, sulfolane is notstable over 353 K under the 3Mpa high pressure due to the decomposed property.

3.3.2.2.2 CH4 pressure
A range of CH4 pressure from 0.5 to 3 Mpa were investigated at the conditions: 10 ml sulfolane,
323 K, 27 mmol H2O2, 50 mg catalyst, 1000 rpm, 2 h. As expected, the productivity of total liquid
products and each liquid product were increased along with CH4 pressure. Among the liquid products,
MeOH achieved the maximum growth rate, as shown in Figure 3.10(b). Meanwhile Figure 3.11(b)displays the selectivity of liquid product and H2O2 conversion. MeOH gave the highest selectivity
among the liquid products and the selectivity of every liquid product was relatively stable at different
CH4 pressure, except the slightly increase for HCOOH and decrease for MeOH and HCOH byincreasing CH4 pressure from 1.5 to 3 Mpa. In addition, H2O2 conversion was increased along with
CH4 pressure. It is clear that the CH4 pressure mainly influences the solubility of methane, i.e. thereactant amount. According to Mather’s paper, the solubility of methane in sulfolane was increased
from 917 to 2460 mg/ 100 g sulfolane with increasing methane pressure from 2.37 to 7.44 Mpa at 313
K [51]. Obviously, high CH4 pressure is beneficial to the reaction, taking into account the maximum
pressure that our reaction device can endure, 3 Mpa of CH4 is better for our research.

3.3.2.2.3 Reaction time
The effect of reaction time variations from 0.5 to 4 h on the reaction performance at a constantreaction condition: 10 ml sulfolane, 3 Mpa CH4, 323 K, 27 mmol H2O2, 50 mg catalyst, 1000 rpm, wasdepicted in Figure 3.10(c) and Figure 3.11(c). It was observed that the productivity of total and each67

Chapter 3

liquid product were increased by prolonging the reaction time, especially the productivity of HCOH,
which was greatly increased by extending reaction time from 2 h to 4 h. Correspondingly, theselectivity of HCOH was increased while MeOH and HCOOH were decreased with the reaction time.
The H2O2 conversion was relatively stable with reaction time varying. HCOH was reported as theoxidation product of MeOH [53]. But according to the Chadwick, HCOH comes from the intermediate
MeOOH [46], which was closer to our research results. As the oxidative production of HCOH [54],HCOOH showed lower selectivity than Hutchings group [1, 4] and Chadwick group [46] due to thesolvent effect. The possible reaction pathway in conversion of methane to methanol with H2O2 overcatalyst in this study is showed in Scheme 3.2.

3.3.2.2.4 H2O2 amountThe production of the liquid products shows a maximum at 27 mmol with increasing H2O2 amount(Figure 3.10(d)). The total liquid products amount was decreased by increasing H2O2 amount from 27
to 38 mmol due to the total H2O2 converted amount reducing from 3.3 to 2.2 mmol. The possible
reason is the liquid products converting to CO2, which reported by Chadwick group in the aqueous
system [46]. With the H2O2 amount change, methanol achieved the highest selectivity all the time,shown in Figure 3.11(d).

3.3.2.2.5 Catalyst amountFigure 3.10(e) shows the relationship between the amount of liquid products and catalyst massunder the conditions: 10 ml sulfolane, 3 Mpa CH4, 323 K, 27 mmol H2O2, 1000 rpm, 2 h. As thecatalyst amount rose from 10 to 100 mg, the amount of the total and each liquid products, and thehydrogen peroxide conversion were increased. MeOH attained the highest selectivity all the time, theselectivity of MeOH was reduced and the selectivity of HCOH and HCOOH were increased alongwith the catalyst amount, as shown in Figure 3.11(e). In general, more catalyst can provide more active
site, thus produce more products.

3.3.2.2.6 Mixed solvent
According to the Mather’s literature, sulfolane is rarely brought into play alone but in admixture
68Chapter 3

with another solution [55]. The use of mixed solvents is an attractive alternative to either the solvent
effect or the economic benefit and environment protection. Sulfolane-water mixed solvent is
extensively applied in the lithium batteries [56] and extraction agent in the petrochemical industry [57].
Here, the performance of aqueous sulfolane with the volume content range from 0 to 100 vol.% were
investigated. Figure 3.12(a) shows the influence of the sulfolane content on the reaction performance.
When pure distilled water (0 vol.% of sulfoalne ) was individually used as solvent, the lowest
MeOH(12.2 µmol) and HCOH (0 µmol), while the highest HCOOH (462.5 µmol) amount wereobtained. The productivity of the total liquid products and MeOH were increased by increasing thesulfolane content from 0 to 50 vol.%. Continually increasing the sulfolane content from 50 to 100
vol.%, the productivity of the total liquid products and MeOH were decreased. Interestingly, theproductivity of HCOH and HCOOH display obvious and opposite regularity along with the sulfolane
content, increase and decrease, respectively. The selectivity of the liquid products were consistent withthe productivity. The conversion of H2O2 was decreased with the sulfolane content.
The highest H2O2 amount was consumed using H2O as solvent, attained the highest HCOOH
amount but the lowest productivity of MeOH and HCOH. In addition, when 50 vol.% sulfolane wasused as solvent, the amount of consumed H2O2 was reduced a half compared with that of H2O, andalso the productivity of the HCOOH was decrease 3 times, but the productivity of MeOH wasextremely increased. Finally, H2O2 converted in a minimum amount in sulfolane, but the productivityof HCOH was significantly increased compared with those of in H2O and 50 vol.% sulfolane. The
abovementioned results can be described in Figure 3.12(b), which illustrated the importance of thesolvent on the selectivity of product. The interesting phenomenon could be explained with the solvent
effect. On the one hand, sulfolane possesses the feature of temporary combination of hydroxyl, which
has been reported in other systems [58]. In Balducci’s research, the selectivity of phenol in sulfolane
is twice than that of other solvents, because the temporary formation of phenol-sulfolane complex
prevents the production of by-products [59]. Murata et al. also has reported that sulfolane is effective
for improving phenol selectivity on oxidation of benzene with oxygen and acetic acid on palladiumcatalyst [60]. One the other hand, water is a protic solvent, which could provide proton, while sulfolane
is an aprotic solvent, which is difficult to provide proton. Hydrogen peroxide has an interestingchemistry because of its ability to function as oxidant as well as a reductant in both acid and alkaline
69Chapter 3

solutions, indicating that protic solvent is beneficial to H2O2 convert. It is the probably reason that
H2O2 showed the highest conversion in water and the lowest conversion in sulfolane.
Actually, the excellent ability of sulfolane-water solution has been revealed in the literatures.
Stewart and co-workers have reported the drastic increase in the effective basicity of hydroxide ion, asa result of the solvents’ inability to effectively drastic increase in the hydrate such small ion [61]. In
other words, sulfolane is a good solvent for improving methane solubility because of “the law ofsimilar mutual solubility” but bad for H2O2 decomposition due to the aprotic nature. However, water
has the opposite characteristics: low methane solubility and beneficial for H2O2 decomposition becauseof the protic nature. The aqueous sulfolane solvent combines the advantages and avoids theshortcomings of both water and sulfolane.

3.3.2.3 The catalytic performance over FS(T)x and FS(TN)y catalysts in MTM reaction
Thus prepared zeolites were applied as catalysts for the MTM reaction under the optimized
reaction conditions. The catalytic results of the FS(T)x and FS(TN)y catalysts are summarized in Table
3.4. When x was decreased from 100 to 15, the productivity of total liquid products and MeOH wereincreased for both FS(T)x and FS(TN)y catalysts, due to the increase of the active Fe species with Fecontent. Meanwhile, the H2O2 conversion for both FS(T)x and FS(TN)y catalysts were increased alongwith Fe content. The selectivity of MeOH for FS(T)x and FS(TN)y were more than 85% and 83%,
respectively. In particular, the selectivity of MeOH for FS(T)100 was high to reach 94% at the expenseof the total production. At the same time, the H2O2 efficiency was involved and calculated using themolar amount of total liquid products dividing by the molar amount of hydrogen peroxide consumed.
Basically the H2O2 efficiency for both FS(T)x and FS(TN)y catalysts were increased with Fe content
and the value was lower than 25%, suggesting that byproducts such as CO2 was produced in the gas
phase or useless self-decomposition of H2O2. Compared with the reaction performance, it was obvious
to see the difference between FS(T)x and FS(TN)y catalysts. FS(T)x zeolites achieved higher
productivity of the total liquid products and MeOH than FS(TN)y ones under the same Si/Fe molar
ratio in the synthesis gel. Meanwhile, the H2O2 conversion of FS(T)x samples were slightly higher thanthat of FS(TN)y, probably indicating the higher CH4 conversion of FS(T)x.
Association the total Fe content, different Fe species and amount, and the productivity of MeOH
and total liquid products, interesting and obvious difference were noticed. As shown in Figure 3.13(a),the change of the different Fe species content was as a function of the total Fe content for FS(T)xcatalysts. In particular the isolated and oligomeric Fe species, and larger Fe clusters on the extra
70Chapter 3

framework, corresponding Fe content were proportional to the total Fe content, but the slope of theisolated and oligomeric Fe species was higher than that of the larger Fe clusters. The proportion of theframework Fe was decreased with the total Fe content in the FS(T)x catalysts, indicating that the Fe
species and content were affected by the Fe distribution. As for the bulk iron oxide in the FS(T)xcatalysts, it appeared only when x reached as low as 15. Figure 3.13(b) shows the relationship between
the total Fe content and the different Fe species content in the FS(TN)y catalysts. At the low total Fe
content (0-2.0 wt.%), there were only framework Fe, and isolated and oligomeric Fe species on theextra framework. Because the positive charge like TPA+ and Na+ were located in the intersection andthe channels, Fe can be separated in and /or around the corresponding position. Positive charge wasenough to balance the Fe species on the low Fe content condition. It was not easy to aggregation due
to the well separation. While at the high Fe content (3.0-6.0 wt.%), other Fe species, i.e. larger Fe
clusters and bulk iron oxide, appeared due to the Fe ions gathered in a more concentrated place in thecorresponding as made samples. The proportion of framework Fe was decreased, and the isolated andoligomeric Fe species became the mainstream.
Figure 3.14 summaries the product amount as a function of the total Fe content in both the FS(T)xand FS(TN)y samples. The amount of both MeOH and the total liquid products for FS(T)x samples
were higher than those of FS(TN)y catalysts under the similar total Fe content, indicating the higher
activity for FS(T)x samples. As for the FS(T)x catalysts, the slope was high in the low Fe content, but
decreased with the total Fe content increasing (Figure 3.14(a)). It implied that the proportion of Fe
species with no or low activity was increase, but the active Fe amount was increased as well due to theincrease of total Fe content. While for FS(TN)y samples, the slope was basically unchanged with thetotal Fe content, suggesting that the proportion of active Fe species was low and unchanged, as shownin Figure 3.14(b).
The change in the product amount as a function of the Fe content of the isolated and oligomeric
Fe species on the extra framework in the FS(T)x and FS(TN)y catalysts are shown in Figure 3.15,
clearly indicating that the product amount was correlated with the content of isolated and oligomeric
extra framework Fe species. Thus, the formation of isolated and oligomeric extra framework Fe
species in the MFI structure is vital for achieving a high yield of methanol. Interestingly, it took morethan two times Fe content of the isolated and oligomeric Fe species on the extra framework for
FS(TN)y samples to achieve the similar product amount than those of FS(T)x catalysts. Perhaps itmeans the Fe species on the channels for FS(TN)y samples with low or no activity in the MTM
reaction.

71

Chapter 3

3.4. Conclusions
The conversion of methane to methanol over Fe-containing MFI zeolites under mild conditions
has been investigated. Fe-silicalite-1 zeolites were directly synthesized using TPAOH as OSDA withor without Na cations in the synthesis gel. The physicochemical properties, especially the states of Fe
species was characterized. Fe-silicalite-1 zeolites synthesized without Na cations displayed more
uniform distribution for each kinds of Fe species and higher proportion of framework Fe than thosesynthesized with Na cations. The impact of solvent on the reaction performance was investigated. We
firstly found that sulfolane was the best one among the solvents used in terms of the improvement in
the production of methanol and its stability during the reaction. Finally, the effects of various reaction
parameters, such as reaction temperature, CH4 pressure, reaction time, catalyst amount and the amountof H2O2 were optimized. Furthermore, the use of mixture of sulfolane-water solvent with anappropriate proportion led to an extremely high methanol production with a high selectivity. Generally
Fe-silicalite-1 zeolites synthesized using TPAOH as OSDA without Na cations (FS(T)x) showed better
catalytic performance than those with Na cations (FS(TN)y). Under the optimal condition, FS(T)15
zeolite displayed the highest methanol yield of 947.8 µmol with the selectivity of 85% in the aqueous
sulfolane solvent.

72

Chapter 3

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75

Chapter 3

(c)

5

10

15

20

25

30

35

(d)

Intensity (cps.)

Intensity (cps.)

(d)

(c)

(b)

(b)

(a)

(a)

40

5

10

15

20

25

30

35

40

2 Theta (Degree)

2 Theta (Degree)

Figure 3.1 XRD patterns of the as-synthesized samples: left: (a) FS(T)100, (b) FS(T)50, (c) FS(T)25,(d) FS(T)15, and right: (a) FS(TN)100, (b) FS(TN)50, (c) FS(TN)25, (d) FS(TN)15.

76

Chapter 3

(b)

(a)

500 nm(e)(c)

500 nm

500 nm(f)

500 nm

(d)

(h)

(g)

500 nm

500 nm

500 nm

500 nmFigure 3.2 FE-SEM images of (a) FS(T)100, (b) FS(T)50, (c) FS(T)25, (d) FS(T)15, (e) FS(TN)100,(f) FS(TN)50, (g) FS(TN)25 and (h) FS(TN)15.

77

Chapter 3
14

14

12

12

(d)
10

8

8

K/M

K/M

(d)
10

(c)

(c)

6

6

4

4

(b)
2

(b)

2

(a)

(a)
0

0
200

300

400

500

600

Wavelength (nm)

200

300

400

500

600

Wavelength (nm)Figure 3.3 UV-vis spectra of left: (a) FS(T)100, (b) FS(T)50, (c) FS(T)25, (d) FS(T)15, and right: (a)FS(TN)100, (b) FS(TN)50, (c) FS(TN)25, (d) FS(TN)15.

78

Chapter 3
1.3
1.2

1.3

PNO

1.2

1867

1.1

1.1

1810

1.0

1.0

1843

0.8

1915

0.7

1810

0.9

(d)

0.6

Absorbance

0.9

Absorbance

1867

PNO

0.5

0.8

1915

0.7

(d)

0.6

1902

0.5

0.4

0.4

(c)

0.3

1843

(c)

0.3

1902

0.2

0.2

(b)

(b)

0.1

0.1

(a)

(a)
0.0
2000

1950

1900

1850

1800

1750

1700

Wavenumbers (cm-1)

0.0
2000

1950

1900

1850

1800

1750

1700

Wavenumbers (cm-1)

Figure 3.4 NO adsorbed FT-IR spectra of left: (a) FS(T)100, (b) FS(T)50, (c) FS(T)25, (d) FS(T)15,
and right: (a) FS(TN)100, (b) FS(TN)50, (c) FS(TN)25, (d) FS(TN)15.

79

Chapter 3
3900

3900

3600

3600

3300

3300

3000

3000
2700

2400

2400

2100
1800

(c)

TCD signal

TCD signal

(d)
2700

2100
1800

1500

1500

1200

1200

900

(b)

600

(d)

(c)

900

(b)

600

300

300

(a)

(a)

0
350 400 450 500 550 600 650 700 750 800 850 900

0350 400 450 500 550 600 650 700 750 800 850 900

Temperature(K)Temperature(K)Figure 3.5 NH3-TPD spectra of left: (a) FS(T)100, (b) FS(T)50, (c) FS(T)25, (d) FS(T)15, and right:(a) FS(TN)100, (b) FS(TN)50, (c) FS(TN)25, (d) FS(TN)15.

80

Chapter 3
0.10

0.08

0.06

0.04

0.02

TMS

HCOH

HCOOH
0.00
8

6

4

2

0

ppm
1

Figure 3.6 H-NMR spectrum of blank test using acetonitrile as solvent (no CH4 as reactant) over
FS(T)15 catalyst, mesitylene was used as internal standard and CD3CN/ TMS mixture was used as
chemical shift calibrator.

81

Chapter 3
0.10

0.08

0.06

TMS

0.04

0.02

HCOOH
0.00
8

6

4

2

0

ppm
1

Figure 3.7 H-NMR spectrum of blank test using ethanol as solvent (no CH4 as reactant) over FS(T)15
catalyst, mesitylene was used as internal standard and CD3CN/ TMS mixture was used as chemical
shift calibrator.

82

Chapter 3
0.10

0.08

sulfolane

0.06

0.04

TMS
0.02

0.00
8

6

4

2

0

ppm
1

Figure 3.8 H-NMR spectrum of blank test using sulfolane as solvent (no CH4 as reactant) over
FS(T)15 catalyst, mesitylene was used as internal standard and CD3CN/ TMS mixture was used as
chemical shift calibrator.

83

Chapter 3
(a)

(b)

TMS
CH3OH

TMSP-D4

sulfolane

1,4-Dioxane

Mesitylene
HCOOH

HCOH

HCOH

CH3OH

HCOOH
8

6

4

2

0

8

ppm

6

4

2

0

ppm

Figure 3.9 (a) 1H-NMR spectrum of methane oxidation over FS(T)15 zeolite catalyst using sulfolane
as solvent, mesitylene was used as internal standard and CD3CN/ TMS mixture was used as chemical
shift calibrator. (b) 1H-NMR spectrum of methane oxidation over FS(T)15 zeolite catalyst using H2O
as solvent, 1,4-dioxane was used as internal standard and D2O/TMSP-D4 mixture was used as
chemical shift calibrator.

84

Chapter 3

2000

Product Amount (mol)

1800

(b)

(a)

Total liquid products
(c)

HCOH

HCOOH

MeOH

(d)

(e)

1600
1400
1200
1000
800
600
400
200
0
300

320

340

Temperature (K)360 0.5 1.0 1.5 2.0 2.5 3.0

CH4 pressure (Mpa)

0

1

2

3

Reaction Time (h)45 10 15 20 25 30 35 40 0

H2O2 amount (mmol)

20

40

60

80 100Catalyst amount (mg)Figure 3.10 Changes of liquid product amount over FMD15 catalyst as a function of (a) temperature, (b) CH4 pressure, (c) reaction time, (d) H2O2 amount,(e) catalyst amount. Reaction conditions: (a) 10 ml sulfolane, 50 mg FS(T)15, 27 mmol H2O2, 2 h, 3 Mpa CH4. (b) 323 K, 10 ml sulfolane, 50 mg FS(T)15,
27 mmol H2O2, 2 h. (c) 323 K, 10 ml sulfolane, 50 mg FS(T)15, 27 mmol H2O2, 3 Mpa CH4. (d) 323 K, 10 ml sulfolane, 50 mg FS(T)15, 2 h, 3 Mpa CH4.
(e) 323 K, 10 ml sulfolane, 27 mmol H2O2, 2 h, 3 Mpa CH4.

85

Chapter 3

MeOH Selectivity

Selectivity or conversion (%)

100

HCOOH Selectivity

HCOH Selectivity

(c)

(d)

(b)

(a)

H2O2 Conversion
(e)

80

60

40

20

0
300

320

340

Temperature (K)360 0.5 1.0 1.5 2.0 2.5 3.0

CH4 pressure (Mpa)

0

1

2

3

Reaction Time (h)45 10 15 20 25 30 35 40 0

20 40 60 80 100

H2O2 amount (mmol)

Catalyst amount (mg)Figure 3.11 Changes of the selectivity of the liquid product and H2O2 conversion over FMD15 catalyst as a function of (a) temperature, (b) CH4 pressure,(c) reaction time, (d) H2O2 amount, (e) catalyst amount. Reaction conditions: (a) 10 ml sulfolane, 50 mg FS(T)15, 27 mmol H 2O2, 2 h, 3 Mpa CH4. (b)323 K, 10 ml sulfolane, 50 mg FS(T)15, 27 mmol H2O2, 2 h. (c) 323 K, 10 ml sulfolane, 50 mg FS(T)15, 27 mmol H2O2, 3 Mpa CH4. (d) 323 K, 10 ml
sulfolane, 50 mg FS(T)15, 2 h, 3 Mpa CH4. (e) 323 K, 10 ml sulfolane, 27 mmol H2O2, 2 h, 3 Mpa CH4.

86

Chapter 3

Consumed H2O2
MeOH
HCOOH
HCOH

800

12000

10000

8000
600
6000
400
4000
200

0

Consumed H2O2 (μ mol)

Produced liquid productμ (mol)( a ) 1000

2000

0

25

50

75

100

0

Sulfolane content (vol.%)selectivity(b)

Figure 3.12 (a) The amount of liquid product and consumed H2O2 under different sulfolane content(b) possible liquid product species distribution under different sulfolane content. Reaction condition:
323 K, 50 mg FS(T)15, 27 mmol H2O2, 2 h, 3 Mpa CH4.

87

Chapter 3
3.0

Isolated and oligomeric Fe species
Large Fe cluster
Bulk iron oxide

2.5

Fe content (wt.%)

Fe content (wt.%)

2.5

3.0

(a)

Framework Fe

2.0

1.5

1.0

0.5

0.0
0.0

(b)

Framework Fe
Isolated and oligomeric Fe species
Large Fe cluster
Bulk iron oxide

2.0

1.5

1.0

0.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

The total Fe content (wt.%)

0.0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

The total Fe content (wt.%)

Figure 3.13 Fe content change of different Fe species as a function of the total Fe content for (a)FS(T)x and FS(TN)y catalysts.

88

Chapter 3
1200

1200

1000

1000

900

900

800
700
600
500
400
300

700
600
500
400
300
200

100

100
0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

The total Fe content (wt.%)

(b)

800

200

0
0.0

MeOH
Total liquid products

1100

Product amount (mol)

Product amount (mol)

(a)

MeOH
Total liquid products

1100

0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

The total Fe content (wt.%)

Figure 3.14 Change of the product amount as a function of the total Fe content for (a) FS(T)x and (b)FS(TN)y catalysts. Reaction conditions: 10 ml 50 vol.% sulfolane, 323 K, 50 mg catalysts, 27 mmol
H2O2, 2 h, 3 Mpa CH4.

89

Chapter 3
1200
1100

1100
1000

900

Product amount (mol)

Product amount (mol)

1000

1200

(a)

MeOH
Total liquid products

800
700
600
500
400
300

900
800
700
600
500
400
300

200

200

100

100

0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

(b)

MeOH
Total liquid products

0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

The Fe content of isolated and oligomeric
Fe species on the extra-framework (wt.%)

The Fe content of isolated and oligomeric
Fe species on the extra-framework (wt.%)

Figure 3.15 Change of the product amount as a function of the Fe content of the isolated andoligomeric Fe species on the extra framework for (a) FS(T)x and (b) FS(TN)y catalysts. Reaction
conditions: 10 ml 50 vol.% sulfolane, 323 K, 50 mg catalysts, 27 mmol H2O2, 2 h, 3 Mpa CH4.

90

Chapter 3

Si source
Fe source
H2O

Si source
Fe source
H2O

Na+

Na+
Na+

Na+

Na+

Na+

Na+
Framework Fe atom

Scheme 3.1 Strategy for preferential distribution of the Fe atoms in intersection and/ or channels of
the MFI framework.

91Chapter 3

CH4

+H2O2

Catalyst

CH3OOH

HCOH

+H2O2

+H2O2

CH3OH

HCOOH

CO2

Scheme 3.2 Assumed reaction pathway in conversion of methane to methanol with H2O2 overcatalyst.

92

Chapter 3

Table 3.1 Physicochemical characteristics of the FS(T)x and FS(TN)y catalysts.
Samples

In gelSi/Fe

In product a
Fe (wt.%)SBET(m2g−1)bSEXT(m2g−1)c

VTotal(cm3g-1)d

440
145
0.39
FS(T)100
100
306
0.3
434
157
0.45
FS(T)50
50
95
1.0
430
142
0.37
FS(T)25
25
44
2.1
474
183
0.46
FS(T)15
15
19
4.7
455
190
0.61
FS(TN)100
100
114
0.8
457
196
0.58
FS(TN)50
50
59
1.6
400
134
0.48
FS(TN)25
25
29
3.1
406
196
0.76
FS(TN)15
15
15
5.8
a
Si/Fe was determined by ICP-AES analysis, Fe content was calculated by Si/Fe ratio.
b

VMicro(cm3g-1)e
0.16
0.16
0.17
0.18
0.14
0.15
0.15
0.13

Specific surface areas of the catalysts were calculated using the Brunauer-Emmett-Teller (BET) equation on the N2
adsorption isotherms.
c External surface area (SEXT) of the catalysts were calculated by the t-plot method based on the adsorption isotherms.
dTotal pore volumes of the catalysts were calculated based on BET equation on the N 2 adsorption isotherms.
eMicropore volumes of the catalysts were calculated by the t-plot method based on the adsorption isotherms.

93Chapter 3

Table 3.2 Percentage of sub-band areas (I1: λ< 250 nm, I2: 250 <λ< 350 nm, I3: 350 <λ< 450 nm, andI4: λ>450 nm) derived by deconvoluting UV-vis spectra and corresponding Fe species content for the
FS(T)x and FS(TN)y catalysts.
Samples
FS(T)100
FS(T)50
FS(T)25
FS(T)15
FS(TN)100
FS(TN)50
FS(TN)25
FS(TN)15

Framework Fe
I1(%)
wt.%
86.7
0.26
73.0
0.73
61.6
1.29
40.9
1.92
78.7
0.63
77.3
1.24
49.8
1.54
35.1
2.04

Isolated and oligomeric Fe
I2(%)
wt.(%)
10.7
0.03
23.3
0.23
31.7
0.67
40.4
1.90
21.3
0.17
22.7
0.36
37.0
1.15
47.3
2.74

94

Iron oxide clusters
I3(%)
wt.%
2.6
0.01
3.6
0.04
6.7
0.14
10.7
0.50
7.8
0.24
12.0
0.70

Bulk iron oxides
I4(%)
wt.%
7.9
0.37
5.5
0.17
5.6
0.33

Chapter 3

Table 3.3 NH3 adsorbed amount of the FS(T)x and FS(TN)y catalysts.
Samples
FS(T)100
FS(T)50
FS(T)25
FS(T)15
FS(TN)100
FS(TN)50
FS(TN)25
FS(TN)15

NH3 adsorbed amount (mmol/g) / Tmax (K)LTMTHT
0.04/436
0.03/474
0.10/565
0.04/615
0.07/432
0.05/472
0.20/570
0.05/713
0.10/433
0.09/470
0.25/568
0.12/683
0.12/433
0.11/472
0.27/563
0.16/678
0.08/433
0.05/467
0.23/564
0.03/702
0.12/431
0.09/463
0.31/562
0.06/707
0.09/431
0.13/469
0.24/543
0.17/668
0.11/436
0.15/472
0.25/548
0.12/669

95

Chapter 3

Table 3.4 Catalytic performance for the FS(T)x and FS(TN)y catalysts using mixed aqueous sulfolane(50 vol.% sulfolane) as reaction solvent for the direct oxidation of methane to methanol with H2O2.
Catalyst

Selectivity (%) a

Products(µmol)
MeOH

HCOOH

HCOH

MeOH

HCOOH

HCOH

H2O2

H2O2

Conv. (%)b

Eff. (%)c

FS(T)100
133
9
0
94
6
0
7
FS(T)50
439
35
11
91
7
2
10
FS(T)25
670
28
29
92
4
4
15
FS(T)15
948
119
51
85
11
5
18
FS(TN)100
191
31
8
83
14
4
6
FS(TN)50
301
25
10
90
7
3
8
FS(TN)25
461
22
25
91
4
5
15
FS(TN)15
828
42
39
91
5
4
17
Reaction conditions: 323K, 10 ml 50 vol.% sulfolane, 50 mg catalyst, 27 mmol H2O2, 2 h, 3 Mpa CH4.
a Each product selectivity= (moles of each product)*100/(moles of total liquid products).
b H O conversion = (moles of H O after reaction)*100/(initial moles of H O ).
2 2
2 2
2 2
c H O efficiency = (moles of total liquid products)*100/(moles of H O conversion).
2 2
2 2

96

8
20
19
24
16
17
13
21

Chapter 4

Chapter 4
The influence of iron and aluminum location in MFI zeolites on the catalytic
performance in hydroxylation of benzene to phenol and methane to methanol
with H2O2
Abstract
Fe-ZSM-5 zeolites were directly synthesized using tetrapropylammonium hydroxide (TPAOH) as
OSDA with and without Na cations. The physicochemical properties, especially the states of Fe species
were characterized. Fe-ZSM-5 zeolites synthesized without Na cations (FZ(T)) displayed more
uniform distribution for each kinds of Fe species than the zeolites synthesized with Na cations(FZ(TN)). Generally FZ(T) zeolites showed slightly better catalytic performance than FZ(TN) in bothbenzene to phenol and methane to methanol reactions with H2O2. The introduction of aluminum in
FZ(T) and FZ(TN) zeolites decrease the proportion of framework Fe and increased the proportion ofisolated and oligomeric Fe species on the extra framework, thus improved the catalytic performancefor FZ(T) zeolites in BTP and MTM reactions. But for FZ(TN) zeolites, only the catalytic performancein BTP reaction was improved.

4.1. Introduction
The location and distribution of Al atoms in the zeolite framework is a hot topic in recent years,
which have been recognized as an important factor for activity and selectivity, because they would
profoundly affect the accessibility of molecules to acid sites and the spatial constraints of the reaction
field in the pores [1-4]. It’s generally recognized that to balance the charge, AlO4− tetrahedra species
are located near cations including organic cations, such as quaternary ammonium ions as OSDAs, or
inorganic ones, for example, Na+ and K+. Hence, the Al content is affected by the number of the cations,and the location of Al atoms may be reliant on their size and type of the cations. A synthetic strategy
to control of the distribution of Al in the Ferrierite-type (FER) zeolite by using different OSDAs in thepresence or absence of Na cations has been established [5-9]. Recently, the relationship between thesize of templates in the synthetic gel and the distribution of acid sites has been reported [7, 8].
97

Chapter 4

Meanwhile, the Al distribution over the RTH-type framework was found to dependent on the type ofthe cations by the high-resolution 27Al MAS NMR and 27Al MQMAS NMR techniques [10]. In
addition, Dedecek and co-workers controlled the distribution of Al in ZSM-5 zeolite by using different
Si, Al and Na sources [2]. In our previous reports, the distribution of Al in ZSM-5 could be controlled
by using different kinds of alcohols as the pore-forming agent and identified by the 27Al MAS NMR,
Co(II) ion UV-vis DRS, and constraint index value [4]. Moreover, the distribution of Al in the CHAtype zeolite can be controlled by varying the starting materials, and be detected by 27Al MAS NMR[11].
Except the distribution of Al in the zeolites, the location of other metals can also be controlled.
The most common method is depending on the parent zeolites with different Al location. Sobalik et al.
used FER-type zeolites with different Si/Al ratios, thus with different amount of Al pairs and single
Al atoms, as the parent zeolite to impregnate with the acetyl acetone solution of FeCl3. The prepared
Fe-FER zeolite with different Fe locations according to the site of Al in the framework [12]. In addition,
Sazama and co-workers applied ZSM-5 with different Al locations as the parent zeolite and thecounterpart impregnated with an anhydrous solution of FeCl3 in acetyl acetone [13]. What is more, theformation of Cu-oxo clusters can be controlled by using ZSM-5 with different Al distribution and ion
exchanging with Cu(CH3COO)2 solution [14].
In addition, similar to aluminum, iron as the positive trivalent transition metal possibly possesses
the same property by direct introducing Fe source into the synthesis gel, which exist and distribute in
the framework according the synthesis conditions. It is possible to form FeO4− tetrahedra in theframework rings of high-silica zeolites and the local negative charge to balance by the Na+, TPA+ andso on. In the presence of both iron and aluminum, situation will be a little complex than in the presenceof only Fe. Both Fe and Al can form the AlO4− and FeO4− tetrahedra in the framework, and meanwhile
Fe in single ions or iron-oxo species can control the negative charge produce by the tetrahedral in theframework as the counter ion species. However, this method has not been published in the open
literatures, and the influence of both the Fe and Al in the presence of the synthesis gel on the catalytic
morphology and performance has not been involved.
The distribution of AlO4− tetrahedra in the framework of aluminosilicate zeolites has contributed
to control the structures and contents of the transition metal counter ion species and their activities in,98Chapter 4

for example, SCR-NOx by hydrocarbons or ammonia [15, 16], decomposition of NO [15, 17] and N2O[18, 19], selective oxidation of benzene to phenol [20, 21] or methane to methanol [22, 23], as well as
the local proton density affecting the acid-catalyzed transformations of hydrocarbons [24]. But therelationship between their structures and distribution of the framework Al atoms has not been
established so far.
Here we report the synthesis of Fe-ZSM-5 zeolite catalysts with the location of aluminum andiron atoms in the framework controlled by using TPA cations as OSDA with or without Na cations.
The distribution of iron and aluminum in the zeolites were estimated by the UV-vis, NH3-TPD and
27

Al MAS NMR techniques. Finally, thus prepared Fe-ZSM-5 catalysts with different iron andaluminum locations were applied in the direct conversion of benzene to phenol and methane tomethanol reactions with H2O2 to clarify the effects of the distributions of iron and aluminum.
Meanwhile, the Fe-silicalite-1 zeolites with different locations of Fe species were applied in the tworeactions to investigate the effect of aluminum.

4.2. Experiments
4.2.1. Materials
Colloid silica (HS-40) and tetrapropylammonium hydroxide (TPAOH) (20-25% in water) werepurchased from Tokyo Chemical Industry. NaOH, Fe(NO3)3·9H2O, Al(NO3)3 ·9H2O, NH4NO3, H2O2
(30 wt.%), benzene, acetonitrile, sulfolane, mesitylene, anisole, 1,4-dioxane, tetramethylsilane (TMS),
and CD3CN were purchased from Wako. Methane (99.99%) gas was purchase from Taiyo Nippon
Sanso Co., Ltd.. All of the reagents were used as received, without further purification.

4.2.2. Catalysts preparation
Fe-ZSM-5 zeolites were directly prepared using TPAOH as OSDA without Na cations. First,
colloid silica was added to the solution containing water, TPAOH and Fe (NO3)3 and Al(NO3)3. The
gel with the molar composition of 1 Si: 0.01-0.04 Fe: 0.04 Al: 0.25 TPA: 20 H2O was crystallized at
443 K for 7 days with 40 rpm before aging at 353 K for 24 h. Then, the solid product was collected byfiltering, rinsing and drying. The as-synthesized samples were calcined at 823 K for 10 h to remove
TPA+ species. Thus obtained zeolites were designated as “FZ(T)x”, where x means the Si/Fe ratio in99

Chapter 4

the synthesis gel. The ZSM-5 without Fe source in the condition was denoted as Z(T).
Fe-ZSM-5 zeolites were also directly prepared using TPAOH as OSDA with Na cations. The gel
was aging at 353 K for 24 h and then crystallized at 443 K for 3 days. The final molar compositionwas 1 Si: 0.01-0.04 Fe: 0.04 Al: 0.25 TPA: 0.25 Na: 20 H2O. The calcined Na-type samples weretreated with 1 M NH4NO3 aq. at 353 K for 3 h twice, and filtered, washed, dry and calcined at 823 K
for 5 h to obtain the H-type samples. Thus obtained catalyst was designated as “FZ(TN)y ” where ymeans the Si/Fe ratio in the synthesis gel.. The ZSM-5 without Fe source in this condition was denotedas Z(TN).
The specific information of “FS(T)x” and “FS(TN)y” zeolites refer Chapter 3.

4.2.3. Characterization of catalysts
XRD patterns were collected on a Rint-Ultima III (Rigaku) using a Cu Kα X-ray source (40 kV,
20 mA). Field-emission scanning electron microscopic (FE-SEM) images of the powder samples wereobtained on S-5200 (Hitachi) microscope operating at 1 kV. Elemental analyses of the samples were
performed on an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu
ICPE-9000). Nitrogen adsorption desorption measurements to obtain information on the micro- andmeso-porosities were conducted at 77 K on a Belsorp-mini II (MicrotracBEL).
UV-vis diffuse reflectance spectra were recorded on a V-650DS spectrometer (JASCO). The
diffuse reflectance spectra were converted into the absorption spectra using the Kubelka-Munkfunction.
Temperature-programmed ammonia desorption (NH3-TPD) profiles were noted down on a
Multitrack TPD equipment (Japan BEL). Normally, 25 mg catalyst was pretreated at 773 K for 1 h ina He flow of 50 mL min-1 and then cooled down to 423 K. The sample was evacuated at 423 K for 1
h prior to the adsorption of NH3. Approximately 2500 Pa of NH3 contacted with the sample at 423 K
for 10 min. Subsequently, the sample was evacuated to remove the weakly adsorbed NH3 at the same
temperature for 30 min. Finally, the sample was heated from 423 to 873 K at a ramping rate of 10 K
min-1 in a He flow of 50 mL min-1.
The high-resolution 27Al MAS NMR and 29Si MAS NMR spectra were obtained on a JEOL ECA600 spectrometer (14.1 T) equipped with an additional 1 kW power amplifier. The 27Al and 29Si
100

Chapter 4chemical shifts were referenced to AlNH4(SO4)2·12H2O at -0.54 ppm and polydimethylsiloxane(PDMS) at -34.12 ppm, respectively. The samples were spun at 15 kHz by using a 4 mm ZrO2 rotor.

4.2.4 Catalytic tests
4.2.4.1 Direct hydroxylation of benzene to phenol with H2O2 (BTP)The BTP reaction was carried out in a 25 mL round-bottom flask equipped with a reflux condenser
and a magnetic stirrer. In a typical run, 50 mg of catalyst was immersed in 10 mL of acetonitrile, 5mmol of benzene and 10 mmol H2O2 was stirred at 333 K for 6 h [25]. After the reaction mixture wascooled down, the catalyst was removed. A certain amount of anisole as internal standard was added to
the resultant liquid, resulting in the formation of a single-phase liquid for the GC analysis [26]. The
remaining H2O2 concentration was quantified by standard titration method with 0.1 mol/L Ce(SO4)2
solution.

4.2.4.2 Direct conversion of methane to methanol with H2O2 (MTM)
The liquid-phase MTM reactions were carried out in a 100 ml PTFE autoclave. The CH4 pressure
was controlled by the pressure gage. The reactants were stirred vigorously by an agitator blade. In atypical run, 10 ml sulfolane, 50 mg catalyst and 27 mmol H2O2 were added to the autoclave and heated
to 323 K. The sealed reactor was then purged with CH4 to 3 Mpa. After reaction 2 h, the autoclave wascooled rapidly to 278 K in an ice bath to minimize any further chemical reaction and reduce loss ofvolatile products. After separation liquid phase and catalyst, the liquid-phase products were analyzed
by 1H NMR spectroscopy on JEOL ECA-600 spectrometer (14.1 T) equipped with an additional 1 kW
power ampliﬁer. Mesitylene and TMS/CD3CN were used as the internal standard a chemical shiftcalibrator, respectively. The detectable products in the liquid phase were CH3OH, HCOH, HCOOH
and CH3OOH. The amount of unconverted H2O2 was quantified by standard titration method with 0.1
mol/L Ce(SO4)2 solution.

4.3. Results and discussion
4.3.1 Characterization of catalysts
4.3.1.1 Physicochemical properties
Figure 4.1 show the XRD patterns of the as-synthesized FZ(T)x and FZ(TN)y zeolites. Both as101

Chapter 4

synthesized Z(T) and Z(TN) showed good crystallinity. The crystallinity of all the Fe-ZSM-5 samples
were decreased with the Si/Fe ratios decreased, indicating that too high Fe and Al contents were notconducive to form MFI structure zeolites. When the Si/Fe and Si/Al ratios both equaled to 25 in thesynthesis gel, the crystallinity of as-FZ(TN)25 was very low but still maintained the MFI structure.
However, it was amorphous for as-FZ(T)25. The possible reason was the limited positive charge
provided by TPA cation. For all the highly crystallized Fe-ZSM-5 zeolites, no features belonging tolarge iron oxide particles were observed, indicating the high Fe dispersion in the as-synthesizedzeolites.
The FE-SEM images of all the samples are shown in Figure 4.2. Z(T) displayed the cylindricalshape with protrusions on the side surface. The diameter of the cylinder was approximately 500 nm.
For Z(TN), it was irregular sphere with the particle size of 200 nm. The FZ(T)x zeolites were sphericaland the particle size was increased from 200 nm to 400 nm with the Fe content. However, the FZ(TN)yzeolites showed the irregular shape and the particle size was decreased from 300 nm to 50 nm with the
Fe content increasing. In addition, some tiny particle was distributed on the surface of FZ(T)50 and
FZ(TN)25, herein the lower crystallinity was displayed in XRD patterns.
The chemical composition and textural properties of the samples are listed in Table 4.1. The Si/Al
ratios of the FZ(T)x and Z(T) were higher than FZ(TN)y and Z(TN), which was consistent with theresults of our previous reports [1, 4], possible due to the limited positive charge proved by TPA cations
for FZ(T)x and Z(T) zeolites. For all the samples, the actual Si/Fe ratios in the products were higher
than those in the synthesis gels, especially FZ(T)100 and FZ(TN)100. Meanwhile, the actual Si/Fe
ratios of FZ(T)x samples were higher than those of FZ(TN)y, possibly due to the less amount of
positive charge provided by TPA cation for FZ(T)x than both TPA and Na cation for FZ(TN)y. The
textural properties of all the samples are listed in Table 4.1. FZ(T)x zeolites showed the BET surface
area (SBET) of 408-379 m2g-1 and external surface area (SEXT) of 175-116 m2g-1, which were lower than
FZ(TN)y, possibly due to the large particle size of FZ(T)x. All the Fe-containing MFI zeolite catalysts
possessed the similar micropore volume (Vmicro) of 0.14-0.16 cm3g-1.

4.3.1.2 Fe states
The Fe coordination of the Fe-containing MFI zeolite catalysts were studied by UV-vis
spectroscopy, as shown in Figure 4.3. According to the reference [27], the multiple bands canadequately be fitted by using single bands corresponding to (1) framework Fe species (200-250 nm,
λ1), (2) isolated and oligomeric extra framework Fe cations within the zeolite channels (250-350 nm,λ2), (3) larger Fe clusters (350-450 nm, λ3,) and finally (4) bulk Fe oxides on the surface of the zeolite
102Chapter 4(>450 nm, λ4). The amount and proportion of different Fe species based on the relative peak areas arelisted in Table 4.2. Similar to the results of FS(T)x zeolites in Chapter 3, the FZ(T)x zeolites contained
every kind of Fe species even at a very low Fe content.
Connection with the character of TPA cation, it was considered that the framework Fe wasdistributed in or close to the intersection in the as-synthesized FZ(T) samples. Influenced by the charge
amount and position, coordinated Fe species were preferentially distributed in close to the intersection.
Because the intersection had larger void (8.9 Å) than the channels (ca. 5.5 Å), the extra framework Fe
species were with different coordination after calcination. Thus the possibility of formation oligomer,
cluster and aggregated Fe species after calcination for FZ(T)x were much higher than FZ(TN)y. In
contrast, no bulk iron oxides appeared for FZ(TN)100, and the proportion of the other Fe species on
the extra framework for FZ(TN)y samples were increased with the Fe content growing. The proportionof each kind of Fe species for FZ(TN) 50 and FZ(TN)25 were similar. It suggested that the proportionof each Fe species did not change when Fe content was increased continuously after the Fe species
being evenly distributed.
4.3.1.3 Acidity
Figure 4.4 shows the NH3-TPD profiles of the Fe-ZSM-5 zeolites with different locations of ironand aluminum. The profiles can be deconvoluted into three kinds of peaks at low (373-473 K), medium(473-573 K), and high temperature (573-873 K) (designated as LT, MT, and HT). The amounts of NH3
adsorbed and the temperature of the maximum peak (Tmax) are listed in Table 4.3. According to thereferences [28, 29], the LT peak corresponds to NH3 adsorbed on the non-acidic-OH groups and NH4+by hydrogen bonding, which was not related to the true acid site and excluded in the discussion. The
MT peak corresponds to NH3 adsorbed on the extra framework Fe or Al species (Lewis acid site), andthe HT peak corresponds to NH3 adsorbed on the bridged Si-O(H)-M (Brønsted acid site), which M
means Fe and Al [29].
The acid amount for FZ(T)x zeolites were increased with the Fe content, but the increased amount
was not much. Instead, the MT and HT were obviously increased from 539 K to 570 K and from 623
K to 673 K for FZ(T)100 and FZ(T)50, respectively, which indicating the different coordination statesof Fe and Al. Meanwhile, the results were consistent with UV-vis of FZ(T)x, which contained eachkind of acid site and the acid amount were slightly increased with Fe content increasing. It was clear
to see that the acid amount of FZ(T)x zeolites were much weaker than Z(T), suggesting that the additionof Fe species affected the coordination state of aluminum in MFI zeolite. The acid amount of FZ(TN)y103

Chapter 4

were closed to each other with Fe content changing. Because there was no change in the amount ofaluminum for FZ(TN)y zeolites that mainly providing the acid amount. However, compared with
Z(TN), the intensity of peaks at around 540 K were increase with Fe content, and the intensity of peaks
at around 650 K were decreased and migrated to the low temperature with Fe content. The possible
reason was that Fe replaced or influenced part of Al in the framework thus decreased the acid amountat the high temperature. At the same time, part of Fe species appeared on the extra framework, as aresults increased the intensity of peaks at around 540 K, although it did not reflect on Table 4.3.

4.3.1.4 The states of Si and Al in Fe-ZSM-5 zeolites
The 29Si NMR spectra of the H-type MFI zeolites are shown in Figure 4.5. All the samples
possessed a main resonance peak at around -111 ppm were assigned to the silicon atoms coordinatedby four silicon atoms (Q4(0Al), Si(OSi)4). And another obvious resonance was at -103 ppm due to theQ3 silicon atoms ((OH)Si(OSi)3), which were mainly attributed to the surface silanol group, or Si(1Al)and Si(1Fe) for the FZ(T)x and FZ(TN)y zeolites [30]. There was a slightly increase in the spectral
intensity at around -103 ppm for FZ(T)x and FZ(TN)y compared with Z(T) and Z(TN), respectively,
possibly due to the increased amount of Si(1Fe) by addition of Fe. However, too much paramagnetic
Fe species in FZ(T)50 affected the measurement and lead to the unsmooth curve.
Figure 4.6 shows the 27Al NMR spectra of the MFI zeolites. The Al atoms were introduced
into the structure in a tetrahedral form at ca. 56 ppm and existed at the extra framework in hexahedron
form at ca. 0 ppm. Z(T) showed smooth curve and typical distribution at ca. 56 and 0 ppm, indicatingthat the vast majority of aluminum was in the form of tetra-coordination. The as-FZ(T)x contained
relative higher intensity at ca. 56 ppm, suggesting that most of aluminum was in the framework as theform of tetra-coordination in the as-synthesized samples, but after calcination, the intensity of chemical
shift at ca. 56 ppm for both FZ(T)100 and FZ(T)50 were greatly decreased. The phenomenon may be
explain the low acid amount of FZ(T)x zeolites. Z(T) and FZ(TN)y displayed the similar spectra,
indicating that most of the aluminum was in the framework as the tetrahedral form at ca. 56 ppm. It
was also consistent with the results of acid amount of FZ(TN)y. Thus the distribution of Fe and Al in
the MFI zeolite can be speculated. As shown in Scheme 4.1, the Fe and Al ions were distributed
preferentially in and/or around the intersection, which was similar to the situation of ZSM-5 with
TPAOH as OSDA in the absence of Na ions [1]. However, when the Na ions was presence in the104

Chapter 4

synthesis gel, the Fe and Al ions were distributed at both the intersection and channels due to thepositive Na+ and TPA+ location.

4.3.2 Direct oxidation of benzene to phenol
The catalytic performances for the direct hydroxylation of benzene with H2O2 over Fe-containing
MFI zeolite catalysts using TPAOH as OSDA without Na cations are detailed in Table 4.4. Z(T)achieved very low phenol yield (0.1%), indicating that ZSM-5 zeolite with the aluminum located in
the intersection was no active or very low activity in BTP reaction with H2O2. However, Fe-containing
MFI zeolites attained phenol yield of 1.4-3.6% with selectivity of higher than 98%, suggesting that theactive site was Fe species, which has been discussed in Chapter 2. The FZ(T)x zeolites achieved higher
phenol yield (2.2-3.6%) than FS(T)x (1.4-2.7%) with the selectivity of 98-99 % possibly due to thehigher proportion of isolated and oligomeric Fe species on the extra framework for FZ(T)x zeolites.
Table 4.5 are listed the catalytic performances for the direct hydroxylation of benzene with H2O2
over Fe-containing MFI zeolite catalysts using TPAOH as OSDA with Na cations. Similarly to Z(T),Z(TN) displayed very low phenol yield (0.1%), indicating that ZSM-5 zeolite with the aluminum
located in both the intersection and channels was no active or very low activity in BTP reaction with
H2O2. However, Fe-containing MFI zeolites attained phenol yield of 1.4-3.7% with selectivity of morethan 97%, suggesting that the active site was Fe species. The FZ(TN)y zeolites achieved higher phenol
yield (1.5-3.7%) than FS(TN)y (1.4-3.2%) with the selectivity of 97-99 % possibly due to the higher
proportion of isolated and oligomeric Fe species on the extra framework for FZ(TN)y zeolites .
Compared with the results in Table 4.4 and Table 4.5, it was clear to see that FZ(T)x and FS(T)xattained higher phenol yield than FZ(TN)y and FS(TN)y. The larger reaction space in the intersection
than the channels was one of the reasons. Besides, the activity of the formed Fe species in theintersection was different from the ones in the intersection and the channels. However, the specifickind of Fe species in the intersection and/ or the channels are still unknown.

4.3.3 Direct oxidation of methane to methanol
The catalytic performances for the direct hydroxylation of methane with H2O2 over Fe-containing
MFI zeolite catalysts using TPAOH as OSDA without Na cations are detailed in Table 4.6. The FZ(T)x105

Chapter 4

zeolites achieved more methanol or total liquid products than FS(T)x possibly due to the higher
proportion of isolated and oligomeric Fe species on the extra framework for FZ(T)x zeolites.
The catalytic performances for the direct hydroxylation of methane with H2O2 over Fe-containing
MFI zeolite catalysts using TPAOH as OSDA with Na cations are detailed in Table 4.7. The FZ(TN)yzeolites achieved more methanol than FS(TN)y due to the higher proportion of isolated and oligomeric
Fe species on the extra framework for FZ(TN)y zeolites .
Compared with the results in Table 4.6 and Table 4.7, the different of catalytic performance inconversion of methane to methanol between FZ(T)x and FZ(TN)y were not obvious. One possible
reason was no space limitation for methane and methanol in MFI zeolites. The H2O2 conversion of all
the samples were between 4-15%, which was low due to the properties of sulfolane.

4.4. Conclusions
Fe-ZSM-5 zeolites were directly synthesized using TPAOH as OSDA with or without Na cations.
The physicochemical properties, especially the states of Fe species were characterized by UV-vis and
NH3-TPD. Fe-ZSM-5 zeolites synthesized without Na cations displayed more uniform distribution foreach kinds of Fe species. Fe-ZSM-5 zeolites synthesized with Na cations presented strong acidity,
since most of aluminum located in the framework. Generally Fe-ZSM-5 zeolites synthesized without
Na cations (FZ(T)x) showed better catalytic performance in BTP reaction than the ones with Na cations(FS(TN)y). The introduction of aluminum in FZ(T)x and FZ(TN)y zeolites increased the isolated andoligomeric Fe species on the extra framework compared with FS(T)x and FS(TN)y, respectively. Thus,
the catalytic performance in BTP and MTM reactions were improved with the introduction ofaluminum in the MFI zeolites.

106

Chapter 4

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[20] K. A. Dubkov, N. S. Ovanesyan, A. A. Shteinman, E. V. Starokon, E. V. Starokon, G. I. Panov, J.
Catal., 207 (2002) 341.
[21] P. Kubanek, B. Wichterlova, Z. Sobalik, J. Catal., 211 (2002) 109.
[22] M. H. Groothaert, P. J. Smeets, B. F. Sels, B. A. Jacobs, R. A. Schoonheydt, J. Am. Chem. Soc.,
127 (2005) 1394.
[23] P. Vanelderen, R. G. Hadt, P. J. Smeets, E. I. Solomon, R. A. Schoonheydt, B. F. Sels, J. Catal.,
284 (2011) 157.
[24] P. Sazama, J. Dedecek, V. Gabova, B. Wichterlova, G. Spoto, S. Bordiga, J. Catal., 254 (2008)180.
[25] G. Wen, S. Wu, B. Li, C. Dai, D. Su, Angew. Chem. Int. Ed., 54 (2015) 4105
[26] M. Sasaki,Y. Sato, Y. Tsuboi, S. Inagaki, Y. Kubota, ACS Catal., 4 (2014) 2653.
[27] C. Hammond, I. Hermans, N. Dimitratos, N. Dimitratos, J. A. Lopez-Sanchez, R. L. Jenkins, G.
Whiting, S. A. Kondrat, M. H. ab Rahim, M. M. Forde, A. Thetford, H. Hagen, E. E. Stangland, J. M.
107

Chapter 4

Moulijn, S. H. Taylor, D. J. Willock, G. J. Hutchings, ACS Catal., 3 (2013) 1835.
[28] G. Wu, F. Hei, N. Zhang, N. Guan, L. Li, W. Grünert, Appl. Catal., A, 468 (2013) 230.
[29] R. Mohammad, Y. Fereydoon, Fuel, 181 (2016) 537.
[30] D. Freude, J. Kärger, Handbook of porous solids, 1 (2002) 465.

108

Chapter 4(d)Intensity (cps.)

(c)

5

10

15

20

25

30

35

Intensity (cps.)

(d)

(c)

(b)

(b)

(a)

(a)

40

5

10

15

20

25

30

35

40

2 Theta (Degree)

2 Theta (Degree)

Figure 4.1 XRD patterns of the as-synthesized left: (a) Z(T), (b) FZ(T)100,(c) FZ(T)50, (d) FZ(T)25,
right: (a) Z(TN), (b) FZ(TN)100,(c) FZ(TN)50, (d) FZ(TN)25.

109

Chapter 4
(a)

(b)

500 nm(d)(c)

500 nm(e)

500 nm

500 nm(g)(f)

500 nm

500 nm

500 nmFigure 4.2 FE-SEM images of (a) Z(T), (b) FZ(T)100, (c) FZ(T)50, (d) Z(TN), (e) FZ(TN)100, (f)FZ(TN)50 and (g) FZ(TN)25.

110

Chapter 4
8

8

6

6

K/M

K/M

(c)

4

4

(b)

(b)

2

2

(a)

(a)

0

0
200

300

400

500

600

Wavelength (nm)

200

300

400

500

600

Wavelength (nm)Figure 4.3 UV-vis spectra of left: (a) FZ(T)100,(b) FZ(T)50, right: (a) FZ(TN)100,(b) FZ(TN)50, (c)FZ(TN)25.

111

Chapter 4
6000

6000

5500

5500

5000

5000

4500

4500

4000

4000

TCD signal

TCD signal

(d)

3500

(c)
3000
2500
2000

3500

(c)
3000
2500
2000

(b)

(b)
1500

1500

1000

1000

500

500

(a)

(a)
0
350 400 450 500 550 600 650 700 750 800 850 900

0350 400 450 500 550 600 650 700 750 800 850 900

Temperature(K)Temperature(K)Figure 4.4 NH3-TPD analyses of left: (a) Z(T), (b) FZ(T)100,(c) FZ(T)50, (d) FZ(T)25, right: (a)Z(TN), (b) FZ(TN)100,(c) FZ(TN)50, (d) FZ(TN)25.

112

Chapter 4

Q4

Q4
(d)

-90

-95

-100

-105

-110

-115

-120

(c)

(c)

(b)

(b)

(a)

(a)

-125

-130

-90

Chemical shift (ppm)

-95

-100

-105

-110

-115

-120

-125

-130

Chemical shift (ppm)

Figure 4.5 29Si MAS NMR spectra of left: (a) Z(T), (b) FZ(T)100, (c) FZ(T)50, right: (a) Z(TN), (b)FZ(TN)100, (c) FZ(TN)50, (d) FZ(TN)25.

113

Chapter 4

Al Ⅳ

(e)

Al Ⅵ

Al Ⅳ

(d)

Al Ⅵ
(d)

(c)

(c)

(b)

(b)

(a)

(a)

100

90

80

70

60

50

40

30

20

10

0

-10 -20 100

90

80

70

60

50

40

30

20

10

0

-10 -20

Chemical shift (ppm)

Chemical shift (ppm)27

Figure 4.6 Al MAS NMR spectra of left: (a) Z(T), (b) as-FZ(T)100, (c) as-FZ(T)50, (d) FZ(T)100,(e) FZ(T)50, right: (a) Z(TN), (b) FZ(TN)100, (c) FZ(TN)50, (d) FZ(TN)25.

114

Chapter 4

as-FZ(T)FZ(T)

as-FZ(TN)FZ(TN)Na+Na+

Si source
Fe source
Al source
H2O

Si source
Fe source
Al source
H2O

Na+

Na+

Na+

Na+

Na+
Framework Fe atom

Framework Al atom

Scheme 4.1 Strategy for preferential distribution of the Fe and Al atoms in FZ(T)x and FZ(TN)yzeolites.

115

Chapter 4

Table 4.1 Physicochemical characteristics of the Fe-containing MFI zeolite catalysts.
SampleIn product a

In gel
Si/Al

Si/Fe

Si/Al

Si/Fe

Z(T)
25
27
FZ(T)100
25
100
28
241
FZ(T)50
25
50
28
66
Z(TN)
25
25
FZ(TN)100
25
100
23
133
FZ(TN)50
25
50
23
62
FZ(TN)25
25
25
24
28
a
Si/Fe was determined by ICP-AES analysis.

Fe (wt.%)SBET(m2g−1)bSEXT(m2g−1)c

VTotal(cm3g-1)dVMicro(cm3g-1)e

0.4
1.3
0.6
1.3
3.1

462
408
379
422
450
444
470

126
175
116
156
212
182
251

0.41
0.50
0.35
0.43
0.48
0.45
0.61

0.15
0.14
0.14
0.17
0.16
0.16
0.16

b

Specific surface areas of the catalysts were calculated using the Brunauer-Emmett-Teller (BET) equation on the N2
adsorption isotherms.
cExternal surface area (SEXT) of the catalysts were calculated by the t-plot method based on the adsorption isotherms.
dTotal pore volumes of the catalysts were calculated based on BET equation on the N 2 adsorption isotherms.
eMicropore volumes of the catalysts were calculated by the t-plot method based on the adsorption isotherms.

116Chapter 4

Table 4.2 Percentage of sub-band areas (I1:λ< 250 nm, I2: 250 <λ< 350 nm, I3: 350 <λ< 450 nm, andI4: λ>450 nm) derived by deconvoluting UV-vis spectra and corresponding Fe species content for Fecontaining MFI zeolite catalysts.
SampleFZ(T)100
FZ(T)50
FZ(TN)100
FZ(TN)50
FZ(TN)25

Framework Fe
I1(%)
wt.%
39.9
0.12
41.1
0.53
60.8
0.36
38.5
0.50
38.7
1.12

Isolated and Oligomeric Fe
I2(%)
wt.(%)
39.9
0.12
44.3
0.58
30.4
0.18
46.5
0.60
46.7
1.45

117

Larger Fe clusters
I3(%)
wt.%
11.9
0.04
9.2
0.12
8.8
0.05
9.1
0.12
9.5
0.29

Bulk Fe oxides
I4(%)
wt.%
8.2
0.02
5.4
0.07
5.9
5.7

0.08
0.18

Chapter 4

Table 4.3 NH3 adsorbed amount of Fe-containing MFI zeolite catalysts.
SampleZ(T)
FZ(T)100
FZ(T)50Z(TN)FZ(TN)100
FZ(TN)50
FZ(TN)25

NH3 adsorbed amount (mmol/g) / Tmax (K)LTMTHT
0.27/434
0.36/622
0.05/434
0.05/462
0.12/539
0.08/623
0.07/440
0.07/469
0.20/570
0.08/673
0.34/458
0.48/633
0.19/432
0.16/486
0.25/620
0.11/660
0.22/433
0.26/472
0.27/610
0.18/643
0.19/440
0.25/484
0.25/573
0.18/668

118

Chapter 4

Table 4.4 Catalytic results of Fe-containing MFI zeolite catalysts using TPAOH as OSDA without
Na cations in the synthesis gel for the direct oxidation of benzene to phenol with H2O2.
SamplePhenolYield (%)a

Product Selectivity (%)b
Phenol

HQ

CL

H2O2
Conv.(%)c

Z(T)
0.1
100
0
0
2
FZ(T)100
2.2
99
1
0
40
FZ(T)50
3.6
98
1
1
53
FS(T)100
1.4
99
1
0
29
FS(T)50
2.7
98
1
1
48
Reaction conditions: 333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
aPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
b
Each product selectivity= (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL).
c H O conversion = (moles of H O after reaction)*100/(initial moles of H O ).
2 2
2 2
2 2

119

Chapter 4

Table 4.5 Catalytic results of Fe-containing MFI zeolite catalysts using TPAOH as OSDA with Na
cations in the synthesis gel for the direct oxidation of benzene to phenol with H2O2.
SamplePhenolYield (%)a

Product Selectivity (%)b
Phenol

HQ

CL

H2O2
Conv.(%)c

Z(TN)
0.1
100
0
0
3
FZ(TN)100
1.5
98
2
1
48
FZ(TN)50
1.7
97
2
1
67
FZ(TN)25
3.7
99
1
0
90
FS(TN)100
1.4
99
1
0
55
FS(TN)50
2.5
98
1
1
63
FS(TN)25
3.2
97
2
1
78
Reaction conditions: 333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
aPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
b Each product selectivity= (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL).
c H O conversion = (moles of H O after reaction)*100/(initial moles of H O ).
2 2
2 2
2 2

120

Chapter 4

Table 4.6 Catalytic results of Fe-containing MFI zeolite catalysts using TPAOH as OSDA without
Na cations in the synthesis gel for the direct oxidation of methane to methanol with H2O2.
SampleLiquid products
MeOH

HCOOH

HCOH

MeOOH

H2O2
MeOH
a
Sele.(%) Conv.(%)b Eff. (%)c

FZ(T)100
131
44
114
0
45
7
FZ(T)50
250
6
187
0
56
10
FS(T)100
80
8
0
0
91
6
FS(T)50
266
30
83
0
70
7
Reaction conditions: 323 K, 27 mmol H2O2, 50 mg catalyst, 10 ml sulfolane, 3 Mpa CH4, 2 h.
aEach product selectivity= (moles of each product)*100/(moles of total liquid products).
b
H2O2 conversion = (moles of H2O2 after reaction)*100/(initial moles of H2O2).
c
H2O2 efficiency = (moles of total liquid products)*100/(moles of H2O2 conversion).

121

16
10
6
9

Chapter 4

Table 4.7 Catalytic results of Fe-containing MFI zeolite catalysts using TPAOH as OSDA with Na
cations in the synthesis gel for the direct oxidation of methane to methanol with H2O2.
SampleLiquid products
MeOH

HCOOH

HCOH

MeOOH

H2O2
MeOH
a
Sele.(%) Conv.(%)b Eff. (%)c

FZ(TN)100
155
12
94
0
59
5
FZ(TN)50
245
9
189
36
51
11
FZ(TN)25
303
0
209
0
59
15
FS(TN)100
116
27
63
0
56
4
FS(TN)50
183
21
76
0
65
6
FS(TN)25
279
19
187
0
58
11
Reaction conditions: 323 K, 27 mmol H2O2, 50 mg catalyst, 10 ml sulfolane, 3 Mpa CH4, 2 h.
aEach product selectivity= (moles of each product)*100/(moles of total liquid products).
b H O conversion = (moles of H O after reaction)*100/(initial moles of H O ).
2 2
2 2
2 2
c H O efficiency = (moles of total liquid products)*100/(moles of H O conversion).
2 2
2 2

122

36
17
15
18
14
16

Chapter 5

Chapter 5
Alkaline treatment on as-synthesized and calcined Fe-silicalite-1 and Fe-ZSM-5
zeolites for hydroxylation of benzene to phenol with H2O2
Abstract
The influence of desilication on as-synthesized and calcined Fe-ZSM-5 and Fe-silicalite-1 zeolites
were investigated. Based on that, Fe-silicalite-1 and Fe-ZSM-5 zeolite were synthesized by in-situ
method. The as-synthesized and calcined zeolites were modified by the same alkaline condition. N2
adsorption and desorption was used to character the texture properties. The UV-vis spectroscopy wasapplied to evaluate the states of Fe species and the corresponding Fe species content derived bydeconvoluting the sub-band areas. The ammonia adsorbed amount measured by NH3-TPD was used
to investigate the environment of iron and aluminum in the zeolites. Finally, the catalytic performanceof thus prepared catalysts were estimated in hydroxylation of benzene to phenol with H2O2. Significant
changes occurred in the calcined Fe-ZSM-5, both as-made and calcined Fe-silicalite-1 zeolites.

5.1. Introduction
Fe-containing zeolites have been widely used and revealed outstanding performance in various
catalytic conversions related to environmental applications, such as direct N2O decomposition [1-3],
selective catalytic reduction of NOx and N2O [4, 5], selective oxidation of NH3 to N2 [6]; and inoxidative transformations of hydrocarbons, including benzene and phenol hydroxylation [7-9],
propane oxidative dehydrogenation [2, 10-12], hydroxylation of methane to methanol [13-15] and soon.
The preparation method of Fe-containing zeolites strongly influences the nature and distributionof the resulting Fe species, which basically determine the catalytic performance. Extensive efforts havebeen dedicated to establishing reliable procedures for iron incorporation, including direct hydrothermal
synthesis [4, 16, 17], ion exchange [18], impregnation [19, 20], chemical vapor deposition (CVD) [21,22] and so on. Introduction of iron from the zeolite matrix is typically followed by high temperature
calcination or steam treatment. The latter is crucial to yield active sites in isomorphously substituted
zeolites by extraction of framework Fe to extra framework [23]. The positive effect of steaming has
123Chapter 5

also been reported for Fe/ZSM-5 zeolites prepared via CVD that were used in benzene hydroxylation
with N2O [21]. Besides, high temperature calcination on Fe-silicalite-1 and Fe-ZSM-5 was also
reported to change the catalytic performance in aqueous phase methane oxidation, but the
improvement only occurred on the Fe-silicalite-1[13].
Desilication has been reported to control the mesopore generation in zeolites with the ultimate
aim of improving transport properties [24]. Adjustable mesoporosity has been achieved with preserved
crystalline, microporous, and acidic properties, since the existence of template and aluminum in thezeolites have great influence. Several studies have showed that templates can be helpful for successful
alkaline treatment. Pérez-Ramírez et al. reported that partial template followed by alkaline treatment
can be used to tune the desilication process and that the addition of template molecules (TPA+ or TBA+)
during desilication can be used as pore-growth moderators [25]. A study by Smirniotis et al. verified
that templated ZSM-12 samples with Si/Al ratio up to 500 were protected from silicon extraction
during alkaline treatment [26]. Perez-Ramirez and co-workers introduced that framework aluminium
controlled the process of framework silicon extraction and made desilication selective towards
intracrystalline mesopore formation [24]. Based on that, the hierarchical pore system can be controlleddependency on the initial Si/Al molar ratio in the zeolites [27, 28]. As a result, mild alkaline
modification was applied to defragment zeolite crystals, and then iron was fully incorporated by liquidphase ion exchange on zeolites, accordingly reducing agglomeration of Fe species, shorting diffusional
path lengths and leading to improve activity [29, 30].
So far, desilication has not been directly applied to Fe-containing zeolites. Moreover, no open
literature reported the effect of template and aluminum in Fe-ZSM-5 and Fe-silicalite-1 on desilication.
In this study, Fe-ZSM-5 and Fe-silicalite-1 zeolites were alkaline treated in the absence or presence of
OSDA. The structural textural properties, the states of Fe species and acid properties of the thus
prepared samples were characterized. The effects of template and aluminum in Fe-ZSM-5 and Fesilicalite-1 on desilication were discussed. Finally, the catalytic performance in BTP reaction wasinvestigated to clarify the influence of alkaline treatment.

5.2. Experiments
5.2.1 Materials
124Chapter 5

Tetraethyl orthosilicate (TEOS), colloid silica (HS-40, 40 wt.%) and tetrapropylammoniumhydroxide (TPAOH) (20-25% in water) were purchased from Tokyo Chemical Industry. NaOH,
Fe(NO3)3·9H2O, Al(NO3)3·9H2O, H2O2 (30 wt.%), acetonitrile, benzene, acetone, acetonitrile andanisole were purchased from Wako. All of the reagents were used as received, without further
purification.

5.2.2 Catalysts preparation
For the synthesis of Fe-ZSM-5 zeolites, TEOS was added into the mixed aqueous solution
containing TPAOH, Fe(NO3)3, Al(NO3)3. The molar ratio of the gel compositions was as follows: 1
Si: 0.025 Al: 0.3 TPA: 0.01 Fe: 45 H2O. After vigorous stirring at room temperature for 1 h, theresulting gel was transferred to a Teflon-lined autoclave and crystallized with 40 rpm at 443 K for 5
days. The solid products were filtered, washed, and dried. The as-made sample, which was noted as“as-FZ”, was calcined at 823 K for 10 h, and then treated by 1 M NH4NO3 aqueous solution at 353 K
for 3 h twice to obtain the NH4+-type ones. H-type sample was made from the NH4+-ones by calcination
at 823 K for 5 h. Thus obtained product was denoted as “H-FZ”.
The Fe-silicalite-1 zeolite was directly prepared with TPA cation in the absence of Na cation as
follows. First, colloid silica was added to the solution containing water, TPAOH and Fe (NO3)3·9H2O.
The gel with the molar composition of 1 Si: 0.067 Fe: 0.25 TPA: 20 H2O was crystallized at 443 K for
7 days with 40 rpm after aging at 353 K for 24 h. Then, the solid product was collected by filtering,rinsing and drying. The as-synthesized material, noted as “as-FS”, was calcined at 823 K for 10 h toremove TPA species and then treated by 1 M NH4NO3 aqueous solution at 353 K for 3 h twice toobtain the NH4+-type ones. H-type sample was made from the NH4+-ones by calcination at 823 K for
5 h. Thus obtained catalyst was designated as “H-FS”
The as-FZ and as-FS samples were treated in alkaline aqueous solution using 0.2 M NaOH at 353
K for 2 h. The resultant samples was calcined at 823 K for 10 h. In order to facilitate comparison, thealkaline treatment samples were NH4+-exchanged, then calcined at 823 K for 5 h. The final products
were designated as “AAT-FZ” and “AAT-FS”, respectively.
Similarly, the calcined Fe-ZSM-5 and Fe-silicalite-1 samples were treated in alkaline aqueous
solution using 0.2 M NaOH at 353 K for 2 h and 1 h, respectively. The resultant samples were washed,dried overnight, NH4+-exchanged and calcined at 823 K for 5 h. The final products were designated as“CAT-FZ” and “CAT-FS”.

125

Chapter 5

5.2.3 Characterization of catalysts
XRD patterns were collected on a Rint-Ultima III (Rigaku) using a Cu Kα X-ray source (40 kV,
20 mA). Field-emission scanning electron microscopic (FE-SEM) images of the powder samples wereobtained on S-5200 (Hitachi) microscope operating at 1 kV. Elemental analyses of the samples were
performed on an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu
ICPE-9000). Nitrogen adsorption and desorption measurements to obtain information on the microand meso-porosities were conducted at 77 K on a Belsorp-mini II (MicrotracBEL).
UV-vis diffuse reflectance spectra were recorded on a V-650DS spectrometer (JASCO). The
diffuse reflectance spectra were converted into the absorption spectra using the Kubelka-Munkfunction.
Temperature-programmed ammonia desorption (NH3-TPD) profiles were noted down on a
Multitrack TPD equipment (Japan BEL). Normally, 25 mg catalyst was pretreated at 773 K for 1 h ina He flow of 50 mL min-1 and then cooled down to 423 K. The sample was evacuated at 423 K for 1
h prior to the adsorption of NH3. Approximately 2500 Pa of NH3 contacted with the sample at 423 K
for 10 min. Subsequently, the sample was evacuated to remove the weakly adsorbed NH3 at the same
temperature for 30 min. Finally, the sample was heated from 423 to 873 K at a ramping rate of 10 K
min-1 in a He flow of 50 mL min-1.

5.2.4 Catalytic tests
The BTP reaction was carried out in a 25 mL round-bottom flask equipped with a reflux
condenser and a magnetic stirrer. In a typical run, the mixture containing 50 mg of catalyst, 10 mL ofacetonitrile, 5 mmol of benzene, and 10 mmol of H2O2 was stirred at 333 K for 6 h. After the reaction
mixture was cooled down, the catalyst was removed. A certain amount of anisole as internal standard
was added to the resultant liquid, and the products were fixed by exhaustive acetylation with excess(CH3CO)2O-K2CO3, the derivative products were analyzed by GC [28, 29]. The amount of
unconverted H2O2 was quantified by standard titration method with 0.1 mol/L Ce(SO4)2 solution. The
phenol yield was based on the amount of phenol produced per the initial amount of benzene. The
product selectivity was based on phenol, hydroquinone and catechol.

5.3. Results and discussion
5.3.1 Characterization of the catalysts
126Chapter 5

5.3.1.1 Physicochemical properties
Figure 5.1 shows the XRD patterns of the different Fe-ZSM-5 and Fe-silicalite-1 catalysts. Bothof as-synthesized Fe-ZSM-5 and Fe-silicalite-1 had the typical MFI structure, and after calcinaion the
crystallinity of Fe-ZSM-5 and Fe-silicalite-1 were increased. However, the Fe-ZSM-5 has higher
crystallinity than Fe-silicalite-1. For AAT-FZ and AAT-FS, the structure and crystallinity remained,
indicating that alkaline treatment on the as-synthesized samples did not make seriously desilication.
For CAT-FZ, the crystallinity was decreased, suggesting the severe desilication. As for CAT-FS, thecrystallinity was not decreased. It was worth to note that the CAT-FS sample was treated in 0.2 M
NaOH for only 1 h. The solid yield of CAT-FS was 69%. If the calcined Fe-silicalite-1 was treated for
2 h, the solid yield was only 9%, which topology structure was broken. The solid yield of AAT-FZ,
CAT-FZ, AAT-FS were 90%, 27% and 62%, respectively.
Figure 5.2 shows the FE-SEM images of the samples. The Fe-ZSM-5 zeolite displayed the square
structure and the flat surface with some tiny cuboids. The particle size of H-FZ was about 3 µm. The
overall appearance of AAT-FZ has not changed, but some small pores was added to the flat surface,which produced by the alkaline treatment. In addition, the particle of a part of CAT-FZ was broken
after alkaline treatment, indicating that the calcined sample was easily desilicated under the same
condition, which was consistent with the result of XRD. The Fe-silicalite-1 zeolite showed the coffin
shape with the particle size about 300 nm. No obvious pore on the zeolite was observed due to thesmall particle size and unsmooth surface. The overall shape of the CAT-FS particle was not destroyedsince the the time of alkaline treatment on this sample was short.
The chemical composition of the samples are listed in Table 5.1. The actual Si/Al and Si/Fe ratiosof H-FZ were higher than those in the synthesis gels, meaning that only part of iron was introduced
into the zeolite. For AAT-FZ, the Si/Al and Si/Fe ratios were slightly increased, possibly due to thealkaline treatment on the as-made samples remove the alumina and iron oxide species on the extra
framework. However, the Si/Al and Si/Fe ratios of CAT-FZ were much lower than those of H-FZ,
suggesting that a large amount of silicon in the calcined sample was removed. For H-FS, the actual
Si/Fe ratio was higher than those of AAT-FS and CAT-FS, which also indicated the removing ofsilicon from as-FS and calcined Fe-silicalite-1.
The N2 adsorption desorption isotherms and BJH pore size distributions are depicted in Figure5.3. The texture properties were listed in Table 5.1. The N2 adsorption isotherm of the H-FZ sample
was of type I with a plateau at higher relative pressures and no distinct hysteresis loop, typical for a
microporous material without significant mesoporosity. The SBET (432 m2g-1) of H-FZ was primarilya result of the presence of micropores, which was in accordance with Ma's results [31]. The N2
127

Chapter 5

adsorption isotherm of the AAT-FZ sample was similar to that of H-FZ, the SEXT, VTotal and VMicrowere no much difference, except that the SBET of AAT-FZ was slightly increased to 473 m2g-1. In thecase of CAT-FZ, the isotherm can be ascribed to the type I isotherm with H4-shaped hysteresis loops,
indicating the formation of considerable amount of mesopores (0.72 cm3g-1) accompanying with slight
destruction of micropores (0.16 cm3g-1) after alkaline treatment [32]. Thereby, the SBET and SEXT of
CAT-FZ were increased to 683 m2g-1 and 366 m2g-1. The results of the pore size distribution revealthat H-FZ and AAT-FZ samples possess a narrow and sharp pore-size distribution in the micropore
range, which is basically consistent with the pore size (0.53 nm×0.56 nm, IUPAC) of main channel
in the ZSM-5 zeolite. The BJH model depending on the adsorption branch demonstrates the presenceof mesopores in the CAT-FZ samples, a pronounced development of mesopores distributed at 5 and12 nm was shown in Figure 5.3(b) [31].
Figure 5.4 shows the N2 adsorption and desorption isotherms and BJH pore size distributions forthe parent and alkali-treated Fe-silicalite-1 catalysts. The textural properties are also listed in Table
5.1. The SBET of H-FS was 476 m2g-1 and Vtotal was 0.55 cm3g-1, which were higher than H-FZ due tothe smaller particle size. The SBET of AAT-FS was increased to 551 m2g-1. The isotherms can be
ascribed to the type I isotherm with H4-shaped hysteresis loops, indicating the formation of
considerable amount of mesopores (1.02 cm3g-1) accompanying with very low micropores (0.05 cm3g1) after alkaline treatment. Among the mesopores, a part of constructional void porosity was contained

due to the sharp risen at the p/p0 over 0.9. The CAT-FS showed closed SBET (536 m2g-1) compared with
AAT-FS. But there was no constructional void porosity observed, thus the measured mesopores (0.78
cm3g-1) was lower than that of AAT-FS. But CAT-FS had mesopores distributed at 13 nm.

5.3.1.2 Fe states
The UV-vis spectra of the Fe-ZSM-5 samples are shown in Figure 5.5, which were deconvoluted
into sub-bands by applying Gauss functions, as listed in Table 5.2. According to the references [33,34], the bands at about 210 nm should be ascribed to the isolated Fe in tetrahedral coordination and
250 nm assigned to the higher coordination. Generally, the bands below 250 nm were assigned toframework Fe species, the bands between 250-350 nm were ascribed to isolated and oligomeric Fe
species on the extra framework (e.g., dimeric, trimeric, and tetrameric Fe species), the bands between
350-450 nm were ascribed to iron oxide clusters, and the bands above 450 nm were ascribed to bulk
iron oxide particles less or larger than 2 nm. The bands for as-FZ sample mainly located at 209 nm
and 256 nm. The high proportion of 83.5% indicated that most of Fe was introduced into the frameworkduring the crystallization. The remaining 16.5% Fe species mainly concentrated at the bands below
128

Chapter 5

350 nm, indicating small amount of isolated and oligomeric Fe species on the extra framework. For
H-FZ, the proportion of bands from 250 nm to 350 nm was increased from 16.5 to 42.8%, indicatingthat a large amount of Fe in the framework immigrated to the extra framework. What interesting was
the composition of Fe species for the AAT-FZ sample, the proportion of isolated and oligomeric Fe
species on the extra framework were lower than that of H-FZ, possibly due to part of Fe and Al specieson the extra framework being removed after alkaline treatment. The distinct difference of CAT-FZ
was the severe aggregated degree of Fe species, isolated and oligomeric Fe species on the extra
framework were increased to 47.6%, large Fe species and bulk iron oxide were increased to 11.5% and7.2%, respectively. Because framework silicon was extracted from the calcined Fe-ZSM-5, the ironand aluminum in the framework were more easily to migrate to the extra framework after calcination
due to the defects formed during the alkaline treatment.
Figure 5.6 shows the UV-vis spectra of the different form of Fe-silicalite-1 zeolites, which were
deconvoluted into several bands by applying Guass functions. The amount and proportion of different
Fe species based on the relative peak areas are listed in Table 5.2. The as-FS zeolite took a strong
sharp peak at 210 nm with shoulder at 250 nm, assignment to tetrahedral coordination and octahedral
coordination. The broad peak at 320 nm was ascribed to oligomeric Fe species on the extra framework,
due to the high Fe content in the synthesis gel, part of the Fe species existed on the surface of thezeolite during the crystallization. After calcination, part of Fe species in the framework migrate to theextra framework, thus the intensity of band from 350 to 600 nm was increased. Alkaline treatment on
as-FS made the proportion of bands from 300 to 600 nm further increased compare with that of H-FS.
Similarly, the proportion of aggregated Fe species for CAT-FS was increased compared with H-FS,
but it was not obvious to compare with AAT-FS.

5.3.1.3 Acidity
The acidity measured by NH3-TPD revealed the Fe environment to a certain degree. Figure 5.7(ac) show the NH3-TPD profiles of the H-FZ, AAT-FZ and CAT-FZ. Most of the profiles can be
deconvoluted into three kinds of peaks at low (373-473 K), medium (473-573 K), and high temperature(573-773 K) (designated as LT, MT, and HT). The amounts of NH3 adsorbed of the samples are listed
in Table 5.3. According to the references [11, 35], the LT peak corresponds to NH3 adsorbed on the
non-acidic-OH groups and NH4+ by hydrogen bonding, which was not related to the true acid site andexcluded in the discussion. The MT peak corresponds to NH3 adsorbed on the extra framework Fe andAl species (Lewis acid site), and the HT peak corresponds to NH3 adsorbed on the bridged Si-O(H)Fe and Si-O(H)-Al (Brønsted acid site) [35].
The acid amount on MT for AAT-FZ was decrease from 0.21 to 0.15 mmol/g compared with H129

Chapter 5

FZ, which indicated the decrease Fe species on the extra framework. It was consistent with the resultof UV-vis, which showed that the isolated and oligomeric Fe species on the extra framework werereduced for AAT-FZ due to the same reason. The acid amount on HT for AAT-FZ was slightly
increased from 0.06 to 0.08 mmol/g compared with H-FZ. The reason was the slight desilication on
as-FZ, which slightly decreased the Si/Fe and Si/Al ratios. However, the acid amount on both MT andHT for CAT-FZ were greatly increased due to the decreased Si/Fe and Si/Al ratios and the increased
proportion of isolated and oligomeric Fe species on the extra framework after desilication, which wereconsistent with the results of ICP and UV-vis.
The NH3-TPD profiles of the H-FS, AAT-FS and CAT-FS are presented in Figure 5.7(d-f). The
amounts of NH3 adsorbed of the Fe-silicalite-1 are listed in Table 5.3. For both AAT-FS and CATFS, the acid amount on MT showed no obvious difference compared with H-FS. However, the acidamount on HT for AAT-FS and CAT-FS were slightly increased. Note that, although the proportionof Fe species for AAT-FS and CAT-FS were similar in UV-vis spectra, the specific states of Fe species
may be different. Meanwhile, the acid amount on MT and HT for CAT-FS were slightly higher than
AAT-FS.

5.3.2. Direct oxidation of benzene to phenol with H2O2 (BTP)
The catalytic performances for the direct hydroxylation of benzene with H2O2 over different formof Fe-ZSM-5 and Fe-silicalite-1 are detailed in Table 5.4. H-FZ achieved only 0.6% yield with 100%
selectivity to phenol, demonstrating the low activity. After alkaline treatment on as-FZ, the phenol
yield of AAT-FZ was only increased to 0.8% due to the almost unchanged Fe species and amount.
However, after alkaline treatment on calcined Fe-ZSM-5, the phenol yield of CAT-FZ was dramaticallyincreased to 5.3% with 98% phenol selectively. The results were greatly depended on the increased Fe
content, which increased the active site. Another possible reason was the function of mesopores,
leading the decrease of mass transfer resistance (Scheme 5.1).
For Fe-silicalite-1 zeolites, H-FS displayed phenol yield of 5.0% with selectivity of 96%,
demonstrating the high activity, which was related to the high Fe content. After alkaline treatment on
as-FS, the phenol yield of AAT-FS was also increased to 6.4% with selectivity of 95% due to theincreased amount of isolated and oligomeric Fe species, the surface area and the pore size. Similarly,
after desilication on calcined Fe-silicalite-1, the phenol yield of CAT-FS was also increased to 7.5%
with phenol selectively of 91%. Note that the content of isolated and oligomeric Fe species for CATFS was close to AAT-FS, but the states of Fe species was somewhat different, which has been discussed
in the acidity. Besides, the textual properties may be another reason, AAT-FS possessed high VMeso(1.02 cm3g-1), but containing part of constructional void porosity. CAT-FS hold a definite VMeso of 0.78
130

Chapter 5

cm3g-1 and mesopores distributed at 13 nm, which was larger than AAT-FS (Scheme 5.2).

5.4. Conclusions
The influence of desilication on as-synthesized and calcined Fe-ZSM-5 and Fe-silicalite-1
zeolites were investigated. Both template and aluminum can prevent desilication and protect the crystal
structure. While the introduction of iron in the zeolite did not avoid desilication to a large extent. Thus
alkaline treatment on Fe-ZSM-5 and Fe-silicalite-1 is an effective post modification method that
affected both the porosity of the zeolite and the nature of Fe species. The increased porosity and surface
area can improve the transport properties, reduce diffusion resistance and well disperse of Fe species.
After alkaline treatment the phenol yield for Fe-ZSM-5 and Fe-silicalite-1 were increased from 0.6%
(H-FZ) to 5.3% (CAT-FZ) and from 5.0% (H-FS) to 7.5% (CAT-FS), respectively.

131

Chapter 5

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133

Chapter 5

(c)

5

10

15

20

25

30

35

(d)

Intensity (a.u.)

Intensity (a.u.)

(d)

(c)

(b)

(b)

(a)

(a)
40

2 theta (degree)

5

10

15

20

25

30

35

40

2 theta (degree)

Figure 5.1 XRD patterns of left: (a) as-FZ, (b) H-FZ, (c) AAT-FZ and (d) CAT-FZ and right: (a) asFS, (b) H-FS, (c) AAT-FS and (d) CAT-FS.

134

Chapter 5
(a)

(c)

(b)

1 µm

(d)

1 µm

1 µm

(f)

(e)

1 µm

1 µm

1 µm

Figure 5.2 FE-SEM images of (a) H-FZ, (b) AAT-FZ, (c) CAT-FZ, (d) H-FS, (e) AAT-FS and (f)CAT-FS.

135

Chapter 5
1000

600

400

200

0
0.0

(a)
(b)
(c)

0.08

dVp/ddp

Va/cm3(STP) g-1

800

0.10

(a)
(b)
(c)

0.06

0.04

0.02

0.00

0.2

0.4

0.6

0.8

1.0

0

2

4

6

8

10

12

14

16

18

20

dp (nm)p/p0

Figure 5.3 Left: N2 adsorption desorption isotherms and right: BJH pore size distributions of the (a)H-FZ, (b) AAT-FZ and (c) CAT-FZ zeolite.

136

Chapter 5
1000

600

400

200

0
0.0

(a)
(b)
(c)

0.08

dVp/ddp

Va/cm3(STP) g-1

800

0.10

(a)
(b)
(c)

0.06

0.04

0.02

0.00

0.2

0.4

0.6

0.8

1.0

0

2

4

6

8

10

12

14

16

18

20

dp (nm)p/p0

Figure 5.4 Left: N2 adsorption desorption isotherms and right: BJH pore size distributions of the (a)H-FS, (b) AAT-FS and (c) CAT-FS zeolite.

137

Chapter 5
20
18

(d)
16
14

K/M

12

(c)
10
8

(b)

6
4
2

(a)
0
200

300

400

500

600

Wavelength (nm)Figure 5.5 UV-vis spectra of (a) as-FZ, (b) H-FZ, (c) AAT-FZ, (d) CAT-FZ.

138

Chapter 5
20
18

(d)
16
14

K/M

12

(c)

10
8

(b)

6
4
2

(a)
0
200

300

400

500

600

Wavelength (nm)Figure 5.6 UV-vis spectra of (a) as-FS, (b) H-FS, (c) AAT-FS, (d) CAT-FS.

139

Chapter 5
1800

(b)

(a)

(c)

1600
1400

Intensity

1200
1000
800
600
400
200
0
400 500 600 700 800 900

400 500 600 700 800 900

400 500 600 700 800 900

Temperature (K)Temperature (K)Temperature (K)

1000

(e)

(d)

(f)

Intensity

800

600

400

200

0
400 500 600 700 800 900

400 500 600 700 800 900

400 500 600 700 800 900

Temperature (K)Temperature (K)Temperature (K)Figure 5.7 NH3-TPD spectra of (a) H-FZ, (b) AAT-FZ, (c) CAT-FZ, (d) H-FS, (e) AAT-FS and(f) CAT-FS.

140

Chapter 5

Alkaline
Treatment

Calcination

NH4 exchange
Calcination

AAT-FZ

as-FZ

Alkaline
Treatment

Calcination

NH4 exchange
Calcination

CAT-FZ

Cal-FZFeEF: Fe species on extra framework
AlEF: Al species on extra framework

Scheme 5.1 Schematic diagram about the formation of calcined, AAT and CAT type from Fe-ZSM-5 zeolite.

141

Chapter 5

Alkaline
Treatment

Calcination

NH4 exchange

Calcination

AAT-FS

as-FS

Alkaline
Treatment

Calcination

NH4 exchange
Calcination

CAT-FS

Cal-FSFeEF: Fe species on extra framework

Scheme 5.2 Schematic diagram about the formation of calcined, AAT and CAT type from Fe-silicalite-1 zeolite.

142

Chapter 5

Table 5.1 Physicochemical characteristics of the alkaline modified Fe-ZSM-5 and Fe-silicalite-1 zeolites.
SampleAlkaline treatment
condition

Solid yield(%)a

In product b
Si/Al

Si/Fe

Fe (wt.%)SBET(m2g−1)cSEXT(m2g−1)dVTotal(cm3g-1)eVMicro(cm3g-1)f

VMeso(cm3g-1)g

0.5
0.4
1.6
4.7
6.5
6.5

432
473
683
476
551
536

40
41
366
223
429
416

0.23
0.25
0.92
0.55
1.07
0.83

0.18
0.19
0.16
0.14
0.05
0.05

0.05
0.06
0.72
0.41
1.02
0.78

H-FZ
63
184
AAT-FZ
0.2 M, 353K, 2h
90
68
202
CAT-FZ
0.2 M, 353K, 2h
27
14
50
H-FS
19
AAT-FS
0.2 M, 353K, 2h
62
14CAT-FS
0.2 M, 353K, 2h
9CAT-FS
0.2 M, 353K, 1h69
14asolid yield = mass after alkaline treatment*100/initial mass.
b
Si/Fe was determined by ICP-AES analysis, Fe content was calculated by Si/Fe.
c

Specific surface areas of the catalysts were calculated using the Brunauer-Emmett-Teller (BET) equation on the N2 adsorption isotherms.
EXT) of the catalysts were calculated by the t-plot method based on the adsorption isotherms.
eTotal pore volumes of the catalysts were calculated based on BET equation on the N2 adsorption isotherms.
fMicropore volumes of the catalysts were calculated by the t-plot method based on the adsorption isotherms.
gMesopore volumes = total pore volumes - micropore volumes.
d External surface area (S143Chapter 5

Table 5.2 Percentage of sub-band areas (I1:λ< 250 nm, I2: 250 <λ< 350 nm, I3: 350 <λ< 450 nm, andI4: λ>450 nm) derived by deconvoluting UV-vis spectra and corresponding Fe species content for Fecontaining MFI zeolite catalysts.
Sampleas-FZ
H-FZ
AAT-FZ
CAT-FZ
as-FS
H-FS
AAT-FS
CAT-FS

Framework Fe
I1(%)
wt.%
83.5
0.42
47.6
0.24
58.1
0.23
33.7
0.54
50.2
2.36
49.1
2.31
32.7
2.13
32.1
2.09

Isolated and Oligomeric Fe
I2(%)
wt.(%)
16.5
0.08
42.8
0.21
30.6
0.12
47.6
0.76
49.8
2.34
30.4
1.43
38.5
2.50
38.0
2.47

144

Larger Fe clusters
I3(%)
wt.%
9.7
0.05
11.3
0.06
11.5
0.18
13.1
0.62
17.5
1.14
17.9
1.16

Bulk Fe oxides
I4(%)
wt.%
7.2
0.12
7.5
0.35
11.4
0.74
11.9
0.77

Chapter 5

Table 5.3 Ammonia adsorbed amount of H-type, AAT and CAT form Fe-ZSM-5 and Fe-silicalite-1
zeolite catalysts
SampleH-typeAAT-FZ
CAT-FZH-FS
AAT-FS
CAT-FS

Ammonia adsorbed amount (mmol/g)/ Tmax (K)LTMTHT
0.12/443
0.11/485
0.21/612
0.06/695
0.10/439
0.11/478
0.15/544
0.08/658
0.21/431
0.25/467
0.28/570
0.33/661
0.08/435
0.07/467
0.23/563
0.08/707
0.11/430
0.10/463
0.20/536
0.13/672
0.09/435
0.09/471
0.22/550
0.15/672

145

Chapter 5

Table 5.4 Catalytic results for the H-type, AAT and CAT form of Fe-ZSM-5 and Fe-silicalite-1
zeolite catalysts for the direct oxidation of benzene to phenol with H2O2.
SamplePhenol Yield(%)a

Product Selectivity (%)b
Phenol

HQ

CL

H2O2
Conv. (%)c

H-FZ
0.6
100
0
0
46
AAT-FZ
0.8
100
0
0
45
CAT-FZ
5.3
98
1
1
78
H-FS
5.0
96
3
1
96
AAT-FS
6.4
95
3
2
75
CAT-FS
7.5
91
8
1
75
Reaction conditions: 333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
aPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
b Each product selectivity= (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL).
c H O conversion = (moles of H O after reaction)*100/(initial moles of H O ).
2 2
2 2
2 2

146

Chapter 6

Chapter 6
Direct synthesis of Fe-containing MWW zeolite for direct hydroxylation ofbenzene to phenol and methane to methanol with H2O2
Abstract
Fe-containing MWW zeolite catalysts (i.e. Fe-MWW and Fe,Al-MWW) were directly synthesized.
The physiochemical properties, especially the states of Fe species was characterized by using UV-vis
and NO adsorbed FT-IR. Fe-MWW showed better performance than Fe,Al-MWW in both MTM and
BTP reaction with H2O2. The effect of calcination temperature on the catalytic performance in BTP
reaction has been studied in detail. Relatively high temperature calcination in the first step to remove
OSDA was beneficial to produce more isolated and oligomeric Fe species on the extra framework for
Fe-MWW, thus activate the catalytic performance but not good for Fe,Al-MWW. However, increasing
the second step calcination temperature to prepare H-type zeolite from NH4+-type made more seriously
Fe aggregation and reduce the terminal hydroxyl (Fe-OH and/or Al-OH on the extra framework) for
both Fe-MWW and Fe,Al-MWW, thus decrease the catalytic performance. The best results wereachieved by FW-1023-823, displaying phenol yield of 8.3% with selectivity of 93% in BTP reaction
and 855.8 µmol methanol yield with 70% selectivity in MTM reaction with H2O2.

6.1. Introduction
C-H activation has become a hot topic in the recent decades due to the huge economic andsustainable value [1]. An early and continuing aspiration of C-H activation is the possibility of methane
conversion to useful partial oxidation products such as MeOH [1]. Of course, it also includes the lightolefins such as ethylene, propylene, and benzene conversion to high-density energy sources or highvalue-added chemicals [2]. However, the cleavage of high energy C-H bond (≈440 kcal·mol-1)
typically requires harsh reaction conditions resulting in limited substrate scope and low functionalgroup tolerance [3]. As such, C-H activation has not yet found widespread applications in the late stage
functionalization of complex molecules, which often contain many functionalities not tolerant of harshreaction conditions. Substantial efforts have been made to develop an efficient method to activate CH. Bao et al. cover the area of direct conversion of methane to value-added chemicals with their review
147

Chapter 6of heterogeneous catalytic reactions in gas phase [2]. Direct oxidation of methane to methanol (MTM)
and benzene to phenol (BTP) in liquid phase with heterogeneous catalysts, especially zeolite catalysts,
are the tiny but popular aspects [4-7]. As a result, the catalytic reaction system with mild reaction
condition and decent performance are exactly what the field needs.
MWW structure zeolite is a kind of layered zeolite with high surface area, complex porosity andcontaining both medium and large pores [8, 9]. The structural complexity derives from the presenceof two independent pore systems accessible through 10-ring openings. One of these systems is definedby two-dimensional sinusoidal channels, maintaining an effective 10-ring diameter throughout thestructure. The second one includes large supercages with 12-ring openings defined by inner diameterof 7.1 Å and inner length of 18.2 Å [8, 10]. This material is particularly interesting as a considerably
higher amount of Fe can be introduced in the T sites of its framework [8]. It is important to mention
that ion exchange or impregnation of MWW with Fe-salts in various ways results in poor Fe states,
mainly because of the more extensive agglomeration of iron [11]. A simple but reproducible approach
to highly active Fe-containing zeolite catalysts is persistently pursued, but it is always hindered by the
intrinsic complexity of Fe species [12]. Hence, this work is devoted to the study of Fe-containing
MWW zeolite catalysts prepared by isomorphous insertion, i.e. in-situ synthesized. Comparison withthe post-synthesis, in-situ synthesized method introduces Fe ions in the framework by mixture Fe-saltin the synthesis gel, and the Fe species including isolated, oligomer, aggregated cluster and iron oxide
particles are formed by calcination in air [13]. Hence, the state of Fe ions in the framework in the assynthesized sample and the conditions of calcination are the key factors for influence the state of Fe
species and improving the catalytic activity.
The presence or absence of aluminum in the framework of Fe-containing MWW zeolite catalysts
is one of the most important factors to affect the Fe state in the framework. According to Hammond etal.[14], the function of aluminum in the Fe-containing MFI zeolite catalysts are (1) assist the extractionof Fe from the framework to the extra framework and hence increase the formation of the active Fe
species and (2) provide an associated negatively charged framework, which is capable of stabilizing
and maintaining the dispersion of the cationic Fe species on the extra framework responsible forcatalytic activity in direct oxidation of methane to methanol with H2O2. In addition, Hensen et al.
pointed out that an extra framework Fe-Al-O species stabilized in the zeolite micropores was the active
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Chapter 6

component in the benzene oxidation with nitrous oxide [15-17]. Meanwhile, Hensen’s group suggested
that the large difference in activity of Fe-silicalite-1 and Fe-ZSM-5 should not be ascribed to a higher
active site density in Fe-ZSM-5 but to the chemical difference of Fe species in the Fe-O-Al and Fe-OFe adducts [18]. Centi et al. indicated that the presence of Al in the zeolite framework is beneficial toreduce the rate of deactivation with respect to Fe-silicalite-1 samples, probably due to the different
position of the sites responsible for phenol synthesis and further conversion inside the zeolite channels[19]. Indeed, the presence of aluminum can influence the state of Fe species due to the effect of charge.
However, the specific effect of Al on the catalytic performance of Fe-containing MWW has not been
investigated in the published literatures.
Besides, the calcination temperature and sequence for Fe-MWW and Fe,Al-MWW samples
should make a huge impact on the Fe species and catalytic performance. According to Berlier, increase
the activation temperature is a general increase in integrated band intensities for Fe-silicalite-1 zeolite[20]. Meanwhile, Hensen et al. thought that high-temperature treatments increased the disordered
nature of the iron oxide aggregates rather than the presence of an abundant fraction of binuclear ironclusters in Fe-ZSM-5 zeolite [21]. The direct oxidation of methane to methanol with H2O2 in liquid
phase reported by Hammond et al. just verified the abovementioned points, increase the pretreatment
temperature with a certain range could increase the catalytic performance of Fe-silicalite-1 but
decrease that of the Fe-ZSM-5 [14]. Furthermore, calcination at different stages have different function,such as removing the template, preparing the proton-type sample and activation the catalysts before
reaction. Thus, temperature at different calcination stages makes different effects.
To our best knowledge, the influence of the presence of Al, calcination temperature and sequencein the Fe-containing MWW zeolite catalysts have not been investigated, let alone the application inhydroxylation of benzene to phenol and methane to methanol with H2O2 in liquid phase. In this study,
Fe-containing MWW-type zeolite catalysts were prepared by in-situ synthesized method. The effectsof aluminum in the synthesis gel, the calcination temperature and sequence on the BTP and MTM
reaction with H2O2 were investigated in details.

6.2. Experiments
6.2.1 Materials
149Chapter 6

Fumed silica (Cab-O-Sil M5, Cabot), sodium aluminate (53.1% Al2O3, 32.3% Na2O, Koso
Chemical Co., Ltd.), NaOH (97%, Wako), Fe(NO3)3·9H2O (99%, Wako), hexamethyleneimine (HMI)(99%, Acros Organics). NH4NO3, H2O2 (30%), benzene, acetone, acetonitrile, anisole, sulfolane,
mesitylene, tetramethylsilane (TMS) and CD3CN were purchased from Wako. Methane (99.99%) gas
was provided by Taiyo Nippon Sanso Co., Ltd.. All of the reagents were used as received, without
further purification.

6.2.2 Catalysts preparation
The Fe,Al-MWW lamellar precursor was hydrothermal synthesized with fumed silica, sodium
aluminate, NaOH, Fe(NO3)3·9H2O, hexamethyleneimine (HMI) and deionized water according to theliterature [10]. Firstly, 1.62 g Fe(NO3)3·9H2O, 0.43 g NaAlO2 and 1.03 g NaOH were dissolved in 81
g deionized water and stirred for 10 min. Then, 8.91 g HMI was added and stirred for 1 h. Finally, 6 g
fumed silica and 0.3 g DeB-MWW zeolite (5% weight based on the weight of SiO2), the detail preparedmethod refer literature [22], were added and further stirred for 1 h to obtain a gel with a molar
composition of 1 SiO2: 0.15 Na2O: 0.016 Al2O3: 0.02 Fe2O3: 0.9 HMI: 45 H2O. The hydrothermal
treatment was carried out in a Teflon-lined stainless steel autoclave at 423 K for 5 d under tumbling
conditions. The Fe-MWW lamellar precursor was hydrothermal synthesized by the same method
without sodium aluminate.
In order to investigate the influence of calcination temperature (K) on the catalytic performance,
the first step calcination temperature for removing OSDA was noted as T1, and the second step for
preparation H-type sample form NH4+-type was donated as T2. The samples were filtered, washed,
and dried at 373K to produce the Fe,Al-MWW and Fe-MWW precursors. The as-synthesized samples
were calcined in air at T1 for 10 h resulted in the product with the 3D MWW structure. Thus obtained
Na-type samples were converted to the H-type one by treating it with 1 M NH4NO3 twice at 353K for3 h then by calcination at T2 for 5 h in air. Thus prepared H-form Fe,Al-MWW and Fe-MWW zeolites
were denoted by FAW-T1-T2 and FW-T1-T2.

6.2.3 Characterization of catalysts
XRD patterns were collected on a Rint-Ultima III (Rigaku) using a Cu Kα X-ray source (40 kV,
20 mA). Field-emission scanning electron microscopic (FE-SEM) images of the powder samples were150

Chapter 6

obtained on S-5200 (Hitachi) microscope operating at 1 kV. The samples for FE-SEM observations
were mounted on a carbon-coated microgrid (Okenshoji Co.) without any metal coating. Elemental
analyses of the samples were performed on an inductively coupled plasma-atomic emission
spectrometer (ICP-AES, Shimadzu ICPE-9000). Nitrogen adsorption and desorption measurements toobtain information on the micro- and meso-porosities were conducted at 77 K on a Belsorp-mini II(Microtrac BEL).
UV-vis diffuse reflectance spectra were recorded on a V-923DS spectrometer (JASCO). The
diffuse reflectance spectra were converted into the absorption spectra using the Kubelka-Munkfunction. FT-IR spectra were obtained by a JASCO FT-IR 4100 spectrometer equipped with atriglycine sulfate (TGS) detector. For FT-IR observation, the sample was pressed into a self-supporting
disk (20 mm diameter, ca. 30 mg) and placed in an IR cell attached to a closed gas-circulation system.
After the sample was pretreated by evacuation at 773 K for 2 h, then adsorbed 5-1000 Pa NO at ambient
temperature. The IR spectra resulting from the subtraction of the background spectra from those with
NO adsorbed are shown unless otherwise noted.
Temperature-programmed ammonia desorption (NH3-TPD) profiles were noted down on a
Multitrack TPD equipment (Japan BEL). Normally, 25 mg catalyst was pretreated at 773 K for 1 h ina He flow of 50 mL min-1 and then cooled down to 423 K. The sample was evacuated at 423 K for 1
h prior to the adsorption of NH3. Approximately 2500 Pa of NH3 contacted with the sample at 423 K
for 10 min. Subsequently, the sample was evacuated to remove the weakly adsorbed NH3 at the same
temperature for 30 min. Finally, the sample was heated from 423 to 873 K at a ramping rate of 10 K
min-1 in a He flow of 50 mL min-1.
The high-resolution 27Al MAS NMR and 29Si MAS NMR spectra were obtained on a JEOL ECA600 spectrometer (14.1 T) equipped with an additional 1 kW power amplifier. The 27Al and 29Si
chemical shifts were referenced to AlNH4(SO4)2·12H2O at -0.54 ppm and polydimethylsiloxane(PDMS) at -34.12 ppm, respectively. The samples were spun at 15 kHz by using a 4 mm ZrO2 rotor.

6.2.4 Catalytic tests
6.2.4.1 Direct hydroxylation of benzene to phenol (BTP)The BTP reaction was carried out in a 25 mL round-bottom flask equipped with a reflux condenser
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Chapter 6

and a magnetic stirrer. In a typical run, 50 mg of catalyst was immersed in 10 mL of acetonitrile, 5mmol of benzene and 10 mmol H2O2 was stirred at 333 K for 6 h. After the reaction mixture wascooled down, the catalyst was removed. A certain amount of anisole as internal standard was added to
the resultant liquid, and the products were fixed by exhaustive acetylation with excess (CH3CO)2OK2CO3, the derivative products were analyzed by GC [23]. The remaining H2O2 concentration wasquantified by standard titration method with 0.1 mol/L Ce(SO4)2 solution.

6.2.4.2 Direct conversion of methane to methanol (MTM)
The liquid-phase MTM reaction was carried out in a 100 ml PTFE autoclave. The CH4 pressure
was controlled by the pressure gage. The reactants were stirred vigorously by an agitator blade. In atypical run, 10 ml sulfolane, 50 mg catalyst and 27 mmol H2O2 were added to the autoclave and heated
to 323 K. The sealed reactor was then purged with CH4 to 3 Mpa. After reaction 2 h, the autoclave wascooled rapidly to 278 K in an ice bath to minimize any further chemical reaction and reduce loss ofvolatile products. After separation liquid phase and catalyst, the liquid-phase products were analyzed
by 1H NMR spectroscopy on JEOL ECA-600 spectrometer (14.1 T) equipped with an additional 1 kW
power ampliﬁer. Mesitylene and TMS/CD3CN were used as the internal standard and chemical shiftcalibrator, respectively. The detectable products in the liquid-phase were CH3OH, HCOH, HCOOH
and CH3OOH. The amount of unconverted H2O2 was quantified by standard titration method with 0.1
mol/L Ce(SO4)2 solution.

6.3. Results and discussion
6.3.1 Characterization of catalysts
6.3.1.1 Physicochemical properties
The XRD patterns of FW and FAW samples calcined at different temperatures are shown in
Figure 6.1. The diffraction pattern of all the samples were consistent with those reported by Rubin
and Chu [24], indicating that they were well-crystallized MWW-type zeolites. No change wasobserved among the samples after calcination at different temperatures, meaning good thermal stability
for the FW and FAW catalysts. Besides, the diffraction peaks ascribed to bulk iron oxides were notobserved in both FW and FAW, indicating the Fe species dispersed well in the samples.
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Chapter 6

Figure 6.2 shows the SEM images of FW and FAW zeolites, revealing that both Fe-MWW and
Fe,Al-MWW samples were composed of very thin flake-like crystals.
Table 6.1 lists the composition and textual properties of the Fe-containing MWW zeolites. The
Fe content of Fe,Al-MWW was higher than that of Fe-MWW possibly due to the existence ofaluminum in the framework providing some attachment points for Fe species.
Figure 6.3 displays the N2 adsorption and desorption isotherms of the FW and FAW zeolites
calcined at different temperatures. The additional N2 uptakes at around 0.8 < p/p0 < 1.0 were due tothe inter particle voids of the flake-like crystals. The average pore diameters displayed no difference
between FW and FAW zeolites after calcined at different temperatures (ca. 2.3 nm). Pore size
distribution data demonstrate that the MWW zeolites contain uniform mesopores, these mesopores
may originate from voids between the zeolite crystals, as shown in Figure 6.4. Meanwhile, the peakof the pore size distribution for FW zeolites were higher those of the FAW zeolites, indicating that theuniformity of mesopores for FW zeolites was higher than that of FAW zeolites. As listed in Table 6.1,
the BET (SBET) and external (SEXT) surface areas of FW were slightly higher than those of FAW zeolite
calcined at different temperatures. Besides, FW and FAW samples possessed the same micropore
volumes (VMicro) and pretty high total pore volumes (VTotal) (0.73-1.15 cm3g -1). The VTotal of FW weredecreased a little (1.15 to 0.99 cm3g -1) with calcination temperature increase, possibly due to theformation of the iron oxide on the pores.

6.3.1.2 Fe states
The coordination state and extent of aggregation of iron in Fe-containing MWW zeolite were
investigated by UV-vis spectroscopy. Generally, the bands below 250 nm are assigned to framework
Fe species, the bands between 250-350 nm are ascribed to the isolated and oligomeric extra framework
Fe species, the bands between 350-450 nm are assigned to aggregated iron oxide, and the bands above
450 nm are ascribed to bulk iron oxide particles [11, 14].
Figure 6.5 shows the UV-vis spectra of FW and FAW zeolites calcined at different temperatures,
which were deconvoluted into several bands by applying Guass functions. The amount and proportionof different Fe species based on the relative peak areas are listed in Table 6.2. The intensity of bands
at high wavenumber for both FW and FAW zeolites were increased with calcination temperature
153

Chapter 6

increase, indicating the formation of more Fe species on the extra framework. Comparison with thedirectly synthesized Fe-containing MFI-type zeolite catalysts, Fe species in the Fe-containing MWW
zeolite did not easily aggregate, probable due to the high surface area and big pore size for MWW
zeolite [25]. Note that the degree of Fe aggregation after the second step calcination for both FW and
FAW were weaker than those of the first step calcination, because most of iron in the framework havealready been moved outside in the process of the first calcination. Meanwhile, it was interesting to findthat the degree of Fe aggregation for FW was more seriously than that of FAW. Possibly because theexistence of aluminum in FAW could separate and stabilize Fe, which has been reported in theliterature [14].
The in-situ FT-IR spectroscopy technique using NO as probe molecule was applied to investigate
the coordinated unsaturated Fe states on the extra framework. Figure 6.6 shows the NO-adsorbed FTIR spectra of FW and FAW samples calcined at different temperatures. For FW catalysts, theasymmetrical peaks at 1862 cm-1 with shoulder at 1840 cm-1 achieved when the NO pressure was 5 Pa.
While the FAW catalysts showed symmetrical peaks at 1867 cm-1, which was assigned to Fe2+(NO).
Upon increasing NO pressure, peaks at 1900 cm-1 and 1815 cm-1 for FW appeared and the intensity
enhanced until saturated, the two peaks were both assigned to Fe2+(NO)3. However, for FAW samples,the corresponding peaks at 1900 cm-1 and 1815 cm-1 slightly right migrated to 1898 cm-1 and left
shifted to 1819 cm-1 and the intensity enhanced along with pressure increase.
For FW samples calcined at higher temperature, the peaks position were basically unchanged
while the intensity of these bands were seriously decreased comparison FW-1023-823 and FW-823823 samples, especially the adsorptions at around 1862 cm-1 dropped almost half. The intensity ofbands for FW-1023-1023 was decreased one third compared with FW-1023-823, meaning that theincrease of the second step calcination temperature was conducive to aggregate, but the degree of Fe
aggregation was weaker than that of the first step calcination. Peaks position for FAW were shifted tohigher wavenumbers with calcination temperature increase and the intensity of bands at 1898, 1867,
1819 and 1790 cm-1 were decreased. Note that the bands at 1867 cm-1 of FAW-1023-823 and FAW1023-1023 migrated left to 1875 cm-1, possibly indicating the formation of Fe-O-Al oligomers [26].
The reduced ratios between FAW-1023-823 and FAW-823-823, FAW-1023-823 and FAW-1023-1023
were less than those of FW. It suggested that the presence of aluminum was prone to disperse Fe and154

Chapter 6

form the unsaturated Fe species [26]. The different intensity between FW and FAW also indicated thedifferent responsible values for NO adsorbed FT-IR just like that aluminum can provide higher acidity
than iron in NH3-TPD.
Summarize the above characterization results, the influence of calcination temperature on FW
and FAW can be displayed in Scheme. 1. For FW, calcination made Fe in the framework migrate to
the extra framework and form the abundant Fe species, such as isolated Fe cations, oligomeric cationic
Fe complexes, and neutral Fe-oxide clusters (FexOy) as well as larger Fe-oxide aggregates. Hightemperature calcination made the migration seriously and caused more severe aggregated between Fe
species. However, owing to the high surface area and large pore size provided by the MWW zeolite,
it was not easy to aggregate for Fe species compared with MFI or other zeolites. The proportion ofisolated Fe and oligomer Fe species were increased. Nevertheless, for FAW, calcination, especiallyhigh temperature calcination migrated not only Fe but also Al to the extra framework and formedclustered FeOx species and FeAlOx species.

6.3.1.3 Acidity
The ammonia adsorbed amount actually reflect the environment of Fe in the zeolite. Figure 6.7
shows the NH3-TPD profiles of Fe-MWW and Fe,Al-MWW zeolite catalysts calcined at different
temperatures. The peaks at 373-473 K are denoted as physically adsorption and 573-873K are assigned
to strongly adsorbed ammonia on Brønsted and/or Lewis acid sites [27, 28]. The peak at 473-573 K
may be due to ammonia weakly adsorbed on the extra framework Fe and/or Al [29]. The ammonia
adsorbed amount calculated according to the peak area at 373-473 K, 473-573 K and 573-873 K aredemonstrated in Table 6.3. It was clear to see that the ammonia adsorbed amount of FW zeolites weremuch lower than those of FAW on account of aluminum [30].
When the first step calcination temperature was increased from 823 to 923 K, the decreased
degree of the ammonia adsorbed amount for both FW and FAW were low, indicating that the amountof the Fe immigration from framework to the extra framework was small. While increasing the firststep calcination temperature from 923 to 1023 K, the decreased amount of ammonia adsorbed amounton the Brønsted and/or Lewis acid sites for both FW and FAW were higher than those of increasing
the first step calcination temperature from 823 to 923 K. Note that the ammonia adsorbed amount on
155

Chapter 6

the extra framework Fe and/or Al and the Brønsted and/or Lewis acid sites for both FW and FAW
were decreased by increasing the second step calcination temperature from 823 to 1023 K. It impliedthat increasing the second step calcination temperature was not only immigration Fe and /or Al to theextra framework but also affect the coordination and saturation of the Fe and Al species on the extra
framework.
The hydroxyl vibrations regions measured by FT-IR can also reflect the Fe states of Fe-containing
MWW zeolite catalysts. Figure 6.8 shows the hydroxyl vibrations regions of FW and FAW samples
calcined at different temperatures. The peak at 3746 cm-1 is assigned to isolated and terminal silanols.
Both FW and FAW samples presented sharp and high peaks at 3746 cm-1, indicating the catalysts withlarge amount of hydroxyl. The bands at about 3637 cm−1 for FW zeolites were ascribed to OH groups
bridged between framework Si and Fe atoms. The bands were decreased with the increase of the firstand second step calcination temperature. The related bands for FAW samples appeared at around 3621
cm-1 correlation with the framework Si, Fe and Al, which were also decreased with calcination
temperature increase. Meanwhile the peaks area of FAW samples were larger than those of FW
catalysts at 3620-3640 cm-1, which was consistent with the results of NH3-TPD due to the similarreason. The bands at around 3670 cm-1 assigned t to the terminal hydroxyl like Fe-OH and Al-OH
were decreased with calcination temperature increasing [31]. The area of the terminal hydroxyl forFW zeolites were slightly higher than those of FAW. According to the reference [32], the binuclear Fe
species on the extra framework with the hydroxyl presented higher activity than the ones withouthydroxyl.

6.3.1.4 The states of Si and Al in Fe-containing MWM zeolites
The 29Si NMR spectra of the H-type FW and FAW zeolites calcined at different temperatures areshown in Figure 6.9(a). All the samples possessed a main resonance peak at around -111 ppm wereassigned to the silicon atoms coordinated by four silicon atoms (Q4(0Al), Si(OSi)4). And anotherobvious resonance was at -103 ppm due to the Q3 silicon atoms ((OH)Si(OSi)3), which were mainly
attributed to the surface silanol group, or Si(1Al) or Si(1Fe) for Fe-MWW and Fe,Al-MWW [33].
There was a slightly increase in the spectral intensity between -95 and -105 ppm for FAW comparedwith FW zeolites. And the substantial broadening of the resonances belonged to Q3 defect sites by156

Chapter 6

breaking Si-O-Si bonds, or Si(1Al) and Si(1Fe) due to the presence of Al atom in the FAW zeolites[34]. There was an apparent peaks at -105 ppm for both FW-1023-823 and FAW-1023-823, possibly
due to the defect sites by breaking Si-O-Si bonds under high temperature calcination [35]. Figure 6.9
does not show any fine structure due to the presence of paramagnetic Fe(III) species, which lead toline broadening [36].
Figure 6.9(b) shows the 27Al NMR spectra of FAW-823-823 and FAW-1023-823 zeolite. The
Al atoms were introduced into the structure in a tetrahedral form at ca. 56 ppm and existed at the extra
framework in hexahedron form at ca. 0 ppm. The same to the 29Si NMR spectra, the curve of the 27Al
NMR spectra was not smooth influenced by the paramagnetic Fe(III) species. The peaks area for the
tetra-coordinated and hexa-coordinated Al of FAW-1023-823 were smaller than those of FAW-823823. The possible reason was that high temperature calcination migrated Al in the framework to extra
framework and formed alumina or other non-coordinated aluminum species [33, 37].

6.3.2 Direct oxidation of benzene to phenol with H2O2 (BTP)
The catalytic performances for the direct hydroxylation of benzene with H2O2 over FW and FAW
zeolite catalysts calcined at different temperatures are detailed in Table 6.4. FW catalysts achieved
higher phenol yield than FAW. The result was different from the literatures, which reported that thepresence of Al in Fe-containing MFI zeolite catalysts was benefit to the dispersion of Fe on the zeolite
thus increasing the catalytic performance [38]. However, the significantly low degree of aggregation
for Fe in FW and FAW zeolites demonstrated that even without the presence of Al, Fe could be welldispersed, possibly due to the high surface area, large pore size and the different T sites of MWW fromMFI zeolite.
For FW catalysts, the phenol yield was increased with the first step calcination temperature
increase from 823 to 1023 K, while it was decreased when the second step calcination temperature wasenhanced from 823 to 1023 K. The possible reason was the severe Fe aggregation, especially for the
Fe species on the extra framework under the second step calcination. In addition, the reduced terminalhydroxyl produced by the Fe species on the extra framework under high temperature may be anotherreason. However, the phenol yield of FAW catalysts were decreased with increase the temperature forboth the first and second step calcination. The Fe aggregation was one possible reason, but the degree
157

Chapter 6

was not as serious as FW, thus the reason may be the formation of FeAlOx on the extra framework,
which possess low activity [37]. Besides, the deceased terminal hydroxyl perhaps involved. In addition,
the phenol selectivity of FW zeolites were at the range of 93-95%, which were lower than those of
FAW (95-99%). It implied that the presence of aluminum can improve the selectivity in BTP reaction
with H2O2.
The phenol yield of FW and FAW were higher than those of Fe-MFI catalysts under the same
Si/Fe ratio in the synthesis gel [BTP1]. One of the possible reasons was the well dispersion of Fe on
the FW and FAW zeolites due to the higher surface area and larger pore size of MWW zeolite than
MFI zeolite. Another reason was the larger reaction space of MWW than MFI zeolite. It was also
possible that the abovementioned reasons common caused the different Fe species thus the different
catalytic performance in BTP reaction with H2O2.
Turnover number (TON) was used to display the catalytic performance, normally the total Fe
content on the used catalyst was took to calculate the value, which was noted as TON1. There was no
doubt that the TON1 for all the catalysts were a little low due to the high Fe amount and low phenol
yield. However, in our research the Fe content of the isolated and oligomeric Fe species on the extra
framework was used to calculate the TON2, because the active site was a part of the isolated andoligomeric Fe species on the extra framework, TON2 was closer to the true value. Obviously, FW
achieved the highest TON among the Fe-containing zeolite catalysts.
To further clarify the importance of the isolated and oligomeric Fe species on extra framework,
Figure 6.10 displays the change in the phenol yield and TON2 as function of the content of isolated
and oligomeric Fe species on extra framework for the FW and FAW catalysts. The phenol yield was
proportional to the content of isolated and oligomeric Fe species on extra framework for FW catalyst.
But the TON was inversely proportional to the corresponding content, indicating that the increase ofphenol yield can not keep up with the increase of analogous Fe content. The relationship of FAW
between phenol yield and Fe content was different from that of FW, decrease with the Fe content,
which suggesting that the migration of Al from the framework to the extra framework was notbeneficial to the catalytic performance or the existence of Al in FAW prevented the movement of Fe
from the framework to the extra framework and the formation of active Fe species. Similarly, the TON
and the Fe content for FAW showed the same relationship.
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6.3.3 Direct oxidation of methane to methanol with H2O2 (MTM)
The essential features of the reaction pathways for the benzene-phenol and methane-methanol
conversions are identical, especially in bonding characters [39]. Thus, the catalytic performance are
the similar to each other theoretically. The influence of the presence of Al in the Fe-containing MWW
zeolite, i.e. FW and FAW, and the calcination temperature on catalytic performance were investigated,the detail results as shown in Table 6.5. Not surprisingly, FW-823-823 and FW-1023-823 achieved
higher methanol yield than FAW-823-823 and FAW-1023-823, respectively. The methanol selectivity
for FW-823-823 and FW-1023-823 maintained above 70%, while those of FAW-823-823 and FAW1023-823 were only 40%. The presence of aluminum in MWW zeolites decreased the selectivity ofmethanol, which was different from the results of BTP reaction but consistent with the results ofHammond [14]. Meanwhile, high temperature calcination improved the catalytic performance in MTM
reaction for FW catalyst, but reduced the catalytic performance for FAW. The results were a little
consistent with the paper of Hammond et al.[14]. In Hammond’s paper, the catalytic performance of
Fe-slicalite-1 zeolite in MTM reaction with H2O2 was increased with the activation temperature
increasing, while the performance of Fe-ZSM-5 was decreased. In addition, the H2O2 conversion for
FW and FAW were in the range of 25-53%, which were much higher than that of Fe-silicalite-1 catalyst
in the same reaction conditions. Meanwhile, the methanol yield of FW-823-823 and FAW-823-823
were higher than those of Fe-silitalite-1 catalysts under the same Si/Fe ratio in the synthesis gel in
Chapter 3. As for the reasons, the particle size of methane and the products were smaller than 0.4 nm,
which was smaller than the reaction space in MWW (ca.0.5 nm), so as to the higher activity of Fe
species in the MWW zeolites was the reason.

6.4. Conclusions
Fe-containing MWW zeolites (i.e. Fe-MWW and Fe,Al-MWW) were synthesized by direct
method and extensively characterized, especially the states of Fe species and the acidity of catalysts.
Fe-MWW showed better performance than Fe,Al-MWW in both MTM and BTP reaction with H2O2.
The effect of calcination temperature on the catalytic performance has been studied in detail. Relatively
high temperature calcination in the first step to remove OSDA was beneficial to produce more isolated
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and oligomeric Fe species on the extra framework for Fe-MWW, thus activate the catalytic
performance. But the presence of aluminum in Fe,Al-MWW dispersed iron and high temperature
calcination produced the FeAlOx species, which may be not beneficial to the catalytic performance.
In addition, increasing the second step calcination temperature to prepare H-type zeolite from NH4+type made more seriously Fe aggregation and reduce the terminal hydroxyl (Fe-OH and/or Al-OH on
the extra framework) for both Fe-MWW and Fe,Al-MWW, thus decrease the catalytic performance.
The best results were achieved by FW-1023-823, phenol yield of 8.3% with selectivity of 93% in BTP
reaction and 855.8 µmol methanol yield with 70% selectivity in MTM reaction with H2O2. Both FeMWW and Fe,Al-MWW realized considerable and better catalytic performance comparison with FeMFI zeolite in BTP and MTM reactions.

160

Chapter 6

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162

Chapter 6

(c)

(b)

(d)

Intensity (cps.)

Intensity (cps.)

(d)

(c)

(b)

(a)

5

10

15

20

25

30

35

(a)

40

2 Theta (Degree)

5

10

15

20

25

30

35

40

2 Theta (Degree)

Figure 6.1 XRD patterns of left: (a) FW-823-823, (b) FW-923-823, (c) FW-1023-823 and (d) FW1023-1023, right: (a) FAW-823-823, (b) FAW-923-823, (c) FAW-1023-823 and (d) FAW-1023-1023.

163

Chapter 6

(a)

(b)

1 µm

1 µm

Figure 6.2 FE-SEM images of as-synthesized (a) FW and (b) FAW.

164

Chapter 6
1800
1600

1400

1200
1000
800
600

1200
1000
800
600

400

400

200

200

0
0.0

0.2

0.4

0.6

0.8

(a)
(b)
(c)
(d)

1600

Va (cm3(STP) g-1)

1400

Va (cm3(STP) g-1)

1800

(a)
(b)
(c)
(d)

1.0

p/p0

0
0.0

0.2

0.4

0.6

0.8

1.0

p/p0

Figure 6.3 N2 adsorption and desorption isotherms of left: (a) FW-823-823, (b) FW-923-823, (c) FW1023-823 and (d) FW-1023-1023, right: (a) FAW-823-823, (b) FAW-923-823, (c) FAW-1023-823 and(d) FAW-1023-1023.

165

Chapter 6
0.10

0.10

(a)
(b)
(c)
(d)

0.08

0.06

dVp/ddpdVp/ddp

0.08

(a)
(b)
(c)
(d)

0.04

0.06

0.04

0.02

0.02

0.00

0.00
0

2

4

6

8

10

12

14

16

18

20

dp (nm)

0

2

4

6

8

10

12

14

16

18

20

dp (nm)Figure 6.4 Pore size distribution of left: (a) FW-823-823, (b) FW-923-823, (c) FW-1023-823 and (d)FW-1023-1023, right: (a) FAW-823-823, (b) FAW-923-823, (c) FAW-1023-823 and (d) FAW-10231023.

166

Chapter 6
14

14

(d)

12

10

10

8

(c)

K/M

K/M

(d)

12

6

8

(c)

6

4

4

(b)

(b)

2

2

(a)

(a)

0

0
200

300

400

500

600

Wavelength(nm)

200

300

400

500

600

Wavelength(nm)Figure 6.5 UV-vis spectra of left: (a) FW-823-823, (b) FW-923-823, (c) FW-1023-823, (d) FW-10231023, and right: (a) FAW-823-823, (b) FAW-923-823, (c) FAW-1023-823, (d) FAW-1023-1023.

167

Chapter 6
4.0

3.5

4.0

PNO

3.5

1900

3.0

1862

1867

PNO
1898

1815

1819

3.0

(c)

2.5

AbsorbanceAbsorbance

(c)

2.0

(b)

2.5

2.0

(b)

1.5

1.5

1.0

1.0

0.5

0.5

(a)
0.0
2000

1950

1900

1850

1800

1750

(a)

1700

Wavenumbers (cm-1)

0.0
2000

1950

1900

1850

1800

1750

1700

Wavenumbers (cm-1)

Figure 6.6 NO adsorbed FT-IR spectra (increasing PNO from 5 Pa to 1000 Pa) at room temperature on
left: (a) FW-823-823, (b) FW-1023-823, (c) FW-1023-1023, and right: (a) FAW-823-823, (b) FAW1023-823, (c) FAW-1023-1023.

168

Chapter 6
7500

7500

7000

7000
6500

(d)

6000

6000

5500

5500

5000

5000

4500

TCD signal

TCD signal

6500

(c)

4000
3500
3000

(d)

4500

(c)
4000
3500
3000

2500

2500

(b)

2000

2000

1500

1500

1000

1000

500

(b)

500

(a)

(a)

0
350 400 450 500 550 600 650 700 750 800 850 900

0350 400 450 500 550 600 650 700 750 800 850 900

Temperature(K)Temperature(K)Figure 6.7 NH3-TPD spectra of left: (a) FW-823-823, (b) FW-923-823, (c) FW-1023-823, (d) FW1023-1023, and right: (a) FAW-823-823, (b) FAW-923-823, (c) FAW-1023-823, (d) FAW-1023-1023.

169

Chapter 6

2.5

3637
(c)

3800

3750

3700

3650

3600

3550

3746

Absorbance (a.u.)3746

Absorbance (a.u.)

2.5

3621
(c)

(b)

(b)

(a)

(a)

3500

Wavenumbers (cm-1)

3800

3750

3700

3650

3600

3550

3500

Wavenumbers (cm-1)

Figure 6.8 FT-IR spectra in the v(OH) vibrations regions of left: (a) FW-823-823, (b) FW-1023-823,(c) FW-1023-1023, and right: (a) FAW-823-823, (b) FAW-1023-823, (c) FAW-1023-1023.

170

Chapter 6

Q4

Al Ⅳ
Al Ⅵ

Q3

(b)
(d)
(c)
(b)

(a)
(a)
-90

-95

-100

-105

-110

-115

-120

-125

-130 100

Chemical shift (ppm)

90

80

70

60

50

40

30

20

10

0

-10 -20

Chemical shift (ppm)

29

Figure 6.9 Left: Si MAS NMR spectra of (a) FW-823-823, (b) FW-1023-823, (c) FAW-823-823, (d)FAW-1023-823; right: 27Al MAS NMR spectra of (a) FAW-823-823 and (b) FAW-1023-823 zeolite
catalysts.

171

Chapter 6
FAW

FW

100

8

90

7

80
70

6

60

5

50
4

40

3

30

2

20

1

10

0
0.4

TON

Phenol yield (%)

9

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0
1.2

The Fe content of isolated and oligomer Fe species (wt.%)

Figure 6.10 Change in the phenol yield and TON as function of the content of isolated and oligomeric
Fe species on the extra framework for the FW and FAW zeolite catalysts. Reaction conditions: 50 mg
catalyst, 10 ml CH3CN, 5 mmol Benzene, H2O2/Benzene=2, 333 K, 6 h. Phenol yield = (moles ofphenol produced)*100/(initial moles of benzene). TON = (moles of phenol + moles of HQ + moles of
CL+ moles of p-BQ)*100/(moles of the used Fe content for the isolated and oligomeric Fe species on
the extra framework).

172

Chapter 6
as-Fe-MWW

Fe-MWW

12 MR Supercage

Sup6

12 MR Supercage

Isolate Fe

Sup6
Sup5

10 MR
sinusoidal

10 MR
sinusoidal

Sup4

Calcination

Fe-O-Fe binuclear

FeOx
oligomers

Fe-O2-Fe
binuclear

Sup5Fe-O-Fe binuclear
Fe
Si

10 MR
Crossing

as-Fe,Al-MWW

FeOx
oligomers

Fe,Al-MWW

12 MR Supercage

Sup6

Sup4

12 MR Supercage

Isolate Fe

Sup6
Sup5

10 MR
sinusoidalO

FeOx
oligomers

FeAlOx

10 MR
sinusoidal

Calcination

Fe-O2-Fe
binuclear

Sup5Fe-O-Fe binuclear
Fe
Al

10 MR
Crossing

Si
FeAlOxO

Scheme 6.1. Schematic representation the formation of Fe species for the directly synthesized FeMWW and Fe,Al-MWW after calcination.

173

Chapter 6

Table 6.1 Physicochemical characteristics of the FW and FAW zeolite catalysts calcined at different
temperatures.
SBET(m2g−1)bSEXT(m2g−1)c

VTotal(cm3g-1)dVMicro(cm3g-1)e

601

264

1.15

0.16

534

266

1.09

0.17

573

342

1.65

0.27

517
569

385
225

0.99
0.73

0.15
0.16

501

214

0.73

0.15

485

276

0.86

0.13

FAW-1023-1023
512
248
Si/Fe was determined by ICP-AES analysis, Fe content was calculated by Si/Fe.

0.83

0.14

SampleSi/AlaSi/FeaFe (wt.%)aFW-823-823
FW-923-823
FW-1023-823

-

40

2.3

FW-1023-1023
FAW-823-823
FAW-923-823
FAW-1023-823
a

31

26

3.2

b

Specific surface areas of the catalysts were calculated using the Brunauer-Emmett-Teller (BET) equation on the N2
adsorption isotherms.
c External surface area (SEXT) of the catalysts were calculated by the t-plot method based on the adsorption isotherms.
dTotal pore volumes of the catalysts were calculated based on BET equation on the N2 adsorption isotherms.
eMicropore volumes of the catalysts were calculated by the t-plot method based on the adsorption isotherms.

174Chapter 6

Table 6.2 Numerical analysis of UV-vis spectra of FW and FAW zeolite catalysts calcined at different temperatures. Percentage of the sub-bands (I1:λ< 250 nm, I2: 250 <λ< 350 nm, I3: 350 <λ< 450 nm, I4:λ> 450 nm) and wt.% Fe of the corresponding species.
SampleFW-823-823
FW-923-823
FW-1023-823
FW-1023-1023
FAW-823-823
FAW-923-823
FAW-1023-823
FAW-1023-1023

Framework Fe
I1(%)
wt.%
75.5
1.74
63.6
1.46
44.2
1.02
33.6
0.77
72.4
2.32
67.6
2.16
47.2
1.51
41.8
1.34

Isolate and oligomer Fe
I2(%)
wt.(%)
21.7
0.50
27.1
0.62
38.0
0.87
39.4
0.91
25.8
0.83
26.8
0.86
34.8
1.11
38.7
1.24

175

Aggregated iron oxide
I3(%)
wt.%
2.7
0.06
5.8
0.13
12.3
0.39
18.8
0.43
1.9
0.06
5.6
0.18
10.2
0.33
13.2
0.42

Bulk iron oxide
I4(%)
wt.%
5.0
0.12
5.5
0.13
8.1
0.19
7.2
0.23
6.4
0.20

Chapter 6

Table 6.3 Ammonia adsorbed amount of FW and FAW zeolite catalysts calcined under different
temperatures.
SampleFW-823-823
FW-923-823
FW-1023-823
FW-1023-1023
FAW-823-823
FAW-923-823
FAW-1023-823
FAW-1023-1023

Ammonia adsorbed amount (mmol/g)/Tmax (K）LTMTHT
0.13/438
0.15/492
0.36/590
0.11/435
0.15/484
0.34/589
0.09/440
0.16/508
0.21/610
0.05/435
0.08/489
0.15/579
0.24/440
0.23/479
0.70/605
0.22/442
0.24/489
0.65/608
0.18/439
0.23/486
0.54/591
0.12/441
0.17/485
0.46/611

176

Chapter 6

Table 6.4 Catalytic results of FW and FAW zeolite catalysts under different temperatures calcination for the direct oxidation of benzene to phenol with
H2O2.
SamplePhenolYield (%)a

Product Selectivity (%)b
Phenol

HQ

CL

p-BQ

H2O2
Conv. (%)c

TON1d

TON2e

Refer.

FW-823-823
6.8
94
5
1
0
88
20.6
94.9
FW-923-823
7.2
95
4
1
0
91
21.4
70.2
FW-1023-823
8.3
93
5
2
0
88
26.4
69.8
FW-1023-1023
5.0
95
3
1
1
82
15.6
39.5
This
FAW-823-823
4.2
95
5
0
0
98
9.5
36.5
Chapter
FAW-923-823
3.0
97
3
0
0
98
6.5
27.6
FAW-1023-823
2.3
97
3
0
0
95
4.8
13.9
FAW-1023-1023
1.2
99
1
0
0
98
2.5
6.6
97
2
1
0
FMD25
3.7
76
6.3
16.3
Chapter
FZP2.5
3.0
99
1
0
0
44
3.9
19.3
2
Reaction conditions: 333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
aPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
b Each product selectivity = (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL+ moles of p-BQ).
c H O conversion = (moles of H O after reaction)*100/(initial moles of H O ).
2 2
2 2
2 2
d TON1=(moles of phenol + moles of HQ + moles of CL+ moles of p-BQ)*100/(moles of the total Fe content used).
e TON2=(moles of phenol + moles of HQ + moles of CL+ moles of p-BQ)*100/(moles of the used Fe content for the isolated and oligomeric Fe species on the extra framework).

177

Chapter 6

Table 6.5 Catalytic performance for Fe-containing MWW zeolite catalysts using sulfolane as reaction solvent for the direct oxidation of methane tomethanol with H2O2.
Catalyst

Product Amount (µmol)
MeOH

HCOOH

HCOH

MeOOH

FW-823-823
729
91
115
0
FAW-823-823
552
151
500
0
FW-1023-823
856
179
144
49
FAW-1023-823
339
273
96
80
FMD25
209
15
115
0
Reaction conditions: 323 K, 27 mmol H2O2, 50 mg catalyst, 10 ml sulfolane, 3 Mpa CH4, 2 h.
aEach product selectivity= (moles of each product)*100/(moles of total liquid products).
b
H2O2 conversion = (moles of H2O2 after reaction)*100/(initial moles of H2O2).
c H O efficiency = (moles of total liquid products)*100/(moles of H O conversion).
2 2
2 2

178

H2O2

MeOH
Selec.(%)

Conv. (%)b

Eff. (%)c

78
46
70
43
55

25
53
43
38
7

16
8
11
8
22

Refer.

This
Chapter
Chapter 2

Chapter 7

Chapter 7
Iron- and copper-exchanged Beta zeolite catalysts for hydroxylation of benzene tophenol and methane to methanol with H2O2
Abstract
Fe-exchanged, Cu-exchanged and Fe-Cu bimetallic exchanged Beta catalysts were prepared by theaqueous ion-exchange method with the loading of metal varied. The metallic states were investigatedby UV-vis, NO adsorbed FT-IR and NH3-TPD. The influence of the reaction conditions in BTP reaction
on the catalytic performance on Fe and/or Cu-containing Beta zeolite catalysts were studied in details.
The stability in the BTP reaction of the Fe and/or Cu-containing Beta zeolite catalysts were
investigated. Fe-Cu bimetallic exchanged Beta catalyst showed a dramatically high catalytic activity
in both the BTP and MTM reaction comparison with the Fe/Beta and Cu/Beta zeolites, the highest
phenol yield of 10.5 % with the selectivity of 90 %, and the highest MeOH yield of 720 µmol with theselectivity 57% were achieved.

7.1. Introduction
Direct hydroxylation of benzene to phenol (BTP reaction) and methane to methanol (MTM) haveattracted increasing attention in recent decades because of the economic outlook, environmental
friendliness and technical difficulty [1-3]. However, take BTP reaction as an example, direct
introduction of hydroxyl functionality into benzene is still full of challenging. Almost all the developed
catalytic systems in the gas phase require an elevated temperature and suffer from low conversion
because of the notoriously low reactivity of aromatic C−H bonds [4-5]. Moreover, in liquid phase
phenol is easily over-oxidized to produce by-products such as catechol (CL), hydroquinone (HQ),
benzoquinones (p-BQ), and tars, which makes the low selectivity [6]. Substantial efforts have been
made to develop an efficient method to direct hydroxylation of benzene to phenol with H2O2.
Bianchi and co-workers used titanium silicalite (TS-1) as catalyst, obtaining 4.5% of benzene
conversion with 43% of phenol selectivity in acetonitrile [7]; when they applied TS-1B, which
modified with NH4HF2 and H2O2, the benzene conversion of 8.6% with phenol selectivity of 94% wasachieved in sulfolane [8]. CuAPO-5 zeolite was applied in the BTP reaction, 5.9% benzene conversion
179

Chapter 7

with 68% phenol selectivity was attained [9]. Besides, the BTP reaction was executed on Fe/TS-1
mixture, a reasonable phenol yield of 7.6% with selectivity of 15% was obtained [10]. It is clear to seethat the Fe, Cu, Ti and so on are widely used in the oxidation of benzene to phenol with H2O2 but the
Fe-Cu bimetallic composite zeolite catalysts are rarely reported.
In addition, studies have shown that the reaction intermediates and the transition states in thisreaction pathway of benzene to phenol are quite similar to those in the methane-methanol conversion
pathway with respect to essential bonding characters [11]. Benzene and methane, especially methaneis recognized as one of the most stable hydrocarbons due to its perfectly symmetrical tetrahedron
structure [12]. The dissociation energy of the methane C-H bond is 440 kJ·mol-1 and hence its
activation may require harsh conditions [12]. Typically, the products of direct conversion methane tomethanol are trace and high requirement on the equipment [13]. It seems that the benzene-phenol
process is slightly easy compared to the methane-methanol process, judging from the published papers
concerning the hydroxylation of methane and benzene [14]. Besides, the products of benzene-phenol
process are easier to detect [15]. Hence, benzene to phenol process can be selected as the screening
reaction to investigate the catalytic performance in this research. The catalysts with good catalytic
performance are then applied in the methane-methanol process.
Beta zeolite has large 12-membered-ring pores with three-dimensional interconnection, isthermally stable, and can be easily obtained with a varied range of Si/Al ratio. Meanwhile its open
crystalline structure decreases the diffusion path of reactants [16]. Fe-exchanged Beta zeolite has been
widely used in the selective catalytic reduction (SCR) of NOx [17-18], N2O decomposition [19-20],hydroxylation of benzene to phenol with N2O and so on [21], but it is rarely reported in direct oxidationof benzene to phenol with H2O2.
Moreover, the introduction of Fe and Cu cations together by ion-exchanged has been establishedin other fields. Gao et al. studied on Fe- and Cu-exchanged Beta for NH3-SCR, a clear synergetic effectbetween Fe and Cu species was found [22]. Hammond and co-workers have reported that the CuFe/ZSM-5 catalyst used as an efficient heterogeneous catalyst for the direct oxidation of methane tomethanol with H2O2 under mild reaction conditions, and the presence of Cu cation maintain the highselectivity to alcohol [23]. However, no published paper involve in both Fe- and Cu-exchanged Beta
for the BTP reaction in liquid-phase with H2O2. Besides, the influence of the Fe and/or Cu contents on
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the catalytic performance has not fully been investigated.
In the study, Fe and/or Cu Beta zeolite catalysts with different metal contents were prepared. The
states of the Fe and Cu species were investigated by UV-vis, NO adsorbed FT-IR, and NH3-TPD. The
influence of metallic type and content on the hydroxylation of benzene to phenol and methane tomethanol with H2O2 were studied. Meanwhile, the effects of reaction conditions on the catalytic
performance of Fe and/ or Cu-containing Beta zeolite catalysts in BTP reaction were investigated.

7.2. Experiments
7.2.1 Materials
NH4+-Beta (Si/Al=12) was provided by Zeolyst. Fe(NO3)3·9H2O, Cu(NO3)2·3H2O, H2O2 (30wt.%), benzene, acetonitrile, sulfolane, mesitylene, anisole, 1,4-dioxane, tetramethylsilane (TMS), andCD3CN were purchased from Wako. Methane (99.99%) gas was purchase from Taiyo Nippon Sanso
Co., Ltd.. All of the reagents were used as received, without further purification.

7.2.2 Catalyst preparation
The Fe/Beta and Cu/Beta catalysts were prepared by aqueous ion exchange of NH4+-Beta (Zeolyst,
Si/Al=12) with Fe(NO3)3 or Cu(NO3)2 solutions at 353 K. After stirring 24 h, the suspension was
vacuum filtered, washed, dried at 373 K overnight and calcined in air at 823 K for 5 h. The obtained
products were denoted as “xFe/Beta” or “yCu/Beta”, where x and y were the inputting Fe and Cu
content (wt.%), respectively (based on the zeolite). The Fe and Cu bimetallic exchanged Beta zeolite
catalysts were prepared by adding Fe(NO3)3 and Cu(NO3)2 aqueous solutions together to NH4+-Beta.
The other steps were the similar to those of Fe/Beta. The obtained products were denoted as “xFeyCu/Beta”, where x and y were the inputting Fe and Cu content (wt.%), respectively.

7.2.3 Characterization of catalysts
XRD patterns were collected on a Rint-Ultima III (Rigaku) using a Cu Kα X-ray source (40 kV,
20 mA). Elemental analyses of the samples were performed on an inductively coupled plasma-atomic
emission spectrometer (ICP-AES, Shimadzu ICPE-9000).
UV-vis diffuse reflectance spectra were recorded on a V-650DS spectrometer (JASCO). The
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Chapter 7

diffuse reflectance spectra were converted into the absorption spectra using the Kubelka-Munkfunction. Fourier Transform Infrared (FT-IR) spectra were obtained by a JASCO FT-IR 4100
spectrometer equipped with a triglycine sulfate (TGS) detector. For FT-IR observation, the sample waspressed into a self-supporting disk (20 mm diameter, ca. 30 mg) and placed in an IR cell attached to aclosed gas-circulation system. After the sample was pretreated by evacuation at 773 K for 2 h, thenadsorbed 5-1000 Pa NO at ambient temperature. The IR spectra resulting from the subtraction of the
background spectra from those with NO adsorbed are shown unless otherwise noted.
Temperature-programmed ammonia desorption (NH3-TPD) profiles were noted down on a
Multitrack TPD equipment (Japan BEL). Normally, 25 mg catalyst was pretreated at 773 K for 1 h ina He flow of 50 mL min-1 and then cooled down to 423 K. The sample was evacuated at 423 K for 1
h prior to the adsorption of NH3. Approximately 2500 Pa of NH3 contacted with the sample at 423 K
for 10 min. Subsequently, the sample was evacuated to remove the weakly adsorbed NH3 at the same
temperature for 30 min. Finally, the sample was heated from 423 to 873 K at a ramping rate of 10 K
min-1 in a He flow of 50 mL min-1.

7.2.4 Catalytic tests
7.2.4.1 Direct conversion of benzene to phenol with H2O2 (BTP)The BTP reaction was carried out in a 25 mL round-bottom flask equipped with a reflux condenser
and a magnetic stirrer. In a typical run, the mixture containing 50 mg of catalyst, 10 mL of acetonitrile,
5 mmol of benzene, and 10 mmol of H2O2 was stirred at 333 K for 6 h. After the reaction mixture wascooled down, the catalyst was removed. A certain amount of anisole as internal standard was added to
the resultant liquid, and the products were fixed by exhaustive acetylation with excess (CH3CO)2OK2CO3, the derivative products were analyzed by GC [24]. The amount of unconverted H2O2 wasquantified by standard titration method with 0.1 mol/L Ce(SO4)2 solution.

7.2.4.2 Direct conversion of methane to methanol with H2O2 (MTM)
The liquid-phase MTM reactions were carried out in a 100 ml PTFE autoclave. The CH4 pressure
was controlled by the pressure gage. The reactants were stirred vigorously by an agitator blade.
Aqueous hydrogen peroxide (30 wt.%) was used as oxidant. In a typical run, 10 ml sulfolane, 50 mg
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Chapter 7

catalyst and 27 mmol H2O2 were added to the autoclave and heated to 323 K. The sealed reactor wasthen purged with CH4 to 3 Mpa. After reaction 2 h, the autoclave was cooled rapidly to 278 K in an
ice bath to minimize any further chemical reaction and reduce loss of volatile products. After
separation liquid phase and catalyst, the liquid-phase products were analyzed by 1H NMR spectroscopy
on JEOL ECA-600 spectrometer (14.1 T) equipped with an additional 1 kW power ampliﬁer.
Mesitylene was used as the internal standard and TMS/CD3CN was took as chemical shift calibrator,
respectively. The detectable products in the liquid-phase were CH3OH, HCOH, HCOOH andCH3OOH. The amount of unconverted H2O2 was quantified by standard titration method with 0.1
mol/L Ce(SO4)2 solution.

7.3. Results and discussion
7.3.1 Characterization of catalysts
7.3.1.1 Physicochemical properties
Figure 7.1 shows the XRD patterns of the catalysts, indicating that all the samples had the typical
BEA structure. No obvious iron and copper oxide particles were observed, suggesting that the Fe andCu species were highly dispersed on the Beta zeolite.
Figure 7.2 shows the SEM image of the parent Beta zeolite. It was clear that the zeolite composedof tiny particles.
The elemental compositions of the zeolites as determined by ICP elemental analysis are listed inTable 7.1. An increase in the Si/Al ratio was observed in all the catalysts after the ion-exchange dueto the leaching of Al under low pH during the ion-exchange process, as well as the isomorphic
substitution of Al by Fe and/or Cu on the octahedral network [25]. The output Fe and Cu contents wereconsiderably different under the same ion-exchange conditions. The output Fe content was regularly
increased with the input Fe content in both Fe/Beta and Fe-Cu/Beta catalysts. The output Cu content
in Cu/Beta was also increased along with the input Cu content (1.3-1.8 wt.%). However, the output Cu
content in Fe-Cu/Beta catalysts was much lower than the input ones (0.2-0.3 wt.%), suggesting that
NH4+ cations on the Beta zeolites were more easily exchanged with Fe cations rather than Cu ones
under such competitive conditions.
Figure 7.3 displays the N2 adsorption and desorption isotherms of the zeolites. The isotherm risen
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Chapter 7

sharply at low relative pressure where the micropore filling occurs in the precapillary condensation
range. A sharp risen at the p/p0 over 0.9 indicated the existence of constructional void porosity so thatthe total pore volumes for some samples were higher than 1.0 cm3g-1, as listed in Table 7.1. The BET
surface area of all the samples were around 600-700 cm2g-1 and the external surface were 200-300
cm2g-1 due to the tiny particles. Besides, different metal species and amount loading on the Beta zeolite
did not change the textural properties. It was concluded that micropores were present along with somemesopores.

7.3.1.2 The states of Fe and Cu species
Figure 7.4 shows the UV-vis spectra of the Fe/Beta, Cu/Beta and Fe-Cu/Beta products with proton
form. For Fe/Beta samples, a shoulder band at 215 nm was related to the electron transfer from O toisolated ferric ions in tetrahedral coordination. The band around 270 nm was derived from isolatedmononuclear Fe3+ species in octahedral coordination, whereas the bands ranging from 300 to 400 nm
were assigned to octahedral Fe3+ species in small oligomeric FexOy clusters, and the bands above 400
nm are ascribed to bulk iron oxide particles less or larger than 2 nm [25]. The Cu/Beta samples showa sharp band below 350 nm. The maximum band at about 209 nm was attributed to monomeric Cu2+ions interacting with oxygen of the zeolite structure, and the maximum band at about 267 nm was
ascribed to oligomeric [Cu2+-O2--Cu2+] species [20, 26-27]. The spectra of Fe-Cu/Beta catalysts werebasically the same to those of the Fe/Beta samples, possibly due to a high contribution of the Fe species.
Because the UV-vis spectra can not distinguish the Fe/Beta and Fe-Cu/Beta zeolites, meanwhile
NO adsorbed FT-IR technology can be applied to investigate the states of Fe and Cu species on theextra framework [28]. The left parts of Figures 7.5(a-c) present the NO adsorbed FT-IR spectra for
the Fe/Beta samples with different Fe loadings at 298 K. At the lowest NO pressure (5 Pa), the bandat 1875 cm−1 was assigned to Fe2+(NO) on Fe2+-O-Al species or oligomer Fe species of the extra
framework [17, 29-30]. The intensity of the band was dramatic increased along with NO pressure,indicating the existence of a large amount of Fe2+-O-Al species or oligomeric Fe species. Meanwhile
the intensity of the bands were increased along with the Fe content, suggesting that the amount of Fe2+O-Al species or oligomeric Fe species were increased with the total Fe content. The high loading ofNO led to the appearance of a broad band centered at around 1820 cm−1, assigned to Fe2+(NO)3 on
isolated ferrous ions, which was highly coordinatively unsaturated [31-32]. Note that the intensity of184

Chapter 7

the bands at 1820 cm-1 were not increased along with the Fe content. 3Fe/Beta zeolite displayed thehighest intensity at 1820 cm-1, implying the highest proportion of highly coordinatively unsaturated
Fe species in the Fe/Beta catalysts. The middle parts of Figures 7.5(a-c) show the NO adsorbed FTIR spectra of the Cu/Beta samples. The band at 1802 cm−1 with a weak shoulder at 1818 cm-1 assigned
to Cu+(NO) were observed at the lowest NO pressure [32]. The intensity of these bands were increased
and then decreased with increasing the NO pressure. Meanwhile, some new bands at 1913 and 1950
cm-1 attributed to Cu2+(NO) appeared [33]. With increasing Cu content, the intensity of bands at 1913
cm-1 and 1802 cm-1 were increased and then maintained. However, the bands at 1913 cm-1 migrated tolower wavenumber with Cu content increase possibly due to the change of coordinatively unsaturated
degree for Cu species. For the Fe-Cu/Beta samples (the right parts of Figures 7.5(a-c)), the intensityof bands at 1875 cm-1 were increased with the metal content increase. But the intensity of Fe-Cu/Betazeolites were lower than Fe/Beta under the similar Fe content, indicating that the states of Fe species
in the Fe-Cu/Beta zeolite were influenced by the introduction of Cu. Compared with the spectra of
Fe/Beta zeolites, the bands at 1815 cm-1 for Fe-Cu/Beta samples shifted to lower wavenumber with ca.
5 cm-1 possibly due to the introduction of Cu. The intensity of the band at 1815 cm -1 of Fe-Cu/Beta
were increased along with metal content, and higher than those of Fe/Beta zeolites. It greatly illustrated
the more highly coordinatively unsaturated Fe species for Fe-Cu/Beta than Fe/Beta zeolites. Amongthem, 6Fe-6Cu/Beta presented the highest intensity. A weak band at 1808 cm-1 appeared, which was
attributed to Cu+(NO). The existence of Cu was also reflected on the band at 1912 cm-1, even though
the intensity was very weak due to the low Cu content in the Fe-Cu/Beta catalysts.

7.3.1.3 Acidity
NH3-TPD can also reflect the condition of the metal species in the zeolites. Figure 7.6 shows the
NH3-TPD profiles of the catalysts. All the profiles can be deconvoluted into three peaks at low,
medium, and high temperatures (designated as LT, MT, and HT). The amounts of NH3 adsorbed andthe temperature of the maximum peak (Tmax) are listed in Table 7.2. According to the references [3435] , the LT peak, which corresponds to NH3 adsorbed on the non-acidic OH groups and NH4+ byhydrogen bonding, was not related to the true acid site and excluded in the discussion. The MT andHT peaks correspond to NH3 adsorbed on the extra framework metal species (Lewis acid site), and toNH3 adsorbed on the bridged Si-O(H)-M species (Brønsted acid site) (M means Fe, Cu and Al),185

Chapter 7

respectively.
The introduction of iron species to the Beta zeolite by ion exchange results in a distinct decreasein the amount at the Brønsted acid sites due to the replacement of protons in Brønsted acid hydroxyl
groups by cationic iron species (Figure 7.6(a)). It can also be noted that the amount of Brønsted acidsites in Fe/Beta were gradually decreased with the accumulative of Fe content, which should be
ascribed to the continuous dealumination in the Beta framework [35]. Meanwhile the HT reached the
maximum value for 3Fe/Beta (576 K), meaning that the Brønsted acid site of 3Fe/Beta was a little
different from those of 1.5Fe/Beta and 6Fe/Beta. Besides, the NH3 adsorbed amount on the extra
framework metal species displayed interesting phenomenon. It was not increased with the Fe content,
but reached the maximum value on 3Fe/Beta (0.23 mmol/g), demonstrating that the NH3 adsorbed
amount on the extra framework metal species was related to the states instead of the amount of metal
species. In addition, incorporation of Cu into Beta zeolite increased strong acid sites from 0.46 to 0.54mmol/g, which was consistent with the Sultana’s report [36]. But the HT was decreased from 568 to
549 K with the increase of Cu content from 0 to 1.8 wt.%, illustrating the changed strength of the acidsite. The NH3 adsorbed amount on the extra framework metal species for Cu/Beta was decreased with
the Cu content, which was different from Fe/Beta. For Fe-Cu/Beta, the NH3 adsorbed amount was thejoint effect of Fe and Cu species. The amount of both Brønsted acid site and Lewis acid site weredecreased with Fe and Cu content increase. One possible reason was the severe dealumination, which
was verified by the ICP result. Another reason was that the presence of Fe influenced the state of Cu
species and cannot increase the strong acid sites, consequently influence the condition of Brønstedacid sites.

7.3.2 Direct oxidation of benzene to phenol with H2O2 (BTP)
Table 7.3 lists reaction results in the BTP reaction with H2O2. H-Beta exhibited 0.9% phenol yieldwith 100% selectivity to phenol, no significant but higher than H-ZSM-5 in Chapter 2. The phenol
yield was in the following order: Fe-Cu/Beta > Fe/Beta > Cu/Beta. For the Fe/Beta catalysts, the phenol
yield was increased from 4.6% to 5.1% with an increase in the Fe content from 0.8 to 2.1 wt.%, andthen it was decreased to 4.4% with Fe content continuously increase to 5.6 wt.%. It indicated that thephenol yield was not only related to Fe content but also to the Fe state, since the 3Fe/Beta containing
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Chapter 7

the highest amount of highly coordinatively unsaturated Fe species. For Cu/Beta, the highest phenol
yield was achieved on 1Cu/Beta, the increase of Cu content resulted in the decrease of phenol yield,
signifying that high Cu content was not beneficial to the phenol production. It may be related to thechange of coordinatively unsaturated condition of Cu species. However, it is still unknown for the
specific influence of Cu state on the catalytic performance. Fe-Cu/Beta catalysts showed dramaticallyhigh catalytic activity compared with both Fe/Beta and Cu/Beta catalysts. The phenol yield wasincreased from 7.4 to 10.5% along with the Fe content, since the Cu contents were maintained at 0.20.3 wt.%. The improvement would be due to the synergetic effect, which could be explained bycooperation between Fe and Cu cations, i.e., so-called “Fenton-like” reaction mechanism [37,38].
Especially 6Fe-6Cu/Beta achieved the highest phenol yield, which could be related to the highest
isolated Fe ions. The high activity in partial oxidation reactions of isolated Fe ions has also beenreported the by Berlier et al. [39].
The phenol selectivity of the catalysts exhibited the following order: Fe/Beta > Fe-Cu/Beta >Cu/Beta. Fe/Beta zeolites achieved 89-96% phenol selectivity. In contrast, Cu/Beta catalysts presented
a low phenol selectivity, 84-78%, since the by-product p-BQ was much produced. For the Fe-Cu/Beta
catalysts, the phenol selectivity was higher than that of Cu/Beta but lower than that of Fe/Beta. The
Cu species in the Beta would show a high activity for not only the hydroxylation of benzene but also
the successive reaction of phenol, resulting in the low yield and selectivity. Actually, the liquid productfor Cu/Beta catalysts were always black, indicating the formation of tar as by-product, which cannot
be analyzed by GC. On the other hand, the Fe species exhibited a high phenol selectivity. For
comparison, the results over other zeolite catalysts reported in the literatures were listed in Table 7.3.
It was clear to show that the higher phenol yield for Fe-Cu/Beta zeolites were achieved.
In order to investigate the influence of reaction time, molar ratio of H2O2/benzene, and reaction
temperature on the reaction performance, 6Fe/Beta, 6Cu/Beta and 6Fe-6Cu/Beta zeolites were used as
the typical catalysts.
As shown in Figure 7.7, the phenol yield of 6Cu/Beta was increased from 1.9 to 2.7% with time
prolonging from 0.33 to 2 h, and then maintained at 2.7% by extending reaction time from 2 to 10 h.
Because H2O2 conversion was up to 85% when reacting for 2 h. The HO· produced from H2O2 waslimited after 2 h, thus the phenol yield maintained at 2.7%. Meanwhile, the phenol selectivity of187

Chapter 7

Cu/Beta was decreased from 89 to 78% with time changing from 0.33 to 2 h, and maintained at 78%
by prolonging reaction time from 2 to 10 h. The increased benzene conversion of 6Cu/Beta indicatedthe formation of undetectable by-products. For 6Fe/Beta, except the result of reaction 6 h, the basic
trend was similar to 6Fe-6Cu/Beta. The phenol yield and benzene conversion gave a massive growth
firstly and then slowly increase. It was also related to the H2O2 conversion. When reaction timeincreased from 0.33 to 6 h, the H2O2 conversion was increased from around 20 to 90%, which produceda large number of HO·. However, when the reaction time prolonging to 10 h, the H2O2 conversion
showed no significant changes. In short, long reaction time was beneficial to the benzene conversion
and phenol yield, but limited by the H2O2 conversion.
Figure 7.8 displays the reaction results under different molar ratios of H2O2/benzene, the benzene
amount was fixed at 5 mmol. For the three samples, the phenol yield and benzene conversion wereboth increased with the molar ratio varying from 2 to 8. But the increased rate of phenol yield was not
significant. The phenol selectivity for the three samples were maintained, except the result of 6Fe/Beta
attaining at molar ratio equal to 2. The results were involved to the sufficient HO· provided by theH2O2. Although, the H2O2 conversion were decreased with H2O2 amount increase, the consumed H2O2
amount were increased. In a word, high molar ratios of H2O2/benzene was conducive to the reaction
under the catalytic systems, but high price of H2O2 was worth considering.
The influence of reaction temperature on the catalytic performance are presented in Figure 7.9.
The benzene conversion and phenol yield for the three samples were increased with temperature
raising from 313 to 333 K due to the dramatically increased H2O2 conversion. Continuing to increase
temperature to 353 K, the benzene conversion of the three samples, the phenol yield of 6Fe/Beta and
6Cu/Beta were increased, while the phenol yield for 6Fe-6Cu/Beta was decreased to from 10.5 to 9.4%.
The phenol selectivity of 6Fe-6Cu/Beta and 6Fe/Beta were reduced by 6%, while that of 6Cu/Beta wasincreased by 12%. Note that the reaction systems were boiling when the reaction temperature was
353K. Thus the H2O2 conversion were up to 90% for the three samples at 333 and 353K. Overall,properly increasing the reaction temperature favored the catalytic performance. However, too highreaction temperature did not improve the performance, but increased the energy consumption.
Besides, from the results of 0.33 h for 6Cu/Beta in Figure 7.7, molar ratio of H2O2/Benzene for
2 in Figure 7.8, and 313K in Figure 7.9, we found that high H2O2 conversion with low phenol
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Chapter 7

selectivity were achieved for 6Cu/Beta under the mild conditions. It implied that Cu/Beta could easilyand quickly react with H2O2 to produce HO· and HO2·, thus the detectable by-products like HQ and
undetectable products like tar were produced. That may be the reason of low phenol selectivity
provided by Cu/Beta.
The stability of the Fe and/or Cu-containing Beta zeolite catalysts and the catalytic performanceof other composite bimetallic catalysts were investigated, showing in Table 7.4. The catalytic
performance of the used and calcined 6Cu/Beta was reduced by 48%, and 6Fe-6Cu/Beta was reduced
38%, due to the severe leaching of Cu species. In contrast, the Fe species was so stable to maintain thephenol yield for 6Fe/Beta catalyst. The catalytic performance of 25mg 6Fe/Beta plus 25mg 6Cu/Beta
was better than 50mg 6Fe/Beta and 50 mg 6Cu/Beta, but worse than 6Fe-6Cu/Beta, indicating that thesynergistic effect was influenced by the states and content of metals. Besides, the reaction result of
imp-6Fe-6Cu/Beta was much worse than both 6Fe-6Cu/Beta and 6Fe/Beta+6u/Beta, though thehighest Fe and Cu content were contained, which confirmed the above statement.

7.3.3 Direct oxidation of methane to methanol with H2O2 (MTM)
3Fe/Beta, 1.5Cu/Beta and 6Fe-6Cu/Beta as the catalysts achieved the highest phenol yield in
Fe/Beta, Cu/Beta and Fe-Cu/Beta catalysts, respectively, were applied in the direct oxidation of
methane to methanol, as a contrast the H/Beta catalyst was also investigated , the results are displayedin Table 7.5. Similarity to the results of direct oxidation of benzene to phenol, 6Fe-6Cu/Beta catalyst
achieved the highest methanol amount which was double times than that of 3Fe/Beta, four times thanthat of 1.5Cu/Beta and almost thirteen times than that of H/Beta. Meanwhile, 3Fe/Beta attained thehighest total liquid product amount and 57% methanol selectivity. The H2O2 conversion of the catalysts
ranged from 4% to 22%, which was not very high affected by the aprotic solvent sulfolane. In addition,
the H2O2 efficiency was determined by the liquid product amount and H2O2 conversion, 6Fe-6Cu/Betarealized the highest H2O2 efficiency 43%. Meanwhile, the methanol yield of 3Fe/Beta and 6Fe6Cu/Beta were higher than Fe-MFI catalyst. The more active Fe species provided by the large surface
area and pore size for Beta zeolites maybe the reason.

7.4. Conclusions
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Chapter 7

In conclusion, Fe and/ or Cu exchanged Beta catalysts with varied metal content were investigatedin hydroxylation of benzene to phenol with H2O2. The states of Fe and Cu species were characterized
by UV-vis and NO adsorbed FT-IR. The influence of the reaction conditions in BTP reaction on the
catalytic performance of Fe and/or Cu-containing Beta zeolite catalysts were studied in details. The
stability of the Fe and/or Cu-containing Beta zeolite catalysts in the BTP reaction were investigated.
The synergetic effect was influenced by the states of the metals instead of the content. Fe-Cu/Beta
catalysts showed dramatically high catalytic activity due to the synergetic effect. 6Fe-6Cu/Betaachieved the highest phenol yield of 10.5 % in BTP reaction and MeOH yield of 720 µmol in MTM
reaction, which was related to the highest isolated Fe species influenced by the introduction of Cu.
Thus, we successfully found that the incorporation of Fe and Cu cation species together into zeolite isa promising method for improving catalytic performance in the BTP and MTM reaction, and suchsynergetic effect by attaining the introduction of different metal cation species will be applicable toother applications.

190

Chapter 7

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264.

192

Chapter 7(j)(i)

Intensity (cps.)

(h)
(g)
(f)
(e)
(d)
(c)
(b)
(a)
5

10

15

20

25

30

35

40

2 theta (degree)

Figure 7.1 XRD patterns of the (a) H-Beta, (b)1.5Fe/Beta, (c) 3Fe/Beta, (d) 6Fe/Beta, (e) 1.5Cu/Beta,(f) 3Cu/Beta, (g) 6Cu/Beta, (h) 1.5Fe-1.5Cu/Beta, (i) 3Fe-3Cu/Beta, (j) 6Fe-6Cu/Beta.

193

Chapter 7

Beta

1 µm
Figure 7.2 SEM image of the parent Beta zeolite.

194Chapter 7
1200

800

1000

Va/cm3(STP) g-1

Va/cm3(STP) g-1

1000

(a)
(b)
(b)
(c)

600

400

200

0
0.0

800

1200

(a)
(b)
(c)
(d)

1000

Va/cm3(STP) g-1

1200

600

400

200

0.2

0.4

0.6

p/p0

0.8

1.0

0
0.0

800

(a)
(b)
(c)
(d)

600

400

200

0.2

0.4

0.6

p/p0

0.8

1.0

0
0.0

0.2

0.4

0.6

0.8

1.0

p/p0

Figure 7.3 N2 adsorption and desorption isotherms of left: (a) H/Beta, (b) 1.5Fe/Beta, (c) 3Fe/Beta, (d)6Fe/Bea; middle: (a) H/Beta, (b) 1.5Cu/Beta, (c) 3Cu/Beta, (d) 6Cu/Bea; right: (a) H/Beta, (b) 1.5Fe1.5Cu/Beta, (c) 3Fe-3Cu/Beta, (d) 6Fe-6Cu/Bea.

195

Chapter 7
0.6

3.5

(a)
(b)
(c)

3.0

0.5

2.5

2.5

K/M

2.0

(a)
(b)
(c)

3.0

0.4

K/M

K/M

3.5
(a)
(b)
(c)

0.3

1.5

2.0
1.5

0.2
1.0

1.0
0.1

0.5
0.0

200

300

400
Wavelength (nm)

500

600

0.0

0.5

200

300

400
Wavelength (nm)

500

600

0.0

200

300

400

500

600

Wavelength (nm)Figure 7.4 UV-vis spectra of left: (a) H/Beta, (b) 1.5Fe/Beta, (c) 3Fe/Beta, (d) 6Fe/Bea; middle: (a)H/Beta, (b) 1.5Cu/Beta, (c) 3Cu/Beta, (d) 6Cu/Bea; right: (a) H/Beta, (b) 1.5Fe-1.5Cu/Beta, (c) 3Fe3Cu/Beta, (d) 6Fe-6Cu/Bea.

196

Chapter 7

2.8

3.2

PNO 1913
2.8

1820

2.4

2.8

1950

1818

1.6
1.2

(c)

2.0
1.6
1.2

(b)
0.8

(b)

0.8

0.4

2.4

1802

(c)

2.0

AbsorbanceAbsorbance

2.4

3.2

1875

PNO

0.4

1875
1815

PNO
1912

1808
(c)

Absorbance

3.2

2.0
1.6
1.2
(b)
0.8
0.4

(a)

(a)
(a)
0.0
0.0
0.0
2000 1950 1900 1850 1800 1750 1700 2000 1950 1900 1850 1800 1750 1700
2000 1950 1900 1850 1800 1750 1700

Wavenumbers (cm-1)

Wavenumbers (cm-1)

Wavenumbers (cm-1)

Figure 7.5 NO-adsorbed FT-IR spectra of left: (a) H/Beta, (b) 1.5Fe/Beta, (c) 3Fe/Beta, (d) 6Fe/Bea;middle: (a) H/Beta, (b) 1.5Cu/Beta, (c) 3Cu/Beta, (d) 6Cu/Bea; right: (a) H/Beta, (b) 1.5Fe-1.5Cu/Beta,(c) 3Fe-3Cu/Beta, (d) 6Fe-6Cu/Bea.

197

Chapter 7
7200

7200

7200

6400

6400

6400

(d)

5600

(c)

3200

2400

4800

4000

(c)

3200

2400

(c)

3200

2400

1600

800

4000

(b)

(b)

(b)
1600

TCD signal

4000

(d)

5600

4800

TCD signal

4800

TCD signal

(d)

5600

1600

800

800

(a)

(a)

(a)
0
350 400 450 500 550 600 650 700 750 800 850 900

0350 400 450 500 550 600 650 700 750 800 850 900

0350 400 450 500 550 600 650 700 750 800 850 900

Temperature(K)Temperature(K)Temperature(K)Figure 7.6 NH3-TPD spectra of left: (a) H/Beta, (b) 1.5Fe/Beta, (c) 3Fe/Beta, (d) 6Fe/Bea; middle: (a)H/Beta, (b) 1.5Cu/Beta, (c) 3Cu/Beta, (d) 6Cu/Bea; right: (a) H/Beta, (b) 1.5Fe-1.5Cu/Beta, (c) 3Fe3Cu/Beta, (d) 6Fe-6Cu/Bea.

198

Chapter 7
12

8

6

4

2

(b)

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

90

Benzene conversion (%)

Phenol yield (%)

10

100

(a)

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

80
70
60
50
40
30
20
10
0

0
0

2

4

6

8

10

0

2

4

6

8

10

Raction time (h)Raction time (h)
100

100

(d)

90

80

80

H2O2 conversion(%)

Phenol selectivity (%)

(c)
90

70

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

60
50
40
30

70
60
50
40

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

30

20

20

10

10
0

0
0

2

4

6

8

0

10

2

4

6

8

10

Raction time (h)Raction time (h)Figure 7.7 Changes of (a) phenol yield, (b) benzene conversion, (c) phenol selectivity and (d) H2O2
conversion over Fe and/ or Cu containing Beta zeolite catalysts as a function of reaction time. Reaction
conditions: 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 333 K. a Phenol yield = (moles of phenol
produced)*100/(initial moles of benzene). b Benzene conversion = (moles of benzene
consumed)*100/(initial moles of benzene). c Phenol selectivity = (moles of each liquid
product)*100/(moles of phenol + moles of HQ + moles of CL+ moles of p-BQ). d H2O2 conversion =(moles of H2O2 consumed)*100/(initial moles of H2O2).

199

Chapter 7
14

Phenol yield (%)

10
8
6
4

(b)

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

90

Benzene conversion (%)

12

100

(a)

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

80
70
60
50
40
30
20

2
10
0

0
2

6

8

2

Molar ratio of H2O2/Benzene

100

90

80

80

60

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

50
40
30

6

8

(d)

90

70

4

Molar ratio of H2O2/Benzene

100

(c)

H2O2 conversion(%)

Phenol selectivity (%)

4

70
60
50
40

20

20

10

10

0

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

30

0
2

4

6

8

2

Molar ratio of H2O2/Benzene

4

6

8

Molar ratio of H2O2/Benzene

Figure 7.8 Changes of (a) phenol yield, (b) benzene conversion, (c) phenol selectivity and (d) H2O2
conversion over Fe and/ or Cu containing Beta zeolite catalysts as a function of molar ratio of
H2O2/Benzene. Reaction conditions: 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 333 K, 6 h. a
Phenol yield = (moles of phenol produced)*100/(initial moles of benzene). b Benzene conversion =(moles of benzene consumed)*100/(initial moles of benzene). c Phenol selectivity = (moles of eachliquid product)*100/(moles of phenol + moles of HQ + moles of CL+ moles of p-BQ). d H2O2
conversion = (moles of H2O2 consumed)*100/(initial moles of H2O2).

200

Chapter 7
12

100

8

6

4

2

(b)

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

90

Benzene conversion (%)

10

Phenol yield (%)(a)

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

80
70
60
50
40
30
20
10

0

0
310

320

330

340

350

360

310

Reaction temperature (K)

320

330

340

350

Reaction temperature (K)

100

100

(c)

(d)

90

90

80

80

H2O2 conversion(%)

Phenol selectivity (%)

360

70
60

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

50
40
30

70
60
50
40

20

20

10

10

0

6Cu/Beta
6Fe/Beta
6Fe-6Cu/Beta

30

0
310

320

330

340

350

360

Reaction temperature (K)

310

320

330

340

350

360

Reaction temperature (K)Figure 7.9 Changes of (a) phenol yield, (b) benzene conversion, (c) phenol selectivity and (d) H2O2
conversion over Fe and/ or Cu containing Beta zeolite catalysts as a function of reaction temperature.
Reaction conditions: 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 10 mmol H2O2, 6 h. a Phenol
yield = (moles of phenol produced)*100/(initial moles of benzene). b Benzene conversion = (moles ofbenzene consumed)*100/(initial moles of benzene). c Phenol selectivity = (moles of each liquid
product)*100/(moles of phenol + moles of HQ + moles of CL+ moles of p-BQ). d H2O2 conversion =(moles of H2O2 consumed)*100/(initial moles of H2O2).

201

Chapter 7

Table 7.1 Composition and texture properties of Fe/Beta, Cu/Beta and Fe-Cu/Beta zeolite catalysts.
SampleInput (wt.%)
Fe

Cu

Output (wt.%) a
Fe

Cu

SBET
(m2g−1)bSEXT(m2g−1)c

1.5Fe/Beta
1.5
0.8
641
286
3Fe/Beta
3.0
2.1
616
264
6Fe/Beta
6.0
5.6
647
275
1.5Cu/Beta
1.5
1.3
629
241
3Cu/Beta
3.0
1.6
657
244
6Cu/Beta
6.0
1.8
645
240
1.5Fe-1.5Cu/Beta
1.5
1.5
0.9
0.3
726
310
3Fe-3Cu/Beta
3.0
3.0
2.1
0.2
608
237
6Fe-6Cu/Beta
6.0
6.0
5.5
0.3
618
242
a
Si/Fe was determined by ICP-AES analysis, Fe content was calculated by Si/Fe.
b

VTotal(cm3g-1)dVMicro(cm3g-1)e

1.02
0.98
1.09
1.05
0.95
0.93
1.07
1.08
1.05

0.24
0.23
0.25
0.22
0.23
0.23
0.23
0.23
0.24

Specific surface areas of the catalysts were calculated using the Brunauer-Emmett-Teller (BET) equation on the N2
adsorption isotherms.
c External surface area (SEXT) of the catalysts were calculated by the t-plot method based on the adsorption isotherms.
dTotal pore volumes of the catalysts were calculated based on BET equation on the N2 adsorption isotherms.
eMicropore volumes of the catalysts were calculated by the t-plot method based on the adsorption isotherms.

202Chapter 7

Table 7.2 NH3 adsorbed amount of the Fe/Beta, Cu/Beta and Fe-Cu/Beta zeolite catalysts.
Samples
H-Beta1.5Fe/Beta
3Fe/Betao6Fe/Beta
1.5Cu/Beta
3Cu/Beta
6Cu/Beta
1.5Fe-1.5Cu/Beta
3Fe-3Cu/Beta
6Fe-6Cu/Beta

NH3 adsorbed amount (mmol/g) / Tmax (K)LTMTHT
0.21/437
0.22/472
0.46/568
0.21/441
0.21/476
0.37/568
0.19/441
0.23/486
0.28/576
0.18/441
0.18/487
0.23/570
0.24/437
0.20/481
0.50/559
0.31/431
0.18/472
0.52/551
0.22/435
0.18/476
0.54/549
0.21/439
0.23/489
0.30/569
0.19/439
0.23/495
0.26/570
0.15/439
0.19/489
0.19/553

203

Chapter 7

Table 7.3 Direct oxidation of benzene to phenol results using Fe or/and Cu-containing Beta catalysts
with H2O2.
Product Selectivity (%)b
H2O2
Refer.
Conv. (%)c
PhenolHQ
CL
p-BQ
H-Beta
0.9
100
0
0
0
19
1.5Fe/Beta
4.6
95
4
1
0
84
3Fe/Beta
5.1
96
4
1
0
80
6Fe/Beta
4.4
89
10
1
0
86
1.5Cu/Beta
3.4
84
8
1
7
85
This
Chapter
3Cu/Beta
2.8
80
10
2
8
92
6Cu/Beta
2.7
78
11
3
7
87
1.5Fe-1.5Cu/Beta
7.4
92
3
4
0
65
3Fe-3Cu/Beta
9.4
94
4
2
0
61
6Fe-6Cu/Beta
10.5
90
7
2
0
96
[7]
TS-1B
8.1
94
97
[8]
CuAPO-5
7.9
62
11
27
0
[9]
TS-1
1.9
43
13
12
93
[10]
Fe/TS-1
7.6
15
96
Reaction conditions: 333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
aPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
b
Product selectivity = (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL+ molesof p-BQ)c H O conversion = (moles of H O consumed)*100/(initial moles of H O ).
2 2
2 2
2 2
SamplePhenolYield (%)a204

Chapter 7

Table 7.4 Direct oxidation of benzene to phenol results using Fe or/and Cu-containing Beta catalysts with H2O2.
Fe Cu
Product Selectivity (%)b
PhenolYield (%)a Phenol HQ CL p-BQ
(mg)
Fresh
0.9
2.7
78
11
3
7
6Cu/Beta
Refresh d
0
0.3
1.4
94
5
1
Fresh
2.8
4.4
89
10
1
0
6Fe/BetadRefresh
1
0
0
2.6
4.3
99
Fresh
2.8 0.2
10.5
90
7
2
0
6Fe-6Cu/Beta
Refresh d
1
0
0
2.6
6.5
99
e
6Fe/Beta+6Cu/Beta
1
2
0
Fresh
1.4 0.5
5.1
97
imp-6Fe-6Cu/Beta f
13
1
20
Fresh
3.0 3.0
2.0
66
Reaction conditions: 333 K, 10 mmol H2O2, 50 mg catalyst, 10 ml CH3CN, 5 mmol Benzene, 6 h.
aPhenol yield = (moles of phenol produced)*100/(initial moles of benzene).
b
Product selectivity = (moles of each liquid product)*100/(moles of phenol + moles of HQ + moles of CL+ moles of p-BQ).
c
H2O2 conversion = (moles of H2O2 consumed)*100/(initial moles of H2O2).
d
Catalysts were used in BTP, washed using Ethanol, dried overnight, calcined at 823K for 5h.
e
25 mg 6Fe/Beta and 25 mg 6Cu/Beta.
f
Prepared by impregnation.
Catalyst

Type

205

H2O2
Conv. (%)c
87
4
86
43
96
27
89
92

Chapter 7

Table 7.5 Direct oxidation of methane to methanol results using Fe or/and Cu Beta catalysts with H2O2.
SampleProducts(µmol)

MeOH
Selec.(%)a

H 2O 2

MeOH HCOOH HCOH
Conv. (%)
Eff.(%)c
57
27
68
4
9
H/Beta
437
51
396
49
22
15
3Fe/Beta
174
0
133
57
4
29
1.5Cu/Beta
720
77
472
57
12
43
6Fe-6Cu/Beta
FMD25
209
15
115
55
7
22
Reaction conditions: 323 K, 28 mmol H2O2, 50 mg catalyst, 10 ml solfulane, 3Mpa CH4 pressure, 2 h.
a Each product selectivity= (moles of each product)*100/(moles of total liquid products).
b H O conversion = (moles of H O after reaction)*100/(initial moles of H O ).
2 2
2 2
2 2
c H O efficiency = (moles of total liquid products)*100/(moles of H O conversion).
2 2
2 2

206

b

Chapter 8

Chapter 8
Summary
Chapter 2 explored Fe-containing MFI zeolite catalysts including Fe-silicalite-1 and Fe-ZSM-5
by direct or post-synthesized method and post-modification by acid or alkaline treatment. In the BTP
reaction with H2O2, direct-synthesized Fe-silicalite-1 achieved much higher phenol yield than the postsynthesized ones at similar Fe content. Based on the specific characterizations, we successfully
clarified that the formation of isolated and oligomeric Fe species on the extra framework in MFI
zeolites were critical for achieving high phenol yield. Among isolated and oligomeric extra framework
Fe species, low nuclear oligomeric Fe species would be more active. Moreover, alkaline treatment was
found to be advantageous to the formation of oligomeric Fe species, and as a result, 7.6% of phenol
yield with 92% selectivity was attained. Our findings would contribute to the improvement of catalytic
activity of Fe-containing zeolite catalysts, and also the development of the catalytic process for direct
production of phenol from benzene.
Chapter 3 investigated the Fe-silicalite-1 zeolites directly synthesized using TPAOH as OSDA
with or without Na cations in the synthesis gel. The physicochemical properties, especially the statesof Fe species was characterized. Fe-silicalite-1 zeolites synthesized without Na cations displayed more
uniform distribution for each kinds of Fe species and higher proportion of framework Fe than thosesynthesized with Na cations. The impact of solvent on the reaction performance was investigated. We
firstly found that sulfolane was the best one among the solvents used in terms of the improvement in
the production of methanol and its stability during the reaction. Finally, the effects of various reaction
parameters, such as reaction temperature, CH4 pressure, reaction time, catalyst amount and the amountof H2O2 were optimized. Furthermore, the use of mixture of sulfolane-water solvent with anappropriate proportion led to an extremely high methanol production with a high selectivity. Generally
Fe-silicalite-1 zeolites synthesized using TPAOH as OSDA without Na cations (FS(T)x) showed better
catalytic performance than those with Na cations (FS(TN)y). Under the optimal condition, FS(T)15
zeolite displayed the highest methanol yield of 947.8 µmol with the selectivity of 85% in the aqueous
sulfolane solvent.
Chapter 4 discussed the Fe-ZSM-5 zeolites directly synthesized using TPAOH as OSDA withor without Na cations. The physicochemical properties, especially the states of Fe species were207

Chapter 8

characterized by UV-vis and NH3-TPD. Fe-ZSM-5 zeolites synthesized without Na cations displayed
more uniform distribution for each kinds of Fe species. Fe-ZSM-5 zeolites synthesized with Na cations
presented strong acidity, since most of aluminum located in the framework. Generally Fe-ZSM-5
zeolites synthesized without Na cations (FZ(T)x) showed better catalytic performance in BTP reaction
than the ones with Na cations (FS(TN)y). The introduction of aluminum in FZ(T)x and FZ(TN)yzeolites increased the isolated and oligomeric Fe species on the extra framework compared with
FS(T)x and FS(TN)y, respectively. Thus, the catalytic performance in BTP and MTM reactions were
improved with the introduction of aluminum in the MFI zeolites.
Chapter 5 researched the influence of desilication on as-synthesized and calcined Fe-ZSM-5 andFe-silicalite-1 zeolites. Both template and aluminum can prevent desilication and protect the crystal
structure. While the introduction of iron in the zeolite did not avoid desilication to a large extent. Thus
alkaline treatment on Fe-ZSM-5 and Fe-silicalite-1 is an effective post modification method that
affected both the porosity of the zeolite and the nature of Fe species. The increased porosity and surface
area can improve the transport properties, reduce diffusion resistance and well disperse of Fe species.
After alkaline treatment the phenol yield for Fe-ZSM-5 and Fe-silicalite-1 were increased from 0.6%
(H-FZ) to 5.3% (CAT-FZ) and from 5.0% (H-FS) to 7.5% (CAT-FS), respectively.
Chapter 6 discovered Fe-containing MWW zeolites (i.e. Fe-MWW and Fe,Al-MWW) directly
synthesized and extensively characterized, especially the states of Fe species and the acidity ofcatalysts. Fe-MWW showed better performance than Fe,Al-MWW in both MTM and BTP reaction
with H2O2. The effect of calcination temperature on the catalytic performance has been studied indetail. Relatively high temperature calcination in the first step to remove OSDA was beneficial toproduce more isolated and oligomeric Fe species on the extra framework for Fe-MWW, thus activate
the catalytic performance. But the presence of aluminum in Fe,Al-MWW dispersed iron and hightemperature calcination produced the FeAlOx species, which may be not beneficial to the catalytic
performance. In addition, increasing the second step calcination temperature to prepare H-type zeolitefrom NH4+-type made more seriously Fe aggregation and reduce the terminal hydroxyl (Fe-OH and/or
Al-OH on the extra framework) for both Fe-MWW and Fe,Al-MWW, thus decrease the catalytic
performance. The best results were achieved by FW-1023-823, phenol yield of 8.3% with selectivityof 93% in BTP reaction and 855.8 µmol methanol yield with 70% selectivity in MTM reaction with
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Chapter 8

H2O2. Both Fe-MWW and Fe,Al-MWW realized considerable and better catalytic performancecomparison with Fe-MFI zeolite in BTP and MTM reactions.
Chapter 7 included Fe and/ or Cu exchanged Beta catalysts with varied metal content forhydroxylation of benzene to phenol with H2O2. The states of Fe and Cu species were characterized by
UV-vis and NO adsorbed FT-IR. The influence of the reaction conditions in BTP reaction on the
catalytic performance of Fe and/or Cu-containing Beta zeolite catalysts were studied in details. The
stability of the Fe and/or Cu-containing Beta zeolite catalysts in the BTP reaction were investigated.
The synergetic effect was influenced by the states of the metals instead of the content. Fe-Cu/Beta
catalysts showed dramatically high catalytic activity due to the synergetic effect. 6Fe-6Cu/Betaachieved the highest phenol yield of 10.5 % in BTP reaction and MeOH yield of 720 µmol in MTM
reaction, which was related to the highest isolated Fe species influenced by the introduction of Cu.

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List of publications

List of publications
Journal articles
1.

2.

3.

4.

Peipei Xiao, Yong Wang, Junko N. Kondo, Toshiyuki Yokoi, Iron- and Copper-exchanged Beta
Zeolite Catalysts for Hydroxylation of Benzene to Phenol with H2O2, Chemistry Letters, 2018,
47(9). (Chapter 7)
Peipei Xiao, Yong Wang, Junko N. Kondo, Toshiyuki Yokoi, Direct synthesis of phenol byhydroxylation of benzene with hydrogen peroxide over Fe-containing MFI zeolite catalysts,
Catalysis Science & Technology, submission preparation. (Chapter 2)
Peipei Xiao, Yong Wang, Toshiki Nishitoba, Junko N. Kondo, Toshiyuki Yokoi, Dramatic impactsof the location of Fe species in Fe-silicalite-1 zeolites and solvent on liquid-phase methane
conversion to methanol with H2O2, in preparation. (Chapter 3)
Peipei Xiao, Yong Wang, Junko N. Kondo, Toshiyuki Yokoi, Direct synthesis of Fe-containing
MWW zeolite for direct hydroxylation of benzene to phenol with H2O2, in preparation. (Chapter
6)

Presentations
1.

2.

3.

4.

5.

6.

7.

8.

Peipei Xiao, Yong Wang, Masato Yoshioka, Ryoichi Otomo, Junko N. Kondo, Toshiyuki Yokoi,
Direct conversion of Methane to Methanol over Fe-containing zeolites/H2O2 system, the 21 st
JPIJS, May 25-26, 2016, Funabori, Tokyo. Poster presentation.
Peipei Xiao, Yoshihiko Kimura, Yong Wang, Junko N. Kondo, Toshiyuki Yokoi, Directconversion Methane to Methanol over Fe-containing zeolites/H2O2 system, the 16th international
congress on catalyst (ICC16), July 3-8, 2016, Beijing, China. Poster presentation.
Peipei Xiao, Junko N. Kondo, Toshiyuki Yokoi, Direct conversion of methane to methanol insolvent/H2O2 system using Fe-containing MFI catalyst prepared by newly developed direct
method, 2016 Zeolite Joint Study Group, December 17-18, 2016, Hakone, Tokyo. Oralpresentation.
Peipei Xiao, Yong Wang, Junko N. Kondo, Toshiyuki Yokoi, Direct conversion of Methane toMethanol over Fe-containing zeolites/H2O2 system, The 3rd Euro-Asia Zeolite Conference,
January 22-25, 2017, Bali, Indonesia. Poster presentation.
Peipei Xiao, Junko N. Kondo, Toshiyuki Yokoi, Direct hydroxylation of benzene to phenol using
H2O2 as an oxidant over Fe- or Cu-containing zeolite catalysts, the 16 th Korea-Japan Symposium
on Catalysis, May 15-17, 2017, Sapporo, Japan. Poster presentation.
Peipei Xiao, Toshiki Nishitoba, Junko N. Kondo, Toshiyuki Yokoi, Direct conversion of Methane
to Methanol over Fe-containing zeolites/H2O2 system, the 22 nd JPIJS, May 23-24, 2017,
Funabori, Japan, Poster presentation.
Peipei Xiao, Ryota Osuga, Yoshihiko Kimura, Junko N. Kondo, Toshiyuki Yokoi, Effect of Al
distribution in the MFI framework on the catalytic performance in the CH4 conversion, the 25th
North American Meeting of the Catalysis Society, June 4-9, 2017, Colorado, American. Poster
presentation.
Peipei Xiao, Junko N. Kondo, Toshiyuki Yokoi, Fe or/ and Cu- Beta catalysts: preparation,
characterization and catalytic performance in oxidation of benzene to phenol, 2017 International
Conference on Nanospace Materials, August 24-28, 2017, Shanghai, China. Poster presentation.
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List of publications

9.

Peipei Xiao, Junko N. Kondo, Toshiyuki Yokoi, One-pot synthesized Fe-containing MWW
zeolite for hydroxylation of methane to methanol with H2O2, the International CLS Forum on
Photo and Catalytic Science for Sustainable Society, March 3-4, 2018, Tokyo, Japan. Poster
presentation.
10. Peipei Xiao, Junko N. Kondo, Toshiyuki Yokoi, The effect of Fe site location in Fe-MFI zeolite
catalysts on the performance of hydroxylation of benzene to phenol with H2O2, 255th ACS
National Meeting, March 18-22, 2018, New Orleans, LA, America. Poster presentation.

211

Acknowledgement

Acknowledgement
The doctoral thesis was conducted at Nomura & Yokoi group, Laboratory for Chemistry and Life
Science & Nanospace Catalysis Unit, Institute of Innovative Research, Tokyo Institute of Technology
during 2015.10-2018.8. I sincerely appreciate all the members in our laboratory, my friends and family.
I am grateful to Assoc. Prof. Junko N. Kondo, Assoc. Prof. Keigo Kamata, Assoc. Prof. Ken
Motokura, Assoc. Prof. Yoshiaki Shoji, and Assoc. Prof. Toshiyuki Yokoi for taking time to review mythesis.
Firstly, I would like to express my deepest gratitude to Assoc. Prof. Junko N. Kondo and Assoc.
Prof. Toshiyuki Yokoi for providing me such an excellent opportunity to carry out my Ph.D. researchin the laboratory.
Nomura-sensei, you always gave me sincere help when I need. I am grateful for your guidance,
constructive suggestion, and encouragement throughout my studies. Thank you very much.
Yokoi-sensei, your tolerance, generosity and humor have always affected me. You supported mewith so many opportunities to attend the international and domestic academic conferences. You helped
me to modify the manuscripts over and over again. Thank you very much.
The financial support from Japan Student Services Organization (JASSO) and the Asahi Glass
Scholarship are highly appreciated.
Wang-san, you have put forward a lot of good suggestions for my research. You taught me to use
the NH3-TPD, FT-IR, TG, N2 adsorption and desorption about zeolite measurements. You helped me
to modify each chapter of the doctoral thesis, each academic manuscript and conference paper. Everyseminar you know my progress and point out my shortcomings. You and your wife have given me a
lot of help and encouragement, invited me in the reunion days, who was far from home. Thank youvery much.
Yunan-san, you carefully taught me to synthesize mesoporous silicon materials and mesoporous
carbon materials in the days when I first came to the lab. You taught me to use the FE-SEM (SU9000),
XRD, UV-vis, N2 adsorption and desorption about mesoporous materials measurements. You provided
me a lot of help in the daily life, I still use the rice cooker that you gave me three years ago. Thank youvery much.
Kunitake-san and Osuga-san, when I met trouble in the experiments and instruments, you guys
can always help me to solve the problems quickly and effectively. Thank you very much. Kunitakekun, the AEI zeolite you provided me, has applied in the BTP and MTM reactions with H2O2, which
proved that the small pore zeolite was not suitable for these two reactions. Osuga-san, the NO and CH4
adsorbed FT-IR on Cu/MFI(TPA) and Cu/MFI(PET+Na) zeolite catalysts that you helped me tomeasure, they were the start that I researched on the influence of aluminum distribution on the statesof the transition metals. Although, the two parts of work were not written in the doctoral thesis, I
believe that they can be published in the future. Thank you very much.
I am also appreciate Otomo-san, Bili-san, Kimura-kun and Fuji-kun who have already graduatedfrom our laboratory. In the old days, you offered me tremendous help and joy. Thank you very much.
Nakamura-sensei, I always admire your colorful life. You are so harmony, optimism and easygoing that make you just a senior friend in our lab.
Nishitoba-san, you are the very hard working person in our lab. Every time I did the experiments
late, only you were still in the lab. Congratulations on your PhD graduate. In the aspect of solid 29Si
MAS NMR and 27Al MAS NMR, and liquid H-NMR, incredible support have been provided by212

Acknowledgement

Nakamura-sensei and Nishitoba-san. Thank you very much.
Raquel-san, thank you very much to give me a worthy suggestion in terms of testing the type ofleaching Fe species, and it has been reported in Chapter 2. You have also set an example for me in the
aspect of publishing paper. It is a kind of driving force for me to make progress. Thank you very much.
Park-san, you have made great efforts to maintain the normal operation of the laboratory. Thankyou very much.
Thanks to all the young members in Nomura &Yokoi laboratory and the secretaries, Hirose-sanand Furuzono-san, thank you very much for maintaining such a harmonious laboratory.
I would like to express heartfelt thanks to my former supervisors, Dr. Chuyi Zeng and Assoc. Prof.
Hong Zhao. You are my advisors and friends. You always encouraged me to pursue my dreams bravely
and you also help me greatly. There is no doubt that I can not be here today without yourencouragement and help. Thank you very much.
My dear friends, who climbed the Mount Fuji together, we have experienced the hardest climb,
as well as enjoyed the most beautiful meteor shower, sunrise and sea of clouds.
I greatly appreciate my boyfriend, Dai Xin. Thank you for making delicious food for me in every
weekend, which can be proved by my double chin. Thank you for walking outside with me in weekend,
for example taste pickled vegetables in Kamakura, I think Yokoi-sensei can be the witness. You always
tell me to relax and then inviting me to watch variety show, anime, World Cup and play Nintendo
games, even at the last day of the deadline. Thank you for taking me so many photos, then I have astack of goofy photos. All of these makes every routine day colorful.
Thanks uncle Dai Fuming and aunt Zhu Aimei. Thank you for sending me new clothes,
comfortable shoes, and a variety of homemade food. You are so attentive about our healthy and daily
life that makes us feel to be concerned about all the time. Thank you very much.
Special thanks to my family, Papa, Mama, Dabo, Xiaobo, Jingge and Ma Diandian, thank you foryour understanding, support and encouragement. On my way of studying these years, my family isalways the warmest haven, let me fill with the anticipation to the future. It is my greatest wish that myfamily will always be healthy and happy!
Aug 21, 2018
Xiao Peipei

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