# Characterization and Catalytic Performance of Niobic Acid Dispersed over Titanium Silicalite

Authors: Didik Prasetyoko; Zainab Ramli; Salasiah Endud; Hadi Nur
Journal: Advances in Materials Science and Engineering
 (2008)

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## Abstract

Niobic acid,Nb2O5⋅nH2O, has been supported on the titanium silicalite by impregnation method. The obtained materials were characterized by X-ray diffraction, infrared, and ultra-violet—visible diffuse reflectance spectroscopy, temperature programmed reduction, pyridine adsorption, and field emission scanning electron microscopy techniques. It was demonstrated that the tetrahedral titanium species still retained after impregnation of niobic acid. The results revealed that niobium species interacted with hydroxyl groups on the surface of TS-1. The niobium species in the catalysts are predominantly polymerized niobium oxides species or bulk niobium oxide with the octahedral structure. All catalysts showed both Brønsted and Lewis acid sites. The catalysts have been tested for epoxidation of 1-octene with aqueous hydrogen peroxide. It was found that the presence of niobic acid in the catalysts enhanced the rate of the formation of epoxide at the initial reaction time. Diol as a side product was also observed due to the acidic properties of the catalysts.


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## Body

## 1. Introduction

Niobium oxides and its
compounds are interesting and important materials in catalysis with various
functions such as promoter, support, redox materials, and acid catalysts [1–3].
The materials-containing niobium have been pointed out as effective catalysts
in the many processes, for example, dehydration of alcohols, hydrolysis, oxidation,
esterification, alkylation, isomerization, and photocatalysis [1, 2, 4].Among the niobium compounds,
hydrated niobium pentoxide (niobic acid, Nb2O5⋅nH2O)
has shown high catalytic activity, selectivity, and stability for acid catalyzed
reactions [5, 6]. The surface acid strength of niobic acid corresponds to the acid
strength (Ho≤−5.6) of 70% sulfuric acid. Niobic acid is an unusual
solid acid since any kinds of acidic metal oxides show decreasing of the acidic
property by absorbing water, while niobic acid shows high acid strength in
spite of its water content [2, 7, 8].The attachment of niobium oxide
over the large surface area of the supports favors the enhancement of the
active sites, since the niobium oxide itself exhibits low surface area. The
different surface species and catalytic activity produced depend on the support
materials, niobium precursors, amount of niobium loading, and the experiment
conditions. Jehng and Wachs [9] claimed that the molecular structure of the surface
niobium species on SiO2 dependent on the preparation method. They
also reported that the surface niobium species on TiO2 gave rise to
acidic products. Low amount of niobium oxide loading (1 wt%) on silica is
predominantly composed of isolated NbO4 units, while the Nb cations
in the supported Nb2O5/SiO2 catalysts possess polymerized
NbOx species and/or bulk Nb2O5 with increasing
Nb loading [10–12]. It has been reported that for supported niobium oxide on
silica, both Brønsted and Lewis acid sites remains bound to the support surface
even in the presence of water and acts as an effective catalyst for
acid-catalyzed reactions [13–17]. We have reported recently in the preliminary
study that the surface hydroxyl groups in the titanium silicalite (TS-1) have bounded
with niobium species in the Nb2O5/TS-1 catalyst [18].
This catalyst is active for epoxidation of 1-octene with aqueous hydrogen
peroxide. However, the effect of the amount of niobium loading to the catalyst
properties and activities has not been studied. This paper reported in detail
of preparation, characterization and catalytic performance of niobic acid
supported on TS-1, NBA/TS-1.


## 2. Experimental

### 2.1. Preparation of Sample

Titanium silicalite, TS-1 (Ti  =  1% mol) was prepared according to the procedure described earlier [19]. Sample
Nb/TS-1 with various loading of niobium was prepared by impregnation technique
using niobium ethoxide Nb(OC2H5)5 (Aldrich, 99.95%) precursor. TS-1 was dried in an
oven at 200°C for 24 hours. After that, the necessary amount of
niobium ethoxide was dissolved in n-hexane (Aldrich, >99%) to obtain the
desired metal loading, and the required quantity of a predried TS-1 was
immediately added to the clear solution with stirring. The mixture was stirred
at room temperature for 3 hours. The solid was recovered by evaporating the
n-hexane at 80°C. The acid hydrolysis was performed by addition of
20 mL solution of 0.5 M HNO3 in distilled water and aged overnight,
followed by drying at 120°C. The solid was then washed with
distilled water for three times and finally dried at 200°C for 24 hours.
Sample denoted as XNb/TS-1, in which X is the percentage of
niobium in wt%.For comparison on the presence
of Nb species, experiments were carried out to obtain niobium oxide and niobic
acid. Niobium oxide was prepared by calcination of ammonium niobium oxalate
(99.9%, Aldrich) at 550°C for 3 hours. The prepared particles were
labeled as NBO. Niobic acid, Nb2O5nH2O was
prepared by hydrolysis of niobium ethoxide in the neutral hydrolysis method.
Niobium ethoxide was first dissolved in the n-hexane and water was then added
to this solution drop by drop with stirring and the stirring was continued for
an hour. The solid was recovered by evaporation of solvent, followed by drying
at 100°C. Finally, the solid was calcined at 200°C for 3
hours and the resulting solid was named as NBA. Table 1 presents the code,
niobium content, and preparation method of the samples.Table 1
Chemical compositions of the samples and
preparation method.
SampleCodeNb/(Nb+TS-1), wt.%MethodTitanium silicaliteTS-10.81 (gel)(*)Direct synthesisNb2O5⋅nH2O/
TS-13Nb/TS-13.42Impregnation &
HydrolysisNb2O5⋅nH2O/TS-110Nb/TS-19.92Impregnation &
HydrolysisNb2O5⋅nH2O/TS-120Nb/TS-119.78Impregnation &
HydrolysisNb2O5NBO100CalcinationNb2O5⋅nH2ONBA100Hydrolysis(*)%Ti  =  Ti/(Ti+Si) in the initial gel.


### 2.2. Characterization

The catalysts structures and
properties were determined by X-ray diffraction, Fourier-transform infrared and
ultra violet—visible diffuse
reflectance spectroscopy, temperature programmed reduction, and pyridine
adsorption techniques. All molecularsieves were characterized by powder X-ray
diffraction technique for identification of the crystalline phases in the
catalysts using a Bruker Advance D8 diffractometer with the Cu Kα (λ=1.5405 Å)
radiation as the diffracted monochromatic beam at 40 kV and 40 mA. The pattern
was scanned in the 2θ range of 5–50° at
a step size 0.020° and step time 1 second. Infrared spectra
of the samples were collected on a Shimadzu Fourier transform Infrared, with a
spectral resolution of 2 cm−1, scans 10 seconds, at 20°C by KBr pellet
method. The framework spectra were recorded in the region of 1400–400 cm−1.
The structure of niobium species in the samples was monitored by UV-vis DR
spectroscopy
technique. UV-vis DR
spectra were recorded under ambient conditions on a Perkin Elmer Lambda 900
UV/VIS/NIR spectrometer. The samples were loaded in a quartz cell. The spectra
were monitored in the wavenumber range 190–600 nm and Kubelka-Munk
axis, using polytetrafluoroethylene polymer as a standard background. Temperature
programmed reduction experiments were performed using TPDRO 1100 Thermo Quest
CE Instrument as follows. The sample (typically about 0.05 g) was pretreated in
nitrogen at a flow rate of 30 mL/min at 200°C for 1 hour and cooled
down to 100°C.
The reduction analysis was performed by heating the sample from 100 up to 1000°C at a
rate of 10°C/min
and held for 5 minutes at 1000°C in a flowing mixture of 5% hydrogen
in nitrogen (H2/N2) at the rate of 40 mL/min. For
hydroxyl groups and acidity study, about 10 mg of sample was pressed at 3 tones
for a minute to obtain a 13 mm disk. The sample was introduced in the infrared
cell with calcium fluorite, CaF2 windows. The sample was heated at
200°C in vacuum condition for 16 hours. The residual vacuum
reached in the IR cell prior to recording spectra was 1×10e−6mbar. The infrared spectra were collected at room temperature
using Shimadzu 2000 FTIR spectrometer at 2 cm−1 resolution. The
types of acid sites were examined using pyridine as a probe molecule. Pyridine
was adsorbed at room temperature for a minute, continued by desorption at 150°C
for an hour. The infrared spectra were recorded at room temperature. Morphology
of the solid samples was monitored by field emission scanning electron
microscopy technique using electron microscope model Philip XL40.


### 2.3. Performance of Catalyst

The catalyst performance was
tested in the epoxidation of 1-octene using aqueous H2O2 (30%) as oxidant. The reaction mixture containing 1-octene (8 mmol), H2O2 (8 mmol), and acetone (10 g) as solvent was put in a round bottom flask
equipped with a condenser. The catalyst (0.05 g TS-1 based) was then added to
the solution. The reaction was carried out in an oil bath under stirring at 70°C.
The products of the reaction were analyzed by a Hewlett-Packard 6890 N gas
chromatograph using an Ultra-1 column and a Hewlett-Packard GC-MSD instrument
using an HP5 column.


## 2.1. Preparation of Sample

Titanium silicalite, TS-1 (Ti  =  1% mol) was prepared according to the procedure described earlier [19]. Sample
Nb/TS-1 with various loading of niobium was prepared by impregnation technique
using niobium ethoxide Nb(OC2H5)5 (Aldrich, 99.95%) precursor. TS-1 was dried in an
oven at 200°C for 24 hours. After that, the necessary amount of
niobium ethoxide was dissolved in n-hexane (Aldrich, >99%) to obtain the
desired metal loading, and the required quantity of a predried TS-1 was
immediately added to the clear solution with stirring. The mixture was stirred
at room temperature for 3 hours. The solid was recovered by evaporating the
n-hexane at 80°C. The acid hydrolysis was performed by addition of
20 mL solution of 0.5 M HNO3 in distilled water and aged overnight,
followed by drying at 120°C. The solid was then washed with
distilled water for three times and finally dried at 200°C for 24 hours.
Sample denoted as XNb/TS-1, in which X is the percentage of
niobium in wt%.For comparison on the presence
of Nb species, experiments were carried out to obtain niobium oxide and niobic
acid. Niobium oxide was prepared by calcination of ammonium niobium oxalate
(99.9%, Aldrich) at 550°C for 3 hours. The prepared particles were
labeled as NBO. Niobic acid, Nb2O5nH2O was
prepared by hydrolysis of niobium ethoxide in the neutral hydrolysis method.
Niobium ethoxide was first dissolved in the n-hexane and water was then added
to this solution drop by drop with stirring and the stirring was continued for
an hour. The solid was recovered by evaporation of solvent, followed by drying
at 100°C. Finally, the solid was calcined at 200°C for 3
hours and the resulting solid was named as NBA. Table 1 presents the code,
niobium content, and preparation method of the samples.Table 1
Chemical compositions of the samples and
preparation method.
SampleCodeNb/(Nb+TS-1), wt.%MethodTitanium silicaliteTS-10.81 (gel)(*)Direct synthesisNb2O5⋅nH2O/
TS-13Nb/TS-13.42Impregnation &
HydrolysisNb2O5⋅nH2O/TS-110Nb/TS-19.92Impregnation &
HydrolysisNb2O5⋅nH2O/TS-120Nb/TS-119.78Impregnation &
HydrolysisNb2O5NBO100CalcinationNb2O5⋅nH2ONBA100Hydrolysis(*)%Ti  =  Ti/(Ti+Si) in the initial gel.


## 2.2. Characterization

The catalysts structures and
properties were determined by X-ray diffraction, Fourier-transform infrared and
ultra violet—visible diffuse
reflectance spectroscopy, temperature programmed reduction, and pyridine
adsorption techniques. All molecularsieves were characterized by powder X-ray
diffraction technique for identification of the crystalline phases in the
catalysts using a Bruker Advance D8 diffractometer with the Cu Kα (λ=1.5405 Å)
radiation as the diffracted monochromatic beam at 40 kV and 40 mA. The pattern
was scanned in the 2θ range of 5–50° at
a step size 0.020° and step time 1 second. Infrared spectra
of the samples were collected on a Shimadzu Fourier transform Infrared, with a
spectral resolution of 2 cm−1, scans 10 seconds, at 20°C by KBr pellet
method. The framework spectra were recorded in the region of 1400–400 cm−1.
The structure of niobium species in the samples was monitored by UV-vis DR
spectroscopy
technique. UV-vis DR
spectra were recorded under ambient conditions on a Perkin Elmer Lambda 900
UV/VIS/NIR spectrometer. The samples were loaded in a quartz cell. The spectra
were monitored in the wavenumber range 190–600 nm and Kubelka-Munk
axis, using polytetrafluoroethylene polymer as a standard background. Temperature
programmed reduction experiments were performed using TPDRO 1100 Thermo Quest
CE Instrument as follows. The sample (typically about 0.05 g) was pretreated in
nitrogen at a flow rate of 30 mL/min at 200°C for 1 hour and cooled
down to 100°C.
The reduction analysis was performed by heating the sample from 100 up to 1000°C at a
rate of 10°C/min
and held for 5 minutes at 1000°C in a flowing mixture of 5% hydrogen
in nitrogen (H2/N2) at the rate of 40 mL/min. For
hydroxyl groups and acidity study, about 10 mg of sample was pressed at 3 tones
for a minute to obtain a 13 mm disk. The sample was introduced in the infrared
cell with calcium fluorite, CaF2 windows. The sample was heated at
200°C in vacuum condition for 16 hours. The residual vacuum
reached in the IR cell prior to recording spectra was 1×10e−6mbar. The infrared spectra were collected at room temperature
using Shimadzu 2000 FTIR spectrometer at 2 cm−1 resolution. The
types of acid sites were examined using pyridine as a probe molecule. Pyridine
was adsorbed at room temperature for a minute, continued by desorption at 150°C
for an hour. The infrared spectra were recorded at room temperature. Morphology
of the solid samples was monitored by field emission scanning electron
microscopy technique using electron microscope model Philip XL40.


## 2.3. Performance of Catalyst

The catalyst performance was
tested in the epoxidation of 1-octene using aqueous H2O2 (30%) as oxidant. The reaction mixture containing 1-octene (8 mmol), H2O2 (8 mmol), and acetone (10 g) as solvent was put in a round bottom flask
equipped with a condenser. The catalyst (0.05 g TS-1 based) was then added to
the solution. The reaction was carried out in an oil bath under stirring at 70°C.
The products of the reaction were analyzed by a Hewlett-Packard 6890 N gas
chromatograph using an Ultra-1 column and a Hewlett-Packard GC-MSD instrument
using an HP5 column.


## 3. Results and Discussion

### 3.1. Catalysts Characterization

In this study, niobium ethoxide
was used as a precursor in the preparation of Nb/TS-1. Since the molecular size
of niobium ethoxide is larger than the pore size of TS-1, theoretically, the
niobium species could not possibly enter the pores of TS-1, thereby should be
dispersed on the surface of TS-1 only. In addition, in the preparation of the
samples, the acidic condition used in the hydrolysis step to increase the
formation of niobic acid, since aqueous solution at pH < 3 produced mainly
of niobic acid [9].Figure1 shows the XRD patterns
of the samples. The structure of the samples and the crystallinity of niobium
containing TS-1 (based on the highest diffraction line of TS-1) are summarized
in Table 2. For niobic acid sample, NBA prepared by hydrolysis of niobium pentoxide,
no diffraction lines can be observed on the XRD pattern, suggesting that the
niobic acid phase is fully amorphous. On the other hand, niobium oxide sample, NBO
prepared by calcination of ammonium niobium oxalate at 550°C shows
splitting of the diffraction line at around 2θ=29°, indicating that
the sample is crystalline niobium oxide phase (T-Nb2O3)
with orthorhombic structure [20]. Similar orthorhombic phase of niobium oxide
has been prepared by Paulis et al. [21] for the preparation of niobium oxide
from a precursor solution of niobium chloride in ethanol, and calcined at 700°C.
The XRD patterns of samples XNb/TS-1 reveal that the samples contain
framework structures of the MFI type zeolite. The observations of samples
XNb/TS-1 show that the structure of TS-1 is not strongly affected by the
presence of impregnated niobium. No diffraction line assigned for crystalline
phase of the niobium oxide is present in the XRD pattern. The result indicated
that niobium was well dispersed over TS-1. This also suggested that the niobium
species was present in the form of amorphous niobium oxide hydrate, since it
was prepared by hydrolysis of niobium ethoxide and was not calcined at
temperature higher than 400°C. In addition, the peak intensity of
TS-1 is drastically decreased to 60% after impregnation of 3.4 wt% of niobium
on the TS-1 (sample 3Nb/TS-1). Moreover, its intensity decreased significantly
as niobium loading increased. It is suggested that niobium could be located on
the surface of TS-1 or covering the surface of TS-1. Since the size of niobium
precursor, that is, Nb(OEt)5 is ca. 1.1 nm is larger than the
pore-entrance size of TS-1 (ca. 0.55 nm), hence, the niobium species
most probably be attached to the external surface of TS-1 [18].Table 2
Structure and crystallinity of samples.
CodeIntensity, cpsStructureTS-11957MFI3Nb/TS-11142MFI10Nb/TS-1942MFI20Nb/TS-1585MFINBO—OrthorhombicNBA—AmorphousFigure 1
XRD pattern of the NBA, NBO, TS-1, andXNb/TS-1 samples.The infrared spectra of the
samples in the lattice vibration region between 1400 and 400 cm−1 are depicted in Figure 2. Niobic acid (sample NBA) shows a major strong band
centered at 586 cm−1 and a shoulder band at 933 cm−1. This spectrum
is consistent with the skeletal vibration previously reported for amorphous
niobic acid [12, 21, 22]. The spectrum of niobium oxide crystals (sample NBO)
exhibits two strong bands at 615 and 850 cm−1. The broad band at a
higher frequency  ca. >850 cm−1 is associated with the stretching
mode of Nb=O bonds (niobyl species) in a highly distorted NbO6 groups, while
the band around 600 cm−1 can be assigned to the stretching of longer bridging
Nb–O–Nb bonds [22]. For the infrared spectra of samples XNb/TS-1, a
small band at around 970 cm−1 characteristic for titanium ions in
the tetrahedral structure is still present after impregnation of niobium. No
additional band after impregnation of niobium into the TS-1 can be observed.
This finding shows that impregnation of niobium has not affected the MFI structure
of TS-1 significantly. Infrared spectroscopy technique was not able to detect
the presence of niobic acid in the samples XNb/TS-1, due to the low
vibration intensity of pure niobic acid itself as compared to sample TS-1.Figure 2
FTIR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples.The UV-vis DR  spectra of the samples NBA,
NBO, TS-1, and Nb/TS-1 are shown in Figure3. The UV-vis spectrum of NBO sample
exhibits a maximum band at around 295 nm and a shoulder band at around 243 nm,
while sample NBA shows a sharp band at around 280 nm and a shoulder band at
around 240 nm. These bands are characteristic for distorted octahedral and
tetrahedral species, respectively, [22, 23]. These bands are attributed to the
charge transfer transitions O2− to Nb5+, which can be associated
to the energy gap between the O 2p-valence band and the Nb 4d-conductance band
[22]. In the supported niobium oxide, the position and width of the UV
absorption bands of niobium oxide species were observed to depend on the
niobium content and the size of niobia crystallites. The decrease in niobium
concentration leads to a blue shift of the charge transfer band, caused by
decreasing of the size of niobium oxide species. With decreasing of niobium
loading, the LMCT peak maximum shifts from a higher to a lower wavelength. Gao et
al. [11] found that in the niobium oxide compounds containing polymerized NbO6 structures with five or six Nb–O–Nb bonds around the central Nb atom, the
decrease of the layer thickness of the layered niobium oxide compounds slightly
increases the edge energy. Sample TS-1 shows a single strong band at 215 nm
characteristic for tetrahedral titanium framework. Impregnation of niobium on
the TS-1 (sample 3Nb/TS-1) shows a strong band at around 240 nm assigned to
octahedral niobium species. The intensity of this band in the samples XNb/TS-1
increased as the niobium content in the samples increased. Meanwhile, the band
characteristic for tetrahedral titanium at about 215 nm cannot be observed due
to the overlapping of this band with the band of niobium oxide that has much
higher intensity. In addition, this charged transfer band shifted to the higher
wavelength as the niobium loading increased. The samples 10Nb/TS-1 and
20Nb/TS-1 showed a shoulder band at around 245 nm and an intense band at around
280 nm, in which sample 10Nb/TS-1 has lower intensity bands than that of sample
20Nb/TS-1. These bands were also observed on bulk niobic acid (sample NBA),
suggesting the presence of similar species in this sample. The shift of the UV-vis
band position as the metal oxide content increases is generally attributed to
the enhancement of degree of polymerization. Therefore, in this study, the
increase of niobium content has led to an increase in the degree of
polymerization of the niobium species. According to Tanaka et al. [24], Nb2O5/Al2O3 with low niobium loading could be represented by monomer or dimmer species,
whereas at high niobium loading, the samples could be identified by the presence
of Nb2O5 particles.Figure 3
UV-Vis DR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples.Figure4 shows the TPR profiles
of the samples. The TPR profile of sample NBO: crystalline niobium oxide
displays a low signal and two high signals of hydrogen consumption centered at
around 680°C, 870°C, and higher than 1000°C
(not complete). Sample NBA prepared by hydrolysis of niobium ethoxide and
calcined at 200°C, exhibited three regions of hydrogen consumption, that
is, a negative peak at around 550°C and the high positive peaks at
around 900°C and at a higher temperature above 1000°C. The
negative peak can be attributed to CO or CO2 formation due to carbon
residue from the niobium ethoxide that was not fully hydrolyzed. The first
positive peak was attributed to the reduction of Nb5+ to Nb4+ in the form of Nb2O5 to NbO2 [1], while for reduction
of NbO2 to lower oxidation state of niobium, NbO needed higher
reduction temperature. The TPR profile of sample 20Nb/TS-1 shows a major peak
at around 925°C, which is also observed on amorphous bulk Nb2O5,
suggesting the presence of similar species of niobium on both samples. The
reduction temperature of sample 20Nb/TS-1 is slightly higher than that of
sample NBA, indicating that the interaction between niobic acid and the surface
of TS-1 exists. On the contrary, samples 3Nb/TS-1 and 10Nb/TS-1 do not show any
significant peak in the reduction temperature range, although the sample
contains niobium. This finding is explained to be due to several factors. Low
amount of niobium loading results in high dispersion of niobium on the surface
of TS-1 that promotes interaction between niobium and TS-1. Pereira et al. [25]
found that high dispersions promote intimate contact between niobium and the
silica surface, Nb–O–Si bonding in
relation to Nb–O–Nb bonding
prevailing. The formation of a surface of niobia monolayer is derived from the
Nb–O–Si bonding [26]. It also can be explained based on the electronegativity
concept that the presence of silicon in the coordination sphere of niobium
should increase the electronic density niobium. Thus, the reduction of niobium
from Nb+5 to Nb+4 was more difficult to occur [25].
Recently, Mendez et al. [27] found that 20 wt% of niobium supported on Al2O3 did not show any hydrogen uptake as an indication of the strong interaction of
niobium with Al2O3.Figure 4
TPR profiles of the NBA, NBO, andXNb/TS-1 samples.The hydroxyl groups and acidity
of the samples were monitored by infrared spectroscopy technique. The wafer
sample was put in the glass cell and evacuated at 200°C for 16 hours
under vacuum. The infrared spectra were recorded at room temperature. Figure 5
shows the infrared spectra of the samples TS-1, NBA, NBO, XNb/TS-1, and
a mechanical mixture of NBA and TS-1 (sample MM-20NBA+TS-1, 20 wt% of NBA) after
evacuation at 200°C. TS-1 shows a sharp peak at around 3743 cm−1 characteristic of hydroxyl silanol groups. Niobic acid (sample NBA) shows a
small peak at around 3700 cm−1 and a broad peak centered at around
3400 cm−1 that can be assigned to hydroxyl stretching mode of free
Nb–O–H groups and hydroxyl with hydrogen bonding, respectively, [22, 28]. On the
other hand, crystalline niobium oxide does not show any peak in the vibration
region of hydroxyl groups indicating that crystalline niobium oxide does not
contain any hydroxyl groups. A different finding was reported recently by Braga
et al. [12] for niobium oxide calcined at 800°C. They observed the
peaks at around 3748 cm−1 and 3526 cm−1 assigned to
terminal hydroxyl bonded to a slightly distorted NbO6 and hydroxyl
with hydrogen bonding, respectively. The peak at around 3743 cm−1 disappeared as niobium was loaded on the TS-1. A small peak at around 3733 cm−1 and a broad peak centered at around 3530 cm−1 were observed in
samples XNb/TS-1. In addition, a peak at around 3733 cm−1 of
sample 20Nb/TS-1 has higher intensity as compared to samples 3Nb/TS-1 and
10Nb/TS-1. Meanwhile, a mechanical mixture of 20 wt% NBA in TS-1 only shows a
peak similar to that of TS-1 at around 3744 cm−1 assigned for
silanol hydroxyl groups. Although this mixture contains 20 wt% of NBA, however,
no peak at around 3700 cm−1 assigned for Nb–O–H can be observed. It suggests
that the amount of hydroxyl groups of Nb–OH in the sample NBA is much lower
than the amount of silanol groups of TS-1. For samples XNb/TS-1, a
drastic decrease of silanol species present on the surface of TS-1 indicates
that there is strong interaction between niobium and TS-1 (niobium bonded with
–O–Si, replacing hydrogen atom of silanol species in the TS-1). Since niobic
acid does not show any peak in this region, the results clearly suggests that
the niobium species has covered the surface of TS-1. Therefore, depositions of
niobium oxide species on TS-1 consume the surface Si–OH groups of TS-1.
Consequently, the amount silanol groups decreased.Figure 5
FTIR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples in the hydroxyl vibration region, after evacuation under vacuum at 200°C.The effect of evacuation
temperature on the hydroxyl groups present in the sample was studied for sample
3Nb/TS-1 at evacuation temperatures 120, 200, and 400°C (Figure 6). The spectrum of sample 3Nb/TS-1 after evacuation at 120°C for 24 hours
shows a small peak at around 3735 cm−1 and a broad peak with high
intensity center at around 3500 cm−1 characteristic for hydroxyl
groups of silanol and hydrogen bonding, respectively. After evacuation at 200°C
for 24 hours, the peaks for silanol groups are still maintained, but the peak
for hydrogen bonding is shifted to a higher frequency at around 3530 cm−1 with lower intensity. Continuing evacuation at 400°C for an hour, results
in slightly higher intensity of silanol but still at similar frequency, while
the hydroxyl of hydrogen bonding becomes a shoulder peak and shifted to the
higher frequency at around 3600 cm−1. The decreasing of hydroxyl
groups of hydrogen bonding and increasing of silanol groups indicates that the
hydroxyl groups of hydrogen bonding have interacted with the silanol groups.
Therefore, as evacuation temperature increased, the hydroxyl groups of hydrogen
bonding decreased, consequently, the silanol increased.Figure 6
FTIR spectra of the sample 3Nb/TS-1 in the hydroxyls region after evacuation under vacuum at temperature 120, 200, and 400°C.The type and amount of acid sites
in the sample were monitored and measured by pyridine adsorption. Pyridine was
adsorbed at room temperature after evacuation of sample at 200°C for
16 hours in vacuum. The infrared spectra were collected after evacuation of
pyridine at 150°C for an hour. Figure 7 shows the infrared spectra
of the samples in the pyridine region. Sample NBO does not show any peak which
indicates that crystalline niobium oxide has no acidity. Sample NBA shows the
peaks at 1636, 1609, 1575, 1540, 1489, and 1448 cm−1. The peaks at
around 1540 and 1448 cm−1 are due to Brønsted and Lewis acid sites,
respectively. Similar findings have been reported by Morais et al. [29] and da
Silva et al. [30] for niobic acid. It has been reported also that the acidity
of niobic acid also depends on the calcination temperature [31]. The number of
Lewis acid sites increased with increasing pretreatment temperatures up to 500°C
and then decreased at higher temperatures. On the other hand, Brønsted acid
sites decreased with increasing temperature above 100°C. As the
number of coordinated water molecules increased, the number of Brønsted acid
sites decreased, the concentration of hydronium ions (H3O+)
increased, and the formation of H2O–HO species is observed [32].Figure 7
FTIR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples after evacuation at 200°C for 24 hours followed by pyridine adsorption at room temperature and evacuation at 150°C for one hour, in the pyridine regions.Niobium oxide samples containing
TS-1 showed the peaks similar to that of sample NBA. However, the intensity of
the characteristic peaks for Brønsted and Lewis acid sites at around 1540 and
1448 cm−1, respectively, was lower than that of sample NBA. Jehng
and Wachs [13] and Shirai et al. [15] have reported the evidence of the presence
of both Brønsted and Lewis acid sites in niobium oxide supported on the silica,
while Mendes et al. [27] only found Lewis acid sites in silica supported niobic
acid. In this study, a mechanical mixture of niobic acid and TS-1 (sample
MM-20NBA+TS-1) only shows the peaks at around 1608, 1489, and 1447 cm−1,
which are similar to those of sample TS-1. This finding indicates that the
sample NBA in sample MM-20NBA+TS-1 contained low amount of acid sites.The infrared spectra shown in
Figure7 confirm that all samples XNb/TS-1 have Brønsted and Lewis acid
sites. The amount of Brønsted and Lewis acid sites calculated on the basis of
the peak at around 1540 and 1448 cm−1 are tabulated in Table 3. It showed
that the amount of Brønsted and Lewis acids increases with the increasing niobium
loading on the support. The area under the peak at around 1490 cm−1 characteristic
for the mixture of Brønsted and Lewis acids also increased as the niobium loading
increased. This finding indicates that niobium plays a role in the formation of
both Brønsted and Lewis acid sites. Similar findings have been reported by
Jehng and Wachs [13] and Shirai et al. [15] for niobium oxide supported on
silica.Table 3
The amount of the Brønsted and Lewis acid sites and
the area under the peak at around 1490 cm−1 of the samples.
SampleBrønsted,μmol/gLewis,μmol/gLewis added,μmol/g(#)Peak area at 1490, cm−2(*)TS-1 (1%)030—0.143Nb/TS-11848180.4610Nb/TS-14763330.5220Nb/TS-16672420.93NBO00—0NBA101145—1.75(#)Additional
amount of the Lewis in the samples Nb/TS-1, calculated by deduction of Lewis
acid in the samples Nb/TS-1 by sample TS-1.
(*)Area of the peak
at around 1490 cm−1, characteristic for the mixture of Brønsted and
Lewis acid sites.Figure8 shows the correlation
between niobium content (wt%) in the solid samples and the amount of the
Brønsted and Lewis acid sites. It shows that addition of niobium onto TS-1 up
to 10 wt% results in the increase amount of Brønsted acid sites. Although the
amount of Brønsted acid sites keep on increasing with further increased in
niobium loading up to 20 wt%, the additional amount of Brønsted acid produced
is slightly lower compared to that of the first addition (sample 10Nb/TS-1).
This finding suggests that not all of the added niobium creates Brønsted acid
sites. Based on this observation, it is suggested that the Brønsted acid sites
may be present only on the surface of niobic acid. Similar pattern is observed
also for the formation of Lewis acid sites.Figure 8
Correlation between the niobium content and the amount of acid sites of the catalysts.According to Jehng and Wachs
[13, 14], interaction between Nb2O5 and acidic surfaces
(SiO2 and Al2O3) resulted in the formation of
slightly distorted NbO6, NbO7, and NbO8 sites.
The slightly distorted NbO6, NbO7, and NbO8 groups only possess Nb–O bonds and are associated with Brønsted acid sites. The
highly distorted NbO6 octahedral possesses Nb=O bonds in which
associated with Lewis acid sites. In this study, it is found that the increased
amount of Lewis acid sites present in the sample with low amount of niobium
loading (3Nb/TS-1) is similar with the increased amount of Brønsted acid sites.
This suggests that the amount of slightly distorted NbO6, NbO7,
and NbO8 groups is similar with those of highly distorted NbO6.
However, at higher niobium loading, it is found that the increased amount of
Lewis acid is slightly lower than that of Brønsted acid. It indicates that at
higher niobium loading, the presence of slightly distorted NbO6, NbO7,
and NbO8 groups is more prominent. This finding was supported by the
results obtained from the TPR and infrared of hydroxyl groups data, which
indicate the interaction between niobium species with the support TS-1
(Nb–O–Si). Since Nb is more electronegative compared to Si or Ti (Pauling value
of Nb, Si, and Ti are 1.6, 1.0, and 1.5, resp.) means that niobia oxide
shows more ionic character. Therefore, generation of Lewis acid sites in the
samples XNb/TS-1 can be a consequence of the higher ionicity of the Nb–O
bond. This suggests that higher Lewis acid could be generated in the first
layer of the supported niobium oxide on TS-1.Further explanation is
demonstrated by pyridine adsorption, which shows that Nb-loaded TS-1 has
Brønsted acid sites. On the other side, sample physical mixing of niobic acid
and TS-1 (20 wt% NBA) only shows Lewis acid sites. In this sample, there is no
chemical interaction between Nb–O–Si. The acid sites in this sample are purely
come from sample NBA with a small amount of acid sites. Meanwhile, in the
Nb/TS-1 sample, the interaction between Nb–O–Si occurs. Therefore, generation of
Brønsted acid sites can be expected since protons are required to balance the
excess negative charges on the oxygen atoms, analog of Al atom in the
aluminosilicate molecularsieves. Generation of Brønsted acid sites is also in accordance with Kung’s assumptions [33, 34]. They explained two models, that is,
(1) an electronic interaction exists between Nb2O5 and
TS-1 components, and (2) Nb–O bonds neighboring the more covalent Si–O possess
a higher ionicity. It means that the electron density of the OH bonds is
reduced by the inductive effect of the nearby electronegative anions. As a
consequence, this weakens the OH bond and generates the Brønsted acid sites.
Furthermore, based on the XPS data by Noronha et al. [35] and Bosman et al. [36]
found the increased in the covalency of Si–O bonds in binary SiO2–TiO2 and SiO2–ZrO2 systems, respectively, that leads to the
creation of Brønsted acid sites.Samples morphology were
monitored using field emission scanning electron microscope. Figure9 shows the
FESEM images of samples TS-1 and 10Nb/TS-1 material. The FESEM image shows that
TS-1 has cubic morphology with the crystal size around 100 nm. Meanwhile, a
grubby TS-1 crystal is observed in the sample 10Nb/TS-1 with the crystal size
similar to that of TS-1 sample, that is, around 100 nm. Based on the UV-vis DR
finding,
this sample contained niobic acid. Therefore, it is concluded that a small particle
observed on the surface of TS-1 is niobic acid. However, the morphology of TS-1
is not affected by impregnation of niobic acid.The FESEM images of the materials (a) TS-1 and (b) 10Nb/TS-1.
(a)(b)


### 3.2. Performance of Catalysts

The catalytic activities of the
TS-1, niobium compounds, and niobium-containing TS-1 samples were tested in the
epoxidation of 1-octene with aqueous H2O2 as oxidant in
acetone at 70°C. Table 4 shows the catalytic results of the samples.
For the conversion of 1-octene after 48 hours of reaction time (Table 4), only
below 0.2 mmol of 1-octene has converted using samples NBO and NBA. This
conversion is much lower as compared to the 1-octene conversion catalyzed by
samples TS-1 and Nb/TS-1, that is, up to 1.6 mmol 1-octene. Meanwhile, it is
found that the catalytic activity of samples TS-1 and Nb/TS-1 in term of
conversion of 1-octene are not much different. This finding suggests that the
niobium content is not taken into account of the 1-octene conversion. The low
conversion of 1-octene on the samples NBO and NBA supported this implication.Table 4
Catalytic activity of theXNb/TS-1 in the
epoxidation of 1-octene with H2O2 at 70°C for
48 h.
CatalystConversion, %(#)Brønsted,μmolDiol,μmolTOF, 10−1 h−1(*)Selectivity, %(‡)NBO0.25————NBA0.19————TS-11.63020—1.213Nb/TS-11.531.073324.7510Nb/TS-11.812.8105155.8220Nb/TS-11.684.6133137.93(#)mmol of 1-octene reacted.
(‡)mmol of 1,2-octanediol per mmol of 1-octene reacted, in %.
(*)TOF is calculated on the basis of the total Brønsted acid in the
catalysts for the production of 1,2-octanediol.Figure10 shows the graph of
the formation of the 1,2-epoxyoctane versus reaction time using various samples
as catalysts. Generally, both amorphous and crystalline form of niobium oxides
(samples NBA and NBO) show no observable activity toward the formation of
1,2-epoxyoctane, that is, 0.04 and 0.01 mmol, respectively, after 48 hours of reaction
time. These findings indicate that both niobium oxides are not active as catalysts
for epoxidation of 1-octene at 70°C. However, some researchers have
reported the capability of niobium oxide for oxidation reactions at high
temperature (>200°C) using oxygen as oxidant. Examples are
methanol oxidation that can be performed by catalyst Nb2O5/SiO2 at temperature as low as 230°C [11, 37] and alkane oxidation at high
temperature, 550°C [38]. Wachs et al. [39] and Védrine et al. [40]
have also reported the redox properties of niobium oxide. Meanwhile, Gallo et
al. [23] have reported that the oxidation activity of niobium-containing MCM-41
(Nb-MCM-41) in the epoxidation of alkene at higher temperature. Nb-MCM-41 also
was applied in the oxidation of butyl sulfide and cyclohexene using H2O2 at low temperature [41–44]. In that case, niobium species is located in the
framework of molecularsieves. Therefore, in this study, it is suggested that
supported niobium oxide become an active catalyst for oxidation reaction at
high temperature.Figure 10
The formation of 1,2-epoxyoctane from 1-octene epoxidation using
H2O2 at 70°C catalyzed by NBA, NBO, TS-1, and XNb/TS-1.The graph of the formation of
1,2-epoxyoctane from the epoxidation of 1-octene catalyzed by samples TS-1 andXNb/TS-1
are shown in Figure 10. All niobium containing TS-1 show the higher rate of the
formation of 1,2-epoxyoctane up to 6 hours of the reaction times as compared to
TS-1. Sample 3Nb/TS-1 shows the highest rate of the formation of 1,2-epoxyoctane,
followed by sample 10Nb/TS-1 that shows similar rate of the formation of
1,2-epoxyoctane in the first reaction time with sample 20Nb/TS-1, and the lowest
rate is observed in sample TS-1. However, at the reaction time longer than 6 hours,
the rates of the formation of 1,2-epoxyoctane of samples niobium containing
TS-1 are lower than that of sample TS-1. It is clearly seen that TS-1 is still
active at reaction time longer than 24 hours.The enhancement of the rate of
the formation of 1,2-epoxyoctane in the initial reaction time can be explained
on the basis of the relationship between the hydrophilic properties of the
catalysts and the formation of the active oxo-titanium intermediate. The
infrared spectra of samplesXNb/TS-1 show the characteristic of hydroxyl
groups of hydrogen bonding at around 3500 cm−1 that does not observe
in sample TS-1. This indicates that samples Nb/TS-1 have higher hydrophilic
properties as compared with sample TS-1. This finding is further suggested that
the capability of samples XNb/TS-1 to adsorb H2O2 solution in water is higher than that of sample TS-1, since H2O2 is hydrophilic. It has been reported that the active species in the epoxidation
reaction using hydrogen peroxide as oxidant was oxo-titanium species [45, 46].
Therefore, it was expected faster formation of the oxo-titanium species in the
catalyst containing hydrophilic character. Consequently, the production of
1,2-epoxyoctane is remarkable faster in samples XNb/TS-1. Meanwhile,
decreasing of the 1,2-epoxyoctane yield in the longer reaction time is mainly
due to 1,2-epoxyoctane has converted into 1,2-octanediol. The formation of
1,2-octanediol versus reaction time using TS-1 and XNb/TS-1 as catalysts
is shown in Figure 11. Samples NBO and NBA showed no noticeable activity toward
the formation of 1,2-epoxyoctane as shown in Figure 10. Consequently, there was
no 1,2-octanediol that can be observed from these reactions. Meanwhile, samples
TS-1 and Nb/TS-1 show their activity toward the formation of 1,2-octanediol.
However, samples Nb/TS-1 gave higher yield of 1,2-octanediol compare to TS-1.
In addition, the yield of 1,2-octanediol increased as the amount of niobium
increased. This may be due to the increase of the amount of Brønsted acid
present in the catalysts as demonstrated by pyridine adsorption study. However, the
selectivity of the diols seems to be lower since TS-1 alone show considerable
acidity, for example, it catalyzes oxidative cyclization [47].Figure 11
The yield of 1,2-octanediol from 1-octene epoxidation of using H2O2 at 70°C catalyzed by NBA, NBO, TS-1, and XNb/TS-1.Moreover, the starting time of
the formation of 1,2-octanediol is different among the catalysts. The highest
of starting time is observed on sample 20Nb/TS-1, followed by sample 10Nb/TS-1
and 3Nb/TS-1, and the last is TS-1, that is, after 1, 2, 4, and 24 hours of reaction
time, respectively. These finding may relate to the amount of Brønsted acid sites
in the catalysts, in which the higher concentration of Brønsted sites, the
higher probability of 1,2-epoxyoctane to convert to 1,2-octanediol at these sites.
Furthermore, the turnover frequency (TOF) decreases about half from sample
3Nb/TS-1 to samples 10Nb/TS-1 and 20Nb/TS-1 (Table4). It suggests that the
Brønsted acid site in the sample 3Nb/TS-1 is more effective compared to the
other samples. It may be due to the sample 3Nb/TS-1 containing less polymerized
niobium oxides species as compared to samples 10Nb/TS-1 and 20Nb/TS-1 as
observed by UV-vis DR
spectroscopy. However, high selectivity of 1,2-octanediol is observed on sample
containing high amount of Brønsted acid sites, that is, sample 20Nb/TS-1.


## 3.1. Catalysts Characterization

In this study, niobium ethoxide
was used as a precursor in the preparation of Nb/TS-1. Since the molecular size
of niobium ethoxide is larger than the pore size of TS-1, theoretically, the
niobium species could not possibly enter the pores of TS-1, thereby should be
dispersed on the surface of TS-1 only. In addition, in the preparation of the
samples, the acidic condition used in the hydrolysis step to increase the
formation of niobic acid, since aqueous solution at pH < 3 produced mainly
of niobic acid [9].Figure1 shows the XRD patterns
of the samples. The structure of the samples and the crystallinity of niobium
containing TS-1 (based on the highest diffraction line of TS-1) are summarized
in Table 2. For niobic acid sample, NBA prepared by hydrolysis of niobium pentoxide,
no diffraction lines can be observed on the XRD pattern, suggesting that the
niobic acid phase is fully amorphous. On the other hand, niobium oxide sample, NBO
prepared by calcination of ammonium niobium oxalate at 550°C shows
splitting of the diffraction line at around 2θ=29°, indicating that
the sample is crystalline niobium oxide phase (T-Nb2O3)
with orthorhombic structure [20]. Similar orthorhombic phase of niobium oxide
has been prepared by Paulis et al. [21] for the preparation of niobium oxide
from a precursor solution of niobium chloride in ethanol, and calcined at 700°C.
The XRD patterns of samples XNb/TS-1 reveal that the samples contain
framework structures of the MFI type zeolite. The observations of samples
XNb/TS-1 show that the structure of TS-1 is not strongly affected by the
presence of impregnated niobium. No diffraction line assigned for crystalline
phase of the niobium oxide is present in the XRD pattern. The result indicated
that niobium was well dispersed over TS-1. This also suggested that the niobium
species was present in the form of amorphous niobium oxide hydrate, since it
was prepared by hydrolysis of niobium ethoxide and was not calcined at
temperature higher than 400°C. In addition, the peak intensity of
TS-1 is drastically decreased to 60% after impregnation of 3.4 wt% of niobium
on the TS-1 (sample 3Nb/TS-1). Moreover, its intensity decreased significantly
as niobium loading increased. It is suggested that niobium could be located on
the surface of TS-1 or covering the surface of TS-1. Since the size of niobium
precursor, that is, Nb(OEt)5 is ca. 1.1 nm is larger than the
pore-entrance size of TS-1 (ca. 0.55 nm), hence, the niobium species
most probably be attached to the external surface of TS-1 [18].Table 2
Structure and crystallinity of samples.
CodeIntensity, cpsStructureTS-11957MFI3Nb/TS-11142MFI10Nb/TS-1942MFI20Nb/TS-1585MFINBO—OrthorhombicNBA—AmorphousFigure 1
XRD pattern of the NBA, NBO, TS-1, andXNb/TS-1 samples.The infrared spectra of the
samples in the lattice vibration region between 1400 and 400 cm−1 are depicted in Figure 2. Niobic acid (sample NBA) shows a major strong band
centered at 586 cm−1 and a shoulder band at 933 cm−1. This spectrum
is consistent with the skeletal vibration previously reported for amorphous
niobic acid [12, 21, 22]. The spectrum of niobium oxide crystals (sample NBO)
exhibits two strong bands at 615 and 850 cm−1. The broad band at a
higher frequency  ca. >850 cm−1 is associated with the stretching
mode of Nb=O bonds (niobyl species) in a highly distorted NbO6 groups, while
the band around 600 cm−1 can be assigned to the stretching of longer bridging
Nb–O–Nb bonds [22]. For the infrared spectra of samples XNb/TS-1, a
small band at around 970 cm−1 characteristic for titanium ions in
the tetrahedral structure is still present after impregnation of niobium. No
additional band after impregnation of niobium into the TS-1 can be observed.
This finding shows that impregnation of niobium has not affected the MFI structure
of TS-1 significantly. Infrared spectroscopy technique was not able to detect
the presence of niobic acid in the samples XNb/TS-1, due to the low
vibration intensity of pure niobic acid itself as compared to sample TS-1.Figure 2
FTIR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples.The UV-vis DR  spectra of the samples NBA,
NBO, TS-1, and Nb/TS-1 are shown in Figure3. The UV-vis spectrum of NBO sample
exhibits a maximum band at around 295 nm and a shoulder band at around 243 nm,
while sample NBA shows a sharp band at around 280 nm and a shoulder band at
around 240 nm. These bands are characteristic for distorted octahedral and
tetrahedral species, respectively, [22, 23]. These bands are attributed to the
charge transfer transitions O2− to Nb5+, which can be associated
to the energy gap between the O 2p-valence band and the Nb 4d-conductance band
[22]. In the supported niobium oxide, the position and width of the UV
absorption bands of niobium oxide species were observed to depend on the
niobium content and the size of niobia crystallites. The decrease in niobium
concentration leads to a blue shift of the charge transfer band, caused by
decreasing of the size of niobium oxide species. With decreasing of niobium
loading, the LMCT peak maximum shifts from a higher to a lower wavelength. Gao et
al. [11] found that in the niobium oxide compounds containing polymerized NbO6 structures with five or six Nb–O–Nb bonds around the central Nb atom, the
decrease of the layer thickness of the layered niobium oxide compounds slightly
increases the edge energy. Sample TS-1 shows a single strong band at 215 nm
characteristic for tetrahedral titanium framework. Impregnation of niobium on
the TS-1 (sample 3Nb/TS-1) shows a strong band at around 240 nm assigned to
octahedral niobium species. The intensity of this band in the samples XNb/TS-1
increased as the niobium content in the samples increased. Meanwhile, the band
characteristic for tetrahedral titanium at about 215 nm cannot be observed due
to the overlapping of this band with the band of niobium oxide that has much
higher intensity. In addition, this charged transfer band shifted to the higher
wavelength as the niobium loading increased. The samples 10Nb/TS-1 and
20Nb/TS-1 showed a shoulder band at around 245 nm and an intense band at around
280 nm, in which sample 10Nb/TS-1 has lower intensity bands than that of sample
20Nb/TS-1. These bands were also observed on bulk niobic acid (sample NBA),
suggesting the presence of similar species in this sample. The shift of the UV-vis
band position as the metal oxide content increases is generally attributed to
the enhancement of degree of polymerization. Therefore, in this study, the
increase of niobium content has led to an increase in the degree of
polymerization of the niobium species. According to Tanaka et al. [24], Nb2O5/Al2O3 with low niobium loading could be represented by monomer or dimmer species,
whereas at high niobium loading, the samples could be identified by the presence
of Nb2O5 particles.Figure 3
UV-Vis DR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples.Figure4 shows the TPR profiles
of the samples. The TPR profile of sample NBO: crystalline niobium oxide
displays a low signal and two high signals of hydrogen consumption centered at
around 680°C, 870°C, and higher than 1000°C
(not complete). Sample NBA prepared by hydrolysis of niobium ethoxide and
calcined at 200°C, exhibited three regions of hydrogen consumption, that
is, a negative peak at around 550°C and the high positive peaks at
around 900°C and at a higher temperature above 1000°C. The
negative peak can be attributed to CO or CO2 formation due to carbon
residue from the niobium ethoxide that was not fully hydrolyzed. The first
positive peak was attributed to the reduction of Nb5+ to Nb4+ in the form of Nb2O5 to NbO2 [1], while for reduction
of NbO2 to lower oxidation state of niobium, NbO needed higher
reduction temperature. The TPR profile of sample 20Nb/TS-1 shows a major peak
at around 925°C, which is also observed on amorphous bulk Nb2O5,
suggesting the presence of similar species of niobium on both samples. The
reduction temperature of sample 20Nb/TS-1 is slightly higher than that of
sample NBA, indicating that the interaction between niobic acid and the surface
of TS-1 exists. On the contrary, samples 3Nb/TS-1 and 10Nb/TS-1 do not show any
significant peak in the reduction temperature range, although the sample
contains niobium. This finding is explained to be due to several factors. Low
amount of niobium loading results in high dispersion of niobium on the surface
of TS-1 that promotes interaction between niobium and TS-1. Pereira et al. [25]
found that high dispersions promote intimate contact between niobium and the
silica surface, Nb–O–Si bonding in
relation to Nb–O–Nb bonding
prevailing. The formation of a surface of niobia monolayer is derived from the
Nb–O–Si bonding [26]. It also can be explained based on the electronegativity
concept that the presence of silicon in the coordination sphere of niobium
should increase the electronic density niobium. Thus, the reduction of niobium
from Nb+5 to Nb+4 was more difficult to occur [25].
Recently, Mendez et al. [27] found that 20 wt% of niobium supported on Al2O3 did not show any hydrogen uptake as an indication of the strong interaction of
niobium with Al2O3.Figure 4
TPR profiles of the NBA, NBO, andXNb/TS-1 samples.The hydroxyl groups and acidity
of the samples were monitored by infrared spectroscopy technique. The wafer
sample was put in the glass cell and evacuated at 200°C for 16 hours
under vacuum. The infrared spectra were recorded at room temperature. Figure 5
shows the infrared spectra of the samples TS-1, NBA, NBO, XNb/TS-1, and
a mechanical mixture of NBA and TS-1 (sample MM-20NBA+TS-1, 20 wt% of NBA) after
evacuation at 200°C. TS-1 shows a sharp peak at around 3743 cm−1 characteristic of hydroxyl silanol groups. Niobic acid (sample NBA) shows a
small peak at around 3700 cm−1 and a broad peak centered at around
3400 cm−1 that can be assigned to hydroxyl stretching mode of free
Nb–O–H groups and hydroxyl with hydrogen bonding, respectively, [22, 28]. On the
other hand, crystalline niobium oxide does not show any peak in the vibration
region of hydroxyl groups indicating that crystalline niobium oxide does not
contain any hydroxyl groups. A different finding was reported recently by Braga
et al. [12] for niobium oxide calcined at 800°C. They observed the
peaks at around 3748 cm−1 and 3526 cm−1 assigned to
terminal hydroxyl bonded to a slightly distorted NbO6 and hydroxyl
with hydrogen bonding, respectively. The peak at around 3743 cm−1 disappeared as niobium was loaded on the TS-1. A small peak at around 3733 cm−1 and a broad peak centered at around 3530 cm−1 were observed in
samples XNb/TS-1. In addition, a peak at around 3733 cm−1 of
sample 20Nb/TS-1 has higher intensity as compared to samples 3Nb/TS-1 and
10Nb/TS-1. Meanwhile, a mechanical mixture of 20 wt% NBA in TS-1 only shows a
peak similar to that of TS-1 at around 3744 cm−1 assigned for
silanol hydroxyl groups. Although this mixture contains 20 wt% of NBA, however,
no peak at around 3700 cm−1 assigned for Nb–O–H can be observed. It suggests
that the amount of hydroxyl groups of Nb–OH in the sample NBA is much lower
than the amount of silanol groups of TS-1. For samples XNb/TS-1, a
drastic decrease of silanol species present on the surface of TS-1 indicates
that there is strong interaction between niobium and TS-1 (niobium bonded with
–O–Si, replacing hydrogen atom of silanol species in the TS-1). Since niobic
acid does not show any peak in this region, the results clearly suggests that
the niobium species has covered the surface of TS-1. Therefore, depositions of
niobium oxide species on TS-1 consume the surface Si–OH groups of TS-1.
Consequently, the amount silanol groups decreased.Figure 5
FTIR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples in the hydroxyl vibration region, after evacuation under vacuum at 200°C.The effect of evacuation
temperature on the hydroxyl groups present in the sample was studied for sample
3Nb/TS-1 at evacuation temperatures 120, 200, and 400°C (Figure 6). The spectrum of sample 3Nb/TS-1 after evacuation at 120°C for 24 hours
shows a small peak at around 3735 cm−1 and a broad peak with high
intensity center at around 3500 cm−1 characteristic for hydroxyl
groups of silanol and hydrogen bonding, respectively. After evacuation at 200°C
for 24 hours, the peaks for silanol groups are still maintained, but the peak
for hydrogen bonding is shifted to a higher frequency at around 3530 cm−1 with lower intensity. Continuing evacuation at 400°C for an hour, results
in slightly higher intensity of silanol but still at similar frequency, while
the hydroxyl of hydrogen bonding becomes a shoulder peak and shifted to the
higher frequency at around 3600 cm−1. The decreasing of hydroxyl
groups of hydrogen bonding and increasing of silanol groups indicates that the
hydroxyl groups of hydrogen bonding have interacted with the silanol groups.
Therefore, as evacuation temperature increased, the hydroxyl groups of hydrogen
bonding decreased, consequently, the silanol increased.Figure 6
FTIR spectra of the sample 3Nb/TS-1 in the hydroxyls region after evacuation under vacuum at temperature 120, 200, and 400°C.The type and amount of acid sites
in the sample were monitored and measured by pyridine adsorption. Pyridine was
adsorbed at room temperature after evacuation of sample at 200°C for
16 hours in vacuum. The infrared spectra were collected after evacuation of
pyridine at 150°C for an hour. Figure 7 shows the infrared spectra
of the samples in the pyridine region. Sample NBO does not show any peak which
indicates that crystalline niobium oxide has no acidity. Sample NBA shows the
peaks at 1636, 1609, 1575, 1540, 1489, and 1448 cm−1. The peaks at
around 1540 and 1448 cm−1 are due to Brønsted and Lewis acid sites,
respectively. Similar findings have been reported by Morais et al. [29] and da
Silva et al. [30] for niobic acid. It has been reported also that the acidity
of niobic acid also depends on the calcination temperature [31]. The number of
Lewis acid sites increased with increasing pretreatment temperatures up to 500°C
and then decreased at higher temperatures. On the other hand, Brønsted acid
sites decreased with increasing temperature above 100°C. As the
number of coordinated water molecules increased, the number of Brønsted acid
sites decreased, the concentration of hydronium ions (H3O+)
increased, and the formation of H2O–HO species is observed [32].Figure 7
FTIR spectra of the NBA, NBO, TS-1, andXNb/TS-1 samples after evacuation at 200°C for 24 hours followed by pyridine adsorption at room temperature and evacuation at 150°C for one hour, in the pyridine regions.Niobium oxide samples containing
TS-1 showed the peaks similar to that of sample NBA. However, the intensity of
the characteristic peaks for Brønsted and Lewis acid sites at around 1540 and
1448 cm−1, respectively, was lower than that of sample NBA. Jehng
and Wachs [13] and Shirai et al. [15] have reported the evidence of the presence
of both Brønsted and Lewis acid sites in niobium oxide supported on the silica,
while Mendes et al. [27] only found Lewis acid sites in silica supported niobic
acid. In this study, a mechanical mixture of niobic acid and TS-1 (sample
MM-20NBA+TS-1) only shows the peaks at around 1608, 1489, and 1447 cm−1,
which are similar to those of sample TS-1. This finding indicates that the
sample NBA in sample MM-20NBA+TS-1 contained low amount of acid sites.The infrared spectra shown in
Figure7 confirm that all samples XNb/TS-1 have Brønsted and Lewis acid
sites. The amount of Brønsted and Lewis acid sites calculated on the basis of
the peak at around 1540 and 1448 cm−1 are tabulated in Table 3. It showed
that the amount of Brønsted and Lewis acids increases with the increasing niobium
loading on the support. The area under the peak at around 1490 cm−1 characteristic
for the mixture of Brønsted and Lewis acids also increased as the niobium loading
increased. This finding indicates that niobium plays a role in the formation of
both Brønsted and Lewis acid sites. Similar findings have been reported by
Jehng and Wachs [13] and Shirai et al. [15] for niobium oxide supported on
silica.Table 3
The amount of the Brønsted and Lewis acid sites and
the area under the peak at around 1490 cm−1 of the samples.
SampleBrønsted,μmol/gLewis,μmol/gLewis added,μmol/g(#)Peak area at 1490, cm−2(*)TS-1 (1%)030—0.143Nb/TS-11848180.4610Nb/TS-14763330.5220Nb/TS-16672420.93NBO00—0NBA101145—1.75(#)Additional
amount of the Lewis in the samples Nb/TS-1, calculated by deduction of Lewis
acid in the samples Nb/TS-1 by sample TS-1.
(*)Area of the peak
at around 1490 cm−1, characteristic for the mixture of Brønsted and
Lewis acid sites.Figure8 shows the correlation
between niobium content (wt%) in the solid samples and the amount of the
Brønsted and Lewis acid sites. It shows that addition of niobium onto TS-1 up
to 10 wt% results in the increase amount of Brønsted acid sites. Although the
amount of Brønsted acid sites keep on increasing with further increased in
niobium loading up to 20 wt%, the additional amount of Brønsted acid produced
is slightly lower compared to that of the first addition (sample 10Nb/TS-1).
This finding suggests that not all of the added niobium creates Brønsted acid
sites. Based on this observation, it is suggested that the Brønsted acid sites
may be present only on the surface of niobic acid. Similar pattern is observed
also for the formation of Lewis acid sites.Figure 8
Correlation between the niobium content and the amount of acid sites of the catalysts.According to Jehng and Wachs
[13, 14], interaction between Nb2O5 and acidic surfaces
(SiO2 and Al2O3) resulted in the formation of
slightly distorted NbO6, NbO7, and NbO8 sites.
The slightly distorted NbO6, NbO7, and NbO8 groups only possess Nb–O bonds and are associated with Brønsted acid sites. The
highly distorted NbO6 octahedral possesses Nb=O bonds in which
associated with Lewis acid sites. In this study, it is found that the increased
amount of Lewis acid sites present in the sample with low amount of niobium
loading (3Nb/TS-1) is similar with the increased amount of Brønsted acid sites.
This suggests that the amount of slightly distorted NbO6, NbO7,
and NbO8 groups is similar with those of highly distorted NbO6.
However, at higher niobium loading, it is found that the increased amount of
Lewis acid is slightly lower than that of Brønsted acid. It indicates that at
higher niobium loading, the presence of slightly distorted NbO6, NbO7,
and NbO8 groups is more prominent. This finding was supported by the
results obtained from the TPR and infrared of hydroxyl groups data, which
indicate the interaction between niobium species with the support TS-1
(Nb–O–Si). Since Nb is more electronegative compared to Si or Ti (Pauling value
of Nb, Si, and Ti are 1.6, 1.0, and 1.5, resp.) means that niobia oxide
shows more ionic character. Therefore, generation of Lewis acid sites in the
samples XNb/TS-1 can be a consequence of the higher ionicity of the Nb–O
bond. This suggests that higher Lewis acid could be generated in the first
layer of the supported niobium oxide on TS-1.Further explanation is
demonstrated by pyridine adsorption, which shows that Nb-loaded TS-1 has
Brønsted acid sites. On the other side, sample physical mixing of niobic acid
and TS-1 (20 wt% NBA) only shows Lewis acid sites. In this sample, there is no
chemical interaction between Nb–O–Si. The acid sites in this sample are purely
come from sample NBA with a small amount of acid sites. Meanwhile, in the
Nb/TS-1 sample, the interaction between Nb–O–Si occurs. Therefore, generation of
Brønsted acid sites can be expected since protons are required to balance the
excess negative charges on the oxygen atoms, analog of Al atom in the
aluminosilicate molecularsieves. Generation of Brønsted acid sites is also in accordance with Kung’s assumptions [33, 34]. They explained two models, that is,
(1) an electronic interaction exists between Nb2O5 and
TS-1 components, and (2) Nb–O bonds neighboring the more covalent Si–O possess
a higher ionicity. It means that the electron density of the OH bonds is
reduced by the inductive effect of the nearby electronegative anions. As a
consequence, this weakens the OH bond and generates the Brønsted acid sites.
Furthermore, based on the XPS data by Noronha et al. [35] and Bosman et al. [36]
found the increased in the covalency of Si–O bonds in binary SiO2–TiO2 and SiO2–ZrO2 systems, respectively, that leads to the
creation of Brønsted acid sites.Samples morphology were
monitored using field emission scanning electron microscope. Figure9 shows the
FESEM images of samples TS-1 and 10Nb/TS-1 material. The FESEM image shows that
TS-1 has cubic morphology with the crystal size around 100 nm. Meanwhile, a
grubby TS-1 crystal is observed in the sample 10Nb/TS-1 with the crystal size
similar to that of TS-1 sample, that is, around 100 nm. Based on the UV-vis DR
finding,
this sample contained niobic acid. Therefore, it is concluded that a small particle
observed on the surface of TS-1 is niobic acid. However, the morphology of TS-1
is not affected by impregnation of niobic acid.The FESEM images of the materials (a) TS-1 and (b) 10Nb/TS-1.
(a)(b)


## 3.2. Performance of Catalysts

The catalytic activities of the
TS-1, niobium compounds, and niobium-containing TS-1 samples were tested in the
epoxidation of 1-octene with aqueous H2O2 as oxidant in
acetone at 70°C. Table 4 shows the catalytic results of the samples.
For the conversion of 1-octene after 48 hours of reaction time (Table 4), only
below 0.2 mmol of 1-octene has converted using samples NBO and NBA. This
conversion is much lower as compared to the 1-octene conversion catalyzed by
samples TS-1 and Nb/TS-1, that is, up to 1.6 mmol 1-octene. Meanwhile, it is
found that the catalytic activity of samples TS-1 and Nb/TS-1 in term of
conversion of 1-octene are not much different. This finding suggests that the
niobium content is not taken into account of the 1-octene conversion. The low
conversion of 1-octene on the samples NBO and NBA supported this implication.Table 4
Catalytic activity of theXNb/TS-1 in the
epoxidation of 1-octene with H2O2 at 70°C for
48 h.
CatalystConversion, %(#)Brønsted,μmolDiol,μmolTOF, 10−1 h−1(*)Selectivity, %(‡)NBO0.25————NBA0.19————TS-11.63020—1.213Nb/TS-11.531.073324.7510Nb/TS-11.812.8105155.8220Nb/TS-11.684.6133137.93(#)mmol of 1-octene reacted.
(‡)mmol of 1,2-octanediol per mmol of 1-octene reacted, in %.
(*)TOF is calculated on the basis of the total Brønsted acid in the
catalysts for the production of 1,2-octanediol.Figure10 shows the graph of
the formation of the 1,2-epoxyoctane versus reaction time using various samples
as catalysts. Generally, both amorphous and crystalline form of niobium oxides
(samples NBA and NBO) show no observable activity toward the formation of
1,2-epoxyoctane, that is, 0.04 and 0.01 mmol, respectively, after 48 hours of reaction
time. These findings indicate that both niobium oxides are not active as catalysts
for epoxidation of 1-octene at 70°C. However, some researchers have
reported the capability of niobium oxide for oxidation reactions at high
temperature (>200°C) using oxygen as oxidant. Examples are
methanol oxidation that can be performed by catalyst Nb2O5/SiO2 at temperature as low as 230°C [11, 37] and alkane oxidation at high
temperature, 550°C [38]. Wachs et al. [39] and Védrine et al. [40]
have also reported the redox properties of niobium oxide. Meanwhile, Gallo et
al. [23] have reported that the oxidation activity of niobium-containing MCM-41
(Nb-MCM-41) in the epoxidation of alkene at higher temperature. Nb-MCM-41 also
was applied in the oxidation of butyl sulfide and cyclohexene using H2O2 at low temperature [41–44]. In that case, niobium species is located in the
framework of molecularsieves. Therefore, in this study, it is suggested that
supported niobium oxide become an active catalyst for oxidation reaction at
high temperature.Figure 10
The formation of 1,2-epoxyoctane from 1-octene epoxidation using
H2O2 at 70°C catalyzed by NBA, NBO, TS-1, and XNb/TS-1.The graph of the formation of
1,2-epoxyoctane from the epoxidation of 1-octene catalyzed by samples TS-1 andXNb/TS-1
are shown in Figure 10. All niobium containing TS-1 show the higher rate of the
formation of 1,2-epoxyoctane up to 6 hours of the reaction times as compared to
TS-1. Sample 3Nb/TS-1 shows the highest rate of the formation of 1,2-epoxyoctane,
followed by sample 10Nb/TS-1 that shows similar rate of the formation of
1,2-epoxyoctane in the first reaction time with sample 20Nb/TS-1, and the lowest
rate is observed in sample TS-1. However, at the reaction time longer than 6 hours,
the rates of the formation of 1,2-epoxyoctane of samples niobium containing
TS-1 are lower than that of sample TS-1. It is clearly seen that TS-1 is still
active at reaction time longer than 24 hours.The enhancement of the rate of
the formation of 1,2-epoxyoctane in the initial reaction time can be explained
on the basis of the relationship between the hydrophilic properties of the
catalysts and the formation of the active oxo-titanium intermediate. The
infrared spectra of samplesXNb/TS-1 show the characteristic of hydroxyl
groups of hydrogen bonding at around 3500 cm−1 that does not observe
in sample TS-1. This indicates that samples Nb/TS-1 have higher hydrophilic
properties as compared with sample TS-1. This finding is further suggested that
the capability of samples XNb/TS-1 to adsorb H2O2 solution in water is higher than that of sample TS-1, since H2O2 is hydrophilic. It has been reported that the active species in the epoxidation
reaction using hydrogen peroxide as oxidant was oxo-titanium species [45, 46].
Therefore, it was expected faster formation of the oxo-titanium species in the
catalyst containing hydrophilic character. Consequently, the production of
1,2-epoxyoctane is remarkable faster in samples XNb/TS-1. Meanwhile,
decreasing of the 1,2-epoxyoctane yield in the longer reaction time is mainly
due to 1,2-epoxyoctane has converted into 1,2-octanediol. The formation of
1,2-octanediol versus reaction time using TS-1 and XNb/TS-1 as catalysts
is shown in Figure 11. Samples NBO and NBA showed no noticeable activity toward
the formation of 1,2-epoxyoctane as shown in Figure 10. Consequently, there was
no 1,2-octanediol that can be observed from these reactions. Meanwhile, samples
TS-1 and Nb/TS-1 show their activity toward the formation of 1,2-octanediol.
However, samples Nb/TS-1 gave higher yield of 1,2-octanediol compare to TS-1.
In addition, the yield of 1,2-octanediol increased as the amount of niobium
increased. This may be due to the increase of the amount of Brønsted acid
present in the catalysts as demonstrated by pyridine adsorption study. However, the
selectivity of the diols seems to be lower since TS-1 alone show considerable
acidity, for example, it catalyzes oxidative cyclization [47].Figure 11
The yield of 1,2-octanediol from 1-octene epoxidation of using H2O2 at 70°C catalyzed by NBA, NBO, TS-1, and XNb/TS-1.Moreover, the starting time of
the formation of 1,2-octanediol is different among the catalysts. The highest
of starting time is observed on sample 20Nb/TS-1, followed by sample 10Nb/TS-1
and 3Nb/TS-1, and the last is TS-1, that is, after 1, 2, 4, and 24 hours of reaction
time, respectively. These finding may relate to the amount of Brønsted acid sites
in the catalysts, in which the higher concentration of Brønsted sites, the
higher probability of 1,2-epoxyoctane to convert to 1,2-octanediol at these sites.
Furthermore, the turnover frequency (TOF) decreases about half from sample
3Nb/TS-1 to samples 10Nb/TS-1 and 20Nb/TS-1 (Table4). It suggests that the
Brønsted acid site in the sample 3Nb/TS-1 is more effective compared to the
other samples. It may be due to the sample 3Nb/TS-1 containing less polymerized
niobium oxides species as compared to samples 10Nb/TS-1 and 20Nb/TS-1 as
observed by UV-vis DR
spectroscopy. However, high selectivity of 1,2-octanediol is observed on sample
containing high amount of Brønsted acid sites, that is, sample 20Nb/TS-1.


## 4. Conclusions

Impregnation of niobic acid
onto titanium silicalite resulted in the formation of Brønsted acid sites. Both
the MFI structure of TS-1 and tetrahedral titanium structure still retained
after impregnation of niobic acid. It was found that the niobium species has interacted
with the surface hydroxyl groups of TS-1. The addition of niobic acid onto TS-1
has resulted an increase of the catalytic activity toward the rate of formation
of epoxide.



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*Source: 345895-2008-09-25.xml*