# Zeolite Encapsulated Nanocrystalline CuO: A Redox Catalyst for the Oxidation of Secondary Alcohols

Authors: Sakthivel Vijaikumar; Thirumeni Subramanian; Kasi Pitchumani
Journal: Journal of Nanomaterials
 (2008)

---

## Abstract

Zeolite encapsulated nanocrystalline CuO is synthesized and characterized by powder XRD and HRTEM analyses which clearly show that the particles are less than 15 nm and the nanoparticles are highly dispersed. This nano CuO encapsulated CuY zeolite is used as catalyst in the oxidation of aromatic secondary alcohols. CuY zeolite acts as an efficient support for nano CuO, by stabilizing it and preventing its aggregation. Plausible mechanisms for the formation of the various products are also given.


---

## Body

## 1. Introduction

Nanostructured
materials present great promise and opportunities for a new generation of
materials with improved and tailorable properties for applications in sensors,
optoelectronics, separation, and catalysis. Catalysts in many cases are
nanoscale particles, and thus catalysis is a nanoscale phenomenon [1, 2].
Significant progress has been made in recent years in controlling the size of
nanoscale materials and in understanding their unique size-dependent catalytic
properties [3]. In this context, controlling the shape of
nanoparticles is also an equally important aspect of desired catalyst synthesis [4]. Nanostructured
materials, especially nanometal oxides such as MgO, CuO, Al2O3,
CaO, BaO, SrO, ZnO, and TiO2, exhibit great promise and
opportunities for a new generation of materials with improved and tailorable
properties for applications in sensor development, optoelectronics, separation,
and catalysis studies [5]. Among these nanometal oxides, CuO has gained
advantages because of its higher catalytic activity in many oxidation processes
[6].Various preparation methods to ultrafine CuO have been reported [7–10]. Bandara et al. [11] have reported the synthesis of highly stable CuO
incorporated TiO2 and these catalysts have higher catalytic activity
in the photocatalytic reduction of H2O under sacrificial conditions.
Liquid-phase oxidation of benzene to phenol catalyzed by CuO supported Al2O3 has been reported by Miyahara et al.
[12]. They have also reported that CuO supported Al2O3 prepared by
coprecipitation of Cu(NO3)2·H2O and Al(NO3)3·9H2O
inhibits the leaching of Cu species in comparison with the Cu catalysts
supported by the impregnation method on Al2O3, SiO2,
MCM-41, and so forth. Mixed oxides of Cu-Mn derived from
calcination of Cu-Mn-hydrotalcite precursors are reported to be promising
catalysts for the total combustion of toluene [13].Oxidative decomposition of benzene
and its methyl derivatives catalyzed by copper and palladium ion-exchanged
Y-type zeolite is reported by Becker et
al. [14]. They have observed
the presence of microcrystalline CuO phases in the catalyst by EXAFS
experiments and the equilibrium between Cu(I) and CuO is probably the rate
determining step for the oxidation reaction.Velu et al. [15] have
utilized MCM-41 mesoporous molecular sieves containing copper and zinc for the
selective oxidation of alcohols to aldehydes. They have also observed the
presence of finely dispersed CuO inside the channels of MCM-41 which produce
overoxidation products, namely, CO2 and H2O. Lin and Wang [16] have reported the oxidation of
2-chlorophenol catalyzed by CuO/ZSM-48 derived from the calcination of Cu2+-exchanged
in zeolite at 500°C. For the cation exchange, they
have used Cu(OCOCH3)2 in aqueous solution. They have also
reported that the oxidation efficiency of 2-chlorophenol in the supercritical
water is efficiently enhanced by the presence of CuO in the zeolite.In earlier reports for preparation of CuO
containing zeolites, calcination procedure is utilized (calcination of Cu2+-exchanged
zeolite at 500°C) and only microcrystalline
CuO phases are observed. However, there
are no reports for the syntheses of nanocrystalline CuO in the supercage
of Y-zeolite. The shape and size of
the zeolite cavities, particularly the cages and polar active sites are
expected to stabilize the CuO nanoparticles and thus prevent their aggregation.
Hence, in the present study, it is proposed to synthesize nanocrystalline CuO
encapsulated inside the supercages of Y-zeolite and to study its catalytic activity in the oxidation of
alcohols.


## 2. Experimental Methods

### 2.1. Preparation of Zeolite Encapsulated Nanocrystalline CuO

Zeolite encapsulated nanocrystalline CuO was synthesized from Cu2+-exchanged Y-zeolite. About 2 g of Cu2+-exchanged
Y-zeolite were taken in a round-bottomed flask containing 10 mL of deionized
water and they were heated to 80°C in a
water bath. To the hot mixture, 2 mL of 10% sodium hydroxide solution was
added with constant stirring. First, the bluish green Cu2+Y-zeolite
becomes dark blue by the formation of Cu(OH)2, then it changed in situ into black colour by the
formation of CuO. The mixture was stirred at room temperature for 6 hours and
then it was separated by centrifuging. It was washed thrice with water,
centrifuged, and the precipitate was dried in an air oven at 100°C.


### 2.2. Oxidation of Secondary Alcohols

Zeolite encapsulated nanocrystalline CuO 300 mg was activated in a muffle furnace at
450°C for 6 hours.
The hot zeolite was immediately transferred into 5 mL of n-hexane to avoid
absorption of water. Then, 100 mg of benzhydrol were loaded into the zeolite by
stirring it in a magnetic stirrer. After loading, n-hexane was evaporated with
rotovapor and then it was dried. Alcohol-loaded zeolite was uniformly packed in
a sample loading unit and it was fitted into the reactor. Oxygen was allowed to
flow into the reactor with a rate of 1 mL/min. The temperature of the reactor
was maintained at 200°C. After 1 hour, the products condensed on the
receiver and also on zeolite were extracted with dichloromethane and the mass
balance was noted. Then, the reaction mixture was analyzed in GC, and the
products were characterized in GC-MS.


## 2.1. Preparation of Zeolite Encapsulated Nanocrystalline CuO

Zeolite encapsulated nanocrystalline CuO was synthesized from Cu2+-exchanged Y-zeolite. About 2 g of Cu2+-exchanged
Y-zeolite were taken in a round-bottomed flask containing 10 mL of deionized
water and they were heated to 80°C in a
water bath. To the hot mixture, 2 mL of 10% sodium hydroxide solution was
added with constant stirring. First, the bluish green Cu2+Y-zeolite
becomes dark blue by the formation of Cu(OH)2, then it changed in situ into black colour by the
formation of CuO. The mixture was stirred at room temperature for 6 hours and
then it was separated by centrifuging. It was washed thrice with water,
centrifuged, and the precipitate was dried in an air oven at 100°C.


## 2.2. Oxidation of Secondary Alcohols

Zeolite encapsulated nanocrystalline CuO 300 mg was activated in a muffle furnace at
450°C for 6 hours.
The hot zeolite was immediately transferred into 5 mL of n-hexane to avoid
absorption of water. Then, 100 mg of benzhydrol were loaded into the zeolite by
stirring it in a magnetic stirrer. After loading, n-hexane was evaporated with
rotovapor and then it was dried. Alcohol-loaded zeolite was uniformly packed in
a sample loading unit and it was fitted into the reactor. Oxygen was allowed to
flow into the reactor with a rate of 1 mL/min. The temperature of the reactor
was maintained at 200°C. After 1 hour, the products condensed on the
receiver and also on zeolite were extracted with dichloromethane and the mass
balance was noted. Then, the reaction mixture was analyzed in GC, and the
products were characterized in GC-MS.


## 3. Results and Discussion

### 3.1. Characterization of Zeolite Encapsulated Nanocrystalline CuO

In this paper, zeolite encapsulated
nanocrystalline CuO is synthesized by the addition of NaOH solution to a suspension of Cu2+-exchanged
zeolites. Here, the procedure similar to that reported by Cao et al. [10] is adopted. They have used
CTAB as capping agent which prevents the aggregation of nanoparticle. But in
our study, the shape of the supercage and its polar active sites are likely to
prevent the aggregation of nanocrystalline CuO and consequently no capping
agent is used. Formation of CuO from Cu2+ is characterized by its
powder XRD and its nano form is confirmed by HRTEM analysis. The prepared
nanocrystalline CuO encapsulated CuY zeolite is characterized by AAS, powder
XRD, SEM, and HRTEM analyses, and the observed results are given below.AAS analysis of CuO encapsulated CuY zeolite
shows 8.4 wt% of copper present in the zeolite. However, with this result
alone, it is difficult to come to a conclusion about the percentage of CuO and
Cu2+ (as a cation) presents inside the zeolite. Powder
XRD pattern of Cu2+-exchanged and zeolite
encapsulated nanocrystalline CuO is  given in Figure 1. The presence of peaks
corresponding to CuO (at 2=34 and 2=38)16 confirms the presence CuO
in the zeolite. Pure Cu2+-Y zeolite also has small peaks that
corresponding to CuO and this may be due to the formation of trace amount of
CuO by partial oxidation of Cu2+ to CuO during cation exchange and
drying the cation-exchanged sample in an air oven. There have been no other
changes in the other diffraction peaks of the zeolite and the crystallinity of
the zeolite is maintained even after the base treatment of the zeolites.Powder XRD patterns of (a) Cu2+ exchange Y-zeolite, and (b) zeolite encapsulated nanocrystalline CuO.
(a)(b)SEM analysis of zeolite encapsulated
nanocrystalline CuO shows the surface morphology of the zeolite (Figure2). Zeolite particles are found to be present as granules of particle size ~500 nm. HRTEM images of zeolite encapsulated nanocrystalline CuO at different magnifications are given in Figure
3. These figures clearly show that the CuO particles are in nanometer range
(<15 nm) and most of the particles (which are less than 2 nm) are embedded
inside the zeolite materials (Figures 3(b) and 3(c)). These CuO nanoparticles
are also spherical in shape. Few other particles in the range of 15 nm are also
found and this is attributed to the passage of high energy beam during the
HRTEM analysis and consequent cage-escape and aggregation of CuO
nanoparticles.SEM images of zeolite encapsulated nanocrystalline
CuO at different magnifications.
(a)(b)(c)(d)HRTEM images of zeolite encapsulated nanocrystalline CuO at different magnifications.
(a)(b)(c)(d)HRTEM images reveal that nanocrystalline CuO particles are highly dispersed and no
aggregation or cluster formation occurs. This may be due to the interaction
between the zeolites active sites and the CuO surface, which have high surface
energy due to its smaller size [17].  Further, the formation of the nanocrystalline
CuO inside the supercages of Y-zeolite was confirmed by the BET pore-size and
pore volume of the CuY-zeolite. The pore size and pore volume of CuY-zeolite
before and after nano CuO formation are 16.11 Å, 0.3179 cc/g and 15.19 Å,
0.2608 cc/g, respectively. The decrease in the average pore diameter and the
pore volume clearly indicates the formation of nanocrystalline CuO within the
supercages of zeolite without damaging the zeolite crystal structure.CuO has been widely used as a catalyst for the
oxidation of alcohols and aromatic hydrocarbons [12–15]
. Here, zeolite encapsulated
nanocrystalline CuO is used for the oxidation of aromatic alcohols.
Since CuO is mildly basic and zeolites are mildly acidic, this combination will
provide an acid-base bifunctional catalyst. In addition, zeolite supercages
will also provide closer proximity between substrates, adjacent acidic, and
basic sites which is expected to increase the rate of the reaction. In general,
catalytic activity of CuO is maximum at higher temperature and hence, we have
developed a solid-phase reactor provided with constant flow of oxygen. The
results obtained for the oxidation of various aromatic alcohols using zeolite encapsulated
nanocrystalline CuO are given in Table 1 and Scheme 1.Table 1
Zeolite encapsulated nanocrystalline CuO catalyzed oxidation of alcohols(a).
S. NoSubstrateCatalyst% Conversion(b)% of products2345X(c)(1)BenzoinCuOY10092.2———7.8(2)BenzhydrolCuOY10029.170.9———(3)CuOY(d)nil—————(4)Cu2+Y21.028.771.3———(5)NaY11.2—100———(6)CuO100100————(7)Y-zeolite-CuO(e)10090.08.4——1.6(8)1-PhenylethanolCuOY100——84.9—15.1(9)CuO78.0100————(10)HydrobenzoinCuOY100———100—(11)Benzyl alcoholCuOY13.7100————(a)Substrate loaded zeolite heated
in solid phase reactor at 200°C for 1 hour under oxygen atmosphere.(b)Analyzed in GC, error limit ±3%.(c)Unidentified products.(d)Reaction
carried out at 100°C.(e)Physical mixture of Y-zeolite and CuO.Scheme 1In the oxidation of benzoin with zeolite encapsulated
nanocrystalline CuO in the solid phase reactor, the oxidation product,
namely, benzil is formed in good yield. But with benzhydrol, benzophenone, and
diphenylmethane, the oxidized as well as reduced products, respectively, are
formed. However, at a lower temperature (100°C), there is no reaction (Table 1, entry 3)
. With
only Cu2+-exchanged zeolite, the conversion is lower and a similar
product ratio is observed. Hence, it is likely that at an elevated temperature,
Cu2+ in oxygen atmosphere may have been oxidized to CuO. Similarly,
when studied in NaY zeolite also, the conversion is very low and only the
reduced product is observed. In the presence of pure CuO, only the oxidized
product is formed exclusively.When 1-phenylethanol is subjected to oxidation
under the same conditions, the oxidative-coupled product an alkene
(1,2-dimethylstilbene) is formed. Hydrobenzoin, an aromatic diol, undergoes
overoxidation to produce benzoic acid. With an aromatic primary alcohol, namely
benzyl alcohol, the conversion is very low (Table1, entry 10). It is likely
that because of its lower boiling point, it may be vaporized before it reacts
with the catalyst as it is found to be condensed on the receiver. Thus, this
method is found to be efficient only for alcohols having higher boiling
points.


### 3.2. Mechanism for the Oxidation of Alcohols

Nagase et al. [18] have studied the dynamic oxidation states of copper in the
course of carbon monoxide oxidation over powdered CuO and Cu2O in a
gas mixture of 5% CO/He and air (O2) by means of an appropriate
combination of evolved gas analysis, thermogravimetry, XRD, and FT-IR in the
temperature range of room temperature to 350°C. They have observed the reduction of CuO to Cu and
the stepwise oxidation of CuCu2OCuO from the
XRD spectra of the products after each process. Hence, the oxidation state of
copper changes thermodynamically between CuO, Cu2O, and Cu as a
function of temperature and oxygen atmosphere. This redox behavior and the difference in oxygen defects, oxygen holes,
and oxygen adsorption species in these oxidation states are thought to be the
reason for the high activity or selectivity of the copper catalyst.Earlier Sheikh and Eadon [19]
have reported
the vapor-phase oxidation of alcohols to aldehydes and ketones by cupric oxide
as a catalyst. They have proposed a homolytic cleavage of C–H and O–H bonds for
the oxidation of alcohols catalyzed by CuO. A similar mechanism may also be
proposed in our case too, when benzoin is oxidized to benzil (Scheme 2). In
this process, CuO is proposed to be reduced to Cu, and this Cu may undergo
aerial oxidation to regenerate CuO.Scheme 2
Proposed mechanism for the oxidation
of benzoin catalyzed by zeolite encapsulated nanocrystalline CuO.Benzhydrol behaves differently with CuO
encapsulated CuY zeolite. With pure CuO, benzhydrol gives benzophenone
as the sole product. However, with CuO encapsulated CuY zeolite, the
oxidized product benzophenone as well as the reduced product, diphenylmethane, is
obtained. It is relevant to note that
under supercritical water, diarylmethanols and its derivatives are transformed
into diarylmethanes and diarylketones [20]
. This disproportionation is
considered to proceed via a
hydrogen transfer from bis(diarylmethyl)ether intermediate. The same group has also reported the
formation of diarylalkanes from diarylketones under supercritical 2-propanol [21]. Earlier, Gribble et al. [22] have reported the
reduction of diarylketone with NaBH4 in CF3CO2H
to yield diarylmethane.Zeolite plays a vital
role in the oxidation/reduction of benzhydrol. In the oxidation of benzhydrol
with pure CuO, benzophenone is formed as the sole product. However, in presence
of CuY zeolite, diphenylmethane, the reduced product is also formed. To account
for this, it is proposed that zeolite supercage provides closer proximity
between the reactants resulting in the formation of the intermediate
bis(diarylmethyl) ether. This ether undergoes disproportionation to form
alcohol and ketone (see Supplementary Figure available online at doi:10.1155/2008/257691). A plausible mechanism is proposed for the oxidation/reduction of benzhydrol
catalyzed by CuO encapsulated CuY zeolite (Scheme3). To confirm this, a
control experiment was also performed
with a physical mixture of Y-zeolite and CuO. Here, a vastly different
products distribution was obtained with only
8.4% of reduced product and the remaining being the oxidized product. This clearly
indicates that the acid-base bifunctionality is due to encapsulation of CuO
inside the zeolite framework.Scheme 3
Proposed mechanism for the
oxidation/reduction of benzhydrol catalyzed by zeolite
encapsulated nanocrystalline CuO.In the oxidation of benzhydrol,
hydrogen transfer to CuO takes place involving homolytic cleavage of hydroxyl group. This
reduces Cu(II) to Cu(I) which can readily eliminate H+.
This H+ will protonate benzhydrol which followed by dehydration
forms the corresponding cation. This cation may be reduced by the Cu(I) to
diphenylmethyl radical. Combination of this with the initially formed diarylmethyleneoxy radical leads to the formation of bis(diarylmethyl) ether intermediate.
Disproportionation of this intermediate proceeds, through intramolecular
hydrogen transfer to give diarylmethane
and diarylketone. The redox behavior of Cu provides a useful
regeneration of the catalyst.In the case of 1-phenylethanol, 1,2-dimethylstilbene is formed as the
major product. To account for this, the following mechanism is proposed (Scheme4). Here, 2,3-diphenyl-2-butanol intermediate is proposed which may undergo
dehydration to form the alkene.Scheme 4
Proposed mechanism for the coupling of 1-phenylethanol
catalyzed by zeolite encapsulated nanocrystalline CuO.


## 3.1. Characterization of Zeolite Encapsulated Nanocrystalline CuO

In this paper, zeolite encapsulated
nanocrystalline CuO is synthesized by the addition of NaOH solution to a suspension of Cu2+-exchanged
zeolites. Here, the procedure similar to that reported by Cao et al. [10] is adopted. They have used
CTAB as capping agent which prevents the aggregation of nanoparticle. But in
our study, the shape of the supercage and its polar active sites are likely to
prevent the aggregation of nanocrystalline CuO and consequently no capping
agent is used. Formation of CuO from Cu2+ is characterized by its
powder XRD and its nano form is confirmed by HRTEM analysis. The prepared
nanocrystalline CuO encapsulated CuY zeolite is characterized by AAS, powder
XRD, SEM, and HRTEM analyses, and the observed results are given below.AAS analysis of CuO encapsulated CuY zeolite
shows 8.4 wt% of copper present in the zeolite. However, with this result
alone, it is difficult to come to a conclusion about the percentage of CuO and
Cu2+ (as a cation) presents inside the zeolite. Powder
XRD pattern of Cu2+-exchanged and zeolite
encapsulated nanocrystalline CuO is  given in Figure 1. The presence of peaks
corresponding to CuO (at 2=34 and 2=38)16 confirms the presence CuO
in the zeolite. Pure Cu2+-Y zeolite also has small peaks that
corresponding to CuO and this may be due to the formation of trace amount of
CuO by partial oxidation of Cu2+ to CuO during cation exchange and
drying the cation-exchanged sample in an air oven. There have been no other
changes in the other diffraction peaks of the zeolite and the crystallinity of
the zeolite is maintained even after the base treatment of the zeolites.Powder XRD patterns of (a) Cu2+ exchange Y-zeolite, and (b) zeolite encapsulated nanocrystalline CuO.
(a)(b)SEM analysis of zeolite encapsulated
nanocrystalline CuO shows the surface morphology of the zeolite (Figure2). Zeolite particles are found to be present as granules of particle size ~500 nm. HRTEM images of zeolite encapsulated nanocrystalline CuO at different magnifications are given in Figure
3. These figures clearly show that the CuO particles are in nanometer range
(<15 nm) and most of the particles (which are less than 2 nm) are embedded
inside the zeolite materials (Figures 3(b) and 3(c)). These CuO nanoparticles
are also spherical in shape. Few other particles in the range of 15 nm are also
found and this is attributed to the passage of high energy beam during the
HRTEM analysis and consequent cage-escape and aggregation of CuO
nanoparticles.SEM images of zeolite encapsulated nanocrystalline
CuO at different magnifications.
(a)(b)(c)(d)HRTEM images of zeolite encapsulated nanocrystalline CuO at different magnifications.
(a)(b)(c)(d)HRTEM images reveal that nanocrystalline CuO particles are highly dispersed and no
aggregation or cluster formation occurs. This may be due to the interaction
between the zeolites active sites and the CuO surface, which have high surface
energy due to its smaller size [17].  Further, the formation of the nanocrystalline
CuO inside the supercages of Y-zeolite was confirmed by the BET pore-size and
pore volume of the CuY-zeolite. The pore size and pore volume of CuY-zeolite
before and after nano CuO formation are 16.11 Å, 0.3179 cc/g and 15.19 Å,
0.2608 cc/g, respectively. The decrease in the average pore diameter and the
pore volume clearly indicates the formation of nanocrystalline CuO within the
supercages of zeolite without damaging the zeolite crystal structure.CuO has been widely used as a catalyst for the
oxidation of alcohols and aromatic hydrocarbons [12–15]
. Here, zeolite encapsulated
nanocrystalline CuO is used for the oxidation of aromatic alcohols.
Since CuO is mildly basic and zeolites are mildly acidic, this combination will
provide an acid-base bifunctional catalyst. In addition, zeolite supercages
will also provide closer proximity between substrates, adjacent acidic, and
basic sites which is expected to increase the rate of the reaction. In general,
catalytic activity of CuO is maximum at higher temperature and hence, we have
developed a solid-phase reactor provided with constant flow of oxygen. The
results obtained for the oxidation of various aromatic alcohols using zeolite encapsulated
nanocrystalline CuO are given in Table 1 and Scheme 1.Table 1
Zeolite encapsulated nanocrystalline CuO catalyzed oxidation of alcohols(a).
S. NoSubstrateCatalyst% Conversion(b)% of products2345X(c)(1)BenzoinCuOY10092.2———7.8(2)BenzhydrolCuOY10029.170.9———(3)CuOY(d)nil—————(4)Cu2+Y21.028.771.3———(5)NaY11.2—100———(6)CuO100100————(7)Y-zeolite-CuO(e)10090.08.4——1.6(8)1-PhenylethanolCuOY100——84.9—15.1(9)CuO78.0100————(10)HydrobenzoinCuOY100———100—(11)Benzyl alcoholCuOY13.7100————(a)Substrate loaded zeolite heated
in solid phase reactor at 200°C for 1 hour under oxygen atmosphere.(b)Analyzed in GC, error limit ±3%.(c)Unidentified products.(d)Reaction
carried out at 100°C.(e)Physical mixture of Y-zeolite and CuO.Scheme 1In the oxidation of benzoin with zeolite encapsulated
nanocrystalline CuO in the solid phase reactor, the oxidation product,
namely, benzil is formed in good yield. But with benzhydrol, benzophenone, and
diphenylmethane, the oxidized as well as reduced products, respectively, are
formed. However, at a lower temperature (100°C), there is no reaction (Table 1, entry 3)
. With
only Cu2+-exchanged zeolite, the conversion is lower and a similar
product ratio is observed. Hence, it is likely that at an elevated temperature,
Cu2+ in oxygen atmosphere may have been oxidized to CuO. Similarly,
when studied in NaY zeolite also, the conversion is very low and only the
reduced product is observed. In the presence of pure CuO, only the oxidized
product is formed exclusively.When 1-phenylethanol is subjected to oxidation
under the same conditions, the oxidative-coupled product an alkene
(1,2-dimethylstilbene) is formed. Hydrobenzoin, an aromatic diol, undergoes
overoxidation to produce benzoic acid. With an aromatic primary alcohol, namely
benzyl alcohol, the conversion is very low (Table1, entry 10). It is likely
that because of its lower boiling point, it may be vaporized before it reacts
with the catalyst as it is found to be condensed on the receiver. Thus, this
method is found to be efficient only for alcohols having higher boiling
points.


## 3.2. Mechanism for the Oxidation of Alcohols

Nagase et al. [18] have studied the dynamic oxidation states of copper in the
course of carbon monoxide oxidation over powdered CuO and Cu2O in a
gas mixture of 5% CO/He and air (O2) by means of an appropriate
combination of evolved gas analysis, thermogravimetry, XRD, and FT-IR in the
temperature range of room temperature to 350°C. They have observed the reduction of CuO to Cu and
the stepwise oxidation of CuCu2OCuO from the
XRD spectra of the products after each process. Hence, the oxidation state of
copper changes thermodynamically between CuO, Cu2O, and Cu as a
function of temperature and oxygen atmosphere. This redox behavior and the difference in oxygen defects, oxygen holes,
and oxygen adsorption species in these oxidation states are thought to be the
reason for the high activity or selectivity of the copper catalyst.Earlier Sheikh and Eadon [19]
have reported
the vapor-phase oxidation of alcohols to aldehydes and ketones by cupric oxide
as a catalyst. They have proposed a homolytic cleavage of C–H and O–H bonds for
the oxidation of alcohols catalyzed by CuO. A similar mechanism may also be
proposed in our case too, when benzoin is oxidized to benzil (Scheme 2). In
this process, CuO is proposed to be reduced to Cu, and this Cu may undergo
aerial oxidation to regenerate CuO.Scheme 2
Proposed mechanism for the oxidation
of benzoin catalyzed by zeolite encapsulated nanocrystalline CuO.Benzhydrol behaves differently with CuO
encapsulated CuY zeolite. With pure CuO, benzhydrol gives benzophenone
as the sole product. However, with CuO encapsulated CuY zeolite, the
oxidized product benzophenone as well as the reduced product, diphenylmethane, is
obtained. It is relevant to note that
under supercritical water, diarylmethanols and its derivatives are transformed
into diarylmethanes and diarylketones [20]
. This disproportionation is
considered to proceed via a
hydrogen transfer from bis(diarylmethyl)ether intermediate. The same group has also reported the
formation of diarylalkanes from diarylketones under supercritical 2-propanol [21]. Earlier, Gribble et al. [22] have reported the
reduction of diarylketone with NaBH4 in CF3CO2H
to yield diarylmethane.Zeolite plays a vital
role in the oxidation/reduction of benzhydrol. In the oxidation of benzhydrol
with pure CuO, benzophenone is formed as the sole product. However, in presence
of CuY zeolite, diphenylmethane, the reduced product is also formed. To account
for this, it is proposed that zeolite supercage provides closer proximity
between the reactants resulting in the formation of the intermediate
bis(diarylmethyl) ether. This ether undergoes disproportionation to form
alcohol and ketone (see Supplementary Figure available online at doi:10.1155/2008/257691). A plausible mechanism is proposed for the oxidation/reduction of benzhydrol
catalyzed by CuO encapsulated CuY zeolite (Scheme3). To confirm this, a
control experiment was also performed
with a physical mixture of Y-zeolite and CuO. Here, a vastly different
products distribution was obtained with only
8.4% of reduced product and the remaining being the oxidized product. This clearly
indicates that the acid-base bifunctionality is due to encapsulation of CuO
inside the zeolite framework.Scheme 3
Proposed mechanism for the
oxidation/reduction of benzhydrol catalyzed by zeolite
encapsulated nanocrystalline CuO.In the oxidation of benzhydrol,
hydrogen transfer to CuO takes place involving homolytic cleavage of hydroxyl group. This
reduces Cu(II) to Cu(I) which can readily eliminate H+.
This H+ will protonate benzhydrol which followed by dehydration
forms the corresponding cation. This cation may be reduced by the Cu(I) to
diphenylmethyl radical. Combination of this with the initially formed diarylmethyleneoxy radical leads to the formation of bis(diarylmethyl) ether intermediate.
Disproportionation of this intermediate proceeds, through intramolecular
hydrogen transfer to give diarylmethane
and diarylketone. The redox behavior of Cu provides a useful
regeneration of the catalyst.In the case of 1-phenylethanol, 1,2-dimethylstilbene is formed as the
major product. To account for this, the following mechanism is proposed (Scheme4). Here, 2,3-diphenyl-2-butanol intermediate is proposed which may undergo
dehydration to form the alkene.Scheme 4
Proposed mechanism for the coupling of 1-phenylethanol
catalyzed by zeolite encapsulated nanocrystalline CuO.


## 4. Conclusions

Zeolite encapsulated nanocrystalline CuO (particle size less than 15 nm) is synthesized successfully and is
characterized by powder XRD, SEM, and HRTEM analyses. The nanoparticles are
highly dispersed. This CuOY is used as catalysts in the oxidation of aromatic
alcohols. Secondary alcohols undergo oxidation to a greater extent and primary
alcohols react very slowly. With benzhydrol, disproportionation takes place
leading to both oxidation as well as reduction products (benzophenone and
diphenylmethane, resp.). CuY zeolite acts as an efficient support for nano CuO,
by stabilizing it and preventing its aggregation. The process is also
significant in that it is a new method for the synthesis of diphenylmethane, a
useful material in spice production and also an important intermediate in drug
and organic synthesis [23]. Plausible mechanisms for the formation of the
various products are also given.



---

*Source: 257691-2008-07-09.xml*