Synthesis, characterization and study of catalytic activity of Silver doped ZnO nanocomposite as an efficient catalyst for selective oxidation of benzyl alcohol

Article abstract of DOI:10.1007/s12039-015-0795-0

Powder samples of Ag/ZnO nanocomposite containing different amounts of Ag were synthesized by co-precipitation method. The synthesized samples were characterized by XRD, SEM, EDX and TEM techniques. The XRD results revealed that all the samples show wurtzite hexagonal phase of ZnO. The TEM micrographs of the samples showed that size of Ag-ZnO nanoparticles was in the range of 30-50 nm. Catalytic activity was tested using liquid-phase selective oxidation of benzylic alcohols to aldehydes. The influence of some parameters such as optimum weight of Ag, catalyst dosage, oxidant and various solvents were studied. The superior catalytic performance of the Ag/ZnO nanocomposite was observed in microwave condition compared to that performed in reflux condition. The catalysts were recycled three times in the oxidation of alcohols and little change in the conversion efficiency was observed. The highly dispersed Ag metal particles on ZnO surface was considered to be responsible for the catalytic activity. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0795-0

J. Chem. Sci. Vol. 127, No. 3, March 2015, pp. 481–491. ꢀc Indian Academy of Sciences.
DOI 10.1007/s12039-015-0795-0
Synthesis, characterization and study of catalytic activity of Silver doped
ZnO nanocomposite as an efficient catalyst for selective oxidation of
benzyl alcohol
∗
A FALLAH SHOJAEI , K TABATABAEIAN, M A ZANJANCHI, H FALLAH MOAFI
and N MODIRPANAH
Department of Chemistry, Faculty of Science, University of Guilan, P. O. Box 1914, Rasht, Iran
e-mail: shoja47@gmail.com
MS received 7 January 2014; revised 21 May 2014; accepted 14 July 2014
Abstract. Powder samples of Ag/ZnO nanocomposite containing different amounts of Ag were synthesized
by co-precipitation method. The synthesized samples were characterized by XRD, SEM, EDX and TEM tech-
niques. The XRD results revealed that all the samples show wurtzite hexagonal phase of ZnO. The TEM
micrographs of the samples showed that size of Ag-ZnO nanoparticles was in the range of 30–50 nm. Catalytic
activity was tested using liquid-phase selective oxidation of benzylic alcohols to aldehydes. The influence of
some parameters such as optimum weight of Ag, catalyst dosage, oxidant and various solvents were studied.
The superior catalytic performance of the Ag/ZnO nanocomposite was observed in microwave condition com-
pared to that performed in reflux condition. The catalysts were recycled three times in the oxidation of alcohols
and little change in the conversion efficiency was observed. The highly dispersed Ag metal particles on ZnO
surface was considered to be responsible for the catalytic activity.
Keywords. Ag/ZnO nanocomposite; selective oxidation; benzyl alcohol; heterogeneous catalysis.
corresponding carbonyl compounds.4,5 Also, molec-
ular oxygen is an interesting oxidant for oxidative
reactions due to their obvious economic and ecolog-
ical advantages, and it has been utilized in oxida-
1
. Introduction
The heterogeneous catalysis is recently extended to
synthetic organic chemistry for production of fine
chemicals and pharmaceuticals. Heterogeneous cata-
1
6
tion of alcohols with transition metals. Heterogeneous
lysts are convenient to use on a large scale, present
a high surface area of the catalytically active phase
and also have many other advantages such as sep-
aration and recycling process compared to homo-
geneous catalysts.2 Among the organic reactions
performed under heterogeneous catalysis, selective oxi-
dation reactions are extensively applied in synthetic
organic chemistry. The selective oxidation reactions
represent one of the most important classes of organic
transformations in chemical synthesis.3 The catalytic
oxidation of alcohols is particularly fascinating because
this reaction can be utilized in the production of car-
bonyl compounds. Carbonyl compounds such as ace-
tophenone are precursors for some pharmaceuticals,
drugs, vitamins, fragrances alcohols, esters and useful
in the food processing or cosmetics industry and are
catalysts for oxidation of alcohols are usually based
7,8
9–11
12,13
14–16
on noble metals such as Pd Au
Ru
Pt
The oxida-
17–23
24–27
and Ag
or combinations of them.
tion reactions are carried out in liquid or liquid-like
phases. Furthermore, heterogeneous oxidations of alco-
hols over metal oxide catalysts have also been studied.
28–30
The studied catalysts are the oxides of magnesium,
31
32–34
35–39
40
41
copper, cerium,
vanadium, molybdenum, niobium,
titanium,
tungsten, cobalt,
42
43
44,45
6,46,47
and zinc.
,4
Recently semiconductor metal oxides nanoparticles
have been widely used for heterogeneous oxidation
of alcohols to the corresponding carbonyl compounds.
Among semiconductor metal oxides, TiO has a great
2
39,48–53
potential in organic synthesis.
For instance,
49,50
Higashimoto et al.
have reported that the photocat-
alytic oxidation of benzyl alcohol and its derivatives
into corresponding aldehydes proceeded at high con-
2
important intermediates in many complex syntheses.
Today, heterogeneous catalytic systems are available
for the selective oxidation of different alcohols to the
version and selectivity on a TiO photocatalyst under
2
53
O atmosphere. Wang et al. studied the selective
2
oxidation of alcohols to aldehydes and ketones over
∗
For correspondence
TiO -supported gold nanoparticles with good yields and
2
481

482
A Fallah Shojaei et al.
selectivities. In another report, Marotta et al.39 have (Zn(Ac)
.2H O) was used as zinc oxide source. The
2
2
studied the selective oxidation of benzyl alcohol to ben- co-precipitation is one of the most successful tech-
zaldehyde in aqueous solution by the TiO /Cu(II)/solar niques for synthesis of ZnO powders. To prepare pure
2
system. Metal oxide nanoparticles have different phys- ZnO and Ag-doped ZnO, the following steps were fol-
ical and chemical properties compared to its bulk mate- lowed. In a typical procedure, 0.01 mol of zinc acetate
rial because of their high surface area. Furthermore, the dihydrate was dissolved in 50 mL of deionized water
surface of semiconductor metal oxides such as TiO2, with stirring for half an hour, thus making precur-
Al O and ZnO exhibit both Lewis acid and Lewis sor solution A. Then, 0.01 mol of sodium carbonate
2
3
base characters. They are good adsorbents for a wide was dissolved in 50 mL of deionized water with stir-
variety of organic compounds, and increase the reac- ring for half an hour, making precursor solution B. In
tivity of the reactants.54 ZnO is a very interesting order to make ZnO nanoparticles, solution A was added
metal oxide as it has surface properties which are dropwise into the solution B while stirring the solu-
potentially favourable for diverse organic reactions. tion for half an hour. Subsequently, a precipitation was
Nano-sized Zinc oxide an inexpensive, non-toxic and obtained. Then, the resultant precipitate was filtered and
easily available Lewis acid catalyst is another semi- washed several times by deionized water. Finally, the
◦
conductor metal oxide which has been widely used as obtained precipitation was dried at 80 C and then calci-
4
7,54–56
◦
catalyst in organic transformations.
In our pre- nated at 300 C for 2 h. To prepare Ag-doped ZnO, the
vious published research works,5
7–63
we synthesized above procedure was repeated. In the synthesis proce-
some catalytic systems and activities of the prepared dure of Ag- ZnO, certain amounts of silver nitrate (3, 5
catalysts were described in relation to different and 7 wt% with respect to zinc acetate dihydrate) were
reactions. In the present work, we investigated the dissolved in deionized water under vigorous stirring
synthesis of efficient Ag-ZnO nanocomposits for oxi- (∼200 rpm) and then the solution was added into the
dation of benzylic alcohols to the corresponding solution of zinc acetate dihydrate/deionized water drop-
aldehydes or ketone under reflux and microwave irradi- wise. In order to make Ag-doped ZnO, solution of zinc
ation. Oxidation reactions are studied in various operat- acetate dehydrate/silver nitrate was added dropwise to
ing conditions such as silver loading, catalyst loading, a solution of sodium carbonate under constant stirring
temperature, solvent, oxidizing agent effect and quan- for half an hour. The resultant precipitate was separated
tities of alcohols. We are interested in exploring the by filtration and washed several times by deionized
◦
utility of Ag-ZnO-nanoparticles as a recyclable, eco- water and so obtained precipitation was dried at 80 C
◦
nomical, cheap and commercially available catalyst and then calcinated at 300 C for 2 h. The nanoparticles
for the oxidation of benzylic alcohols. Operational thus synthesized were then used for further experimental
simplicity, non-toxic catalytic system, practicability, exploration, i.e., catalytic oxidation of benzylic alcohols.
high yields of products, good reusability, easy separa-
2
.3 Catalytic test
tion cost efficient and simple work-up procedure are the
worthy benefits of this protocol.
A certain amount of catalysts (30 mg) was placed
in the two necked, round-bottomed flask and then
1
mmol of alcohol and 10 mL of solvent (acetonitrile,
2
. Experimental
dichloromethane and ethanol) were introduced into the
reactor through a septum. The catalytic experiments
were carried out under reflux and microwave condi-
2.1 General remarks
Zinc acetate, silver nitrate and sodium carbonate (from tion under atmospheric pressure and magnetic stirring,
Merck, Germany) are all analytical grades and used using t-BHP 70%, (0.3–0.5 mL) or hydrogen peroxide
without further purification. Deionized water was used 30% (0.3 mL) as oxidant. The progress of the reaction
in all the experiments.
was followed using thin layer chromatography. After
the reaction was run for the desired time, the products
were filtered out from the catalyst and then oxidation
products were analyzed using a gas chromatograph with
a GC capillary column HP 6890 and a FID detector.
2.2 Preparation of ZnO and Ag/ZnO heterostructure
nanocrystals
The pure and Ag-doped ZnO nanoparticles were pre-
pared by co-precipitation method using the precur-
2
.4 Characterization techniques
2+
+
sors of zinc, silver by the reaction of Zn , Ag and To investigate the morphology of the pure and Ag-
−
OH in an aqueous medium. Zinc acetate dihydrate doped samples scanning electron microscopy (SEM)

Benzyl alcohol oxidation using Ag-ZnO nanocomposite
483
images were obtained on a Philips XL30 equipped with “ꢀ” can be indexed to hexagonal wurtzite ZnO, while
an energy dispersive (EDS) microanalysis system for the others marked with “*” can be indexed to face-
compositional analysis of the doped samples. For the centered-cubic (fcc) metallic Ag. No other crystalline
SEM analysis, samples were covered with Au. The impurities are observed in the patterns. The XRD pat-
particles sizes were obtained by transmission electron terns of 5% and 7% Ag-ZnO shows that the intensity
microscope (TEM) images on a Philips CM10 instru- of the Ag diffraction peak increased, showing aggrega-
ment with an accelerating voltage of 100 kV. X-ray tion of the Ag particles on the surface of ZnO. In addi-
diffraction measurements were recorded by a Philips tion, there is no remarkable shift of Ag-ZnO diffraction
PW1840, X-ray diffractometer using Cu Kα radiation peaks, implying that no Zn1 Ag O solid solution is
−x
x
(
2
λ = 1.54 Å), scan rate 0.02 2θ/s and within a range of formed and the change of the lattice parameters of ZnO
64
θ of 10 to 70 degree at room temperature.
nanocrystals should be negligible. After three cycles,
the catalyst 3% Ag-ZnO was washed, and then char-
acterized using XRD. The results showed that Ag-ZnO
still showed a hexagonal wurtzite crystal structure. On
the other hand, the XRD patterns of Ag-ZnO nanocom-
posite after three runs showed that silver nanoparti-
cles were present on the surface of the catalyst and the
3
. Results and Discussion
3
.1 Crystal structure of ZnO and Ag/ZnO
heterostructure nanocrystals
The XRD patterns of the as-synthesized samples are grain-boundary stability was similar to the as-prepared
shown in figure 1. All the materials showed a hexag- samples.
onal wurtzite crystal structure and high crystallinity of
The preparation of nano Ag-ZnO was carried out
ZnO. The XRD patterns of all the Ag-doped ZnO cat- by zinc acetate dihyrate as zinc precursor. The excess
alysts are almost similar to that of ZnO, suggesting acetate anion adsorbed on the surface of Ag-ZnO could
that there is no change in the crystal structure upon act as a capping agent, which favoured grain-boundary
silver doping process. All XRD patterns in Ag-ZnO stability in the as-prepared catalyst. This type of com-
samples obviously show two sets of diffraction peaks, plexation of acetate anion on the surface of Ag-ZnO
indicating that the as-synthesized products are compos- may be responsible for the decrease in the crystallite
ite materials with high crystallinity. Those marked with size of catalyst in this synthesis.
Figure 1. XRD patterns of ZnO and Ag-ZnO nanocomposite with different Ag contents and after three cycles.

484
A Fallah Shojaei et al.
3.2 Morphology analysis
In order to investigate the morphology of the obtained
samples, comparison between the SEM images of the
ZnO and 3% wt Ag-ZnO is illustrated in figure 2. In
figure 2A, typical SEM image of ZnO nanoparticles
is shown. The image reveals that most of them are
irregularly shaped and relatively spherical with dimen-
sions that are less than 100 nm. Figure 2B shows the
SEM image of 3% Ag-ZnO nanoparticles. The Ag-
ZnO sample shows a surface morphology similar to that
of the bare ZnO. The particles in this sample have
relatively a sphere-like morphology and the nanopar-
ticles were composed of agglomerates of Ag-ZnO
particles. To check the chemical composition of the
synthesized ZnO nanocomposite, an energy-dispersive
X-ray (EDX) spectroscopy analysis was performed.
Figure 3. EDS analysis of 3 wt% Ag-ZnO.
Figure 3 shows the EDX spectroscopy for 3% Ag-ZnO consisting of metallic Ag nanoparticles. The black dots
nanocomposite. The existence of the Ag dopant can be are Ag nanoparticles and Ag aggregates are not found
clearly observed in the spectrum.
in our TEM observations, indicating that all metallic
In order to obtain detailed information about the Ag nanoparticles are highly dispersed in ZnO nanocrys-
exact size and morphology of the as-synthesized sam- tals. It is obvious that the diameters of Ag nanoparti-
ples, TEM images are presented. Figure 4 depicts cles are in the range of 20–40 nm. After three reaction
transmission electron micrographs of ZnO nanoparti- cycles, the catalyst 3% Ag-ZnO was washed, and then
cles, 3% wt Ag-ZnO and 3% wt Ag-ZnO after three characterized using TEM analysis. Figure 4C shows the
runs. TEM image of ZnO particles (figure 4A) shows TEM image of 3% wt Ag-ZnO after three cycles of
that the ZnO consists of nanoparticles with aver- reaction. The TEM image of sample shows the Ag/ZnO
age size of 20–70 nm. The TEM image of Ag-ZnO heterostructure consisting of metallic Ag nanoparticles
shows the shape of the Ag-doped ZnO nanoparticles, and the grain-boundary stability was achieved in the
observed as aggregates with average particle size of as prepared catalyst. On the other hand, TEM image
3
0–50 nm (figure 4B). The TEM image of the sam- showed no obvious changes in the surface morphology
ple shows the Ag/ZnO heterostructure nanocrystals indicating that the catalyst had good mechanical stability.
Figure 2. SEM images of: (A) ZnO and (B) 3 wt% Ag-ZnO nanocomposite.

Benzyl alcohol oxidation using Ag-ZnO nanocomposite
485
Figure 4. TEM images of: (A) ZnO, (B) 3% wt Ag-ZnO nanocomposite and (C) 3% wt Ag-ZnO after three cycles.
3.3 Catalytic activity study
oxidation process. The oxidation reaction takes place
on the pure ZnO with low conversion, but with the
increase of Ag content in Ag-ZnO catalysts, the conver-
sion of alcohol increased significantly. In fact, the cat-
alyst properties are closely associated with the amount
of metal active sites and the size of Ag. Higher Ag load-
ing obviously ensures sufficient active Ag sites.10 As
shown in table 1, the 3 wt% Ag-doped ZnO sample had
suitable catalytic efficiency and the optimal Ag doping
is 3 wt%. It is reported that the supported Ag catalyst
needs an induction period to reach its optimal activity.
We studied the oxidation of different types of alcohols,
including primary and secondary benzylic alcohols.
They were efficiently oxidized to their corresponding
carbonyl compounds with Ag-ZnO as the catalyst. The
catalytic oxidation was carried out at both reflux and
microwave condition. In the presence of pure ZnO cata-
lyst, the oxidation of alcohols was also tested and result
is shown in table 1.
3
.3a Effect of Ag doping concentration on catalytic The presence of induction time indicates that the metal-
properties of Ag/ZnO: To evaluate the catalytic activ- lic Ag particles on the catalyst surface are required to be
ity of the ZnO and Ag-doped ZnO and determine the activated at the initial stage and the oxygen molecules
optimum amount of Ag doping, oxidation experiments adsorbed on the metallic Ag is dissociated to create an
+
on benzhydrol (Diphenylmethanol) were carried out by oxidized Ag surface (Ag ) for the oxidation of ben-
17,18
different Ag amounts of doped samples ranging from zyl alcohol.
to 7 wt%, and the results are given in table 1. As constant value of more than 85%.
shown in table 1, Ag loading had a remarkable effect
Since, support ZnO has very low catalytic activ-
After 15 min, the conversion reaches a
1
on benzhydrol conversion. It can be seen that the con- ity (table 1), it is believed that the pre-adsorbed oxy-
version increased with increasing load of Ag during the gen and oxygen species from oxidized Ag surface are

486
A Fallah Shojaei et al.
Table 1. Oxidation of benzhydrol in presence of ZnO and Ag-ZnO with different
Ag doping concentration under reflux condition (1 mmol benzhydrol, 30 mg catalyst,
0.4 mL TBHP and 10 mL acetonitrile).
Samples
Time (min)
Conversion (%)
No catalyst
ZnO
45
30
15
15
15
15
5
10
60
85
85
90
1
3
5
7
wt% Ag-ZnO
wt% Ag-ZnO
wt% Ag-ZnO
wt% Ag-ZnO
alcohol occurs not only on the Ag particle surface but
is also dependent on the ZnO support. If the metal and
support are separated from each other, transportation of
oxygen species would be inhibited.
3.3b Effect of catalyst weight: The effect of 3 wt%
Ag-ZnO weight on the oxidation of the benzhydrol was
studied in the weights of 20, 30 and 40 mg (table 2). It
is notable that catalytic activity is proportional to the
amount of the catalysts. A negligible conversion of ben-
zhydrol (only 5%) was obtained in the absence of the
catalysts, indicating that the presence of catalyst is nec-
essary for the benzhydrol oxidation (table 1). A small
but significant increase in the rate of the catalytic oxi-
dation observed with an increase in the catalyst weight
from 20 to 40 mg. On the other hand, the conversion
increased from 60 to 90% by increasing the catalyst
amount from 20 to 40 mg. In the higher amount of the
catalyst, the chances are better for the reactants (ben-
zhydrol and TBHP) to react before diffusing out of the
catalysts surface. Consequently, the extent of oxida-
tion of benzhydrol increases on increasing the catalyst
amount and suitable efficiency (85%, after 15 min) of
benzhydrol was obtained at catalyst dosage of 30 mg.
Scheme 1. Schematic reaction mechanism of benzyl alco-
hol oxidation on Ag-ZnO nanocomposite.
responsible for the alcohol oxidation. Oxidized Ag
species are dispersed on the surface of dominant metal-
17
lic Ag particles. Metallic Ag seems to have a higher
affinity for gaseous O . The oxidized Ag surface on the
2
supported metallic Ag was proposed to be an active site
for the oxidation of benzyl alcohol.18
Based on the aforementioned considerations, a plau-
sible schematic model of the active sites of the sup-
ported Ag catalyst is proposed for catalytic oxidation of 3.3c Effect of reaction temperature on oxidation pro-
benzyl alcohol and illustrated in scheme 1. cess: The catalytic activity of 3 wt% Ag-ZnO catalyst
In the selective oxidation of benzyl alcohol, the was investigated on the oxidation of the alcholos at
◦
◦
gaseous oxygen molecules can be adsorbed on the reflux condition (80 C) and room temperature (25 C)
surface of the metallic Ag particle and dissociate to (tables 3 and 4). The results show that the catalytic
−x
form the atomic oxygen species such as Oad through activity of Ag-ZnO nanocomposits at reflux condition
17,18
electron transfer from the metallic Ag particles.
is much higher than the room temperature. It can be
The surface of supported metallic Ag will be positive- seen that conversion of benzhydrol and benzyl alco-
+
charged with formation of Ag species.
hol largely increased at reflux condition. Therefore,
The atomic oxygen species on the surface of Ag high temperature favours the reaction as the benzhydrol
first migrate to the ZnO surface to react with the acti- conversion increased from 40 to 85% and also benzyl
vated alcohol molecules adsorbed on the ZnO, leading alcohol conversion increased from 30 to 90% when the
+
−
◦
◦
to the formation of an electron-rich state (Ag + e → temperature was elevated from 25 C to 80 C. There-
0
◦
Ag ) on the Ag particle surfaces. The generated atomic fore, reflux condition (80 C) was considered the opti-
oxygen species with nucleophilic character activate the mum reaction temperature. As shown in table 1, under
C–H and O–H bonds in benzyl alcohol to produce cor- reflux condition, benzyl alcohol and its derivatives sub-
responding aldehyde. However, the oxidation of benzyl stituted by –Cl, –Br, –NO , –CH(CH ) , –CH(OH)CH
2
3
2
3

Benzyl alcohol oxidation using Ag-ZnO nanocomposite
487
Table 2. Oxidation of benzhydrol in presence of different amount of 3 wt% Ag-ZnO
(1 mmol benzhydrol, 30 mg catalyst, 0.4 mL TBHP and 10 mL acetonitrile).
Catalyst weight
Conversion (%)
20 mg
60%
30 mg
85%
40 mg
90%
Table 3. Oxidation of benzhydrol and benzyl alcohol in presence 3 wt% Ag-ZnO at room temperature (1 mmol alcohols,
0 mg catalyst, 0.4 mL TBHP and 10 mL acetonitrile).
3
Entry
Substrate
Product
Time(min)
Conversion %
GC Yield %
1
2
60
60
40
30
35
27
Table 4. Oxidation of benzylic alcohols to their corresponding carbonyl compounds using 3 wt% Ag-ZnO under reflux
condition (1 mmol alcohols, 30 mg catalyst, 0.4 mL TBHP and 10 mL acetonitrile).
aEntry
Substrate
Product
Time (min)
Conversion
GC Yield %
1
2
30
15
90
85
77
80
3
4
5
95
85
90
80
15
5
6
15
45
90
40
85c
35
7
8
45
30
30
90
25
86
9
30
90
84

488
A Fallah Shojaei et al.
Table 5. Oxidation of benzhydrol and benzyl alcohol in presence different solvents
(
3 wt% Ag-ZnO, reflux, (1 mmol alcohols, 30 mg catalyst, 0.4 mL TBHP and 10 mL
acetonitrile, 15 min).
Solvents
Acetonitrile
Ethanol
Dichloromethane
Benzhydrol (conversion %)
Benzyl alcohol (conversion %)
85
90
60
40
40
30
Table 6. Oxidation of benzhydrol and benzyl alcohol in presence t-BHP and H2O2
3 wt% Ag-ZnO, reflux, 1 mmol alcohols, 30 mg catalyst, 0.4 mL TBHP and H2O2
(
and 10 mL acetonitrile).
Oxidant
Alcohols
t-BHP
conversion%
H2O2
time
time
Conversion %
Benzhydrol
Benzhyl alcohol
15
15
85
70
60
60
40
30
Table 7. Reusability of 3 wt% Ag-ZnO for oxidation of benzhydrol and benzyl alcohol (reflux, 1 mmol alcohols, 30 mg
catalyst, 0.4 mL TBHP and 10 mL acetonitrile).
Run 1
Conversion (%)
Run 2
Conversion (%)
Run 3
Conversion (%)
Reusability
Alcohols
Time (min)
Time (min)
Time (min)
Benzhydrol
Benzhyl alcohol
15
15
80
70
15
15
75
65
15
15
67
55
and–CH(OH)C H groups were converted to corres- e.g., tetrabuthyl hydroperoxide (t-BHP) and hydrogen
2
5
ponding aldehydes with different conversion and yield peroxide (H O ) (table 6). The results show that the cat-
2
2
on Ag-ZnO, while no other products were observed.
alytic activity of Ag-ZnO nanocomposits in presence
of t-BHP (t-BHP/alcohol molar ratio of 0.25) is much
3
.3d Effect of different solvents: To investigate the higher than that of H
effect of solvents in the catalytic activity of the Ag- good reaction time was obtained by t-BHP while H
doped ZnO, oxidation of the benzhydrol and benzyl is not found to be effective for the oxidation of alcohols.
alcohol was studied in various other solvents e.g., ace- Actually, when H was utilized for the oxidation of
tonitrile, ethanol and dichloromethane (table 5). It can alcohols, most of H molecules were decomposed
O . Maximum conversion and
2 2
O
2
2
O
2
2
O
2
2
60
be seen that, acetonitrile is a suitable solvent for the into O
oxidation reaction. As shown in table 5, the best results
and the reaction time was prolonged.
2
were obtained when acetonitrile was used as the sol- 3.3f Reusability of catalytic systems: The reusabil-
vent. However, in the pretence of acetonitrile 85% and ity of the synthesized catalysts is a significant factor
70% conversion were achieved for benzhydrol and ben- from environmental and economical points of view.
zyl alcohol. Ethanol and dichloromethane were also In heterogeneous catalysts the leaching of metal is a
used as the solvent where a 60% and 40% of conver- main problem, which leads to loss of catalyst activ-
sion was observed in benzhydrol in 15 min. It can be ity on subsequent uses. In order to investigate the sta-
concluded that the polarity index of acetonitrile (ace- bility of the catalytic systems, recycling experiments
tonitrile: 5.8, ethanol: 5.2 and dichloromethane: 3.1) is were performed. The catalyst after reaction were recov-
higher than other solvents and strong polarity of solvent ered by centrifugation and washed with deionized water
may be useful for the oxidation of alcohols (table 5).
and ethanol, then dried and reused for catalytic reac-
tion under the similar conditions. The catalytic activ-
3.3e Effect of oxidants: The utilization of oxidants ity of the Ag-doped ZnO decreased when the recovered
plays a crucial role in the oxidation process. To eval- catalytic system was reused for further catalytic reac-
uate the oxidants’ effect in the catalytic activity of tions and in the third cycle, the conversion was reduced
the Ag-doped ZnO, oxidation of the benzhydrol and to nearly 67% and 55% for benzhydrol and benzyl
benzyl alcohol was studied in presence of two oxidants respectively, as shown in table 7.

Benzyl alcohol oxidation using Ag-ZnO nanocomposite
489
Table 8. Oxidation of benzylic alcohols to their corresponding carbonyl compounds using 3 wt%Ag-ZnO under microwave
condition (1 mmol alcohols, 30 mg catalyst, 0.4 mL TBHP and 10 mL acetonitrile).
Entry
Substrate
Product
Time(min)
Conversion
GC Yield %
1
2
3
3
70
80
65
75
3
4
2
90
90
87
82
2.5
5
6
2.5
3
90
50
85
40
7
8
3
30
90
25
85
2.5
9
2.5
80
78
3.3g Effects of reaction conditions and reaction time reduced times of reaction (from 30 h to 3 min) as well
(comparison of oxidation of alcohols under microwave as increased the conversion of starting material in the
and reflux conditions: In order to study the influ- oxidation under relatively similar reaction conditions.
ence of the reaction conditions, oxidation experiments This demonstrates that the microwave irradiated syn-
were assayed in different conditions e.g., reflux and thesis is superior to that of conventional methods in
microwave (tables 4, 8). Microwave method has shown terms of reaction time and energy consumption. The
the best results in shorter time. The reaction rate is high- production yields for the microwave reaction were a lit-
est in the microwave state and is lower in the reflux tle lower but comparable to those achieved by reflux
condition. Table 8 summarizes the main results for the process. Whereas the microwave reaction were mean-
oxidation of benzyl alcohol under microwave irradia- ingfully shorter, it was difficult to maintain a con-
tion using Ag-ZnO catalyst. As shown in table 8, com- stant temperature; though, the reflux condition was
pared with reflux procedure, in microwave reaction, heated more slowly. The advantages of microwave syn-
benzylic alcohol and its derivatives were successfully thesis are that the reactions under dielectric heating
converted to their corresponding aldehydes with higher show higher yields and purities, and in many cases
conversion at very short time. Results showed that good the microwave synthesis is considered as environment
conversions of starting material (>50%) were obtained friendly method for organic transformation.65 The
after 2–3 min of reaction. The use of microwaves advantage of microwaves as novel and cleaner reaction

490
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4. Frassoldati A, Pinel C and Besson M 2011 Catal. Today
4
. Conclusion
1
73 81
In order to perform effective, selective liquid-phase oxi-
dation of benzyl alcohols, Wurtwize ZnO nanocompos-
1
5. Gangwal V R, Van Wachem B G M, Kuster B F M and
Schouten J C 2002 Chem. Eng. Sci. 57 5051
ite that was doped with silver was synthesized using 16. Frassoldati A, Pinel C and Besson M 2013 Catal. Today
2
03 133
7. Jia L, Zhang S, Gu F, Ping Y, Guo X, Zhong Z and Su F
012 Microporous Mesoporous Mater. 149 158
co-precipitation method in a single step. The Ag-ZnO
showed high activity and conversion in the oxida-
tion of benzyl alcohol to benzaldehyde. The catalytic
activity increased as the silver loading increased. The
most suitable conditions such as shorter reaction time,
1
1
1
2
2
2
2
8. Yamamoto R, Sawayama Y, Shibahara H, Ichihashi Y,
Nishiyama S and Tsuruya S 2005 J. Catal. 234 308
9. Deng M, Zhao G, Xue Q, Chen L and Lu Y 2010 Appl.
Catal. B: Environ. 99 222
0. Raji, Vadakkekara, C Mousumi and Parikh P A 2012
Ind. Eng. Chem. Res. 51 5691
1. Sawayama Y, Shibahara H, Ichihashi Y, Nishiyama S
and Tsuruya S 2006 Ind. Eng. Chem. Res. 45 8837
2. Ma L, Jia l, Guo X and Xiang L 2014 Chin. J. Catal. 35
108
◦
acetonitriles as solvent, temperature of 80 C, catalyst
amount of 30 mg, TBHP/alcohol molar ratio of 0.25,
Ag loading of 3% were obtained in reflux condition.
It is a useful strategy for the synthesis of aldehydes
or ketones, without the formation of carboxylate com-
pounds. Results also showed that the catalytic activ-
ity of Ag-ZnO nanocatalyst at microwave condition is 23. Chen D, Qu Z, Shen S, Li X, Shi Y, Wang Y, Fu Q and
Wu J 2011 Catal. Today 175 338
much higher than that of reflux condition. The reactiv-
ity order of the alcohols examined in this work was that
aromatic alcohols like benzhydrol were more reactive
than aliphatic alcohols. The reusability of the ZnO sup-
ported with silver was ascertained and catalytic perfor-
2
4. Miedziak P, Sankar M, Dimitratos N, Lopez-Sanchez J
A, Carley A F, Knight D W, Taylor S H, Kiely C J and
Hutchings G J 2011 Catal. Today 164 315
5. Chen Y, Wang H, Liu C J, Zeng Z, Zhang H, Zhou C,
Jia X and Yang Y 2012 J. Catal. 289 105
2
mance of the regenerated catalyst was comparable to 26. Liotta L F, Venezia A M, Deganello G, Longo A,
Martorana A, Schay Z and Guczi L 2001 Catal. Today
that of the fresh catalyst.
66 271
2
7. Ma C Y, Dou B J, Li J J, Cheng J, Hu Q, Hao Z P and
Qiao S Z 2009 Appl. Catal. B 92 202
Acknowledgements
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8. Hu J Sun K He D and Xu B 2007 Chin. J. Catal. 28 1025
9. Choudhary V R and Dumbre D K 2009 Catal. Commun.
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The authors are grateful to the Research Council of
University of Guilan for partial support of this study.
1
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