A novel and active catalyst Ag/ZnO for oxidant-free dehydrogenation of alcohols

Article abstract of DOI:10.1016/j.materresbull.2015.07.019

Nano Ag/ZnO catalysts were prepared by varying load of Ag on ZnO supports using a new and very simple method. The structure of nano Ag/ZnO has been confirmed by various techniques. The Ag/ZnO with 7.4 × 10^-5 mol% of Ag has pore size distribution about 2.74 nm and this nano Ag/ZnO is found to be the best catalyst for oxidation of primary and secondary benzyl alcohols into corresponding aldehydes and ketones in oxidant-free at the atmospheric pressure. The influence of various parameters such as: solvent, base, temperature, time of reaction, etc. has been systematically studied on nano Ag/ZnO catalyst.

Full text of DOI:10.1016/j.materresbull.2015.07.019

Accepted Manuscript
Title: A Novel and Active Catalyst Ag/ZnO for Oxidant-Free
Dehydrogenation of Alcohols
Author: Mona Hosseini-Sarvari Tahereh Ataee-Kachouei
Fatemeh Moeini
PII:
S0025-5408(15)30040-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2015.07.019
MRB 8327
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Accepted date:
5-2-2014
30-6-2015
16-7-2015
Please cite this article as: Mona Hosseini-Sarvari, Tahereh Ataee-
Kachouei, Fatemeh Moeini, A Novel and Active Catalyst Ag/ZnO for
Oxidant-Free Dehydrogenation of Alcohols, Materials Research Bulletin
http://dx.doi.org/10.1016/j.materresbull.2015.07.019
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A Novel and Active Catalyst Ag/ZnO for Oxidant-Free
Dehydrogenation of Alcohols
Mona Hosseini-Sarvari,*Tahereh Ataee-Kachouei and Fatemeh Moeini,
Department of Chemistry, Shiraz University, Shiraz 71454, I. R. Iran.
To whom correspondence should be addressed:
Dr. Mona Hossseini-Sarvari
TEL: +98-711-6137169
FAX: +98-711-6460788
E-mail: hossaini@shirazu.ac.ir; monahosseini20@gmail.com
Graphical abstract
Synthesis and characterization of nano Ag/ZnO by varying loading of Ag on ZnO support using a new and
very simple method has been investigated.
1

Highlights
 Synthesis and characterization of small amount of Ag supported on ZnO
 Nano Ag/ZnO as highly efficient heterogeneous catalyst
 Nano Ag/ZnO for oxidant-free oxidation
 nano Ag/ZnO has been confirmed by various methods
Abstract
Nano Ag/ZnO catalysts were prepared by varying load of Ag on ZnO support using a new and
very simple method. The structure of nano Ag/ZnO has been confirmed by various techniques.
The Ag/ZnO with 7.4×10-5 mol% of Ag has pore size distribution about 2.74 nm and this nano
Ag/ZnO is found to be the best catalyst for oxidation of primary and secondary benzyl alcohols
into corresponding aldehydes and ketones in oxidant-free at the atmospheric pressure. The
influence of various parameters such as: solvent, base, temperature, time of reaction, and etc. has
been systematically studied on nano Ag/ZnO catalyst.
Keywords: Nano Ag/ZnO; heterogeneous catalyst; oxidation; oxidant-free oxidation
2

1. Introduction
Oxidation of alcohols to carbonyl compounds is one of the important transformations in organic
synthesis, since the oxidation products are essential intermediates in the manufacture of high-value fine
chemicals, agrochemicals, pharmaceutical and high-tonnage commodity chemicals [1,2]. To accomplish
economical and environmental acceptability, much effort has been applied to the oxidation of alcohols
with environmentally friendly oxidants such as oxygen (O2) or hydrogen peroxides [3], because of the
necessity replacing stoichiometries reactions involving toxic oxidants such as chromium and manganese
salts and other classical procedures [4]. Even though in many of these aerobic oxidation methods,
homogeneous systems were used and share common disadvantages because work up, and catalyst reuse is
difficult.
In fact, from the safety and environmental issues, using more atom-efficient catalyst systems, which
do not use molecular oxygen or air for dehydrogenation of alcohols to carbonyl compounds and
molecular hydrogen would be ideal. So, using an oxidant-free methodology is particularly interesting both
from an environmental and a practical point of view because i) the formation of H2O, as a byproduct is
eliminated, which often tedious purification of products from the aqueous reaction mixture is needed, ii) it
is useful for alcohols bearing functional groups which are sensitive to O2, iii) H2 is produced, which it is
an attractive feedstock for energy generation, and iv) it is avoided possible over oxidation of the substrate
to carboxylic acids. Therefore, a series of transition metal catalysts (Ru, Ir, Au, Ag, Pt, Cu and etc.) have
been developed for oxidant-free dehydrogenation of alcohols. Homogeneous and heterogeneous Ru [5-9]
and Ir [10–13] catalysts have been reported for dehydrogenation of alcohols, but they variously suffer
from drawbacks such as difficulties in catalyst reuse, the requirement of acid or base additives, difficulties
in catalyst synthesis, manipulation, high cost, and low activity for the primary alcohol dehydration.
Heterogeneous Pt [14,15], Au [16], Cu [17,18], Co [19], Ni [20], and Ag [21,22] catalysts were also
3

reported to be effective. Among them, there are a few reports on using Ag catalysts for the oxidant-free
dehydrogenation of alcohols. Although these supported silver nanoparticles are attractive as the most
active catalyst for this reaction, but they suffer from drawbacks. In these few methods before using the
catalyst it should be treated under H2 in order to reduce AgI to Ag0, the amount of silver used as catalysts
are 0.005 and 1-10 mole%, respectively, and the reactions were placed under Ar or N2 flows.
ZnO, as one of the most semiconductor oxide nanostructures is attracting significant attention
because they can act as potential candidates for applications in energy generators, sensors, lasers,
optoelectronics and light-emitting diodes [23-27]. In particular, nano zinc oxide has been proven to
exhibit better photocatalytic proficiency for the degradation of some organic compounds than that of the
TiO2, which is currently the most extensively-studied photocatalysis. ZnO is also biodegradable, nontoxic
and biocompatible for environmental applications. Recent researches revealed that modification with
noble metals, such as Au, Ag, or Pt, is an effective way to prohibit the recombination of charge carriers
and to raise the photocatalytic performance of ZnO photocatalyst for degrading toxic organic pollutants.
Especially, Ag/ZnO nanostructures with various morphologies have been obtained with different
synthesis strategies [28-31].
Being an eco-friendly and high performance catalyst for the oxidation of alcohols, herein, in this
study, the catalysts Ag/ZnO was prepared by a simple method, and it was used as an active and efficient
catalyst for the oxidation of a wide range of benzylic alcohols for the first time.
2. Experimental
2.1. Preparation of nano Ag/ZnO
All the reagents, including, Zinc nitrate (Zn(NO3)2.6H2O), silver nitrate (AgNO3) and urea were all
obtained from Fluka or Merck and used as received without further purification. The synthesis of nano
4

Ag/ZnO is as follows: Zn(NO3)2.6H2O, AgNO3 and CO(NH2)2 were dissolved in a minimum quantity of
deionized water to form a transparent solution. In practice, the addition of 10% more than the
stociometric amount of urea was preferred. The mixture was refluxed for 6 h at 100◦C. The product was
centrifuged and washed with deionized water and absolute ethanol. Then the Ag/ZnO nano powders were
◦
allowed to dry at 80◦C in an oven. Then the nanoparticles were annealed at 500 C for 3 h in high
temperature furnace. According to this method, four kinds of nano Ag/ZnO were prepared in more than
95% isolated yields which the molar ratio of AgNO3: Zn(NO3)2.6H2O was 1:1, 1:2, 1:4, and 1:8,
respectively.
2.2. General procedure for the dehydrogenation of benzylic alcohols using Ag/ZnO nanoparticles
A mixture of alcohol (1 mmol), KOH (1 mmol), and 0.005g Ag/ZnO nanoparticles (2) (containing 8x10-5
o
g Ag) in toluene (2 mL) was stirred at 100 C for the appropriate time. The reaction was monitored by
thin layer chromatography (TLC) or GC. After completion of the reaction, the reaction mixture was
diluted with EtOAc and centrifuged to remove the catalyst. The filtrate was extracted with water, the
organic layer dried over CaCl2 and evaporated under reduced pressure. The resulting product was purified
by column chromatography on silica-gel to afford the pure product. All compounds are known and were
characterized by comparison of their physical and spectroscopic data with the already described in the
literature.
3. Results and Discussion
3.1. Catalyst preparation
5

Nano Ag/ZnO catalyst was prepared in high yield by precipitation method (more than 95% isolated
yield). Based on related literature [32-37], a possible mechanism is proposed, as schematically in
equations 1-9.
+
CO(NH2)2 + 3 H2O
2NH4 + CO2 +2OH-
(1)
(2)
(3)
Zn2++ 2NO3
-
Zn(NO3)2 .6 H2O + H2O
-
Ag+ + NO3
AgNO3 + H2O
Zn2+ + 2OH-
Ag+ + OH-
(4)
(5)
(6)
(7)
Zn(OH)2
AgOH
Zn(OH)4
Zn(OH)2 + 2OH-
AgOH+ OH-
2-
2-
-
Ag(OH)2
2H O
4OH-
-
Ag O / ZnO
(8)
(9)
Zn(OH)4 + 2Ag(OH)2
+
2
+
2
+
Ag2O / ZnO + NH4
Ag/ZnO
At first, Zn(NO3)2 and AgNO3 were reacted with H2O and corresponding Zn2+ and Ag+ ions were
formed (eq. 2, 3). By the addition of urea, Zn(OH)2 and AgOH were formed (eq. 4,5). When the amount
of urea increased (addition of 10%) more than the stoichiometric amount, Zn(OH)2and AgOH will be
2-
-
gradually dissolved into Zn(OH)4 and Ag(OH)2 , respectively (eq. 6,7). Under the alkaline thermal
2-
-
condition, intermolecular dehydrolysis between Zn(OH)4 and Ag(OH)2 may occur, and Ag2O/ZnO
formed (eq. 8). Urea here plays a dual role, acting not only as a co-solvent but also as a reducing agent to
reduce Ag+ into Ag. Finally, nano Ag/ZnO would be produced.
According to this method, four kinds of nano Ag/ZnO were prepared. The content of Ag in nano
Ag/ZnO was varied from 10-4 g (9.3×10-5 mol%) (Ag/ZnO (1)), 8x10-5 g (7.4×10-5 mol%) (Ag /ZnO (2)),
4x10-5 g (3.6×10-5 mol%) (Ag/ZnO (3)), and 3.5x10-5 g (3.3×10-5 mol%) (Ag/ZnO (4)), which the molar
6

ratio of AgNO3: Zn(NO3)2.6H2O was 1:1, 1:2, 1:4, and 1:8, respectively. The physical appearance of
these catalysts can be seen in Figure 1. The natural color of ZnO is white. By increasing the amount of Ag
doped on ZnO, the colors of the catalysts were changed from white to dark-brown.
3.2. Characterization of the catalyst
X-ray diffraction (XRD) patterns of nano Ag/ZnO with various amounts of Ag doped on ZnO, nano Ag
and nano ZnO are shown in Figure 2. The diffraction peaks could be categorized into two sets. Those
marked with “#” are the diffraction peaks at 2θ = 32o, 34o, and 36o and are indexed to (100), (002), and
(101) diffractions of hexagonal structured ZnO (wurtzite) (JCPD No. 36-1451) respectively, while the
others marked with “*” and located at 2θ = 38o and 44o are characteristic to (111) and (200) diffractions
of FCC structured Ag (JCPDS No. 04-0783), respectively. The sharp diffraction peaks indicate that the
nano Ag/ZnO is highly crystallized. Besides, the diffraction peaks corresponding to silver oxide species,
for example Ag2O at 32.97◦ are not detected, so it can be concluded that there is no silver oxide species
in the samples as well [38]. Furthermore, from (a) to (f) a consistent decrease in the intensity of silver
peaks can be noted with the decrease in concentration of Ag from 9.3×10-5, 7.4×10-5, 3.6×10-5 and
3.3×10-5 mol%, respectively. Additionally, the average sizes of the ZnO and Ag nanoparticles, calculated
by the Scherrer formula [39] applied to the major peaks (2θ = 34o for ZnO) were estimated to be
approximately 36.28, 36.02, 39.6, 30.13 and 65.04 nm for Ag/ZnO (1), Ag/ZnO (2), Ag/ZnO (3), Ag/ZnO
(4) and nano ZnO, respectively, and (2θ = 38o for Ag) were estimated to be approximately 45.83 nm,
43.79 nm, 53.31 nm, 31.55 nm and 25.57 nm for Ag/ZnO (1), Ag/ZnO (2), Ag/ZnO (3), Ag/ZnO (4) and
nano Ag, respectively. With the obtained XRD results, indicated that the present catalyst (Ag/ZnO) shows
the presence of Ag only, and not any peaks for silver oxides (AgO, or Ag2O) were observed. In
subsequent paragraphs we will prove this by XPS method.By comparing the XRD pattern of pure
7

hexagonal ZnO and nano Ag/ZnO (Figure 3), there is a shift in the peak position of ZnO for the prepared
Ag/ZnO samples (~0.15o). Generally this shift in peak position value is observed when a doping element
having larger ionic radius than Zn2+ is replaced at the substitutional sites of the ZnO crystal lattice. Zn2+
o
o
ion (0.71 A) has lower ionic size than Ag+ (1.22 A). So, this shift toward the XRD peak position can
suggest that Ag ion has occupied the interstitial sites of ZnO [40].
The surface structure of the Ag/ZnO sample with an Ag content of 7.4× 10-5 % was investigated
by using X-ray photoelectron spectroscopy (XPS) analysis, and the results are shown in Figure 4. All of
the peaks in the curve can be ascribed to Ag, Zn, O, and C elements, while C 1s at 285 eV is due to the
hydrocarbon from the XPS instrument itself [41]. Clearly all the peaks are ascribed to Zn, O, and Ag, and
no peaks characteristics of impurities are observed. Figure 4b, shows the Zn 2p XPS spectra of the
catalyst. Because of strong spin-orbit coupling, the Zn 2p peak split into Zn 2p3/2 and Zn 2p1/2 with a
doublet peak energy separation of ~23 eV. The peak positions at 1022.8 and 1045.7 eV corresponds to
the Zn 2p3/2, and Zn 2p1/2, respectively, which confirm that the Zn in the catalyst mainly exists for the
form of Zn2+ [42]. Figure 4c provides the XPS spectra of Ag. The Ag 3d5/2 and 3d3/2 peaks appear at a
binding energy of 367.5 and 373.1 eV, respectively. Here, interestingly, the peaks of Ag in our work were
found to shift to the lower binding energy compared with the standards value (about 368.2, and 374.1
eV). This confirms the interaction between Ag and ZnO nano crystals. When the Ag and ZnO
nanoparticles attach together, they adjust the position of their corresponding Fermi energy levels to the
same value. Thus, there are many free electrons above the new Fermi level of metallic Ag nanoparticles.
Because the conduction band (CB) of ZnO nanoparticles is vacant, the free electrons could tunnel into the
CB, resulting in the higher valence of Ag [43,44]. This is in consistent with the results from XRD. Figure
4d shows the O 1s peaks of Ag/ZnO catalyst. The O 1s peak can be divided into two peaks at about 530
and 532 eV, corresponding to the lattice oxygen and surface hydroxyl oxygen, respectively.
8

The Brunauer-Emmett-Teller analysis (BET) gas adsorption–desorption measurements of nano
Ag/ZnO (2) catalyst synthesized in this work is measured and the results are shown in Table 1.
The morphology of nano Ag/ZnO (2) obtained from transmission electron microscopy (TEM)
analysis was displayed in Figure 5. The size of Ag/ZnO nanoparticle that was seen from TEM image was
about in the range of 34 nm.
3.3. Catalytic activity of nano Ag/ZnO for oxidation of benzyl alcohols
After successful synthesis and fully characterize of nano Ag/ZnO, we examined its catalytic activity for
oxidant-free oxidation of alcohols (Scheme 1).
Our first attempt in this study was to find a proper and efficient optimum reaction conditions. So,
3-methoxy benzylalcohol was chosen as a model compound and various parameters such as bases,
solvents, temperature, the amount of catalyst, and different Ag loading were studied. The results are
described in Tables 2-4.
According to Table 2, nano Ag/ZnO (2) was chosen as an effective catalyst for the oxidant-free
oxidation. According to these results, Ag/ZnO (1) (9.3× 10-5mol% Ag) and Ag/ZnO (2) (7.4 × 10-5mol%
Ag (entries 1 and 6) showed the same yields, but we chose nano Ag/ZnO (2) because of the lower amount
of Ag loading on ZnO. From the results obtained from Tables 2-4, we can conclude that when 3-methoxy
benzyl alcohol (1 mmol) was treated in the presence of nano Ag/ZnO (2) (0.005 g), KOH (1 mmol) at 100
oC in toluene (2 mL) for 6 h, 3-methoxy benzaldehyde was produced in 80% isolated yield with 99%
selectivity.
Thus, we chose Ag/ZnO (2) as the standard catalyst and the scope and generality was explored
next. The results are summarized in Table 5. According to Table 5, various substituted alcohols bearing
electron-donating and withdrawing groups were examined; substituted alcohols with electron-donating
9

groups had more yield than that of withdrawing groups. In addition, p-substituted alcohols with electron-
donating groups had better yields in comparing with the others. According to Table 5, the lowest yield
was observed for 2-nitrobenzaldehydes because of the o-substituted and a strong electron withdrawing
group. It is important to note that secondary alcohols were also tolerated with good yields (entries 13, 17,
18). In addition, heterocyclic alcohols, such as 2-thiophenemethanol, and 2-pyridinemethanol (entries 15,
16), were oxidized to the corresponding aldehydes in high yields. No oxidation was observed with S and
N hetero atoms.
The heterogeneity of the catalyst was evaluated to study whether the reaction using solid catalysts
or was catalyzed by nano Ag/ZnO species in the liquid phase. To address this issue, two separate
experiments were conducted with 3-methoxy benzyl alcohol. In the first experiment, the reaction was
terminated after two hours; at this juncture, the catalyst was separated from the reaction mixture and the
reaction was continued with the filtrate for an additional three hours. In the second experiment, the
reaction was terminated after two hours. In both cases, the desired product was obtained in the same yield
(50%). Ag was not detected in the filtrate in either experiment by ICP analyzer. These studies
demonstrate that only the Ag support on ZnO during the reaction is active, and the reaction proceeds on
the heterogeneous surface. We also confirmed that no leaching of Ag from the Ag/ZnO catalyst occurred
during the dehydrogenation of 3-methoxy benzyl alcohol. No Ag can be detected in the filtrate after the
reaction.
A proposed mechanism for the role of nano Ag/ZnO as catalyst is shown in Scheme 2. The first
step of the reaction is proton abstraction from the alcohol by KOH to yield an alkoxide group. The ZnO
has Lewis acid sites (Zn+2) and also Lewis basic sites (O2-) [45]. The Lewis acid site of ZnO is
coordinated to the oxygen of the alkoxide group, and also a hydrogen-bonded with water molecules,
which generated from the proton abstraction by KOH. In view of the role of the silver, we propose that
Ag atoms play an important role in the a-C-H activation step. So, in the next step, alkoxide species at the
10

interface between a silver and ZnO undergo hydride abstraction to yield hydride species on the Ag and
aldehydes via a transition state with a positive charge at the-carbon atom. Then, a hydride ion on the Ag
reacts with the water molecule, which absorbed on Lewis acid sites of ZnO, to yield H2. Bronsted
acidities of the hydrogen atoms of absorbed water on the surface of ZnO are increased because of the
Lewis acid sites of ZnO. This acid-base mechanism can facilities the release of the H2.
A comparison of the present catalyst, using nano Ag/ZnO, with some of previously known Ag
catalysts is collected in Table 6 to demonstrate that the present catalyst is indeed superior to several of the
other protocols. According to Table 5, the selective oxidation of benzylalcohol to benzaldehyde is
completed in 8 h at 120 8C in 98% isolated yield by using Ag/ZnO (2). Most of the other Ag catalysts
listed either take a longer reaction time for completion, higher reaction temperature or require to use
under inert atmosphere with generally reduced isolated yields. Thus, nano Ag/ZnO was found to be the
better choice for this reaction.
Finally, The catalyst Ag/ZnO after reaction was characterized by XRD and the patterns images is
shown in Figure 6.
4. Conclusions
In conclusion, the ZnO supported small amount of silver nano catalyst prepared by a very simple
and low cost method with small particles sizes around 36 nm and characterized in terms of its
structural aspects. This nano Ag/ZnO acts as an efficient heterogeneous catalyst for oxidant-free
dehydrogenation of benzyl alcohols in a very mild reaction conditions without the use of any Ar
or N2 flow. The Ag/ZnO afforded high yields of the corresponding carbonyl compounds with co-
production of equivalent molar amounts of H2.
11

Acknowledgements
We gratefully acknowledge the support of this work by the Shiraz University Research
Council.
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15

Table 1. Results of BET surface area measurements for Ag/ZnO (2)
BET surface area (m2/g)
4.204
4.859
0.033
Surface area
Pore volume
BJH adsorption cumulative surface area of pores (m2/g)
Single point adsorption total pore volume of pores (cm3/g)
BJH adsorption cumulative volume of pores (cm3/g)
0.032
31.266
2.740
Mean pore diameter (nm)
Pore size
Pore size distribution (nm)[a]
[a] The pore size distribution (PSD) is defined as the statistical distribution of the radius of the largest sphere that can be fitted
inside a pore at a given point.
16

Table 2. Oxidation of 3-methoxy benzyl alcohol with different catalysts[a].
Entry
Catalyst
Nano Ag/ZnO (2)[b]
Nano Ag
Time (h) Yield (%)
1
2
3
4
5
6
7
8
5
5
5
5
5
4
6
6
80
50
65
40
10
80
75
70
Nano ZnO
AgNO3
Zn(NO3)2
Nano Ag/ZnO (1)[c]
Nano Ag/ZnO (3)[d]
Nano Ag/ZnO (4)[e]
[a] Reaction condition: 3-Methoxybenzyl alcohol (1 mmol), toluene (2 mL), KOH (1 mmol), catalyst
(0.005 g), 100 oC. [b] The amount of Ag is 8× 10-5 g. [c] 10-4 g . [d] 4×10-5 g and [e] 3.5 × 10- g.
17

Table 3. Base, solvent, and temperature optimization[a]
Entry Base (1 mmol)
Temp.(oC) Time (h) Yield (%)
Solvent
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DMF
1
2
K2CO3
Cs2CO3
KOH
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
80
10
10
6
20
20
80
15
60
15
15
10
5
3
4
Ba(OH)2
NaOH
Na2CO3
Ca(OH)2
KF
10
8
5
6
8
7
10
8
8
9
Et3N
7
10
11
12
13
14
15
16
17
18
19
20
21
none
9
15
30
80
15
60
0
K3PO4
KOH
8
6
KOH
7
KOH
DMSO
H2O
9
KOH
6
KOH
EtOH
6
0
KOH
CH3CN
none
6
15
50
20
80
60
KOH
8
KOH
THF
8
KOH
Toluene
Toluene
6
KOH
9
18

22
23
KOH
KOH
Toluene
Toluene
60
11
10
40
10
r.t.
[a] Reaction conditions: 3-Methoxy benzyl alcohol (1 mmol), nano Ag/ZnO (2) (0.005 g), base (1 mmol)
solvent (2 mL)
19

Table 4. Amount of catalyst optimization[a]
Entry
nano Ag/ZnO (2) (g)
Time (h)
Yield (%)
1
2
0.05
0.025
0.0125
0.0063
0.005
0.004
0.003
0.002
0.001
none
6
6
6
5
5
5
5
5
5
5
80
80
80
80
80
70
50
50
50
50
3
4
5
6
7
8
9
10
[a] Reaction conditions: 3-Methoxy benzyl alcohol (1mmol), KOH (1 mmol), toluene (2 mL), 100 oC..
20

Table 5. Oxidation of benzyl alcohols using nano Ag/ZnO (2)[a]
Entry
1
Substrate
Product
Time (h) Yield (%)
O
H
OH
8
3
98
95
O
OH
2
3
H
Me
Me
O
OH
OH
H
6
7
95
90
O
4
5
H
MeO
MeO
O
O
OH
H
3
80
Me
Me
OH
6
5
7
80
H
O
OMe
50[b]
OMe
OH
7
8
7
6
85
45
H
Cl
Cl
O
OH
H
Cl
Cl
O
Cl
Cl
9
H
OH
3
30
Cl
Cl
O
OH
10
11
H
Br
16
8
85
10
Br
O
OH
NO2
H
NO2
21

O
OH
12
H
O2N
6.5
40
O2N
OH
O
H
13
14
6
5
90
60
HO
O
15
16
H
OH
6
4
90
98
S
S
O
OH
H
N
N
O
OH
O
17
18
6
6
60
80
OH
O
O
O
[a] Reaction condition: alcohol (1 mmol), toluene (2 mL), KOH (1 mmol), nano Ag/ZnO (2) (0.005g),
100 oC. [b] Large scale test.
22

Table 6. Oxidation of benzylalcohol catalyzed by various Ag catalysts.
Entry
1
Catalyst
Solvent
Temp. (oC)
Time (h)
8
Yield (%)
Ref.
Ag/ZnO (2)
(0.005 g)
Toluene (2 mL)
100
Isolated 98 Present work
2
3
4
Ag/HT(Hydrotalcites) p- Xylene ( 5 mL)
(0.1 g)
130/
Ar atmosphere
100
10
24
24
[46]
[22]
[47]
Conversion
99
Ag/Al2O3
(2 mol%)
Toluene (3 mL)
82
Fe3O4@SiO2-Ag
( 0.5 mol%)
Toluene (3 mL)
Reflux/
Conversion
98
N2 atmosphere
23

Figure captions
Figure 1. Physical appearance of various nanoAg/ZnO
Figure 2. The XRD patterns of the Ag/ZnO nanoparticles with various Ag doped on ZnO.
Figure 3. X-ray diffraction patterns of nano ZnO (red curve) and nano Ag/ZnO (2) (blue curve) for
comparison.
Figure 4. XPS spectra of the sample with a Ag content of 7.391× 10-5mol %: (a) XPS full spectrum of the
sample; (b) Zn 2p spectrum; (c) Ag 3d; (d) O 1s spectrum spectra.
Figure 5. Transmission electron microscopic (TEM) image of nano Ag/ZnO (2).
Figure 6. XRD pattern of catalyst a) before reaction b) after reaction
Scheme 1. Oxidant-free oxidation of benzyl alcohols using nano Ag/ZnO as catalyst
Scheme 2. A proposed mechanism
24

Figure 1.
25

Figure 2.
26

`
Figure 3.
27

28

Figure 4.
29