Gold nanoparticles supported on metal oxides as catalysts for the direct oxidative esterification of alcohols under mild conditions

Article abstract of DOI:10.1016/j.ica.2014.11.024

Gold nanoparticles supported on metal oxides were used to catalyze the direct oxidative esterification of alcohols; esters were obtained using molecular oxygen as an oxidant under ambient temperature and pressure. Higher activities for the reaction between the benzyl alcohol and methanol were obtained over Au/CeO₂ and Au/ZrO₂ than Au/TiO₂ (anatase), Au/HT (hydrotalcite) and Au/Al₂O₃. These catalysts were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS) and X-ray photoelectron spectroscopy (XPS). The distribution of gold nanoparticles was uniform; no change of chemical states occurred for supports and gold nanoparticles after the catalysts were reused 10 times. The oxidative esterification of various alcohols over these catalysts could also occur under optimized reaction conditions. The substituted benzyl alcohols and cinnamyl alcohols were more active than heterocyclic alcohols and aliphatic alcohols because their α-H could be eliminated more easily during the induction of adsorbed dioxygen. Self-oxidative esterification was available for benzyl alcohol in inert solvents with low polarity indexes. A reaction mechanism was proposed for the synthesis of esters from alcohols with molecular oxygen.

Full text of DOI:10.1016/j.ica.2014.11.024

Accepted Manuscript
Gold Nanoparticles Supported on Metal Oxides as Catalysts for the Direct Ox-
idative Esterification of Alcohols under Mild Conditions
Huili Wei, Jingyi Li, Jing Yu, Jianwei Zheng, Haiquan Su, Xiaojing Wang
PII:
S0020-1693(14)00720-8
http://dx.doi.org/10.1016/j.ica.2014.11.024
ICA 16308
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
10 August 2014
20 November 2014
28 November 2014
Please cite this article as: H. Wei, J. Li, J. Yu, J. Zheng, H. Su, X. Wang, Gold Nanoparticles Supported on Metal
Oxides as Catalysts for the Direct Oxidative Esterification of Alcohols under Mild Conditions, Inorganica Chimica
Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.2014.11.024
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Gold Nanoparticles Supported on Metal Oxides as Catalysts for the Direct
Oxidative Esterification of Alcohols under Mild Conditions
∗
Huili Wei, Jingyi Li , Jing Yu, Jianwei Zheng, Haiquan Su, Xiaojing Wang
College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot,
Inner Mongolia, 010021, P. R. China
Abstract: Gold nanoparticles supported on metal oxides were used to catalyze the
direct oxidative esterification of alcohols; esters were obtained using molecular
oxygen as an oxidant under ambient temperature and pressure. Higher activities for
the reaction between the benzyl alcohol and methanol were obtained over Au/CeO
2
and Au/ZrO than Au/TiO (anatase), Au/HT (hydrotalcite) and Au/Al . These
2
2
2 3
O
catalysts were characterized using transmission electron microscopy (TEM), X-ray
diffraction (XRD), atomic absorption spectroscopy (AAS) and X-ray photoelectron
spectroscopy (XPS). The distribution of gold nanoparticles was uniform; no change of
chemical states occurred for supports and gold nanoparticles after the catalysts were
reused 10 times. The oxidative esterification of various alcohols over these catalysts
could also occur under optimized reaction conditions. The substituted benzyl alcohols
and cinnamyl alcohols were more active than heterocyclic alcohols and aliphatic
alcohols because their α-H could be eliminated more easily during the induction of
adsorbed dioxygen. Self-oxidative esterification was available for benzyl alcohol in
∗
Corresponding author. Fax: +86-471-4992981, Telephone: +86-13848129221,
E-mail: lijingyicn@163.com.
1

inert solvents with low polarity indexes. A reaction mechanism was proposed for the
synthesis of esters from alcohols with molecular oxygen.
KEYWORDS: gold nanomaterial, catalysis, ester synthesis, dioxygen, polarity index
1. Introduction
Esterification is one of the most important reactions in fundamental organic
synthesis. The broad application of esters in fragrances [1], pharmaceuticals [2] and
adhesives [3], as well as the concept of green chemistry, require the improvement of
ester synthesis methods. In traditional methods [4], carboxylic acids or carboxylic
derivatives were first obtained using the corresponding alcohols before being mixed
with alcohols to prepare esters under reflux. Those processes are usually operated at
higher temperatures, inevitably generating by-products [5]. To simplify the operation
and improve efficiency, the catalytic one-pot oxidative esterification of alcohols and
aldehydes was proposed [6, 7] using I
2
5 2 2 2
[8, 9], KHSO [10], MnO [11, 12] or H O
(
33%) [13] as an oxidant. Later, the direct oxidative esterification of alcohols was
achieved under O
2
. Fangzheng Su et al. [14] reported that alcohols or aldehydes could
on Au/β-Ga . The
conversions and selectivities were high. Yajuan Hao et al. [15] reported that benzyl
alcohols and methanol could form methyl benzoate with Au/SBA-16 and K CO at
5°C under 6 atm of O . PI-CB/Au-Pd [16], Au-Ag [17] and [PdCl (CH CN )] [18]
form homologous methyl esters with methanol at 90°C, 5 atm O
2
2
O
3
2
3
2
2
2
3
2
were applied as catalysts in many studies. These efficient protocols for aerobic
oxidative esterification required higher temperatures or pressures; these conditions
should be avoided for sustainable development in chemistry. Thus, the preparation of
esters via direct oxidative esterification of alcohols using O
and pressure is attractive.
2
at ambient temperature
In recent years, gold nanoparticles have been widely applied during the selective
oxidation of alcohols [19-22], the reduction of nitroaromatic compounds [23] and
2

Suzuki-Miyaura cross-coupling reactions [24]. Considering the interaction between
the substrates and supports [25-27] as well as the oxygen molecular adsorption on 2-5
nm gold nanoparticle [17, 28], we decided to use gold nanoparticles supported on
metal oxides as catalysts.
In this investigation, we prepared gold nanoparticles supported on metal oxides
(
CeO
of benzyl alcohol [29-31], using O
2
We found that Au/CeO and Au/ZrO showed excellent catalytic efficiencies with
2
, ZrO
2
, TiO
2
, HT and Al
2
O
3
) and tested their oxidative esterification activities
2
as an oxidant at ambient temperature and pressure.
2
good recyclabilities; they were suitable for the catalytic oxidative esterification of
various alcohols and the self-oxidative esterification of benzyl alcohol. More
importantly, based on previous research achievements and the present experimental
studies, a reaction mechanism was proposed.
2. Experimental Section
2.1. Materials. Methanol, ethanol, hydrochloric acid, acetone, petroleum ether and
DMSO were obtained from Fengchuan Chemical Reagent Technologies Co., Ltd.,
Tianjin, China, while CeO , ZrO , TiO (anatase), NaBH , L-Lysine and benzyl
alcohol were purchased from Sigma-Aldrich. The hydrotalcite (Its shortening name is
HT) was prepared by Jing Yu. Most alcohols, HAuCl ·3H O, Cs CO , 1, 4-dioxane,
benzotrifluoride and mesitylene were purchased from J&K Scientific LTD.
2
2
2
4
4
2
2
3
2.2. Preparation of catalysts with 3 wt% gold [32]. The gold nanoparticles were
supported on a metal oxide powder (CeO , ZrO , TiO , HT and Al ). A quantity of
2
2
2
2 3
O
metal oxide (2.5 g) was weighed and dispersed in deionized water (50 mL). To ensure
a uniform dispersion, the suspension was stirred for 10 min and ultrasonically
-
3
-1
dispersed for 10 min. Then, HAuCl
4
(100 mL, 3.8×10 mol·L ), L-Lysine (20 mL,
-
1
-1
0
.53 mol·L ) and NaBH
4
(10 mL, 0.35 mol·L ) were added sequentially to the
above-mentioned suspension with a slow dripping speed and stirring. Afterward,
dilute hydrochloric acid was added to adjust the pH value to approximately 9.5. The
3

mixed solution was stirred for 1 hour and left standing for 24 hours to obtain a
precipitate that could be used as a catalyst after being separated, washed and dried.
2.3. Characterization. Transmission electron microscopy (TEM) was performed on a
FEI Tecnai F20 TEM at 200 kV. X-ray diffraction (XRD) patterns were recorded with
an Empyrean diffractometer using CuΚα radiation at 40 kV and 40 mA (λ=1.5406 Å).
The gold content of the supports was determined using a GBC AVANTA XY-05
atomic absorption spectroscopy (AAS) from the Inner Mongolia Minerals Experiment
Research Institute. X-ray photoelectron spectroscopy (XPS) data were obtained on a
Kratos XSAM800 with AlΚα radiation at the Chengdu Institute of Organic Chemistry,
Chinese Academy of Sciences. The conversions and selectivities of the alcohol
reactions were calculated according to the results determined by Gas Chromatograph
-
2014C (GC-2014C). The speciation of the products was measured with a Trace
DSQII gas chromatograph-mass spectrometer (GC-MS) at Inner Mongolia University.
2
.4. Activity test. Known amounts of catalyst, base and alcohol (RCH
were mixed with methanol in a round bottomed flask. The reaction device was sealed
after having been filled with O at ambient pressure and then kept at a specified
temperature with a magnetic stirrer. During the reaction, the O was supplemented; a
2
OH, 2 mmol)
2
2
small amount of the mixture was extracted and filtered to remove the catalysts. The
filtrate was analyzed using the GC-2014C.
3. Results and Discussion
3.1. TEM
To reveal the size, shape and distribution of gold nanoparticles over catalysts,
these samples were characterized by TEM. As shown in Figure 1, the following
features of the catalysts were identified. (1) The gold particles were spherical and
uniformly distributed on the CeO
2
and ZrO
2
(Figure 1a and 1c). (2) Most gold
particles were within 3-6 nm for Au/CeO
2
and 4-7 nm for Au/ZrO . (3) However, for
2
the recycled catalysts, the number of gold particles was slightly reduced on the
4

support surfaces (Figure 1b and 1d). (4) Moreover, part of the gold particle sizes
became slightly larger as a result of aggregation due to reuse. (5) The
electron-diffraction pattern (Figure 1e) showed that the gold existed in
mono-crystalline form and that its lattice spacing was close to 0.238 nm,
corresponding to the Au(111) crystal plane.
(
Figure 1 here)
3.2. XRD
The X-ray diffraction patterns of the supports and the catalysts are shown in Figure
. Clearly, the structures of the supports remained nearly unchanged after gold
2
loading and reuse, revealing that they were stable enough for practical applications.
No gold diffraction peaks were detected due to the smaller particle sizes and lower
gold contents [33-35]. The gold contents of the supports were assessed by AAS: they
were 2.8% for Au/CeO
recycled Au/ZrO . Some of the gold was lost from the catalysts after they were reused
0 times, as observed by TEM and AAS.
2 2 2
, 2.4% for recycled Au/CeO , 2.9% for Au/ZrO and 2.1% for
2
1
(
Figure 2 here)
.3. XPS
The surface composition and chemical states of the catalysts were investigated by
3
XPS. The full-range XPS spectra (Figure 3a) clearly showed characteristic signals of
O, Ce or Zr for the supports, as well as Au for the gold nanoparticles. The O signals
could be attributed to CeO
2 2 2 2
or ZrO , as well as O and H O adsorbed on the catalyst
surfaces. For Au/CeO , the signals of Ce 3d (Figure 3b) were fitted to two sets of
2
triplets (3d3/2 and 3d5/2): one set of binding energies (BEs) was 900.9, 907.1 and 916.5
eV, while the other was 882.5, 888.3 and 898.1 eV. The difference between the main
signals was 18.4 eV (900.9﹣882.5=18.4 eV), similar to the standard values for CeO
2
(
18.3 eV). The other signals, marked “S” in Figure 3b, correspond to the satellite lines
of CeO
2
2
3d3/2 and 3d5/2. The two signals from Au/ZrO (Figure 3c) were 184.3 and
5

4
+
1
81.9 eV, respectively, corresponding to 3d3/2 and 3d5/2 of Zr , respectively. The Au
4
f
5/2 and 4f7/2 BEs of Au/CeO were 87.4 eV and 83.7 eV, respectively, equaling the
2
values of metallic gold [24, 32] (Figure 3d). Therefore, the supported gold
nanoparticles existed in the metallic state. As shown in Figure 3e, the Au 4f BEs of
Au/CeO
Au/ZrO
2
and the recycled Au/CeO
2
(87.4 and 83.7 eV) were similar to that of
2
and recycled Au/ZrO (87.2 and 83.5 eV). Although the signals of the
2
recycled samples were significantly decreased and jagged due to their lower gold
contents, the gold nanoparticles remained consistently in the metallic state. Therefore,
CeO
2 2
, ZrO and Au state are stable because their XPS spectra remained unchanged
after 10 reuses (not shown here).
(
Figure 3 here)
3.4. Activity test
3
.4.1. Activity test of benzyl alcohol and methanol
To assess the activity of the catalyst, a primary study on the oxidative esterification
of benzyl alcohol (2 mmol) and methanol (10 mL) under the initial conditions
Au/CeO (50 mg), Cs CO (20 mol% , 0.2606 g), at 30°C, and filled with O ) was
(
2
2
3
2
undertaken. The products were detected by GC; 98.3% conversion of benzyl alcohol
and 99.7% selectivity of methyl benzoate were obtained after 3 hours. Encouraged by
this excellent result, we optimized the reaction conditions: 5 mL of methanol as a
solvent, 10 mol% of Cs
2
CO
3
as a base [36], 25 mg of catalyst and filled with O under
2
ambient temperature and pressure.
Benzyl alcohol and methanol were chosen as the model substrates, and the
reactions were carried out under various conditions (Table 1). When the reaction was
carried out without any catalyst, gold, base or dioxygen, almost no methyl benzoate
formed (Table 1, entries 1, 2, 9 and 10). Therefore, these components were essential
for the oxidative esterification reaction. The reaction on micron-size gold particles
gave a lower conversion of benzyl alcohols and a moderate selectivity for methyl
6

benzoate (Table 1, entry 3). Additionally, when comparing a set of catalysts, ideal
catalytic activities were exhibited on Au/CeO and Au/ZrO . Slightly lower catalytic
2
2
activities were obtained on Au/TiO and Au/HT. However, even lower catalytic
2
activities were detected on Au/Al O (Table 1, entries 4-8). The gold nanoparticle
2
3
sizes ranged from 3-6 nm, 4-7 nm, 5-7 nm, 10-13 nm and 5-7 nm for Au/CeO
2
,
Au/ZrO , Au/TiO , Au/HT and Au/Al , respectively (Figure 1 and Figure S1).
2
2
2
O
3
Therefore, the catalytic performance was determined not only by the size of the gold
nanoparticles but also by the interaction of substrate and the support [17, 28].
Generally, the smaller the size of gold nanoparticles is, the higher the reactivity of the
2 3 2 3
catalyst is. Au/Al O exhibited poor catalytic activity, because Al O is easily
damaged under alkaline conditions as an amphoteric oxide. The production of methyl
benzoate declined in air due to the deficiency of dioxygen compared to that in pure
oxygen (Table 1, entry 11).
(
Table 1 here)
3.4.2. The recyclability of catalysts
Based on the excellent activities over Au/CeO
2
and Au/ZrO , further experiments
2
were performed to test their recyclabilities. The test results are displayed in Figure 4.
The conversion of benzyl alcohol and the selectivity of methyl benzoate reached 100%
2 2
on Au/CeO after 4 cycles and on Au/ZrO after 5 cycles. After each cycle, the
catalysts were washed with ethanol, centrifuged and dried at 60°C for the next cycle.
When the catalysts were reused 10 times, their conversions were 94.0% and 92.0% on
Au/CeO
Au/CeO
2
2
and Au/ZrO
2
, respectively. The selectivities were 100.0% and 88.4% on
and Au/ZrO , respectively. This slight decline was attributed to the shedding
2
of some gold nanoparticles, as observed in the TEM images (Figure 1b and 1d) and
AAS data. In summary, their high conversions and selectivities reflect the
recyclability of Au/CeO
Figure 4 here)
2 2
and Au/ZrO .
(
7

3.4.3. Activity test of primary alcohols (or benzyl aldehyde) and methanol (or
ethanol)
To study the scope of this procedure, various primary alcohols and benzyl
aldehyde were examined under optimized conditions. The experimental results are
presented in Table 2. The results were as follows: (1) when comparing benzyl
aldehyde with benzyl alcohol, benzyl aldehyde reacted more easily than benzyl
2 2
alcohol (Table 2, entries 1, 2). (2) Catalytic activities on Au/CeO and Au/ZrO were
similar in methanol. The same trend was observed in ethanol. (3) For most alcohols
1
(
R CH
2
OH), their reactions with methanol (at 25°C) were easier than that with
ethanol (at 30°C) due to the stronger nucleophilicity of the methoxy group relative to
the ethoxy group. (4) Substituted benzyl alcohols (Table 2, entries 2-5) exhibited
excellent activities, especially when reacting with methanol (conversions>99.0%).
Based on the results with ethanol, their substituent effects can be estimated to have
3 3
the following conversion order: p-OCH > p-CH > p-H> p-Cl. This sequence shows
that when the electron-donating ability of the substituent group is stronger, the
reactivity is higher. Conversely, p-nitrobenzyl alcohol (Table 2, entry 6) exhibits a
stronger electron-withdrawing ability than p-chlorobenzyl alcohol and exhibits a
higher reactivity in ethanol. (5) Cinnamyl alcohol was also successfully applied to the
oxidative esterification system (entry 7). The selectivities of methyl cinnamate on
Au/CeO
2 2
and Au/ZrO reached 94.9% and 97.1% in methanol but only 53.3% and
51.5% in ethanol, with high conversions. The major by-product in ethanol was
cinnamyl aldehyde, which was also the intermediate used to form methyl cinnamate;
this species was stable due to its conjugated structure. (6) Heterocyclic alcohols and
aliphatic alcohols exhibited low reactivities (entries 9-11) due to the difficulty
encountered during α-H elimination [37, 38]. (7) Under these conditions, the
oxidative esterification of 2-phenylethanol was difficult (entry 12). When the
temperature was increased to 50°C, conversion reached only 7.9%, while the
selectivity dropped to 78.1%. In addition, other methods were attempted to improve
the conversion of 2-phenylethanol, such as the use of solvents with different polarities
8

(
petroleum ether, toluene, 1, 4-dioxane, mesitylene, benzotrifluoride, acetone, DMSO
and DMF), the application of a stronger oxidant (H ) or the introduction of visible
2
O
2
light [39, 40]. However, those methods remained ineffective. Therefore, a better
solution is required. Acetic acid esters were also detected in the ethanol system, but
no formic acid esters were detected in the methanol system.
(
Table 2 here)
3.4.4. Each species produced during the reaction of benzyl alcohol and methanol
To detect the contents of each species during the oxidative esterification of benzyl
alcohol with methanol, an experiment with a slower reaction rate was designed in air.
As observed in Figure 5, benzyl alcohol was completely converted into methyl
benzoate after 8 hours. Benzaldehyde originally existed in the substrates and finally
transformed into methyl benzoate. Moreover, a small amount of benylate appeared in
the intermediate process. However, this species finally disappeared due to its
transesterification with methanol.
(
Figure 5 here)
3.4.5. The self-oxidative esterification of benzyl alcohol
To study the self-oxidative esterification of benzyl alcohol, some inert solvents
were employed to replace methanol or ethanol, preventing cross-reactions with the
two types of alcohols. The conversion of benzyl alcohol in different solvents
increased when the reaction time was prolonged (Table 3). Analysis of the products
revealed that the conversion of benzyl alcohol in petroleum ether reached 47.4%, 56.7%
and 66.7% after 4, 8 and 12 hours, respectively, and the selectivities of benzaldehyde
and benzylate were 25.7% and 74.3%, respectively (Table 3, entry 1). Obviously, the
conversion of benzyl alcohol decreased as the solvent polarity index increased due to
the solvation effect (Excluding CCl
4
and toluene; we will study the unusual
phenomenon observed in CCl and toluene in the future). However, the selectivity of
4
benzaldehyde increased as the solvent polarity index increased due to an induction
9

effect caused by the solvent. The higher the solvent polarity index, the stronger
interaction of the solvent with alcohol or aldehyde, and the reaction occurs more
difficult. In conclusion, the polarity index of the solvent strongly affected the
self-oxidative esterification of benzyl alcohol. A lower solvent polarity index
corresponded to a higher ester yield. Thomas P. Wells et al. [41] reported that a polar
solvent would slow the rate of esterification. Therefore, solvents with lower polarity
indexes favor the self-reaction of benzyl alcohol.
(
Table 3 here)
3.5. Reaction mechanism
As shown in Table 1 (entries 2, 3, 9 and 10), the gold, supports, base and O were
2
all needed during the aerobic oxidative esterification of alcohols. Moreover, the gold
nanoparticle surfaces can adsorb O to reduce the O=O bond energy or help dissociate
to form active oxygen species [17, 28]. Therefore, a tentative mechanism was
2
O
2
proposed for the oxidative esterification of alcohols and the transfer process for O
Scheme 1) [17, 26, 28, 42].
2
(
(
Scheme 1 here)
1
First, the alcohol molecules (R CH
2
OH) were adsorbed on the support surface due
to the difference in electronegativity between the two species. The interaction
between the oxygen atoms of the alcohols and the metal cations of the supports, as
well as the interaction between the hydrogen atoms of alcohol hydroxyl groups and
oxygen anions of the supports, weakened the O—H bonds of the alcohols. Under
+
basic conditions provided by Cs
2
CO
3
, the H from the alcohols’ —OH was captured
2
-
by CO
3
, forming structure III with a negative charge on the oxygen atom. The α-H
adsorbed on the gold nanoparticles surface, biasing the
was attracted by the O
2
electron cloud of the oxygen atom toward the carbon atom. Then, the α-H was
-
removed as H from structure III, forming an oxygen-carbon double bond.
Concurrently, the O double bond was broken, and an O—H bond was formed to
2
10

-
-
produce Oad ←O—H. (Oad represents the oxygen ion adsorbed on the gold
-
nanoparticles surface, and ← represents the coordination bond between Oad and O.)
2
Another un-adsorbed alcohol (R OH) and structure IV rapidly undertook a
nucleophilic addition reaction to form hemiacetal VI (structure V was quickly
1
converted into structure VI). Similar to the interaction of R CH
2
OH with the supports,
an interaction between the hemiacetals and the supports weakened the bond of the
+
hemiacetal hydroxyl group. Consequently, the H from the —OH of the hemiacetal
2
-
was captured by CO
3
, forming structure VII. The α-H of structure VII was also
-
attracted by the oxygen atom of Oad ←O—H (or adsorbed O
2
) on the gold
-
nanoparticles. Simultaneously, the two bonding electrons over Oad ←O in
-
-
O
ad ←O—H moved from the oxygen atom (O) to the adsorbed oxygen anion (Oad ).
+
Concurrently, the H in the Oad-←O—H was strongly attracted by the electron-rich
-
+
-
-
oxygen anion (Oad ). Finally, the H was transferred to Oad , and the bond in Oad ←O
-
was broken; 2OH and an ester were formed, leaving the gold nanoparticles. The gold
nanoparticle surfaces could then re-adsorb oxygen from the reaction environment,
beginning a new reaction process.
As observed from scheme 1, negatively charged structure III was generated during
the reaction. An electric field existed between structure III and the polar solvent,
surrounding structure III with solvent molecules. Therefore, structure III became
relatively stable, and its reactivity decreased, lowering the conversion of the alcohol.
The interaction between the charged structure and the solvent molecules is the
solvation effect mentioned above. An induction effect existed between structure IV
and the polar solvents, reducing the electropositivity of the C on the C=O of structure
IV and reducing the opportunities for nucleophilic addition reaction between aldehyde
and alcohol when increasing the solvent polarity. Finally, ester formation was
prevented, making the product more likely to remain an aldehyde. Therefore, in polar
solvents, the alcohol conversion decreased, and the selectivity of the ester also
decreased for the self-oxidative esterification of the alcohol. The interactions between
11

the solvent and reactant molecules always existed, but they were strongest in the
appearance of structure III and structure IV mentioned above.
4. Conclusion
In summary, excellent catalytic performance was demonstrated during the direct
2 2
oxidative esterification of alcohols to form esters over Au/CeO and Au/ZrO with the
use of dioxygen as an oxidant under ambient temperature and pressure. Practical
application is possible due to the simple process, mild conditions, excellent yields and
environmental friendliness. A possible mechanism was provided, and the reaction
pathways from the alcohols to the esters were described in detail.
Acknowledgements
This work was financially supported by NSFC (Nos. 20567002 and Nos. 21067007),
as well as by the Scientific Research Startup Fund of Inner Mongolia University
(
(
203044), the Education Department of Inner Mongolia Autonomous Region
NJ04093), Chunhui Plan of the Education Ministry (Z2004-2-15030 and
Z2009-1-01005), the Natural Science Fund of Inner Mongolia (2010MS0203 and
014MS0201) and “513 talents” Plan of Inner Mongolia University and 2013 Annual
2
Grassland Talents Project of Inner Mongolia Autonomous Region and 2013 Annual
Inner Mongolia talent development fund.
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the online version
at http:// xxxx.xx.xxx.
12

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17

Figure captions
2
Figure 1. TEM images and gold particle size distributions (insets) of (a) Au/CeO , (b)
2 2 2
recycled Au/CeO after 10 reuses, (c) Au/ZrO , (d) recycled Au/ZrO after 10 reuses,
and (e) HRTEM image and electron-diffraction of gold particle of recycled Au/ZrO
2
after 10 reuses.
Figure 2. XRD patterns of (a) CeO and recycled Au/CeO
2
, Au/CeO
2
2
after 10 reuses,
(
b) ZrO , Au/ZrO and recycled Au/ZrO after 10 reuses.
2
2
2
Figure 3. XPS spectra of (a) the full-range XPS spectra of Au/CeO
Au/CeO after 10 reuses, Au/ZrO and recycled Au/ZrO after 10 reuses, (b) Ce 3d
XPS spectrum of Au/CeO , (c) Zr 3d XPS spectrum of Au/ZrO , (d) Au 4f XPS
spectrum of Au/CeO , and (e) Au 4f XPS spectra of Au/CeO , recycled Au/CeO after
0 reuses, Au/ZrO and recycled Au/ZrO after 10 reuses.
2
, recycled
2
2
2
2
2
2
2
2
1
2
2
Figure 4. The recyclability test of Au/CeO
of benzyl alcohol with methanol. The reactions were conducted at 25°C in 1 atm O
using CH OH (5 mL) with benzyl alcohol (2.0 mmol), catalyst (100 mg), and Cs CO
10 mol%) in 3 hours. The conversion and selectivity were determined by GC.
2
and Au/ZrO for oxidative esterification
2
2
3
3
2
(
Figure 5. The oxidative esterification of benzyl alcohol with methanol in air. The
reaction was conducted at 25°C in air using CH OH (5 mL) with benzyl alcohol (2.0
mmol), Au/CeO (25 mg), andCs CO (10 mol%). The concentration was
determined by GC.
3
2
2
3
18

Table captions
Table 1. The oxidative esterification of benzyl alcohol with methanol in different
[a]
conditions.
Table 2. Oxidative esterification of alcohols (or aldehydes) on Au/CeO
2
and
[
a]
Au/ZrO
2
.
[a]
Table 3. The self-oxidative esterification of benzyl alcohol in different solvents.
19

Scheme captions
Scheme 1. A proposed mechanism for aerobic oxidative esterification of alcohols by
gold nanoparticles supported on metal oxides.
20

21

Figure2(a)
CeO2
Au/CeO2
Recycled Au/CeO2
10
20 30
40
50
60 70
80
90
2Theta(degree)
22

Figure2(b)
ZrO2
Au/ZrO2
Recycled Au/ZrO2
10
20
30
40
50
Theta(degree)
60
70
80
2
23

O(KLL)
Ce3d
Figure3(a)
Ce(MNN)
Au/CeO2
O1s
O1s
C1s
Ce4d
Au4f
Cs3d
Au/ZrO2
Recycled Au/CeO
2
O(KLL)
Zr3p
Zr3d
C1s
Au4f
Cs(MNN)
Recycled Au/ZrO2
Cs3d
1
000 800
600
Binding energy(eV)
400
200
0
24

Figure3(b)
3
d3/2900.9
Ce3d
3d5/2882.5
S
S
S
S
1
8.4
920
910
900
890
880
Binding energy(eV)
25

Figure3(c)
3d5/2181.9
Zr3d
3d3/2184.3
2.4
188
184
180
176
Binding energy(eV)
26

Figure3(d)
Au4f
4
f7/283.7
4
f5/287.4
3
.7
9
3
90
87
84
81
Binding energy(eV)
27

Au4f
Figure3(e)
Au/ZrO2
Recycled Au/ZrO2
4
4
f7/2
f5/2
Au/CeO2
Recycled Au/CeO2
90
87 84
Binding energy(eV)
81
28

Figure4(a)
1
00
100
80
60
40
20
0
80
60
40
20
0
1
2
3
4
5
6
7
Cycle times
8
9
10
Conversion% of Au/CeO2
Selectivity% of Au/CeO2
29