Gold supported on nanocrystalline β-Ga2O3 as a versatile bifunctional catalyst for facile oxidative transformation of alcohols, aldehydes, and acetals into esters

Article abstract of DOI:10.1002/chem.200800982

Facile oxidative transformation of alcohols, aldehydes, and acetal into esters has been demonstrated using bifunctional catalyst of gold supported nanocrystalline β-Ga²O³. The study used alcoholic gel-precipitation method to prepare nanocrystalline support and X-ray diffraction (XRD) to characterize the diffraction features of synthesized nanocrystalline support. The study observed that the Au/β-Ga ₂O₃ catalyst can be reused after aerobic oxidation with maintaining catalytic activity and selectivity for oxidative esterification. The study also observed that aromatic and aliphatic primary alcohols can be converted into their methyl, ethyl, or propyl esters with high selectivities. The study also found that the esters produced after esterification were in good yields.

Full text of DOI:10.1002/chem.200800982

COMMUNICATION
DOI: 10.1002/chem.200800982
Gold Supported on Nanocrystalline b-Ga O as a Versatile Bifunctional
2
3
Catalyst for Facile Oxidative Transformation of Alcohols, Aldehydes, and
Acetals into Esters
[
a]
Fang-Zheng Su, Ji Ni, Hao Sun, Yong Cao,* He-Yong He, and Kang-Nian Fan
[
7]
Esterification is one of the most fundamentally important
Solid catalysts based on supported gold nanoparticles
[1]
reactions in organic synthesis. Although a number of
methods have been developed, the search for new, facile,
cost-effective, and environmentally friendly procedures that
avoid the use of large excess of reagents and expensive acti-
have attracted tremendous recent attention owing to their
unique catalytic properties for a broad spectrum of organic
transformations, especially for aerobic oxidation of alco-
[8]
[9]
hols under mild conditions. Over the last few years, our
[2]
[10]
[11]
[12]
vators has attracted substantial interest. An attractive al-
ternative is the direct catalytic transformation of alcohols or
aldehydes to esters, without the use of the corresponding
group,
Hutchings et al.,
Baiker et al.,
and Corma
[
13]
et al. reported that gold nanoclusters deposited on TiO2,
CeO , and Cu-Mg-Al or Ga-Al mixed-metal oxides are
2
[3]
acid or acid derivative. In particular, the direct oxidative
conversion of alcohols or aldehydes under mild conditions is
an attractive goal. As opposed to the traditional esterifica-
tion method, in which a two-step synthetic procedure first
involving the synthesis of carboxylic acids or activated car-
boxylic acid derivatives, such as acid anhydrides or chlo-
highly effective for aerobic alcohol oxidation under solvent-
free conditions. One critical issue associated with the gold-
catalyzed primary-alcohol oxidation process is the selectivity
toward aldehydes. Most recently, several studies have re-
vealed that, depending on the substrate or acidic nature of
the supports, the yielding of esters may severely reduce the
[4]
[14]
A
C
H
T
R
E
U
N
G
rides, is required, the single-step nature of the oxidative
selectivity toward target products of aldehyde, with hemi-
esterification procedure has economic and environmental
benefits in the synthesis of esters. However, relevant reports
are limited to a few nonselective heterogeneous reactions or
homogeneous systems, which utilize stoichiometric amount
of oxygen donors and long reaction times. In general, het-
erogeneous systems capable of catalyzing oxidative esterifi-
acetal being identified as the key intermediate for ester for-
mation. Taking into account that inorganic oxides contain
some appropriate acid sites that are able to facilitate the
[13,14a]
hemiacetal formation,
it appeared to us that a new con-
[5]
cept of catalyst could be brought forward if the gold nano-
particles in the presence of these acidic sites can coopera-
tively work together by introducing a solid bifunctional cata-
lyst that facilitates the generation and consecutive oxidation
of the intermediate hemiacetals to the corresponding esters.
To explore this possibility, we chose a solid catalyst formed
by gold supported on nanocrystalline b-Ga O (denoted as
cation of alcohols or aldehydes using molecular oxygen (O )
2
[6]
as oxidant are relatively scarce. One notable exception is
the ethylene glycol to methyl glycolate (MGC) process
based on a specific Au-based catalyst system with propriet-
ary formulations recently developed by the company
2
3
[6a–c]
Nippon Shokubai;
this result represents a milestone to-
Au/b-Ga O ). The reason for this choice is that gallium
2 3
wards greener commercial process for clean and efficient
production of carboxylic esters.
oxide has emerged as an exceptional catalytic or supporting
material that is highly efficient for a wide range of acid-cata-
[15]
lyzed reactions;
it is also known that significantly in-
creased surface Lewis acidity can be achieved on nanocrys-
[
a] F.-Z. Su, J. Ni, H. Sun, Prof. Dr. Y. Cao, Prof. Dr. H.-Y. He,
Prof. K.-N. Fan
[16]
talline b-Ga O .
2
3
Department of Chemistry &
Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials
Fudan University, Shanghai 200433 (P. R. China)
Fax : (+86)21-6564-2978
E-mail: yongcao@fudan.edu.cn
The nanocrystalline b-Ga O support was prepared by an
2 3
[17]
alcoholic gel-precipitation method. The X-ray diffraction
XRD) pattern of the as synthesized nanocrystalline support
shows well-defined diffraction features characteristic of b-
Ga O (Figure S1 in the Supporting Information). Transmis-
(
2
3
sion electron microscopy (TEM) shows that the support is
highly porous in nature; it consists of interconnected parti-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800982.
Chem. Eur. J. 2008, 14, 7131 – 7135
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7131

cles with average crystallite size of 7–8 nm (Figure S2 in the
Supporting Information). Owing to the nanocrystalline
nature of the support, the BET (Brunner–Emmett–Teller)
surface area of the as synthesized b-Ga O is very high
Table 1. Oxidative esterification of benzyl alcohol in the presence of
methanol over various gold catalysts.
[
a]
Entry
Catalysts
Conv.
%]
Selectivity [%]
Aldehyde
[
Ester
Acetal
2
3
2
À1
1
2
Au/b-Ga
Au/b-Ga
2
O
O
O
3
3
3
90
71
77.4
97
70
36.8
5.5
88
93.3
22.6
81.6
85
64.5
45
4.1
32.1
14.9
9.2
20.7
27.1
31.2
3.7
1.2
43.4
2.5
(
84 m g ). Surface acidity measurements by temperature-
2
-c
programmed desorption of NH (NH -TPD) and pyridine
3
3
3Au/
g-Ga
2
adsorption coupled with FTIR measurements (for details in
the Supporting Information, Table S1 and Figure S3) reveal
4
5
6
7
8
Au/Ga
3
Al
3
O
9
4.4
Au/TiO2
13.1
26.3
54.2
2.1
an abundance of surface Lewis acidity of medium strength
Au/Fe
Au/C
2
O
3
À1
12
93.9
with a site density of ca. 0.60 mmol NH (gcat) in the as
3
[
b]
Au/b-Ga
2
O
3
synthesized b-Ga O material, which is much higher than
2
3
[
(
a] Reaction conditions: benzyl alcohol (3.5 mmol) and Au catalysts
0.29 mol%) in methanol (10 mL) at 908C, 5 atm O , time=2 h; conver-
that previously reported for conventional calcination-de-
À1
2
rived b-Ga O (~0.24 mmol NH (gcat )).
2
3
3
sion (conv.) and selectivity were determined by GC by using n-nonane as
the external standard. [b] Results for the fifth run.
When gold nanoparticles were deposited onto the nano-
crystalline b-Ga O (see Supporting Information for the de-
2
3
[18]
tailed preparation procedure),
the XPS spectrum of the
0
Au 4 f7/2 core level shows a contribution from Au at a bind-
ing energy of 83.1 eV (Figure S4 in the Supporting Informa-
tion). This value is significantly lower than the reported
value (84.0 eV) for a bulk gold metal, suggesting the pres-
ence of a strong metal–support interaction in the Au/b-
Ga O system. The sole presence of metallic gold was also
activity for the oxidative esterification of benzyl alcohol.
Note that Au/C showed only very little activity under pres-
ent additive-free conditions.
After the catalytic oxidative esterification of benzyl alco-
hol was completed under the conditions in Table 1, the reac-
tion mixture was filtered to remove the Au/b-Ga O . It was
2
3
2
3
confirmed by the IR band of adsorbed CO (Figure S5 in
the Supporting Information). An almost identical XRD pat-
tern was obtained for Au/b-Ga O relative to b-Ga O , indi-
confirmed that no gold was present in the filtrate by induc-
tively coupled plasma atomic emission spectroscopy (ICP-
AES; detection limit of 2.5 ppb). In addition, the oxidation
process was terminated by the removal of Au/b-Ga O from
2
3
2
3
cating that the crystal structure of the nanocrystalline sup-
2
3
port is maintained. TEM analysis of the Au/b-Ga O catalyst
the reaction solution (Figure S6 in the Supporting Informa-
tion). These results indicate that any contribution to the ob-
served catalysis from gold species that leached into the reac-
tion solution can be ruled out and that the observed cataly-
sis is truly heterogeneous. The Au/b-Ga O catalyst could be
2
3
reveals randomly dispersed particles, and energy-dispersive
X-ray (EDX) analyses confirmed that the particles corre-
sponded to gold with an average diameter of about 2–4 nm.
It should be noted that, upon the introduction of gold nano-
2
3
particles, similar NH -TPD results were obtained for the
reused after aerobic oxidation, and both the catalytic activi-
ty and selectivity toward the oxidative esterification reac-
tions were retained (entry 8 in Table 1).
3
Au/b-Ga O catalyst (Table S1), thus revealing that the sur-
2
3
face acidity of b-Ga O was largely retained after deposition
2
3
of gold nanoparticles.
The gold nanoparticles supported on nanocrystalline b-
Oxidative esterifications of other primary alcohols were
examined by using the Au/b-Ga O -catalyzed protocol, and
2
3
Ga O were initially tested as a catalyst for the selective oxi-
the results are depicted in Table 2. A family of aromatic and
aliphatic primary alcohols, including benzyl alcohol, cinnam-
yl alcohol, and 1-octanol, can be converted into their corre-
sponding methyl, ethyl, or propyl esters with essentially
complete conversion and high selectivities (Table 2, en-
tries 1, 4, 5, 9, and 10), although longer reaction times are
required for the aliphatic alcohols. The results in Table 2
also indicate that the benzyl alcohols substituted with an
2
3
dation of 1-phenylethanol with O as the oxidant in the ab-
2
sence of solvent (see Supporting Information for experimen-
tal details). The Au/b-Ga O catalyst is found to be highly
2
3
effective for alcohol oxidation, with very high turnover fre-
À1
quencies (~15,000 h ) achievable for the exclusive synthesis
of acetophenone. Subsequent experiments exploring the oxi-
dative esterification of benzyl alcohol in the presence of
methanol have shown that the Au/b-Ga O3 catalyst was
electron-donating group (ÀOCH ) tend to be oxidized more
2
3
highly active and selective for the facile oxidative transfor-
easily than those with electron-withdrawing substituents, the
reaction rate for which was also slower than unsubstituted
benzyl alcohol (entries 2 and 3). Furthermore, it was estab-
lished that oxidative esterification of heteroaromatic pri-
mary alcohols such as furfuryl alcohol under the Au/b-
Ga O -catalyzed conditions also proceeded efficiently, and
[19]
mation of benzyl alcohol into methyl benzoate. The bene-
fit of using nanocrystalline b-Ga O as a bifunctional sup-
2
3
port for gold catalysts is evident when comparing the cata-
lytic activity of Au supported on conventional b-Ga O (Au/
2
3
b-Ga O -c), g-Ga O (Au/g-Ga O ) and gold reference cata-
2
3
2
3
2
3
2
3
lysts (Au/TiO , Au/Fe O and Au/C) supplied by the World
Gold Council (Table 1). A clear advantage of the Au/b-
the ester products were produced in good yields (>75%,
Table 2 entry 6). In contrast, only moderate selectivities to
the corresponding methyl esters was obtained for allylic pri-
mary alcohols such as crotonyl alcohol and allyl alcohol
under the same condition as described above (Table 2, en-
tries 7 and 8). The lower selectivity obtained for the oxida-
2
2
3
Ga O catalyst over Au/Ga Al O was also noticed when the
2
3
3
3
9
reaction was conducted by using Au/Ga Al O under other-
3
3
9
wise identical conditions. Among various Au catalysts
tested, the Au/b-Ga O catalyst showed the highest catalytic
2
3
7132
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Chem. Eur. J. 2008, 14, 7131 – 7135

Oxidative Esterification
COMMUNICATION
[
a]
2 3
Table 2. Au/b-Ga O -catalyzed oxidative esterification of alcohols (aldehydes).
[
e]
Entry
Substrate
Product
t [h]
Conv. [%]
Select. [%]
1
2
benzyl alcohol (benzaldehyde)
anisalcohol (anisaldehyde)
methyl benzoate
methyl anisate
2.5 (1)
2 (1)
98 (>99)
97 (96.5)
96.5 (94.3)
98 (94.8)
3
4
5
6
7
8
9
0
4-nitro benzyl alcohol (4-nitro benzaldehyde)
methyl 4-nitro benzoate
4 (2.5)
(90)
4.5 (2.5)
6 (1)
8 (4)
4 (3)
5.5 (2)
3.5 (3)
3.5 (2)
83 97 (80.9)
cinnamyl alcohol (cinnamaldehyde)
methyl cinnamate
methyl octanoate
methyl pyromucate
methyl crotonate
methyl acrylate
>99 (93.4)
81 (93)
89 (91)
92.5 (98)
>99 (>99)
94 (92)
94 (97.5)
94.7 (91)
84.4 (92.3)
73.5 (81.7)
65.5 (84)
95.5 (95)
94.3 (95.8)
[
b]
1-octanol (octanal)
furfuryl alcohol (furfural)
crotyl alcohol (crotonaldehyde)
allyl alcohol (acrolein)
benzyl alcohol (benzaldehyde)
benzyl alcohol (benzaldehyde)
[
[
c]
ethyl benzoate
n-propyl benzoate
d]
1
93 (95)
[
a] Reaction conditions: 2.5 mmol alcohol (or aldehyde) and Au catalysts (0.62 mmol%) in methanol (10 mL) at 908C, 5 atm O
selectivity (select.) were determined by GC with n-nonane as the external standard. [b] Alcohol (1 mmol). [c] Ethanol (10 mL) instead of methanol.
d] n-Propanol (10 mL) instead of methanol. [e] Selectivity of the ester was measured based on substrate (alcohol or aldehyde) conversion. As noted by
2
; conversion (conv.) and
[
[
6c]
Hayashi et al., the excess methanol (ethanol or propanol) as a solvent may also undergo the oxidative esterification reaction, leading to undesired for-
mation of appreciable amount of self-esterification products, that is, methyl formate, ethyl acetate, or propyl propanoate. We confirmed, however, that
the yield of the self-esterification products is always less than 2% in the present reactions.
tive esterification of allylic primary alcohols is probably due
to an undesired formation of oligomers of allyl acrylate,
since there is no other low-molecular products detected by
GC and GC-MS.
dized than the aliphatic and allylic ones (entries 1, 5, and 8).
Similarly, less active allylic aldehydes such as crotonalde-
hyde and acrolein also afford good yields of around 80%
for the corresponding esters (entries 6 and 7). Notice that
these values constitute a significantly higher selectivity com-
pared with their alcohol counterparts. Additionally, the cata-
lytic system efficiently oxidizes aliphatic aldehydes to the
corresponding ester compounds: for the oxidation of octa-
nal, methyl octanoate was produced in a 91% yield in 1 h
(entry 8, Table 2). This observation is in contrast to the oxi-
dative esterification of 1-octanol, for which a much longer
reaction time is required to complete the reaction
Another issue from Table 2 worth noting is the high sub-
strate selectivity of Au/b-Ga O to promote the oxidative
2
3
transformation of various primary alcohols to their corre-
sponding esters. In contrast, it has been reported that under
these conditions the excess methanol as a solvent may also
[6c]
undergo the oxidation reaction.
Particularly relevant is
the undesired formation of appreciable amount of methyl
formate by competitive oxidative esterification. To make a
straightforward comparison of the substrate selectivity as re-
The present Au/b-Ga O catalyst also has high catalytic
2
3
[6c]
ported by Hayashi et al.,
we examined the oxidative
activity for the direct conversion of acetals to the corre-
sponding esters with molecular oxygen. With benzaldehyde
dimethyl acetal as the model substrate, the results obtained
for the conversion into methyl benzoate with various gold
catalysts (compared in Figure 1) showed a high versatility of
the b-Ga O -supported gold catalyst. The Au/b-Ga O3
esterification using n-propanol and n-butanol as the sole re-
action substrates (Table S2 in the Supporting Information).
We found that under identical conditions both alcohols are
oxidized at higher rates than the Au/TiO or Au/ZrO cata-
2
2
[6a–c]
lysts used by Hayashi et al.,
propanoate or n-butyl butanoate always being higher than
0%. Moreover, by decreasing the catalyst loading it is pos-
with selectivities to n-propyl
2
3
2
system is found to be highly active and selective for the
aerobic oxidation; in most cases benzaldehyde was obtained
as the sole byproduct. Using Au/TiO , Au/Fe O or Au/C
9
sible to achieve a low conversion of these substrates within
a short time-frame. This result, together with the intrinsical-
ly more active substrate relative to the excess solvent (i.e.,
2
2
3
reference samples provided by the World Gold Council as
the catalysts, we found that less than 56% of the benzalde-
hyde dimethyl acetal was oxidized to methyl benzoate
within 2 h at 908C. It should be mentioned that under the
same reaction conditions, benzaldehyde was obtained as the
major product over the least active Au/C catalyst, presuma-
bly due to the presence of excess amount of water in the as-
[10,19]
methanol, ethanol or n-propanol),
may account for the
high substrate selectivity of present Au/b-Ga O for alcohol
2
3
oxidative esterification.
Further to oxidative transformation of aldehydes, we ex-
amined the catalytic activity of Au/b-Ga O for the oxida-
2
3
[20]
tive esterification of aldehydes, which is also a synthetically
useful transformation. In general, the oxidation of the alde-
hydes proceeded towards esters much more efficiently than
the corresponding alcohols (results are given in parentheses
in Table 2 under the same reaction conditions), indicating
that the first step of oxidation of alcohols to corresponding
aldehydes may play an important role in the whole catalytic
process. The aromatic aldehydes tend to be more easily oxi-
received WGC sample. These results clearly indicate that
bifunctional Au/b-Ga O catalyst is much more active and
2
3
selective than conventional oxide-supported Au catalysts
and that the collaborative interaction between gold and the
nanocrystalline b-Ga O support are essential for the oxida-
2
3
tive esterification of acetals.
Based on the reaction pathway for alcohol oxidation as
[6c]
proposed by Hayashi et al., a possible mechanism for the
Chem. Eur. J. 2008, 14, 7131 – 7135
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7133

Y. Cao et al.
for step 4, as indicated by the significantly higher efficiency
of the Au/b-Ga O catalysts for the oxidative esterification
2
3
of acetals as compared with that of other Au-based refer-
ence catalysts. Indeed, we have seen that the introduction of
nanocrystalline b-Ga O , which acts as an effective hetero-
2
3
geneous Lewis acid for step 2 or 4, into the reaction mixture
greatly facilitates the oxidative transformation of aldehydes
or acetals over the less effective Au/TiO catalyst (Figure S8
2
in the Supporting Information).
In conclusion, we have shown that the combination of
gold nanoparticles and nanocrystalline b-Ga O can allow
2
3
the devise of new versatile gold catalyst for greener organic
synthesis under mild conditions. Catalyst design has resulted
in the novel bifunctional catalyst that is an outstanding ma-
terial for the additive-free direct catalytic transformation of
primary alcohols, aldehyde,s or acetals to esters under mild,
neutral conditions, providing an environmentally benign
protocol for the direct synthesis of carboxylic esters.
Figure 1. Oxidative esterification of benzaldehyde dimethyl acetal over
various gold catalysts. Reaction conditions: substrate (2.5 mmol), 1 equiv
H
O
2
O and Au catalysts (0.47 mol%) in methanol (10 mL) at 908C, 5 atm
, time=2 h; Conversion and selectivity were determined by GC using
2
n-nonane as the external standards.
Acknowledgements
Au/b-Ga O -catalyzed oxidative esterification of primary al-
2
3
cohols, aldehydes, and acetals is illustrated in Scheme 1. The
one-pot aerobic conversion of primary alcohols into methyl
The authors thank the NSF of China (20421303, 20473021, 20633030), the
National High Technology Research and Development Program of China
(
(
(
2006AA03Z336), the State Key Basic Research Program of PRC
2003CB615807), the Shanghai Science Technology Committee
07QH14003), and Shanghai Education Committee (06SG03) for finan-
&
cial support.
Keywords: esterification
·
gallium oxide
·
gold
·
heterogeneous catalysis · Lewis acids
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2 3
Scheme 1. A proposed mechanisms for Au/b-Ga O -catalyzed oxidative
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[10,21]
from alcohol.
Then, the condensation reaction between
aldehyde and alcohol takes place, leading to the formation
[6a,14,22]
of hemiacetal species
rectly formed by the direct oxidation of hemiacetals
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2
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[
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1
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[
[
4
2
Received: May 22, 2008
Published online: July 14, 2008
Chem. Eur. J. 2008, 14, 7131 – 7135
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7135