Magnesia-supported gold nanoparticles as efficient catalysts for oxidative esterification of aldehydes or alcohols with methanol to methyl esters

Article abstract of DOI:10.1016/j.cattod.2013.12.012

Magnesia-supported gold nanoparticles were found to be highly efficient catalysts for the oxidative esterification of methacrolein (MAL) with methanol in the presence of molecular oxygen into methyl methacrylate (MMA) under liquid base-free conditions. MAL conversion of 98% and MMA selectivity of 99% were obtained over the Au/MgO catalyst at 343 K after a 2 h reaction. Besides the Au nanoparticles, the support also played pivotal roles in the oxidative esterification of MAL. The support with higher density of basic sites, particularly stronger basic sites, showed better performances for the formation of MMA. The enhancement of the intermediate formation by the basic sites is proposed to be the key reason for the superior activity of the Au/MgO catalyst. Our studies on the size effect of Au nanoparticles reveal that smaller Au nanoparticles favor the transformation of MAL, and the turnover frequency increases with decreasing mean size of Au nanoparticles. This suggests that the Au-catalyzed oxidative esterification of MAL is a structure-sensitive reaction. We have demonstrated that the Au/MgO catalyst is also applicable to the oxidative esterification of different aldehydes and alcohols.

Full text of DOI:10.1016/j.cattod.2013.12.012

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Contents lists available at ScienceDirect
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Magnesia-supported gold nanoparticles as efficient catalysts for
oxidative esterification of aldehydes or alcohols with methanol to
methyl esters
∗
∗
Xiaoyue Wan, Weiping Deng , Qinghong Zhang, Ye Wang
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering
Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen
361005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 October 2013
Received in revised form
Magnesia-supported gold nanoparticles were found to be highly efficient catalysts for the oxidative
esterification of methacrolein (MAL) with methanol in the presence of molecular oxygen into methyl
methacrylate (MMA) under liquid base-free conditions. MAL conversion of 98% and MMA selectivity of
2
3 November 2013
9
9% were obtained over the Au/MgO catalyst at 343 K after a 2 h reaction. Besides the Au nanoparticles, the
Accepted 2 December 2013
support also played pivotal roles in the oxidative esterification of MAL. The support with higher density
of basic sites, particularly stronger basic sites, showed better performances for the formation of MMA.
The enhancement of the intermediate formation by the basic sites is proposed to be the key reason for
the superior activity of the Au/MgO catalyst. Our studies on the size effect of Au nanoparticles reveal that
smaller Au nanoparticles favor the transformation of MAL, and the turnover frequency increases with
decreasing mean size of Au nanoparticles. This suggests that the Au-catalyzed oxidative esterification of
MAL is a structure-sensitive reaction. We have demonstrated that the Au/MgO catalyst is also applicable
to the oxidative esterification of different aldehydes and alcohols.
Available online xxx
Keywords:
Oxidative esterification
Gold nanoparticles
Magnesia
Bifunctional catalyst
Methacrolein
Methyl methacrylate
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
improve the selectivity of MMA [2], but Pb is quite toxic. Further-
more, the use of liquid base additives is required for the Pd-based
catalysts.
Methyl methacrylate (MMA) is an important chemical, which is
widely used for producing acrylic plastics (polymethyl methacry-
late) or polymer dispersions for paints and coatings [1,2]. Currently,
MMA is mainly produced by the acetone cyan hydrine (ACH)
method [1,2]. However, this process suffers from the problems of
using highly toxic hydrogen cyanide as the raw material and hand-
ling the resulting ammonium sulfide waste [2]. Therefore, many
efforts have been devoted to developing environmentally benign
alternatives for the production of MMA. The oxidative esterifi-
cation of methacrolein (MAL) with methanol in the presence of
O2 is one the most attractive routes for the production of MMA
On the other hand, Au catalysts have attracted considerable
interest because of their unique catalytic behaviors, especially
unique selectivity in many reactions [7–11]. Generally, the cat-
alytic behavior of supported Au catalysts depends on the size and
chemical state of Au particles, the property of the support, and the
Au-support interaction. Au-based catalysts have been successfully
used for the direct oxidative esterification of primary alcohols with
methanol to methyl esters [12–24]. A wide range of materials such
as Al O [12], TiO [13–15], CeO [16–18], Ga O [19], SiO [20–22],
2
3
2
2
2
3
2
polymers [23], and hydrotalcite [24] have been exploited as sup-
ports of Au catalysts for these reactions. However, in most cases, a
liquid base additive, e.g., alkali carbonate (K CO , Na CO ), alkali
(Scheme 1).
Supported Pd catalysts have been reported to be efficient for the
2
3
2
3
direct oxidative esterification of MAL and other aldehydes or alco-
hols with methanol to the corresponding esters [2–6]. However,
the selectivity of MMA is typically low over supported Pd catalysts.
hydroxide (NaOH, KOH), or NaOCH3 is required to achieve high
yields of the target ester [16,18,19,24]. This makes the process less
green and less cost-effective.
The modification of Pd catalysts by Pb forming Pd Pb phase could
Recently, we have aimed at developing efficient Au catalysts
capable of working for the oxidative esterification of MAL with
methanol to MMA in the absence of any liquid bases. Here, we
report our finding that Au nanoparticles loaded on a solid base,
i.e., MgO, are highly efficient catalysts for liquid base-free oxida-
tive esterification reactions. The effects of the support and the Au
3
∗ Corresponding authors. Tel.: +86 592 2186156; fax: +86 592 2183047.
E-mail addresses: dengwp@xmu.edu.cn (W. Deng), wangye@xmu.edu.cn
(
Y. Wang).
0
920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.12.012
Please cite this article in press as: X. Wan, et al., Magnesia-supported gold nanoparticles as efficient catalysts for oxidative esterification of
aldehydes or alcohols with methanol to methyl esters, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.12.012

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O
Catalyst
O
+
CH OH
O2
+ H O
2
3
+
OCH3
Scheme 1. Oxidative esterification of methacrolein with methanol to methyl methacrylate.
particle size will be discussed in detail. The substrate scope of the
Au/MgO catalyst will also be investigated.
2.3. Catalytic reaction
The oxidative esterification of MAL and other aldehydes or alco-
hols to methyl esters was performed in a batch-type Teflon-lined
stainless-steel autoclave. Typically, MAL purchased from Alfa Aesar
2
. Experimental
(
12 mmol) and the catalyst (typically, 0.50 g) were added into the
2
.1. Catalyst preparation
reactor pre-charged with methanol (20 mL). After the introduction
of O2 with a certain pressure (typically 0.2 MPa), the mixture was
heated up to a reaction temperature (typically 343 K) in an oil bath,
and then the catalytic reaction was started by vigorously stirring.
After a fixed time (typically 2 h, recorded as reaction time), the reac-
tion was stopped by cooling down the reactor to room temperature
in cold water. The products were analyzed by a gas chromatograph
equipped with a FID detector and a capillary column (DB-FFAP,
60 m × 0.32 mm × 0.25 ␮m) using ethanol as an external standard
for quantification.
The supported Au catalysts were prepared by
a
deposition–precipitation (DP) method using urea as a homo-
geneous precipitant. In brief, urea was first added into an aqueous
solution of HAuCl4 (typically 0.48 mmol L ) with a molar ratio
of urea/Au of 100:1. Then, the support was added into the mixed
aqueous solution, and the suspension was vigorously stirred at
−1
3
53 K. After aging for a certain time (typically 4 h), the solid product
was recovered by centrifugation followed by thorough washing
with deionized water to remove the remaining chloride anion. The
resulting powdery solid was dried in air at 353 K for 1 h, and finally
reduced in H at 523 K for 2 h. The concentration of HAuCl and the
3
. Results and discussion
2
4
aging time were regulated to control the size of Au nanoparticles
as reported previously [25]. The target Au loading in each catalyst
used in this work was 0.5 wt%.
3
.1. Catalytic behaviors of Au nanoparticles loaded on different
supports
Table 1 shows the catalytic performances of Au catalysts loaded
on different supports for the direct oxidative esterification of MAL
with methanol in the presence of O2 to MMA. Au catalysts loaded
on some metal oxides without basicity such as SiO , TiO , and
2
.2. Catalyst characterization
Transmission electronic microscopy measurements were car-
2
2
SBA-15 showed poorer activity for the conversion of MAL. The
selectivities of MMA over these catalysts were also quite lower.
Many side reactions occurred over these catalysts, and the main
by-products included isobutyric acid, isobutyl aldehyde, dimers of
the methacrolein, and CO . The employment of Al O , ZnO, ZrO ,
and hydroxyapatite as the supports of Au catalysts provided mod-
erate MMA selectivities (68–83%) although the conversions of MAL
over these catalysts were not high. Among all of the catalysts exam-
ined in the present work, the Au/MgO and Au/hydrotalcite (HT)
exhibited the best performance for the conversion of MAL to MMA;
the yields of MMA were 97% and 90% over the Au/MgO and Au/HT,
ried out on a JEM-2100 electron microscope operated at an
acceleration voltage of 200 kV. The mean size of Au nanoparticles
in the sample was estimated from the TEM micrographs by count-
ing around 150–200 particles. By assuming a spherical model of Au
nanoparticles, the Au dispersion (D) could be estimated by using
the following relationship, D = 1.17/d(nm) × 100%, where d is parti-
cle diameter [26]. Inductively coupled plasma mass spectrometry
2
2
3
2
(
4
ICP-MS) measurements were performed with an Agilent ICP-MS
500 instrument to measure the content of Au in each sample. Spe-
cific surface areas of the catalysts were measured by N adsorption
2
with Micromeritics Tristar II 3020.
CO2 adsorption measurements were performed on
a
Micromeritics ASAP 2020 instrument. Typically, the sample
loaded in a quartz tube was first pretreated with high-purity He
at 423 K for 2 h. After the sample was cooled down to 323 K, the
Table 1
Catalytic performances of Au catalysts loaded on different supports for the oxidative
esterification of MAL with CH3OH in the presence of O2.
a
reactor was evacuated for 1 h. Then, CO adsorption was performed
2
Selectivityb (%)
Catalyst
MAL conv. (%)
by dosing certain amount of CO . After adsorption for 30 min, the
2
MMA
MIB
MAA
Others
gas phase and the weakly adsorbed CO2 were evacuated. Then,
CO2 adsorption amount was evaluated by the difference between
the CO2 amounts injected and evacuated.
Au/SiO2
Au/SBA-15
Au/Al2O3
Au/TiO2
Au/ZrO2
Au/ZnO
Au/CeO2
Au/MgO
Au/HT
16
13
42
25
33
28
58
98
99
31
16
8.8
80
39
67
78
99
99
91
83
0.5
0.3
1.6
1.5
0.2
2.7
0.3
0.4
0.4
0.6
1.1
0.8
3.0
0.8
0.2
0.1
0.3
0.1
0.1
0.2
82
90
16
59
33
19
0.4
0.4
8.4
16
The strength of basicity for different supports was estimated
by Hammett titration. The methyl red, bromothymol blue, phenol-
phthalein, 2,4-dinitroaniline, and 4-nitroaniline were used as the
Hammett indicators with the pKa values of +4.8, +7.2, +9.3, +15.0,
and +18.4, respectively. The indicators were dissolved in petroleum
ether of reagent grade and its concentration was ∼0.1 wt%. After
drying in vacuum, 100 mg of grinded sample was rapidly trans-
ferred into a tube and 1 mL of petroleum ether was injected to cover
the sample. Subsequently, several drops of indicator solution were
added followed by vigorous shaking. The strength of the basicity of
supports was determined by the color change.
Au/HAP
a
Reaction conditions: catalyst (Au loading, ∼0.45 wt%), 0.50 g; CH3OH/MAL = 40:1
molar ratio); CH3OH, 20 mL; P(O2) = 0.2 MPa; T = 343 K; t = 2 h.
MMA, methyl methacrylate; MIB, methyl isobutyrate; MAA, methacrylic acid;
others include isobutyric acid, isobutyl aldehyde, dimers of the methacrolein, CO2,
and some unknown products.
(
b
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Table 2
were also similar, ranging from 1.9 to 3.2 nm. These clearly suggest
that the DP method using urea as a homogeneous precipitant is an
effective technique to successfully deposit Au nanoparticles with
similar sizes onto different supports. Thus, we speculate that the
differences in the catalytic performances among the catalysts with
different supports mainly arise from the property of the support.
The BET surface area for each catalyst is also displayed in Table 2.
The Au/SBA-15 possesses the largest surface area, but this catalyst
has shown very poor activity for the conversion of MAL (Table 1).
The Au/TiO2, Au/MgO, and Au/HT catalysts possess moderate sur-
face areas, but only the latter two catalysts are efficient for the
oxidative esterification of MAL to MMA. This indicates that the
BET surface area of the catalyst is not a key factor influencing the
catalytic performances.
Properties of Au catalysts loaded on different supports.
Au contenta
wt%)
Au particle
size (nm)
Surface area
(m
c
Catalyst
b
2
−1
g )
(
Au/SiO2
Au/SBA-15
Au/Al2O3
Au/TiO2
Au/ZrO2
Au/ZnO
Au/CeO2
Au/MgO
Au/HT
0.43
0.44
0.43
0.45
0.43
0.42
0.45
0.45
0.45
0.44
2.2
3.2
2.5
2.3
2.5
1.9
2.3
2.2
2.5
2.4
180
227
104
22
1.6
2.7
2.7
14
13
30
Au/HAP
a
Obtained from ICP-MS.
Evaluated from TEM.
Calculated from N2 physisorption.
b
c
The Hammett titration method can be used to determine the
strength of acidity and basicity for solid acids and solid bases [27].
The change in color of an indicator in the suspension containing
a solid base indicates that the Hammett value (basicity) of this
respectively, at 343 K after 2 h of reaction. The Au/CeO catalyst also
2
showed very high selectivity to MMA (99%), but the MAL conver-
sion over this catalyst was inferior to those over the Au/MgO and
Au/HT catalysts.
base was higher than the pK of the indicator. Table 3 displays the
results obtained from the Hammett titration for different supports.
a
Among all the supports, only MgO and HT could induce a color
change for phenolphthalein (pKa = 9.3), indicating that these two
supports possess stronger basic sites. The titration results also sug-
gest that the strength of basicity decreases in the order of MgO,
HT > HAP, Al O > CeO , ZnO > ZrO , TiO , SiO and SBA-15. MgO-
3.2. Characterizations of Au catalysts loaded on different
supports and the effects of the support
2
3
2
2
2
2
The ICP-MS analysis showed that the Au content in each cata-
lyst was approximately 0.45 wt% (Table 2), close to the target value
and HT-supported Au catalysts were the most efficient catalysts for
the conversion of MAL to MMA, while the use of SiO2, SBA-15, or
TiO2 as the support provided much poorer catalytic performances
(Table 1). This may suggest that the strength of the basicity of the
support plays a role in the oxidative esterification reaction.
We also quantified the density of basic sites on each support by
measuring the adsorption amount of CO2. As displayed in Table 4,
Al2O3, MgO, HT, and HAP are capable of adsorbing larger amounts
of CO2 per gram of catalysts. In contrast, no adsorption of CO2 was
(
0.5 wt%) used in the preparation gel. Fig. 1 shows the TEM micro-
graphs for some typical supported Au catalysts used in this work.
From Fig. 1, it is clear that the Au particles are well dispersed on
the supports in these catalysts. The mean sizes of the Au parti-
cles estimated by counting 150–200 particles over these catalysts
were quite similar. As summarized in Table 2, the mean sizes of
Au nanoparticles in all of the catalysts used in the present work
Fig. 1. TEM micrographs of Au nanoparticles loaded on different supports. (a) MgO, (b) HT, (c) Al2O3, (d) ZrO2, (e) TiO2, and (f) SiO2.
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Table 3
Results obtained from the Hammett titration with various indicators for different supports.
Support
Indicators (pKa)
Methyl red (4.8)
Bromothymol blue (7.2)
Phenolphthalein (9.3)
2,4-Dinitro-aniline (15.0)
4-Nitroaniline (18.4)
MgO
HT
+
+
+
+
+
+
−
−
−
−
+
+
+
+
−
−
−
−
−
−
+
+
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
␥
-Al2O3
−
−
−
−
−
−
−
−
HAP
CeO2
ZnO
TiO2
ZrO2
SiO2
SBA-15
+
, color change occurs; −, no color change occurs.
Table 4
product from MAL conversion, during the whole reaction process.
In addition, the leaching of MgO, which might have been caused
by the reaction with MAA, an acid, if it is formed during the reac-
tion, has not occurred. Thus, we speculate that the reaction over our
Au/MgO catalyst may proceed via route 1 in Scheme 2. It has been
reported that the presence of a base can accelerate the formation of
hemiacetal [29] and is also essential for the consecutive oxidation of
the intermediate [13]. Therefore, for the oxidative esterification of
MAL, it is understandable that MgO or HT-supported catalyst con-
taining high density of basic sites, particularly the stronger basic
sites, favors the formation of MMA.
The adsorption amounts of CO2 over different supports.
Support
CO2 adsorption amount
Per gram
Per surface area
(␮mol m
␮mol g 1)
−
−2
)
(
MgO
HT
CeO2
Al2O3
HAP
ZnO
ZrO2
TiO2
SiO2
44.0
43.5
3.9
80.1
41.8
3.3
10.9
1.2
0
3.2
3.4
1.4
0.8
1.4
1.2
0.9
0.1
0
SBA-15
0
0
3.3. Effect of size of Au nanoparticles over Au/MgO catalysts
It is well known that the size of Au nanoparticles significantly
affects their catalytic behaviors in many reactions [7–11,30–32].
observed over SBA-15 and SiO , indicating the absence of basic sites
2
over these two supports. Medium to lower adsorption amounts
of CO2 were observed over ZrO , CeO , ZnO, and TiO , by taking
By regulating the concentration of Au precursor ([HAuCl ]), the
4
aging time, and the calcination conditions used in the catalyst
preparation by the DP method, we have succeeded in preparing
the MgO-supported Au catalysts containing Au nanoparticles with
mean sizes ranging from 2.2 to 10 nm. Table 5 shows the condi-
tions used in the preparation and the corresponding mean sizes of
Au nanoparticles derived from TEM in the final catalysts after H2
reduction. Typical TEM micrographs and the Au particle size distri-
butions for these catalysts are shown in Fig. 2. Table 5 shows that
the increase in the concentration of HAuCl4 decreases the mean
size of Au nanoparticles. The prolonging of aging time also sig-
nificantly increased the size of Au nanoparticles probably due to
the Ostwald ripening, which was caused by the dissolved smaller
particles precipitating onto the larger particles [25].
2
2
2
account into the surface area, the density of basic sites per sur-
face area was also evaluated. Among all the supports, MgO and HT
bearing stronger basicity also showed the highest densities of basic
sites. CeO , HAP, ZnO, ZrO , and Al O possessed medium densities
2
2
2
3
of basic sites, whereas TiO possessed a lower density of basic sites.
2
This trend correlates well with that for the change of catalytic per-
formance by changing the support (Table 1). These results allow
us to propose that the basicity of support plays key roles in the
oxidative esterification of MAL with CH OH to MMA.
3
Recently, several groups have investigated the reaction mech-
anism for the oxidative esterification of aldehydes with alcohols
over both Pd- and Au-based catalysts [6,19,28]. The reaction has
been proposed to proceed through the initial formation of a hemi-
acetal intermediate, followed by the oxidation of the hemiacetal to
ester (Scheme 2, route 1). The oxidation of aldehyde to carboxylic
acid in the first step followed by the esterification of carboxylic
acid with alcohol to ester in the second step may also be possible
We have investigated the catalytic performances of the Au/MgO
catalysts with different mean sizes of Au nanoparticles for the
oxidative esterification of MAL with CH OH in the presence of O .
3
2
Table 6 shows the results obtained at 343 K after 2 h of reaction.
Without Au, no formation of MMA was observed. After the load-
ing of Au particles, the conversion of MAL increased significantly,
and MMA was formed as a major product, suggesting that the Au
(Scheme 2, route 2). In our case, we could not observe the formation
of significant amount of methacrylic acid (MAA), a direct oxidative
OH
O
O
1
Catalyst
R OH
R1
R1
O2
Route 1
Route 2
R
O
R
O
R
H
O
1
O2
R OH
Catalyst
R
OH
Scheme 2. Possible mechanism for oxidative esterification of aldehydes to esters.
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Table 5
Effect of preparation conditions on the mean size and dispersion of Au nanoparticles over Au/MgO catalysts.
Entry
[HAuCl4] (mol/L)
Aging time (h)
Mean Au sizea (nm)
Au dispersionb (%)
1
2
3
4
5
6
0.96
0.48
0.24
0.12
0.12
0.12
0.12
4
4
4
4
8
2.2
2.8
3.5
4.1
5.0
6.1
10
53
42
33
29
23
19
12
16
16
c
7
a
Obtained from TEM.
Estimated from the mean size of Au.
The sample was calcined in air at 673 K for 2 h.
b
c
nanoparticles accelerated the conversion of MAL and were crucial
for the formation of MMA. The catalyst with smaller Au nanopar-
ticles provided higher MAL conversion. To gain further insights
into the Au particle size effect, we have performed the conver-
sion of MAL at short reaction time over the Au/MgO catalysts with
different mean sizes of Au nanoparticles. We measured the MAL
conversions at the initial reaction stage for these catalysts and
evaluated the initial rate of MAL conversion by using the slopes
of the straight lines obtained in the plots of the conversion versus
reaction time (Fig. 3). The initial rate of MAL conversion increased
markedly as the size of the Au particles decreased from 10 nm to
MAL (Fig. 4). Our results suggest that the oxidative esterification
of MAL with methanol over the Au/MgO catalyst is a structure-
sensitive reaction. The facile activation of O2 or the intermediate
may be the reason for the higher TOFs over the smaller Au nanopar-
ticles, which contains higher fraction of coordinatively unsaturated
Au sites [25].
3.4. Optimization of Au loading, recycling uses and substrate
scope of Au/MgO catalyst
2
.2 nm (Table 7). The selectivity of MMA was >93% over each cata-
The results described above suggest that the basicity and the
Au particle size are two crucial factors determining the catalytic
behaviors of the supported Au nanoparticles for the oxidative ester-
ification of MAL with methanol in the presence of O2 under liquid
base-free conditions. The Au/MgO with strong basicity and a mean
lyst under these conditions. We have also calculated the turnover
frequency (TOF), i.e., the moles of MAL converted per unit time at
the initial stage per mole of surface Au atoms. The catalysts with
smaller Au nanoparticles exhibit higher TOFs for the conversion of
Fig. 2. TEM micrographs of Au/MgO catalysts prepared under different conditions and the corresponding particle size distributions. The conditions are displayed in Table 5;
the conditions used for the preparation of samples (a), (b), (c), (d), (e), and (f) correspond to Entries of 1, 3, 4, 5, 6, and 7 in Table 5, respectively.
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Table 6
Table 8
Catalytic performances of MgO-supported Au nanoparticles with different mean
sizes for oxidative esterification of MAL with CH3OH in the presence of O2.
Effect of Au loadings on catalytic performances of Au/MgO catalysts for oxidative
esterification of MAL with CH3OH in the presence of O2.
a
a
Catalyst
MAL conv. (%)
Selectivityb (%)
Au loadings (wt%)
MAL conv. (%)
Selectivityb (%)
MMA
MIB
MAA
Others
MMA
MIB
MAA
Others
MgO
1.7
62
57
44
19
0
98
98
97
93
0
0
0
0
0
1.7
1.6
1.4
2.4
6.7
0
1.7
30
65
86
98
0
98
98
99
99
0
0
1.7
1.2
1.2
0.3
0.4
0.3
Au/MgO (Au, 2.8 nm)
Au/MgO (Au, 3.5 nm)
Au/MgO (Au, 4.1 nm)
Au/MgO (Au, 6.2 nm)
0.4
0.6
0.6
0.3
0.11
0.25
0.36
0.45
0.7
0.6
0.5
0.4
0.2
0.2
0.2
0.1
0.1
0
0.1
0.55
100
>99
a
Reaction conditions: catalyst (Au loading 0.45 wt%) = 0.30 g; CH3OH:MAL = 40:1
a
(
molar ratio); CH3OH, 20 mL; P(O2) = 0.2 MPa; T = 343 K; t = 2 h.
Reaction conditions: Au/MgO, 0.50 g; CH3OH:MAL = 40:1 (molar ratio); CH3OH,
b
MMA, methyl methacrylate; MIB, methyl isobutyrate; MAA, methacrylic acid;
20 mL; P(O2) = 0.2 MPa; T = 343 K; t = 2 h.
b
others includes isobutyric acid, isobutyl aldehyde, dimers of the methacrolein, CO2
and other unknown products.
MMA, methyl methacrylate; MIB, methyl isobutyrate; MAA, methacrylic acid;
others includes isobutyric acid, isobutyl aldehyde, dimers of the methacrolein, CO2
and other unknown products.
Table 7
Effect of the mean size of Au nanoparticles on the initial rate of MAL conversion.
Au particle size of 2.2 nm could afford a MMA yield of 98% at 343 K
after 2 h of reaction.
Mean Au size (nm)
Initial MAL conversion
rate (␮mol g
−
1
−1
s )
We have investigated the effect of the loading amount of Au on
catalytic performances for the oxidative esterification of MAL with
methanol. As shown in Table 8, in the absence of Au the conversion
of MAL was very low (1.7%) and no formation of MMA was observed.
After the loading of a small amount of Au particles onto MgO, the
conversion of MAL increased remarkably, and MMA was formed as
a major product. The increase in the Au loading led to an increase in
the MAL conversion. A high yield of MMA (>95%) could be attained
as Au loading was ≥0.45 wt%. These results provide further evi-
dence that the Au nanoparticles accelerate the conversion of MAL
and are responsible for the formation of MMA. Taking the MMA
yield and the cost of catalyst into consideration, it is clear that the
Au loading of 0.45 wt% is appropriate for the oxidative esterification
of MAL with methanol to MMA.
2
3
4
6
.2
.5
.1
.2
5.6
3.1
2.2
1.1
0.55
1
0.0
Reaction conditions: catalyst (Au loading 0.45 wt%) = 0.30 g; CH3OH:MAL = 40:1
molar ratio); CH3OH, 20 mL; P(O2) = 0.2 MPa; T = 343 K.
(
Besides the yield of MMA, the stability of the catalyst is also a key
factor for its practical application. Thus, we performed the recycling
uses of the Au/MgO catalyst with an Au loading of 0.45 wt% for the
oxidative esterification of MAL with methanol. After each run, the
catalyst was recovered by filtration, washing and drying, and then
was re-used in the next run. Fig. 5 shows that the MAL conver-
sion decreased only slightly in the initial 4 recycles, and then kept
almost unchanged in the 5th recycle. Meanwhile, the selectivity to
MMA was kept as high as 99%. After 5 recycles, the MAL conversion
and the MMA yield could still be sustained at >90%. This indicates
that the Au/MgO catalyst is stable during the recycling uses for the
conversion of MAL with methanol in the presence of O2.
Fig. 3. Catalytic performances of Au/MgO catalysts with different mean sizes
of Au particles for oxidative esterification of MAL. Reaction conditions: catalyst
(
Au loading 0.45 wt%), 0.30 g; CH3OH:MAL = 40:1 (molar ratio); CH3OH, 20 mL;
We have also examined the catalytic performances of the
Au/MgO (Au, 2.2 nm) catalyst for the liquid base-free oxidative
P(O2) = 0.2 MPa; T = 343 K.
Fig. 4. Dependence of TOF for MAL conversion over Au/MgO catalysts on the mean
size of Au particles. Reaction conditions: catalyst (Au loading, 0.45 wt%) = 0.30 g;
CH3OH:MAL = 40:1 (molar ratio); CH3OH, 20 mL; P(O2) = 0.2 MPa; T = 343 K.
Fig. 5. Recycling uses of Au/MgO catalyst for oxidative esterification of MAL. Reac-
tion conditions: catalyst (Au loading, 0.45 wt%) = 0.50 g; CH3OH:MAL = 40:1 (molar
ratio); CH3OH, 20 mL; P(O2) = 0.2 MPa; T = 343 K; t = 2 h.
Please cite this article in press as: X. Wan, et al., Magnesia-supported gold nanoparticles as efficient catalysts for oxidative esterification of
aldehydes or alcohols with methanol to methyl esters, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.12.012

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CATTOD-8788; No. of Pages8
ARTICLE IN PRESS
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7
Table 9
a
Oxidative esterification of various substrates with methanol over the Au/MgO catalyst.
Entry
Reactant
Product
Reaction time (h)
Conv. (%)
99
Select. (%)
87
O
O
1
3
OCH3
2
3
3
3
65
96
92
>99
4
5
6
6
94
92
97
>99
6
7
12
81
12
80
66
O
OCH3
6
F3C
8
9
3
39
89
97
98
12
Reaction conditions: catalyst (Au loading, 0.45 wt%) = 0.50 g; CH3OH:substrate = 40:1 (molar ratio); CH3OH, 20 mL; P(O2) = 0.5 MPa; T = 353 K.
esterification of a wide scope of aldehydes and alcohols with
methanol under O2 atmosphere. As shown in Table 9, the conver-
sion of benzaldehyde to methyl benzoate could be catalyzed by the
Au/MgO catalyst at 353 K with a good efficiency and the yield of
methyl benzoate reached ∼87% (Entry 1). Benzyl alcohol could also
be selectively transformed to methyl benzoate although the con-
version was relatively low (Entry 2). Notably, o- and p-methoxy
benzyl alcohols and p-methyl benzyl alcohol could facilely be con-
verted to the corresponding methyl esters in high yields under
appropriate reaction conditions (Entries 3–5). The conversions of 4-
alcohol, increased from 39% to 89% with prolonging the reaction
time from 3 to 12 h over the Au/MgO catalyst, while the selec-
tivity of methyl octanate kept at 97–98% at the same time. Thus,
the present Au/MgO catalyst favors the oxidative esterification of
benzylic alcohols containing electron-donating groups to the cor-
responding methyl esters, while it is difficult for the conversion of
benzylic alcohols containing electron-withdrawing groups.
4. Conclusions
(
(
trifluoromethyl)benzyl alcohol to the corresponding methyl esters
Entry 7) became difficult. The conversion of octanol, an aliphatic
The comparison among Au catalysts loaded on a series of sup-
ports reveals that the support plays significant roles in determining
Please cite this article in press as: X. Wan, et al., Magnesia-supported gold nanoparticles as efficient catalysts for oxidative esterification of
aldehydes or alcohols with methanol to methyl esters, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.12.012

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8
X. Wan et al. / Catalysis Today xxx (2013) xxx–xxx
both the activity and selectivity for the oxidative esterification of
MAL to MMA under liquid base-free conditions. We have clearly
demonstrated that the support with a larger density of basic sites
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Please cite this article in press as: X. Wan, et al., Magnesia-supported gold nanoparticles as efficient catalysts for oxidative esterification of
aldehydes or alcohols with methanol to methyl esters, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.12.012