Highly efficient mesostructured Ag/SBA-15 catalysts for the chemoselective synthesis of methyl glycolate by dimethyl oxalate hydrogenation

Article abstract of DOI:10.1016/j.catcom.2013.06.022

Ag/SBA-15 catalyst is found to exhibit excellent catalytic activity and long-term stability for the chemoselective hydrogenation of dimethyl oxalate to methyl glycolate. The size of Ag crystallites, which is markedly affected by the Ag loading levels and catalyst pretreatment conditions, is a key factor determining the reaction rate of the structure-sensitive hydrogenation but hardly influenced the product distribution. The best catalytic hydrogenation activity is obtained over the Ag/SBA-15 catalyst with an average Ag crystallite size of around 3.9 nm.

Full text of DOI:10.1016/j.catcom.2013.06.022

Catalysis Communications 40 (2013) 129–133
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly efficient mesostructured Ag/SBA-15 catalysts for the
chemoselective synthesis of methyl glycolate by dimethyl
oxalate hydrogenation
⁎
Jianwei Zheng, Haiqiang Lin, Xinlei Zheng, Xinping Duan, Youzhu Yuan
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols–Ethers–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 13 April 2013
Received in revised form 17 June 2013
Accepted 18 June 2013
Available online 24 June 2013
Ag/SBA-15 catalyst is found to exhibit excellent catalytic activity and long-term stability for the chemoselective
hydrogenation of dimethyl oxalate to methyl glycolate. The size of Ag crystallites, which is markedly affected
by the Ag loading levels and catalyst pretreatment conditions, is a key factor determining the reaction rate
of the structure-sensitive hydrogenation but hardly influenced the product distribution. The best catalytic hydro-
genation activity is obtained over the Ag/SBA-15 catalyst with an average Ag crystallite size of around 3.9 nm.
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Ag/SBA-15 catalyst
Dimethyl oxalate
Methyl glycolate
Hydrogenation
Effect of metal crystallite size
1. Introduction
Ordered mesoporous materials (OMMs) based silicas like MCM-41,
SBA-15, and HMS with tunable and regular mesopores, easily accessible
Methyl glycolate (MG) is usually used as a raw material for the
synthesis of polyglycolic acid and other important organic chemicals.
Several approaches to MG production have been developed, but
weaknesses of these methods such as severe reaction conditions or
lower yields (b70%) markedly hinder their practical applications [1–4].
The chemoselective hydrogenation of dimethyl oxalate (DMO) has led
to the development of a more efficient and greener catalytic procedure
for the challenging feat of efficient MG synthesis [5–8]. Copper-based
catalysts are usually preferred for ethylene glycol (EG) and ethanol
(EtOH) syntheses by the DMO hydrogenation [9–14]. However, excellent
selectivity for the mono-hydrogenation of DMO to MG is difficult to
achieve using analogous copper catalyst systems mainly because of the
high thermodynamic constant of the subsequent hydrogenation of MG
to EG. Studies have shown that Au and Ag have superior catalytic capabil-
ity over Cu for the chemoselective hydrogenation of DMO to MG [5–8].
From the viewpoint of practical application, developing Ag-based cata-
lysts that match the urgent demands for catalytic efficiency, long-term
stability, and easy handling is a challenge. Undoubtedly, probing into
the relationship between the properties of Ag crystallites and the
catalytic performance could significantly help advance the design and
fabrication of Ag-based catalysts with promising features.
internal surface, and relatively high surface areas have been extensively
used as carriers for preparing supported metal catalysts. These catalysts
are efficient for many catalytic reactions in which hydrogen is required,
such as photocatalysis, hydrogenation, oxidative dehydrogenation,
hydrodesulfurization, and the Fischer–Tropsch synthesis [15–18].
The high surface area and uniform pore size distribution in OMMs
allow better dispersion and stabilization of crystallites in oxide or
metal phases with easily controlled sizes. Moreover, the narrow pore
size distribution of OMMs is evidently beneficial for preventing the
sintering effect of metal crystallites, which is important for catalyst sta-
bility. Different from previously reported silica or MCM-41 supported
Ag catalysts [5,6], the Ag/SBA-15 designed in the present study is an
attractive catalyst that displays excellent activity, selectivity, and stability
for the DMO hydrogenation to MG, as well as crystallite size dependence
that may help reveal the importance of nanocatalysis.
2. Experimental
2.1. Catalyst preparation
Mesoporous SBA-15 was synthesized according to literature [19].
Classic volumetric impregnation was adopted to fabricate Ag/SBA-15
catalysts. Typically, a specific amount of AgNO3 was dissolved in
deionized water with one drop of 37 wt.% HNO3 solution. The calcined
SBA-15 and AgNO3 solution were quickly mixed and subjected to
⁎
Corresponding author. Tel.: +86 592 2181659; fax: +86 592 2183047.
E-mail address: yzyuan@xmu.edu.cn (Y. Yuan).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.06.022

130
J. Zheng et al. / Catalysis Communications 40 (2013) 129–133
Table 1
aging at room temperature for 10 h. Prior to calcination in air at 623 K
for 4 h, the obtained powder was dried at 383 K in darkness. Before
catalytic evaluation, the Ag/SBA-15 catalyst precursors were reduced
at 623 K at different temperature ramping rates (2, 6, and 10 K/min)
by 5% H2–95% N2 gas for 4 h, affording the catalysts denoted as
x%Ag/SBA-15-2 K, x%Ag/SBA-15-6 K, and x%Ag/SBA-15-10 K (where x
stands for Ag mass loading).
Textural and physicochemical characteristics of SBA-15 and Ag/SBA-15 catalysts.
Catalyst
Metal loading SBET
Vpore
Dpore
Crystallite size
(wt. %)a
(m2/g) (cm3/g) (nm)b (nm)
by XRDc by TEM
SBA-15
−
833.4
439.5
439.1
410.6
397.4
287.8
1.07
0.58
0.63
0.58
0.55
0.39
5.56
5.05
5.23
5.24
4.99
4.86
−
−
5%Ag/SBA-15
7.5%Ag/SBA-15
10%Ag/SBA-15
12.5%Ag/SBA-15 12.2
15%Ag/SBA-15
4.3
6.9
9.3
3.0
3.1
3.6
15.2
26.0
2.8
3.0
3.9
2.2. Catalyst characterization
n. m.d
n. m.
13.8
Powder X-ray diffraction (XRD) analysis was performed with a
PANalytical X’pert Pro Super X-ray diffractometer using Cu Kα radiation
(λ = 0.15418 nm). Nitrogen adsorption–desorption isotherms were
measured by static N2 physisorption at 77.3 K with a Micromeritics
TriStar II 3020 surface area and pore analyzer. Transmission electron
microscopy (TEM) images were obtained on a Tecnai F30 apparatus op-
erated at 300 kV. The metallic loadings of the samples were determined
by inductively coupled plasma optical emission spectrometry (ICP-
OES) using a Thermo Elemental IRIS Intrepid II XSP. The hydrogen-
temperature-program reduction (H2-TPR) was performed on a
Micromeritics Autochem II 2920 instrument. O2 chemisorption experi-
ments were conducted on a Micromeritics Autochem 2020 instrument
following a method described in previous literature [20]. Before O2
chemisorption measurement at 443 K, the samples were reduced and
then evacuated for 30 min at 623 K, thereafter cooled to 443 K. Metal dis-
a
Metal loading was determined by ICP-OES.
Obtained from P/P0 = 0.99.
Determined by the Scherrer equation from XRD patterns.
Not measured.
b
c
d
of Ag crystallites and subsequent treatments of high-temperature calci-
nation and hydrogen reduction (Fig. S2). The Ag/SBA-15 catalysts showed
a broad peak at 23° ascribed to amorphous silica and five other diffraction
peaks that all corresponded to the characteristic diffraction of cubic Ag
metallic crystals (Fig. 1). Different Ag loadings induced obvious variations
in the intensity and sharpness of these diffractions. Using the Scherrer
equation, the size of Ag crystallites were found to increase gradually
from 3 to 26 nm with increased Ag loading from 5 wt.% to 15 wt.%
(Table 1). The easy aggregation of Ag crystallites at high surface con-
centrations and the limited capability of SBA-15 to accommodate excess
Ag atoms may be responsible for the large crystallite sizes at high
Ag loadings. Anand et al. [21] found similar phenomena on Ag/SBA-15
synthesized by an impregnation method.
persion was also estimated by the metal size according to D = 1.17/dAg
,
where dAg was calculated from the Ag diffraction at a 2θ = 38.06° using
the Scherrer equation.
2.3. Catalytic test
The ordered hexagonal and straight channels were clearly observed
in the HRTEM images of SBA-15 (Fig. 2). In the case of 5%Ag/SBA-15-2 K
and 10%Ag/SBA-15-2 K catalysts reduced at a low temperature ramping
rate, spherical Ag crystallites were found to be homogeneously dis-
persed on the pore surface of SBA-15. The statistical values for the
average crystallite size were consistent with the XRD results. As for
the 10%Ag/SBA-15-6 K and 10%Ag/SBA-15-10 K catalysts reduced at
an elevated temperature ramping rate, apparent aggregation of Ag crys-
tallites occurred and the amount of large Ag crystallites was noticeable.
However, the calcination of Ag/SBA-15 catalyst precursors in air did not
induce the growth of Ag crystallites. The formation of surface Ag species
and their interaction with SBA-15 surface could be responsible for the
effect of reductive atmosphere and temperature on the size and shape
of Ag crystallites.
DMO hydrogenation reaction was carried out in continuous flow
mode in a stainless steel tubular reactor equipped with a computer-
controlled auto-sampling system. Typically, 200 mg of a catalyst precur-
sor (40–60 meshes) was loaded into the center of the reactor, and both
sides of the catalyst bed were packed with quartz powders (40–60
meshes). 0.02 g/mL DMO methanol solution was pumped into the
catalyst bed with a Series III digital HPLC pump (Scientific Systems,
Inc.). The products were analyzed by an on-line gas chromatograph
(Agilent 7890A) equipped with a flame ionization detector and a capil-
lary column (KB-Wax, 30 m). Turnover frequency (TOF) was calculated
according to the following equation:
CDMO⋅V⋅XDMO
Therefore, a series of Ag/SBA-15 catalysts with different Ag crystallite
sizes were prepared by changing the temperature ramping rate during
reduction. Initially, the Ag+ ions in AgNO3 solution were adsorbed on
the pore surface of SBA-15 by strong electrostatic adsorption. Calcination
TOF ¼
;
D⋅NAg
where CDMO is the DMO concentration in the DMO methanol solution,
V is the flow rate of the DMO methanol solution, XDMO is the DMO con-
version measured, NAg is the total amount of Ag and D is the Ag disper-
sion obtained from O2 chemisorption analyses. Through adjusting the
LHSVDMO, the DMO conversion was controlled to lower than 30% to
provide appropriate data for the TOF calculation (Table S1).
Ag
Amorphous silica
e
3. Results and discussion
d
c
3.1. Texture characterization
The actual Ag loading determined by ICP-OES was slightly lower
than the preset values, indicating that most of the Ag precursors
were successfully loaded onto the pore surface of SBA-15 (Table 1).
The surface area, pore volume, and average pore size (getting from
Fig. S1) progressively decreased with increased Ag loading compared
with the pure SBA-15, which could be mainly ascribed to the exacerba-
tion of pore blocking by Ag crystallites.
b
a
15
30
45
60
75
90
2θ / 0
The XRD patterns indicated that the structure of the well-ordered
mesoporous SBA-15 did not significantly change after the immobilization
Fig. 1. Wide-angle XRD patterns of (a) 5%Ag/SBA-15-2 K, (b) 7.5%Ag/SBA-15-2 K,
(c) 10%Ag/SBA-15-2 K, (d) 12.5%Ag/SBA-15-2 K, and (e) 15%Ag/SBA-15-2 K.

J. Zheng et al. / Catalysis Communications 40 (2013) 129–133
131
a
b
c
d
e
f
Fig. 2. TEM images of (a) SBA-15, (b) 5%Ag/SBA-15-2 K, (c) 10%Ag/SBA-15-2 K, (d) 15%Ag/SBA-15-2 K, (e) 10%Ag/SBA-15-6 K, and (f) 10%Ag/SBA-15-10 K.
in air decomposed the weakly absorbed Ag precursors and formed highly
dispersed Ag2O clusters strongly anchored on the SBA-15 surface by the
bond of Si\O\Ag, thereby strongly inhibiting the surface mass transfer
and formation of large Ag crystallites. Under the atmosphere of hydro-
gen, the positively charged Ag species to metallic Ag atoms may signifi-
cantly reduce the strength of the metal-support interaction and the
Ag clusters were apt to move and coagulate because of the low melting
points of nano-Ag and the weak interaction with the supports. High
temperature and rapid temperature ramping inevitably led to the quick
growth of Ag crystallites. Kazuma et al. [22] reported a similar finding
after sintering Ag crystallites on graphene sheets. For a catalyst with an
extremely high Ag loading such as 15 wt.%, even a low ramping rate
for hydrogen reduction cannot afford highly dispersed Ag crystallites
because of the too high surface concentration of Ag atoms. In this case,
many worm-like Ag wires located in the straight channels of SBA-15
emerged in 15%Ag/SBA-15-2 K.
which was ascribed to the transformation of Ag+ to Ag0 (Fig. 4). When
the silver loading was increased to 15 wt.%, a large reduction peak
appeared at a low temperature of 401 K, although the 417 K peak still
emerged on the shoulder of 401 K peak. The reducibility of Ag+ species
is generally affected by both the interaction between metal and support
and crystallite size. On the basis of TEM, XRD and O2 chemisorption anal-
yses, most of the Ag crystallites on 5%Ag/SBA-15 and 10%Ag/SBA-15
catalysts might be confined in the nanochannels of SBA-15 and possess
small sizes. These Ag species were believed to strongly interact with
the internal surface of SBA-15. They were relatively difficult to be re-
duced by hydrogen as compared to unsupported Ag2O particles which
usually showed a reduction peak at ca. 400 K [23]. However, high Ag
loading in the 15%Ag/SBA-15 catalyst could not favor the efficient disper-
sion of Ag on the internal surface of SBA-15 and lead to the formation of
many large Ag crystallites on the external surface. Therefore, the 401 K
3.2. Effect of Ag loading
DMO conv.
MG selec.
EG selec.
The performances of a series of Ag/SBA-15 catalysts with different
Ag loadings for the DMO hydrogenation are shown in Fig. 3. It was not
strange that pure SBA-15 completely did not catalyze the hydrogena-
tion of DMO, suggesting that the active metal Ag played a vital role in
the catalysis of DMO hydrogenation. Increased Ag loading from 5 wt.% to
10 wt.% considerably improved DMO conversion, whereas the selectivity
to MG was almost unchanged (>94%). However, further increased Ag
loading to 12.5 wt.% and 15 wt.% significantly decreased the hydrogena-
tion activity. Extra high Ag loading did not promote the catalytic hydro-
genation of DMO but instead considerably suppressed the activity that
may be due to the variations in the actual size of Ag crystallites and
the amount of Ag active sites.
100
80
60
40
20
0
5.0
7.5
10.0
12.5
15.0
Ag loading / wt%
3.3. H2-TPR
Fig. 3. DMO hydrogenation performance over x%Ag/SBA-15-2 K as a function of Ag loading.
Reaction conditions: Temperature = 473 K, LHSV(DMO) = 0.6 h−1
H2/DMO = 100.
, P(H2) = 3.0 MPa,
The H2-TPR profiles of 5%Ag/SBA-15 and 10%Ag/SBA-15 catalysts
gave one reduction peak centered at ca. 408 K and 417 K, respectively,

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J. Zheng et al. / Catalysis Communications 40 (2013) 129–133
3.5. Effect of Ag crystallite size
401
417
To better understand the essence of the effect of Ag crystallite
size on catalytic behavior, the TOF values for different catalysts are
also calculated using the metal dispersion from O2 chemisorption
(Table S1). Variations in TOF in the DMO hydrogenation as a function
of the Ag crystallite size are shown in Fig. 6. All Ag/SBA-15 catalysts
exhibited excellent selectivity to MG; thus, the Ag crystallite size
only influenced the catalytic activity rather than the selectivity.
417
c
b
a
408
Ag crystallites smaller than 3 nm displayed a low TOF of ca. 13 h−1
,
which increased to 34 h−1 when the mean Ag crystallite size was
3.6 nm. Further increased Ag crystallite size resulted in a gradual
decline in TOF, indicating that in addition to the simple relationship
between the amount of surface metal atoms and the catalytic hydro-
genation activity, some complicated correlations on the formation
of catalytic sites with high efficiency for the chemoselective hydro-
genation of DMO also existed. The dependence of TOF on the average
size of Ag crystallites suggested that DMO hydrogenation was a structure-
sensitive reaction. In our previous work, a bimetallic Au–Ag catalyst
showed a synergistic effect in which the Au sites helped activate the
substrate ester molecules whereas the Ag sites contributed to activating
the H2 molecules by chemisorption [9]. Ag catalysts requiring special
sites to serve as Au sites help activate the substrate ester molecules.
Special low-coordinated Ag atoms are believed to be the active site for
the chemoselective hydrogenation of DMO, and their surface concen-
tration may reach the maximum when the average size of Ag crystallites
is close to 3.9 nm.
350 400 450 500 550 600 650 700 750 800
Temperature / K
Fig. 4. H2-TPR profiles of Ag/SBA-15 catalysts with different Ag loadings. (a) 5%Ag/SBA-15;
(b) 10%Ag/SBA-15; (c) 15%Ag/SBA-15.
peak should be due to the reduction of these large Ag crystallites located
on the external surface of SBA-15.
3.4. Effect of pretreatment
The pretreatment conditions for the Ag/SBA-15 catalyst precursors
were also found to influence the catalytic performances remarkably.
Fig. 5 shows that a slow temperature ramping rate of 2 K/min produced
an excellent 10%Ag/SBA-15-2 K catalyst with DMO conversion
(99.5%) and MG selectivity (95%). By contrast, 10%Ag/SBA-15-6 K
and 10%Ag/SBA-15-10 K catalysts activated at higher temperature
ramping rates had increasingly inferior hydrogenation activities
but retained high MG selectivity (~99%). Combining the results
of the effect of pretreatment conditions on the Ag crystallite size
from TEM observation (Fig. 2) and the textural factors for the
Ag/SBA-15 catalysts with different Ag loadings (Table 1), it is supposed
that Ag crystallites appropriately loaded on the surface of SBA-15 in
smaller crystallite size may provide more active sites and then enhance
the rate of catalytic hydrogenation.
The mean sizes of the interesting worm-like Ag rods filling the
channels of SBA-15 formed at a high temperature ramping rate were
also calculated using the Scherrer equation (Fig. S3). These catalysts
with obviously coagulated Ag crystallites displayed poor performance
in the hydrogenation of DMO to MG (Fig. 4). However, the inaccessibility
of these Ag rods for the DMO substrate could not be excluded to under-
stand the effect of metal crystallite size.
3.6. Long-term performance
The catalyst 10%Ag/SBA-15-2 K with the best activity was subjected
to a comprehensive catalytic evaluation for the DMO hydrogenation.
The DMO liquid hourly space velocity (LHSV) markedly affected the
catalytic performances similar to other kinetic factors such as reaction
temperature and pressure (Fig. S4). At a DMO LHSV of 0.6 h−1, the
best performance was reached at 473 K. Moreover, the catalytic behavior
of the 10%Ag/SBA-15-2 K catalyst was stable for no less than 100 h
(Fig. S5). The XRD characterization on the recycled catalyst confirmed
that the long-term reaction did not obviously enlarge the size of Ag
crystallites (Fig. S5 inlet). Therefore, the Ag/SBA-15 catalyst fabricated
by optimal synthesis and treatment methods is potential for the MG
production by chemoselective hydrogenation of DMO.
100
50
DMO conv.
MG selec.
EG selec.
100
80
60
40
20
0
80
40
30
20
10
0
TOF
MG selectivity
EG selectivity
60
40
20
0
5
10
15
20
25
Diameter of Ag crystallites / nm
Fig. 5. DMO hydrogenation performance over 10%Ag/SBA-15 reduced with different
Fig. 6. TOF and product selectivity for the hydrogenation of DMO as a function of average
temperature ramping rates. The reaction conditions are the same as in Fig. 3.
Ag crystallite size.

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