Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted Cu/SiO2

Article abstract of DOI:10.1016/j.jcat.2019.10.001

Hydrogenation of dimethyl oxalate (DMO) on Cu/SiO₂ catalysts is the final reaction step in a growing industrial syngas to ethylene glycol (StEG) process. In the present work, indium species are found to dramatically enhance the performance of a Cu/SiO₂ catalyst prepared by urea-assisted gelation for the conversion of DMO to EG. Characterization reveals that indium species have negligible effect on the content of Cu⁺ species on the surface. Rather, the promotional effect likely originates from interactions between reduced indium species and Cu⁰ species though enhanced ability of the latter to activate H₂.

Full text of DOI:10.1016/j.jcat.2019.10.001

Journal of Catalysis xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over
indium promoted Cu/SiO2
Xinbin Yu a, Trey A. Vest a, Nicolas Gleason-Boure a, Stavros G. Karakalos a, Gregory L. Tate a,
Michael Burkholder b, John R. Monnier a, Christopher T. Williams a,
⇑
a Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA
b Department of Chemical Engineering, Virginia Commonwealth University, Richmond, VA 23284-3068, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrogenation of dimethyl oxalate (DMO) on Cu/SiO2 catalysts is the final reaction step in a growing
industrial syngas to ethylene glycol (StEG) process. In the present work, indium species are found to dra-
matically enhance the performance of a Cu/SiO2 catalyst prepared by urea-assisted gelation for the con-
version of DMO to EG. Characterization reveals that indium species have negligible effect on the content
of Cu+ species on the surface. Rather, the promotional effect likely originates from interactions between
reduced indium species and Cu0 species though enhanced ability of the latter to activate H2.
Ó 2019 Elsevier Inc. All rights reserved.
Received 9 August 2019
Revised 30 September 2019
Accepted 1 October 2019
Available online xxxx
Keywords:
Hydrogenation
Syngas
Dimethyl oxalate
Ethylene glycol
Copper catalyst
1. Introduction
ester group in DMO, while the latter serves to activate and dissoci-
ate H2 molecules. Different preparation methods (e.g., ammonia
The hydrogenation of dimethyl oxalate (DMO) has attracted
attention because it can bridge the gap between synthesis gas
and methyl glycolate (MG), ethylene glycol (EG) and ethanol, etc.
[1–3]. EG is widely used as a solvent and anti-freezing agent, and
in the production of polyester fiber, among many other applica-
tions [1]. As a versatile building block, synthesis gas can be readily
produced from coal, natural gas and biomass, making the DMO-to-
EG process more economically and technologically feasible [4].
evaporation, sol-gel, precipitation-deposition, impregnation), silica
supports (e.g., MCM-41, SBA-15, HMS), and process variables (e.g.,
pH, temperature) have been explored to prepare catalysts with
high metallic dispersion and moderate interaction between copper
and silica [4,7–12].
Such catalytic systems suffer from inherent issues such as sus-
ceptibility to Cu agglomeration due to its low Hüttig temperature,
and loss of Cu+ species by valence transition during the reaction
[13]. These problems cause diminution of active sites, collapse of
the synergistic effect between Cu0 and Cu+ species, and eventually
the degradation of catalytic performance. One strategy to address
these issues is through modification of the properties of silica sup-
ported copper catalyst with appropriate promoters. A variety of
species (e.g., Au, Ag, Ni, La, Zn, B2O3, etc.) have been found to
improve the catalytic performance of silica supported copper cata-
lysts by increasing the dispersion and/or enhancing the stability of
Cu0 and/or Cu+ species [11,14–18]. The forms of promoters and
their specific role in enhancing catalytic performance are a topic
still under intense investigation.
H3COOC-COOCH3 + 2H2 ! H3COOC-CH2OH + CH3OH
ð1Þ
H3COOC-CH2OH + 2H2 ! HOH2C-CH2OH + CH3OH
ð2Þ
Various catalytic materials have been designed to facilitate the
efficient hydrogenation of DMO to EG. The most widely used cata-
lyst in industry is the well-established copper-chromium, which
exhibits excellent activity and stability [5,6]. However, the haz-
ardous Cr6+ motivates exploration of other effective and yet more
environmentally friendly alternatives. Silica supported copper cat-
alysts have been found to be active in this process due to a unique
Cu-O-Si structure that balances the concentration of surface Cu+
and Cu0 species. The former species facilitates the activation of
Here we report the promotional effect of indium species on the
performance of hydrogenation of DMO to EG on silica supported
copper catalysts. The highly dispersed catalysts are synthesized
in a simple, robust method with a low amount of the cheap, non-
toxic promoter. The effect of weight hourly space velocity (WHSV)
⇑
Corresponding author.
E-mail address: willia84@cec.sc.edu (C.T. Williams).
https://doi.org/10.1016/j.jcat.2019.10.001
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
Cu/SiO2, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.001

2
X. Yu et al. / Journal of Catalysis xxx (xxxx) xxx
and pressure on the catalytic performance and the stability of cat-
alysts are studied in detail. Various characterization methods are
conducted to elucidate the role of indium species to establish the
structure-performance correlation and reveal how the size distri-
bution of promoter and the interaction between two metals modify
the properties of main catalytic components.
sumption assuming 1.46 ꢂ 1019 copper atoms/m2. The result from
N2O titration on 1 wt% In/SiO2 shows that reduced indium species
do not consume N2O.
H2 temperature-programmed reduction (H2-TPR) is performed
on a Micromeritics Autochem II 2920 instrument. For H2-TPR,
100 mg of an calcined catalyst is loaded into the U-shape
quartz tube. The sample is reduced with 10% H2/Ar flowing at
40 mL/min at a ramping rate of 10 °C/min from RT to 600 °C. A
TCD detector is used to monitor the consumption of H2.
The Fourier-transform infrared (FT-IR) spectra of calcined cata-
lysts are collected on Perkin Elmer Spectrum 100 FT-IR spectrom-
eter with 4 scans for each spectrum at room temperature. The
spectrometer is equipped with a diamond internal reflection ele-
ment (IRE) and operated in ATR mode. Sample powders are spread
on the IRE and tightened by an alumina cylinder to ensure a close
contact with IRE.
2. Experimental
2.1. Materials
All the reagents are commercially available and used without
further purification. Copper (II) nitrate trihydrate (99%, Acros
Organics), Indium (III) nitrate hydrate (99.99%, Alfa Aesar), 40 wt%
Ludox AS-40 colloidal silica (Sigma-Aldrich), dimethyl oxalate
(99%, Sigma-Aldrich), concentrated nitric acid (68-70%, BDH), urea
(99.7%, Fisher Chemical), ammonia aqueous solution (BDH, 28%)
are used as received. H2 (UHP), He (UHP), Ar (UHP), N2(UHP), and
air (Zero grade) are supplied by Praxair. Deionized water
X-ray diffraction (XRD) patterns of catalysts are collected on a
Rigaku SmartLab SE X-ray diffractometer using Cu K
a radiation
(k = 0.15418 nm) with a scanning angle (2h) range of 10° to 90°.
The tube voltage and current were 40 kV and 44 mA, respectively.
The XRD patterns for calcined catalysts are scanned at a speed of
15°/min. For in situ XRD measurement, calcined catalysts are
reduced in an in situ reactor with 20% H2-80% N2 mixture
(100 mL/min) at 350 °C for 2 h. XRD patterns are collected when
the temperature is held at 350 °C or decreases to 190 °C at a scan-
ning speed of 5°/min. The Cu crystallite size is calculated according
to Scherrer equation using the full width at half-maximum
(FWHM) of the Cu(1 1 1) diffraction peak at 2h = 43.2°.
Transmission electron microscopy (TEM) images are obtained
on a Hitachi HT7800 transmission electron microscope operating
at an acceleration voltage of 100 kV. Reduced catalyst powders
are passivated with 0.5 vol% O2/He (100 mL/min) at RT for 12 h.
The passivated samples are first grinded to fine particles and then
ultrasonically dispersed in ethanol at RT for 1 h. The solution is
dipped onto carbon-coated copper grid (300 mesh) and evaporat-
ing the solvent before it is loaded into the chamber. More than
300 particles are counted for the distribution of particle size. The
mean diameter of particles is calculated with the following
formula:
(18 MX) is obtained by using a Milli-Q water system with Barnant
B-pure filter.
2.2. Catalyst preparation
10 wt% Cu/SiO2 catalyst is prepared by urea-assisted gelation.
Briefly, 2.03 g of Cu(NO3)2ꢀ3H2O, 3.5 mL of ammonia aqueous solu-
tion is dissolved in 96.5 mL of DI water in a round-bottom flask.
Subsequently, 3 g of urea is added before slowly dropping 12 g of
40 wt% Ludox AS-40 colloidal silica under stirring. The mixture is
vigorously stirred and aged at 80 °C in water bath for 4 h. The
obtained light blue precipitate is filtrated, washed three times with
deionized water, dried overnight at 120 °C in an oven. Copper-
indium bimetallic catalysts are prepared by incipient wetness
impregnation. For example, to prepare the sample with 0.5 wt%
of In, 5 g of oven-dried10 wt% Cu/SiO2 is mixed with 6.5 mL solu-
tion containing 0.0777 g indium nitrate. The slurry is dried at room
temperature for 48 h and then at 120 °C for 12 h. Finally, all sam-
ples are calcined at 400 °C for 3 h in air to obtain calcined catalysts.
1 wt% In/SiO2 is prepared with incipient wetness impregnation
to determine if metallic indium species consume N2O during N2O
titration process as urea assisted gelation does not work for
depositing indium species on colloidal silica. 40 wt% Ludox AS-40
colloidal silica is first dried to remove water, grinded to fine parti-
cles and then mixed with a certain amount of aqueous solution
containing indium nitrate. The slurry is dried at room temperature
for 48 h and then at 120 °C for 12 h. Finally, the sample is calcined
at 400 °C for 3 h in air.
P
ꢁ
d ¼
di
n
where di is the diameter of the i-th particle and n is the total num-
ber of particles.
X-ray photoelectron spectroscopy measurements are per-
formed using a Kratos AXIS Ultra DLD XPS system, with a
monochromatic Al K
a source, operated at 15 keV and 150 W and
2.3. Catalyst characterization
a hemispherical energy analyzer. The X-rays are incident at an
angle of 45° with respect to the surface normal and analysis is per-
formed at a pressure of 1x10-9 mbar. High resolution core level
spectra are measured with a pass energy of 40 eV and survey scans
with a pass energy of 160 eV. The in situ reduction of the catalysts
is performed in a reaction cell (Model
ES-009R01) directly attached to the XPS chamber, which allows
the sample to be treated under H2 flow conditions. The reaction
cell is evacuated and then the sample transferred back to the anal-
ysis chamber without exposure to the external atmosphere. Since
the SiO2 support is an insulator, a charge neutralizer is used to
compensate the substrate charge during XPS measurements. Cal-
cined catalysts are reduced at 350 °C in an in situ cell for 1.5 h.
XPS analysis is conducted on the same instrument after the sample
cooling down to RT. All spectra are corrected using 284.8 eV as a
reference for C1s binding energy (BE).
Nitrogen adsorption-desorption isotherms at ꢁ196 °C are mea-
sured with a Micromeritics ASAP 2020 surface area and pore ana-
lyzer. Samples are degassed at 120 °C for 1.5 h to remove
physically adsorbed impurities before each measurement. The
specific surface areas are calculated following the Brunauer-
Emmett-Teller (BET) method. Pore size distribution is calculated
by Barrett-Joyner-Halenda method (BJH) method using the adsorp-
tion isotherm branch.
The actual metallic contents of catalysts are analyzed by induc-
tively coupled plasma-optical emission spectroscopy (ICP-OES) on
a Perkin Elmer Avio 200 instrument. The surface area of copper on
catalysts is measured by pulsed N2O titration method at 60 °C. The
consumption of N2O is detected by TCD and the specific area of
metallic copper is calculated from the total amount of N2O con-
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
Cu/SiO2, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.001

X. Yu et al. / Journal of Catalysis xxx (xxxx) xxx
3
2.4. Catalytic tests
range showing a potential to decrease the operating pressure in
industry.
Catalytic evaluation of vapor-phase DMO hydrogenation is con-
ducted in a continuous flow mode using a stainless-steel tubular
reactor. In a typical run, 0.8 g of calcined catalyst (40–60 mesh)
is placed in the center of the reactor (9.4 mm internal diameter),
and the top and bottom side of the catalyst bed are packed with
adequate amount of quartz powder (20–40 and 40–60 meshes).
The calcined catalyst is activated in a flow of 20% H2-80% N2 stream
(75 mL/min) controlled by mass flow controllers at 350 °C and
ambient pressure for 4 h with a ramping rate of 2 °C/min. The sys-
tem pressure is increased and precisely controlled at 2.5 MPa with
a back-pressure regulator when the reactor is cooled to the target
reaction temperature (190 °C). A 10 wt% DMO solution (in metha-
nol) is pumped into the reactor using an ISCO pump (Teledyne
ISCO). The products are collected after the reaction reaches steady
Turnover frequency is calculated according to the DMO conver-
sion data (less than 20% at 190 °C) and the surface amount of Cu0
sites determined by N2O titration method. As shown in Table 1,
the TOF value has a volcano shape with respect to the increasing
amount of In content. The value is larger on bimetallic catalysts
when the In content is not higher than 1 wt%, reaching a maximum
(70.3 5.9 hꢁ1) at 0.25 wt% In doping.
The comparison of stability is performed on 10Cu0.5In/SiO2 and
10Cu/SiO2 catalyst (Fig. 1(f)) over a 100 h run test. While both cat-
alysts are stable, the activity of 10Cu0.5In/SiO2 and its selectivity to
EG are much higher than the monometallic Cu counterpart.
3.2. Material characterizations
state and analyzed on
equipped with a flame ionization detector and RTX-Wax capillary
m). The calculation of the con-
version of DMO and selectivity to various products (methyl glyco-
late and ethylene glycol) follows formula below:
a
gas chromatograph (Agilent 5890)
Various characterization methods have been conducted to
reveal why an appropriate amount of indium species can promote
the performance of Cu/SiO2 in the hydrogenation of DMO to EG.
The chemical compositions and textural properties of all catalysts
are summarized in Table 1. ICP-OES proves the effectiveness of
the preparation method which can deposit most of the copper
and indium species onto the silica support. The N2 physisorption
measurement shows that all the calcined samples exhibit type IV
isotherms with mesopore diameter of ~20 nm, as well as some
microporosity (Fig. 2). The BET surface areas of samples with low
In loading amount (0–1 wt%) are all close to each other. The excep-
tion is the material with 2 wt% loading of indium species which
shows a 23% decrease in the BET surface area compare with
monometallic Cu catalyst, possibly arising due to pore blockage
by indium oxide particles at such high loading. Indeed, the intro-
duction of indium species decreases the pore volume by about
10–15% when compared with the pore volume of monometallic
Cu catalyst.
H2-TPR was conducted to understand the reducibility of the cat-
alysts and to elucidate surface chemical information such as the
distribution of metal species and the interaction between them.
Fig. 3(A) shows the H2-TPR profiles of 10 wt% Cu/SiO2 with differ-
ent amounts of In. For 10 wt% Cu/SiO2, the peak centered at
192 °C is assigned to the collective reduction of Cu2+ species to
Cu0/Cu+. The reduction of bulk cupric oxide usually occurs at much
higher temperatures of around 243–255 °C [11,18]. Thus, the low
reduction temperature of 10 wt% Cu/SiO2 indicates a high disper-
sion of copper species on silica, which is further supported by
TEM measurements, and a weak interaction between certain
amount of copper species (e.g. CuO) with SiO2. Interestingly, the
reduction temperature gradually increases with the amount of
indium species as listed in the inset except for the sample with
0.25 wt% In. The gradual shift of reduction temperature suggests
the existence of an interaction between copper and indium species.
Thus, the incorporation of In hinders the reduction of 10 wt%
Cu/SiO2, which is similar to modification of copper catalysts with
other promoters [11,17,18,21]. The growing peak with respect to
increasing indium content at 270 °C is ascribed to the partial
reduction of indium species, which is further confirmed by XPS
(see below). As indium species are very difficult to be well dis-
persed on silica, the size distribution is broad with a certain
amount of large grains [22,23]. The reduction behavior of indium
species is highly dependent on the particle size, with small grains
able to be reduced below 300 °C and large grains being reduced
only at very high temperatures [23,24]. The reduction of indium
species could also be initiated by the neighboring copper species
when they are in close contact. We also cannot rule out that part
of the reduction profile of indium species with small particle size
may be overlapped with the reduction profile of copper species,
given the low loading of indium species in this study.
column (30 m ꢂ 0.25 mm ꢂ 0.25
l
nDMO;in ꢁ nDMO;out
XDMO
¼
ꢂ 100
nDMO;in
nMG
SMG
¼
ꢂ 100
nDMO;in ꢁ nDMO;out
nEG
SEG
¼
ꢂ 100
nDMO;in ꢁ nDMO;out
The selectivity to other products including ethanol, 1,2-
butanediol (1,2-BDO) and 1,2-propanediol (1,2-PDO) is negligible.
Turnover frequency (TOF, molDMO/(molCu on the surfaceꢂh) or hꢁ1
)
of the reaction is measured when the DMO conversion is lower
than 20% (T = 190 °C, P(H2) = 2.5 MPa, H2/DMO molar ratio = 95,
WHSV = 4.8 hꢁ1). The TOF value is calculated based on the number
of surface Cu0 atoms determined by N2O titration method.
3. Results and discussion
3.1. Catalytic performance
The hydrogenation performance DMO over silica supported
copper and copper-indium catalysts was investigated in a fixed
bed reactor. Fig. 1(a) shows the effect of weight percentage of In
on the performance at 190 °C. Further increasing the amount of
In above 0.5 wt% decreases both the conversion and EG selectivity,
which may result from covering active sites by excess amount of
indium species.
The effect of WHSV on the catalytic performance (10Cu/SiO2
and 10Cu0.5In/SiO2) was investigated to measure the production
capacity on different catalysts. Fig. 1(b and c) shows that the per-
formance in terms of conversion and selectivity to EG decreases
on both catalysts with the increasing WHSV due to the shortening
of contact time of the reactant with catalysts [19]. However, the
performance of 10Cu0.5In/SiO2 is superior to that obtained with
10Cu/SiO2 catalyst under different WHSV, confirming the advan-
tage of bimetallic catalyst in this case. The results show that incor-
poration of an appropriate amount of In can improve the
performance of 10 wt% Cu/SiO2.
The effect of H2 pressure on the performance of catalysts was
also examined. As shown in Fig. 1(d and e), the selectivity to EG
increases with elevated H2 pressure, possibly arising from the
increased concentration of surface absorbed H2 species [20]. How-
ever, the performance of hydrogenation of DMO on 10Cu0.5In/SiO2
is much less influenced than that on 10Cu/SiO2 in the low-pressure
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
Cu/SiO2, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.001

4
X. Yu et al. / Journal of Catalysis xxx (xxxx) xxx
c
100
80
60
40
20
0
Conversion
100
80
60
40
20
0
Conversion
S(EG)
100
b
a
S(EG)
S(MG)
S(MG)
80
60
40
20
0
Conversion(190 oC)
S(EG,190 oC)
S(MG,190 oC)
0.5
0.75
1.0
2.0
3.0
0.0 0.2 0.4 0.6 0.8 1.0
Weight percentage of In/%
0.5
0.75 1.0
WHSV/h-1
2.0
3.0
WHSV/h-1
100
80
60
40
20
0
100
80
60
40
20
0
d
100
80
60
40
20
e
f
10Cu0.5In/SiO2 10Cu/SiO2
Conversion
S(EG)
Conversion
S(EG)
S(MG)
S(MG)
1.0
1.5
2.0
2.5
3.0
1.0
1.5
2.0
2.5
3.0
0
10 20 30 40 50 60 70 80 90 100
Time/h
Pressure/MPa
Conversion
Pressure/MPa
Conversion
S(EG)
S(MG)
S(EG)
S(MG)
Fig. 1. (a) Effect of In weight percentage on the performance of hydrogenation of DMO to EG on different catalysts, reaction condition: T = 190 °C, P(H2) = 2.5 MPa, H2/DMO
molar ratio = 95, WHSV = 0.5 hꢁ1. (b, c) Effect of WHSV on the DMO hydrogenation performance for catalysts (b) 10Cu/SiO2 and (c) 10Cu0.5In/SiO2, reaction condition.
T = 190 °C, P(H2) = 2.5 MPa, H2/DMO molar ratio = 95. (d, e) effect of H2 pressure on the DMO hydrogenation performance for catalysts (d) 10Cu/SiO2 and (e) 10Cu0.5In/SiO2,
reaction condition. T = 190 °C, WHSV = 0.5 hꢁ1, H2/DMO molar ratio = 95. f. Stability of catalysts in DMO hydrogenation, reaction condition. T = 190 °C, P(H2) = 2.5 MPa,
H2/DMO molar ratio = 95, WHSV = 1 hꢁ1
.
Table 1
Physicochemical and catalytic properties of Cu/SiO2 catalyst with different In loading amount.
Sample
Cu content,a (wt %)
In content,a (wt %)
SBET
,
b/m2 gꢁ1
Vp,c/cm3 gꢁ1
SCu,d/m2 gꢁ1
dp,e/nm
dp,f/nm
TOF,g/hꢁ1
10Cu/SiO2
11.4
10.6
10.2
10.5
9.9
0
256
242
256
244
198
0.81
0.74
0.71
0.73
0.70
3.9
4.5
4.7
4.3
4.8
4.6
4.2
4.4
4.2
4.5
2.9
3.3
3.4
3.4
3.3
38.8 4.6
70.3 5.9
64.2 2.4
54.3 2.3
14.6 0.3
10Cu0.25In/SiO2
10Cu0.5In/SiO2
10Cu1In/SiO2
10Cu2In/SiO2
0.30
0.53
1.04
1.93
a
b
c
d
e
f
Determined by ICP-OES analysis.
BET specific surface area.
Pore volume that obtained from P/P0 = 0.99.
Cu metallic surface area per gram of catalyst determined by N2O titration method.
Average particle size determined by TEM images.
Average particle size determined by Cu(1 1 1) peak from in situ XRD patterns at 190 °C.
g
TOF value calculated according to surface number of Cu0 atoms from N2O titration method.
XRD patterns of the calcined samples and the in situ reduced
stretching vibration band (m
SiO) of Si–O at 1045 cmꢁ1 in the FTIR
catalysts were collected to understand the existing phase and crys-
tallite sizes within the catalysts. The XRD patterns of calcined sam-
ples (Figure S2(A)) do not exhibit noticeable peaks belonging to
metal oxides implying their high dispersion and amorphous char-
acter. After in situ reduction at 350 °C, the peaks (Fig. 3(B)) at 43.5°
and 74.4° can be ascribed to Cu0(1 1 1) and Cu0(2 2 0) [4,18,25].
The average particle size of Cu crystallite is calculated in terms of
Scherrer equation and listed in Table 1. The crystallite sizes for
all the samples are around 3 nm, which shows that the metallic
species are well-dispersed after reduction.
spectra prove the existence of cupric phyllosilicate (Cu2Si2O5(OH)2,
Figure S1), suggesting the existence of Cu+ during the reduction
[11,26]. The relative content of cupric phyllosilicate in each sample
can be estimated using the intensity ratio of the deformation band
(dOH) of cupric phyllosilicate at 670 cmꢁ1 and the symmetric
stretching vibration band (m
SiO) of silica at 800 cmꢁ1, denoted as
I670/I800 [27]. However, the I670/I800 ratio only gives a qualitative
estimation of the content of cupric phyllosilicate as the extinction
coefficients of the corresponding IR bands are not known. Never-
theless, cupric phyllosilicate exists in all the catalyst samples.
TEM images (Fig. 3(D), Figure S4) of reduced catalysts were
obtained to reveal the distribution of particle sizes in the catalysts.
The histograms of particle size (inset) exhibit a very narrow
distribution with the mean particle diameter about 4 nm for all
ATR-FTIR was utilized to reveal structural information for the
calcined samples shown in Fig. 3(C). The band at 1118 cmꢁ1 is
ascribed to asymmetric stretching vibration band (
mSiO) of SiO2.
The deformation band (dOH) of OH at 670 cmꢁ1 and asymmetric
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
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X. Yu et al. / Journal of Catalysis xxx (xxxx) xxx
5
(A)
(B)
0.08
70
60
50
40
30
20
10
0.06
0.04
0.02
0.00
e
e
d
c
b
d
c
b
a
a
0.0
0.2
0.4
0.6
0.8
1.0
0
20 40 60 80 100 120 140 160
P/P
Pore Diameter/nm
0
Fig. 2. (A) N2 adsorption-desorption isotherms and (B) BJH pore size distribution of the calcined catalysts of different copper loadings. a. 10Cu/SiO2, b. 10Cu0.25In/SiO2, c.
10Cu0.5In/SiO2, d. 10Cu1In/SiO2, e. 10Cu2In/SiO2.
Fig. 3. (A) H2-TPR profiles, a. 10Cu/SiO2, b.10Cu0.25In/SiO2, c. 10Cu0.5In/SiO2, d. 10Cu1In/SiO2, e. 10Cu2In/SiO2. (B) in situ XRD patterns of 350 °C reduced sample at 190 °C, a.
blank XRD reactor, b. 10Cu/SiO2, c. 10Cu0.25In/SiO2, d. 10Cu0. 5In/SiO2, e. 10Cu1In/SiO2, f. 10Cu2In/SiO2. (C) FT-IR spectra of calcined samples, a. 10Cu/SiO2, b.10Cu0.25In/
SiO2, c. 10Cu0.5In/SiO2, d. 10Cu1In/SiO2, e. 10Cu2In/SiO2, f. SiO2. (D) TEM images of reduced 10Cu0.5In/SiO2 catalysts.
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
Cu/SiO2, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.001

6
X. Yu et al. / Journal of Catalysis xxx (xxxx) xxx
the samples. Thus, copper species are highly dispersed on silica,
which is consistent with the results of H2-TPR and in situ XRD
measurements.
is reasonable since the Pauling electronegativity of copper (1.90) is
higher than that of indium (1.78). Thus, an interaction between a
certain amount of copper and reduced indium species, possibly
by alloying, may exist in the bimetallic samples.
Surface properties of reduced catalysts were investigated by
XPS and with X-ray induced Auger electron spectroscopy (XAES).
Figure S4 shows the Cu 2p XPS spectra of calcined samples. The
peak at around 936 eV is assigned to Cu2+ which is confirmed by
the characteristic satellite peak of Cu2+ species at 945 eV. No peaks
belonging to Cu0 or Cu+ are observed suggesting that copper exists
as Cu2+ species in the calcined sample. After reduction, only two
peaks at 932.1 and 951.9 eV are observed (Fig. 4(A)), attributed
to Cu 2p3/2 and Cu 2p1/2, respectively [9,11]. Furthermore, the
satellite peak and the peak with Cu 2p binding energy of 936 eV
assigned to Cu2+ species are absent. These results show that Cu2+
species on the surface are reduced to Cu0 and/or Cu+ species which
is consistent with H2-TPR result. The binding energy for Cu 2p3/2 in
10Cu0.5In/SiO2 is lower (0.4 eV) than that in 10Cu/SiO2 indicating
an electron transfer from reduced In species to copper species. This
As the difference in Cu 2p3/2 and Cu 2p1/2 binding energies
between Cu+ and Cu0 are negligible, Cu LMM XAES was recorded
to discriminate these peaks [9–11]. Two peaks with Auger kinetic
energy of 914 eV and 917 eV are observed for the reduced catalysts
(Fig. 4(B)), confirming the simultaneous existence of Cu0 and Cu+
species on these catalysts [11–14,28]. Deconvolution results
(Table S1) show that the near-surface content of Cu+ species for
all samples with various In loading amounts is almost the same.
The high content of Cu+ species further confirms the assumption
of well-dispersed copper species on silica and the existence of
interaction between copper species and SiO2 as indicated by H2-
TPR and TEM.
Fig. 4(C) shows In 3d XPS spectra of calcined and reduced
10Cu0.5In/SiO2. The peaks at approx. 446 eV and 453.6 eV are
Cu 2p3/2
Cu0
Cu+
(B) e
(A)
e Cu 2p
1/2
d
c
b
a
d
c
b
a
960 955 950 945 940 935 930 925
920 918 916 914 912 910 908 906
Binding Energy/eV
Kinetic energy/eV
In3+ 3d3/2 In3+ 3d5/2
a
(C)
b
In3+ 3d3/2 In3+ 3d5/2
In 3d5/2
In 3d3/2
In 3d3/2
In 3d5/2
i
i
ii
ii
456
452
448
444
456
452
448
444
Binding energy/eV
Binding energy/eV
In3+ 3d5/2
c
In3+ 3d5/2
d
In3+ 3d3/2
In3+ 3d3/2
In 3d5/2
In 3d5/2
In 3d3/2
In 3d3/2
i
ii
i
ii
456
452
448
444
456
452
448
444
Binding energy/eV
Binding energy/eV
Fig. 4. (A) Cu 2p XPS spectra and (B) Cu LMM XAES spectra of reduced catalysts, a. 10Cu/SiO2, b. 10Cu0.25In /SiO2, c. 10Cu0.5In/SiO2, d. 10Cu1In /SiO2, e. 10Cu2In/SiO2, (C) In
3d XPS spectra of calcined and reduced bimetallic samples, a. 10Cu0.25In/SiO2, b. 10Cu0. 5In/SiO2, c. 10Cu1In /SiO2, d. 10Cu2In/SiO2, i. calcined samples, ii. reduced samples.
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
Cu/SiO2, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.001

X. Yu et al. / Journal of Catalysis xxx (xxxx) xxx
7
assigned to In 3d5/2 and In 3d3/2, indicating that the indium spe-
cies in the calcined sample are in oxide form [23,29]. However, two
new peaks located at 444.1 eV and 451.7 eV attributed to metallic
indium species emerge after reduction [30–33]. The result shows
that indium species are partially reduced to metallic form after
reduction consistent with the analysis in H2-TPR test.
3.3. Discussion
High metal dispersion is generally acknowledged to enhance
the hydrogenation activity of Cu/SiO2 catalysts by promoting the
formation of Cu+ species for the cooperation between Cu0 and
Cu+ sites [34,35]. It is proposed that Cu0 species adsorb and disso-
ciate hydrogen molecules, while the Cu+ species stabilize the meth-
oxy and acyl species from the activated ester molecules [11,15,18].
An excellent hydrogenation performance could be achieved via the
synergetic effect of balanced Cu0 and Cu+ sites on the surface,
which can explain the superior performance of 10Cu/SiO2 in our
case. The amount of Cu+ sites has been proposed previously to be
directly related to the selectivity to EG and the effect of promoters
is usually ascribed to the increase in the content of Cu+ species
[11,15,18]. However, XPS results show that the near-surface
amount of Cu+ for the catalysts in our case (at least prior to reac-
tion) is roughly the same. Furthermore, N2O titration results reveal
that doping indium species (0.25–2 wt%) can slightly increase the
surface amount of Cu0 sites which might be another evidence for
the existence of the interaction between copper species and
indium species. However, the performance of bimetallic samples
with low loading of indium species (10Cu0.25In/SiO2 and
10Cu0.5In/SiO2) are significantly improved over that of 10Cu/
SiO2. We propose that the increased performance of these In pro-
moted samples might be attributed to the enhanced ability of
adsorption and activation of H2 on bimetallic surface sites.
Fig. 5. Schematic illustration of promotional effect of indium species on 10 wt% Cu/
SiO2 in the hydrogenation of DMO to EG.
ing a balance between the amount of In0 species and In2O3 species
can improve the intrinsic activity of 10Cu/SiO2.
The detailed reaction pathways in the hydrogenation of DMO on
Cu/SiO2 catalysts are given in Fig. S5. Fig. 5 depicts the role of
indium species in the hydrogenation of DMO to EG. The reduction
of cupric phyllosilicate (Cu2Si2O5(OH)2) and copper oxide (CuO)
can yield Cu+ and Cu0 sites, respectively. The reduction of indium
species can be initiated by In2O3 particles with small crystallite
size and the intimate contact with neighboring Cu0 species. The
reduced indium species interacts with the Cu0, possibly by alloy-
ing, to form a new type of metallic sites. These sites enhance the
adsorption and activation of H2, resulting in the efficient refilling
of activated H2 species consumed in the reaction and thus enhanc-
ing the number of turnover.
Indeed, the estimated TOF value of bimetallic catalysts
(10Cu0.2In/SiO2 and 10Cu0.5In/SiO2) based on Cu0 content, are
much higher than the TOF of 10Cu/SiO2 which indicates the nature
of the active sites are significantly different. Thus, a new active
phase attributed to the modification of Cu0 crystallites with In0
species is created on the surface. Wang et al. find that when the
accessible metallic Cu surface area is below a certain value
(approximately 20 m2/g in their case), the catalytic activity of
hydrogenation of DMO on silica supported copper catalysts lin-
early increases with increasing Cu0 surface area. This was attribu-
ted to insufficient H2 decomposition on Cu0 limiting the reaction
rates [36]. Considering that in the present case, the majority of cop-
per species are Cu+ and that the surface area of Cu0 for all the sam-
ples is very low, it is likely that the incorporation of In0 with Cu0
clusters forms bimetallic sites that promote dissociation of H2.
The TOF value of 10Cu2In/SiO2 is much smaller than the TOF value
of 10Cu/SiO2 even though they have similar Cu+ content and the
bimetallic catalyst has ~22% higher amount of Cu0 sites. It is gener-
ally accepted that the dispersion of supported metal oxides
decreases with increased metal oxide loading. The absolute
amount of unreduced In2O3 species tends to increase with the
incremental amount of indium species on the catalysts. Indeed,
indium species are difficult to disperse on silica [22,23] and only
small In2O3 particles and/or In2O3 particles close to Cu0 sites can
be reduced. Moreover, the generation of an excess amount of In0
species on bimetallic catalysts with high loading of indium might
also be a factor for deteriorating catalytic activity. This significant
decrease in TOF from overpromotion of In is likely a result of dis-
ruption of the balance between Cu0 and Cu+ sites on the surface
by In2O3 species and/or excess In0 species (e.g. covering sites with
high activity and separating metallic sites from Cu+ sites). Thus,
incorporating only an appropriate amount of indium species offer-
4. Conclusions
Silica supported Cu-In bimetallic catalysts are successfully pre-
pared via urea-assisted gelation followed by incipient wetness
impregnation. The incorporation of a low amount (less than 1 wt
% in this study) of cheap, nontoxic indium species dramatically
enhances the intrinsic activity of Cu/SiO2 for the conversion of
DMO to EG. Two types of indium species, In0 and In3+, are observed
after reduction via XPS. An interaction among copper species and
indium species are confirmed by H2-TPR, N2O titration and XPS.
However, the surface content of Cu+ which is related to the activa-
tion of ester group is not significantly affected by indium species.
Instead, In0 species might be alloyed with Cu0 species to form a
new type of metallic sites with enhanced ability to dissociate H2
which might account for the observed increase in the intrinsic
activity of bimetallic catalysts.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
The project is sponsored by National Science Foundation (Uni-
ted States, CBET-1510157). In addition, NGB thanks support from
the NSF (United States)-IGERT program at the University of South
Carolina (DGE-1250052).
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
Cu/SiO2, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.001

8
X. Yu et al. / Journal of Catalysis xxx (xxxx) xxx
dimethyl oxalate to ethylene glycol through zinc incorporation, Catal. Comm.
Appendix A. Supplementary material
108 (2018) 68–72.
[18] Z. He, H.Q. Lin, P. He, Y.Z. Yuan, Effect of boric oxide doping on the stability and
activity of a Cu-SiO2 catalyst for vapor-phase hydrogenation of dimethyl
oxalate to ethylene glycol, J. Catal. 277 (2011) 54–63.
[19] J.X. Chen, Y. Yang, H. Shi, M.F. Li, Y. Chu, Z.Y. Pan, X.B. Yu, Regulating product
distribution in deoxygenation of methyl laurate on silica-supported Ni–Mo
phosphides: effect of Ni/Mo ratio, Fuel 129 (2014) 1–10.
[20] X.B. Yu, J.X. Chen, T.Y. Ren, Promotional effect of Fe on performance of Ni/SiO2
for deoxygenation of methyl laurate as a model compound to hydrocarbons,
RSC Adv. 87 (2014) 46427–46436.
[21] S. Zhao, H.R. Yue, Y.J. Zhao, B. Wang, Y.C. Geng, J. Lv, S.P. Wang, J.L. Gong, X.B.
Ma, Chemoselective synthesis of ethanol via hydrogenation of dimethyl
oxalate on Cu/SiO2 enhanced stability with boron dopant, J. Catal. 297 (2013)
142–150.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.10.001.
References
[1] H.R. Yue, Y.J. Zhao, X.B. Ma, J.L. Gong, Ethylene glycol properties, synthesis and
applications, Chem. Soc. Rev. 41 (2012) 4218–4244.
[2] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, High activity and selectivity of Ag/SiO2
catalyst for hydrogenation of dimethyl oxalate, Chem. Comm. 46 (2010) 4348–
4350.
[22] A. Gervasini, J. Perdigon-Melon, C. Guimon, A. Auroux, An in-depth study of
supported In2O3 catalysts for the selective catalytic reduction of NOxthe
influence of the oxide support, J. Phys. Chem. B 110 (2006) 240–249.
[23] M. Chen, J. Xu, Y. Cao, H.Y. He, K.N. Fan, J.H. Zhuang, Dehydrogenation of
propane over In2O3–Al2O3 mixed oxide in the presence of carbon dioxide, J.
Catal. 272 (2010) 101–108.
[24] P.W. Park, C.S. Ragle, C.L. Boyer, M.L. Balmer, M. Engelhard, D. McCready,
In2O3/Al2O3 catalysts for NOx reduction in lean condition, J. Catal. 210 (2002)
97–105.
[3] J.L. Gong, H.R. Yue, Y.J. Zhao, S. Zhao, L. Zhao, J. Lv, S.P. Wang, X.B. Ma, Synthesis
of ethanol via syngas on Cu/SiO2 catalysts with balanced Cu0–Cu+ sites, J. Am.
Chem. Soc. 134 (2012) 13922–13925.
[4] L.F. Chen, P.J. Guo, M.H. Qiao, S.R. Yan, H.X. Li, W. Shen, H.L. Xu, K.N. Fan, Cu/
SiO2 catalysts prepared by the ammonia-evaporation method: texture,
structure, and catalytic performance in hydrogenation of dimethyl oxalate to
ethylene glycol, J. Catal. 257 (2008) 172–180.
[5] L.R. Zehner, R.W. Lenton, Process for the preparation of ethylene glycol, US
patent 4112245, 1978.
[25] J. Ding, T. Popa, J.K. Tang, K.A.M. Gasem, M.H. Fan, Q. Zhong, Highly selective
and stable Cu/SiO2 catalysts prepared with a green method for hydrogenation
of diethyl oxalate into ethylene glycol, Appl. Catal. B Environ. 209 (2017) 530–
542.
[26] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, The nature of active copper species in Cu-
HMS catalyst for hydrogenation of dimethyl oxalate to ethylene glycol: New
insights on the synergetic effect between Cu0 and Cu+, J. Phys. Chem. C 113
(2009) 11003–11013.
[27] T. Toupance, M. Kermarec, J.F. Lambert, C. Louis, Conditions of formation of
copper phyllosilicates in silica-supported copper catalysts prepared by
selective adsorption, J. Phys. Chem. B 106 (2002) 2277–2286.
[28] H.H. Yang, Y.Y. Chen, X.J. Cui, G.F. Wang, Y.L. Cen, T.S. Deng, W.J. Yan, J. Gao, S.
H. Zhu, U. Olsbye, J.G. Wang, W.B. Fan, A highly stable copper-based catalyst
for clarifying the catalytic roles of Cu0 and Cu+ species in methanol
dehydrogenation, Angew. Chem. Int. Ed. 57 (2018) 1836–1840.
[29] S. Rasul, D.H. Anjum, A. Jedidi, Y. Minenkov, L. Cavallo, K. Takanabe, A highly
selective copper–indium bimetallic electrocatalyst for the electrochemical
reduction of aqueous CO2 to CO, Angew. Chem. Int. Ed. 54 (2015) 2146–2150.
[30] P. Sen, M.S. Hegde, C.N.R. Rao, Surface oxidation of cadmium, indium, tin and
antimony by photoelectron and Auger spectroscopy, Appl. Surf. Sci. 10 (1982)
63–74.
[6] M. Haruhiko, H. Kouichi, U. Taizou, N. Yasuo, I. Seizou, T. Takanori, Preparation
of ethylene glycol, JP Patent 57123127, 1982.
[7] J.W. Zheng, J.F. Zhou, H.Q. Lin, X.P. Duan, C.T. Williams, Y.Z. Yuan, CO-mediated
deactivation mechanism of SiO2-supported copper catalysts during dimethyl
oxalate hydrogenation to ethylene glycol, J. Phys. Chem. C. 119 (2015) 13758–
13766.
[8] X.Y. Guo, A.Y. Yin, W.L. Dai, K.N. Fan, One pot synthesis of ultra-high copper
contented Cu/SBA-15 material as excellent catalyst in the hydrogenation of
dimethyl oxalate to ethylene glycol, Catal. Lett. 132 (2009) 22–27.
[9] X.B. Ma, H.W. Chi, H.R. Yue, Y.J. Zhao, Y. Xu, J. Lv, S.P. Wang, J.L. Gong,
Hydrogenation of dimethyl oxalate to ethylene glycol over mesoporous Cu-
MCM-41 catalysts, AlChE J. 59 (2013) 2530–2539.
[10] A.Y. Yin, J.W. Qu, X.Y. Guo, W.L. Dai, K.N. Fan, The influence of B-doping on the
catalytic performance of Cu/HMS catalyst for the hydrogenation of
dimethyloxalate, Appl. Catal. A Gen. 400 (2011) 39–47.
[11] X.L. Zheng, H.Q. Lin, J.W. Zheng, X.P. Duan, Y.Z. Yuan, Lanthanum oxide-
modified Cu/SiO2 as
a high-performance catalyst for chemoselective
hydrogenation of dimethyl oxalate to ethylene glycol, ACS Catal. 3 (2013)
2738–2749.
[12] B.W. Wang, X. Zhang, Q. Xu, G.H. Xu, Preparation and characterization of Cu/
SiO2 catalyst and its catalytic activity for hydrogenation of diethyl oxalate to
ethylene glycol, Chin. J. Catal. 29 (2008) 275–280.
[31] R.I. Hegde, Core level binding energy shifts in dilute tin alloys, Surf. Interface
Anal. 4 (1982) 204–207.
[13] H.R. Yue, Y.J. Zhao, L. Zhao, J. Lv, S.P. Wang, J.L. Gong, X.B. Ma, Hydrogenation of
[32] A.W.C. Lin, N.R. Armstrong, T. Kuwana, X-ray photoelectron/Auger electron
spectroscopic studies of tin and indium metal foils and oxides, Anal. Chem. 49
(1977) 1228–1235.
dimethyl oxalate to ethylene glycol on
catalyst: enhanced internal mass transfer and stability, AlChE J. 58 (2012)
2798–2809.
a Cu/SiO2/cordierite monolithic
[33] A.C. Parry-Jones, P. Weightman, P.T. Andrews, The M4,5N4,5N4,5 auger spectra of
Ag, Cd, In and Sn, J. Phys. C. 12 (1979) 1587–1600.
[34] Y.Y. Zhu, S.P. Wang, L.J. Zhu, X.L. Ge, X.B. Li, Z.Y. Luo, The influence of copper
particle dispersion in Cu/SiO2 catalysts on the hydrogenation synthesis of
ethylene glycol, Catal. Lett. 135 (2010) 275–281.
[35] S. Lambert, C. Cellier, P. Grange, J.P. Pirard, B. Heinrichs, Synthesis of Pd/SiO2,
Ag/SiO2, and Cu/SiO2 cogelled xerogel catalysts: study of metal dispersion and
catalytic activity, J. Catal. 221 (2004) 335–346.
[36] Y. Wang, Y.L. Shen, Y.J. Zhao, J. Lv, S.P. Wang, X.B. Ma, Insight into the balancing
effect of active Cu species for hydrogenation of carbonꢁoxygen bonds, ACS
Catal. 3 (2015) 6200–6208.
[14] J.W. Zheng, H.Q. Lin, Y.N. Wang, X.L. Zheng, X.P. Duan, Y.Z. Yuan, Efficient low-
temperature selective hydrogenation of esters on bimetallic Au-Ag/SBA-15
catalyst, J. Catal. 297 (2013) 110–118.
[15] Y. Huang, H. Ariga, X.L. Zheng, X.P. Duan, S. Takakusagi, K. Asakura, Y.Z. Yuan,
Silver-modulated SiO2-supported copper catalysts for selective hydrogenation
of dimethyl oxalate to ethylene glycol, J. Catal. 307 (2013) 74–83.
[16] A.Y. Yin, C. Wen, X.Y. Guo, W.L. Dai, K.N. Fan, Influence of Ni species on the
structural evolution of Cu/SiO2 catalyst for the chemoselective hydrogenation
of dimethyl oxalate, J. Catal. 280 (2011) 77–88.
[17] W. Qi, L. Qiu, D. Ding, Y.Z. Chen, C.W. Shi, P. Cui, Y. Wang, Q.H. Zhang, R. Liu, H.
Shen, Performance enhancement of Cu/SiO2 catalyst for hydrogenation of
Please cite this article as: X. Yu, T. A. Vest, N. Gleason-Boure et al., Enhanced hydrogenation of dimethyl oxalate to ethylene glycol over indium promoted
Cu/SiO2, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.001