Synthesis of a high surface area and highly dispersed Cu-O-Si complex oxide used for the low-temperature hydrogenation of dimethyl oxalate to ethylene glycol

Article abstract of DOI:10.1016/j.catcom.2021.106310

A highly dispersed Cu-O-Si composite oxide framework catalyst with high surface area (619 m²/g) and very small particle size (~3 nm) has been synthesized using an improved precipitation method, which showed high catalytic activity toward the hydrogenation of dimethyl oxalate (DMO) (99.9% conversion of DMO and 96.7% selectivity for ethylene glycol) even at low temperature (175 °C). In addition, its chemical stability was significantly enhanced due to the formation of a Cu-O-Si composite structure and the low reaction temperature, and the catalyst did not show any significant inactivation during the stability test over 140 h.

Full text of DOI:10.1016/j.catcom.2021.106310

Catalysis Communications 154 (2021) 106310
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Synthesis of a high surface area and highly dispersed Cu-O-Si complex
oxide used for the low-temperature hydrogenation of dimethyl oxalate to
ethylene glycol
a
,
*
,
1
a,
1
a
a
a
a,*
Yu Zhao
, Xian Kan , Hongfei Yun , Dongliang Wang , Ning Li , Guixian Li
,
b
Jianyi Shen
a
School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu, China
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu, China
b
A R T I C L E I N F O
A B S T R A C T
2
Keywords:
A highly dispersed Cu-O-Si composite oxide framework catalyst with high surface area (619 m /g) and very small
particle size (~3 nm) has been synthesized using an improved precipitation method, which showed high catalytic
activity toward the hydrogenation of dimethyl oxalate (DMO) (99.9% conversion of DMO and 96.7% selectivity
Cu-O-Si complex oxide
Atomic dispersion
Hydrogenation
◦
for ethylene glycol) even at low temperature (175 C). In addition, its chemical stability was significantly
Ethylene glycol
enhanced due to the formation of a Cu-O-Si composite structure and the low reaction temperature, and the
catalyst did not show any significant inactivation during the stability test over 140 h.
Low-temperature reaction
1
. Introduction
Ethylene glycol (EG) is an important chemical product that has
forming a metal solid solution or strengthen the interactions formed
between the second metal species and Cu [14–16]. In addition, the
synergistic effect of the second metal can also improve the catalytic
activity and affect the product distribution. However, the incorporation
of a second component makes the catalytic system complicated and
increases the difficulty in understanding the catalytic reaction. More-
over, the technical control points in the catalyst preparation process are
more complicated. Therefore, it is more advantageous to synthesize
versatile applications during the manufacture of polyester, lubricant and
anti-freeze [1]. At present, the industrial production of EG is conducted
via the hydration of ethylene oxide produced from petroleum-derived
ethylene. With the depletion of petroleum resources and increasing
demand for EG, the search for a new EG production method is increas-
ingly imperative. Recently, the production of EG from coal has attracted
more and more attention, in which the hydrogenation of dimethyl ox-
alate (DMO) to EG is the crucial step [2,3]. A considerable number of
single metal Cu/SiO
2
catalysts with high stability. A possible method to
catalysts is to
improve the chemical stability of single metal Cu/SiO
2
embed the copper species into the framework of the silica support to
form an atomically dispersed Cu-O-Si composite oxide framework. The
strong interactions formed between copper, silicon and oxygen can limit
the movement of copper, thus effectively improving the chemical sta-
bility of the copper species.
2
studies on catalysts have shown that the Cu/SiO catalyst is the most
promising candidate from the perspective of its environmental friend-
liness and high selectivity toward EG [4]. However, the poor stability of
copper is an important factor affecting the wide use of Cu/SiO
2
catalysts
[
5]. As the Hüttig and Tamman temperatures of copper are relatively
Herein, a series of highly dispersed Cu-O-Si composite oxide frame-
work materials with high surface areas were successfully synthesized
using a simple improved precipitation method. Based on the high cat-
alytic activity as expected, a reduced reaction temperature (even at
low, it easily moves under the reaction conditions and small granules
may undergo aggregation to form large particles, resulting in a decrease
in the catalytic activity [6]. Numerous methods have been developed to
enhance the stability of copper-based catalysts [7–13]. Among them, the
most common method is to dope a second component into the catalyst to
stabilize the copper species and extend the lifespan of the catalyst by
◦
175 C) would be used for the hydrogenation of DMO. Thus, the
chemical stability of the Cu-O-Si catalyst would be significantly
enhanced due to the formation of the Cu-O-Si skeleton structure and the
*
Corresponding authors at: School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu, China.
E-mail addresses: yzhao@lut.edu.cn (Y. Zhao), lgxwyf@163.com (G. Li).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.catcom.2021.106310
Received 30 January 2021; Received in revised form 9 March 2021; Accepted 24 March 2021
Available online 26 March 2021
1
566-7367/© 2021 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(
http://creativecommons.org/licenses/by-nc-nd/4.0/).

Y. Zhao et al.
Catalysis Communications 154 (2021) 106310
Table 1
Physicochemical properties of the calcined catalyst samples.
Samples
Cu loading
Cu dispersion
S
BET
d
p
V
p
a
b
(m ⋅ gꢀ 1
2
c
c
(cm ⋅ gꢀ 1
)
3
cy
(
wt%)
(%)
)
(nm)
CuSi
CuSi
CuSi
CuSi
1
2
3
4
8.75
/
354.7
385.0
619.5
564.1
272.9
13.0
14.7
10.3
9.2
1.49
1.83
2.37
1.96
0.32
16.73
24.09
29.18
23.92
/
40.95
/
IM-CuSi
9.72
3.8
a
Cu loading determined by ICP-OES.
b
c
2
Cu dispersion determined by N O titration.
Obtained by N adsorption-desorption.
2
low reaction temperature. To the best of our knowledge, the strategy of
enhancing the catalytic stability of the Cu/SiO catalyst via reducing the
reaction temperature has been rarely reported [17].
2
2
. Experimental
2
.1. Catalyst preparation
Cu-O-Si composite oxides with a preset copper-silicon atomic ratio
varying from 0.1 to 0.4 were prepared using a precipitation method.
Typically, 400 mL of deionized water was added to a three-necked flask
◦
and heated to 80 C in an oil bath. An appropriate amount of Cu
(
NO
3
)
2
⋅3H
2
O was dissolved in 100 mL of 2 mol/L nitric acid solution and
O was dissolved in 150 mL of deionized water. The
2
8.42 g NaSiO
3
⋅9H
2
two solutions were added dropwise at the same time to the three-necked
flask with stirring. The stirring was stopped after 1 h and the mixture
◦
was maintained at 80 C for 12 h for crystal maturation. The precipitate
was filtered and washed. It was then added to a beaker containing 200
◦
mL of n-butanol and the mixture was heated to 110 C under stirring to
evaporate the solvent. After the evaporation process was completed, the
◦
◦
precursor was dried at 110 C overnight and then calcined at 450 C in
air for 4 h. Finally, the resulting solid was crushed and sieved to 40–60
1 2 3 4
mesh. The catalysts were denoted as CuSi , CuSi , CuSi and CuSi ,
x
Fig. 1. XRD patterns obtained for the CuSi and IM-CuSi samples (a) calcinated
where one to four indicated that the copper‑silicon atomic ratio of the
samples was 0.1 to 0.4, respectively.
◦
◦
at 450 C and (b) reduced at 350 C.
To compare the differences in the preparation method used, a
contrast sample was prepared via an impregnation method and the
sample obtained was labeled as IM-CuSi.
exhibited the largest specific surface area and pore volume among all the
2
3
samples studied, which were 619 m /g and 2.37 cm /g, respectively,
even if the Cu content was 24 wt% (see ICP results in Table 1). To the
best of our knowledge, this is the largest specific surface area reported
for single Cu-Si-O catalysts prepared using ammonia evaporation,
precipitation-deposition, sol-gel, ion-exchange and impregnation
2
.2. Characterization
The obtained samples were characterized using N
2
adsorption-
methods. In contrast to CuSi
sample with the same Cu content was only 273 m /g. The contrast be-
tween CuSi and IM-CuSi clearly showed the superiority of our synthesis
method. A large specific surface area and pore volume are pivotal bases
for the good dispersion of Cu species on the catalyst [6]. Thus, CuSi
with the maximum specific surface area among all the CuSi samples was
3
, the specific surface area of the IM-CuSi
2
2 2
desorption, XRD, TEM, H -TPR, XPS and XAES, N O chemisorptions,
ICP-OES, etc. The detailed characterization process can be found in the
supplementary information.
3
3
2
.3. Catalytic activity
x
selected as the catalyst for catalytic activity test described below.
The hydrogenation of DMO was performed in a continuous-flow
fixed-bed reactor and the condensed products were analyzed on an
Agilent 3500GC instrument equipped with a flame-ionization detector,
which can be seen in detail in the supplementary information.
3.2. XRD analysis
The XRD patterns obtained for the calcined and reduced CuSi and
x
IM-CuSi samples are shown in Fig. 1(a) and (b). For the calcined CuSi
x
3
. Results and discussion
samples, with the exception of the distinct diffraction peak observed at
◦
2
2
θ = 22 , which was ascribed to amorphous SiO , no diffraction peaks
3
.1. N adsorption-desorption
2
related to the copper species were detected, indicating that the copper
species were well dispersed in the support. The high specific surface area
of the catalysts should be responsible for the good dispersion of the
copper species at a high copper content. An increase in the copper
content usually deteriorates the dispersion of the copper species on the
support, but the catalysts we synthesized maintain the good dispersion
of the copper species even when the copper content was close to 30%.
The results of N
calcined samples are shown in Table 1, Figs. S1 and S2. The adsorption
isotherms of the CuSi samples exhibited typical type IV isotherms with
H1-type hysteresis loops when the copper loading was low and a type-
H3 hysteresis loop upon increasing the copper content [18]. CuSi
2
adsorption-desorption measurements for all the
x
3
2

Y. Zhao et al.
Catalysis Communications 154 (2021) 106310
+
Cu
0
Cu
CuSi3
IM-CuSi
580
575
570
565
Binding energy (eV)
Fig. 3. Cu LMM Auger spectra obtained for the reduced CuSi
3
and IM-
Fig. 2. The TEM image of the reduced CuSi
3
catalyst.
CuSi catalysts.
Those data indicated that the measures we took in the catalyst synthesis
were effective toward improving the dispersion of the copper species.
However, the XRD pattern of the calcined IM-CuSi sample showed two
sample. Thus, the high copper dispersion of the CuSi
understand.
3
sample was easy to
◦
◦
sharp peaks at 2θ = 35.5 and 38.7 , accompanied by satellite peaks at
3.5. H
2
-TPR analysis
◦ ◦
4
8.8 and 61.5 , which clearly indicated that there was a large amount
of blocky CuO in the IM-CuSi sample [19].
2
H -TPR was performed to evaluate the reducibility of the calcined
The XRD patterns of the reduced samples are shown in Fig. 1(b). The
catalysts. Fig. S4 showed that the reduction peaks in CuSi
x
were all
◦
diffraction patterns of the reduced CuSi
x
samples exhibited a very weak
O species [19]. The
located in the low temperature region (200–300 C), indicating that the
copper species in these samples were made up of small particles that
◦
peak at 37.5 , which could be attributed to the Cu
2
peaks were weak and diffuse, indicating that the particle size of Cu
2
O
were easy to reduce. The H
the Cu contents from CuSi
CuSi sample was quite different from that of the CuSi
2
consumption increased with an increase in
was small and the crystallinity was poor. During the reduction process,
the forming Cu metallic species leave the silica framework, so sintering
should always occur. However, due to the stable Cu-O-Si structure and
enhanced strong metal-support interactions (SMSI) formed during the
1
to CuSi . The reduction behavior of the IM-
4
x
samples. The TPR
◦
curve of the IM-CuSi sample showed a reduction peak at 219 C, cor-
responding to the reduction of copper species with small particle size in
the IM-CuSi sample [21]. The reduction temperature was much lower
x
calcination treatment, the copper particle size in the CuSi samples after
the reduction step was still small. However, the XRD pattern of the
reduced IM-CuSi sample showed sharp diffraction peaks at 43.3, 50.4
3
than that of CuSi , indicating that the interactions formed between the
copper species and the support were weak and these copper species were
prone to be reduced. There were two additional broad hydrogen con-
◦
0
and 74.1 , which belonged to the Cu species. The XRD pattern also
◦
◦
showed weak diffraction at 37.5 corresponding to the Cu
2
O species,
sumption peaks in the high temperature regions of 260–330 C and
◦
indicating that the reduced IM-CuSi samples were mainly composed of
large metallic Cu particles and a small amount of Cu O species.
375–425 C, which could be attributed to the large particles of CuO with
2
2
poor dispersion in the IM-CuSi sample. In addition, the H consumption
observed for the IM-CuSi sample was larger than that for the CuSi
3
+
3
.3. Transmission electron microscopy (TEM)
sample with the same Cu content, indicating that some Cu species may
3
already exist in the calcined CuSi sample due to the strong interactions
The morphologies of the reduced CuSi
3
and IM-CuSi samples were
sample (Fig. 2),
among the Cu-O-Si composite structure.
3.6. XPS analysis
investigated using TEM. In the case of the reduced CuSi
3
a large number of copper species were uniformly dispersed on the sup-
port and the particle size distribution showed that the average diameter
of copper species on the sample was only 2.96 nm, which is consistent
with the XRD pattern. However, the average diameter of copper species
on the IM-CuSi catalyst was ~18 nm (Fig. S3). Apparently, the disper-
sion of copper species on the support for the IM-CuSi catalyst was poor,
which may be a result from the weak interaction formed between the
impregnated copper species and the support, and the severe agglomer-
ation of the copper species during the calcination and reduction steps.
3
The surface chemical states of the reduced CuSi and IM-CuSi sam-
2
+
ples were determined using XPS. As seen in Fig. S5, Cu was completely
reduced to its low valence state after the reduction process [22]. Further
research on the Cu Auger LMM spectra shown in Fig. 3 indicates that all
the spectra present a series of asymmetrical and broad peaks, indicating
the coexistence of Cu and Cu0 on the surface of the catalysts. Decon-
+
volution of the Cu LMM peaks was carried out and the peak located at
0
5
69.8 eV corresponded to the Cu species, while the peak with a higher
+
3
.4. N
2
O titration
BE value was the peak corresponding to the Cu species [23]. The
+
deconvolution results showed that the Cu content on the surface of the
0
+
The well dispersed Cu catalyst can produce high surface Cu and Cu
reduced CuSi
3
(58.3%) and IM-CuSi samples (28.6%) differed greatly. It
+
0
+
concentrations combined with a high Cu /(Cu + Cu ) ratio to achieve
was the special Cu-O-Si structure in the CuSi
3
sample, which strongly
+
high catalytic activity [10]. Thus, the dispersion of surface copper in the
interacted with the support, retarding the further reduction of Cu into
0
CuSi
20] and the results are listed in Table 1. The CuSi
high dispersion of copper (40.95%), which was much higher than that in
3
and IM-CuSi sample were compared using N
2
O titration studies
Cu .
[
3
sample possessed a
3.7. Catalytic performance during the hydrogenation of DMO
the IM-CuSi sample (9.72%). The N
TEM analyses showed that the CuSi
2
adsorption-desorption, XRD and
sample exhibited a significantly
3
The hydrogenation of DMO was performed to compare the catalytic
performance of the catalysts and the reaction can be seen in Scheme S1
higher surface area and much smaller Cu particle size than the IM-CuSi
3

Y. Zhao et al.
Catalysis Communications 154 (2021) 106310
4
. Conclusions
100
Cu-O-Si catalyst materials with large specific surface areas and
highly dispersed metal species have been prepared using an improved
precipitation method. The CuSi catalyst prepared with a Cu content of
80
60
40
20
0
3
Conversion of DMO
Selectivity to MG
Selectivity to EG
2
24 wt% possessed a much higher specific surface (619 m /g) and very
small particle size (~3 nm), which exhibited high catalytic activity to-
ward the hydrogenation of DMO with a DMO conversion of 99.9% and
Selectivity to EtOH
Selectivity to others
◦
EG selectivity of 96.7% even at 175 C. In addition, the formation of the
Cu-O-Si composite structure and the low-temperature reaction signifi-
cantly enhanced its chemical stability, and the catalyst did not show any
significant inactivation during the stability test over 140 h. These results
provide a new strategy to prepare highly stable single metal Cu/SiO
2
170
180
190
200
210
catalysts with high catalytic activity, which was much simpler than the
conventional method of doping a second metal species to improve the
stability of copper catalysts.
o
Reaction temperature ( C)
Fig. 4. Effect of reaction temperature on the catalytic performance over CuSi
3
ꢀ 1
catalyst. Reaction conditions: P = 2.5 MPa, LHSV = 0.5 h and H
120.
2
/DMO
Declaration of Competing Interest
The authors declare that there is no conflict of interest.
Acknowledgements
=
1
00
9
9
8
8
8
6
4
2
0
5
0
5
0
Financial supports from Key research and development program in
Gansu Province (18YF1GA062) and NSFC (21763016) are
acknowledged.
Conversion of DMO
Selectivity to MG
Selectivity to EG
Selectivity to EtOH
Selectivity to others
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.catcom.2021.106310.
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