Structure evolution of mesoporous silica supported copper catalyst for dimethyl oxalate hydrogenation

Article abstract of DOI:10.1016/j.apcata.2017.04.001

A modified ammonia evaporation method with an ordered mesoporous silica as the precursor of the support was applied to prepare the well dispersed copper-based catalysts. Appropriate amount of ammonia was used during the aging stage to prevent the destruction of the ordered mesoporous structure, which can ensure the homogeneous pre-distribution of the copper precursor ([Cu(NH₃)₄]²⁺) in the mesopores. Then the formation of copper phyllosilicate or surface Cu–O–Si species can be prompted during the ammonia evaporation stage, resulting in large surface areas of both Cu⁰ and Cu⁺ species in the final catalysts. It was also revealed that the formation of copper phyllosilicate led to the destruction of mesoporous silica structure in the ammonia evaporation stage especially at the higher copper loading. The catalysts with various copper loading were systematically characterized and applied in the hydrogenation of dimethyl oxalate to ethylene glycol (EG). An excellent low-temperature catalytic performance and stability were achieved on 20Cu/OMS with EG selectivity of 98.2% at 453?K, due to the superior surface areas of both Cu⁰ and Cu⁺, as well as the highest ratio of Cu⁺/(Cu⁰?+?Cu⁺).

Full text of DOI:10.1016/j.apcata.2017.04.001

Accepted Manuscript
Title: Structure evolution of mesoporous silica supported
copper catalyst for dimethyl oxalate hydrogenation
Author: Yujun Zhao Yaqing Zhang Yue Wang Jian Zhang Yan
Xu Shengping Wang Xinbin Ma
PII:
S0926-860X(17)30149-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.04.001
APCATA 16199
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
1-2-2017
24-3-2017
1-4-2017
Please cite this article as: Y. Zhao, Y. Zhang, Y. Wang, J. Zhang, Y. Xu,
S. Wang, X. Ma, Structure evolution of mesoporous silica supported copper
catalyst for dimethyl oxalate hydrogenation, Applied Catalysis A, General (2017),
http://dx.doi.org/10.1016/j.apcata.2017.04.001
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Page 1 of 43

Highlights
1
2
3
4
. High efficient copper-based catalyst was prepared with ordered mesoporous silica
as support precursor.
. Appropriate pH value of the solution can ensure the high dispersion of copper
species.
. The presence of mesoporous structure enhanced the formation of copper
phyllosilicate.
. 20Cu/OMS presented excellent catalytic performance in dimethyl oxalate
hydrogenation.
Page 2 of 43

Structure evolution of mesoporous silica supported copper
catalyst for dimethyl oxalate hydrogenation
Yujun Zhao*, Yaqing Zhang, Yue Wang, Jian Zhang, Yan Xu, Shengping Wang
and Xinbin Ma
Key Laboratory for Green Chemical Technology of Ministry of Education,
Collaborative Innovation Center of Chemical Science and Engineering, School of
Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
*
Corresponding author: E-mail: yujunzhao@tju.edu.cn; Phone: +86-022-87401818
Page 3 of 43

ABSTRACT:
A modified ammonia evaporation method with an ordered mesoporous silica as
the precursor of the support was applied to prepare the well dispersed copper-based
catalysts. Appropriate amount of ammonia was used during the aging stage to prevent
the destruction of the ordered mesoporous structure, which can ensure the
2+
homogeneous pre-distribution of the copper precursor ([Cu(NH ) ] ) in the
3
4
mesopores. Then the formation of copper phyllosilicate or surface Cu-O-Si species
can be prompted during the ammonia evaporation stage, resulting in large surface
0
+
areas of both Cu and Cu species in the final catalysts. It was also revealed that the
formation of copper phyllosilicate led to the destruction of mesoporous silica structure
in the ammonia evaporation stage especially at the higher copper loading. The
catalysts with various copper loading were systematically characterized and applied in
the hydrogenation of dimethyl oxalate to ethylene glycol (EG). An excellent
low-temperature catalytic performance and stability were achieved on 20Cu/OMS
0
with EG selectivity of 98.2% at 453K, due to the superior surface areas of both Cu
+
+
0
+
and Cu , as well as the highest ratio of Cu / (Cu +Cu ).
Keywords: copper phyllosilicate, mesoporous silica, hydrogenation, copper
catalyst
Page 4 of 43

Introduction：
Ethylene Glycol (EG) is an important chemical product, which has been
versatilely applied in the manufacturing of polyester, lubricant and antifreeze [1, 2].
Traditionally, EG is industrially produced by the direct hydration of ethylene oxide.
Nevertheless, the shrinking of the fossil-fuel resource and the sustained increasing of
demand of EG hinder its wide application. The indirect route for the production of EG
from syngas via dimethyl oxalate (DMO) is one of the most attractive approach owing
to its advantages in economy and environmental benign feature [3-6]. This route
includes two main steps: the oxidative coupling of CO to DMO and then the
hydrogenation of DMO to EG. Plentiful works have been involved in the
homogeneous hydrogenation of oxalate upon ruthenium-based homogeneous catalysts
[
7, 8]. However, the lower selectivity and the difficulties in the products separation
limited its practical applications.
Due to the excellent activity in the hydrogenation of C=O group and lower
ability to dissociative adsorption of the C-C bonds, copper-based catalysts have been
applied in the hydrogenation of DMO [9, 10]. Although CuCr catalysts show
reasonable performance [11], the toxicity of the chromium restricts its application.
Accordingly, increasing research focused on Cr-free Cu-based catalysts with various
supports, such as SiO , Al O , ZnO, and La O [12, 13]. Strong acid sites of the
2
2
3
2
3
support would lead to the intramolecular dehydration of EG to ethanol, and strong
basic sites would promote the formation of 1, 2-butanediol (1, 2-BDO) by Guerbet
reaction between EG or DMO and ethanol [14, 15]. Up to now, the Cu/SiO catalysts
2
Page 5 of 43

showed the superior performances due to the neutral properties of SiO and the
2
suitable interaction between the copper species and silica. Various method, including
impregnation [16], sol–gel [17], deposition precipitation [18], ion-exchange [19] and
ammonia evaporation (AE) method [20] have been applied to prepare copper-based
catalysts for the hydrogenation of DMO. Among these preparation methods, the AE
method was found to be an efficient method and the as-prepared catalyst showed
higher activity in the hydrogenation reactions.
It is well known that the hydrogenation of DMO involved three main reactions:
first the hydrogenation of DMO to methyl glycolate (MG), then the further
hydrogenation of MG to EG, and the deep hydrogenation of EG to ethanol. (Equation
1
-3)
CH OOCCOOCH3
+
2H2
2H2
→
→
CH OOCCH OH
+
CH OH
(1)
(2)
(3)
3
3
2
3
CH OOCCH OH
+
HOCH CH OH + CH OH
2 2 3
3
2
HOCH CH OH + H2 → CH CH OH + H O
3 2 2
2
2
Raising temperature benefits the further hydrogenation of MG to EG, while the
copper species would irreversibly deactivate due to the particle agglomeration under
high temperature. Moreover, the Guerbet reaction can occur easily at high
temperature with the formation of 1, 2-BDO byproduct, which is difficult to be
separated from EG. As reported in the earlier literatures, temperature around 473K is
favored for the hydrogenation of DMO to EG [5, 16, 19]. The preparation of catalyst
with perfect hydrogenation performance at lower temperature is still a challenge.
Mesoporous materials have drawn much attention in many fields of chemistry
Page 6 of 43

since the successful synthesis of the M41S materials [21]. The ordered mesoporous
materials are used as the catalyst support, due to its high specific surface area and
pore volume, regular structure, uniform pore size distribution, and excellent thermal
stability [22]. Immobilization of copper species into ordered mesoporous materials
would improve copper dispersion and promote the catalytic performance in DMO
hydrogenation. Yin et al [16] reported HMS as support could promote the uniform
dispersion of copper species. Guo et al [18] reported Cu/SBA-15 catalyst with
ultra-high copper loading had high dispersion and small nanoparticle size. Ma et al
[
20] identified that copper could be well incorporated into MCM-41 through the AE
method and Cu/MCM-41 showed high dispersion and excellent catalytic performance
in DMO hydrogenation. However, although it is crucial important in designing more
efficient catalyst, the structure effect of these mesoporous materials on the formation
of the final catalyst has not yet revealed up to now.
In this work, series of ordered mesoporous silica (OMS) were synthesized and
used for the preparation of Cu/OMS by a modified AE method under weaker alkaline
environment. The structure evolution of the ordered mesoporous structure at different
stage during the preparation process was systematically study. The structural affect on
the dispersion and formation of copper species were explored. The catalytic
performance of the Cu/OMS was examined in the low-temperature hydrogenation of
DMO to EG, and the structure-activity relationship was revealed.
2
2
. Experimental
.1 Catalyst preparation
Page 7 of 43

Ordered Mesoporous Silica (OMS): tetraethyl orthosilicate (TEOS),
cetyltrimethylammonium bromide (CTAB), water and ammonia aqueous were mixed
at room temperature in a ratio of 1:0.2:160:1.5 to obtain a homogeneous mixture.
Then the mixture was stirred continuously until turning into a white gel. The white gel
was loaded into a PTFE-lined stainless steel autoclave and heated at 373K for 48h.
After cooling to room-temperature, the products were separated by filtering, washed
with deionized water, and dried in air at 373K for 3h. The resulting solid was calcined
in air by heating at a rate of 2K/min to 823K and then maintained at this temperature
for 6 h.
Cu/OMS: Cu/OMS catalysts were prepared by the AE method described as
follows: a certain amount of Cu(NO ) ·3H O was dissolved in deionized water, and
3
2
2
then 25 wt% ammonia aqueous was added and stirred for 0.5 h at 303 K, followed by
the addition of ordered mesoporous silica powder and stirring for 4 h with the initial
pH value of 10. This process was named aging stage. The suspension was heated from
3
03 to 353 K to remove ammonia. The temperature was kept at 353 K about 2 h until
the pH value of the suspension decreased to 6-7. This process is called evaporation
stage. The filtrate was washed with deionized water and then dried at 373 K overnight.
Finally, the resulting solid was calcined at 673 K in air for 4 h. The catalysts were
denoted as xCu/OMS. The x represents the weight percentage of copper in the
catalyst.
During the aging and evaporation process for the preparation of 20Cu/OMS
catalyst, sampling of the mixture was conducted at different time for examination.
Page 8 of 43

The samples were denoted as 20Cu-AGE-y and 20Cu-EVA-z. Both y and z mean
treatment time (h).
2
0Cu/SiO : About 15.4 g Cu(NO ) ·3H O was dissolved in deionized water, and
2 3 2 2
then 52 mL ammonia aqueous solution (25 wt%) was added and stirred for 0.5 h at
room temperature. Then, 45 mL silica sol (30 wt%) was added and stirred for 4 h. The
suspension was heated to 353 K to evaporate ammonia until the pH value of the
suspension decreased to 6-7. The precipitate was filtrated and washed with deionized
water and then dried at 373 K overnight. The resulting solid was finally calcined at
6
73 K for 4 h.
2
.2 Catalyst Characterization
Textural properties were measured by the method of nitrogen adsorption at 77 K
using a Micromeritics Tristar II 3000 analyzer instrument. The sample was deaerated
at 573 K for 3 h in vacuum before analysis. Mesoporous surface areas were calculated
from desorption isotherms through the method of Brunauer−Emmett−Teller (BET).
And pore size distribution was calculated by the method of Barrett−Joyner−Halenda
(BJH) from desorption branches of the adsorption isotherms.
The actual copper loading in the catalysts was determined by the inductively
coupled plasma optical emission spectrometry (ICP-OES). The sample was dissolved
in the HF solution, neutralized with HBO , and diluted with water before analysis.
3
Transmission electron microscopy (TEM) images were obtained with a Philips
TECNAI G2 F20 system electron microscope equipped with a field emission gun.
The samples powders were scattered in ethanol solution by ultrasonic. The resultant
Page 9 of 43

was dropped and dried on copper grid-supported transparent carbon foil. The particle
size distributions of copper were obtained by counting more than 80 nanoparticles for
all the catalysts.
X-ray powder diffraction (XRD) measurement was carried out by Rigaku
C/max-2500 diffraction meter using the graphite filtered Cu Kα radiation (λ = 1.5406
Å) at room temperature. Data were collected by scanning from 2θ = 10° to 2θ = 90° at
a rate of 8°/min. To avoid the phase transformation for the reduced samples, catalysts
were carefully collected under hydrogen atmosphere and sealed before XRD analysis.
The Scherrer equation was used to calculate the particle size of the copper species.
Fourier Transform infrared spectra (FT-IR) were carried out on a Nicolet 6700
spectrometer. The catalysts were disked with KBr and placed into sample holder. The
-1
spectra region was recorded from 4000 to 400 cm by averaging 32 scans with
-1
resolution ratio of 4 cm .
Temperature-programmed reduction (TPR) was carried out on a Micromeritics
Autochem II 2920 equipped with a thermal conductivity detector (TCD). The samples
of about 50mg were placed into quartz tubes and dried in Ar atmosphere at 393K for
1
h. The samples were then heated in 10%H /Ar to 1073K by the heating rate of
2
1
0K/min. The TCD was employed to quantify the amount of hydrogen consumption
during this process.
The specific surface area of metallic copper was measured by N O using the
2
pulse titration method [23]. 50 mg of sample was reduced in 10% H /Ar at 573 K for
2
2
h and then cooled to 363 K. The consumed H was calculated and denoted as A1.
2
Page 10 of 43

Then 15% N O/He was introduced at a rate of 30 mL/min for 1h. During this process,
2
surface metallic Cu atoms were completely oxidized to Cu O. Then the surface Cu O
2
2
was reduced by hydrogen pulse to metallic Cu. The consumed H in this step was
2
denoted as A2. The consumed amount of hydrogen was obtained by Micromeritics
Autochem II 2920 apparatus with a TCD. Then the surface metallic Cu areas of
1
9
sample were calculated with 1.46 × 10 Cu atoms per square meter [24]. All the
reactions in these processes were described by following equations:
CuO + H → Cu + H O； 2CuO + H → Cu O + H O
(1)
(2)
(3)
2
2
2
2
2
2
Cu + N O → Cu O + N
2
2
2
Cu O + H → 2Cu + H O
2
2
2
The dispersion of metallic copper was calculated by follow equation：
D = 2A /A ×100%
(4)
2
1
X-ray photoelectron spectra (XPS) and Auger electron spectroscopy (XAES)
were carried out to analyze copper species on a PHI 1600 ESCA instrument (PE
Company) equipped with an Al Kα X-ray source (hν = 1486.6 eV). Before testing, the
samples were reduced at 573 K under a flow of H for 4 h, and cooled to room
2
temperature. At last, the samples were transferred into glass bottle without air.
2
.3 Catalyst activity tests
The gas-phase hydrogenation of DMO to EG was performed in a stainless steel
fixed-bed tubular reactor with an internal diameter of 8 mm. The catalyst sample of
0
.4 g (40-60 mesh) was placed in the middle of the reactor tube with a height of about
5 mm. The catalyst was reduced in hydrogen atmosphere at 573 K for 4 h under 2.5
3
MPa, and then cooled to the desired reaction temperature (453K). Then the reactant
Page 11 of 43

(10 wt % DMO (99.9% purity) in methanol (AR purity) solution) was injected
continuously through a high-pressure pump (Lab Alliance Series II Pump). The
reactions were performed at 453K and system pressure of 2.5MPa. The molar ratio of
H /DMO was 80/1 and the weight hourly space velocity (WHSV) of DMO was
2
-1
changed from 0.2 to 2.0 h .
The reaction products were analyzed by a gas chromatograph (Agilent Micro GC
6
820). This gas chromatograph has an HP-INNOWAX capillary column
(Hewlett–Packard Company, 30 m × 0.32 mm × 0.50 µm) equipped with a flame
ionization detector (FID). The main by-products included MG, ethanol and 1, 2-BDO.
Four to six separate samples under the same experiment conditions were taken and
analyzed by GC to ensure repeatability. The results were obtained by averaging the
experimental data for each point, and the uncertainties were within 3%.
3
3
. Results
.1 Textural Properties of Cu/OMS
The physicochemical properties of the ordered mesoporous silica and Cu/OMS
catalysts with different copper loading are listed in Table 1. The ordered mesoporous
2
3
silica has a BET surface area of 1019 m /g and pore volume of 1.10 cm /g ,which are
much higher than those of HMS and MCM-41 reported in previous literatures [16, 20],
implying the presence of more abundant pore structure in the as-prepared OMS. All of
the actual copper contents in different samples are close to their desired value,
indicating the complete precipitation of copper species. While the uploading of copper
species cause tremendous changes to the physical properties of OMS. The specific
Page 12 of 43

surface area of the catalyst shows a big decrease compared to that of OMS, probably
due to the blockage and destruction of the ordered mesoporous structure during the
preparation process. Interestingly, the specific surface area of the catalyst rise up from
2
3
94 to 574 m /g with the increase of copper loading. Meanwhile, the pore volume and
diameter gradually increase as well. These changes could be assigned to the
generation of more copper phyllosilicate in the catalyst with the increasing copper
loading [25, 26].
The N adsorption−desorption isotherms are illustrated in Fig. 1(A). The
2
adsorption curve of the support is very similar to its desorption curve, suggesting the
narrow and uniform pore distribution in the support [20]. The five copper containing
catalysts present Langmuir type IV isotherms with a H1-type hysteresis loop [27],
showing the typical mesoporous structure with 2-dimensional cylindrical channels of
the catalysts. As shown in Fig. 1(B), copper loading has a great impact on the
pore-size distribution. The average pore diameter rises with the increase of copper
loading, indicating the expansion of some mesopores or the destruction of some
mesopores followed by the formation of accumulated pores [17]. During the
2
+
preparation, the copper precursor (copper tetraammine complex, [Cu(NH ) ] ) could
3
4
react with ordered mesoporous silica to form copper phyllosilicate which has a
lamellar structure [10, 28]. When copper was loaded on the support, pore-size
distributions changed from monomodal to bimodal with two ranges of pores, one
estimated at 2.4 nm and the other at around 3.8 nm. Further increasing the copper
loading resulted in the emergence of another pores at around 6.1nm. The 10Cu/OMS
Page 13 of 43

shows highest peak intensity at about 2.4nm, however, this peak disappears with the
increase of copper loading. This structure change might be attributed to the blockage
of initial ordered mesopores by copper species, and the ordered mesopores were
gradually consumed and eventually disappeared with the increment of copper loading.
The pores around 3.8 nm and 6.1 nm should belong to the accumulated pores resulted
from the destruction of the ordered structure in the alkaline environment. The
formation of more tube and lamellar structure of copper phyllosilicate lead to the
augment of the accumulated pores, which could be further proved in the following
TEM results.
Copper dispersion and surface area of the various catalysts were measured by
N O titration. As showed in Table 1, the Cu dispersion declines with the increase of
2
copper loading, while the metallic copper surface area gives a volcano variation trend
and maximizes at the copper loading of 20%. This could be attributed to the
agglomeration of the metallic copper when the copper exceeds the optimum amount
that the ordered mesoporous silica could burden.
3
.2 TEM images
Fig. 2 shows the TEM images of calcined catalysts and the support. The support
exhibits a long-range ordered mesoporous structure with uniform pore diameter due to
the self-assembly of TEOS in the synthesis process. All the catalysts have lamellar
structure, but the ordered mesoporous structure disappears gradually when copper
loading is increased. During the catalyst synthesizing process, the copper precursor
2+
(
[Cu(NH ) ] ) accessed into the pores of the support, and reacted with surface silica
3 4
to form copper phyllosilicate or surface Cu-O-Si structure, leading to the destruction
Page 14 of 43

of some ordered mesopores. Because of the lower copper loading, many ordered
structure were maintained in 10Cu/OMS and 15Cu/OMS. A few ordered mesoporous
channels could be also observed in 20Cu/OMS, but they completely disappeared and
more lamellar or tubular structure were formed in those catalysts with more copper
loading. These results are in good accordance with the physical properties described
by the BET characterization.
Fig. 3 shows TEM images of reduced catalysts. It is observed that copper species
are uniformly distributed on the support and copper phyllosilicate with lamellar or
tubular structure vanished after reduction. The ordered mesopores can be still found
clearly on the catalysts with lower copper content. As listed in Table 1, when the
copper loading increases from 10% to 30%, the mean particle size of copper grows
from 2.08 to 3.16 nm. The ordered mesoporous structure was believed to contribute to
the high copper dispersion. When the copper loading reached certain degree, the
surface of the OMS has no enough capability to well disperse all of these copper
species. So the inevitable aggregation of copper species led to the increment of the
copper nanoparticle size.
3
.3 XRD profiles
The XRD patterns of all the calcined samples are shown in Fig. 4(A). The broad
and diffuse diffraction peak at 22° belongs to the feature peak of amorphous SiO . No
2
characteristic peaks of any crystalline phase for copper species are observed on these
catalysts, even with 30% copper loading, indicating the high dispersion of copper
species.
Page 15 of 43

Fig. 4(B) shows the XRD patterns of the reduced xCu/OMS catalysts. Except for
the characteristic peak of SiO , no diffraction peak is observed when the copper
2
loading is lower than 20%. However, with the continuous increment in copper loading,
the diffraction peak at 2θ = 36.4° arises gradually, which could be ascribed to the
+
characteristic peak of Cu O (JCPDS05-0667). It suggested the existence of Cu in the
2
reduced catalysts. When copper loading arrives at 30%, the characteristic peak of
0
Cu O becomes intensive, and a weak diffraction peak at 2θ= 43.3° assigned to Cu
2
(JCPDS04-0836) can be also observed.
Generally, when the crystalline particles exist below 3 nm in size, their
characteristic peaks cannot be detected by the XRD [16]. The particle size of all the
reduced catalysts are too small to be detected by XRD method, which is consistent to
the results of TEM images. Therefore, it could be deduced that the structure of
ordered mesoporous silica promoted the dispersion of the copper precursor and the
formation of more copper phyllosilicate during AE process, which in turn restrained
the growth of copper nanoparticle during the reduction process. The presence of Cu O
2
species might be attributed to the strong interaction between copper species and the
silica support, which inhibited the completely reduction of these copper species to
metallic copper.
3
.4 FT-IR spectra
The FT-IR spectroscopy was applied to determine the existence of the copper
hydroxide and copper phyllosilicate species and evaluate the amount of copper
phyllosilicate in the catalysts [29, 30]. As shown in Fig. 5(A), all the samples have the
-1
adsorption band of symmetric stretching vibrations of Si-O-Si at 800 cm and the
Page 16 of 43

-1
band of asymmetric stretching vibrations of Si-O-Si bond at 1110 cm in amorphous
-1
SiO . Interestingly, a shoulder peak at 970cm which assigned to stretching vibrations
2
of surface Cu-O-Si bond can be observed at lower copper loading [16]. When the
copper content exceeded 20%, this peak becomes less obvious. The formation of
copper phyllosilicate can be further verified by the existence of the adsorption band at
-1
6
70 cm , which can be assigned to the δ bands in copper phyllosilicate. Besides,
OH
-1
-1
the red shift of Si-O-Si bond vibrations frequency from 1110 cm to 1040 cm could
also certified the presence of more copper phyllosilicate [28].
The relative amount of copper phyllosilicate could be estimated qualitatively by
-1
considering the integrated intensity of the bands at 670 cm normalized to the
-1
integrated intensity of the bands at 800 cm , which was defined as I /I [31, 32].
6
70 800
Fig. 5(B) clearly shows that the ratio of I /I rises with the increment of copper
6
70 800
loading, indicating much more copper phyllosilicate was formed. Obviously, the
FT-IR spectra results strongly proved the coexistence of Cu-O-Si structure and copper
phyllosilicate on the calcined Cu/OMS, which has never been reported before for
copper-based catalysts. The mesoporous structure of OMS should play an important
role on their formation, which will be further confirmed in the discussion section.
3
.5 TPR results
TPR measurements were implemented to investigate the reducibility of the
catalysts. As presented in Fig. 6, the reduction curves of all the samples have a main
and almost symmetrical reduction peak at around 473K, indicating a narrow particle
size distribution. This has also been revealed by the TEM images and the XRD
Page 17 of 43

0
patterns. Chen et al. [28] reported that the reduction of well dispersed CuO to Cu
occurred at about 515K. Van Der Grift et al. [23] reported that the reduction peak at
+
about 510 K belonged to the reduction of copper phyllosilicate to Cu . And further
+
reduction of these Cu species having strong interaction with silica requires a
temperature above 873 K.[5, 33] Therefore, the main reduction peak at about 473 K
for Cu/OMS catalysts could be assigned to the overlapped of the reduction peak of
0
+
well dispersed CuO to Cu and copper phyllosilicate or Cu-O-Si species to Cu .
Notably, a shoulder peak at about 553K is observed, and gradually vanished with
the increase of copper loading. And this variation trend happened to be similar with
-1
that of Cu-O-Si species revealed by FT-IR spectra (the peak at 970 cm ). Meanwhile,
no bulk CuO was found in the calcined catalysts according to the TEM and XRD
results. Therefore, the peak at about 553K might belong to the reduction of a fraction
+
+
of Cu , likely the Cu formed from the reduction of Cu-O-Si species. While the
+
further reduction of those Cu derived from copper phyllosilicate commonly requires
a temperature about 873 K.
3
.6 XPS
The chemical composition and valence state of the reduced catalysts were
determined by XPS and XAES. The Cu 2p XPS spectra are illustrated in Fig. 7(A). It
shows only two peaks at around 932.5eV and 952.3eV, which belong to Cu 2p3/2 and
Cu 2p , respectively [34]. The peak at 942-944eV which is assigned to Cu 2p
1
/2
2+
satellite peak can not be found in all the samples [35]. This indicated that all Cu
+
0
species had been completely reduced to a low valence state of Cu or Cu [28].
Page 18 of 43

+
0
Nevertheless, it is difficult to distinguish between Cu and Cu by XPS spectra due to
the almost identical BE between these two species.
The Cu LMM X-ray induced Auger spectra (XAES) was used to distinguish the
chemical states by their difference in the peak position. As shown in Fig. 7(B), all the
samples present an asymmetric and broad peak, suggesting a stable coexistence of
+
0
Cu and Cu on the catalyst surface [17, 20]. The two overlapping peaks at 916.7 eV
0
+
and 913.6 eV are assigned to kinetic energy of Cu and Cu , respectively. From the
+
+
0
deconvolution results, the ratios of the surface Cu / (Cu + Cu ), which is denoted as
+
+
+
0
XCu , are listed in Table 2. The ratios of Cu / (Cu +Cu ) presents a volcanic curve
with the increasing copper loading due to the hindered reduction effect. Assuming
+
0
that the Cu ions and the Cu atoms occupy the same areas and have identical atomic
+
sensitivity factors, the surface area of Cu can be calculated based on surface area of
0
+
+
0
+
Cu and the ratio of Cu / (Cu +Cu ) [28, 35]. The surface area of Cu first rises and
then declines with the increasing copper loading, and the maximum value reaches
2
0
+
4
1.6m /g at 20% copper loading. The amount of both Cu and Cu on various
catalysts obey the rules: 20Cu/OMS > 25Cu/OMS > 30Cu/OMS > 15Cu/OMS >
+
1
0Cu/OMS. However, the sequence for the amount of Cu is not consistent with the
amount of copper phyllosilicate in the calcined catalysts, which was generally
+
believed to be the origin of Cu . This might be an important clue to further
+
demonstrate that the Cu specie was derived from not only the reduction of copper
phyllosilicate but also the reduction of surface Cu-O-Si species, both of which were
also found by the FT-IR characterization.
Page 19 of 43

3
.7 Catalytic performance in the hydrogenation of DMO
The low-temperature gas-phase hydrogenation of DMO was carried out in a
fixed-bed reactor at 453K to discuss the catalytic performance of the xCu/OMS
catalysts. As shown in Fig. 8(A-C), the activity of the catalysts can be sequenced as:
2
0Cu/OMS > 25Cu/OMS > 30Cu/OMS > 15Cu/OMS > 10Cu/OMS. Under a low
WHSV, the conversion of DMO on all catalysts reached 100%, and it decreased with
the increasing of WHSV. The superior catalytic performance can be achieved on
2
0Cu/OMS catalysts, whose DMO conversion was much higher than other catalysts at
-1
the WHSV above 1.0 h . As shown in Fig. 8(B) and (C), the selectivity to EG show
volcanic curves except 10Cu/OMS, and the selectivity to MG rise gradually for all the
catalysts with the increasing WHSV. The resident time should be not long enough for
the further hydrogenation of MG to EG at the relatively higher WHSV. While the
lower selectivity to EG under low WHSV is attributed to the deep hydrogenation of
EG to ethanol. The 20Cu/OMS catalyst shows the highest EG selectivity of 98.2%
-1
and 100% DMO conversion at the WHSV of 1.0 h under 453 K. Furthermore, as
shown in Fig. 8(D), 20Cu/OMS catalyst exhibits an excellent stability in the
chemoselective hydrogenation of DMO to EG, owing to the advantage in
low-temperature reaction, and thus displays a feasible prospect in the industrial
application.
4
4
. Discussion
.1 Correlation between copper species and catalytic performances
As proved by the characterization results, the catalyst with higher copper
dispersion and larger copper surface areas can be achieved by the use of ordered
mesoporous silica as the support and the modified AE method. It was believed that the
ordered mesoporous structure of OMS should play important role in the formation of
Page 20 of 43

0
active species. The optimal 20Cu/OMS catalyst had the highest surface areas for Cu
+
2
2
and Cu species, which were 36.9m /g and 41.6m /g respectively. These characters led
to the remarkable catalytic performance in the hydrogenation of DMO to EG. For low
copper loading catalysts, the amount of active species was not enough for the
activation of DMO and H at higher WHSV. When copper loading exceeded 20%, the
2
inner surface area of ordered mesoporous silica was not enough to well disperse these
copper species, thus the aggregation of copper nanoparticles happened. The larger
copper nanoparticle and lower copper surface area led to a worse activity.
0
+
Many work reported the viewpoint that the synergic effect of Cu and Cu might
be the reason for the excellent performance in hydrogenation of DMO [16, 28]. Our
previous work indicated the balanced of the copper species contributed to the catalytic
0
+
activity in ester hydrogenation [10, 35]. The synergy mechanism of Cu and Cu was
further proved by a kinetics study for DMO hydrogenation [36]. In this work, the
change of the catalytic performance showed the same trend as that of the surface area
0
+
of both Cu and Cu , indicating that the apparent catalytic activity was strongly
0
+
dependent on the amount of Cu or Cu . This result is consistent with the previous
+
0
findings: Cu and Cu are the active sites for the ester hydrogenation. The space time
yield of EG (STY ) also presented a positive correlation with the surface areas,
EG
0
0
especially Cu species (Fig. 9). Therefore, Cu species might be the determining
factors for the DMO hydrogenation on these catalysts.
4
.2 Comparison between Cu/OMS and Cu/SiO2
For a comparison, the 20Cu/SiO catalyst was prepared by the same method as
2
2
0Cu/OMS except silica sol instead of OMS as support. Obviously, 20Cu/OMS
0
+
catalyst had the higher surface areas of Cu and Cu than 20Cu/SiO . (Table 2) The
2
Page 21 of 43

copper species could be highly dispersed in the ordered mesoporous silica during the
preparation process due to its abundant pore structure. On the contrary, the silica sol
could not provide enough surfaces to fully contact with copper precursor
2
+
[
Cu(NH ) ] , leading to the formation of less copper phyllosilicate (Fig. S2a) and
3 4
larger particle size of copper species after reduced (Fig. S2b). Therefore, 20Cu/OMS
+
+
0
possessed the higher ratios of surface Cu / (Cu + Cu ) and surface areas of both
copper species than 20Cu/SiO . As a result, the 20Cu/OMS showed a remarkable
2
higher catalytic performance than 20Cu/SiO (Fig. S1).
2
4
.3 Structure evolution during the preparation process
In order to further reveal the function of the ordered mesoporous silica, real time
sampling technique was employed to investigate the structure evolution of catalyst
during the prepared process. The samples were taken at different time during the
aging and evaporation process.
The physicochemical properties of these samples for 20Cu/OMS catalyst were
listed in Table S1. The actual copper loading in different samples during the AE
process were close to the designed value, implying copper species were well uploaded
on the support in all the stages. For the samples obtained from the evaporation stage,
the BET surface area showed a slight decline compared to aging samples, and the
pore volume and pore diameter gradually ascent. The mesoporous structure of OMS
can be maintained in the aging stage, but destroyed in the evaporation stage (Fig. 10).
These can be further evidenced by TEM images (Fig. 11a-d), the mesoporous
structure of the support was clearly found in the aging samples (Fig. 11a and b) and
Page 22 of 43

gradually disappeared with time in the evaporation stage (Fig. 11c and d). The
presence of ordered mesoporous structure ensured the uniform distribution of the
copper precursor in their regular channels. Thus, the reaction between copper
2+
precursor ([Cu(NH ) ] ) and silica can be enhanced during the evaporation stage,
3
4
leading to the structural destruction of the mesoporous silica and the formation of
more copper phyllosilicate. The morphology of tubular and lamellar structure in the
final catalyst (Fig. 11d) should be attributed to the character of copper phyllosilicate
[
28]. This might be one of the important reasons for the excellent dispersion of copper
species in the final catalyst.
On basis of above results, the structure evolution of the Cu/OMS catalysts during
the preparation process could be easily understood. The copper precursor
2+
(
[Cu(NH ) ] ) highly dispersed in the mesopores of OMS during the aging stage and
3 4
reacted with support during the evaporation stage, which enhanced the formation of
copper phyllosilicate in the catalyst. Thus, the presence of the mesoporous structure of
OMS in the aging stage should be the key factor in improving the dispersion and
copper surface areas.
4
.4 Effect of pH on the structure evolution of Cu/OMS
It has been reported that the morphology and structure of Cu/SiO catalysts can
2
be greatly affected by the evaporation temperature [28]. While rare reports focused on
the effect of the pH value on the structure evolution of Cu/OMS. In this study, one
catalyst was prepared by the same method as above mentioned except the initial pH
value was changed to 12 instead of 10. This catalyst was denoted as 20Cu/OMS(12)
and the sample 20Cu-AGE-4(12) was also obtained by sampling after aging for 4 h
Page 23 of 43

during the catalyst preparation process.
The ordered mesoporous silica was almost destroyed after aging stage under
higher pH value (Fig. 11e, Fig. S3). After ammonia evaporation process, the obtained
2
2
0Cu/OMS(12) catalyst presented less amount of copper phyllosilicate (Fig. 11f) than
0Cu/OMS (Fig. 11d), as well as the specific surface area and pore volume (Table S1).
Moreover, according to the lower actual copper loading in 20Cu-AGE-4(12) (Table
S1), the copper precursor could not be well immobilized on the support in the aging
stage when the pH of the mixture was 12.
Obviously, the ammonia containing mixture with higher pH destroyed the OMS
structure during the aging stage, so that the copper species could not be well
pre-distributed in the support. Thus, less copper phyllosilicate can be formed during
the ammonia evaporation stage, because of the less contact between the copper
precursor with silica surface. Therefore, the final catalyst gave the lower specific
surface area and pore volume, as well as the worse catalytic performance compared to
the 20Cu/OMS catalyst (Fig. S4).
5
. Conclusion
A high efficient copper-based catalyst for the hydrogenation of DMO to EG has
been successfully prepared by a modified ammonia evaporation method using ordered
mesoporous silica as the support. The optimized copper loading would benefit the
0
+
formation of more copper species including both Cu and Cu . While, excessive
copper loading would result in the aggregation of copper species due to the total
+
destruction of OMS. Moreover, for the first time it was found that Cu specie was
derived from not only the reduction of copper phyllosilicate but also the reduction of
surface Cu-O-Si species. Given appropriate alkaline environment, the copper
Page 24 of 43

2
+
precursor ([Cu(NH ) ] ) can be well pre-distributed in the ordered mesoporous silica
3
4
during the aging stage and reacted with the silica during the evaporation stage, leading
0
+
to the formation of more copper phyllosilicate. Thus the surface areas of Cu and Cu
were significantly improved in the final reduced catalyst. The appropriate pH value
was the key factor to reserve the ordered mesoporous structure during the aging stage.
The catalyst with optimized copper loading presented higher copper dispersion, larger
+
+
0
copper surface areas and proper ratio of Cu / (Cu +Cu ), which showed remarkable
low-temperature catalytic performance with more than 98% yield of EG at 453K.
These findings and synthesized strategy showed a prospective future in fabricating a
catalyst with higher loading of active component.
Acknowledgement
The authors are grateful to the financial support from the National Nature
Science Foundation of China (21276186, 21325626, 91434127, U1510203), and the
Tianjin Natural Science Foundation (13JCZDJC33000).
Page 25 of 43

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Page 28 of 43

Tables of content
Table 1. Textural Parameters of xCu/OMS Catalysts and support
Table 2. Characterization of Cu Species and space time yield of EG of the catalysts
Figure 1. (A) N adsorption−desorption isotherms, (B) pore-size distribution curves.
2
(a)10Cu/OMS,(b)15Cu/OMS,(c)20Cu/OMS,(d)25Cu/OMS,(e)30Cu/OMS,(f)OMS
(signal of f in figure1B was attenuated to 1/5).
Figure 2. The TEM images of the calcined xCu/OMS catalysts and support.
(
a)10Cu/OMS, (b)15Cu/OMS, (c)20Cu/OMS, (d)25Cu/OMS, (e)30Cu/OMS,
f)OMS.
(
Figure 3. The TEM images of the reduced xCu/OMS catalysts. (a)10Cu/OMS,
b)15Cu/OMS, (c)20Cu/OMS, (d)25Cu/OMS, (e)30Cu/OMS.
Figure 4. XRD patterns of the xCu/OMS catalysts (A) calcined and (B) reduced.
(
(a)10Cu/OMS, (b)15Cu/OMS, (c)20Cu/OMS, (d)25Cu/OMS, (e)30Cu/OMS.
Figure 5. (A) FT-IR spectra (B) I670/800 of the calcined xCu/OMS catalysts.
(
a)10Cu/OMS, (b)15Cu/OMS, (c)20Cu/OMS, (d)25Cu/OMS, (e)30Cu/OMS.
Figure 6. The TPR profiles of the xCu/OMS catalysts. (a)10Cu/OMS, (b)15Cu/OMS,
c)20Cu/OMS, (d)25Cu/OMS, (e)30Cu/OMS.
(
Figure 7. (A) Cu 2p XPS spectra (B) Cu LMM Auger spectra of the reduced
xCu/OMS catalysts. (a)10Cu/OMS, (b)15Cu/OMS, (c)20Cu/OMS, (d)25Cu/OMS,
(e)30Cu/OMS.
Figure 8. The catalytic performance and stability of the catalysts. Reaction condition:
p = 2.5 MPa, T = 453 K, H /DMO = 80. (A) conversion of DMO, (B) selectivity of
2
EG, (C) selectivity of MG. (a)10Cu/OMS, (b)15Cu/OMS, (c)20Cu/OMS,
Page 29 of 43