Influence of support on the performance of copper catalysts for the effective hydrogenation of ethylene carbonate to synthesize ethylene glycol and methanol

Article abstract of DOI:10.1039/c6ra06464g

Hydrogenation of ethylene carbonate (EC) is an attractive and promising approach for indirect hydrogenation of CO₂ to co-produce ethylene glycol (EG) and methanol (MeOH). In this work, thermodynamics of EC hydrogenation was firstly calculated, and the results disclosed that EC hydrogenation is exothermic (Δ_rHθm = -71.59 kJ mol^-1) and thermodynamically favorable (Δ_rGθm = -25.62 kJ mol^-1). The mesoporous silica supported copper catalysts were successfully prepared by facile ammonia evaporation method using KIT-6, MCM-41 and SBA-15 as support materials. The as-prepared Cu/KIT-6, Cu/MCM-41 and Cu/SBA-15 were thoroughly characterized by N₂ physisorption, ICP-AES, N₂O titration, FT-IR, XRD, H₂-TPR, TEM, XPS and XAES. It was found that copper nanoparticles were well dispersed on these supports. Interestingly, a larger metallic Cu surface area with higher dispersion was obtained on SBA-15 when compared with KIT-6 and MCM-41. Furthermore, Cu⁰ and Cu⁺ species in various ratios were verified to co-exist on the surfaces of the three catalysts, which originated from CuO and copper phyllosilicate, respectively. The catalytic performances showed that Cu/SBA-15 exhibited a superior catalytic activity with a TON of 22.0 and 11.4 (mol mol^-1) towards EG and MeOH, respectively. Importantly, 100% of EC conversion, 94.7% of EG yield and 62.3% of MeOH yield were achieved within a short reaction time of 4 h over Cu/SBA-15 under optimized conditions. The synergetic effect between Cu⁰ and Cu⁺ species, in which it was proposed that Cu⁰ species dissociated H₂, while Cu⁺ species absorbed the carbonyl group of EC, was responsible for the higher catalytic activity of Cu/SBA-15.

Full text of DOI:10.1039/c6ra06464g

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PAPER
Influence of support on the performance of copper catalysts for
the effective hydrogenation of ethylene carbonate to synthesize
ethylene glycol and methanol
Received 00th XX 20xx,
Accepted 00th XX 20xx
Fengjiao Li,ab Liguo Wang,*a Xiao Han,a Peng He,a Yan Cao,a and Huiquan Li*a
DOI: 10.1039/x0xx00000x
Hydrogenation of ethylene carbonate (EC) is an attractive and promising approach for indirect hydrogenation of CO2 toco-
www.rsc.org/
produce ethylene glycol (EG) and methanol (MeOH). In this work, thermodynamics of EC hydrogenation was firstly
ꢀ Hθ
calculated, and the results disclosed that EC hydrogenation is exothermic (
m = -71.59 kJ/mol) and thermodynamically
r
ꢀ Gθ
favorable (
= -25.62 kJ/mol). The mesoporous silica supported copper catalysts were successfully prepared by facile
r
m
ammonia evaporation method using KIT-6, MCM-41 and SBA-15 as support materials. The as-prepared Cu/KIT-6, Cu/MCM-
41 and Cu/SBA-15 were detailedly characterized by N2 physisorption, ICP-AES, N2O titration, FT-IR, XRD, H2-TPR, TEM, XPS
and XAES. It was found that copper nanoparticles were well dispersed on these supports. Interestingly, larger metallic Cu
surface area with higher dispersion was obtained on SBA-15 when compared with KIT-6 and MCM-41. Furthermore, Cu0
and Cu+ species with varied ratio were verified to co-exist on the surfaces of the three catalysts, which originated from CuO
and copper phyllosilicate, respectively. The catalytic performances showed that Cu/SBA-15 exhibited a superior catalytic
activity with TON of 22.0 and 11.4 (mol/mol) towards EG and MeOH, respectively. Importantly, 100% of EC conversion,
94.7% of EG yield and 62.3% of MeOH yield were achieved within a short reaction time of 4 h over Cu/SBA-15 under an
optimized condition. The synergetic effect between Cu0 and Cu+ species, which was proposed that Cu0 species dissociated
H2, while Cu+ species absorbed the carbonyl group of EC, was responsible for the higher catalytic activity of Cu/SBA-15.
1. Introduction
As an abundant, nontoxic and renewable carbon source, CO2 complex catalyst by Ding et al,15 which has been considered as
has been widely used as a C1 feedstock for manufacturing a very attractive and promising approach to coꢀproduce
chemicals and fuels in terms of environmental concerns and valuable methanol and ethylene glycol indirectly from CO2.
sustainable society.1ꢀ5 Particularly, significant efforts have been Because EC can be largely and industrially available from
made to transform CO2 with renewable hydrogen to methanol, ethylene oxide (EO) and CO2, more attention has been focused
which is considered to be a vital step for building up a methanol on developing effective catalyst systems especially recyclable
economy,6 as methanol is an excellent hydrogen carrier,7 an heterogeneous catalysts for EC hydrogenation. Up to now,
alternative transportable liquid fuel and additive for fuel cells, 8, merely a few heterogeneous Cuꢀbased catalysts such as
9
and a useful feedstock for producing varied valuable CuCr2O4,8 CuꢀSiO2ꢀPG16 and Cu/HMS17 have been reported to
chemicals such as olefins and aromatics.10,
Very recently, catalyze the hydrogenation of EC to synthesize EG and
11
several approaches for indirect hydrogenation of CO2 via methanol. However, there has been no clear awareness of
organic carbonates, carbamates and formates to methanol are thermodynamic of EC hydrogenation to coꢀproduce EG and
drawing growing interest in the viewpoint of both concept and methanol. Moreover, it is often controversial with respect to the
industrialization.12ꢀ14 Among these approaches, indirect roles of copper species such as Cu0 and Cu+ on copperꢀbased
hydrogenation of CO2 via ethylene carbonate (EC) intermediate catalysts, which are required to explain the high catalytic
to coꢀproduce ethylene glycol (EG) and methanol (MeOH) was activity of copperꢀbased catalysts. Hence, it is still highly
proposed and realized over a homogeneous pincerꢀtype RuII desirable to develop more effective catalysts and give insight
into the catalytic mechanism for EC hydrogenation.
Copperꢀbased catalysts have been extensively used in the
hydrogenation of esters due to the merits of the selective
hydrogenation of CꢀO bonds and the lower ability to cleave the
CꢀC bonds, thus resulting in less undesired degradation
products.18ꢀ21 Moreover, the catalytic activity and properties of
copperꢀbased catalysts are significantly influenced by supports.
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In order to obtain high catalytic activity, a variety of supports Copperꢀbased catalysts using different mesoporous silicas
have been employed for preparing highly dispersed copperꢀ including KITꢀ6, MCMꢀ41 and SBAꢀ15 as supports were
based catalysts. As is known, mesoporous silicas are prepared by facile ammonia evaporation method, briefly
extensively utilized as suitable supports for various highly described as follows. First, the desired amount of
dispersed metal and metalꢀoxide nanoparticles due to their Cu(NO3)2·3H2O was dissolved in distilled water. Then, the
properties of interesting framework topologies, high surface required amount of 25 wt.% ammonia aqueous solution was
area, tunable pore size and pore volume, and high thermal added into the above copper nitrate solution and vigorously
stability.22ꢀ26 Therefore, it will be helpful and useful to prepare stirred for 10 min. Subsequently, 10.0 g mesoporous silica
highly dispersed copperꢀbased catalysts using mesoporous support was added into the above copper ammonia complex
silicas as supports. Furthermore, the catalyst activity and solution under vigorous stirring. After the initial pH was
properties will be greatly influenced by different mesoporous adjusted to 11ꢀ12 by ammonia, the resulting suspension was
silica supports, helping to have a better understanding of the continuously stirred for 4 h. All the above operations were
key influencing factors for the high catalytic activity of copperꢀ performed at ambient temperature. After that, the suspension
based catalysts in EC hydrogenation.
was transferred into an oil bath to evaporate ammonia and
In the present work, thermodynamic calculation of EC finally to deposit copper species on the support at 353 K until
hydrogenation to coꢀproduce EG and methanol was employed the pH of the suspension decreased to 6ꢀ7. The resulting solid
for the first time to predict the reaction spontaneity as a was filtered, washed with deionized water and ethanol, dried in
function of temperature. The influences of three ordered a vacuum oven at 353 K overnight, calcined at 673 K in air
mesoporous silicas including KITꢀ6, MCMꢀ41 and SBAꢀ15 as atmosphere for 4 h, and finally reduced in 10 vol.% H2/N2 at
the supports for copperꢀbased catalysts were investigated on the 623 K for 4 h. The theoretical copper loading based on the
hydrogenation of EC to synthesize EG and methanol. Three weight of the total catalyst was set as 10 wt.%. Hereinafter, for
different catalysts Cu/KITꢀ6, Cu/MCMꢀ41 and Cu/SBAꢀ15 clarity, the dried precursors were denoted as Cu/KITꢀ6ꢀD,
were prepared by facile ammonia evaporation method, keeping Cu/MCMꢀ41ꢀD and Cu/SBAꢀ15ꢀD, the calcined samples were
the same theoretical copper loading of 10 wt.%. To our best denoted as Cu/KITꢀ6ꢀC, Cu/MCMꢀ41ꢀC and Cu/SBAꢀ15ꢀC,
knowledge, such support influence investigation using facile and the finally reduced catalysts were denoted as Cu/KITꢀ6,
ammonia evaporation method has not yet been reported for EC Cu/MCMꢀ41 and Cu/SBAꢀ15.
hydrogenation in literature. The catalyst properties were
systematically characterized by different techniques such as N2
physisorption, ICPꢀAES, N2O titration, FTꢀIR, XRD, H2ꢀTPR,
Quantachrome Autosorbꢀ1 at liquid nitrogen temperature (77
TEM, XPS and XAES. The catalytic performances of catalysts,
2.4 Catalyst characterization
N2 physisorption measurements were conducted on
K) after the samples were outgassed at 573 K in vacuum for 3 h
effects of reaction conditions and catalyst reusability were
to remove physically adsorbed species. The special surface
studied. Moreover, the correlation between the catalytic
activities of catalysts and the Cu0 and Cu+ sites was also
investigated to disclose the roles of different copper species.
areas (
(BET) equation. Total pore volumes (
relative pressures ( 0) close to unity. Pore size distributions
SBET) were calculated by the BrunauerꢀEmmettꢀTeller
V
pore) were evaluated at
P/P
were estimated by the Barrett, Joyner and Halenda (BJH)
method according to the desorption branch of the isotherms.
Copper content of the catalysts was measured with
inductively coupled plasmaꢀatomic emission spectroscopy
(ICPꢀAES) on PerkinElmer Optima 5300DV.
2. Experimental
2.1 Materials
Ethylene carbonate (EC, 99%) was purchased from Alfa Aesar.
Fourierꢀtransform infrared spectra (FTꢀIR) were recorded
on a Bruker Tensor 27 spectrometer at room temperature. For
the FTꢀIR characterizations, the samples were finely ground
and dispersed in KBr to make pellets. The spectral resolution
was 4 cmꢀ1. All spectra were recorded over 4000ꢀ400 cmꢀ1 for
16 scans.
Tetrahydrofuran (THF, 99.8%),
pꢀxylene (98.85%) and
Cu(NO3)2·3H2O (>99%) were purchased from Sinopharm
Chemical Reagent Co., Ltd., China. Ammonia (25 wt.%) was
purchased from Xilong Chemical Co., Ltd., China. Hydrogen
(99.999%) was purchased from Beijing Haikeyuanchang
Practical Gas Co., Ltd., China. SBAꢀ15 and MCMꢀ41 were
commercially available from Nanjing Xianfeng Nano Co., Ltd.,
China. KITꢀ6 was prepared according to the method reported
by Kleitz et al.23 Other reagents were of analytical grade and
used as received.
Xꢀray diffraction (XRD) patterns of the samples were
obtained with a PANalytical Empyrean diffractometer using Cu
K
α
radiation. Smallꢀangle XRD patterns were acquired in the
2
θ range of 0.6° to 10°. Wideꢀangle XRD patterns were
recorded in the 2θ range of 10° to 90°.
2.2 Thermodynamic evaluation
Temperatureꢀprogrammed reduction of hydrogen (H2ꢀTPR)
studies of the catalysts was performed on a Micromeritics
Autochem 2920 instrument. Before tests, about 50 mg catalyst
was outgassed under flowing Ar at 473 K for 0.5 h and then
cooled to ambient temperature. The reduction was carried out
from 323 K to 773 K at a heating rate of 10 K minꢀ1 in 10 vol.%
Calculation of thermodynamic evaluation for the processes
associated with EC hydrogenation was carried out according to
densityꢀfunctional theory (DFT) implemented with 6ꢀ31G(d, p)
Gaussian type basis set.27
2.3 Catalyst preparation
2 |RSC Adv., 2016, 00, 1-3
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H2/Ar (50 ml minꢀ1). The amount of hydrogen consumption was
determined using a thermal conductivity detector (TCD).
3. Results and discussion
3.1 Thermodynamic evaluation
N2O titration was performed on Micromeritics Autochem
2920 with a TCD to determine the Cu dispersion and the
metallic Cu specific surface area of catalysts. About 100 mg
catalyst was first outgassed at 473 K under He for 1 h. After
cooling to ambient temperature under He, the gas was switched
to the 10 vol.% H2/Ar mixture stream (30 mL minꢀ1). Then, the
sample was heated to 623 K at a ramping rate of 10 K minꢀ1 and
retained 623 K for 2 h. The reduced sample was cooled to 363
K and isothermally purged with He for 1 h, and then exposed to
pure N2O (30 mL minꢀ1) for 1 h to ensure complete oxidation of
naked metallic copper. The sample was then flushed with He to
remove N2O and cooled to room temperature. Finally, the gas
was switched to the 10 vol.% H2/Ar mixture stream (30 mL
minꢀ1) and the sample was heated to 623 K at a ramping rate of
10 K minꢀ1. The amount of consumed H2 during the reduction
Up to now, no open literatures have been reported on
thermodynamic of EC hydrogenation to coꢀproduce EG and
methanol. Therefore, the thermodynamic data of EC
hydrogenation (Reaction 3 in Scheme 2) were calculated by
DFT for the first time. In addition, thermodynamic data of other
related reactions (shown in Schemes 1ꢀ3) were also computed,
in order to have a clear picture on EC hydrogenation. The
results for thermodynamic calculation were summarized in
Table 1. As can be seen, methanol synthesis via direct
hydrogenation of CO2 (Reaction 1) is a moderately exothermic
ꢀr Hθm
reaction (
= ꢀ46.98 kJ/mol) and thermodynamically
= 6.01 kJ/mol). In contrast, indirect
ꢀrGθm
unfavorable (
hydrogenation of CO2 via EC intermediate to coꢀproduce
θ
methanol and EG (Reaction 4) was exothermic (
131.9 kJ/mol) and thermodynamically favorable (
= ꢀ
= ꢀ
ꢀr H m
steps was monitored by TCD. The Cu dispersion (
DCu) and the
ꢀrGθm
metallic Cu specific surface area (
equations described by Liu et al.16
SCu) were calculated by the
36.32 kJ/mol). Hence, it is easier to synthesize methanol and
EG by indirect hydrogenation of CO2 via EC intermediate. It
should be noting that EC hydrogenation is also exothermic (
Transmission electron microscopy (TEM) and high
resolution transmission electron microscopy (HRTEM) images
were obtained by fieldꢀemission transmission electron
microscopy (JEOL, JEMꢀ2100F) operating at 200 kV. The
samples were dispersed in ethanol. Drops of the suspension
were deposited on copper grids coated with transparent carbon
foil and dried prior to measurement.
ꢀr Hθm
= ꢀ71.59 kJ/mol) and thermodynamically favorable (
ꢀrGθm
= ꢀ25.62 kJ/mol). Furthermore, EC hydrogenation is
more favored at lower temperatures, while higher temperatures
will facilitate the deep hydrogenation of EG to ethanol.
Surface chemistry of samples was examined by Xꢀray
photoelectron spectroscopy (XPS) and Xꢀray Auger electron
spectroscopy (XAES) analyses. XPS and XAES were
performed under an ultrahigh vacuum using an ESCALAB
3 H
+
H2O
CO2 +
CH3OH
2
(1)
Scheme 1. The direct hydrogenation of CO2 to produce MeOH
O
250Xi spectrometer with Al Kα radiation (1486.6 eV) and a
multichannel detector. The collected binding energies were
calibrated using the C1s peak at 284.6 eV as the reference. The
experimental error was within ±0.2 eV.
O
+ CO2
O
O
(2)
O
2.5 Catalytic performance
Hydrogenation of ethylene carbonate (EC) was carried out in a
50ꢀmL stainless steel autoclave. In a typical run, the autoclave
was charged with 10 mmol EC, 0.176 g catalyst and 20 mL
HO
OH +
CH3OH
3
H
+
O
O
2
(3)
(4)
O
THF. pꢀXylene (100 ꢁL) was also added to the reaction mixture
3 H
+ CO2 +
HO
OH +
CH3OH
2
as the internal standard. After being sealed and flushed with H2
five times, the autoclave was pressurized with H2 to 5.0 MPa at
room temperature, and then heated up to 453 K for 4 h under
vigorous magnetic agitation. After the hydrogenation reaction
was finished, the autoclave was cooled in an iceꢀwater bath.
The residual H2 was released carefully in a hood. The reaction
Scheme 2. Reactions involved in the indirect hydrogenation of
CO2 via EC intermediate to coꢀproduce MeOH and EG: (2) the
cycloaddition reaction of EO and CO2 to produce EC, (3) the
hydrogenation of EC to produce MeOH and EG, and (4) the
sum of reaction (2) and reaction (3)
mixture with pꢀxylene as the internal standard was analyzed by
gas chromatography (Shimadzu GCꢀ2014) with a GsBPꢀ1
column (30 m × 0.32 mm × 1.0 ꢁm) and a flame ionization
detector (FID). The injection and detector temperature were
523 K and 553 K, respectively. The temperature program for
GC analysis was first held at 313 K for 5 min, then from 313 K
to 503 K at a rate of 15 K minꢀ1, and finally held at 503 K for 4
min. Relative standard deviation (RSD) for GC analysis in the
measurement was less than 0.2%.
HO
OH
H O
2
+
C2H5OH
+ H2
(5)
Scheme 3. The deep hydrogenation of EG to form ethanol
3.2 Characterization of catalysts
Pore structures of mesoporous silicas and the three copperꢀ
based catalysts were analyzed by N2 physisorption. The
corresponding isotherms and pore size distribution curves were
shown in Fig. 1, and the derived pore structure parameters
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Table 1 Reaction enthalpy and Gibbs free energy for EC hydrogenation and other related reactions at different temperatures
ꢂr
H
(kJ/mol)
ꢂrG (kJ/mol)
Reaction
298 K
ꢀ46.98
ꢀ60.31
ꢀ71.59
ꢀ131.9
ꢀ94.81
413 K
ꢀ51.97
ꢀ60.77
ꢀ75.87
ꢀ136.6
ꢀ96.22
433 K
ꢀ52.77
ꢀ60.79
ꢀ76.57
ꢀ137.3
ꢀ96.49
453 K
ꢀ53.54
ꢀ60.80
ꢀ77.23
ꢀ138.0
ꢀ96.75
473 K
ꢀ54.28
ꢀ60.80
ꢀ77.87
ꢀ138.7
ꢀ97.02
493 K
ꢀ55.00
ꢀ60.78
ꢀ78.51
ꢀ139.3
ꢀ97.30
298 K
6.01
413 K
27.34
433 K
31.20
11.92
ꢀ3.78
8.14
453 K
35.10
15.28
ꢀ0.41
14.87
473 K
39.02
18.63
3.00
493 K
42.98
21.99
6.43
1
2
3
4
5
ꢀ10.70 8.56
ꢀ25.62 ꢀ7.13
ꢀ36.32 1.43
ꢀ105.1 ꢀ108.8
21.63
ꢀ110.6
28.43
ꢀ111.2
ꢀ109.4 ꢀ110.0
were listed in Table 2. As shown in Fig. 1A, N2 physisorption
isotherms for KITꢀ6, MCMꢀ41 and SBAꢀ15 supports distinctly
exhibited typical type IV isotherms (IUPAC classification).23, 24,
28
In addition, KITꢀ6 presented a sharp capillary condensation
at a relative pressure of 0.60ꢀ0.80 and an H1 type hysteresis
loop, indicating that KITꢀ6 possessed large channelꢀlike pores
with a narrow pore size distribution.23 As shown in Table 2, a
narrow pore size distribution with mean value of 5.88 nm was
determined for KITꢀ6. SBAꢀ15 displayed a sharp capillary
condensation at a relative pressure of ~0.65ꢀ0.80 and an H1
type hysteresis loop, suggesting that SBAꢀ15 had a uniform
mesoporous structure with a narrow pore size distribution. It
could be seen in Table 2 that SBAꢀ15 had an average pore size
of 7.53 nm. Compared with KITꢀ6 and SBAꢀ15, MCMꢀ41
presented a capillary condensation in a wider P/P0 range of
0.30ꢀ0.95 and a smaller H1 type hysteresis loop, indicating that
MCMꢀ41 possessed smaller pores with a more narrow pore size
distribution. As exhibited in Table 2, MCMꢀ41 possessed the
smallest average pore size of 3.79 nm. Based on the above
analysis results, KITꢀ6, MCMꢀ41 and SBAꢀ15 supports
employed in this work possessed obvious mesoporous structure.
The total pore volumes of KITꢀ6, MCMꢀ41 and SBAꢀ15
supports were distributed in a very narrow range of 1.10ꢀ1.21
cm3 gꢀ1. However, there were great differences in BET surface
areas and average pore sizes for KITꢀ6, MCMꢀ41 and SBAꢀ15
supports. Specifically, the BET surface area followed the order
of MCMꢀ41> KITꢀ6 > SBAꢀ15, while the average pore size
followed the order of SBAꢀ15 > KITꢀ6 > MCMꢀ41.
After loading copper onto the three mesoporous silicas
supports, the corresponding copperꢀbased catalysts Cu/KITꢀ6,
Cu/MCMꢀ41 and Cu/SBAꢀ15 still possessed typical type IV
isotherms with H1 type hysteresis loops due to capillary
condensation (Fig. 1A), characteristic of mesoporous materials.
Upon loading copper onto SBAꢀ15, the resulted Cu/SBAꢀ15
exhibited a sharp capillary condensation in a slightly wider P/P0
range of ~0.60ꢀ0.85 with an H1 type hysteresis loop similar to
SBAꢀ15, resulting in a slight increase on the average pore
diameter of Cu/SBAꢀ15 to 9.13 nm. Upon loading copper onto
KITꢀ6 and MCMꢀ41, the resulted Cu/KITꢀ6 and Cu/MCMꢀ41
exhibited H1 type hysteresis loops which were greatly different
from those of their supports. In contrast to KITꢀ6, Cu/KITꢀ6
clearly showed a much wider H1 type hysteresis loop with a
capillary condensation at a relative pressure of 0.60ꢀ0.95,
demonstrating that Cu/KITꢀ6 possessed wider pore size
distribution. As shown in Table 2, the average pore diameter of
Cu/KITꢀ6 was increased to 9.69 nm. Compared with MCMꢀ41,
Cu/MCMꢀ41 displayed a more evident H1 type hysteresis loop
Fig. 1 N2 physisorption isotherms (A) and pore distribution curves (B)
of mesoporous silicas and the corresponding copperꢀbased catalysts.
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Table 2 Textural and structural properties of mesoporous silicas supports and copperꢀbased catalysts
Sample
Cu loadinga (%)
SBETb (m2 gꢀ1)
820
Vporeb (cm3 gꢀ1)
1.21
Dporeb (nm)
5.88
DCuc (%)
SCuc (m2 gꢀ1)
KITꢀ6
ꢀ
ꢀ
ꢀ
MCMꢀ41
SBAꢀ15
ꢀ
1163
628
1.10
3.79
ꢀ
ꢀ
ꢀ
1.18
7.53
ꢀ
ꢀ
Cu/KITꢀ6
Cu/MCMꢀ41
10.0
9.7
10.0
438
1.06
9.69
8.2
6.8
26.2
53
44
170
312
0.73
9.41
Cu/SBAꢀ15
446
1.02
9.13
a
b
c
Cu loading determined by ICPꢀAES. Pore properties calculated from N2 physisorption. Cu dispersion (
obtained from N2O titration.
D )
Cu) and Cu0 surface area (SCu
with a capillary condensation at a relative pressure of 0.45ꢀ0.95.
As for Cu/MCMꢀ41, almost all the mesopores at ~2.7 nm
disappeared and a large number of mesopores at a wide range
of ca. 6.5ꢀ12.3 nm emerged upon loading copper onto MCMꢀ41
(Fig. 1B), indicating that agglomerated copper particles filled
up the mesopore of ~2.7 nm in MCMꢀ41. Therefore, the
average pore diameter of Cu/MCMꢀ41 was greatly increased to
9.41 nm. The total pore volumes of Cu/KITꢀ6 and Cu/SBAꢀ15
were slightly decreased to 1.06 cm3 gꢀ1 and 1.02 cm3 gꢀ1,
respectively, while that of Cu/MCMꢀ41 was remarkably
decreased to 0.73 cm3 gꢀ1. Notably, the BET surface area of
Cu/KITꢀ6, Cu/MCMꢀ41 and Cu/SBAꢀ15 was 438 m2 gꢀ1, 312
m2 gꢀ1and 446 m2 gꢀ1, respectively, which showed a dramatic
decrease compared with pure supports. It was worth noting that
after loading copper, the decreasing degrees on BET surface
areas of the three supports were very different, e.g., the
decreasing degree for MCMꢀ41 and KITꢀ6 was about 73% and
47%, respectively, while that for SBAꢀ15 was only about 29%.
Based on the remarkable decrease on total pore volume and the
great increase on average pore diameter in Cu/MCMꢀ41 in
comparison with MCMꢀ41, it could be deduced that copper
particles blocked a large number of smaller mesopores,
resulting in the drastic decrease on BET surface area of
Cu/MCMꢀ41. Cu/KITꢀ6 and Cu/SBAꢀ15 possessed larger BET
surface areas than Cu/MCMꢀ41, mainly attributing to the less
amount of smaller mesopores blocked by copper particles over
Cu/KITꢀ6 and Cu/SBAꢀ15. The Cu loadings, Cu dispersion and
metallic Cu surface areas of the three copperꢀbased catalysts
were also summarized in Table 2. The actual Cu loadings of the
three catalysts were nearly equal, but the metallic Cu surface
areas of the three catalysts were very different. The metallic Cu
surface area clearly followed the order of Cu/SBAꢀ15 >
Cu/KITꢀ6 > Cu/MCMꢀ41. Specifically, the metallic Cu surface
area of Cu/SBAꢀ15 was 170 m2 gꢀ1 with high Cu dispersion of
26.2%, while Cu/KITꢀ6 and Cu/MCMꢀ41 possessed metallic
Cu surface areas of 53 m2 gꢀ1 and 44 m2 gꢀ1, respectively.
FTꢀIR characterization was used to investigate the evolution
of copper species. The FTꢀIR spectra of the three mesoporous
silica supports, dried precursors, calcined samples and reduced
copperꢀbased catalysts were depicted in Fig. 2. The intensive
absorption band at 1080 cmꢀ1 and a large shoulder peak at 1250
cmꢀ1 were presented in all samples, which aroused from
asymmetric stretching vibrations of SiꢀOꢀSi bonds.28 Besides,
similar phenomena were also observed in all samples for
symmetric stretching vibration at 800 cmꢀ1 and bending mode at
Fig. 2 FTꢀIR of (A) KITꢀ6, Cu/KITꢀ6ꢀD, Cu/KITꢀ6ꢀC and Cu/KITꢀ6,
(B) MCMꢀ41, Cu/MCMꢀ41ꢀD, Cu/MCMꢀ41ꢀC and Cu/MCMꢀ41, and
(C) SBAꢀ15, Cu/SBAꢀ15ꢀD, Cu/SBAꢀ15ꢀC and Cu/SBAꢀ15 samples.
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457 cmꢀ1 of SiꢀOꢀSi bonds.28 In all the dried precursors (Fig. (200) and (210) crystal planes of MCMꢀ41, indicative of the
32
2A), the presence of the characteristic δOH band at 670 cmꢀ1 for typical hexagonal structure of MCMꢀ41.31,
After loading
copper phyllosilicate28 confirmed that copper phyllosilicate was copper onto MCMꢀ41, the intensity of (100) plane of Cu/MCMꢀ
produced by ammonia evaporation method in Cu/KITꢀ6ꢀD, 41 slightly shifted toward higher angle and nearly disappeared,
Cu/MCMꢀ41ꢀD and Cu/SBAꢀ15ꢀD precursors. In addition, the presumably resulting from the collapse of the structure of
absence of representative bands of the δOH at 938 cmꢀ1 and 694 MCMꢀ41. As presented in Fig. 3C, a strong diffraction peak at
cmꢀ1 for copper hydroxide evidenced that copper hydroxide was
2θ θ
of 1.0o and two weak diffraction peaks at 2 of 1.7o and 1.9o
not formed in the three dried precursors. The weaker δOH band were observed in SBAꢀ15, corresponding to the (100), (110)
at 670 cmꢀ1 for copper phyllosilicate was also observed in and (200) crystal planes of SBAꢀ15 with twoꢀdimensional of
calcined samples. Copper phyllosilicate could be partially highly ordered mesoporous structure (p6mm).33ꢀ35 Compared
decomposed to CuO when calcined at 723 K.29 Therefore, apart with SBAꢀ15, all the diffraction peaks of Cu/SBAꢀ15 slightly
from copper phyllosilicate, CuO should also exist on the shifted toward higher angle and became much weaker. It could
calcined samples, since the calcination temperature of 673 K be deemed that the twoꢀdimensional ordered mesoporous
applied in this work could only enable partial copper structure of SBAꢀ15 was almost retained after loading copper
phyllosilicate to decompose to CuO. The characteristic bands of onto SBAꢀ15.
CuO at 575 cmꢀ1, 500 cmꢀ1 and 460 cmꢀ1 were overlapped with
The wideꢀangle XRD patterns of dried precursors, calcined
the intensive adsorption band of SiꢀOꢀSi at 457 cmꢀ1, and just samples and reduced copperꢀbased catalysts were depicted in
exhibited a shoulder peak at ~580 cmꢀ1 in the calcined samples. Fig. 4. There were no evident diffraction peaks of copper
From the above results, it was concluded that copper phyllosilicate in the three dried precursors (Fig. 4A). In
phyllosilicate existed in the three dried precursors, while copper combination with FTꢀIR characterization results, it could be
phyllosilicate and CuO coꢀexisted in the calcined samples. considered that copper phyllosilicate was well dispersed on the
Furthermore, the characteristic δOH band at 670 cmꢀ1 for copper three supports. As for the calcined samples Cu/KITꢀ6ꢀC,
phyllosilicate was absent in the three reduced copperꢀbased Cu/MCMꢀ41ꢀC and Cu/SBAꢀ15ꢀC (Fig. 4B), no obvious
catalysts, suggesting that copper phyllosilicate was entirely diffraction peaks corresponding to crystalline CuO clusters or
reduced after treatment of reduction.
copper phyllosilicate could be detected, suggesting that both
The smallꢀangle XRD patterns of mesoporous silica CuO and copper phyllosilicate in the calcined samples were
supports, calcined samples and reduced copperꢀbased catalysts highly dispersed on the three supports. After reduction of the
were displayed in Fig. 3. KITꢀ6 showed an intense diffraction calcined samples, great differences were observed in the three
peak at 2θ θ
of 1.1o with two weak diffraction peaks at 2 of 1.2o reduced copperꢀbased catalysts (Fig. 4C). As for Cu/KITꢀ6, no
and 1.9o indexed to (211), (220) and (420) reflections (Fig. 3A), metallic Cu or other copper species was obviously observed,
corresponding to the biꢀcontinuous cubic threeꢀdimensional indicating that metallic Cu or other copper species were highly
pore structure of KITꢀ6.23, 30 In comparison with KITꢀ6, the dispersed on KITꢀ6. As for Cu/MCMꢀ41, an obvious diffraction
diffraction peaks at 2
Cu/KITꢀ6 became more intense due to the incorporation of plane of Cu2O (JCPDS 05ꢀ0667). Besides, an evident
copper species. A strong diffraction peak at 2
of 2.3o and three diffraction peak at 2 of 43.5o and two weak diffraction peaks
weak diffraction peaks at 2
of 4.0o, 4.6o and 6.0o were at 2 of 50.7o and 74.1o appeared in Cu/MCMꢀ41, which were
θ θ
of 1.1o, 1.2o and 1.9o of Cu/KITꢀ6ꢀC and peak appeared at 2 of around 36.5o indexed to the (111) crystal
θ
θ
θ
θ
observed in MCMꢀ41 (Fig. 3B), corresponding to (100), (110), assigned to the (111), (220) and (220) crystal planes of fcc
Fig. 3 Smallꢀangle XRD patterns of (A) KITꢀ6, Cu/KITꢀ6ꢀC and Cu/KITꢀ6, (B) MCMꢀ41, Cu/MCMꢀ41ꢀC and Cu/MCMꢀ41, and (C) SBAꢀ15,
Cu/SBAꢀ15ꢀC and Cu/SBAꢀ15 samples.
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Fig. 5 H2ꢀTPR of Cu/KITꢀ6, Cu/MCMꢀ41 and Cu/SBAꢀ15 catalysts.
Cu (JCPDS 04ꢀ0836), suggesting that relatively large Cu2O and
metallic Cu particles were dispersed on MCMꢀ41. As for
Cu/SBAꢀ15, there was an extremely weak diffraction peak
indexed to the (111) crystal plane of Cu2O, characteristic of
very small Cu2O particles. Besides, no obvious diffraction
peaks of metallic Cu were observed in Cu/SBAꢀ15, implying
that metallic Cu was highly dispersed on SBAꢀ15.
H2ꢀTPR profiles of copperꢀbased catalysts were depicted in
Fig. 6. According to the previous work,21 reduction of wellꢀ
dispersed CuO to Cu0 and reduction of copper phillosilicate to
Cu2O occurred at the same temperature of ~516 K, while the
reduction of bulkier CuO particles to Cu0 occurred at a higher
temperature of 540 K. Therefore, in combination with FTꢀIR
and XRD results, the single reduction peak at 468 K for
Cu/KITꢀ6 in this work was attributed to the collective
contribution of the reduction of copper phillosilicate and wellꢀ
dispersed CuO particles. As for Cu/MCMꢀ41, the major sharp
reduction peak at lower temperature of 466 K was attributed to
the collective contribution of the reduction of both copper
phillosilicate and wellꢀdispersed CuO particles, while the very
wide weak reduction peak at higher temperature of 559 K was
ascribed to the reduction of larger CuO particles which was
more difficult. As for Cu/SBAꢀ15, the single sharp reduction
peak at 467 K was also attributed to the collective contribution
of the reduction of copper phillosilicate and wellꢀdispersed
CuO particles.
TEM images of the mesoporous silica supports and copperꢀ
based catalysts were exhibited in Fig. 6. Orderly mesoporous
channel structure could be clearly seen in KITꢀ6, MCMꢀ41 and
SBAꢀ15 supports. For the three copperꢀbased catalysts Cu/KITꢀ
6, Cu/MCMꢀ41 and Cu/SBAꢀ15, the corresponding lattice
fringe spacings of 0.21 nm in HRTEM were clearly observed,
which were assigned to the Cu (111) plane, confirming the
presence of Cu nanoparticle. The size distribution histograms
Fig. 4 Wideꢀangle XRD patterns of (A) dried precursors, (B) calcined
samples and (C) reduced copperꢀbased catalysts.
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within the range of 942ꢀ945 eV. According to the reported
literatures,19, 36ꢀ38 it was very difficult to distinguish CuO and
copper phyllosilicate, because the Cu 2p3/2 binding energy of
CuO was located at 933.5 eV, while that of copper
phyllosilicate was located at 934.9 eV. In combination with FTꢀ
IR, XRD and H2ꢀTPR results, both copper phyllosilicate and
CuO coꢀexisted in the form of Cu2+ on the surfaces of the
calcined samples. XPS and XAES spectra of the three reduced
copperꢀbased catalysts were shown in Fig. 8. After reduction of
the three calcined samples Cu/KITꢀ6ꢀC, Cu/MCMꢀ41ꢀC and
Cu/SBAꢀ15ꢀC, the corresponding reduced copperꢀbased
catalysts Cu/KITꢀ6, Cu/MCMꢀ41 and Cu/SBAꢀ15 possessed Cu
2p3/2 binding energy located around 933.2 eV with no satellite
peaks of Cu2+ detected at 942ꢀ945 eV (Fig. 8A), demonstrating
Cu2+ in the calcined samples were completely reduced to Cu0
and/or Cu+. Since the Cu 2p energies of Cu0 and Cu+ were
almost identical from XPS spectra, XAES was employed to
further distinguish between Cu0 and Cu+ species. As displayed
in Fig. 8B, two overlapping Cu LMM Auger kinetic energy
peaks at ca. 912 eV and 915 eV were detected in the three
copperꢀbased catalysts. The modified Auger parameter and
Cu+/(Cu++Cu0) molar ratio were listed in Table 3. The Auger
parameter value at ca. 1845 eV was attributed to Cu+, while that
at ca. 1848 eV was attributed to Cu0, suggesting that Cu0 and
Cu+ coꢀexisted on the surfaces of the three copperꢀbased
catalysts. In addition, Cu+ was attributed to the reduction of
copper phyllosilicate, and Cu0 was ascribed to the reduction of
CuO. It was also found that Cu/SBAꢀ15 catalyst possessed a
higher Cu+/(Cu++Cu0) molar ratio of 0.65, while Cu/KITꢀ6 and
Cu/MCMꢀ41 possessed lower Cu+/(Cu++Cu0) molar ratio of
0.56 and 0.57, respectively.
Fig. 6 TEM of KITꢀ6, MCMꢀ41 and SBAꢀ15 supports (left) and
Cu/KITꢀ6, Cu/MCMꢀ41 and Cu/SBAꢀ15 catalysts (right).
for the three copperꢀbased catalysts were also obtained by
manually measuring the particles from TEM images. As for
Cu/KITꢀ6, copper particles were uniformly distributed on KITꢀ
6 support with the average size of 3.5 nm. As for Cu/SBAꢀ15,
copper particles were uniformly distributed on SBAꢀ15 support
with the average size of 4.3 nm. Compared with Cu/KITꢀ6 and
Cu/SBAꢀ15, Cu/MCMꢀ41 possessed larger copper particles
with the average size of 6.3 nm which were also uniformly
distributed on MCMꢀ41. The tendency for the average sizes of
copper particles followed the sequence of Cu/MCMꢀ41 >
Cu/SBAꢀ15 > Cu/KITꢀ6, which was in accordance with wideꢀ
angle XRD results.
The surface chemical states and surface compositions were
analyzed by XPS and XAES. XPS spectra of the calcined
samples were shown in Fig. 7. Cu 2p3/2 binding energy of the
three calcined samples Cu/KITꢀ6ꢀC, Cu/MCMꢀ41ꢀC and
Cu/SBAꢀ15ꢀC were located within a wide range of 933.7 eV to
Fig. 7 XPS spectra of calcined Cu/KITꢀ6ꢀC, Cu/MCMꢀ41ꢀC and
Cu/SBAꢀ15ꢀC samples.
936.0 eV, accompanying with 2p
→
3d satellite peaks of Cu2+
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Table 3 Assignments of copper species in copperꢀbased catalysts derived from Cu LMM XAES spectra
Binding Energy (eV)
Kinetic Energy (eV)
Auger parametera (eV)
Cu+/(Cu++Cu0)b
(mol ratio)
Catalyst
Cu 2p3/2
933.5
Si 2p
103.6
103.5
103.5
Cu+
Cu0
Cu+
Cu0
Cu/KITꢀ6
911.8
912.1
912.7
915.0
915.6
915.3
1845.3
1845.3
1845.7
1848.5
1848.8
1848.3
0.56
0.57
0.65
Cu/MCMꢀ41
Cu/SBAꢀ15
933.2
933.0
a obtained by sum of binding energy and kinetic energy. b calculated from Cu LMM XAES spectra.
3.3 Catalytic performances
The catalytic performances of Cu/KITꢀ6, Cu/MCMꢀ41 and
Cu/SBAꢀ15 employed as heterogeneous catalysts for EC
hydrogenation were investigated, and the results were
summarized in Table 4. When the reaction was conducted at
453 K for 4 h, the activity for EC hydrogenation clearly
followed the sequence of Cu/SBAꢀ15 > Cu/KITꢀ6 > Cu/MCMꢀ
41. It was worth noting that the TON towards EG and MeOH
for Cu/SBAꢀ15 were 22.0 and 11.4, respectively. In addition,
8
TON of two reported CuCr2O4 and CuꢀSiO2ꢀPG16 catalysts for
EC hydrogenation under the optimized condition in the batch
reactor were also calculated for comparison. It was found that
compared with the TON towards EG and MeOH for Cu/SBAꢀ
8
15 used in this work, those for the two reported CuCr2O4 and
CuꢀSiO2ꢀPG16 catalysts were relatively lower. The TON
towards EG and MeOH over CuCr2O4 were 4.1 and 2.7,
respectively. As for CuꢀSiO2ꢀPG, the TON towards EG and
MeOH were 8.8 and 8.7, respectively. The above results
indicated that Cu/SBAꢀ15 was a superior catalyst for EC
hydrogenation. When further increasing the reaction
temperature from 453 K to 473 K, EC was completely
converted over Cu/SBAꢀ15, while EC conversion over Cu/KITꢀ
6 and Cu/MCMꢀ41 were 81.6% and 65.0%, respectively.
Moreover, the selectivities of EG and MeOH achieved over the
three catalysts followed the order of Cu/SBAꢀ15 > Cu/KITꢀ6 >
Cu/MCMꢀ41. Especially, the EG selectivity and MeOH
selectivity achieved over Cu/MCMꢀ41 were respectively less
than 60% and 25%, while those over Cu/SBAꢀ15 and Cu/KITꢀ6
were more than 90% and 48%. Therefore, SBAꢀ15 is a better
support for obtaining higher activity and selectivity of copperꢀ
based catalysts than the other two mesoporous KITꢀ6 and
MCMꢀ41 supports. Notably, the sequence for catalytic
performances of the three catalysts accorded with the metallic
Cu surface area of catalysts. Additionally, Cu+/(Cu++Cu0)
molar ratio of Cu/KITꢀ6 and Cu/MCMꢀ41 was almost identical
(Table 3), but the TON towards EG and MeOH for Cu/KITꢀ6
which possessed larger metallic Cu surface area were higher
than that of Cu/MCMꢀ41. Therefore, Cu0 was considered to be
the active site for the activity of catalysts. On the other hand,
the metallic Cu surface area of Cu/SBAꢀ15 was more than three
times higher than that of Cu/KITꢀ6 and Cu/MCMꢀ41, but the
TON towards EG and MeOH for Cu/SBAꢀ15 were less than
three times of that of Cu/KITꢀ6 and Cu/MCMꢀ41, implying that
Cu+ also had great impact on EC hydrogenation. As shown in
Table 3, Cu/SBAꢀ15 possessed higher Cu+/(Cu++Cu0) molar
ratio of 0.65, which could facilitate the faster conversion of
Fig. 8 Cu 2p XPS spectra (A) and Cu LMM XAES spectra (B) of Cu/KITꢀ6,
Cu/MCMꢀ41 and Cu/SBAꢀ15 catalysts.
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Table 4 Catalytic performances of copperꢀbased catalysts Cu/KITꢀ6, Cu/MCMꢀ41 and Cu/SBAꢀ15 in EC hydrogenation
b
Catalyst
Cu
loadinga
SCu
Total amount on the
T
EC
Yield
Sel.
(%)
EG
TONd
(m2 catalyst surfacec (ꢁmol)
gꢀ1)
(K)
Conv. (%)
(mol/mol)
Cu0 Cu+
Cu0+Cu+
EG
MeOH
21.1
41.1
7.7
MeOH
48.2
50.4
18.6
23.4
50.7
62.3
EG
MeOH
(%)
(%)
453
473
453
473
453
473
43.8
81.6
41.4
65.0
62.1
100
39.4
75.7
22.8
29.6
60.9
94.7
90.0
92.8
55.1
45.5
98.1
94.7
14.2 7.6
Cu/KITꢀ6
10.0
9.7
53
23
18
29
52
8.5
2.5
Cu/MCMꢀ41
Cu/SBAꢀ15
44
24
42
15.2
31.5
62.3
22.0 11.4
10.0
170 73
136
209
Reaction Conditions: 10 mmol EC, 5 MPa H2, 20 mL THF, 20 wt.% catalyst loading (based on the weight of EC), and 4 h.
a Cu loading obtained by ICPꢀAES; b Cu0 surface area obtained from N2O titration; c Total amount of Cu0 species on the catalyst surface determined
from N2O titration, while total amount of Cu+ species on the catalyst surfacedetermined from Cu LMM XAES; d TON calculated from the products
generating per total copper amount (mol/mol).
intermediates during the process of EC hydrogenation. In order 413 K to 473 K (Entries 1ꢀ4), EC conversion was remarkably
to have a deeper insight into the relation between the catalytic enhanced from
activity and the total amount of Cu0 and Cu+ species on the
catalyst surface, the total amount of Cu0 and Cu+ species on the Table 5 Effects of reaction conditions on the activity and selectivity
catalyst surface was also calculated. As shown in Table 4, the over Cu/SBAꢀ15 in EC hydrogenation
total amount of Cu0 species on the surface of Cu/SBAꢀ15,
Yield (%)
Sel. (%)
T
t
EC Conv.
Entry
Cu/KITꢀ6 and Cu/MCMꢀ41 were 73, 23 and 18 ꢁmol,
respectively. The total amount of Cu+ species on the surface of
Cu/SBAꢀ15, Cu/KITꢀ6 and Cu/MCMꢀ41 were 136, 29 and 24
ꢁmol, respectively. As could be seen, the total amount of Cu0
and Cu+ species on the catalyst surface followed the order of
Cu/SBAꢀ15 > Cu/KITꢀ6 > Cu/MCMꢀ41. It should be noted that
the total amount of Cu0 species and Cu+ species on the surface
of Cu/KITꢀ6 was 1.3 times and 1.2 times higher than that of
Cu/MCMꢀ41, respectively, but the TON towards EG and
MeOH over Cu/KITꢀ6 were 1.7 times and 3.0 times higher than
those over Cu/MCMꢀ41, suggesting that the synergetic effect of
Cu0 and Cu+ species had influenced the catalytic performances.
It was suggested that in methyl acetate hydrogenation, Cu0
species activated H2 and Cu+ species adsorbed the carbonyl
group.39 Cu+ could act as the stabilizer of the methoxy and acyl
species. Besides, Cu+ could also function as electrophilic or
Lewis acidic sites to polarize the C=O bond via the electron
lone pair in oxygen, thus improving the reactivity of the ester
group.39 Therefore, based on the above results, the synergetic
effect between Cu0 and Cu+ species was responsible for the
higher activity of Cu/SBAꢀ15. Moreover, the different roles that
Cu0 species dissociated H2 and Cu+ species absorbed the
carbonyl group of EC could be proposed in EC hydrogenation
over Cu/SBAꢀ15 catalyst.
(K) (h) (%)
EG
MeOH
EG
MeOH
1
2
3
4
5
6
7
8
413
433
453
473
493
473
473
473
4
4
4
4
4
2
3
6
16.0
23.8
62.1
100
15.2 4.2
23.2 12.9
60.9 31.5
94.7 62.3
92.4 46.1
55.4 39.8
80.0 59.9
94.8 59.8
95.0 26.3
97.5 54.2
98.1 50.7
94.7 62.3
92.4 46.1
96.9 69.6
93.3 69.9
94.8 59.8
100
57.2
85.7
100
Reaction Conditions: 10 mmol EC, 5 MPa H2, 20 mL THF, 20 wt.%
catalyst loading (based on the weight of EC).
16.0% to 100%, while EG yield was greatly improved from
15.2% to 94.7% and MeOH yield was dramatically increased
from 4.2% to 62.3%. Further increasing the reaction
temperature to 493 K (Entry 5), EC was completely converted,
EG yield was slightly decreased to 92.4%, and MeOH yield
was also decreased to 46.1%
，indicating that too high reaction
temperature was not favorable to obtain high selectivities of EG
and MeOH. Hence, from the viewpoints of activity and
selectivity, 473 K was selected as the suitable reaction
temperature.
3.4 Effects of reaction conditions
Then, the effect of reaction time was further investigated in
the range of 2 hꢀ6 h at 473 K. When the reaction was conducted
at 473 K for 2 h (Entry 6), 57.2% of EC conversion with 55.4%
of EG yield and 39.8% of MeOH yield was obtained. As the
reaction time was prolonged to 3 h (Entry 7), EC conversion
was increased to 85.7%, while EG yield and MeOH yield were
increased to 80.0% and 59.9%, respectively. Furthermore, EC
was completely converted with good EG selectivity of 94.7%
and MeOH selectivity of 62.3% at a short reaction time of 4 h
(Entry 4). However, no evident improvement of EG yield and
MeOH yield were observed when further increasing reaction
The effects of reaction conditions such as reaction temperature
and reaction time on the activity and selectivity in EC
hydrogenation were further investigated using Cu/SBAꢀ15 as
the catalyst, and the results were summarized in Table 5.
At first, the effect of reaction temperature was investigated
in the range of 413 Kꢀ493 K. As expected, the higher reaction
temperature accelerated the EC hydrogenation rate.
Specifically, when the reaction temperature was increased from
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time to 6 h (Entry 8). Therefore, 4 h was regarded as the proper
reaction time.
In a word, among the reaction conditions investigated,
100% of EC conversion, 94.7% of EG yield and 62.3% of
MeOH yield were obtained within a short reaction time of 4 h
over Cu/SBAꢀ15 catalyst with a low copper loading of 10 wt.%.
3.5 Catalyst reusability
The reusability of Cu/SBAꢀ15 catalyst was preliminarily
evaluated, and the results were listed in Table 6. The catalyst
after the first run was centrifuged, washed with THF, and then
directly used in the second run with fresh reactants. In the
second run, EC conversion, EG yield and MeOH yield were
decreased to 68.8%, 58.0% and 46.5%, respectively (Run 2). In
order to eliminate the likely oxidation of the used catalyst due
to the inevitable exposure to air during recycling experiments,
then the catalyst after the second run was centrifuged, washed
with THF, dried in a vacuum oven, reduced in 10 vol.% H2/N2
at 623 K for 4 h, and then reused in the third run with fresh
reactants. However, continuous decreases with lower EG and
MeOH yields were still observed (Run 3). Therefore, XRD and
TEM characterization of the catalyst after the third run
(Cu/SBAꢀ15ꢀ3rd) were used to check the changes of the used
catalyst. As depicted in Fig. 9A, the diffraction peak of Cu2O
was more intensified for the used Cu/SBAꢀ15ꢀ3rd catalyst than
the fresh Cu/SBAꢀ15 catalyst, indicating of larger Cu2O
particles in the used Cu/SBAꢀ15ꢀ3rd catalyst. In addition,
compared with the fresh Cu/SBAꢀ15 catalyst (Fig. 6), copper
particles of used Cu/SBAꢀ15ꢀ3rd catalyst were a bit larger with
an average size of 4.5 nm (Fig. 9B). Hence, the agglomeration
of Cu and Cu2O nanoparticles were responsible for the
decreased catalytic activity of Cu/SBAꢀ15 during recycling
experiments. It was reported that the catalytic activity and
stability of Cu/SiO2 catalyst could be successfully enhanced
after doping with lanthanum oxide40, boric oxide41, Ag42 or
Au43. Hence, it is anticipated that the reusability and structure
stability of Cu/SBAꢀ15 catalyst can be improved by doping
suitable amount of catalyst promoters such as lanthanum oxide,
boric oxide, Ag or Au into Cu/SBAꢀ15.
Fig. 9 XRD pattern (A) and TEM image (B) of the fresh and used
Cu/SBAꢀ15 catalysts.
4. Conclusions
In summary, thermodynamic data of different reactions
associated with EC hydrogenation were calculated by DFT for
the first time. The thermodynamic calculation showed that
indirect hydrogenation of CO2 via EC intermediate is more
thermodynamically favored than direct hydrogenation of CO2.
Table 6 Reusability of Cu/SBAꢀ15 in EC hydrogenation
Yield (%)
EG
Sel. (%)
EG
EC Conv.
(%)
Run
MeOH
62.3
46.5
7.8
MeOH
62.3
1
100.0
68.8
32.6
94.7
94.7
2a
3b
58.0
84.3
67.6
ꢀr Hθm
ꢀrGθm
In addition,
and
of EC hydrogenation are ꢀ71.59
21.2
65.0
23.9
kJ/mol and ꢀ25.62 kJ/mol, respectively. Three copperꢀbased
catalysts Cu/SBAꢀ15, Cu/KITꢀ6 and Cu/MCMꢀ41 were
successfully prepared by facile ammonia evaporation method
using ordered mesoporous SBAꢀ15, KITꢀ6 and MCMꢀ41 as
supports. The asꢀprepared catalysts were utilized as
heterogeneous catalysts for EC hydrogenation to coꢀproduce
EG and MeOH. The properties of the three copperꢀbased
catalysts were characterized by various techniques and were
significantly influenced by supports. Catalyst characterization
revealed that the ordered structures of Cu/SBAꢀ15 and Cu/KITꢀ
6 catalysts were retained, while that of Cu/MCMꢀ41 was
Reaction Conditions: 10 mmol EC, 5 MPa H2, 20 mL THF, 20 wt.%
catalyst loading (based on the weight of EC), 473 K, and 4 h.
a Catalyst obtained after the first run and washed with THF.
b
Catalyst obtained after the second run, washed with THF, dried at 353
K in a vacuum oven, and reduced in 10 vol.% H2/N2 at 623 K for 4 h.
This journal is © The Royal Society of Chemistry 2016
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DOI: 10.1039/C6RA06464G
RSC Advances
Paper
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The authors gratefully acknowledge financial supports from
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12 |RSC Adv., 2016, 00, 1-3
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DOI: 10.1039/C6RA06464G
Influence of support on the performance of copper catalysts for the
effective hydrogenation of ethylene carbonate to synthesize
ethylene glycol and methanol
Fengjiao Li,ab Liguo Wang,*a Xiao Han,a Peng He,a Yan Cao,a and Huiquan Li*a
a.Key Laboratory for Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of
Process and Engineering, Chinese Academy of Sciences, Beijing, 100190, P. R. China
b.University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
* Corresponding authors:
E-mail: lgwang@ipe.ac.cn; hqli@ipe.ac.cn; Fax: +86 10 62621355; Tel: +86 10 82544825
Graphical contents entry:
Mesoporous silica supported copper catalysts were prepared by ammonia evaporation
method using KIT-6, MCM-41 and SBA-15 as support materials. Cu/SBA-15 with higher
copper dispersion exhibited a superior activity for the hydrogenation of ethylene carbonate to
co-produce ethylene glycol and methanol. The synergistic effect between metallic Cu0 and Cu+
species was considered to play a crucial role for obtaining a higher catalytic activity.
1