Continuous heterogeneous hydrogenation of CO2-derived dimethyl carbonate to methanol over a Cu-based catalyst

Article abstract of DOI:10.1039/c6ra14447k

Catalytic mild hydrogenation of carbonates synthesized from the CO₂ captured by alcohols is of significant interest both conceptually and practically, which provides alternative approaches to the indirect hydrogenation of CO₂ to methanol. However, the catalytic efficiency of the process of dimethyl carbonate (DMC) hydrogenation still remains unsatisfactory. Unprecedented gas-solid phase fix-bed catalyzed hydrogenation of DMC to methanol has been made. Herein, we address this issue with facile Cu/SiO₂ catalysts, reaching complete conversion and highly selective formation of methanol (up to 80% yield) under mild conditions (503 K, 2.5 MPa), which is the highest yield that has been reported. Characterization results indicate that the copper dispersion and the synergetic effect between balanced Cu⁰ and Cu⁺ sites are considered to play critical role in attaining high yield of methanol. The optimized Cu/SiO₂ catalyst displayed excellent catalytic performance in the hydrogenation of diethyl carbonate (DEC) and di-n-propyl carbonate (DPC) as well, presenting a broad substrate scope. Moreover, the comparative catalytic activity (～76% methanol yield) in the hydrogenation of DMC under solvent-free conditions shows promise for its scalable application in industry.

Full text of DOI:10.1039/c6ra14447k

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Continuous heterogeneous hydrogenation of CO2-derived
dimethyl carbonate to methanol over Cu-based catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Yuanyuan Cui a, Chen Xi,a and Wei-Lin Daia*
Catalytic mild hydrogenation of carbonates synthesized from the CO2 captured by alcohols is of significant interest both
conceptually and practically, which provides alternative approaches to the indirect hydrogenation of CO2 to methanol.
However, the catalytic efficiency of the process of dimethyl carbonate (DMC) hydrogenation still remains unsatisfactory.
Unprecedented gas-solid phase fix-bed catalyzed hydrogenation of DMC to methanol has been made. Herein, we address
this issue with the facile Cu/SiO2 catalysts, reaching complete conversion and highly selective formation of methanol (up to
80% yield) under mild condition (503 K, 2.5 MPa), which is the highest yield that has been reported. Characterization
results indicate that the copper dispersion and the synergetic effect between balanced Cu0 and Cu+ sites are considered to
play a critical role in attaining high yields of methanol. The optimized Cu/SiO2 catalyst displayed excellent catalytic
performance in the hydrogenation of diethyl carbonate (DEC) and di-n-propyl carbonate (DPC) as well, presenting a broad
substrate scope. Moreover, the comparative catalytic activity (~76% methanol yield) in the hydrogenation of DMC under
solvent-free condition shows promise its scalable application in the industry.
DOI: 10.1039/x0xx00000x
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carbon dioxide into the atmosphere is pushing toward the use of
‘‘renewable carbon”, so to avoid as much as possible burning ‘‘fossil
carbon”.5 Thus, it would be more valuable to develop new industrial
1. Introduction
CO2, as a so-called greenhouse gas, has the effect of trapping the
sun’s heat and is believed to be one of the main contributors to the
presently observed global warming phenomena. The input of large
amounts of anthropogenic CO2 emitted has exceeded the needed
amount of the natural carbon cycle.1 Capturing and converting CO2
gas which as an economical, safe and renewable carbon resource to
a liquid fuel has captured much attention during the past four
decades, mainly due to its synthetic significance as an important
pathway outlined in “The Methanol Economy”2 to fundamentally
synthetic feed stock such as methanol. Nevertheless, the
transformation of chemically stable CO2 to methanol represents a
grand challenge in exploring new concepts and solutions for the
academic and industrial development of catalytic processes due to
the high activation energy barrier needed for the cleavage of the
C=O bond.3 As we know, traditional industrial Cu/ZnO/Al2O3 exhibits
promising performance for MeOH production from CO and H2
(syngas) with a space-time yield up to 0.842 gMeOH·g-1Catal·h-1 under
certain conditions, and this process is the dominant one for MeOH
production.4 However, syngas is a synonym of ‘‘fossil carbon”, as it
is generated from fossil fuels, like coals, crude oils and CH4. Along
with the resource exhaustion, the need to reduce the emission of
processes for converting CO2 that can be captured from
atmosphere into “working carbon”. The solar-driven reduction of
CO2 into chemicals and fuels such as formic acid, methane or
methanol has been considered as a promising approach to address
the problems of global warming and energy crisis.6 However, most
of reported solar-active catalysts for CO2 photo-reduction suffer
from low energy conversion efficiency, uncontrollable selectivity,
instability and incapability to completely suppress the competing
hydrogen evolution reaction in the presence of water. The
conversions of CO2 into more valuable products such as CH4, CH3OH
or C2H4, which involve multiple proton-coupled electron transfer,
have only been demonstrated with low conversion efficiency and
selectivity.7 Furthermore, many possible products of CO2 photo-
reduction could be presented both in the gaseous and liquid
phases, which makes the product separation and detection quite a
complex process. Hence, the design and fabrication of highly active
photocatalytic systems with high conversion efficiency and
selectivity for CO2 reduction remain a grand challenge. Electrolysis
of water, driven by electricity that is derived from renewable
energy sources, could provide hydrogen as an ideal energy carrier
for clean and sustainable energy technologies, which has been
industrialized. Besides, the research of photocatalytic
decomposition of water to hydrogen energy with solar energy has
made some progress, which can also provide an inexpensive
approach for the H2 production.
a.Department of chemistry & Shanghai key laboratory of molecular catalysis and
innovative material, Fudan University, Shanghai 200433, P. R. China. E-mail:
wldai@fudan.edu.cn; Fax: +86 021 5566 5572; Tel: +86 021 5566 4678
† Footnotes relaꢀng to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Conventional processes for CO2 hydrogenation involving gas
phase catalytic conversion over Cu-Zn based catalysts was
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the problem of excess CO2, but also provide a low cost pathway for
the DMC production. Up to now, no gas-solid phase heterogeneous
continuous hydrogenation of DMC has been reported until now.
Herein, we report our preliminary results in the vapor phase
hydrogenation of DMC with copper-silica catalysts prepared by
evaporating ammonia (AE) method which can not only conveniently
and effectively disperse copper species on silica but also the high
catalytic activity for hydrogenation of DMC to MeOH results from
the copper particle size and the cooperative effect between Cu0 and
Cu+. Accordingly, a series of silica supported copper catalysts with
different copper contents were fabricated and the relationship
between the catalysts structure and activities were investigated.
proceeded at high pressure and temperatures (503-543 K), where
methanol production was unfavourable in consideration of the
reaction is exothermic (ΔH298K= −49.5 kJ/mol).8-11 CuO/ZnO/Al2O3
catalysts presents methanol productivity of 0.311 g·g-1Catal·h-1 at 553
K and 5 Mpa.12 Traditional Cu/ZnO/Al2O3 catalysts would lead to
huge amount of CO as a by-product of methanol via the reverse
water-gas shift (rWGS) reaction. As we know, the production of CO
not only reduces the yield of methanol but also has a negative
effect when methanol is used in fuel cells because CO poisons the
metal-based catalyst used.13 In view of the great challenge exists in
highly efficient production of methanol from CO2 by direct
hydrogenation under mild conditions,14,15 indirect hydrogenation of
CO2 has been successfully achieved resulting in establishing a bridge
from CO2 to methanol.
2. Experimental
The hydrogenation of a series of CO2 derivatives including
carbonates, carbamates, urea derivatives, formates, polycarbonates
and so on, were investigated in the laboratory and in industry.16-19
Milstein and coworkers pioneered this reaction process and
reported the highly catalytic hydrogenation of dimethyl carbonate
to methanol using a homogeneous dearomatized PNN/RuII pincer
complex under serious conditions (383 K and 5 MPa H2).20 Ding and
coworkers have developed a novel CO2 indirect conversion process
by selective hydrogenation of cyclic carbonates from readily
available CO2 and epoxides to give methanol and the corresponding
ethylene glycol with excellent catalytic efficiency.17 Considering the
difficulties in industrial production of homogenous catalysts in
terms of stability, separation, handling, and reuse, heterogeneous
catalyst system for the hydrogenation of carbonates under mild
conditions with ease of catalyst separation and recycling from
industrial viewpoint is in urgent demand. Recently, Li and
coworkers firstly achieved the heterogeneous hydrogenation of
cyclic ethylene carbonate (EC) over a copper-chromite nanocatalyst
with moderate selectivity (60%) for the formation of methanol and
good selectivity for production of EG (93%) at 453 K and 5 MPa H2,21
however, its practical application might be hindered to some extent
due to environmentally unfriendly condition. Liu22 and Chen23
reported the application of copper-silica nanocomposite in the
hydrogenation of EC, affording methanol and EG in higher yields in
batch and fixed-bed continuous flow reactors under relatively mild
conditions respectively. Nevertheless, the solid state of EC at room
temperature and the separation of hydrogenation products may
limit its industrial production. Notably, the liquid phase product of
DMC hydrogenation need no separation owing to the unitary MeOH
as product compared with other carbonates, while the
hydrogenation of linear dimethyl carbonate is particularly difficult
due to the two adjacent active methoxyl groups.24-25 Very recently,
M. Tamura reported the Cu/CeO2 as an effective catalyst for
methanol synthesis from DMC hydrogenation with 94% yield in a
batch reactor under 433 K and 6-8 MPa.26 The hydrogenation of
DMC technology can be combined with the synthesis of DMC to
accomplish the fixation and indirect transformation of CO or CO2.
DMC obtained from the first step can be directly pumped into the
hydrogenation device without separation and purification process,
which can greatly reduce the production costs. Thus, the methanol
from this technology shows advantages in the separation and
purification compared with the traditional methanol process. DMC
can also be obtained easily by transesterification of ethylene
carbonate (EC) with methanol. The transesterification of ethylene
carbonate (EC) synthesized from ethylene oxide and carbon dioxide
with methanol, ethylene glycol as main product and DMC as co-
production of the whole process. This technology can not only solve
2.1 Catalyst preparation
A series of copper catalysts with different Cu loading were
prepared by the AE method. Briefly, 4.3 g, 6.4 g, 8.6 g and 10.7 g of
Cu(NO3)2·3H2O (A.R., Sinopharm Chemical Reagent Ltd.) yielding 20,
30, 40, and 50 wt.% of Cu in the final catalysts were dissolved in 200
mL deionized water in a beaker, into which 25 wt.% ammonia
solution (A.R., Sinopharm Chemical Reagent Ltd.) was added by a
dropper in 5 min to form a clear [Cu(NH3)4]2 solution. This step was
followed by drop-wise addition of 30 wt.% silica sol (Qingdao Grand
Chemical Co.) solution over a period of 5 min to form a light-blue
precipitate and stirred for 4 h. The initial pH of the suspension was
11-12. The suspension was heated in a water bath preheated to 363
K to allow the evaporation of ammonia. Then, the samples were
dried at 373 K for 12 h, followed by calcination at 723 K for 4 h in
air. The catalysts were pelletized, crushed, sieved to 40-60 meshes,
and denoted as xCu/SiO2 , where x represents the copper mass
fraction. The samples with 40 wt.% CuO dispersed by different
oxides (TiO2, ZrO2 and CeO2) were prepared by the same method.
The CuZnAl catalysts with 1:9 mole ratio of Cu/Zn-Al (the mole ratio
of Zn/Al was fixed at 4:5) were synthesized by the deposition-
precipitation method using Na2CO3 as the precipitant according to
previous report,27 which was denoted as CuZnAl (1/4/5).
2.2 Catalyst characterization
The BET surface area (SBET) was measured using N2 physisorption
at77 K on a Micromeritics TriStar 3000 apparatus. The samples were
outgassed for 2 h at 523 K before each measurement. The X-ray
diffraction (XRD) patterns were collected on a Bruker AXS D8
Advance X-ray diffractometer using Cu Kα radiation (λ = 0.15418
nm) with angle (2θ) range of 20-80°, a scanning speed of 4° min-1, a
voltage of 40 kV, and a current of 20 mA. The full width at half
maximum (FWHM) of Cu (111) diffraction at a 2θ of 43.3° was used
for calculating the Cu crystallite sizes by using the Scherrer
equation.
For the XRD examination of reduced catalysts, to avoid and
diminish phase transformation from surface oxidation, a variety of
measures were taken to suppress the occurrence of oxidization.
Firstly, these catalysts which were reduced one at a time in tube
furnace by 5 vol.% H2/Ar would not be taken out until the
temperature of the tube furnace was below 303 K. What more
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2.3 Activity measurements
important is that, to prevent them from being oxidized, the reduced
catalysts were loaded immediately into a centrifuge tube full of
absolute ethanol which could provide a great effect of isolation
from air for these catalysts after reduction. And then the sample
was sent to the laboratories for the XRD measurements as soon as
possible.
The activity test was conducted on a continuous flow unit
equipped with a stainless-steel fixed-bed tubular reactor. The
catalyst bed had an inner diameter of 10 mm with a height of
approximately 40 mm. Both sides of the catalyst bed were packed
with quartz powders (20-40 meshes). Typically, the sample was
loaded into a stainless steel tubular reactor with the thermocouple
inserted into the catalyst bed for better control of the temperature.
The catalyst was activated in a 5 vol.% H2/Ar atmosphere at 573 K
for 4 h at a temperature ramping rate of 2 K/min from 303 to 573 K.
0.8g of catalyst and 10 wt.% DMC (purity 99.0%) in THF were used
in this probe reaction make it convenient for investigation the
activities of different samples. After cooling to the reaction
temperature, 10 wt.% DMC and H2 were fed into the reactor at a
H2/DMC molar ratio of 260 and a system pressure of 2.5 MPa. The
reaction temperature was first set at 503 K and the room-
temperature liquid hour space velocity (LHSV) of reactant was set at
corresponding value for MeOH production. The methanol content
in liquid products was analyzed using 1-butanol as internal
standard. The gas products collected in the condenser were
analyzed offline by SHIMADZU GC-2010 Plus gas chromatography
using a flame ionization detector and the tail gas was analyzed
online using a flame ionization detector (FID) with a six-way valve as
gas sampler.
The particle size and distribution were observed by transmission
electron microscopy (TEM; JEOL JEM2011) with acceleration voltage
of 200 kV. The reduced samples for electron microscopy were
prepared by grinding and subsequent dispersing the powder in
ethanol and applying a drop of very dilute suspension on carbon-
coated grids and then quickly moved into the vacuum chamber. The
surface species were detected by X-ray photoelectron spectroscopy
(XPS; Perkin Elmer PHI 5000C). The spectra were recorded with Mg
Kα line as the excitation source (hν = 1253.6 eV) at 14 kV and 20
mA. The binding energy (BE) values were referenced to the C 1s
peak of contaminant carbon at 284.6 eV with an uncertainty of ±
0.2 eV. XPSPeak 4.1 was employed to deconvolute the Cu 2p peaks
using the Shirley-type baseline and an iterative least-squared
optimization algorithm. The FWHMs of peaks devided ( for Cu+ and
Cu0) are almost consistent. The ratio of Cu+/Cu0 was calculated from
the ratio of the peak area. Fourier transform infrared spectroscopy
(FT-IR) experiments of the catalysts were performed using a Bruker
Vector 22 spectrometer equipped with a DTGS detector and a KBr
beam splitter.
The TPR profiles were conducted with a homemade apparatus.
During the experiments, each sample (20 mg) was outgassed under 3. Results and Discussion
flowing Ar at 473 for and then cooled to ambient
K
1 h
3.1 Characterization of catalysts
temperature. The TPR profiles were obtained with 5 vol.% H2/Ar
flow (40 mL/min). The temperature was increased from 303 to 773
K at a rate of 10 K/min. The H2 consumption was monitored using a
TCD detector. The Cu dispersion was determined by dissociative
N2O adsorption- H2-TPR reverse titration. The N2O chemisorption
process consists of three sequential steps:
Fig. S1† showed the wide-angle XRD patterns of the supported
Cu catalysts. Evident diffraction peaks of CuO at 35.5° and 38.7°
(PDF#45-0937) were observed and the peaks intensify with the
increase in copper content. After reduction (Fig. 1), peaks from
copper oxides disappeared along with the appearance of the peaks
CuO + H2→ Cu + H2O (1) (hydrogen consumption = A1)
from metallic copper.
A strong diffraction peak at 43.3°
2Cu + N2O → Cu2O + N2 (2)
characteristic of fcc Cu (111) (JCPDS04-0836) could be observed. In
addition, a diffraction peak at 37.0° (JCPDS34-1354) attributed to
Cu2O was also observed in the reduced catalyst. These findings
demonstrated that the metallic copper and Cu2O coexist in the
working catalysts. Cu crystallite sizes calculated by the Scherrer
formula were listed in Table 1. The average Cu particle size
increased from approximately 10.7 to 15.5 nm with an increase in
Cu2O + H2→ 2Cu + H2O (3) (hydrogen consumption = A2)
Step 1 represents the reduction of CuO in the catalysts. In this
step, a flow of 5 vol.% H2/Ar (40 mL/min) was used as the reducing
agent, and the temperature was risen from 298 to 773 K with a
heating rate of 10 K/min. Step 2 represents the oxidation of surface
Cu to Cu2O by N2O, which is a well-known method to evaluate the
dispersion of Cu based catalysts. 28,29 This step was conducted after
the reduced catalyst was cooled to 333 K in Ar (30 mL/min) and
purged with pure Ar flow for 30 min. After then, pure N2O (30
□
C u
C u O
□
○
2
mL/min) is introduced to the catalyst at 333
K for 0.5 h.
Subsequently, the catalyst is purged with Ar (30 mL/min) for 0.5 h
to remove the residual N2O. Step 3 represents the reduction of
surface Cu2O species. In this step, a flow of 5 vol.% H2/Ar (40
mL/min) is also used as the reducing agent, and the temperature
was risen from room temperature to 773 K with a heating rate of 10
K/min. The dispersion (D) of Cu was calculated as the following
equation, which has extensively been used in literatures elsewhere.
D = 100%×2A2/A1
D
□
○
□
C
B
A
30
40
50
60
70
The specific area of metallic copper was calculated from the
amount of H2 consumption with 1.46 × 1019 copper atoms per m2.30
2 Theta(D egree)
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Fig.
1 XRD patterns of xCu/SiO2 reduced at 573 K: A) 20Cu/SiO2, B)
30Cu/SiO2, C) 40Cu/SiO2, D) 50Cu/SiO2.
Table 1 Structural parameters and catalytic properties of the Cu/SiO2 catalysts.
Catalyst
SBET(m2g-1)
367.6
Vp(cm3g-1)
1.03
dp(nm)
11.8
11.1
8.8
adCu(nm)
10.7
bDCu(%)
31.5
bSCu0(m2g-1)
cCu+/Cu0
0.56
bCu+/Cu0 dTOF(h-1)
20Cu/SiO2
30Cu/SiO2
40Cu/SiO2
50Cu/SiO2
4.1
5.6
5.9
4.2
0.51
1.01
1.73
1.43
7.9
9.1
279.8
0.81
14.6
28.7
0.69
257.6
0.54
15.1
22.6
1.08
13.7
8.6
232.6
0.49
10.8
15.5
13.0
0.87
a Cu average diameter of particle size calculated from the XRD. b Cu dispersion, surface area of Cu0 and Cu+/Cu0 determined by N2O titration. c Calculated from
d
XPS data of reduced samples. TOF (grams DMC reacted per gram surface Cu per hour) at temperature of 433 K, at conversion below 30%.
Cu content from 10 to 50 wt.%. The coexistence of metallic copper
H2-temperature-progammed reduction (H2-TPR) was performed
and Cu2O in the working catalyst inferred that the inadequate to further investigate the reducibility and structural evolutions of
reduction of Cu2+ species in the calcined catalysts upon H2- calcined samples (Fig. 2). The calculations of hydrogen consumption
reduction. For the well-established Cu/SiO2 catalysts or other silica added in Table S1†. It can be observed that the uptake of H2
supported catalysts, the generation of the copper phyllosilicate increased along with the increasing in copper content. The
species, which could stabilize the Cu+ species, was known to form reduction peak covered a range from approximately 460 to 560 K,
during the preparation of Cu/SiO2 catalyst by AE method. The which could be attributed to the reduction of CuO species. Only one
incomplete reduced copper species were also found to secure the single reduction peak at about 530 K could be observed in
high catalytic efficiency in the hydrogenation of dimethyl oxalate the20Cu/SiO2 catalyst, indicating that the copper species are in
(DMO), due to the fact that Cu+ could adsorb carbonyl group and well-dispersed state.
inhibit the aggregation of copper species.31,32
appeared when the copper content was higher than 30 wt.%. The
A shoulder peak at higher temperature
Table 1 summarizes the chemical compositions and textural high temperature reduction peak was ascribed to the reduction of
properties of xCu/SiO2 catalysts. The increase of copper content the bulk CuO, and the low temperature one could be assigned to
resulted in the gradual loss in both surface area and the Cu the reduction of small isolated highly dispersed CuO species on the
dispersion, indicating that the excess amount of copper species in surface.35-37 With an increase in copper mass ratio, the amount of
the catalyst adversely impact on the structure of the supports. BET highly dispersed CuO species decreased gradually, resulting in the
surface area and Cu dispersion with varying Cu amount significantly relatively large Cu particle. The results were in good accordance
ranged from 367.6 to 232.6 m2g-1 and 31.5 to 13.0%, respectively. with the observations made by XRD and N2O measurement.
Copper dispersion and Cu0 surface area were crucial factors
determining the catalytic performance of copper-based catalysts.33
The highest Cu0 surface area (5.9 m2g-1) was obtained on 40Cu/SiO2.
N2 adsorption-desorption results (Fig. S2 in ESI†) showed that all the
catalysts displayed the hysteresis loop with type IV, confirming the
presence of the mesoporous structures in the catalysts. The
D
elevated Cu content resulted in a decrease of pore volume, as well
as BET surface area. The corresponding pore size distributions are
C
shown in Fig. S2†. The average pore size of 20Cu/SiO2 catalyst
derived from the desorption branch was at 11.8 nm. With the
increase of copper content, the average pore size decreased to 8.8
nm for the 40Cu/SiO2 catalyst, while it can be seen from the pore
size distribution that the presence of more smaller pores in this
catalyst probably due to the porosity provided by the laminar
structure of copper phyllosilicate. With the copper loading
increased to 50 wt.%, agglomeration copper particles filled up some
smaller pores and covered the surface of the support, which
therefore had relatively larger average pore size.34
B
A
400 450 500 550 600 650 700
T/K
Fig. 2 H2-TPR of calcined samples: A) 20Cu/SiO2, B) 30Cu/SiO2, C) 40Cu/SiO2,
D) 50Cu/SiO2.
Table 2 Catalytic activities of various catalysts
Entry
Catalyst
20Cu/SiO2
30Cu/SiO2
40Cu/SiO2
DMC conv. (%)
MeOH sel. (%)
60.4
STYMeOH (h-1)
0.43
Gas products COx (%)
Gas products CH4 (%)
1
2
3
100
100
100
33.0
23.1
16.7
1.4
1.1
0.9
73.1
0.52
80.0
0.57
4
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4
50Cu/SiO2
100
100
100
100
100
71.5
13.5
25.8
72.1
64.7
0.54
0.10
0.18
0.51
0.46
21.8
73.5
60.3
25.4
27.6
1.7
1.9
0.1
0.4
1.2
5
6
7
8
40Cu/ZrO2
40Cu/TiO2
40Cu/CeO2
CuZnAl(1/4/5)
Reaction Conditions: liquid hour space velocity (LHSV of DMC) ~0.2 h-1, H2/DMC ~ 260 (mol/mol), 503 K, 2.5 MPa. COx means that CO and CO2. STY represents
the space time yield of MeOH for the catalysts, grams of product per gram of catalyst per hour (g·g-1Catal·h-1).
values of Cu 2p3/2 core levels were located in the range of 932.4-
932.7 eV and the absence of Cu 2p satellite peak strongly
Cu+
Cu0
demonstrated that all Cu2+ species had been reduced to a low
valence state of +1 or 0 after the reduction. The Cu+ species was
mainly formed upon the reduction of the copper phyllosilicate
under the experimental conditions (reduced at 573 K), since the
further reduction of Cu+ to Cu0 required a temperature higher than
873 K 38. The result was consistent with those from FT-IR and XRD.
Because the BE values of Cu+ and Cu0 were almost identical, the
distinction between these two species present on the catalyst
surface was feasible only through the examination of XAES spectra.
Two overlapping Cu LMM Auger kinetic energy peaks centered at
about 916.5 and 913.6 eV were observed in the reduced catalyst
(Fig. 3). Deconvolution of the original Cu LMM peaks was thus
carried out and the Cu+/Cu0 ratios extracted from the deconvolution
were listed in Table 1. As shown in Table 1, the Cu+/Cu0 ratio
derived by fitting the Cu LMM peak first rose and then decreased
with the increment of copper loading, and the highest Cu+/Cu0
(1.08) was obtained on 40Cu/SiO2. This finding agreed well with N2O
titration.
D
C
B
A
908 910 912 914 916 918 920 922
Kinetic Energy(eV)
Fig. 3 Cu LMM XAES with different catalysts reduced at 573 K: A)20Cu/SiO2,
B) 30Cu/SiO2, C) 40Cu/SiO2, D) 50Cu/SiO2.
The TEM images and copper particles sizes distributions of
different Cu/SiO2 catalysts were shown in Fig. 4. Light gray spherical
silica particles are identified along with dark ones assignable to CuO
particles. The increased Cu particle sizes and decreased dispersion
of copper species with the increase of the copper content can be
directly observed in Fig. 4, which is consistent with the XRD and N2O
titration results.
Fourier-transform IR (FTIR) spectroscopy is usually adopted to
discriminate the species from copper hydroxide, copper nitrate
hydroxide, and copper hydrosilicate according to the structural OH
groups.38,39 As shown in Fig. S3†, the band near 1640 cm-1 appeared
corresponding to the bending mode of OH groups of adsorbed
water.40 The appearance of the δOH vibration at 668 cm−1 and the
νSiO shoulder peak at 1039 cm−1 illustrated that the structure of
copper phyllosilicate exists in the samples.
3.2 Catalytic Activity
The XPS spectra of the samples as well as the X-ray induced
Auger spectra (XAES) of the reduced Cu/SiO2 catalysts were
displayed in Fig. S4† and Fig. 3. The intense and broad
photoelectron peak at above 933.4 eV (Cu 2p3/2) along with the
presence of the characteristic shake-up satellite peaks suggested
that the copper oxidation state was +2 in all the calcined samples
(Fig. S4†). In the case of the reduced samples (Fig. S4†), the BE
It is well known that Cu is active for methanol synthesis, serving
in the dissociative chemisorption of hydrogen and the dissociative
adsorption of C=O. Therefore, a detailed catalytic performance
study of the Cu-based catalysts system was performed by changing
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Fig. 4 TEM images of the reduced catalyst: (A)20Cu/SiO2, (C)30Cu/SiO2, (E)40Cu/SiO2, (G)50Cu/SiO2 with Cu particle size distribution of (B)20Cu/SiO2,
(D)30Cu/SiO2, (F)40Cu/SiO2, (H)50Cu/SiO2.
the amount of Cu. As shown in Table 2, at liquid hourly space 40Cu/SiO2 catalyst was tested, as shown in Fig. S5†, the Cu species
velocity (LHSV) of 0.2 h-1, all catalysts achieved 100% DMC was not sintered in a 100 h test, besides, the yield of methanol
conversion, which manifested the high activity of Cu in the could achieve ~76% when neat DMC as reactant indicating super
hydrogenation of DMC. Different Cu/SiO2 catalysts showed volcano- stability of catalyst and the potential for further industrial
type catalytic behavior in terms of MeOH yield. Along with the Cu applications. The conversion of DMC and yield of MeOH could
loading, the yield of MeOH and STYMeOH increased slightly and was maintain at ~100% and >75% respectively when the LHSV based on
maximized on the 40Cu/SiO2 catalyst (80.0%). Further increase in DMC is lower than 1.4 h-1 for the 40Cu/SiO2 catalyst. Considering
the Cu loading led to a decrease in MeOH yield. For all the Cu/SiO2 the calculation method on the yield of MeOH is complex if the DMC
catalysts, methanol was the main reaction product with CH4, CO, was not converted totally (<100%), the influence of LHSV was not
and CO2 as main by-products produced through methanation, discussed in detail. The Cu component dispersed on oxide supports
decarbonylation and decomposition side-reactions. The stability of including TiO2, ZrO2, CeO2 shows large amount of gas products,
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which illustrated that the role of active copper dispersion and the the report that Cu/ZnO/Al2O3 catalyst did not work well in the
acidic-basic property on the surface.22 Interestingly, when the hydrogenation of CO.13
CuZnAl catalyst was introduced into this reaction, the gas products
mainly consist of CO demonstrating that the conventional CuZnAl
catalyst showed higher catalytic activity in the hydrogenation of CO2
derived from DMC hydrolysis, while the CO produced cannot be
efficiently converted to methanol, which was in accordance with
Fig. 5 demonstrated the dependence of the yield of methanol on
reaction temperature from 473 to 553 K. The methanol yield
increased with reaction temperature and a maximum was reached
at 503 K, when the temperature was lower than 473 K, the
conversion could not reach 100%, which lowered the methanol
100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
473
503 523
553
483
bDPC
aDEC
Reaction T/K
Fig. 5 The yield of methanol as a function of reaction temperature on
40Cu/SiO2 catalyst. Reaction Conditions: liquid hour space velocity (LHSV of
DMC) ~0.2 h-1, H2/DMC ~260 (mol/mol), 2.5 MPa.
Fig. 6 Catalytic performance of 40 wt.%Cu/SiO2 catalyst in the gas-phase
a
hydrogenation of other carbonates. liquid hour space velocity (LHSV of
DEC) ~0.2 h-1, H2/DEC = 260 (mol/mol), 503 K, 2.5 MPa; b liquid hour space
velocity (LHSV of DPC) ~0.1 h-1, H2/DPC ~ 260 (mol/mol), 503 K, 2.5 MPa.
yield. Nevertheless, the side reactions were favourable at high interaction significantly. The central copper ions isolated and
temperature. Hence, we speculate that the hydrogenation of DMC surrounded by silica can be hardly reduced completely and
over 40Cu/SiO2 catalyst is reversible and equilibrium conversion is aggregated to sinter upon high-temperature calcination, which
reached at temperature of 503 K.
endows copper phyllosilicate with superior catalytic activity and
thermal stability. Copper phyllosilicate is known to form during the
preparation of Cu/SiO2 catalyst by the AE method with selective
adsorption of Cu(NH3)42+ on SiO2.39 In the Cu/SiO2 catalyst fabricated
via the AE method, initially Cu2+ reacted with NH3·H2O to form a
copper ammonia complex, while the colloid silica was dissolved to
yield silicic acid (Si(OH)4). The neutral Cu complex Cu(OH)2·(H2O)4
generated gradually increased with decreasing pH resulted from the
evaporation of ammonia. When the pH of the suspension
progressively declined to 7, the evaporation process was
terminated. Meanwhile, the heterocondensation reaction of silicic
acid with the Cu complex Cu(OH)2·(H2O)4 occurred and
subsequently generated copper phyllosilicate monomer.41 The
formation of copper phyllosilicate was also corroborated by FT-IR
and XRD. Partial copper phyllosilicate transformed to high dispersed
CuO during the calcination process, hence the copper ions were
homogeneously distributed over the silica support to some extent,
even at elevated metal loading. It was generally accepted that the
high Cu surface area was beneficial for the catalytic performance of
copper-based catalyst. A proper loading amount of copper could be
dispersed on the support more uniformly, and larger copper surface
area was achieved, which could induce the higher hydrogenation
activity. The further increase of copper content would decrease the
metal dispersion because the particle sintering dominated over the
catalyst. The remaining copper phyllosilicate after calcination in
samples was reduced to Cu2O upon reduction at ~623 K. For the
ion-exchanged Cu-O-Si species and copper phyllosilicate, the
3.3 Gas-Phase hydrogenation of other carbonates
In order to investigate the catalytic capacity of Cu/SiO2 catalysts
by the AE method, other two linear CO2 derivatives including diethyl
carbonate and di-n-propyl carbonate were selected to undergo the
gas-phase hydrogenation to generate methanol. The 40Cu/SiO2
catalyst was also very effective in the hydrogenation of these
carbonates (Fig. 6), exhibiting 76.5% and 82.0% methanol yield for
the hydrogenation of DEC and DPC respectively. The selectivities of
ethanol and propanol reach above 99% (Table S2†). The above
finding indicates excellent catalytic performance of 40Cu/SiO2
catalyst in the hydrogenation of a series of linear carbonates. It was
known that higher melting point (such as diphenyl carbonate)
would easily result in the reaction pipe blocking at incomplete
conversion due to the low temperature of liquid products collector
(270 K). Furthermore, for dibutyl carbonate, it is mainly synthesized
from the ester exchange of DMC with butanol in industry not from
the reaction of CO2 and butanol. Considering the economical
efficiency, reaction value and implementation possibility, we
choose DMC, DEC and DPC as substrates for this reaction.
3.4 Discussion
High dispersion of copper species and strong metal-support
interactions are vital for the high activity and super stability in
hydrogenation of esters. In particular, copper phyllosilicate with a
lamellar structure can enhance the dispersion and metal-support
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8
7
6
5
4
3
Main reaction:
1.2
H
H
O
O
O
O
H2
CH3OH
0.8
Step 1
H3CO
OCH3 H3CO
OCH3
H
OCH3
0.4
H2
Step 2
CH3OH
CH3OH
0.0
H
OCH3
Side reaction:
-0.4
O
H2O
OCH3
20
25
30
35
40
45
50
CH3OH
Gas products (CO2, CO, CH4)
Cu Loading/%
H3CO
Fig. 7 SCu/m2g-1cat and the mole ratio of Cu+/Cu0 as a function of Cu loading.
Fig.
8 Proposed hydrogenation mechanism over Cu/SiO2 nanocatalys
which was probably due to that the Cu+ contributed to the
adsorption of methoxy and acyl species of DMC during the
hydrogenation reactions, while Cu0 facilitated the decomposition
of H2. SCu/m2g-1cat and the mole ratio of Cu+/Cu0 as a function of Cu
loading were shown in Fig. 7. It was notable that the 40Cu/SiO2
catalyst showed the highest Cu+/Cu0 ratio and the largest Cu0
surface area. Correspondingly, the methanol yield increased
significantly owing to the dissociation of H2 molecules on highly
dispersed catalytically active Cu species and the stronger methoxy
and acyl species adsorption on the Cu+ sites. Remarkably, the TOF
increased steadily at low Cu+/Cu0, reached a maximum at Cu+/Cu0
of 1.08, and decreased with further increases in the ratio (Table 1).
Therefore, the optimal TOF on the 40Cu/SiO2 catalyst lies in the
high surface Cu0 site density and the cooperative effect of Cu0 and
Cu+.
On the basis of “The omega process”, a possible process for the
DMC hydrogenation over Cu/SiO2 catalyst was proposed (Fig. 8).
No gas product could be found when the reactant was methanol
instead of DMC with 40Cu/SiO2 as catalyst, indicating that the
methanol could not decompose to CO2 on the active Cu sites under
the H2 atmosphere. No gas product and methanol was observed
for the hydrogenation of DMC without any catalysts, showing that
the active Cu species are essential in the hydrolysis process of
DMC at relatively high temperature. This transformation begins
reduction is ceased at Cu+ under the present reduction condition
since the progressive reduction of Cu2O to Cu0 required a higher
temperature than 873 K. 41 Notably, the catalyst precursor appears
also to be the key factor in determining the Cu species distribution
and surface chemical state of the Cu/SiO2 catalyst. The chemical
states of surface properties were also changed with the variation
of structural parameters. Except for the reducibility of CuO, the
proportions of surface Cu+ sites of reduced catalyst were
modulated. It has been reported that the efficiency of ester
hydrogenation greatly relies on the synergistic cooperation of Cu0
and Cu+ present on the catalyst surface. Poels and Brands31
suggested that the Cu0 species activated H2 while the Cu+ species
adsorbed intermediate species in ester hydrogenation. Moreover,
Cu+ might function as electrophilic or Lewis acid sites to polarize
the C=O bond via the electron lone pair on oxygen, thus improving
the reactivity of the ester group. Fridman et al. mentioned that
the dissociative adsorption of cyclohexanol on Cu0 sites were
accompanied by formation of cyclohexanol alcoholate and
phenolate species, leading to the poor chemi-selectivity on this
site, which did not occur on the Cu+ species.42 Hence, it can be
deduced that the Cu+ species is more suitable for the stabilization
of intermediate product. Presumably, excessive Cu0 species might
induce the side reactions, such as decomposition of the reaction
intermediates, most probably ethyl formate, which likely result in
the increasing amount of gas products. Liu and coworkers22
discovered that the synergistic effect between Cu0 and Cu+ plays a
critical role for attaining high yields of methanol and diols during
the process of EC hydrogenation. The adsorption of ethyl acetate
on Cu under reduction conditions has been hypothesized in
previous report to proceed via the cleavage of the C-O bond
adjacent to the carbonyl group,43 here we tentatively propose that
the adsorption of DMC also proceeds in this manner. The yield of
methanol rapidly increased with an increase in the Cu+/Cu0 value,
with the initial reduction of DMC to methyl formate (Step 1)
,
followed by the further reduction of methyl formate to methanol
(Step 2). Methanol from the methoxy group was produced in this
process. It seems that these competing reaction paths exist in the
hydrogenation of DMC. CO2 could be produced in the analogous
“Omega process”, in which MeOH is reacted with CO2 to first
afford DMC, followed by catalytic hydrolysis of the DMC with H2O
in the system which comes from solvent and environment to
selectively produce MeOH.17 CO is a problematic by-product in
heterogeneously catalyzed hydrogenation of methyl formate.16,44-
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46
Inevitably, the co-existence of Cu0 and Cu+ can promote the
6
7
S. N. Habisreutinger, L. Schmidt-Mende, J. K. Stolarczyk,
Angew. Chem., Int. Ed., 2013, 52, 7372-7408.
hydrogenation of CO2 to CO, methanol and small amount of CH4.
Wang’s work47 demonstrated the conversion of CO2 rapidly
increased with an increase in the Cu+/(Cu0+Cu+) value and reached
the maximum when Cu+/(Cu0 + Cu+) is 1.00, which showed that the
Cu+ species was the active component in the Cu/SiO2-AE
nanocatalyst for activation and conversion of CO2. Moderate
higher reaction temperature is conducive to the methanol
production from direct hydrogenation of DMC and indirect
hydrogenation of CO2. The side reactions would be promoted
which resulted in more gas products if the reaction temperature
was too high. However, methanol produced from CO2 and DMC
was inhibited at lower temperature. Both of these factors will
result in the decline of methanol yield. Hence, a proper reaction
temperature is essential in the gas-solid phase hydrogenation of
DMC.
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heterogeneous continuous hydrogenation of DMC, diethyl
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low-cost Cu/SiO2 catalysts prepared by the facile AE method. The
catalysts possessed remarkable stability and efficiency even
though the neat DMC as reactant, which could be ascribed to the
high copper species dispersion and the synergistic effect of Cu0
and Cu+. The Cu0 and Cu+ site densities suggested that Cu+ was the
main active site and primarily responsible for the catalytic
performance in the hydrogenation of DMC to methanol.
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temperature. This indirect synthetic methodology using
inexpensive Cu-based catalysts under mild conditions shows
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industry.
17 Z. B. Han, L. C. Rong, J. Wu, L. Zhang, Z. Wang, K. L. Ding,
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We would like to thank financial support by the Major State
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NSFC (Project 21373054, 21173052), the Natural Science
Foundation of Shanghai Science and Technology Committee
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Continuous heterogeneous hydrogenation of CO2-
derived dimethyl carbonate to methanol over Cu-
based catalyst
Yuanyuan Cui[a], Chen Xi[a], and Wei-Lin Dai*[a]
Department of Chemistry, Fudan University
Shanghai, 200433, China
E-mail: wldai@fudan.edu.cn
Copper content played significant roles in the catalytic performance of
Cu/SiO2 catalysts in dimethyl carbonate hydrogenation to methanol.
Optimized hydrogenation activity was achieved over the 40Cu/SiO2 sample.