Cu Sub-Nanoparticles on Cu/CeO2 as an Effective Catalyst for Methanol Synthesis from Organic Carbonate by Hydrogenation

Article abstract of DOI:10.1021/acscatal.5b02258

Cu/CeO₂ works as an effective heterogeneous catalyst for hydrogenation of dimethyl carbonate to methanol at 433 K and even at low H₂ pressure of 2.5 MPa, and it provided 94% and 98% methanol yield based on the carbonyl and total produced methanol, respectively. This is the first report of high yield synthesis of methanol from DMC by hydrogenation with H₂ over heterogeneous catalysts. Characterization of the Cu/CeO₂ catalyst demonstrated that reduction of Cu/CeO₂ produced Cu metal with <1.0 nm (subnanoparticles). Cu metal subnanoparticles are easily formed by the interaction with the CeO₂ surface, which is responsible for the high catalytic performance.

Full text of DOI:10.1021/acscatal.5b02258

Letter
pubs.acs.org/acscatalysis
Cu Sub-Nanoparticles on Cu/CeO2 as an Effective Catalyst for
Methanol Synthesis from Organic Carbonate by Hydrogenation
Masazumi Tamura,* Takahisa Kitanaka, Yoshinao Nakagawa, and Keiichi Tomishige*
Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan
S
* Supporting Information
ABSTRACT: Cu/CeO2 works as an effective heterogeneous catalyst for
hydrogenation of dimethyl carbonate to methanol at 433 K and even at low
H2 pressure of 2.5 MPa, and it provided 94% and 98% methanol yield based
on the carbonyl and total produced methanol, respectively. This is the first
report of high yield synthesis of methanol from DMC by hydrogenation with
H2 over heterogeneous catalysts. Characterization of the Cu/CeO2 catalyst
demonstrated that reduction of Cu/CeO2 produced Cu metal with <1.0 nm
(subnanoparticles). Cu metal subnanoparticles are easily formed by the
interaction with the CeO2 surface, which is responsible for the high catalytic
performance.
KEYWORDS: Cu subnanoparticle, Cu/CeO2, hydrogenation, methanol synthesis, organic carbonate
Recently, Milstein and co-workers proposed a new process
for methanol synthesis from CO2, which is an indirect one via
hydrogenation of carbonate derivatives such as carbonates,
carbamates, and ureas (Scheme 1),7 because these carbonate
O2 can be regarded as an inexpensive, abundant, nontoxic,
Cand renewable resource, and development of efficient
methods to transform CO2 to valuable chemicals is promising
for constitution of a carbon neutral process. Various target
chemicals such as formic acid, methanol, carbonates, and ureas
are known1 and among these target chemicals, methanol has
attracted much attention because methanol is used in the
production of various useful chemicals such as formaldehyde,
dimethyl ether, methyl tert-butyl ether, and acetic acid and is
expected to be one of the most effective hydrogen carriers in
the future.2
Scheme 1. Indirect Conversion of CO2 to Methanol by
Carboxylation of Methanol with CO2 to DMC and
Hydrogenation of DMC to Methanol
Methanol is industrially prepared by hydrogenation of CO
and CO2 with H2, where Cu−Zn-based catalysts are known as
the most effective catalysts.3 The reaction generally requires
severe conditions such as high temperature (typically 523−573
K) and pressure (typically 5−10 MPa).1b,3 High temperature is
not preferable in terms of the reaction equilibrium and
endothermic reverse water−gas shift reaction, bringing about
a decrease of the selectivity.4 Recently, Urakawa et al. achieved
excellent selectivity (>98%) at high conversion (>95%) using
coprecipitated Cu/ZnO/A2O3 at high pressure (36 MPa) and
also reported quite high activity with commercial Cu/ZnO/
A2O3.4l The activity is the highest among the reported Cu-
based catalysts (Table S1). However, the severe reaction
conditions (high pressure and high temperature) are a serious
problem. On the other hand, effective homogeneous catalysts at
low temperature like Ru complexes5 or effective heterogeneous
catalysts at low H2 pressure (0.05 MPa) like MnOx/m-Co3O4
(<433 K)6 were reported. However, these catalytic processes
also have some drawbacks such as low activity, low yield, and/
or high temperature (>473 K).
derivatives can be comparatively easily synthesized from CO2
by nonreductive transformation. Ru-based homogeneous
catalysts are effective for hydrogenation of the carbonyl moiety
in these chemicals to methanol at low temperature of 383 K,7
suggesting the possibility for low-temperature synthesis of
methanol. Other researchers also reported on Ru complexes for
hydrogenation of cyclic carbonates to methanol and diols.8
Heterogeneous catalysts are favorable from the viewpoints of
green sustainable chemistry. Cu-based heterogeneous catalysts9
such as CuCr2O4,9a Cu/mesoporous-silica,9b and Cu-SiO2
nanocomposite9c were reported to be efficient for hydro-
genation of ethylene carbonate to methanol and ethylene
Received: October 8, 2015
Revised: December 1, 2015
© XXXX American Chemical Society
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a
Table 1. Effect of Cu Amount over Cu(x)/CeO2 Catalyst in Hydrogenation of Dimethyl Carbonate (DMC)
b
selectivity (%)
c
x
conv. (%)
yield (%)
CH3OH
HCOOCH3
CH4
CO
CO2
CH3OH/Cu (mol/mol)
0
0.1
0.5
1
1.2
2.1
1.0
1.0
82.3
48.7
63.0
80.8
84.5
81.7
82.0
11.9
24.3
23.3
12.1
7.8
0.0
0.8
1.0
1.0
1.3
1.6
1.7
0.8
2.2
1.5
0.9
1.3
1.6
1.7
5.0
24.1
11.2
5.2
-
99
5.4
3.4
129
164
125
70
10.6
15.4
13.5
14.4
8.6
2
13.0
11.0
11.8
5.4
3
8.6
6.7
5
8.0
7.1
45
a
b
Reaction conditions: DMC 30 mmol, Cu(x)/CeO2 100 mg (x = 0−5), THF 5 g, H2 8 MPa, 433 K, 4 h. Selectivity based on the carbonyl of DMC.
c
CH3OH/Cu ratio (mol/mol) = (Produced methanol amount based on the carbonyl (mmol))/(Total Cu amount (mmol)).
glycol, where the size of Cu metal particles is comparatively
large (≥8 nm) and both Cu0 and Cu+ were proposed as the
active site (Table S2). However, these catalysts have drawbacks
such as low selectivity (<80%), high temperature (453 K), low
turnover frequency (TOF < 3 h−1), and/or low turnover
number (TON < 10) (Table S2). In addition, the Cu-SiO2
nanocomposite showed low selectivity and activity in hydro-
genation of DMC to methanol,9c which was explained by the
lower reactivity of linear carbonates than cyclic carbonates
(Table S2).8b,9c It is essential to develop the effective catalysts
with high activity under mild reaction conditions.
Transition metal subnanoclusters, particularly Ag or Au
subnanocluster, have attracted much attention in the field of
catalysis due to the unique catalytic feature,10 and Cu
subnanocluster is also predicted as a promising unique catalyst
by DFT calculations.11 However, in hydrogenation of CO2, a
large Cu loading amount is generally used for the reaction, and
a large particle of Cu metal species is formed (≥3 nm) (Table
S1); subnanoparticles of Cu metal species (<1 nm) have not
been applied to the reaction. Recently, Curtiss and co-workers
first reported that the heterogeneous Cu4 cluster on Al2O3 is
effective for hydrogenation of CO2 at low CO2 pressure (0.013
MPa) based on the experimental and theoretical results.12
However, the preparation method of the Cu4 clusters is very
specific and cumbersome. Therefore, development of an easy
preparation method of such Cu subnanoparticles will enable a
wide range of applications of Cu subnanoparticles.
We have developed CeO2-catalyzed organic reactions
including transformation of CO2 or nitriles,13,14 and we have
demonstrated that CeO2 can readily activate CO2 or esters on
the surface of CeO2.13 Recently, we reported that the
combination catalyst of CeO2 and 2-cyanopyridine is effective
for the synthesis of DMC from CO2 and methanol to give 94%
DMC yield based on methanol at mild reaction temperature of
393 K, which can be related to more environmentally benign
and economical synthesis of DMC from CO2.14 Considering
the reversibility of DMC formation from CO2 and methanol,
introduction of transition metal species for activation of H2
onto the CeO2 surface without losing the property of CeO2 will
enable the transformation of DMC to methanol at low
temperature. Herein, we report that Cu/CeO2 is an effective
catalyst for hydrogenation of DMC to methanol, providing high
methanol yield. Characterization of Cu/CeO2 catalyst reveals
that Cu species on CeO2 are metallic Cu subnanoparticles.
At first, the catalytic activity for hydrogenation of dimethyl
carbonate (DMC) to methanol at 433 K and 8 MPa H2 was
investigated with CeO2-supported 1 wt % metal catalysts
(M(1)/CeO2, MCu, Pt, Ag, Au, Ir, Ni, Rh, Co, Pd, Ru;
prepared by impregnation) and conventional carbon-supported
5 wt % noble metal catalysts (M′(5)/C, M′Ru, Rh, Pd, Pt)
(Table S3). In this reaction, three moles of methanol are
produced by hydrogenation from the two moles of the methoxy
moiety and one mole of the carbonyl moiety in DMC. To
precisely estimate the catalytic performance, the selectivity was
calculated on the basis of the carbonyl amount of DMC. The
details of the calculation method are described in the
Supporting Information. M(1)/CeO2 catalysts except for
Cu(1)/CeO2 showed low conversion (<3%). Similarly,
M′(5)/C catalysts showed low conversion and/or low
selectivity. Cu(1)/CeO2 exhibited high conversion and
selectivity to methanol, providing higher yield than other
M(1)/CeO2 catalysts. It should be noted that only CeO2
provided quite low conversion with comparatively high
selectivity (80.8%). Therefore, Cu can be regarded as a suitable
metal species over CeO2 in this reaction. Next, effect of
supports was also investigated in the same reaction using metal
oxides such as ZrO2, MgO, TiO2, γ-Al2O3, SiO2−Al2O3 (SAL),
and SiO2 as supports (Table S3). Among these metal oxides,
CeO2 showed higher yield and selectivity to methanol than
other metal oxides. Therefore, the combination of CeO2 and
Cu is the most effective for the reaction. In addition, Cu(1)/
CeO2 catalyst could be reused without large loss of selectivity
and activity (Table S3).
The effect of Cu loading amount was investigated using
Cu(x)/CeO2 catalysts with various amount of Cu (x = 0−5 wt
%) (Table 1). The conversion increased with increasing Cu
amount from 0 to 2 wt %, but the conversion was almost
constant at 2 wt % and larger amount of Cu. On the other
hand, the selectivity increased with increasing Cu amount from
0.1 to 1 wt %, and leveled off at more than 1 wt %. The low
selectivity at small Cu amount (0.1 and 0.5 wt %) will be due to
the low conversion, and alternatively the selectivity to methyl
formate, which will be a main intermediate of the reaction, was
high. CH3OH/Cu ratio (mol/mol) provided a volcano pattern
with respect to the Cu amount, and the highest CH3OH/Cu
ratio was obtained at 1 wt % Cu amount (Cu(1)/CeO2). The
formation of large Cu metal particle was observed at 5 wt % Cu
loaded CeO2 catalyst (Cu(5)/CeO2) by XRD analysis (Figure
S1), although a part of Cu metal species forms large metal
particles in consideration of the peak intensity of Cu metal.
This result at least implies that the large Cu particle is
unfavorable for the reaction. From the viewpoints of selectivity
and activity, Cu(1)/CeO2 was selected as the optimal catalyst.
Figure 1 shows the time-course of hydrogenation of DMC to
methanol over Cu(1)/CeO2 catalyst (details are shown in
Table S4). The reaction proceeded smoothly to reach 100%
conversion after 24 h, resulting in the high methanol yield of
94% and 98% based on the carbonyl of DMC and total
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85% conversion of DEC with 98% selectivity to methanol based
on the carbonyl and 98% selectivity to ethanol (Scheme 2). On
the other hand, hydrogenation of CO2 showed no product
(Scheme 3). This behavior supports that Cu(1)/CeO2 is
effective specifically for hydrogenation of DMC and DEC.
Scheme 2. Hydrogenation of Diethyl Carbonate (DEC) over
a
Cu(1)/CeO2
a
Reaction Conditions: DEC 30 mmol, Cu(1)/CeO2 1 g, THF 5 g, H2
8 MPa, 433 K, 48 h.
Figure 1. Time-course of hydrogenation of DMC over Cu/CeO2
a
Scheme 3. Hydrogenation of CO2 over Cu(1)/CeO2
●
○
△
catalyst ( : conversion, : methanol selectivity, : methyl formate
□
selectivity, : others). Selectivity to methanol was calculated on the
basis of the carbonyl of DMC. Others include CO, CO2, and CH4.
Reaction conditions: DMC 15 mmol, Cu(1)/CeO2 400 mg, THF 5 g,
H2 8 MPa, 433 K.
a
Reaction Conditions: Cu(1)/CeO2 0.2 g, 1,4-dioxane 20 g, CO2 2
MPa, H2 6 MPa, 433 K, 16 h.
produced methanol, respectively, and the TON based on total
Cu amount reached 224 and 701 based on the carbonyl and
total produced methanol, respectively. The number of the
TON based on total Cu amount is more than 20 times higher
than those in the previous reports on the hydrogenation of
carbonates using Cu-based catalysts (Table S2). The selectivity
to methanol based on the carbonyl of DMC was high
throughout the reaction and gradually increased from 85% to
95%. At the initial stage, formation of methyl formate was
observed, and the selectivity gradually decreased with the
reaction time, implying that the reaction proceeds via two
consecutive reactions: (i) hydrogenation of DMC to methyl
formate and (ii) hydrogenation of the methyl formate.
To clarify the state of Cu and CeO2 in Cu(1)/CeO2 catalyst,
Cu(x)/CeO2 catalysts (x = 0−5) were characterized by XRD,
TEM, TPR and XAFS. No Cu species such as CuO, Cu2O or
Cu metal in Cu(1)/CeO2 after the reaction were observed by
XRD and TEM analyses (Figures S1 and S3), which indicates
that the size of Cu species are very small. Considering the
detection limit of particles by TEM analysis, the size of Cu
species will be smaller than 1 nm. In situ XANES and EXAFS
measurements were conducted in order to determine the state
of Cu species. Figure 2a shows Cu K-edge XANES spectra of
Cu(1)/CeO2 reduced under H2 flow at 433 K and standard Cu
samples, which are known to be significantly influenced by the
oxidation state and local symmetry of Cu species.16 The
XANES spectrum of the Cu(1)/CeO2 is quite similar to that of
Cu foil. The first derivatives of the XANES spectra were studied
to clarify the difference of the spectra (Figure 2b). The peaks
are due to the edge energy for each Cu compounds (Cu0: 8978
eV, Cu+: 8980 eV, Cu2+: 8982 eV);16a the main peak of the
Cu(1)/CeO2 agreed well with that of Cu foil, and the peaks
due to Cu2+ and Cu+ were not observed. These results indicate
that Cu species in Cu(1)/CeO2 catalyst after reduction is in the
metallic state. In addition, Fourier transform of k3-weighted Cu
K-edge EXAFS of the Cu(1)/CeO2 showed the signal
corresponding to the Cu metal (Figure 2c), and the curve-
fitting analysis provided the presence of Cu−Cu bond with a
coordination number (CN) of 5.5 (Figure 2d and Table S6),
which is in good accordance with the result that Cu was in the
metallic state by XANES analysis. This CN value is much
smaller than the CN of bulk Cu metal (CN = 12), which can be
supported by the result that Cu species cannot be observed by
XRD and TEM analyses. Based on the CN of the Cu−Cu
bonds, the size of Cu metal particle will be at subnanometer
scale (<1 nm). TPR analyses of Cu(x)/CeO2 and Cu(1)/SiO2
catalysts (Figure S4 and Table S7) showed the shift of H2
consumption signal to lower temperature (473−493 K) by
addition of Cu species on CeO2 compared with the main signal
(580 K) of Cu/SiO2, suggesting that CeO2 facilitates the
reduction of CuOx species. In addition, the amount of H2
consumption calculated by the area of the main signal was
Effect of H2 pressure was investigated with Cu(1)/CeO2 in
the range of 2.5−9.5 MPa (Table S5). Dependence of H2
pressure on the conversion and selectivity is very low, although
the conversion and selectivity gradually decreased with
decreasing H2 pressure, indicating that operation at low H2
pressure will be possible with Cu(1)/CeO2 catalyst. This
dependence of H2 pressure on Cu(1)/CeO2 is quite different
from that over Cu-based catalysts used in hydrogenation of
CO2, where the conversion largely depended on the H2
pressure.15 This means that the coverage of the active hydrogen
species will be high on Cu/CeO2. In order to estimate the
methanol formation rate (mmol g−Cu h−1) and TOF (h−1)
−1
based on the carbonyl of DMC and total Cu metal, the time-
courses at 8 and 2.5 MPa H2 were examined below 15%
conversion of DMC (Figure S2). From the slope of the time-
course, the methanol formation rate (TOF) was calculated to
−1
−1
be 417 mmol g−Cu h−1 (26.5 h−1) and 323 mmol g−Cu h−1
(20.5 h−1) at 8 and 2.5 MPa H2, respectively. The activity of
Cu(1)/CeO2 is about 90 times higher than that of the reported
Cu-SiO2 nanocomposite for hydrogenation of DMC (Table
S2). In addition, the reaction rate is more than 2 orders of
magnitude higher than those of previously reported Cu-based
catalysts for hydrogenation of CO2 to methanol at similar
temperature (433 and 453 K), and moreover, the reaction rate
is similar to or higher than that at much higher temperature of
>523 K (Table S1).
In addition, the hydrogenation of diethyl carbonate (DEC)
was also carried out under similar reaction conditions, showing
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and DMC concentration hardly influence the reaction rate. The
two reaction mechanisms for hydrogenation of CO2 over Cu-
based catalysts have been proposed (Scheme 4a): (i)
Scheme 4. Typical Mechanism of Hydrogenation of CO2
over Cu-Based Catalysts (a) and Proposed Mechanism of
DMC over Cu Metal Particles (b)
Figure 2. (a) Cu K-edge XANES spectra, (b) first derivatives of
XANES spectra ,and (c) Fourier transform of k3-weighted Cu K-edge
EXAFS for Cu(1)/CeO2 after reduction and Cu foil, CuO, and Cu2O
as references, FT range 30−120 nm−1. (d) Curve-fitting results of Cu
K-edge of Cu(1)/CeO2 after reduction. (The detailed data were
shown in Table S7 and Figure S5.) Reduction conditions: 100% H2
flow, 433 K.
hydrogenation of CO2 to formate adspecies and successive
hydrogenation of the formate adspecies, and the latter step is a
rate-determining step; (ii) hydrogenation of CO2 to carboxyl
adspecies (OCOH) and decomposition of the carboxyl
adspecies to CO and OH, and the former step will be a rate-
determining step.4m In general, the reaction rate in hydro-
genation of CO2 strongly depends on the H2 pressure over Cu-
based catalysts.15 On the other hand, the hydrogenation of
DMC will proceed through adsorption of molecular DMC
instead of the carbonate formation and successive hydro-
genation of the molecular DMC (Scheme 4b). The rate-
determining step will be the hydrogenation of the molecular
DMC adspecies. The molecular DMC adspecies is more
reactive than the formate adspecies or CO2 from the viewpoint
of the electronic density of the carbonyl group, so that the
hydrogenation of DMC will proceed at lower reaction
temperature than hydrogenation of CO2. In addition, no
activity for hydrogenation CO2 over Cu(1)/CeO2 suggests that
Cu(1)/CeO2 is especially effective for DMC hydrogenation.
The higher activity of Cu subnanoparticles than those of
reported Cu-SiO2 in hydrogenation of DMC (>90-fold, Table
S2) indicates that the particle size of Cu metal is largely
connected to the reaction rate. Considering the zero reaction
orders with respect to the H2 pressure and DMC concentration,
adsorption of DMC and dissociation of H2 will be very fast, and
the adspecies of DMC and hydrogen will be saturated on the
Cu particles. Therefore, the active hydrogen species can be
produced on Cu subnanoparticles, which leads to the high
activity of Cu-nanoparticles on Cu(1)/CeO2 in the hydro-
genation of DMC.
much larger than the loading amount of Cu (Table S7),
indicating that Cu species was reduced to Cu metal and that a
part of CeO2 was reduced (Ce4+ → Ce3+). Compared with the
signal of CeO2, the signals of Cu(x)/CeO2 were shifted to
lower temperature, indicating that Cu species promote the
reduction of CeO2. From these results, Cu species and CeO2
were strongly interacted, leading to promotion of reduction of
both CeO2 and CuOx species. According to the previous works
about Cu-supported CeO2 catalysts in wet oxidation of phenol
or gas shift reaction,17 CuOx species were easily highly
dispersed on CeO2 by the strong interaction between CeO2
and Cu ion. This good affinity of Cu ion with CeO2 will be
related to the formation of Cu metal subnanoparticles. As
above, we first disclosed that Cu subnanoparticles can be easily
formed on CeO2 by H2 reduction owing to the strong
interaction between Cu species and CeO2. To confirm the
stability of the Cu metal subnanoparticles, XRD patterns and
XAFS before and after the reaction were compared (Figures
S5−7), which provided similar results. Taking the reusability of
Cu(1)/CeO2 catalyst into consideration (Table S3), Cu metal
subnanoparticles are stable under the reaction conditions. This
catalyst is quite different from the reported Cu-based catalysts
in hydrogenation of carbonates in terms of the particle size of
Cu and the active species.9 In particular, both Cu0 and Cu+ can
be active species in the reported catalysts; in contrast, the Cu
metal subnanoparticles are responsible for this high catalytic
performance.
In summary, we have demonstrated that Cu/CeO2 is an
effective heterogeneous catalyst for hydrogenation of DMC to
methanol at the low reaction temperature of 433 K and even at
low H2 pressure of 2.5 MPa. The methanol yield reached 94%
and 98% based on the carbonyl of DMC and total produced
methanol, respectively, and the TON based on total Cu metal
was 224 and 701 based on the carbonyl and total produced
methanol, respectively, which are 2 orders of magnitude higher
than those in the previous reports. The activity of Cu(1)/CeO2
in this reaction is also pretty high, and the methanol formation
To discuss the mechanism over Cu subnanoparticles, kinetic
parameters of H2 pressure and DMC concentration were
investigated. The reaction orders with respect to H2 pressure
and DMC concentration were calculated to be 0.1 and 0.2,
respectively (Figure S8). These results indicate that H2 pressure
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hydrogenation of CO2 (more than 2 orders of magnitude at
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analyses such as XAFS, XRD, TEM, and TPR showed that
Cu species on Cu(1)/CeO2 were reduced to Cu metal and that
the size of Cu metal is below 1 nm (Cu subnanoparticles). We
can conclude that heterogeneous Cu metal subnanoparticles
can be easily formed on CeO2, exhibiting the high catalytic
performance for hydrogenation of DMC.
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ASSOCIATED CONTENT
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* Supporting Information
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The Supporting Information is available free of charge on the
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Experimental details of the catalyst preparation, reaction
and catalyst characterization, and the details of results
(XRD, TEM, TPR, XAFS, kinetics, and GC-chart)
(PDF)
Elam, J. W.; Meyer, R. J.; Redfern, P. C.; Teschner, D.; Schlogl, R.;
̈
AUTHOR INFORMATION
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Corresponding Authors
*E-mail: mtamura@erec.che.tohoku.ac.jp.
*E-mail: tomi@erec.che.tohoku.ac.jp.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acscatal.5b02258
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