Efficient hydrogenation of alkyl formate to methanol over nanocomposite copper/alumina catalysts

Article abstract of DOI:10.1002/cctc.201402560

The production of methanol, an important fuel and chemical feedstock, from carbon dioxide is an important process for CO₂ utilization. We describe herein a mild and efficient method for the indirect hydrogenation of carbon dioxide to methanol via a CO₂-derived formate ester intermediate by using a simple heterogeneous catalyst system comprising Cu highly dispersed in an alumina matrix under solvent-free conditions. This catalyst is also effective for the hydrogenation of other formate esters, such as ethyl formate, propyl formate, and butyl formate.

Full text of DOI:10.1002/cctc.201402560

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DOI: 10.1002/cctc.201402560
Efficient Hydrogenation of Alkyl Formate to Methanol over
Nanocomposite Copper/Alumina Catalysts
[
a]
[a]
[a]
[a]
[b]
Xian-Long Du, Xue-Ping Sun, Chan Jin, Zheng Jiang, Dang Sheng Su,* and Jian-
Qiang Wang*
[
a]
[
5]
The production of methanol, an important fuel and chemical
feedstock, from carbon dioxide is an important process for CO2
utilization. We describe herein a mild and efficient method for
the indirect hydrogenation of carbon dioxide to methanol via
Cu–Zn oxide based catalysts. However, these process require
high operating temperatures (200–2508C), which limits the
theoretical yield of methanol, as the CO hydrogenation reac-
2
tion is thermodynamically favored at low temperature. So, it is
desirable to find a new strategy for the efficient conversion of
a CO -derived formate ester intermediate by using a simple
2
heterogeneous catalyst system comprising Cu highly dispersed
in an alumina matrix under solvent-free conditions. This cata-
lyst is also effective for the hydrogenation of other formate
esters, such as ethyl formate, propyl formate, and butyl
formate.
CO into methanol at a relatively low reaction temperature.
2
Very recently, Milstein and co-workers developed an alterna-
tive, indirect approach from CO to methanol through hydro-
2
genation of CO -derived organic carbonates, carbamates, and
2
formates by using Ru-based homogeneous catalysts under
[6]
mild conditions. In their experiments, the formation of meth-
anol is almost quantitative through hydrogenation of the
Carbon dioxide levels in the atmosphere have reached approx-
imately 400 ppm for the first time in recorded history as a con-
above-mentioned CO derivatives under relatively mild condi-
2
tions (1.0–6.0 MPa H , 110–1458C). In particular, the hydrogena-
2
[
1a]
sequence of human activities.
Converting CO2 into useful
tion of formate ester has been established as a desirable ap-
feedstock chemicals and fuels is an important strategy for re-
proach from CO to methanol, as efficient transformation of
2
moving CO from the atmosphere and for reducing depend-
CO into formic acid and its derivatives [e.g., methyl formate
2
2
[
1]
[7]
ence on petrochemicals. Among the products derived from
(MF)] is known and well investigated. Later on, Huff and San-
CO , methanol is an extremely important basic energy chemi-
ford also reported the hydrogenation of MF to methanol cata-
lyzed by a Ru-based molecular catalyst system in a cascade
2
cal, as it is a feedstock for the production of formaldehyde, ole-
fins, dimethyl ether, methyl tert-butyl ether, acetic acid, and
catalysis for the homogeneous hydrogenation of CO to meth-
2
[
2]
[8]
a wide variety of other products. In 2005, Olah and co-work-
anol. From synthetic and economic point of views, these sys-
[
3]
ers initiated a “methanol economy” concept. According to
this proposal, methanol can serve as an efficient energy-stor-
age chemical and a fuel substitute. For example, methanol can
be converted into gasoline (MTG process), aromatics (MTA pro-
cess), ethylene and propylene in the MTO (methanol to olefins)
tems are not practically useful because of the inherent prob-
lems of non-reusability, the processing cost, as well as addi-
tional handling problems. Hence, the successful development
of an excellent reusable solid catalyst would represent a signifi-
cant advancement in the hydrogenation of formate ester to
methanol.
[
4]
process, as well as other useful petrochemicals. Therefore,
the synthesis of methanol from CO hydrogenation can reduce
Heterogeneous catalyst systems reported for the hydrogena-
tion of formate esters to methanol have mainly focused on
copper-based catalysts. Wainwright et al. reported the gas-
phase hydrogenation of methyl formate to methanol over
a copper chromite catalyst in a U-tube reactor to give >90%
2
the dependence on fossil fuels and control greenhouse gas
emissions in the future. Currently, much work in this area has
focused on the direct hydrogenation of CO into CH OH on
2
3
[9]
[
a] Dr. X.-L. Du, X.-P. Sun, Dr. C. Jin, Prof. Z. Jiang, Prof. J.-Q. Wang
Key Laboratory of Interfacial Physics and Technology
Shanghai Institute of Applied Physics
Chinese Academy of Sciences
conversion to MF at 2008C. However, the selectivity to meth-
anol reached only 70% because of the formation of 30% CO
as a byproduct. Then, their group reported the liquid-phase
hydrogenation of MF in a three-phase slurry reactor by using
Jialuo Road 2019
Shanghai 201800 (P.R. China)
E-mail: wangjianqiang@sinap.ac.cn
a copper chromite catalyst. Measurements at 1708C showed
[10]
very high selectivity to methanol.
Upon evaluating Raney
[
b] Prof. D. S. Su
copper as a catalyst for the transformation, a similar yield of
Shenyang National Laboratory for Materials Science
Institute of Metal Research
Chinese Academy of Sciences
Wenhua Road 72
Shenyang 110016 (P.R. China)
E-mail: dssu@imr.ac.cn
methanol was achieved at 110–1608C under a hydrogen pres-
[11]
sure of 5.2 MPa. MF hydrogenation has also been studied
over heterogeneous rhenium catalysts to give 46.1% conver-
sion with 90% selectivity to methanol under severe conditions
[12]
of temperature and pressure (2008C and 10 MPa H2). Recent-
Supporting information (full experimental procedures, characterization,
hydrogenation reaction, and catalyst recovery) for this article is available
on the WWW under http://dx.doi.org/10.1002/cctc.201402560.
ly, Iwasa reported the hydrogenation of MF over various sup-
[13]
ported Pd and Pt catalysts. The Pd/ZnO catalyst exhibited
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high selectivity (ꢀ95%) for methanol formation with 10%
conversion at 1508C because of the formation of Pd–Zn alloys.
Taken together, the copper chromite catalyst shows effective
catalytic activity for formate ester hydrogenation to methanol.
However, a drawback of chromium-containing catalysts is the
[
a]
Table 1. Hydrogenation of MF to methanol with various catalysts.
[
b]
[b]
Entry Catalyst
Metal loading Conv.
Selectivity [%]
Methanol CO +CO
[
wt%]
[%]
2
1
2
3
4
5
6
7
8
9
1
Cu/Al O -OG
2
2
2
2
2
3
3
3
3
3
50
40
30
20
10
20
20
20
20
50
20
30
57
76
85
92
79
90
60
14
8
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
98
<1
[
14]
Cu/Al
Cu/Al
Cu/Al
Cu/Al
O
O
O
O
-OG
-OG
-OG
-OG
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
2
toxicity of the chromium compounds.
Alumina has been
widely identified as a promising support material for many
metal catalysts, especially for catalysts used in liquid-phase hy-
drogenation reactions, because of its textural properties and
[
c]
[d]
e]
Cu/Al O -OG
2
2
2
3
3
3
[
15]
Cu/Al
Cu/Al
O
O
-CP
-IMP
acid/base characteristics. In this work, we show that a nano-
composite Cu/Al O -OG catalyst prepared by the oxalate–gel
[
2
3
Cu/ZnO-OG
Cu/ZnO–Al
Cu/ZrO -OG
(OG) coprecipitation method is an active and highly selective
0
2
O
3
12
39
51
heterogeneous catalyst for the hydrogenation of formate ester
to methanol under mild conditions (100–1508C). To the best of
our knowledge, this Cu/Al O -OG catalyst represents the most
11
2
12
2 3
Cu/Cr O
2
3
[a] Reaction conditions: MF (162 mmol), nMF/nmetal =54, H (3 MPa), 1308C,
2
efficient, simple, and ecofriendly catalytic system for the con-
venient and controlled synthesis of methanol from formate
esters to date.
6 h. [b] Conversion and selectivity were determined by GC and GC–MS.
[
c] Results for the third run for the Cu/Al
reaction conditions described in entry 4. [d] Cu/Al
precipitation with Na CO . [e] Cu/Al -IMP prepared by impregnation.
2
O
3
catalyst recycled under the
2
3
O -CP prepared by co-
2
3
2 3
O
At the start of our work, a series of copper-based catalysts
(see the Supporting Information for the preparation of the cat-
alysts) were applied to the selective hydrogenation of MF to
methanol in the absence of solvent. The problem of this partic-
ular transformation is the large variety of reduction products
that can possibly be formed (Scheme 1). Besides methanol,
decrease in the Cu loading to 10 wt% led to a clear decrease
in the yield of methanol (Table 1, entry 5). We also tested the
repeated use of the 20-Cu/Al O -OG catalyst for MF hydrogena-
2
3
tion. Much to our delight, MF conversion and methanol yield
decreased only slightly after three repeated uses (Table 1,
entry 6); inductively coupled plasma (ICP) analysis showed that
no Cu was present in the filtrate, and this is indicative of its
high stability and reusability. It is noted that the total turnover
number (TON) based on three successive runs was as high as
1092, and this is the highest TON ever reported for the hydro-
genation of MF to methanol by using a heterogeneous catalyst
(
see Table S1, Supporting Information).
To investigate the origin of the catalytic activity of the Cu/
Al O -OG catalysts, the present Cu/Al O -OG catalysts with
2
3
2
3
loadings of 10–50 wt% were characterized. Typical data such
as metal loadings, BET surface areas, and Cu metal surface
Scheme 1. Simplified pathway for the hydrogenation of MF.
areas (determined from chemical N O titration) are summarized
2
carbon monoxide and carbon dioxide are also typical products
in Table 2. It can be seen that there is only a weak relationship
between the BET surface area or the Cu particle sizes and the
performance of the Cu/Al O -OG samples; this indicates that
[
16]
formed in the hydrogenation process. As one of the main
requisites for subsequent technical applications is methanol,
the key point in MF hydrogenolysis would be to avoid the for-
2
3
the external texture is not a key factor in determining the cata-
mation of CO and CO byproducts. We first tested
2
Cu-Al O -OG catalysts with different loadings, which
2
3
2 3
Table 2. Physicochemical properties of the Cu/Al O catalysts with different Cu load-
ings and preparation methods.
were prepared by an oxalate–gel coprecipitation
method. Prior to the reaction, the copper-based cata-
lysts were reduced by a flow of 5% hydrogen in
argon at 3008C for 2 h and successively passivated
by 2% oxygen diluted with helium (see the Support-
ing Information for the experimental details). Table 1
demonstrates that selective production of methanol
in 57% yield could be achieved if 50 wt% Cu/Al O -
[
a]
[
b]
[b]
[c]
Entry Catalyst Cu loading SBET
SCu
SCu
DCu
[%]
dCu
2
catÀ1
2
catÀ1
2
CuÀ1
[%]
[m g
]
[m g
]
[m g
]
[nm]
1
2
3
4
5
6
7
Cu/Al O -OG
2
2
2
2
2
3
3
3
3
3
10
20
30
40
50
44
43
32
29
27
19
18
6.0
17.8
19.7
21.2
24.1
6.8
60.0
89.0
65.6
53.0
48.2
22.6
3.3
9.2
–
Cu/Al
Cu/Al
Cu/Al
Cu/Al
O
O
O
O
-OG
-OG
-OG
-OG
13.5 14.3
10.1 15.2
8.2 21.3
7.5 27.8
4.1 45.1
0.5 80.6
2
3
OG was used (Table 1, entry 1). A decrease in the Cu
loading to 40 or 30% significantly increased the yield
of methanol (Table 1, entries 2 and 3). The highest
product yield was 92%, which was obtained by using
the Cu/Al O -OG catalyst containing 20 wt% copper
Cu/Al O -CP
30
2
2
3
3
Cu/Al
O
-IMP 30
1.0
[
a] SBET =BET surface area, SCu =Cu metal surface area, DCu =copper dispersion, dCu
size of copper metallic particles. [b] Determined by N O titration method. [c] Estimat-
ed by the Scherrer equation from the diffraction peaks of Cu in the XRD patterns.
=
2
2
3
(
20-Cu/Al O -OG; Table 1, entry 4). However, a further
2 3
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lytic activity of the present samples prepared by oxalate–gel
coprecipitation. However, by careful correlation of the Cu dis-
persion data included in Table 2, it was found that there is
a good relationship between the metallic copper dispersion
and the performance of the Cu/Al O -OG catalysts with various
2
3
Cu loadings. Notably, the data of the metallic Cu surface area
as measured by N O titration demonstrate clearly that the 20-
2
Cu/Al O -OG catalyst exhibits the highest copper dispersion of
2
3
all the samples. Therefore, the fact that the specific composi-
tion of the 20-Cu/Al O -OG sample can substantially maximize
2
3
the exposure of the fraction of catalytically active species at
the catalyst surface is crucial for achieving high activity in MF
hydrogenation, which is in line with the large amount of litera-
ture documenting the structure–activity relationship for sup-
2 3
Figure 2. HRTEM images of Cu particles in the reduced 20-Cu/Al O -OG cata-
lyst.
[
17]
ported copper catalysts.
In situ XRD measurements were used to provide further in-
sight into the genesis of the active phases in the 20-Cu/Al O -
2
3
OG catalyst. The calcined catalyst was reduced by passing
vol% H in argon while increasing the temperature with si-
ther investigated by collecting the X-ray absorption fine struc-
ture (XAFS) data at the Cu K edge. Both the X-ray absorption
near-edge structure (XANES) region, used to determine the oxi-
dation state of the Cu species, and the extended X-ray absorp-
tion fine structure (EXAFS) region of the data were analyzed.
The XANES region of the Cu K edge (Figure 3) shows a clear
pre-edge peak maximum at 8982.2 eV, which is characteristic
5
2
multaneous XRD analysis. As shown in Figure 1, for the sample
0
of a 1s-to-3d transition in the Cu compound. This can be fur-
ther confirmed by the presence of a main peak at 2.22 ꢁ in the
Fourier transform EXAFS region, which can be attributed to
the CuÀCu metal bond (Figure S4). This scenario can be further
corroborated by transmission electron microscopy (TEM), XRD,
and temperature-programmed reduction (TPR) analyses (see
Figure S1–S3).
Figure 1. In situ XRD patterns of the 20-Cu/Al
2
O
3
-OG catalyst taken during
/Ar.
reduction of the calcined Cu/Al oxide precursors in 5% H
2
O
3
2
calcined at 4008C, two peaks exist at 2q=35.5 and 38.88,
which correspond to the (002) and (111) characteristic peaks
of CuO. No peaks of Al O are observed, which can be ascribed
2
3
[
18]
to particles that are too fine to be detected. If the reduction
temperature was increased to 2008C, a clear peak positioned
at 2q=43.38 appeared, which was ascribed to the (111) lattice
plane of metal Cu. Along with a continuous increase in the re-
duction temperature to 3008C, the intensity of the Cu peak in-
creased gradually and no diffraction features of CuO could be
identified; this indicates that CuO was completely reduced into
metallic Cu. The average size of the resultant Cu particles was
estimated to be approximately 14 nm on the basis of the
Scherrer equation. High-resolution transmission electron mi-
croscopy (HRTEM) investigation of the reduced catalysts
showed that Cu nanoparticles approximately 14 nm in size
were observed (Figure 2), which is consistent with the XRD re-
sults. An ordered lattice fringe with an interplanar spacing of
Figure 3. XANES at the Cu K absorption edge of the 20-Cu/Al
Cu foil, Cu O, and CuO.
2 3
O -OG catalyst,
2
In a comparison of other Cu/Al O catalysts with identical Cu
2
3
loadings prepared by conventional coprecipitation and im-
pregnation methods, it was observed that the Cu/Al O -OG
2
3
catalyst featuring a higher component dispersion prepared by
the oxalate–gel coprecipitation method had the best MF hy-
drogenation activity (Table 1, entries 6–8). The hydrogenation
reaction did not occur in the absence of the catalysts and
<2% methanol was formed upon using Al O , Cu, Cu O, or
0.209 nm is clearly seen at the crystal edge, which can be as-
2
3
2
cribed to the (111) crystal plane of Cu with a face-centered
cubic structure. The structure of Cu at 20-Cu/Al O -OG was fur-
a mixture of Cu–Al O3 and Cu O–Al O as catalysts directly
2 2 2 3
(Table S2). Therefore, the stronger interaction between copper
2
3
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and the alumina support generated during catalyst preparation
by using oxalate–gel coprecipitation should be the major
Table 3. Selective hydrogenation of various formate esters into methanol
[a]
over a fresh 20-Cu/Al
2
O
3
-OG catalyst.
[
19]
reason for its high activity.
As the choice of the support
[
b]
Entry
Substrate
T
t
[h]
Conv.
[%]
Methanol yield
[%]
plays an important role in metal catalysis, we also evaluated
copper supported on other supports prepared by the oxalate–
gel coprecipitation method and commercial copper-based cat-
[
8C]
1
2
3
4
methyl formate
ethyl formate
propyl formate
butyl formate
130
130
150
150
8
10
10
12
100
98
97
99
92
91
91
alysts. Conventional Cu/ZnO-OG and industrial Cu/ZnO–Al O3
2
methanol synthesis catalysts gave low activity (Table 1, en-
95
tries 9 and 10). Upon using the Cu/ZrO -OG catalyst, the yield
2
[
[
a] Reaction conditions: MF (162 mmol),
b] Yield determined by GC and GC–MS.
n
MF/nmetal =54,
H
2
(3 MPa).
of methanol was approximately as high as 39%. Considering
that Cu/Cr O is widely used in the hydrogenation of MF to
2
3
methanol, we also tested its catalytic activity. The use of this
commercial catalyst resulted in the formation of the desired
methanol product in moderate yield (Table 1, entry 12). Among
the different supported copper catalysts examined, the Cu/
Al O -OG catalyst exhibited the best catalytic performance. To
presents the results for the synthesis of methanol from differ-
ent formate esters under liquid-phase conditions. For example,
ethyl formate, propyl formate, and butyl formate were selec-
tively hydrogenated to methanol, and the corresponding alco-
hols were obtained in high yields (Table 3, entries 2–4).
2
3
clarify the origin of the enhanced activity of the catalyst to-
wards MF conversion upon using Al O as a support, HCOOCH
3
2
3
temperature-programmed desorption (HCOOCH -TPD) meas-
In summary, we demonstrated that a simple heterogeneous
catalyst system comprising Cu highly dispersed in an alumina
matrix can efficiently catalyze the selective hydrogenation of
3
urements were conducted over different supported copper
catalysts. These measurements revealed that a significantly
greater amount of MF was desorbed from the Cu/Al O -OG
CO -derived formate esters into methanol under solvent-free
2
3
2
catalyst relative to that desorbed from other catalysts
Figure 4), which implies that the high adsorption capacity of
MF associated with the Cu/Al O -OG catalyst was responsible
conditions. The sequence of catalytic activity for the synthesis
of methanol from the hydrogenation of methyl formate is in
good agreement with the results of the physicochemical prop-
erties; the 20-Cu/Al O -OG catalyst with the highest copper dis-
(
2
3
for its superior performance in the transformation of MF into
methanol.
2
3
persion prepared by the oxalate–gel coprecipitation method
exhibited the highest activity. The observation that the copper-
based metal oxide catalyst could be reused suggests that this
catalyst is compatible with a high yield and continuous opera-
tion, which thus paves a new way toward a renewable chemi-
cal industry. The findings in this work could inspire the devel-
opment of new efficient heterogeneous catalysts for the con-
version of CO2 into methanol through indirect synthetic
methodology.
Having established that 20-Cu/Al O -OG was the best cata-
2
3
lyst for the hydrogenation of MF to methanol, our attention
shifted toward optimization of the reaction parameters by
varying the reaction temperature and pressure (see Figure S5
and S6). First, the effect of the reaction temperature was inves-
tigated at a hydrogenation pressure of 3.0 MPa. An increase in
reaction temperature from 100 to 1308C led to a dramatic in-
crease in the MF conversion from 25 to 92% after a reaction
time of 6 h. As the temperature was further increased to
1
408C, complete MF conversion was obtained within 6 h.
Experimental Section
Moreover, studies on the effect of the hydrogen pressure (pH2)
in the range from 1 to 3 MPa at 1308C revealed that the reac-
tion proceeded much more rapidly at higher pressures. Build-
ing upon these results, we started to examine the scope of the
hydrogenation of formate esters with 20-Cu/Al O -OG. Table 3
Preparation of Cu/Al O -OG catalysts
2
3
A series of Cu/Al O -OG catalysts were prepared by the oxalate–gel
2
3
[17]
coprecipitation method following the reported procedure.
A
2
3
mixed alcoholic solution (each 0.1m) of copper nitrate and alumi-
num nitrate was injected rapidly into an alcoholic solution of
a 20% excess amount of oxalic acid at room temperature under
vigorous stirring. The resultant gel-like precipitates were separated
by centrifugation followed by drying at 1108C overnight. Then, cal-
cination of the as-obtained materials was performed in a muffle
oven at 4008C for 4 h. Following calcination, the catalysts were re-
duced at 3008C under 5 vol% H /Ar atmosphere for 2 h. After re-
2
duction, the catalysts were allowed to cool to room temperature
and were then passivated in flowing 2% oxygen in helium for 3 h.
Hydrogenation of MF
The hydrogenation of MF was performed in a 50 mL high-pressure
Parr reactor. First, MF (162 mmol) was added into the autoclave.
The prereduced copper-based catalyst was quickly added into the
reactor after replacing the air in the autoclave with approximately
3
Figure 4. HCOOCH TPD profiles for Cu catalysts supported on different
supports.
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Published online on September 12, 2014
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