Selective Hydrogenation of CO2 to Ethanol over Cobalt Catalysts

Article abstract of DOI:10.1002/anie.201800729

Methods for the hydrogenation of CO₂ into valuable chemicals are in great demand but their development is still challenging. Herein, we report the selective hydrogenation of CO₂ into ethanol over non-noble cobalt catalysts (CoAlO_x), presenting a significant advance for the conversion of CO₂ into ethanol as the major product. By adjusting the composition of the catalysts through the use of different prereduction temperatures, the efficiency of CO₂ to ethanol hydrogenation was optimized; the catalyst reduced at 600 ° gave an ethanol selectivity of 92.1 % at 140 °C with an ethanol time yield of 0.444 mmol g^?1 h^?1. Operando FT-IR spectroscopy revealed that the high ethanol selectivity over the CoAlO_x catalyst might be due to the formation of acetate from formate by insertion of *CH_x, a key intermediate in the production of ethanol by CO₂ hydrogenation.

Full text of DOI:10.1002/anie.201800729

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Selective hydrogenation of CO2 into ethanol over cobalt catalysts
Authors: Lingxiang Wang, Liang Wang, Xiaolong Liu, Hai Wang, Wei
Zhang, Qi Yang, Jingyuan Ma, Xue Dong, Seung Jo Yoo, Jin-
Gyu Kim, Xiangju Meng, and Feng-shou Xiao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800729
Angew. Chem. 10.1002/ange.201800729
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http://dx.doi.org/10.1002/ange.201800729

Angewandte Chemie International Edition
10.1002/anie.201800729
COMMUNICATION
Selective hydrogenation of CO
2
into ethanol over cobalt catalysts
Lingxiang Wang, Liang Wang,* Jian Zhang, Xiaolong Liu, Hai Wang, Wei Zhang,* Qi Yang, Jingyuan
Ma, Xue Dong, Seung Jo Yoo, Jin-Gyu Kim, Xiangju Meng, and Feng-Shou Xiao*
Abstract: Hydrogenation of CO
demand but still challengeable. Here we report the selective
hydrogenation of CO into ethanol over non-noble cobalt catalysts
CoAlO ), presenting a significant advancement for the conversion of
CO into ethanol as a major product. By adjusting the Co-CoO
composition of the catalysts under different pre-reduction
temperatures, the efficiency of CO hydrogenation to ethanol was well
2
into valuable chemicals is in great
bond and selective formation of desired chemicals are still
challenging.
2
A scope of effective catalysts have been developed for the
[
2-4]
(
x
hydrogenation of CO
Au,[3a] Cu,[3b] industrial Cu/ZnO/Al
O
2 3
2
,
typically the ceria-titania decorated with
,[2b,c] Ni-Ga,[3d] Zn-Zr,[4a] Mn-
2
x
Co,[4b] Fe-Co[4c] catalysts. However, these catalysts provide
methanol as major product. In the past few years, the
homogeneous catalysts assembled with air-sensitive organic
ligand emerged to be selective catalysts for the production of C2+
2
optimized over the catalyst reduced at 600 °C, giving ethanol
selectivity of 92.1% at 140 °C with the ethanol time yield at 0.444
-
1
-1
products from CO
2
hydrogenation,[5] but these homogeneous
catalysts are difficult to be regenerated or even separated from
the reaction system. Heterogeneous catalysts are favorable for
mmol g h . Operando FT-IR spectroscopy reveals that the high
selectivity of ethanol over the CoAlO catalyst is ascribed to the
formation of acetate from formate with the insertion of *CH , the key
intermediate to produce ethanol from CO hydrogenation.
x
x
2
the CO
catalyst regeneration. Therefore, supported noble metal catalysts
Pd and Pt) have attracted much attention in CO hydrogenation
2
hydrogenation and can easily overcome the issues in
(
2
Ethanol is a very useful chemical in both industry and our daily
life, which has been widely used as a clean fuel additive, solvent,
due to their extraordinary ability in C-C coupling, an important step
of C≥2 molecule production.[6] But the co-existence of mixed C2+
[
1]
and disinfectant. The conventional route of ethanol production
relies on the fermentation of cellulosic feedstock, which competes
for food with the human beings.[1c] Developing new routes of
ethanol production with industrial potentiality is of great
importance, and a scope of successes have been achieved from
syngas.[1a,b] Particularly, an attractive route is the employment of
alcohol products leads to limited ethanol selectivity. The
replacement of the scarce and expensive noble metals with cheap
and abundant first-row transition metals, such as cobalt, which is
well known as efficient catalyst for the C-C coupling-based
reactions (e.g. Fischer-Tropsch synthesis),[7] would enhance the
sustainability of the CO
synthesis from hydrogenation of CO
2
hydrogenation. In regard to the ethanol
, the challenges of potentially
2
CO as feedstock since its significant advantages including easy
availability and renewability, which is also attractive for eliminating
2
practical and sustainable routes rely on employing abundant
metal catalysts and forming ethanol at high selectivity, which is in
great demand but extremely difficult to achieve.
the greenhouse gas by converting CO
2
into valuable chemicals.[
2-
4
]
However, the CO
2
-to-ethanol transformation is difficult owing to
. Effective cleavage of the C=O
the chemical inertness of CO
2
2
Herein, we reported the selective hydrogenation of CO to
ethanol with abundant cobalt catalysts. Key to this success is the
employment of cobalt catalysts derived from Co-Al layered double
hydroxide (LDH), followed by calcination and reduction to form
[
[
[
a]
b]
c]
L. Wang, Dr. L. Wang, J. Zhang, H. Wang, X. Meng, Prof. F.-S. Xiao
Department of Chemistry, Zhejiang University
Hangzhou 310028, China
E-mail: liangwang@zju.edu.cn; fsxiao@zju.edu.cn
X. Liu
Wuhan Institute of Physics and Mathematics
Chinese Academy of Sciences
Wuhan 430071, China
alumina supported cobalt particles (CoAlO
reduction temperature to adjust the Co-CoO
CoAlO catalysts, the efficiency of CO hydrogenation to ethanol
x
). By optimizing the
x
phases on the
x
2
was well optimized with the catalyst reduced at 600 °C, showing
ethanol selectivity of 92.1% at 140 °C with the ethanol time-yield
at 0.444 mmol g-1 h-1. This result is even comparable with the Pt
Prof. W. Zhang
catalysts,[6a] outperforming all abundant metal catalysts in CO
2
Key Laboratory of Mobile Materials MOE & School of Materials
Science and Engineering, Jilin University
Changchun 130012, China
E-mail: weizhang@jlu.edu.cn
Q. Yang, J. Ma
Shanghai Institute of Applied Physics, Chinese Academy of
Sciences
Shanghai 201204, China
hydrogenation to ethanol reported previously. Mechanism study
demonstrates that the transformation undergoes CO adsorption
2
and activation, formate intermediates formation, insertion to
acetate, and hydrogenation to produce ethanol. More importantly,
the catalyst is stable and exhibits constant performances in
continuous recycle tests.
[
d]
[
e]
f]
X. Dong
2
The cobalt catalysts for CO hydrogenation were derived from
Department of Chemistry and Biochemistry, Texas Tech University
Texas 79409, United States
S. Yoo, J. G. Kim
Electron Microscopy Research Center, Korea Basic Science
Institute
Daejeon 34133, South Korea
the Co-Al layered double hydroxide, which was synthesized by a
co-precipitation assisted with urea, followed by calcination to Co-
Al mixed oxides. After reduction in hydrogen at different
temperatures from 300 to 650 °C, the alumina supported cobalt
[
catalysts were obtained, which are denoted as CoAlO
reduction temperature).
x
-T (T is the
[g]
J. Zhang, Prof. F.-S. Xiao
Beijing Advanced Innovation Center for Soft Matter Science and
Engineering, Beijing University of Chemical Technology
Beijing, 100029, China
Figure 1 shows the catalytic data of various catalysts in the
hydrogenation of CO at 140 and 200 °C . The pure alumina
without cobalt species is inactive for the reaction. Interestingly,
2
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Angewandte Chemie International Edition
10.1002/anie.201800729
COMMUNICATION
the CoAlO
product, but methanol, n-propanol and n-butanol are also
detectable. The time-yield and selectivity of ethanol over CoAlO
catalysts are sensitive to the reduction temperature performed on
the catalysts, and the best performance was achieved over
x
catalysts are always active, giving ethanol as a major
S10). X-ray absorption near-edge structure (XANES)
spectroscopy was carried out to investigate the coordination and
x
the oxidation state of Co on the CoAlO
that the Co K-edge feature changed from Co O
x
samples. Figure 2b shows
to CoO by
3 4
reduction at 400 °C. After the reduction 600 °C, the metallic Co
appeared with the co-existence of oxide. This transformation was
further evidenced by the extended X-ray absorption fine structure
(EXAFS) results (Figure 2c). With the increase of reduction
temperature from 300 °C to 400 °C, the intensity of Co-O
x
CoAlO -600 by optimizing the solvents, catalyst amount, and
cobalt content (Figures S1-S6, Table S1), giving ethanol
selectivity of 92.1% at 140 °C, where the time-yield of ethanol can
reach as high as 0.444 mmol gcat h-1. Further increasing the
-1
reaction temperature to 200 °C gave enhanced alcohol
x
decreased, the Co-Co (CoO ) transformed from peaks at 2.86 and
productivity of 1.003 mmol gcat h-1 with ethanol selectivity of
-
1
3.37 Å to a single peak at 3.00 Å (phase shift corrected),
suggesting the formation of CoO as a dominant phase. The edge
features of metallic Co (Co-Co with R = 2.51 Å) obviously
appeared after reduction at 600 °C with co-existence of CoO (Co-
O with R = 1.97 Å and Co-Co with R = 2.98 Å), which might explain
the higher activity of CoAlO -600 than CoAlO -300 and CoAlO -
x x x
400 because the activation of hydrogen normally requires metallic
sites. In the end, the Co-Co coordination in metallic Co
8
8.9% (Figure S7). In this case, the ethanol productivity (0.892
mmol gcat h-1) is even higher than the time yield of all the C2+
-1
alcohols over Pt-based catalysts,[6a] which are regarded to be
superior catalysts for CO
importantly, CO, methane, and higher hydrocarbons, which are
general gaseous products in CO hydrogenation, were
undetectable after the reaction over CoAlO -600. Lower reduction
2
-to-high alcohol transformation. More
2
x
temperature led to a significant decrease in activity and ethanol
selectivity. When the catalyst was reduced at 300 °C, the total
x
(coordination number at 3.6) became dominant on CoAlO -600
with a slight signal of Co-Co coordination in CoO (coordination
number at 1.0, Table S4). We speculate that the presence of
time-yield of alcohol products was only 0.269 mmol gcat h-1 over
-
1
CoAlO
In contrast, the commercial Co
poor selectivity to ethanol of 25.8%. The Cu/ZnO/Al
which has been widely utilized for CO
hydrogenation,[
only methanol as the product at 140 °C and CO existed at 200 °C
with undetectable ethanol. The alumina supported Fe- and Ni-
catalysts exhibited methanol as major product with a slight
amount of ethanol under the equivalent reaction conditions
x
-300 catalyst with ethanol selectivity of 64.3% at 140 °C.
metallic Co is necessary for the efficient hydrogenation of CO
ethanol, and the co-existence of Co and CoO phases might be
favorable, because the reduction at higher temperature for
CoAlO -650 with more metallic Co species and slight amount of
CoO (Figure 2a) showed enhanced methanol yields and
decreased ethanol yields compared to CoAlO -600 (Figure S11).
2
to
O
3 4
reduced at 600 °C displayed
x
2
O
3
catalyst,
2b,c]
2
gave
x
x
x
(
Figure S8, Tables S2 and S3). All these data confirm the
extraordinary catalytic performances of CoAlO catalysts for the
CO -to-ethanol transformation (Figure S9).
x
2
Figure 2. Structure characterization of CoAlO
CoAl-LDH, Co-Al oxides and CoAlO catalysts reduced at different
temperatures. (b) Co K-edge XANES. (c) EXAFS of various CoAlO samples.
x
catalysts. (a) XRD patterns of
x
x
Figure 3a shows scanning electron microscopy (SEM) image
of CoAlO -600. Although the layered structure was destroyed by
x
calcination (see the XRD patterns), the plate-like morphology still
remained. Transmission electron microscopy (TEM) provides a
direct observation of the CoAlO
dispersed Co particles with a mean size of 4.6 nm on the
amorphous Al support. The Co nanoparticles were well
identified on Al O support by using high-angle annular dark field-
scanning transmission electron microscopy (HAADF-STEM)
image (Figure 3c), where the Co species display brighter contrast
x
-600 sample, showing highly
2
O
3
Figure 1. Data showing the catalytic performances of various catalyts in CO
hydrogeantion. Reaction conditions: 20 mg of catalyst, 2 mL of H O solvent,
initial pressure 4.0 MPa (H /CO = 3:1), 15 h, reaction temperature at (a) 140
2
2
3
2
2
2
and (b) 200 °C. The yields and selectivities were based on the moles of carbon.
than Al
2 3
O . The Co and Al distribution was further confirmed by
Figure 2a shows the XRD patterns of various samples, where
the reduction at high temperatures (e.g. 500-650 °C) led to the
subsequent formation of CoO and Co as dominant phases (Figure
using EDX elemental maps in Figures 3d-f (Figure S12), obviously
showing the dispersion of Co nanoparticles on Al
which are in good agreement with the STEM image.
2 3
O support
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Angewandte Chemie International Edition
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COMMUNICATION
The surface geometry of the CoAlO
studied by HR-TEM characterization. For CoAlO
the faceting of Co particles changed along the line where the
boundary terminates at the surface, assigning to the co-existing
phases of metallic Co and CoO (Figure 3h), as confirmed by a
Fast Fourier transform (FFT) analysis (Figure 3i). The electron
x
catalysts was further
-600 catalyst,
which
selectivity/productivity over CoAlO
CoAlO -500, CoAlO -600 and CoAlO
Cu/ZnO/Al catalyst gave the bands of various C
intermediates with completely undetectable C -species (Figures
S21 and S22),[9] which might explain the sole production of
might
explain
the
relatively
-400 catalyst than those over
-650. On the contrary, the
-oxygen
lower
ethanol
x
x
x
x
x
2
O
3
1
2
energy loss spectrum (EELS) of CoAlO
x
-600 gives the
2 3
methanol on Cu/ZnO/Al O catalyst.
approximate ratio 4.2 of Co L3 and L2 edges, locating between
metallic Co and CoO. It indicates the Co valence locates between
0
and +2, suggesting the CoO and Co are the dominating phases
of CoAlO -600 catalyst after five
-600.[8] Notably, the used CoAlO
runs in CO hydrogenation still exhibits similar phases to the fresh
catalyst (Figure S13), demonstrating its good stability. The
CoAlO reduced at different temperatures gives distinguishable
phases. The CoAlO -300 has pure Co , and metallic Co is
almost undetectable. Increasing the reduction temperatures at
00-650 °C led to the subsequent formation of CoO and metallic
Co (Figures S14-S18), which is proved by the HR-TEM images.
x
x
2
x
x
3 4
O
4
1
Figure 4. In situ FTIR and H NMR spectroscopy investigation. (a and b) In situ
FTIR spectra of (A) CoAlO
x
-400, (B) CoAlO
x
-500, (C) CoAlO
x
-600, and (D)
and H
2
CoAlO
x
-650 at 350 °C with different feed gases of (a) CO
2
and (b) CO
2
with molar ratio at 1/3. (c and d) H NMR spectrum (c) and 1H COSY NMR
1
spectrum (d) of the mixture in the reactor during the reaction. (e) In situ FTIR
Figure 3. Electronic microscopy characterization of CoAlO
x
-600. (a) SEM image.
b) TEM image (insert: size distribution of Co NPs). (c-f) HAADF-STEM image
c), EDX elemental maps (d: CoAlO -600; e: Al, f: Co). (g-i) HRTEM images (g
x 2 2
spectra of CoAlO -600 in a continuous CO flow with pulsed H .
(
(
1
The formation of C
MAS NMR spectrum of the reaction mixture (Figure 4c), which
gives signals at 0.72 and1.35 ppm assigned to *CH group and
2
x
*CH species bonded to the oxygen on the CoAlO -600 catalyst
2
-species was further confirmed by the H
x
and h), The corresponding FFT image of Co NPs (i) in (h). (j) EELS result. The
pink cycles highlighted the cobalt particles.
3
To determine how the Co phases influence the catalytic
surface (*C -oxygen), respectively. 1H homonuclear chemical
performances, we investigated the adsorption of CO
catalysts by in-situ FTIR. As shown in Figure 4a, the biggest
difference among the CO adsorption on CoAlO samples is that
CoAlO -400 and CoAlO -500 have weak signals at 1396 and 1434
cm , while CoAlO -600 and CoAlO -650 give a single and sharp
peak at 1434 cm-1 with strong intensity, demonstrating that a large
amount of CO (CO
2-) was asymmetrically adsorbed on CoAlO
00 and CoAlO -650 samples (Figure S19).
To investigate the reaction intermediates, the operando FTIR
was carried out to understand the interaction between CO and
hydrogen on the cobalt catalysts (Figure 4b), where the bands at
2
on the
1
shift correlation (COSY) spectrum (Figure 4d) showed cross
contours at (1.35, 0.72) and (0.72, 1.35), indicating that the
2
x
x
x
aforementioned *CH3 and *C -oxygen groups on the catalyst
1
-1
x
x
surface are interrelated. These two signals should come from the
-C-C bond structure with characteristics of the two signals. These
results suggest that the *CH prefers to couple with *C -oxygen
2
3
x
-
3
1
6
x
species to form C -oxygen species, rather than deep
2
hydrogenation for the formation of methane product, which might
explain the undetectable methane in the gaseous products.
In order to gain more detailed information on the reaction
intermediates, we performed the operando FTIR spectra of
CoAlO -600 in a continuous CO2 flow with pulsed hydrogen,
x
where the hydrogen are insufficient to decelerate the
transformation. As shown in Figure 4e, the CO2δ-, CO32-, and
2
-
1
1
1
038, 1362, 1471, 1584, and 2895 cm appeared. The bands at
-1
038 and 1362 cm correspond to the ethoxy species, and the
-
1
bands at 1471, 1584, and 2895 cm can be assigned to the
acetate species.[9-11] All these data confirm the formation of C
oxygen of ethoxy and acetate species on CoAlO -600 from CO
hydrogenation, which are regarded as important intermediates for
ethanol production. Similar bands assigned to -oxygen
intermediates were also observed in the in-situ IR spectra of CO
hydrogenation over CoAlO -500 and CoAlO -650 catalysts. It
seems that the Co-CoO phases are important to the formation of
-oxygen intermediate, and CoAlO -400 without metallic Co
phases gives very weak bands of C
2
-
δ-
-1
x
2
HCO3 species (1229, 1430, 1649 cm ) appeared at the
beginning of the reaction due to the primary adsorption and
activation of CO (Figure S23).[ After pulsing slight amount of
11]
C
2
2
2
hydrogen, the above signals are remarkably decreased, giving
-1
-1
x
x
new bands at 1373, 1594 and 2904 cm . The band at 2904 cm
confirmed the presence unsaturated C-H species (*CH ), and the
bands at 1373 and 1594 cm-1 are assigned to symmetric and
x
x
C
2
x
-
[12]
2
-oxygen (Figures 4b and S20), asymmetric OCO stretches of formate (HCOO ) species.
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Angewandte Chemie International Edition
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COMMUNICATION
Generally, the formate species is stable and difficult to be
hydrogenized,[3b] thus hinders methanol formation over the
Natural Science Foundation of China (21720102001, 91634201
and 91645105), Natural Science Foundation of Zhejiang Province
(
LR18B030002), and the Jilin Province/Jilin University co-
CoAlO
x
-600 catalyst. Interestingly, further introduction of
Construction Project-Funds for New Materials (SXGJSF2017-3,
Branch-2/No.440050316A36). We acknowledge beamtime at
Shanghai Synchrotron Radiation Facility (SSRF, beamline
BL14W1).
hydrogen into the catalyst caused the decrease of formate bands,
-1
but giving the new bands at 1521-1584 cm , which are assigned
to the asymmetric OCO stretches in the acetate species.[13] The
formation of acetate is further confirmed by another new band at
-
1
2
896 cm assigned to saturated C-H. Meanwhile, the bands of
2
Keywords: CO hydrogenation · cobalt catalyst · ethanol
-1
x
unsaturated C-H species (*CH , 2904 cm ) and formate (1373
cm ) were significantly reduced (Figure 4e), giving the new bands
associated with C -oxygen species. These results suggest that
-1
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x
2
is indeed transformed into C -oxygen species via the
insertion,[
2d,12]
which might hinder further hydrogenation for the
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acetate group is weakened, benefiting the further hydrogenation
-1
to form ethoxy species (1360 cm ) when more hydrogen was
introduced.[14] All these data confirmed that the ethanol formation
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hydrogenation ability to produce *CH intermediate to transform
x
-600 might be attributed to the
x
[
formate to acetate, an important intermediate for ethanol
production. On the other hand, poorly reduced oxide surface,
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x
selectivity to ethanol but enhanced methanol selectivity, which is
likely due to the weak hydrogenation activity to form acetate
intermediate (Figure 4b).
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The improved performances over CoAlO -600 is likely due to the
optimized surface oxides with co-existence of Co-CoO phases,
which benefit the production of *CH for converting formate into
acetate via insertion, an important intermediate for ethanol
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hydrogenation for selective production of ethanol, but also open
2
by using cobalt catalysts
(
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[
[
[
1
2
1
an avenue for developing non-noble metal catalysts for CO
conversion.
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This work is supported by National Key Research and
Development Program of China (2017YFC0211101) and National
403.
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Angewandte Chemie International Edition
10.1002/anie.201800729
COMMUNICATION
COMMUNICATION
Transformation of greenhouse gases
CO into ethanol: A persuasive CO -to-
2
2
ethanol transformation over designed
cobalt catalysts gives superior ethanol
selectivity, which is reasonably ascribed to
the formation of acetate from formate with
Lingxiang Wang, Liang Wang,* Jian
Zhang, Xiaolong Liu, Hai Wang, Wei
Zhang,* Qi Yang, Jingyuan Ma, Xue
Dong, Seung Jo Yoo, Jin-Gyu Kim,
Xiangju Meng, and Feng-Shou Xiao*
the insertion of *CH
to produce ethanol
hydrogenation.
x
, the key intermediate
from CO
2
Page No. – Page No.
Selective hydrogenation of CO
ethanol over cobalt catalysts
2
into
This article is protected by copyright. All rights reserved.