Synthesis of ethanol via syngas on Cu/SiO2 catalysts with balanced Cu0-Cu+ sites

Article abstract of DOI:10.1021/ja3034153

This paper describes an emerging synthetic route for the production of ethanol (with a yield of ～83%) via syngas using Cu/SiO₂ catalysts. The remarkable stability and efficiency of the catalysts are ascribed to the unique lamellar structure and the cooperative effect between surface Cu⁰ and Cu⁺ obtained by an ammonia evaporation hydrothermal method. Characterization results indicated that the Cu⁰ and Cu⁺ were formed during the reduction process, originating from well-dispersed CuO and copper phyllosilicate, respectively. A correlation between the catalytic activity and the Cu⁰ and Cu⁺ site densities suggested that Cu⁰ could be the sole active site and primarily responsible for the activity of the catalyst. Moreover, we have shown that the selectivity for ethanol or ethylene glycol can be tuned simply by regulating the reaction temperature.

Full text of DOI:10.1021/ja3034153

Communication
pubs.acs.org/JACS
Synthesis of Ethanol via Syngas on Cu/SiO Catalysts with Balanced
2
0
+
Cu −Cu Sites
†
†
Jinlong Gong, Hairong Yue, Yujun Zhao, Shuo Zhao, Li Zhao, Jing Lv, Shengping Wang,
and Xinbin Ma*
Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300072, China
*
S Supporting Information
Scheme 1. Alternative Approach for the Synthesis of EtOH
from Syngas
ABSTRACT: This paper describes an emerging synthetic
route for the production of ethanol (with a yield of ∼83%)
via syngas using Cu/SiO catalysts. The remarkable
2
stability and efficiency of the catalysts are ascribed to the
unique lamellar structure and the cooperative effect
0
+
between surface Cu and Cu obtained by an ammonia
evaporation hydrothermal method. Characterization re-
0
+
sults indicated that the Cu and Cu were formed during
the reduction process, originating from well-dispersed
CuO and copper phyllosilicate, respectively. A correlation
0
+
between the catalytic activity and the Cu and Cu site
0
densities suggested that Cu could be the sole active site
explored by correlating the surface concentrations of catalyst
species with initial rates of reaction.
and primarily responsible for the activity of the catalyst.
Moreover, we have shown that the selectivity for ethanol
or ethylene glycol can be tuned simply by regulating the
reaction temperature.
Cu-based catalysts were chosen primarily because of their
7
high activity for vapor-phase hydrogenation reactions. It has
been reported that Cu sites account for the selective
hydrogenation of C−O bonds and are relatively inactive for
7a
the hydrogenolysis of C−C bonds. Our previous work and
thanol (EtOH) is a versatile feedstock for the synthesis of
the literature on Cu catalysts supported on silica (e.g., SiO ,
2
E
various products (e.g., chemicals, fuels, and polymers) and
has also been commercially used as an additive or a potential
hexagonal mesoporous silica, SBA) indicated that high
dispersion of copper species and strong metal−support
interactions are vital for the high activity and stability in
1
substitute for gasoline. The primary method for EtOH
production is based on microbial fermentation processes
using agricultural feedstocks, such as sugar cane, potato,
8
hydrogenation of oxalates to ethylene glycol (EG). In
2
particular, the formation of copper phyllosilicate
manioc, and corn. Conversion of cellulosic materials, although
3
[
Cu SiO (OH) ] with a lamellar structure can enhance the
promising, requires considerable R&D effort prior to the
2 5 2
8a,d
1
dispersion and metal−support interactions significantly.
Accordingly, we have developed the ammonia evaporation
hydrothermal (AEH) method [see the Supporting Information
achievement of commercial scale. Alternatively, a direct
synthesis of EtOH from syngas could complement the existing
4
technologies. However, major disadvantages of this catalytic
(
SI)] to produce catalysts with phyllosilicate phases.
Table 1 illustrates the physicochemical properties and
process include low yields and poor selectivity of EtOH due to
slow kinetics of the initial formation of the C−C bond and fast
4a,5
chain growth of the C intermediates.
catalytic performance of Cu nanoparticles (NPs) supported
on oxides. Al was chosen as a representative acidic oxide
support, and ZrO (or TiO ) and SiO were selected as weakly
amphoteric and neutral oxides. Cu/TiO , Cu/Al , and Cu/
ZrO catalysts possess lower activities than the Cu/SiO
2
This paper describes an alternative approach for the synthesis
of EtOH via syngas (Scheme 1). This process is an integrated
technology consisting of the coupling of CO with methanol to
form dimethyl oxalate (DMO) and subsequent hydrogenation
O
2 3
2
2
2
O
2 3
2
2
2
catalyst, which could be ascribed to the lower Cu dispersions,
to give EtOH. The byproduct of the second step, CH OH, can
3
numbers of surface Cu atoms, and Brunauer−Emmett−Teller
be separated and used in circulation as the feedstock for the
coupling step. In fact, the first step has been scaled up into
commercial production with a capacity of 10 000 tons per year
(
BET) surface areas, as indicated by N O titration (Table 1),
2
H temperature-programmed reduction (TPR) (Figure S1 in
2
6
the SI), and N adsorption, respectively. Indeed, the number of
as of 2010. Thus, this paper focuses on understanding the
2
accessible surface Cu atoms was reported to account for the
origin of the high efficiency and stability of Cu-based catalysts
for hydrogenation of DMO to EtOH. In particular, the
formation of the bifunctional catalytic sites of Cu species (i.e.,
Received: April 10, 2012
Published: May 24, 2012
+
0
Cu and Cu ) and their contribution to the reaction were
©
2012 American Chemical Society
13922
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Journal of the American Chemical Society
Communication
a
Table 1. Physicochemical Properties and Catalytic Performance
yields (%)
0
+
M loading
C −C
Cu dispersion
S
SCu
dM
S
dpore
(nm)
V
3
4
Cu
BET
pore
b
c
(m g−1)c (m g−1)d (nm)
2
2
e
(m g−1)
2
(m g−1)
3
entry
catalyst
(%)
EtOH
EG
MG
diols
(%)
1
2
3
4
5
6
7
8Cu/SiO2
7.6
14.7
19.2
23.6
27.3
36.8
19.1
28.1
49.3
83.0
70.7
58.7
53.5
17.9
70.7
35.6
0
0
0
1.2
15.1
17.0
27.7
2.3
28.5
27.3
25.8
20.1
14.4
12.7
11.1
15.6
28.2
35.3
34.7
26.3
23.8
15.3
57.8
76.2
72.6
85.7
42.2
28.5
0.5
2.1
2.7
285.9
355.1
483.0
424.5
389.4
424.0
283.6
8.1
8.7
9.2
6.8
5.9
5.2
4.7
0.74
0.93
0.89
0.84
0.69
0.72
0.81
15Cu/SiO2
20Cu/SiO2
25Cu/SiO2
30Cu/SiO2
40Cu/SiO2
0
2.8
1.6
0
4.4
39.8
34.5
54.0
0
4.9
0
12.0
6.2
7.8
20Cu/SiO −
21.9
12.4
2
IM
8
9
20Cu/TiO2
13.4
17.2
14.6
18.3
17.6
4.6
9.8
39.4
61.3
40.3
0
54.7
20.7
52.1
0
1.3
2.5
2.0
0
2.4
5.6
3.2
−
3.3
7.6
4.9
−
0.2
0.7
0.3
−
26.3
10.7
15.2
8.4
62.1
300.1
54.2
18.2
6.1
0.25
0.56
0.15
1.34
0.41
20Cu/Al O3
2
10
11
12
20Cu/ZrO2
3.3
10.2
13.0
8.9
f
20Ni/SiO2
47.0
64.2
384.4
378.6
f
20Co/SiO2
0
0
1.8
−
−
−
4.3
a
−1
Conditions: liquid hour space velocity (LHSV) = 2.0 h , H /DMO = 200, 553 K. MG: methyl glycolate. C −C diols: 1,2-propanediol and 1,2-
butanediol. Determined by ICP-AES analysis. M denotes Cu, Ni, or Co. Cu dispersion and surface area of Cu (S ) were determined by N O
titration. Surface area of Cu (S ) was determined from irreversible CO adsorption isotherms. Diameter of metallic particles calculated from the
XRD data of the (111) peak broading of Cu by Scherrer equation. Other gaseous products (e.g., CH , CO, and CO ) were also formed.
2
3
4
b
c
0
0
Cu
2
d
+
e
+
Cu
f
4
2
8b
catalytic hydrogenation performance. Other nanoclusters
e.g., Ni and Co) supported on silica were also tested: Ni/
SiO and Co/SiO were found to be active for the reaction but
(
2
2
suffer from a low EtOH selectivity as a result of alcohol
decomposition to form CO and subsequent methanation of
9
CO to form methane. Notably, neutral SiO -supported Cu
2
catalysts prepared by the AEH method exhibited better activity
than those prepared by conventional impregnation.
To understand the origin of the catalytic reactivity over Cu/
SiO catalysts, we further investigated the catalytic performance
2
for the gas-phase hydrogenation of DMO by tuning the Cu
loading. Table 1 shows that the 20% Cu/SiO (20Cu/SiO )
2
2
catalyst exhibited superior catalytic reactivity in comparison
with the others. Figure 1 presents the performance of the
2
0Cu/SiO catalyst for hydrogenation of DMO as a function of
2
reaction temperature (453−573 K) and the results of a stability
test at 553 K for 200 h. Remarkably, the DMO conversion
remained at 100% at temperatures above 473 K, and the
selectivity for EtOH increased with increasing temperature,
exceeding 80% at 553 K. One interesting feature is that
formation of EG is favorable at temperatures lower than 553 K,
whereas elevated temperatures (e.g., above 553 K) induce the
intermolecular dehydrogenation of EtOH and methanol with
8
a
EG to form C −C diols, which diminishes the selectivity for
Figure 1. Performance of the 20Cu/SiO2 catalyst in catalytic
hydrogenation of DMO. (A) DMO conversion and product
selectivities as functions of reaction temperature (453−573 K). (B)
DMO conversion and EtOH selectivity vs time on stream at 553 K.
3
4
EtOH. The catalyst was quite stable at 553 K, with constant
conversion of DMO (100%) and selectivity for EtOH (∼83%).
Transmission electron microscopy (TEM) and powder X-ray
diffraction (PXRD) results for the reduced and used samples
−1
Reaction conditions: liquid hour space velocity (LHSV) = 2.0 h , H /
2
DMO = 200 (mol/mol). Trace byproducts include MG and C −C
3
4
(
Figures S2 and S3) provide basic clues concerning the high
diols.
activity and stability of the catalysts . The Cu species was not
sintered in a 200 h test, indicating that the unique structure of
phyllosilicate hampers the sintering of the Cu NPs. In addition,
the change in content of surface Cu and the chemical
environment was nearly negligible as determined by X-ray
photoelectron spectroscopy (XPS) (Figure 2 and Table S1 in
the SI). The excellent stability lies in the formation of copper
phyllosilicates, as confirmed by the lamellar structure in the
II
of the silica surface via hydrolytic adsorption to form SiOCu
8a,b,10
monomer and subsequent further polymerization.
Because
of the strong interaction between the metal and SiO , this
2
complex could supply stable Cu species on the surface that
would remain intact even after our experimental calcination,
reduction, and reaction.
We hypothesize that the high catalytic activity for hydro-
genation of DMO to EtOH results from a cooperative effect
TEM image of the calcined 20CuSiO catalyst (Figure 3A). The
2
structure could stem from its unique preparation method; it is
2
+
0
+
formed via reaction of the Cu complex with the silanol groups
between Cu and Cu . To understand the structure of the Cu
1
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Journal of the American Chemical Society
Communication
the calcined sample, as evidenced by the Cu 2p3/2 peak at
∼
933.3 eV, the Cu 2p1/2 peak at ∼952.5 eV, and the 2p → 3d
8a
satellite peak between 942 and 944 eV. The asymmetry of the
Cu 2p3/2 envelope allowed the peak of the calcined sample to
be deconvoluted into two contributions entered around 933.4
and 936.0 eV, corresponding to dispersed CuO and copper
12
phyllosilicate, respectively.
+
0
The formation of Cu and Cu occurs during the reduction
process, as indicated by the disappearance of the Cu 2p satellite
peak at 942−944 eV in the XPS spectrum and the appearance
of two overlapping peaks at 334.8 and 338.2 eV in the Cu
LMM X-ray excited Auger spectroscopy (XAES) spectrum for
8b
the sample after reduction (Figure 2). The distinguishability
was also confirmed via diffraction peaks in the PXRD patterns
(
Figure S3). Strong diffraction peaks of the reduced catalysts at
3
0
5.8° are ascribable to the Cu O(111) plane (JCPDS no. 05-
2
667), while the diffraction peaks at 43.3, 50.4, and 74.1° are
0
+
characteristic of Cu (JCPDS no. 04-0836). The Cu species is
formed upon the reduction of the copper phyllosilicate under
the experimental conditions (e.g., 623 K), since the further
Figure 2. Cu 2p XPS spectra and (inset) Cu LMM XAES spectra of
+
0
calcined, reduced, and used 20Cu/SiO samples.
reduction of Cu to Cu requires a temperature higher than 873
8a 13
2
0
K. Since the reduction of CuO to Cu occurs at ∼510 K, the
single TPR peak at ∼516 K (Figure S1) could be attributed to
the collective contribution of the stepwise reduction of the well-
0
+
13,14
dispersed CuO to Cu and copper phyllosilicate to Cu .
To further gain insight into the effect of the Cu valence state
on the hydrogenation of DMO to EtOH, we examined the
effect of Cu loading on the EtOH yield and turnover frequency
0
0
+
(
TOF) of EtOH as a function of Cu /(Cu + Cu ) (Figure 4).
Figure 3. (A) TEM image of the calcined 20CuSiO catalyst. (B)
2
FTIR spectra of as-prepared, calcined, and reduced 20Cu/SiO , SiO ,
2
2
and pure Cu(OH) samples.
2
species and the formation process leading to the coexistence of
0
+
Cu and Cu in our Cu/SiO catalysts prepared by the AEH
2
method, we characterized the Cu species in the synthetic
process. Fourier-transform IR (FTIR) spectroscopy was used to
discriminate the copper hydroxide and copper phyllosilicate
Figure 4. (A) Yield of EtOH as a function of Cu loading. (B)
Correlation of the TOF of EtOH with Cu /(Cu + Cu ).
0
0
+
1
0b,11
species.
Because of their structural OH groups, the
frequencies of the δ bands for copper hydroxide are at 938
OH
−1
11
and 690 cm , while those for copper phyllosilicate are at
It is notable that the TOF of EtOH increased steadily at low
Cu /(Cu + Cu ), reached a maximum at Cu /(Cu + Cu ) =
0.33, and decreased with further increases in the ratio (Figure
−
1
8a,10b
0
0
+
0
0
+
1
040 and 670 cm .
For the as-prepared 20Cu/SiO2
catalyst (Figure 3B), the peaks appearing at 690 and 937
−
1
cm suggest the presence of Cu(OH) . This was also
4B). Therefore, the optimal TOF of EtOH on the 20Cu/SiO
2
2
0
confirmed by the diffraction peaks for Cu(OH)2 species
catalyst lies in the high surface Cu site density (Table 1) and
0
+
(
JCPDS no. 13-0420) in the PXRD pattern (Figure S3). The
the cooperative effect of Cu and Cu . Moreover, the strong
correlation between the surface Cu0 site density and the
formation rates of EtOH and C −C diols and the intercept of
−
1
appearance of the δOH vibration at 670 cm and the νSiO
shoulder peak at 1040 cm indicated that the structure of
copper phyllosilicate existed in the samples; these were not
−1
3
4
the trend line (Figure S5A) suggest that the catalytic activity
0
seen for the 20Cu/SiO −IM sample prepared by the
depends on the number of Cu sites. In comparison, there is no
2
impregnation method (Figure S4). Upon calcination, the
absorption bands associated with Cu(OH)2 disappeared,
clear correlation between the specific activity and the number
+
of Cu sites (Figure S5B).
indicating the complete decomposition of Cu(OH) to CuO.
Two general mechanisms were accordingly proposed and
tested on the basis of our experimental results indicating a
2
−1
However, the presence of the δOH vibration at 670 cm in the
calcined 20Cu/SiO2 sample suggests the existence of the
structure of copper phyllosilicate even after calcination at 673
K. The XPS data for the calcined samples (Figure 2) shows that
synergetic effect. In the first, the catalyst has only one active
0
site, Cu , on which H is activated, and the reactant is adsorbed
2
through the hydroxyl group and cleaved. The second is a two-
9
0
the Cu oxidation state is +2 with a d electron configuration in
site mechanism in which the Cu species activates H and the
2
1
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Journal of the American Chemical Society
Communication
+
15
Cu species adsorbs the carbonyl group. The correlation in
ACKNOWLEDGMENTS
■
0
Figure 4 and Figure S5 indicates that Cu is the active site but
This research was supported by the Program for New Century
Excellent Talents in University (NCET-10-0611), the China
Postdoctoral Science Foundation (20090450090), the Seed
Foundation of Tianjin University (60303002), and the Program
of Introducing Talents of Discipline to Universities (B06006).
+
cannot rule out the role of Cu in a two-site mechanism.
However, it has been suggested that in methyl acetate
hydrogenation, Cu acts as the stabilizer of the methoxy and
acyl species, which are intermediates in DMO hydrogenation.
In addition, Cu sites could function as electrophilic or Lewis
acidic sites to polarize the CO bond via the electron lone
pair in oxygen, thus improving the reactivity of the ester group
+
15
+
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(
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(
(
(
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(
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ASSOCIATED CONTENT
Supporting Information
Details of experimental procedures, catalyst characterization,
■
2371.
*
S
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AUTHOR INFORMATION
Corresponding Author
xbma@tju.edu.cn
■
Author Contributions
†
J.G. and H.Y. contributed equally.
Notes
The authors declare no competing financial interest.
1
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