Preferential synthesis of ethanol from syngas: Via dimethyl oxalate hydrogenation over an integrated catalyst

Article abstract of DOI:10.1039/c9cc02372k

An integrated catalyst that contains Fe₅C₂ and CuZnO-SiO₂ with a dual-bed configuration was designed for the preferential synthesis of ethanol via dimethyl oxalate hydrogenation. The cooperation of the two catalyst compone

Full text of DOI:10.1039/c9cc02372k

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DOI: 10.1039/C9CC02372K
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Preferential synthesis of ethanol from syngas via dimethyl oxalate
hydrogenation over an integrated catalyst
Xin Shang, Huijiang Huang, Qiao Han, Yan Xu, Yujun Zhao,* Shengping Wang, and Xinbin Ma
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An integrated catalyst that contains Fe5C2 and CuZnO-SiO2 with a catalyst.16 As higher reaction temperature (~553 K) is necessary
dual-bed configuration was designed for the preferential synthesis for the synthesis of ethanol from DMO hydrogenation, the
of ethanol via dimethyl oxalate hydrogenation. The cooperation of formation of C3-4OH via the Guerbet reaction would be highly
the two catalysis components remarkably inhibited the formation facilitated by the surface basic sites on the Cu-based catalysts.17
of various byproducts, resulting in a significantly high ethanol yield Li’s group18 achieved a high ethanol yield of 95% by using an
of about 98%.
bifunctional catalyst (Cu nanoparticles inlaid mesoporous Al2O3)
and 1,4-dioxane instead of methanol as the solvent. The
influence of support has been also reported by the same
group.19 These works provide a meaningful instruction for
developing new technology for ethanol production. Although
Cu-based catalysts perform well in the conversion of DMO, how
to further improve its selectivity to ethanol is still a big challenge
for a process with the presence of methanol from an industrial
viewpoint.20, 21
Recently, Liu et al.22 developed a molybdenum carbide
catalyst for the hydrogenation of DMO to ethanol with an
ethanol yield of 70% at 473 K. They found that high activity of
Mo2C-based catalysts in C-C cleavage can lead to the
decomposition of DMO and lower ethanol selectivity. In our
previous work, a highly efficient iron carbide catalyst was
prepared for DMO hydrogenation to ethanol with a selectivity
of 90%.23 Meanwhile, it was found that the presence of Fe5C2
lead to the formation of ethanol via the hydrogenation of
intermediate MA instead of EG. The Fe5C2 catalyst exhibited a
different reaction path in comparison with Cu-based catalyst
(scheme 1).
Ethanol has attracted wide attention due its important
application in chemical production and potential to replace
gasoline as an environmentally friendly fuel.1 ethanol
production from syngas has been proposed and considered as a
promising process other than fermentation of biomass or
hydration of ethylene. Direct synthesis of ethanol from syngas
over Rh-based,2 Mo-based,3 Cu-based4 or modified F-T
synthesis catalysts5 has made great advances in recent years.
However, poor selectivity of ethanol limits its industrial
application. Thus, a new ethanol synthesis route from syngas via
dimethyl ether (DME) and methyl acetate (MA) is developed6, 7
The DME synthesized from syngas is carbonylated to MA by
zeolite catalysts8, and MA is sequentially hydrogenated to
ethanol by Cu-based catalysts.10 Relatively higher yield of
ethanol makes it a promising route for industrial production of
ethanol from syngas.11
Another ethanol production process from syngas via the
hydrogenation of dimethyl oxalate (DMO) has been proposed
as an alternative method.12 Cu-based catalysts have been
widely investigated in the hydrogenation of DMO with a
selectivity of ethylene glycol (EG) up to 95%.13, 14 The synergy of
Cu species (Cu0 and Cu+) for H2 dissociation and C=O adsorption
on Cu-based catalysts was reported to be responsible for the
higher catalytic performance in DMO hydrogenation.15 When
the hydrogenation of DMO was performed at higher
temperature and H2/DMO, more ethanol can be formed with a
selectivity of 83% by deep hydrogenation of EG on Cu-based
.
9
a. Key Laboratory for Green Chemical Technology of Ministry of Education,
Collaborative Innovation Center of Chemical Science and Engineering, School of
Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.
E-mail: yujunzhao@tju.edu.cn Address here.
† Electronic Supplementary Information (ESI) available: Experimental details and
characterization data. See DOI: 10.1039/x0xx00000x
Scheme 1 The reaction pathway of hydrogenation of DMO on Fe5C2 and Cu-based
catalyst.
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Fig. 1 Catalytic performances of various Cu-based catalysts and the combination of Fe5C2
and Cu-based catalysts. Reaction conditions: T=533 K, H2/DMO=180, WLHSV=0.8 h-1,
P=2.5 MPa, Fe5C2 /Cu-based catalysts was 5/2 (mass).
Fig. 2 Catalytic performances of the combination of Fe5C2 and CuZnO-SiO2 with different
mass ratios. Reaction conditions: T=533 K, H2/DMO=180, WLHSV=0.8 h-1, P=2.5 MPa.
significant change. The major products change to ethanol and
MA in these two cases and the selectivity of ethanol was
significantly improved from 40-60% to 80-90%. These results
suggested a cooperation effect of the integrated catalyst, that
Fe5C2 catalyst ensure the conversion of DMO and Cu-based
catalysts prompt the further hydrogenation of intermediate MA
to ethanol. The absence of methyl glycolate (MG) and EG
implied that the Fe5C2 is enough active for the conversion of
DMO and MG, preventing the formation of EG via the DMO/MG
hydrogenation on Cu-based catalyst. Obviously, the formation
of C3-4OH has also been efficiently inhibited on the integrated
catalyst.
To further understand how the integrated catalyst performs
in the hydrogenation of DMO, the mass ratio of two
components (Fe5C2 and CuZnO-SiO2) was tuned to investigate
its effect on the catalytic performance. As shown in Fig. 2, when
various mass ratios of Fe5C2 to CuZnO-SiO2 were applied in the
hydrogenation, the product distribution changed significantly at
the same reaction conditions. When the mass ratio of Fe5C2 to
CuZnO-SiO2 increased from 1/5 to 5/5, the selectivity of ethanol
kept increasing, accompanied by a drastic dropping of EG and
C3-4OH selectivity. In this range of mass ratio, the amount of
Fe5C2 was not yet enough to provide active sites for the
conversion of DMO or MG, so unreacted DMO or MG was
hydrogenated to EG on the CuZnO-SiO2 catalyst bed, leading to
the generation of C3-4OH as well on the basic sites of CuZnO-
SiO2.17 However, as Fe5C2/CuZnO-SiO2 increased further (from
5/2 to sole Fe5C2), there was a significant decrease of the
ethanol selectivity (from 90.5% to 75.9%) accompanied by the
In addition, some bifunctional catalysts or integrated
systems for the conversion of C1 molecules to lower olefins24 or
liquid fuels25 have been reported, but the concept of integrated
catalysis has never been reported in selective synthesis of
ethanol via DMO hydrogenation. Since Fe5C2 is an effective
catalyst for the DMO conversion and MA is the key intermediate
product in the consecutive hydrogenation system, a coupling of
Fe5C2 and copper-based catalyst could be a rational strategy to
ensure a higher ethanol selectivity in DMO hydrogenation.
Herein, an integrated catalyst that contains Fe5C2 and CuZnO-
SiO2 with a dual-bed configuration is fabricated for the
hydrogenation of DMO and a significantly high selectivity to
ethanol (98%) was achieved, which is beyond all of the reported
results. The effect of integration manner and the reaction paths
over the integrated catalyst were also well investigated. The
achievements suggest a promising prospect for its practical
application.
To gain an insight into the effect of Fe5C2, the performance
of Cu-based catalysts before and after Fe5C2 introduced was
investigated in the hydrogenation of DMO. And the sole Fe5C2
was also tested in the hydrogenation for a comparison. As
shown in Fig. 1, with the Cu-SiO2 as the sole catalyst, ethanol,
EG and C3-4OH were the major products. The selectivity of
ethanol was very low (58.7%) and considerable amount of EG
was produced due to relatively lower reaction temperature
(533 K). C3-4OH (10.5%) was formed as a result of Guerbet
reaction over the surface basic sites of catalyst (Fig. S3, ESI†).
The selectivity of ethanol over the sole CuZnO-SiO2 was similar
as that of Cu-SiO2, and the higher selectivity of C3-4OH (21.8%)
could be ascribed to the formation of more basic sites because
of the introduction of Zn (Fig. S3, ESI†). The sole Fe5C2 were also
tested for the hydrogenation of DMO at the same conditions
and the obtained ethanol selectivity was 75.9% with MA as
major by-product. These results indicated that it was quite a
different reaction pathway over Fe5C2 catalysts that the DMO
was first hydrogenated to MA instead of EG on the Fe5C2
surface. However, it was difficult for the further hydrogenation
of MA to ethanol on Fe5C2, so that higher selectivity to MA
(24.1%) was obtained. On the other hand, when the Fe5C2 was
introduced on the top of the Cu-based catalyst bed, the product
distribution for the two Cu-based catalysts presented a
increase of MA. In fact, the ratio of 5/2 presented better ethanol
Fig. 3 Influence of the integration manner of the active components on catalytic
behaviours under the same conditions. Reaction conditions: T=533 K, H2/DMO=180,
WLHSV=0.8 h-1, P=2.5 MPa, Fe5C2/CuZnO-SiO2 was 5/2 (mass).
2 | J. Name., 2012, 00, 1-3
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ethanol. While, a copper-based catalyst packed following the
View Article Online
Fe5C2 can effectively enhance MA hydrogDeOnaI:t1i0o.n10.3T9o/Cg9eCtCi0n2s3ig72hKt
into the reaction mechanism, the kinetic properties (reaction
rate and apparent activation energy) of the two catalyst on MA
hydrogenation were examined and calculated. As shown in Fig.
4, much higher reaction rate was observed on CuZnO-SiO2,
whose apparent activation energy (42.1 kJ∙mol-1) was much
lower than that of Fe5C2 (51.7 kJ∙mol-1). It indicated that CuZnO-
SiO2 can provide much more efficient active sites than Fe5C2 for
MA conversion. These results offered reasonable explanations
for better catalytic performance of integrated catalyst.
Fig. 4 The kinetic study. (a) Reaction rate for two components at various temperatures.
(b) Reaction rate constant for Arrhenius plots. Reaction condition for CuZnO-SiO2:
H2/MA=30, WLHSV=10 h-1, P=2.5 MPa. Reaction condition for Fe5C2: H2/MA=30,
WLHSV=4 h-1, P=2.5 MPa.
For the in-series packed Fe5C2&CuZnO-SiO2 catalyst, the
resident time of the reactants on various catalyst components
could affect the product distribution. Therefore, Fe5C2&CuZnO-
SiO2 was tested by carrying out the reaction at varying WLHSV
(weight liquid hourly space velocity). As shown in Fig. 5a, the
ethanol selectivity decreases gradually as increasing the space
velocity. While the selectivity of MA exhibited an increasing
trend at first followed by a decrease. No any EG or C3-4OH was
detected until the WLHSV was beyond 1.0 h-1, when the
selectivity of EG and C3-4OH showed a quick increase. These
results suggested that the conversion of DMO over Fe5C2 was
completed at WLHSV lower than 1.0 h-1. In contrast, DMO
cannot be fully converted over Fe5C2 at much higher WLHSV,
since the contact time was not long enough, so that unreacted
DMO or MG was hydrogenated on CuZnO-SiO2 with the
formation of more EG and C3-4OH. Therefore, an optimal
contact time is necessary to ensure the total conversion of DMO
and MG over Fe5C2, which is vital for obtaining higher ethanol
selectivity on the integrated catalyst. In addition, the integrated
catalyst was tested at varying reaction temperature from 513 K
to 533 K (Inset of Fig. 5a). When the reaction was performed at
lower temperature (513 K), a large amount of EG and C3-4OH
were formed and no MA was found in the products. However,
higher temperature benefits the conversion of DMO to MA over
Fe5C2 catalyst, as well as the further hydrogenation of MA to
ethanol on CuZnO-SiO2. And, the selectivity to EG and C3-4OH
was obviously inhibited as well. As a result, the higher
temperature gave the higher ethanol selectivity in the
temperature regime. It indicated that lower temperature could
not ensure the total conversion of DMO or MG over Fe5C2,
leading to the formation of EG and C3-4OH over CuZnO-SiO2, as
well as the poor ethanol selectivity. Consequently, sufficient
selectivity than any other ratios. Therefore, it can be concluded
that Fe5C2 plays an important role in the total conversion of
DMO and MG, due to the extremely high selectivity to MA in
hydrogenation of MG (as shown in Scheme 1). But it presented
relatively lower activity in hydrogenation of MA than CuZnSiO2
catalyst. A cooperative effect is enhanced by an optimal ratio
which ensures the complete conversion of DMO/MG into
MA/ethanol on Fe5C2 and inhibits the formation of by-products
via DMO/MG on CuZnO-SiO2. Moreover, enough and efficient
sites for MA conversion might request the presence of certain
amount of Cu-based catalyst, otherwise, it will be hard to
improve ethanol selectivity, e.g. the sole Fe5C2 catalyst strategy.
To further verify the effect of the integrated catalyst on
ethanol selectivity and products distribution, the reaction using
Fe5C2 and CuZnO-SiO2 with different integration manners was
performed the same reaction conditions. As a result, the
selectivity of ethanol and the product distribution were affected
markedly by the integration manner of the active components.
As shown in Fig. 3, by using the dual-bed configuration with
CuZnO-SiO2 packed below Fe5C2 (Fe5C2&CuZnO-SiO2), the
selectivity to ethanol reached 90.5%. On the contrary, the dual-
bed configuration with Fe5C2 packed below CuZnO-SiO2 (CuZnO-
SiO2&Fe5C2) showed a much lower selectivity to ethanol
(47.8%). The drastic drop of the ethanol selectivity should be
attributed to the formation of more EG and C3-4OH. For the
reaction on the CuZnO-SiO2&Fe5C2, the conversion of DMO
occurred on CuZnO-SiO2 with the generation of EG and C3-4OH,
which is difficult to be further converted to ethanol over Fe5C2
(Fig. S6, ESI†), resulting in an extremely lower ethanol
selectivity. As for the reaction on granular mixture of Fe5C2 and
CuZnO-SiO2, DMO contacted with two components randomly.
However, more DMO was preferentially converted into EG and
C3-4OH on CuZnO-SiO2 catalyst, which could be attributed to
the higher adsorption ability of copper species than Fe5C2 site.
These results suggest that a reasonable integration manner can
ensure the conversion of DMO on Fe5C2 with high selectivity of
ethanol.
As for the different products distribution obtained on
various integrated catalyst, the different reaction paths on
Fe5C2 and CuZnO-SiO2 should be the main reason. The
hydrogenation of DMO over Fe5C2 favours the formation of MA
instead of EG. Unfortunately, Fe5C2 seems not a high active
catalyst for the further hydrogenation of intermediate MA to
Fig. 5 Catalytic performances of the Fe5C2&CuZnO-SiO2 integrated catalyst at (a) Varying
WLHSV (T=533 K, H2/DMO=180, P=2.5 MPa). Inset: Varying temperatures (H2/DMO=180,
P=2.5 MPa, WLHSV=0.8 h-1). (b) Stability of the Fe5C2&CuZnO-SiO2 integrated catalyst.
(T=573 K, H2/DMO=180, WLHSV=0.6 h-1, P=2.5 MPa). Fe5C2 and CuZnO-SiO2 was 5/2
(mass).
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J. Name., 2013, 00, 1-3 | 3
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design of industrial technology for the preferential synthesis of
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DOI: 10.1039/C9CC02372K
ethanol via DMO hydrogenation.
We are grateful to the financial support from the National
Natural Science Foundation of China (21878227, U1510203).
Conflicts of interest
There are no conflicts to declare.
Scheme 2 The reaction mechanism of hydrogenation of DMO over integrated catalyst.
contact time and suitable reaction temperature are necessary
to ensure high selectivity of ethanol. An unexpected high
ethanol yield of 98% was obtained on the integrated catalyst
Fe5C2&CuZnO-SiO2 at 0.2 h-1 and 533 K. This is beyond any of
reported results for the synthesis of ethanol via DMO
hydrogenation. Fig. 5b exhibits an excellent stability of the
integrated catalyst Fe5C2&CuZnO-SiO2 in the synthesis of
ethanol via DMO hydrogenation (Fig. S9, ESI†). The selectivity of
ethanol kept constant with the total conversion of DMO during
the lifespan test. These results suggest a promising potential for
the industry application of the integrated catalyst strategy.
A possible scheme was proposed for the highly synthesis of
ethanol via consecutive DMO hydrogenation reaction over
Fe5C2&CuZnO-SiO2 integrated catalyst (Scheme 2). The higher
ability of Fe5C2 in activating the hydroxyl group of MG than its
carbonyl group was reported in our previous work23. As a result,
the total conversion of DMO and MG is confined on Fe5C2 of the
integrated catalyst, and the intermediate MA is formed instead
of EG. Thus, the side reactions, such as the Guerbet reaction on
Cu-based catalyst, is also avoided since both DMO and EG are
hardly existed. The CuZnO-SiO2 component integrated following
Fe5C2 presents a high activity for the hydrogenation of MA to
ethanol because of its lower energy barrier for MA activation.
The coupling of two reactions is achieved with unexpected high
yield of ethanol by using a suitable integrated catalyst
Fe5C2&CuZnO-SiO2, on which the limitation of Fe5C2 in MA
conversion, as well as the drawback of CuZnO-SiO2 in the
formation of by-products can be well overcome.
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In summary, we presented a new strategy to achieve
significantly high ethanol yield by applying the Fe5C2&CuZnO-
SiO2 integrated catalyst in the hydrogenation of DMO to
ethanol. The main intermediate product was MA instead of EG
due to the presence of Fe5C2 at top of Cu-based catalyst. Then,
the CuZnO-SiO2 help convert the generated MA into ethanol
with a higher activity and selectivity because of the lower
energy barriers for MA conversion. The formation of undesired
products such as C3-4OH and EG was efficiently suppressed by
ensuring the total conversion of DMO and MG on Fe5C2 catalyst
bed. The practical cooperation between Fe5C2 and CuZnO-SiO2
is necessary to obtain a high ethanol selectivity. At the
optimized reaction conditions, the selectivity of ethanol can
reach approximately 98% at DMO conversion of 100% by using
the integrated catalyst. The integrated catalyst also exhibited
excellent stability in the DMO hydrogenation. This feasible and
efficient strategy may provide an inspiration on the rational
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