Study on the reaction pathway in the vapor-phase hydrogenation of biomass-derived diethyl succinate over CuO/ZnO catalyst

Article abstract of DOI:10.1016/j.catcom.2010.06.007

The reactivity of biomass-derived diethyl succinate and its main reaction intermediates was separately investigated in a fixed-bed reactor over CuO/ZnO catalyst, and some interesting results were obtained. Ethyl 4-hydroxybutyl succinate as an intermediate of diethyl succinate hydrogenation reaction was first detected, and it could be converted to 1,4-butanediol or polyesters. Additionally, propanol and butanol are mainly derived from 1,4-butanediol rather than gamma-butyrolactone. A comprehensive reaction pathway for the hydrogenation of diethyl succinate is proposed, which is significant for the design of new catalytic formulations. A CuO/ZnO + HY admixed catalyst exhibiting excellent performance was prepared according to the proposed reaction pathway.

Full text of DOI:10.1016/j.catcom.2010.06.007

Catalysis Communications 11 (2010) 1120–1124
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Study on the reaction pathway in the vapor-phase hydrogenation of biomass-derived
diethyl succinate over CuO/ZnO catalyst
c
Guoqiang Ding a,b, Yulei Zhu a,c, , Hongyan Zheng , Wei Zhang a,b, Yongwang Li a,c
⁎
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, PR China
Graduate University of the Chinese Academy of Science, Chinese Academy of Sciences, Beijing, 100039, PR China
b
c
Synfuels CHINA Co. Ltd, Taiyuan, 030032, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactivity of biomass-derived diethyl succinate and its main reaction intermediates was separately
investigated in a fixed-bed reactor over CuO/ZnO catalyst, and some interesting results were obtained. Ethyl
4-hydroxybutyl succinate as an intermediate of diethyl succinate hydrogenation reaction was first detected,
and it could be converted to 1,4-butanediol or polyesters. Additionally, propanol and butanol are mainly
derived from 1,4-butanediol rather than gamma-butyrolactone. A comprehensive reaction pathway for the
hydrogenation of diethyl succinate is proposed, which is significant for the design of new catalytic
formulations. A CuO/ZnO+HY admixed catalyst exhibiting excellent performance was prepared according to
the proposed reaction pathway.
Received 3 March 2010
Received in revised form 1 June 2010
Accepted 7 June 2010
Available online 13 June 2010
Keywords:
Diethyl succinate
Hydrogenation
© 2010 Elsevier B.V. All rights reserved.
Reaction pathway
Ethyl 4-hydroxybutyl succinate
1. Introduction
pressures and low temperatures favoring BDO [5]. However, Turek et
al. [12] suggested that THF could also be converted from GBL.
Gamma-butyrolactone (GBL), 1,4-butanediol (BDO) and tetrahy-
drofuran (THF) are significant intermediate chemicals and solvents in
food, pharmaceutical and textile industries [1]. These four-carbon
chemicals are currently produced commercially through the Davy
McKee process from maleic anhydride [2] or the traditional Reppe
process [3]. Alternatively, these chemicals can be produced from the
hydrogenation of biomass-derived succinic acid or its esters (dialkyl
succinate), which is regarded as a promising and environmental-
friendly route [4]. Succinic acid and its esters as important biomass-
derived platform molecules can be produced through the fermenta-
tion of a number of renewable sources e.g. sugars and starches [4].
The vapor-phase hydrogenation of dialkyl succinate (DAS) has
been subjected to numerous studies which mainly focus on screening
and characterization of catalysts [5–12]. However, systematic studies
addressing the reaction pathway of DAS hydrogenation reaction have
not been published in the literature. There is still a lack of the full
understanding and a consensus on the reaction pathway. Generally, it
is thought that GBL, BDO and THF are consecutively formed with
alkanol as co-product [5–12] in the hydrogenation reaction of DAS.
There is equilibrium between GBL and BDO, and the ratio of BDO to
GBL can be controlled by the choice of reaction conditions, with high
Varadaraja [13] reported that THF was primarily formed via direct
conversion of DAS over a silica surported copper-based catalyst. On
the other hand, the origin of by-products such as propanol and
butanol was still debated. Messori et al. [14] reported that these by-
products were mainly derived from GBL. However, Deshpande et al.
[15] suggested that propanol and butanol were converted from BDO.
Additionally, it was suggested that DAS could react with BDO to form
polymers which resulted in fouling and plugging of the unit [5,7],
which indicated the complexity of reaction pathway. Scheme 1
illustrates the reaction pathway generally proposed for the hydroge-
nation of DAS, which is not comprehensive and contains some
conflicting standpoints [5–15].
The aim of this work is to shed light on the reaction pathway of
DAS hydrogenation. The reactivity of biomass-derived diethyl
succinate (DES) and its main reaction intermediates was separately
investigated in a fixed-bed reactor over CuO/ZnO catalyst to
complement the reaction mechanism study.
2. Experimental
2.1. Catalyst preparation
The CuO/ZnO catalyst was prepared by the co-precipitation
method. In a typical preparation procedure, a 1 M mixed solution of
Cu(NO3)2·3H2O and Zn(NO3)2·6H2O was added into a 1 M Na2CO3
solution under stirring at 80 °C, until the pH reaching 7.5. After
precipitation, the suspension was washed and filtered. The precipitate
⁎
Corresponding author. State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, PR China. Tel.:
+86 351 7117097; fax: +86 351 7560668.
E-mail address: zhuyulei@sxicc.ac.cn (Y. Zhu).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.06.007

G. Ding et al. / Catalysis Communications 11 (2010) 1120–1124
1121
}
Scheme 1. Reaction pathway proposed in the literature for the hydrogenation of dialkyl succinate.
was dried at 110 °C for 12 h and then calcined at 450 °C for 5 h. The
resulting catalyst was shaped by a tablet machine and then crushed to
20–40 mesh for catalytic test. The CuO/ZnO+HY catalyst was
prepared by the physical-mixed method. The CuO/ZnO catalyst and
HY zeolite were crashed to fine powders in agate mortar, respectively,
and then the fine powders of both catalysts with appropriate mass
ratio were further carefully kneaded for 30 min in agate mortar to
form apparently homogeneous mixture of micro powders. The
resulting mixture was pressed, crashed, and sieved to the particles
(20–40 mesh).
conversion of DES increases with increasing residence time, the
selectivity of GBL decreases, while the selectivity of BDO increases
(Table 1). This indicates that GBL is firstly formed, and then converted
to BDO, which is consistent with previous studies [6–9]. The effect of
residence time on the selectivity of THF is inconspicuous due to the
weak acidity of CuO/ZnO catalyst [6].
Interestingly, in addition to GBL, BDO and THF, a product was first
detected in the product mixture of DES hydrogenation, and it was
identified to be ethyl 4-hydroxybutyl succinate (EBS) by GC/MS
technique. EBS shows a relative low selectivity at short residence
time, and the highest selectivity of EBS is obtained at a residence time
of 0.9 s. However, the selectivity of EBS decreases to zero when
residence time is improved to 1.8 s. These results suggest that EBS is
an intermediate of the DES hydrogenation reaction because it can be
further converted to BDO at long residence time.
Schlander et al. [5] attributed the dramatic carbon deficit to the
formation of polyesters in the hydrogenation of dimethyl maleate, and
proposed that 4-hydroxybutyl methyl succinate (BMS) could be
formed via the transesterification of dimethyl succinate with BDO
(Scheme 2(a)). However, BMS was not detected by the authors.
Accordingly, EBS may be formed through the transesterification of
DES with BDO (Scheme 2(b)). The transesterification of DES with BDO
was investigated over CuO/ZnO catalyst under the reaction conditions
applied for the hydrogenation of DES. However, the used temperature
is low to avoid the occurrence of DES hydrogenation. It can be seen
that DES shows a low conversion under the applied reaction
conditions (Table 2). However, the conversion of DES is obviously
improved when BDO is added to DES, and the main products are EBS
and ethanol. These results prove that EBS is formed via transester-
ification of DES with BDO (Scheme 2(b)). In fact, EBS could further
react with DES and BDO to polyesters [7], and some white polymeric
materials were indeed found in the catalyst bed and colder parts of the
unit.
2.2. Catalytic test
The catalytic tests were carried out in a fixed-bed reactor. Prior to
reaction, all the catalysts were reduced in situ at atmospheric pressure
with a 10% H2/N2 (v/v) gas mixture at 295 °C for 24 h. After reduction,
the reactants were pumped to a vaporizer, mixed with pure hydrogen
and then introduced into the reactor. The reaction products were
analyzed using a SP-2000 gas-chromatograph and identified by a GC/
MS (GC6890N/5973MSD, Agilent, USA).
3. Results and discussion
3.1. The effect of residence time on product distribution
Table 1 shows the DES conversion and product distribution at
various residence times over CuO/ZnO catalyst. Variation of the
residence time was achieved by changing the catalyst mass. As the
Table 1
Effect of the residence time on DES hydrogenation over CuO/ZnO catalysta.
Catalyst bed
length (cm)
Residence
time (s)
Conversion
(%)
Selectivityb (%)
GBL
BDO
THF
EBSc
Othersd
3.2. Reactivity of GBL, BDO, THF and butanol
1.7
3.4
4.9
6.8
0.5
0.9
1.3
1.8
36.0
51.4
69.9
100
49.2
35.8
18.4
8.8
39.0
47.1
69.1
86.1
3.4
4.1
4.4
3.1
7.6
11.7
7.8
0
0.8
1.3
0.3
2.0
Usually, the by-products in the hydrogenation of the DES were
reported to be butanol and propanol. However, a little of these by-
products were detected over CuO/ZnO catalyst due to the low
temperature applied. Additionally, it is difficult to determine the
origin of by-products, because the mutual conversion between GBL
and BDO takes place with a relatively high rate [5] in the process of
DES hydrogenation, particularly at high pressure. Thus, the reactivity
a
Reaction conditions: 170 °C, 4.0 MPa, H2/DES=100 (molar ratio).
Selectivity (%)=Ci ·ni /∑(Ci ·ni)×100, Ci and ni are the carbon atoms and molar
b
quantity of liquid product i at the reactor outlet, respectively [12]. Ethanol and water
are not taking into account.
c
EBS = ethyl 4-hydroxybutyl succinate.
d
Other products mainly propanol and butanol, etc.

1122
G. Ding et al. / Catalysis Communications 11 (2010) 1120–1124
Scheme 2. The formation pathway of 4-hydroxybutyl methyl succinate and ethyl 4-hydroxybutyl succinate.
of GBL and BDO was investigated separately at 260 °C, 0.1–2.0 MPa
over CuO/ZnO catalyst to clarify the formation pathway of by-
products. The reactivity of THF and butanol was investigated as well.
The hydrogenation GBL to BDO is thermodynamically limited at
low pressure, and no BDO is detected at 0.1 MPa [5]. In addition, GBL
shows low yields of propanol and butanol suggesting the high stability
of GBL (Table 3). When pressure is improved to 2.0 MPa, the yield of
BDO increases to 6.0% because the high pressure favors BDO formation
[5]. At the same time, the yields of THF, propanol and butanol are
obviously improved. These results suggest that THF and by-products
are mainly derived from BDO rather than GBL [14]. When BDO is used
as reactant, the yields of THF, propanol and butanol are higher than
those of GBL under any reaction conditions. The equilibrium ratio of
BDO to GBL is favored at high pressure, thus a lower conversion of
BDO is observed at 2.0 MPa [5], and a lot of THF, propanol and butanol
are obtained. It can be seen that the formations of THF, butanol and
propanol are suppressed at 0.1 MPa, because most of BDO is converted
to GBL [5,16,17]. This further confirms that BDO is the origin of THF
and by-products. The hydrogenation of THF is characterized by low
conversion value suggesting the high stability of THF [14,18]. Most of
butanol is converted to propane and carbon monoxide over CuO/ZnO
catalyst. Accordingly, BDO may be converted to propanol and carbon
monoxide. In addition to THF, BDO can also be dehydrated to 3-buten-
1-ol. Igarashi et al. [19,20] reported that 3-buten-1-ol could be
produced from BDO over CeO2 and ZrO2 catalysts. It can be speculated
that the 3-buten-1-ol formed from BDO is further hydrogenated to
butanol over CuO/ZnO catalyst in the DES hydrogenation process.
Therefore, on the basis of the experimental results in this work and
considering the existing literature, a proper reaction pathway for the
hydrogenation of DES is proposed, which suggests that BDO plays a
key role due to its high reactivity (Scheme 3). Besides cyclodehydra-
tion to THF, BDO can react with DES to EBS which can be further
converted to polyesters or hydrogenated to BDO and GBL. Addition-
ally, by-products such as propanol and butanol are also derived from
BDO.
In industrial plant, the deposit of polyesters may result in fouling
and plugging of catalyst bed and lines of unit [7], which may shorten
the operation period and increase the production cost. However, how
to avoid the formation of polyesters is still an open question in the
hydrogenation of dialky succinate. In addition, the reduction of the
yields of by-products is also significant to decrease the production
cost. According to the proposed reaction pathway, the formation of
polyesters and by-products can be suppressed if the formed BDO can
be immediately converted to THF. However, CuO/ZnO catalyst
exhibits a low reactivity for the cyclodehydration of BDO to THF due
to its weak acidity [6]. It was reported that BDO could be selectively
converted to THF by HY zeolite [21]. Thus, a CuO/ZnO+HY catalyst
was prepared by the physical-mixed method for DES hydrogenation.
3.3. Catalytic performance of CuO/ZnO+HY admixed catalyst
Over CuO/ZnO+HY admixed catalyst, EBS is not detected, and no
white polymeric materials are found in the catalyst bed or colder parts
of the unit, which indicates the formation of polyesters may be
avoided (Table 4). At 200 °C, a THF selectivity of 98.5% is obtained
with complete conversion of DES, as the equilibrium between GBL and
BDO is completely disturbed. In addition, the selectivities of by-
products of CuO/ZnO+HY admixed catalyst are much lower than
those of CuO/ZnO catalyst at high temperature because the formed
BDO is immediately converted to THF.
Table 3
Table 2
Reactivity of the main intermediates of DES hydrogenationa.
Product distribution of transesterification of DES with BDOa.
Feed
stocks (MPa)
Pressure Conversion Yield (%)b
(%)
Catalyst
DES conversion
(%)
Yield (%)b
EtOHc
GBL BDO THF EtOH PrOHc BuOHc Others
GBL
BDO
THF
EBS
Others
GBL
BDO
THF
0.1
2.0
0.1
2.0
2.0
7.7
–
0
6.0
–
0
9.6
2.1
20.7
–
0
0
0
0
1.4
0
3.1
10.8
6.7
15.0
0.4
3.6
17.8
5.3
20.2
3.1
1.0
2.8
5.2
7.5
0.1
CuO/ZnO
CuO/ZnO
5.8d
31.6
2.0
13.9
1.5
1.8
0.4
–
0.1
2.1
0
1.8
1.6
47.0
96.5
86.0
5.0
–
12.2e
77.2
22.6
–
–
a
Reaction conditions: 150 °C, 4.0 MPa, WHSV=0.12 h−1
, H2/DES=200 (molar
–
ratio), and DES/BDO=1:0.7 (molar ratio).
d
BuOH 2.0
45.5
–
–
–
0
–
45.5
b
Yield (%)=Ci ·ni /Cfeed ·nfeed ×100, Cfeed and nfeed are the carbon atoms and molar
quantity of feed, respectively [12].
a
Other reaction conditions: 260 °C, WHSV=0.12 h−1
The same as Table 2.
PrOH = n-propanol, BuOH = n-butanol.
.
c
b
c
EtOH = ethanol.
Reactant is only DES without BDO.
The Ci of EBS is defined as 6 as only 6 carbon atoms are directly derived from DES.
d
e
d
Other products mainly propane and carbon monoxide, etc.

G. Ding et al. / Catalysis Communications 11 (2010) 1120–1124
1123
Scheme 3. Proper reaction pathway proposed for diethyl succinate hydrogenation. The broken lines represent the difference standpoints from Scheme 1.
4. Conclusions
significantly suppressed over CuO/ZnO+HY admixed catalyst, be-
cause the formed BDO is immediately converted to THF.
The results of this work indicate that transesterification and
hydrogenation reactions take place competitively in the process of
DES hydrogenation. In addition to the consecutive formation of GBL,
BDO and THF from DES hydrogenation, the formed BDO can react with
the unconverted DES to EBS and polyesters. Interestingly, the EBS can
be further converted to BDO and GBL, which indicates the hydroge-
nation of DES processes via a complicated netted reaction pathway. In
addition, by-products such as propanol and butanol are mainly
derived from BDO rather than GBL. These results are important to
understand the mechanism and reaction pathway of DES hydrogena-
tion, and provide an instruction for the design of new catalytic
formulations. The formations of polyesters and by-products are
Acknowledgement
The authors gratefully thank the financial support of the Natural
Science Foundation of China (No. 20976185).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.catcom.2010.06.007.
Table 4
Catalytic performances of CuO/ZnO and CuO/ZnO+HY admixed catalystsa.
Catalyst
Temperature
(°C)
Conversion
(%)
Selectivityb (%)
GBL
BDO
THF
EBS
PrOH
BuOH
Others
CuO/ZnO+HYc
170
200
260
260
44.9
99.5
100
38.3
0.2
31.6
0
0
0
2.8
0
59.7
98.5
22.7
95.2
0
0
0
0
0.6
0.5
7.8
1.8
1.3
0.6
16.3
2.7
0.1
0.2
18.8
0.3
CuO/ZnO
CuO/ZnO+HYd
100
a
Other reaction conditions: 4.0 MPa, WHSV=0.12 h−1, H2/DES=100 (molar ratio).
The same as Table 1.
HY zeolite content is 30 wt.%.
b
c
d
HY zeolite content is 10 wt.%.

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G. Ding et al. / Catalysis Communications 11 (2010) 1120–1124
[12] T. Turek, D.L. Trimm, D.StC. Black, N.W. Cant, Appl. Catal. A: Gen. 116 (1994)
References
137–150.
[13] S. Varadarajan, D.J. Miller, Biotechnol. Prog. 15 (1999) 845–854.
[14] M. Messori, A. Vaccari, J. Catal. 150 (1994) 177–185.
[15] R.M. Deshpande, V.V. Buwa, C.V. Rode, R.V. Chaudhari, P.L. Mills, Catal. Commun. 3
(2002) 269–274.
[16] Y. Zhu, J. Yang, G. Dong, H. Zheng, H. Zhang, H. Xiang, Y. Li, Appl. Catal. B: Environ.
57 (2005) 183–190.
[17] Y. Zhu, H. Xiang, G. Wu, L. Bai, Y. Li, Chem. Commun. 3 (2002) 254–255.
[18] H.Y. Zheng, Y.L. Zhu, B.T. Teng, Z.Q. Bai, C.H. Zhang, H.W. Xiang, Y.W. Li, J. Mol.
Catal. A: Chem. 246 (2006) 18–23.
[19] A. Igarashi, S. Sato, R. Takahashi, T. Sodesawa, M. Kobune, Catal. Commun.
8 (2007) 807–810.
[1] A. Küksal, E. Klemm, G. Emig, Appl. Catal. A: Gen. 228 (2002) 237–251.
[2] K. Turner, M, Sharif, C, Rathmell, J.W. Kippax, A.B. Carter, J. Scarlett, A.J. Reason, N.
Harris, US patent 4,751,334 (1988).
[3] W. Reppe, E. Keyssner, US patent 2,232,867 (1941).
[4] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411–2502.
[5] J.H. Schlander, T. Turek, Ind. Eng. Chem. Res. 38 (1999) 1264–1270.
[6] S.P. Müller, M. Kucher, C. Ohlinger, B. Kraushaar-Czarnetzki, J. Catal. 218 (2003)
419–426.
[7] C. Ohlinger, B. Kraushaar-Czarnetzki, Chem. Eng. Sci. 58 (2003) 1453–1461.
[8] Q. Zhang, Z. Wu, L. Xu, Ind. Eng. Chem. Res. 37 (1998) 3525–3532.
[9] M. Mokhtar, C. Ohlinger, J.H. Schlander, T. Turek, Chem. Eng. Technol. 24 (2001)
423–426.
[10] P.J. Guo, L.F. Chen, S.R. Yan, W.L. Dai, M.H. Qiao, H.L. Xu, K.N. Fan, J. Mol. Catal. A: Chem.
256 (2006) 164–170.
[20] A. Igarashi, N. Ichikawa, S. Sato, R. Takahashi, T. Sodesawa, Appl. Catal. A: Gen. 300
(2006) 50–57.
[21] M. Aghaziarati, M. Kazemeini, M. Soltanieh, S. Sahebdelfar, Ind. Eng. Chem. Res. 46
(2007) 726–733.
[11] L.F. Chen, P.J. Guo, L.J. Zhu, M.H. Qiao, W. Shen, H.L. Xu, K.N. Fan, Appl. Catal. A: Gen.
356 (2009) 129–136.