Effect of promoters on Cu-ZnO-SiO2 catalyst for gas-phase hydrogenation of maleic anhydride to γ-butyrolactone at atmospheric pressure

Article abstract of DOI:10.1016/j.molcata.2014.04.038

Cu-ZnO-SiO₂ (CZS) catalysts, prepared by the coprecipitation method, were investigated for gas-phase hydrogenation of maleic anhydride (MA) to γ-butyrolactone (GBL) at atmospheric pressure and characterized by XRD, N₂O chemisorption, SEM and TGA. Effects of the compositions and the promoters on the catalytic performance and stability of CZS catalysts were investigated, in which CZS111 catalyst with the molar ratio of Cu:Zn:Si = 1:1:1 showed better catalytic performance and stability. The results show that the introduction of Ba as a promoter to CZS111 catalyst can further improve greatly the catalyst stability by increasing Cu surface area and ZnO dispersion with more additional adsorption sites for succinic anhydride (SA) to suppress the adsorption and polymerization of SA on the active sites of Cu⁰. The possible reaction pathways were proposed for gas-phase hydrogenation of MA to GBL, which could explain well the effect of Ba as a promoter on the stability of CZS111 catalyst.

Full text of DOI:10.1016/j.molcata.2014.04.038

Accepted Manuscript
Title: Effect of promoters on Cu-ZnO-SiO catalyst for
2
gas-phase hydrogenation of maleic anhydride to
␥
-butyrolactone at atmospheric pressure
Author: Yang Yu Yanglong Guo Wangcheng Zhan Yun Guo
Yunsong Wang Guanzhong Lu
PII:
S1381-1169(14)00177-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.04.038
MOLCAA 9101
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
25-2-2014
29-4-2014
30-4-2014
Please cite this article as: Y. Yu, Y. Guo, W. Zhan, Y. Guo, Y. Wang, G. Lu, Effect of
promoters on Cu-ZnO-SiO catalyst for gas-phase hydrogenation of maleic anhydride
2
to rmgamma-butyrolactone at atmospheric pressure, Journal of Molecular Catalysis
A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.04.038
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Effect of promoters on Cu-ZnO-SiO catalyst for gas-phase hydrogenation
2
of maleic anhydride to γ-butyrolactone at atmospheric pressure
Yang Yu, Yanglong Guo*, Wangcheng Zhan, Yun Guo, Yunsong Wang, Guanzhong Lu*
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East
China University of Science and Technology, Shanghai 200237, P. R. China
ABSTRACT
Cu-ZnO-SiO
gas-phase hydrogenation of maleic anhydride (MA) to γ-butyrolactone (GBL) at atmospheric
pressure and characterized by XRD, N O chemisorption, SEM and TGA. Effects of the
2
(CZS) catalysts, prepared by the coprecipitation method, were investigated for
2
compositions and the promoters on the catalytic performance and stability of CZS catalysts
were investigated, in which CZS111 catalyst with the molar ratio of Cu:Zn:Si=1:1:1 showed
better catalytic performance and stability. The results show that the introduction of Ba as a
promoter to CZS111 catalyst can further improve greatly the catalyst stability by increasing
Cu surface area and ZnO dispersion with more additional adsorption sites for succinic
anhydride (SA) to suppress the adsorption and polymerization of SA on the active sites of
0
Cu . The possible reaction pathways were proposed for gas-phase hydrogenation of MA to
GBL, which could explain well the effect of Ba as a promoter on the stability of CZS111
catalyst.
Keywords: Maleic anhydride; γ-Butyrolactone; Gas-phase hydrogenation; Cu-ZnO-SiO
2
catalyst; promoter
*
Corresponding authors. Fax: +86 21 64252923. E-mail addresses: ylguo@ecust.edu.cn (Y. Guo), gzhlu@ecust.edu.cn (G.
Lu).
1
Page 1 of 19

1
. Introduction
Many researches focus on syntheis of γ-butyrolactone (GBL) due to its versatile
applications. For instance, GBL is used largely as a solvent of polyethylene, polyvinyl
chloride and polystyrene, and as an extracting agent of aromatic hydrocarbon, water-
immiscible ethanol and cyclic ether. GBL, with oxygen-containing five-membered molecular
structure, is an important intermediate for synthesis of N-methylpyrrolidone [1], N-
vinylpyrrolidone [2], and some medicines [3, 4]. Furthermore, GBL can also serve as the cell
electrolyte solvent instead of the strong corrosion acid liquid [5, 6].
The production of GBL mainly includes two processes, i.e., dehydrogenation and cyclization
of 1, 4-butanediol (BDL) [7, 8] and direct hydrogenation of maleic anhydride (MA). The
direct hydrogenation of MA to GBL is an economically promising process due to the
availability of low-price MA from the large-scale industrialization of the partial oxidation of
n-butane to MA.
Some works were reported on the direct hydrogenation of MA to GBL in gas phase, liquid
phase, or supercritical CO
metal catalysts, such as Pd-MoO
complexes [12, 13], and Pd/Al
2
. The latter two processes were carried out preferably over noble
x
3
-NiO/SiO
2 2 2 2
[9], Pd/SiO [10], Pd-SnO /SiO [11], Ru
2
O
[14], Pd/C [15, 16] under higher reaction pressures.
However, gas-phase hydrogenation of MA to GBL at atmospheric pressure over transition
metal catalysts is a better choice because of mild reaction conditions and low-cost catalysts.
Some transition metal catalysts were reported with good catalytic performance for gas-phase
hydrogenation of MA to GBL at atmospheric pressure, such as Cu-ZnO-Al
TiO [18], Cu-ZnO-ZrO [19], Cu-ZnO-CeO [20], Cu-TiO -Al [21], Cu/metal oxides
Al , ZrO or ZnO) [22], Ni or Co/SiO [23], Ni/SiO , /SiO -Al or /H-BEA [24, 25], ect,
2 3
O [17], Cu-ZnO-
2
2
2
2
2 3
O
(
2
O
3
2
2
2
2
2 3
O
in which Cu-based catalysts have better catalytic performance, especially the selectivity to
GBL. Most works of Cu-based catalysts focus on the structure-activity relationship, however,
2
Page 2 of 19

the stability of Cu-based catalysts and how to improve the catalyst stability have been rarely
reported.
According to our previous study [26], the deactivation of Cu-based catalyst for gas-phase
hydrogenation of maleic anhdyride to γ-butyrolactone was mainly attributed to the formation
of carbonaceous species on the catalyst surface. To retard generation of surface carbonaceous
species and improve the stability of Cu-based catalyst for the hydrogenation reactions, neutral
SiO
2 2 3
support was usually selected as a substitute for acidic supports, such as Al O support.
Moreover, Garetto et al. [27] investigated gas-phase hydrogenation of MA over Cu/SiO
2
catalyst and derived a deactivation model with residual activity, in which high conversion of
MA and selectivity to succinic anhydride (SA) were obtained, whereas the yield of GBL was
2
always lower than 2~3 %. In this work, Cu-ZnO-SiO (CZS) catalysts, prepared by the
coprecipitation method, were investigated for gas-phase hydrogenation of MA to GBL at
atmospheric pressure. Effects of the compositions and the promoters on the catalytic
performance and stability of CZS catalysts were also investigated, and the possible reaction
pathways for gas-phase hydrogenation of MA to GBL at atmospheric pressure were proposed.
2. Experimental
2.1. Preparation of catalyst
CZS and CZS+P (P = Mg, Ca, Ba, Ce, Co, Cr or Fe) catalysts were prepared by the
-
1
coprecipitation method with 1.0 molxL Na
2 3
CO aqueous solution as precipitating agent. The
nitrates of copper, zinc and P (P:Cu=1:10, molar ratio) with the given molar ratios were
-
1
dissolved in deionized water to obtain 0.5 molxL aqueous solution, and then the precipitating
agent was added dropwise into the nitrates solution under vigorous stirring at room
temperature until pH≈8. After being aged for 0.5 h under gentle stirring, the precipitate was
+
washed thoroughly by deionized water to remove residual Na and followed by vacuum
3
Page 3 of 19

filtration. Then the resultant precipitate was dispersed in 200 mL of absolute ethanol under
o
vigorous stirring to form a suspension at 60 C in oil bath, and then a certain quantity of silica
2
sol with 10 wt.% SiO was added dropwise into the suspension. The final precipitate was aged
o
o
for 0.5 h, filtrated in vacuum, dried at 120 C overnight and calcined in air at 450 C for 4 h.
At last, the as-prepared catalyst was pressed and then crushed into 20~40 mesh particles for
evaluation of its catalytic performance. The compositions and physicochemical properties of
CZS catalysts are shown in Table 1.
Table 1
The compositions and physicochemical properties of CZS catalysts.
Molar ratios
a
2
-1
b
Catalysts
S
Cu (m xg )
D
Cu (nm)
Cu
1.0
1.5
2.0
1.0
1.0
1.0
1.0
Zn
1.0
1.0
1.0
1.5
2.0
1.0
Si
CZS111
CZS1511
CZS211
CZS1151
CZS121
CZS1115
CZS112
1.0
1.0
1.0
1.0
1.0
1.5
2.0
9.5
8.0
7.8
14
16
17
12
10
10
9
9.6
10.0
10.3
10.8
1.0
a
b
Cu surface area of the fresh catalyts; Crystallite size of Cu in the fresh catalysts.
2.2. Test of catalytic performance
Gas-phase hydrogenation of MA to GBL was carried out in a tubular quartz fixed-bed
reactor (inside diameter = 13 mm, length = 650 mm) at atmospheric pressure. 6 mL of catalyst
(height = 45 mm) was packed in the constant temperature part of the reactor, and then 15 mL
o
of quartz sand pretreated at 600 C in air was loaded above the catalyst for the complete
gasification of the raw material (20 wt.% MA dissolved in GBL). Prior to the test, the as-
-
1
o
prepared catalyst was activated by 60 mLxmin of 5 vol.% H
2
/N
2
at 300 C for 6 h
o
(
designated as the fresh catalyst). Then the temperature was lowered to 220 C, and 27.5
-
1
-1
2
NmLxmin of pure H and 0.2 h (liquid hourly space velocity, LHSV) of raw material, were
fed into the reactor. The products were sampled using the conical beakers cooled by an ice-
bath at intervals of 1 h and analyzed by Perkin-Elmer Clarus 500 gas chromatograph equipped
with a FID detector and a SE-54 capillary column (25 m × 0.32 mm × 1.0 μm). The
4
Page 4 of 19

o
temperatures of the injector and detector were 220 C, and the temperature of the column
o
o
oven was kept at 100 C for 1 min and then increased programmedly to 120 C at the rate of 5
o
-1
Cxmin .
2.3. Characterization of catalyst
The powder XRD patterns were recorded on a Bruker AXS D8 Focus diffractometer
operated at 40 kV, 40 mA (Cu Kα radiation, λ=0.15406 nm), and the diffraction patterns were
o
o
o
-1
taken in the range of 10 <2θ<80 at the scanning rate of 6 xmin . The Cu surface area of the
o
fresh catalyst was measured by N
2
O chemisorption at 60 C assuming a molar stoichiometry
1
9
-2
of Cu:N
2
O=2 and a surface atomic density of 1.46×10 Cu atomsxm [28]. SEM images were
recorded on a JEOL JSM-6360LV scanning electron microscope. TGA profiles were recorded
on a PerkinElmer Pyris Diamond TG-DTA analyzers, in which the sample was heated
o
o
-1
programmedly from 40 to 800 C at the rate of 10 Cxmin in the atmosphere of air of 100
-
1
mLxmin .
3
. Results and discussion
3.1. Catalytic performance of CZS catalyst
Table 2 shows the catalytic performance of CZS catalysts for gas-phase hydrogenation of
MA to GBL at atmospheric pressure. Compared with Cu/SiO
hydrogenation of MA [27], the introduction of ZnO to Cu-SiO
enhanced the selectivity to GBL. Taking CZS111 catalyst as a reference, within reaction time
of 1 h, an increase in SiO or ZnO content in CZS catalyst led to a slight increase in the
2
catalyst for gas-phase
2
catalyst considerably
2
conversion of MA and a slight decrease in the selectivity to GBL. However, a reverse
variation tendency of the catalytic performance occurred with an increase in Cu content in
CZS catalyst. Moreover, few by-products were produced, such as tetrahydrofuran (THF), n-
5
Page 5 of 19

butylaldehyde (BD) and n-butyric acid (BA).
Over all CZS catalsyts, after reaction for 5 h, the conversion of MA and the selectivity to
GBL hardly changed, however, a few amout of SA appeared; after reaction for 10 h, both the
conversion of MA and the selectivity to GBL showed a drastic drop, and meanwhile, SA
became the only by-product. After reaction for 10 h, the residual catalytic performance of
CZS111 catalyst was the highest among all CZS catalysts in terms of the yield of GBL.
Table 2
The catalytic performance of CZS catalysts for gas-phase hydrogenation of MA to GBL at atmospheric
pressure.
MA conversion vs.
reaction time (%)
Selectivity vs. reaction time (%)
THF SA BD and BA
1h 5h 10h 1h 5h 10h 1h 5h 10h 1h 5h 10h
Molar ratios
GBL
Catalysts
Cu Zn Si
1h
5h
10h
CZS111 1.0 1.0 1.0 96.0
CZS1511 1.5 1.0 1.0 95.0
CZS211 2.0 1.0 1.0 94.0
CZS1151 1.0 1.5 1.0 97.6
CZS121 1.0 2.0 1.0 98.4
CZS1115 1.0 1.0 1.5 98.7
CZS112 1.0 1.0 2.0 99.0
96.0
95.0
94.0
96.9
98.4
98.2
98.5
55.0 98.0 98.0 33.0 0.8 0.7
30.0 97.0 97.0 31.0 1.0 0.8
35.0 98.6 97.7 16.0 0.6 0.4
20.0 96.8 95.9 21.0 1.2 0.8
16.0 95.0 95.0 35.0 2.1 1.7
25.0 93.0 92.7 19.0 3.9 2.5
40.0 91.0 90.5 10.0 5.1 3.0
-
-
-
-
-
-
-
- 0.3 67.0 1.2 1.0
- 0.4 69.0 2.0 1.8
- 1.7 84.0 0.8 0.2
- 2.6 79.0 2.0 0.7
- 0.8 65.0 2.9 2.5
- 2.8 81.0 3.1 2.0
- 3.9 90.0 4.0 2.6
-
-
-
-
-
-
-
3.2. Effect of reaction temperature
Table 3 shows the effect of the reaction temperature on the catalytic performance of CZS111
catalsyt. At the same reaction time, with an increase in reaction temperature, the relationship
between the conversion of MA and the reaction temperature displayed a volcano shape
correlation, and the highest conversion of MA appeared at the reaction temperature of 260 ºC.
However, the selectivity to GBL gradually decreased, which was consistent with Castiglioni
et al.’s results [17] that higher reaction temperatures led to the further hydrogenation and/or
hydrogenolysis of GBL. Taking the comprehensive performance of CZS111 catalsyt into
account, the optimum reaction temperature for gas-phse hydrogenation of MA to GBL over
CZS111 catalyst was 220 ºC.
6
Page 6 of 19

Table 3
The catalytic performance of CZS111 catalyst for gas-phase hydrogenation of MA to GBL at atmospheric
pressure at different reaction temperatures.
MA conversion
%)
5h
Selectivity (%)
THF
10h 1h 5h 10h 1h 5h 10h 1h 5h 10h
Reaction
(
GBL
5h
SA
BD and BA
temperature
(ºC)
1h
10h
1h
2
2
2
2
20
40
60
80
96.0 96.0 55.0 98.0 98.0 33.0 0.8 0.7
96.4 96.5 56.2 97.6 97.5 31.2 1.1 0.9
97.0 97.2 57.6 96.4 96.0 27.6 1.7 1.5
96.5 96.1 55.8 94.8 94.9 24.5 2.8 2.1
-
-
-
-
-
-
-
-
0.3 67.0 1.2 1.0
0.4 68.8 1.3 1.2
0.8 72.4 1.9 1.7
1.1 75.5 2.4 1.9
-
-
-
-
3.3. Effect of promoters
CZS111 catalyst showed better catalytic performance for gas-phase hydrogenation of MA to
GBL at atmospheric pressure. However, the catalyst stability was unsatisfactory. To improve
the stability of CZS111 catalyst, several promoters (P = Mg, Ca, Ba, Ce, Co, Cr and Fe) were
investigated. The physicochemical properties of CZS111+P catalsyts are shown in Table 4.
Table 4
The physicochemical properties of CZS111+P catalysts.
2
-1
a
Catalysts
CZS111
S
Cu (m xg )
9.5
D
Cu (nm)
14
10
12
12
9
13
12
D
ZnO (nm)
15
9
CZS111+Ba
CZS111+Mg
CZS111+Ca
CZS111+Ce
CZS111+Co
CZS111+Cr
CZS111+Fe
15.9
10.4
10.7
12.6
8.2
13
10
11
14
9
10.2
7.6
13
13
a
The crystallite size of ZnO in the fresh CZS111+P catalysts determined by Scherrer’s equation using the
diffraction peak of ZnO (101).
Fig. 1 shows the catalytic performance of CZS111+P catalysts for gas-phase hydrogenation
of MA to GBL at atmospheric pressure, in which the selectivities to byproducts, such as SA,
THF, BD or BA, are not shown. As shown in the inset at the lower left corner of Fig. 1, the
introduction of Mg, Ca, Co or Fe decreased the catalytic activity of CZS111 catalyst. The
introduction of Cr led to a slight increase in the catalytic activity. However, the Ce- or Ba-
promoted CZS111 catalyst had an obvious increase in the catalytic activity, in which the
7
Page 7 of 19

conversion of MA could reach 100% over CZS111+Ce catalyst and 97.8% over CZS111+Ba
catalyst, respectively. As for the selectivity, as shown in the inset at the upper right corner of
Fig. 1, the introduction of Ce or Ba to CZS111 catalyst could also increase the selectivity to
GBL. Most importantly, with the introduction of Ba to CZS111 catalyst, the decline rates of
the catalytic activity and the selectivity to GBL were both suppressed to a great extent. After
reaction for 10 h, 90.0% of the conversion of MA and 69.8% of the selectivity to GBL
remained over CZS111+Ba, whereas the corresponding percentages were only 55.0% and
33.0% over CZS111 catalyst at the same reaction time, respectively.
Fig. 1. The catalytic performance of CZS111+P catalysts for gas-phase hydrogenation of MA to GBL at
atmospheric pressure.
To further investigate the effect of a promoter on the catalytic stability of CZS111 catalyst,
the stability tests for 50 h were carried out and the results are shown in Fig. 2. The catalytic
performance of CZS111 catalyst and three representative CZS111+P (Ba, Ce, Mg) catalysts
gradually declined and then reached a steady state after reaction for about 22 h. The
conversion of MA and selectivity to GBL of CZS111+Ba catalyst and CZS111 catalyst after
reaction for 50 h were 59.1%/35.8% and 33.9%/11.7%, respectively.
8
Page 8 of 19

Fig. 2. The catalytic stability of CZS111 and three representative CZS111+P catalysts for gas-phase
hydrogenation of MA to GBL at atmospheric pressure.
3.4. Characterization of catalyst
Fig. 3 shows XRD patterns of the as-prepared and the fresh CZS catalysts. As shown in Fig.
, XRD patterns of all as-prepared CZS catalysts exhibited well-resolved characteristic
2
diffraction peaks of CuO (PDF#65-2309) and ZnO (PDF#36-1451), respectively. Three
o
o
o
characteristic diffraction peaks of Cu (PDF#65-9743) located at 2θ=43.4 , 50.6 and 74.3 ,
corresponding to the crystal planes of Cu (111), (200) and (220), respectively, and the
characteristic diffraction preaks of ZnO were identified in XRD patterns of all fresh CZS
0
catalysts. It indicated that CuO was completely reduced by 5 vol.% H
2
/N
2
to Cu and the state
0
of ZnO did not change, in which Cu is the active site for gas-phase hydrogenation of MA to
o
o
GBL [18-20, 26]. In all XRD patterns, a very broad peak in the 2θ range from 15 to 30
should be assigned to the characteristic diffraction peak of amorphous SiO [29]. The
2
crystallite size of Cu in the fresh CZS catalysts, calculated by Scherrer’s equation with the full
peak width at half maximum height of the characteristic diffraction of Cu (111), is shown in
9
Page 9 of 19

Table 1. As shown in Table 1, the crystallite size of Cu decreased with an increase in ZnO or
SiO content in CZS catalysts. Combined with the catalytic performance shown in Table 2,
2
there was a correlation between the catalytic performance and the crystallite size of Cu over
CZS catalyst for gas-phase hydrogenation of MA to GBL, in which with a decrease in the
crystallite size of Cu, the conversion of MA increased but the selectivity to GBL decreased.
Fig. 4 shows XRD patterns of the as-prepared and the fresh CZS111+P catalysts. Similar to
2 2
CZS catalysts, after reduced by 5 vol.% H /N , CuO in the as-prepared CZS111+P catalyst
0
was completely reduced to Cu and ZnO remained. Except for a characteristic diffraction peak
o
of CeO
2
at 2θ = 28.5 in XRD pattern of CZS111+Ce catalyst, no characteristic diffraction
peaks related to the promoters were observed in XRD patterns of other CZS111+P catalysts.
Table 4 shows the crystallite size of Cu in the fresh CZS111+P catalysts. As shown in Table 3,
the introduction of the promoters could decrease the crystallite size of Cu in CZS111 catalyst,
especially the introduction of Ba or Ce.
Fig. 3. XRD patterns of the as-prepared and the fresh CZS catalysts.
1
0
Page 10 of 19

Fig. 4. XRD patterns of the as-prepared and the fresh CZS111+P catalysts.
The Cu surface area (SCu), a measure indicative of the quantity of exposed Cu atmos per
gram of catalyst, is a significant parameter in the hydrogenation reaction over Cu-based
catalysts [30]. According to SCu data shown in Table 1 and Table 4, compared with CZS111
catalyst, with an increase in Cu content in CZS catalyst, SCu decreased. However, with an
2
increase in ZnO or SiO content in CZS catalyst, SCu had a slight increase. SCu of CZS111+P
catalysts were affected by the introduction of the promoters. Co or Fe as promoter led to a
decrease in SCu, but Mg, Ca or Cr as promoter resulted in a slight increase in SCu. In
particular, with the introduction of Ba or Ce to CZS111 catalyst, the corresponding SCu
2
-1
increased by 6.4 and 3.1 m xg than that of CZS111 catalyst, respectively. Combined with the
catalytic performances of CZS111+P catalysts shown in Fig. 1 and 2, it indicates that SCu is an
important parameter for screening a better Cu-based catalyst for gas-phase hydrogenation of
MA to GBL at atmospheric pressure, especially taking the catalyst stability into consideration.
The surface morphologies of the fresh and the used CZS111 catalysts were investigated by
SEM. As shown in Fig. 5, it was bright and clean on the surface of the fresh CZS111 catalyst.
However, with an increase in the reaction time, some fuscous cotton-like deposition appeared
11
Page 11 of 19

on the catalyst surface and gradually covered most of the surface. It indicates that the
deactivation of CZS111 catalyst mainly results from formation of the surface deposition,
0
which covers the active sites of Cu .
(a)
(b)
(c)
Fig. 5. SEM images of the fresh CZS111 catalyst (a), the used CZS111 catalyst for reaction for 10 h (b) and
the used CZS111 catalyst for reaction for 50 h (c).
After reaction for 10 h, the contents of the surface deposition over used CZS111 and
CZS111+Ba catalysts were determined by TGA. As shown in Fig. 6, the weight loss (△wt) of
the used CZS111+Ba catalyst was lower than that of the used CZS111 catalyst, which
indicates that the introduction of Ba to CZS111 catalyst reduces the formation rate of the
surface deposition over the catalyst and thus improves the catalyst stability.
Fig. 6. TGA profiles of the used CZS111 and CZS111+Ba catalysts.
3.5. The reaction pathways over CZS111 and CZS111+Ba catalysts
1
2
Page 12 of 19

Table 2 shows that the selectivity to SA, as a transition species in the catalytic hydrogenation
of MA to GBL [10], rapidly increased when CZS111 catalyst started to deactivate after
reaction for several hours, and at the same time, the selectivity to GBL decreased. It indicates
that SA can not be hydrogenated to GBL promptly and is strongly adsorbed then polymerized
on the catalysts surface, which results in the fast deactivation of the catalyst [26]. Herrmann et
al. [31] and Emig et al. [32] investigated the effect of ZnO over Cu-based catalysts in the
catalytic hydrogenation of MA and proposed that ZnO without catalytic activity could provide
the additional adsorption sites for SA to alleviate the competitive adsorption of GBL and SA
0
on the active sites of Cu . In this work, the introduction of Ba could significantly increase the
Cu surface area of CZS111 catalyst. Meanwhile, ZnO was also highly dispersed, as implied
by its smaller crystallite size shown in Table 4, that was to say, ZnO could provide more
adsorption sites for SA, which could alleviate the adsorption and polymerization of SA on the
0
active sites of Cu and thus improved the stability of CZS111 catalyst.
The reaction pathways for gas-phase hydrogenatio of MA to GBL at atmospheric pressure
over CZS111+Ba (a) and CZS111 (b) catalysts were proposed in Scheme 1. First, the gasified
MA in the flow of hydrogen diffused to the catalyst surface and started to react in the
0
sequence of (1)~(3) on the active sites of Cu of the catalyst surface. It was different from the
results reported by Emig et al. [32] that no 1, 4-butanediol was produced over CZS or
CZS111+Ba catalysts in reaction (3). Reaction (1) is a quick process over a catalyst [14].
Initially, except that some SA were reversibly adsorbed on the adjacent ZnO surface, most of
SA were hydrogenated to GBL (2). With the reaction progressing, more SA were produced.
0
The excessive SA were strongly adsorbed and then polymerized on the active sites of Cu (4).
Meanwhile, based on the competitive adsorption between GBL and SA on the active sites of
0
Cu , reaction (3) was suppressed to a certain extent, which led to a decrease in the selectivity
to by-products, such as THF, BA and BD. Compared with CZS111 catalyst (b), the
1
3
Page 13 of 19

introduction of Ba (a) could increase Cu surface area and ZnO dispersion with more
additional adsorption sites for SA, which decreased the rate of SA polymerization on the
0
active sites of Cu and thus improved the stability of CZS111 catayst.
Scheme 1. The reaction pathways proposed for gas-phase hydrogenation of MA to GBL over CZS and
CZS111+Ba catalysts at atmospheric pressure.
4. Conclusions
CZS catalysts with different compositions were investigated for gas-phase hydrogenation of
MA to GBL at atmospheric pressure. CZS catalysts showed better catalytic performance, in
which CZS111 catalyst showed better stability. Effects of promoters on the catalytic
performance of CZS111 catalysts were also investigated, in which Ba or Ce as a promoter
was favorable for increasing the catalytic performance of CZS111 catalyst. In particular, the
introduction of Ba as a promoter to CZS111 catalyst could significantly improve the catalyst
stability by increasing Cu surface area and ZnO dispersion.
The possible reaction pathways were proposed for gas-phase hydrogenation of MA to GBL
at atmospheric pressure over CZS111 and CZS111+Ba catalysts, which could explain well the
effect of Ba as a promoter on the stability of CZS111 catalyst.
Acknowledgements
This project was supported by National Basic Research Program of China (2013CB933201,
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2010CB732300), Program for New Century Excellent Talents in University (NCET-09-0343),
Shu Guang Project of Shanghai Municipal Education Commission and Shanghai Education
Development Foundation (10SG30).
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Highlights
2
Cu-ZnO-SiO catalyst showed better catalytic performance and stability.
Ba as a promoter can further improve greatly the catalyst stability.
Higher Cu surface area and ZnO dispersion can suppress the catalyst deactivation.
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