Highly selective one-pot continuous synthesis of 2-methoxyethanol via hydrogenation of dimethyl oxalate on Cu/ZrO2 catalysts with balanced acid sites

Article abstract of DOI:10.1039/c4ra05014b

The distribution of 2-methoxyethanol and ethylene glycol can be controlled by regulating the surface acidity of copper based zirconia catalysts for hydrogenation of dimethyl oxalate. The yield of 2-methoxyethanol can reach 68% due to the synergistic effec

Full text of DOI:10.1039/c4ra05014b

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Highly selective one-pot continuous synthesis of 2-
methoxyethanol via hydrogenation of dimethyl
Cite this: RSC Adv., 2014, 4, 31162
oxalate on Cu/ZrO catalysts with balanced acid
2
sites†
Received 28th May 2014
Accepted 11th July 2014
Yuanyuan Cui, Chao Wen, Xi Chen and Wei-Lin Dai*
DOI: 10.1039/c4ra05014b
www.rsc.org/advances
The distribution of 2-methoxyethanol and ethylene glycol can be properties of the support would induce some side reactions.
controlled by regulating the surface acidity of copper based zirconia Strong acid sites would induce the intermolecular dehydration
catalysts for hydrogenation of dimethyl oxalate. The yield of 2- of EG, whereas strong basic sites would help to catalyze the
11
methoxyethanol can reach 68% due to the synergistic effect of the Guerbet reaction into the formation of 1,2-butanediol. EtOH
surface acid site and active copper site.
might be generated from intramolecular dehydration over weak
12
acidic sites and Cu sites. Zhu et al. proposed that the Cu/ZrO
catalyst exhibited different behavior since the etherication to
2
Considering the depletion of petroleum resources, the trans-
formation of the single-carbon (C1) raw materials carbon
monoxide and carbon dioxide into various high-value-added
2
-methoxyethanol (2-ME) from EG was found to be the main
4+ 2ꢀ
rival side reaction over the surface Zr O cationic Lewis acid
sites when methanol was chosen as solvent, while the yield of
8

ne chemicals has emerged as a versatile synthetic tool in
ꢁ
2-ME was below 15% at reaction temperature of 220 C using
1
–3
organic methodology both on laboratory and industrial scale.
Cu/ZrO
2
as the catalyst.
An efficient production of ethylene glycol (EG) from syngas
through a two-step process consisting of the coupling of CO
with nitrite esters to dimethyl oxalate (DMO) and heteroge-
neous catalytic hydrogenation of DMO has been extensively
2
-ME, which can be widely used in printing, coating, ink and
many other industries because of the co-existence of the
hydroxyl group and ether group in its structure. By 2007 the
annual domestic consumption of glycol ethers reached 200 000
tons. Due to rapid development of China's paint industry,
water-based paint takes the place of oil-based paint gradually,
which means greater demand for glycol ether products.
Commercial 2-ME is mainly produced from petroleum-derived
ethylene oxide through the reaction of ethylene oxide and
methanol. Nevertheless, because of the long-term shortage and
the increased price of crude oil, direct synthesis of 2-ME from
syngas could complement the existing technologies. Further-
more, products 2-ME and EG are easy to be separated due to the
big difference of their boiling points. The objective of this work
is to give fundamental insight into the role of acidity on the
distribution of 2-ME and EG for the hydrogenation of DMO.
Sulfated solid acidic oxides have many applications in acid-
4
researched and industrialized. Copper-based catalysts with
excellent catalytic performance have been extensively explored
for the hydrogenation of DMO considering that the Cu active
sites are accounting for the selective hydrogenation of the C–O
bond, and relatively inactive for the hydrogenolysis of C–C
5,6
bond.
Among the various supports, SiO was found to afford the
2
7
excellent yield of EG in the hydrogenation of DMO. While as a
weakly acidic carrier, silica poorly interacts with metallic Cu,
thereby resulting in the coalescence of Cu NPs and the short
lifespan of the Cu–SiO
2 2 3
catalysts. Some other supports (Al O ,
8
ZrO ) were once studied. Zirconia has recently emerged as a
2
particularly attractive support material for the activation and
stabilization of copper other than ZnO or alumina, which is well
13
catalyzed reactions in industry, such as isomerization, nitra-
9,10
documented in the eld of methanol synthesis, mainly due to
the possession of both amphoteric and redox functions.
Previous researches have demonstrated that the surface acidic
14
15
16
tion, reduction and Friedel Cras acylation. Among solid
acids and superacids, sulfated zirconia has gained much
attention due to its non-toxicity, high strength of acidity and
17
high activity. Li et al. has reported that the catalytic perfor-
mance of sulfated zirconia depended on the sulfation proce-
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Fudan University, Shanghai 200433, P.R. China. E-mail: dure, sulfating concentration and calcination temperature.
wldai@fudan.edu.cn; Fax: +86 21 55665701; Tel: +86 21 55664678
Herein, we demonstrate for the rst time that the Cu-based
catalysts supported on sulfated ZrO show excellent catalytic
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra05014b
1
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activity for the hydrogenation of DMO to EG and 2-ME
(
Scheme 1). ZrO supports were prepared via the reux
2
18
method. The copper supported on zirconia catalysts were
labeled as 10Cu–xSZT catalysts, where x and T denote the
various concentrations of sulfuric acid solution and the calci-
nation temperature of sulfated supports.
The specic surface areas of the catalysts synthesized under
different conditions are shown in Fig. 1. The 10Cu–0.2SZ
sample with uncalcined sulfated support was found to possess a
2
ꢀ1
relatively high BET surface area of 256 m g , whereas further
increasing calcination temperature resulted in a gradual
decline in surface area. The BET surface area initially increased
and reached its maximum at the sulfate concentration of 0.2 M,
2
ꢀ1
with value of 170 m
g . At higher sulfate content, SBET
decreased, very likely due to restricted access to the porosity by
19
pore blockage.
Fig. 1 Specific surface areas of the prepared catalysts.
The morphologies and structural details of the catalyst were
examined using TEM (Fig. S1†). HR-TEM images of 10Cu–
0
.2SZ300 sample clearly showed the copper particles located at
the support surface with the crystalline phase of CuO (111)
0.209 nm). In addition, EDS data showed that sulfur appeared
ꢀ
1
ꢀ1
mmol g then decreased to 2.18 mmol g corresponding to
ꢁ
ꢁ
the uncalcined sulfated supports, calcined at 300 C and 500 C,
respectively.
It's obvious that the calcination temperature of support had
great inuence upon the catalyst acidity. At a lower calcination
temperature the acid site did not completely form. Therefore
(
along with copper clusters on the samples. The presence of
sulfate on the surface was also conrmed by XPS (Fig. S2†).
XRD patterns of 10Cu–Z500, 10Cu–0.2SZT and 10Cu–xSZ300
samples were measured and recorded in Fig. S3.† The sample
ꢁ
the desorption temperature of ammonia (ꢂ584 C) of the
10Cu–Z500 was observed to be predominantly tetragonal phase
1
1
0Cu–0.2SZ catalyst is lower than the 10Cu–0.2SZ300 and
in nature, while the sulfated catalysts has transformed from
monoclinic phase and tetragonal phase to amorphous. This
ꢁ
0Cu–0.2SZ500 (both above 620 C) catalysts. However, when
ꢁ
the calcination temperature was increased to 500 C, the slightly
decreased amount of strong acid site could be due to the inev-
itable initial decomposition of sulfate groups. In agreement
with previous report, the activation of sulfate species in these
samples can be understood according to the following
2
ꢀ

nding is explained in terms of the presence of SO
4
ions,
for the
. Similar mechanism was also proposed
2
ꢀ
which necessitate into higher thermal energy of SO
crystallization of ZrO
4
2
22
20
by Scurrell. A decrease in surface area is observed when the
calcination process of the sulfated support was introduced,
which could contribute to the emerging of CuO peak. The low
intensity of the XRD peaks for the catalysts suggested that CuO
supported on sulfated zirconia mainly consisted of small
crystallites.
ꢁ
sequence. Firstly, aer drying at 100 C, the sulfate groups
replace hydroxyl groups to form bidentate chelation or bridging
sulfate groups. Secondly, aer calcination at a relatively low
temperature, these species are inclined to migrate to the surface
and create potentially active sites. Thirdly, these surface species
As surface acidity plays critical roles on reaction route and
x
start to decompose and evolve as SO around high temperature.
5
product distribution in DMO hydrogenation, NH
3
-TPD experi-
ments were performed to measure the acidity of the catalyst
samples. TPD prole based on desorption temperature maxima
is divided into three regions corresponding to (i) weak (100–200
ꢁ
ꢁ
C), (ii) medium or intermediate (200–450 C), and (iii) strong
ꢁ
21
acid strengths (>450 C). The 10Cu–Z500 catalyst exhibits one
peak in the intermediate range (296 C), while for the sulfated
ꢁ
catalysts, peaks in the range of strong acid strength emerged
ꢀ
1
(
Fig. 2). The total amount of acid sites was about 2.18 mmol g
ꢀ
1
for 10Cu–0.2SZ500 catalyst higher than that 1.29 mmol g
for 10Cu–Z500 (Table S1†). The results showed that the
ꢀ1
total amount of acid sites increased from 1.35 mmol g to 2.36
Fig. 2 NH
3
-TPD of: (a) 10Cu–Z500, (b) 10Cu–0.2SZ, (c) 10Cu–
Scheme 1 Reaction pathway for the conversion of DMO.
0.2SZ300, and (d) 10Cu–0.2SZ500.
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a
Table 1 The catalytic performance of different catalysts
notable increase could be due to the presence of induced acidity
in the catalyst matrix by treatment with sulfuric acid, which is
Selectivity (%)
2-ME
conrmed by NH -TPD results. The catalyst with the sulfated
3
ꢁ
Catalysts
Conversion (%)
EG support calcined at 300 C exhibited highest selectivity to 2-ME.
It is also worthy to note that the as-prepared 10Cu–0.2SZ300
catalyst was very stable under the reaction conditions. Aer 100
hours' lifetime test, no obvious deactivation was observed. It is
also interesting to nd that the sulfur content or surface acid
1
1
1
1
1
1
1
0Cu–Z500
0Cu–0.2SZ
100
100
100
100
100
100
100
9
56
40
37
68
55
34
91
44
60
53
32
45
66
0Cu–0.2SZ500
0Cu–0.1SZ300
0Cu–0.2SZ300
0Cu–0.4SZ300
0Cu–0.8SZ300
amount and/or strength is all kept constant as conrmed by XPS
and NH -TPD. This nding is not strange considering the
3
special stability of the ZrO support and the strong interaction
2
a
ꢁ
ꢀ1
Reaction conditions: T ¼ 220 C, LHSV ¼ 0.4 h , P(H
2
) ¼ 3.0 MPa, H
2
/
between sulfate and the support.
DMO ¼ 150.
In this study, various treatment conditions of the support
inuence not only the physicochemical properties but also the
surface acidity of the as-prepared catalysts, the former deter-
mines the dispersion of copper while the latter affects the
etherication process. Although the 10Cu–0.2SZ exhibits the
smallest surface acid amount, the small copper particle and the
higher SBET value make it be a high-efficiency catalyst. Taken the
discussion into consideration, the relatively poor catalytic
performance of 10Cu–0.2SZ500 seems much more acceptable. It
is interesting to nd that the surface weak-medium acid
amounts of 10Cu–xSZ300 catalysts were related to the acidi-
cation concentration of the support, which was well consistent
with the selectivity to 2-ME, as can be seen in Fig. 3. These
results indicate that the surface weak-medium acid amount has
a signicant effect on the yield of 2-ME. Through the simple
modulation of the acidity on the catalysts, a maximum yield of
2-ME (68%) was obtained. Nevertheless, study to further
Fig. 3 STY of 2-ME with different weak-medium acid sites on the enhance the product yield needs to proceed.
surface of 10Cu–xSZ300 catalysts.
Conclusions
Based on the above discussion, the calcination temperature for In summary, the ndings in this study show that with the
ꢁ
sulfated zirconia was determined as 300 C.
introduction of acidication process, the 10Cu–0.2SZ300 derived
The total acid amount of 10Cu–xSZ300 catalysts (when x is catalyst exhibits remarkable enhancement in its catalytic
greater than 0.2) had no obvious change as shown in Table S2 performance for the hydrogenation of DMO to 2-ME. Highly
and Fig. S4,† which indicated that the sulfate on the surface of dispersed copper species and appropriate acid sites on the
zirconia had reached saturated monolayer dispersion. However, surface mainly contribute to the hydrogenation of DMO, while
the amount of surface weak and medium acid sites on the weak-medium surface acid sites of the catalysts promote the
10Cu–0.2SZ300 catalyst reached the maxima compared with generation of 2-ME. The facile synthesis of the low-cost catalysts
other catalysts. It was clear that the sulfate content played a key makes them promising candidates for practical application.
role in the formation of the amount of acid sites and the
distribution of acid strength.
Acknowledgements
The catalytic activities for the hydrogenation of DMO were
investigated under the same reaction condition, as presented in We would like to thank nancial support by the Major State
Table 1. Generally, hydrogen-transfer reactions occur on acid Basic
Research
Development
Program
(Grant
no.
sites, while hydrogenation reactions are greatly accelerated by 2012CB224804), NSFC (Project 21373054, 21173052), the
the existence of a metal. The conversion of the reaction was Natural Science Foundation of Shanghai Science and Tech-
ꢀ
1
found all to be 100% under relatively low LHSV (0.4 h ) for nology Committee (08DZ2270500).
hydrogenation of DMO as a consequence of better Cu disper-
sion and the appropriate amount of surface acid, which can be
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3
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