Effect of supercritical fluid of CO2 drying during Cu/ZnO catalyst preparation on methanol synthesis from syngas at low temperature

Article abstract of DOI:10.1007/s11164-011-0253-7

A copper-based catalyst can be utilized to synthesize methanol from syngas containing carbon dioxide as well as water at low temperature and low pressure. However, the agglomeration of the metallic copper and zinc oxide decreased the catalyst surface area and the Cu-specific surface area. In order to prevent the sintering, the supercritical CO₂ was used to extract water from the catalyst precursor. Our results demonstrate that the Cu-specific surface area was the essential factor to affect the catalytic activity. A larger Cu-specific surface area would cause higher methanol synthesis activity. The optimized supercritical CO₂ drying condition was at 308 K and 8.0 MPa for 3 h when the methanol yield reached 44.8%.

Full text of DOI:10.1007/s11164-011-0253-7

Res Chem Intermed (2011) 37:397–403
DOI 10.1007/s11164-011-0253-7
Effect of supercritical fluid of CO2 drying during
Cu/ZnO catalyst preparation on methanol synthesis
from syngas at low temperature
•
Fanzhi Meng Xingang Li Ming Meng
•
•
•
Yikuho Ishizaka Noritatsu Tsubaki
Received: 24 July 2010 / Accepted: 29 November 2010 / Published online: 3 February 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A copper-based catalyst can be utilized to synthesize methanol from
syngas containing carbon dioxide as well as water at low temperature and low
pressure. However, the agglomeration of the metallic copper and zinc oxide
decreased the catalyst surface area and the Cu-specific surface area. In order to
prevent the sintering, the supercritical CO2 was used to extract water from the
catalyst precursor. Our results demonstrate that the Cu-specific surface area was the
essential factor to affect the catalytic activity. A larger Cu-specific surface area
would cause higher methanol synthesis activity. The optimized supercritical CO2
drying condition was at 308 K and 8.0 MPa for 3 h when the methanol yield
reached 44.8%.
Keywords Methanol ꢀ Cu/ZnO ꢀ Syngas with CO2 ꢀ Supercritical fluid of CO2
Introduction
Methanol, as the main product of the C1 chemistry, is a kind of chemical feedstock
which has broad applications. Currently, the commercial methanol synthesis method
uses copper–zinc-based oxide catalyst developed by ICI Co. Ltd. under high
F. Meng ꢀ Y. Ishizaka ꢀ N. Tsubaki (&)
Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190,
Toyama City, Toyama 930-8555, Japan
e-mail: tsubaki@eng.u-toyama.ac.jp
X. Li (&) ꢀ M. Meng
Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of Chemical
Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China
e-mail: xingang_li@tju.edu.cn
N. Tsubaki
JST, CREST, Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan
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temperature and high pressure (523–573 K, 50–100 bars). However, the efficiency
of methanol synthesis is severely limited by thermodynamics because it is an
extremely exothermic reaction [1, 2]. With the rapid increase in the demand of
methanol, in particular, the requirement for methanol as a fuel for transport energy,
the development of more efficient methanol synthesis becomes a new challenge. In
order to solve this problem, the present authors developed a new low-temperature
process for methanol synthesis from syngas containing CO2 by using alcohol as both
solvent and promoter over a Cu/ZnO catalyst at 443 K and 5.0 MPa [3–7]. The new
process consists of three steps: (1) formic acid synthesis from CO2 and H2; (2)
esterification of formic acid by promoter butanol; and (3) hydrogenation of the
butoxy formate to methanol and butanol. The fundamental equations are as follows:
CO þ H2O ¼ CO2 þ H2
CO2 þ 1=2H2 þ Cu ¼ HCOOCu
HCOOCu þ ROH ¼ HCOOR þ CuOH
HCOOR þ 2H2 ¼ ROH þ CH3OH
CuOH þ 1=2H2 ¼ H2O þ Cu
CO þ 2H2 ¼ CH3OH
The catalyst was prepared by a co-precipitation method. The swelled precursor
experiences remarkable sintering during drying in air at 393 K. The agglomeration
of the precursor is caused by large liquid–vapour interfacial forces, which destroy
the delicate surface structure during an evaporative drying procedure [8]. Recently,
a new drying technique, supercritical drying, has been developed. The supercritical
drying uses another solvent, which is above the critical point of the mixture of two
solvents, to extract the solvent involved in the swelled catalyst [9]. Theoretically,
the skeletal structure of the swelled catalyst can be maintained through supercritical
drying [10, 11]. Alcohol and carbon dioxide are usually used as supercritical drying
media [12, 13], and the latter has a relatively low critical temperature (Tc = 304 K)
and is not flammable. In this work, we used the supercritical fluid of CO2 (SC-CO2)
to extract the solvent from the catalyst precursor to avoid the agglomeration of the
surface structure of the Cu/ZnO catalyst. The influence of the supercritical fluid
drying conditions to methanol synthesis was also investigated and discussed.
Experimental
The supercritical dried Cu/ZnO catalyst was prepared by a co-precipitated method.
An aqueous solution containing copper and zinc nitrates with a molar ratio of 1:1,
and an aqueous solution of sodium carbonate were added simultaneously into
300 ml deionized water with constant stirring. The obtained precipitate was aged
overnight, filtrated, and washed with adequate deionized water. The deposit was
placed into an autoclave. Then, the liquid carbon dioxide was fed into the autoclave
for 3 h at a rate of 20 mL/min at a temperature of 308 K to dry the precursor. The
pressure was kept at 8.0 MPa during this operation (the supercritical dried catalyst is
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Effect of supercritical fluid of CO2 drying
399
called CZS in this work). To investigate the influence of the supercritical drying
conditions on the catalytic activity, the catalysts were also dried at 8.0 MPa 308 K
for different periods, i.e. 1–5 h, and dried at 8.0 MPa at different temperatures for
3 h, i.e. 323–363 K. The catalyst was also dried at 7.5 MPa 308 K for 3 h. To
compare with the CZS catalyst, the catalyst which dried at 393 K for 12 h instead of
the supercritical drying processing was also prepared (called CZO in this work).
These precursors as mentioned above were then calcined in air at 623 K for 1 h,
reduced by a flow of 5% hydrogen diluted by nitrogen, and successively passivated
by 1% oxygen diluted by argon.
The BET surface area of Cu/ZnO catalysts was measured by the N2 adsorption
method (AUTOSORB-1). Specific copper surface areas were determined by a well-
known decomposition method in which N2O reacts with surface Cu0 [14].
The activity of the catalysts was tested with a semi-batch type reactor. Amounts
of 3.0 g of the catalyst and 40 ml of 2-butanol were poured into the reactor. After
purging the system with feeding gas (CO/CO2/Ar/H2 = 31.8/5.21/3.04/59.95) at
room temperature, the internal pressure of the reactor was increased to 5.0 MPa.
The experiment was conducted at 443 K with the feeding gas of 20 ml/min under
continuous stirring. The products were collected after a 20-h semi-batch activity
test, and then analyzed by Shimadzu GC320 and GC-8A with FID detector.
The CO, CO2 and total carbon conversion were calculated by the equations as
follows:
CO conv: ¼ 100 ꢁ ðCO=Ar in feed ꢂ CO=Ar in effluentÞ=ðCO=Ar in feedÞ
CO2conv: ¼ 100 ꢁ ðCO2=Ar in feed ꢂ CO2=Ar in effluentÞ=ðCO2=Ar in feedÞ
Total carbon conv: ¼ CO conv: ꢁ a=ða þ bÞ þ CO2conv: ꢁ b=ða þ bÞ
(a, b are the contents of CO, CO2 in the feed gas).
Results and discussion
The BET surface area and the specific copper surface area of the prepared CZO
catalysts for methanol synthesis at low temperature from syngas containing CO2
using alcohol as promoter, as proposed in our previous work [4, 15], are listed in
Fig. 1.
The BET surface area and Cu-specific surface area of the CZO catalyst were
55.0 m2/g and 18.9 mC2 u/gcat, respectively. When the supercritical CO2 (SC-CO2)
drying procedure was carried out at 8.0 MPa and 308 K, the BET surface area
increased continuously following the drying period, and reached 63.5 m2/g after
SC-CO2 drying for 5 h. For the variation of the Cu-specific surface area, the biggest
maximum value was obtained after SC-CO2 for 3 h, i.e. 23.3 m2Cu/gcat. After the SC-
CO2 drying for 5 h, the Cu-specific surface area of the precursor decreased to
18.7 m2Cu/gcat. The results suggest that the SC-CO2 drying conditions enhanced the
BET surface area and also altered the Cu-specific surface area. The aim of the SC-
CO2 drying treatment is to remove water from the catalyst precursor. The phase
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F. Meng et al.
Surface area (m2 /g)
2
60
50
40
30
20
10
0
Cu-specific area ꢀ m /gꢁ
Fig. 1 The surface area of the Cu/ZnO catalyst under the various SC-CO2 drying conditions
change of the residual water, i.e. liquid–vapour transformation, happened during the
calcination of the catalyst precursor at high temperature, such as 623 K in this work.
It will enlarge the pore size and destroy the delicate surface structure of the catalyst
inducing the suppressed BET surface area. Therefore, the extension of the SC-CO2
drying period enhanced the BET surface area. The decrease of the Cu-specific area
may be due to the weak strength of the catalyst precursor, which was prepared by
the co-precipitation process without high temperature calcination. So, some copper
species on the surface of the precursor may be flushed away after a long SC-CO2
treatment period, inducing a lower Cu surface area. This hypothesis will be
investigated in our future work. With the increase of the SC-CO2 drying
temperature, the BET surface area decreased continuously to 53.8 m2/g, while the
Cu-specific area kept constant at around 18.5 m2Cu/gcat. A possible mechanism, in
this sense, is that a higher SC-CO2 drying temperature might collapse the
framework of the precursor before calcination, and the surface areas decrease. We
also investigated the effect of the pressure of the SC-CO2 drying for the surface area
as shown in Fig. 1. The lower pressure could greatly enhance the BET surface area,
i.e. 63.9 m2/g, but the Cu-specific surface area was slightly decreased, i.e. 22.7 m2Cu
cat. It also suggests that the mild supercritical conditions are favorable for the
surface areas.
/
g
A flow-type semi-batch reactor was employed for methanol synthesis. The results
of the catalytic activity test for the methanol synthesis are listed in Table 1. Among
the catalyst precursors dried by the SC-CO2 at 308 K for 1, 3 and 5 h, the precursor
dried by SC-CO2 for 3 h presented the highest catalytic activity, i.e. MeOH yield of
44.8%, and total conversion of 46.4%. Here, the MeOH selectivity kept constant at
around 96%. It indicates that the prolonged SC-CO2 drying period could enhance
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Effect of supercritical fluid of CO2 drying
401
the BET surface area continuously, but the best catalytic activity was achieved after
the SC-CO2 drying pretreatment for 3 h, corresponding to the biggest Cu-specific
surface area possessed by the series of CZS catalysts as shown in Fig. 1. It seems
that the Cu-specific surface area of the catalyst was the key factor affecting the
catalytic performance instead of the BET surface area. Our results show that the
metallic Cu is the catalytic active center for methanol synthesis, which is in good
agreement with the previous studies [16, 17]. Thereafter, we also investigated the
influence of the SC-CO2 drying temperature to the catalytic performance. When the
SC-CO2 drying temperature increased from 308 to 363 K, the MeOH yield, the total
conversion, and the MeOH selectivity dropped simultaneously, following the
decrease of the BET surface area and the Cu-specific surface area. The higher SC-
CO2 drying temperature sintered the catalyst precursor leading to the suppressed
BET surface area as well as the Cu-specific surface area. It is observed that the
larger the Cu-specific surface area, the higher the MeOH synthesis activity. Here,
the Cu-specific surface area was the important factor in obtaining the high catalytic
activity for the CZS catalyst, although the BET surface area might improve the
dispersion of the metallic Cu and increase the number of active Cu sites.
In order to compare with the SC-CO2 drying pretreatment prepared catalyst at
308 K and at 8.0 MPa, the catalysts, which were dried in air at 393 K or dried by
SC-CO2 at 308 K and at 7.5 MPa, were also prepared. The results of the catalytic
activity tests are listed in Table 2. The CZO catalyst showed the lowest catalytic
activity, i.e. total conversion of 35.1% and MeOH yield of 33.8%, among three
Table 1 Catalytic performance of the Cu/ZnO catalysts with the various SC-CO2 treatments
SC-CO2 drying conditions
Total conv. (%)
MeOH sel. (%)
MeOH yield (%)
308 K, 8.0 MPa, 1 h
308 K, 8.0 MPa, 3 h
308 K, 8.0 MPa, 5 h
323 K, 8.0 MPa, 3 h
343 K, 8.0 MPa, 3 h
363 K, 8.0 MPa, 3 h
37.1
46.4
39.2
37.4
36.9
36.4
96.0
96.6
96.2
94.9
95.3
94.7
35.6
44.8
37.7
35.5
35.2
34.5
Reaction conditions: T 443 K; P 5.0 MPa; time 20 h; solvent 2-butanol, 40 ml; W 3.0 g; feeding gas CO/
CO2/Ar/H2 = 31.8/5.21/3.04/59.95, 20 ml/min
Table 2 Catalytic performance between the Cu/ZnO catalysts with and without the SC-CO2 drying
Catalyst
SC-CO2 drying
pressure (MPa)
Conversion (%)
MeOH
yield (%)
MeOH
sel. (%)
CO
CO2
Total
CZO
CZS
0.0
7.5
8.0
40.8
48.2
51.9
–1.2
8.2
35.1
42.7
46.4
33.8
42.1
44.8
96.3
98.6
96.6
12.4
Reaction conditions T 443 K; P 5.0 MPa; time 20 h; solvent 2-butanol, 40 ml; W 3.0 g; feeding gas CO/
CO2/Ar/H2 = 31.8/5.21/3.04/59.95, 20 ml/min
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catalysts. Here, the CO2 conversion of the CZO catalyst was about -1.2%. The
negative CO2 conversion means that a little CO was converted to CO2 through the
water–gas shift reaction. Nevertheless, the CO2 conversion of the CZS catalysts was
8.2 and 12.4%, respectively. It suggests that the Cu/ZnO catalyst, which was
pretreated by SC-CO2 drying, easily involves the CO2 in methanol production,
inducing an improved catalytic activity. If the pressure of the SC-CO2 drying was
decreased from 8.0 to 7.5 MPa, the MeOH yield of the catalyst dried at 7.5 MPa
decreased from 44.8 to 42.1%, and the total conversion also decreased. Referring to
Fig. 1, it indicates that the BET surface area did not directly affect the catalytic
activity of the catalysts, while the increase of the Cu-specific surface area, which
correlated to the number of the active species, i.e. Cu0, on the catalyst, could
enhance the catalytic activity. This result is in good agreement with the data in
Table 1. Moreover, the MeOH selectivity on the catalyst dried at 7.5 MPa showed
the highest catalytic performance,indicating that the lower pressure of SC-CO2
drying pretreatment might favor an increase in the selectivity of MeOH.
Conclusions
For MeOH synthesis from syngas using 2-butanol as solvent at low temperatures,
the Cu/ZnO catalyst prepared by the SC-CO2 drying method showed excellent
catalytic performances. This is because the SC-CO2 could extract water under mild
conditions thus maintaining the structure of the catalyst precursors. Therefore, the
dispersion of the active sites, i.e. copper, on the catalyst surface was improved
significantly even after the calcination of Cu/ZnO catalyst at 623 K. By the SC-CO2
drying preparation method, the large surface area and Cu-specific surface area were
achieved. Here, the Cu-specific surface area was the key factor to affect the catalytic
activity. Therefore, the SC-CO2 drying conditions should be adjusted to the
appropriate conditions to obtain the high Cu-specific surface area.
Acknowledgments This work was financially supported by the ‘‘863 Program’’ of the Ministry of
Science & Technology of China (No. 2008AA06Z323), the National Natural Science Foundation of
China (No. 20806056), the Doctoral Fund of Ministry of Education of China (No. 200800561002), the
Program for Introducing Talents of Discipline to Universities of China (No. B06006), and the NSF of
Tianjin (No. 09ZCGHHZ00400).
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