Download New process of low-temperature methanol synthesis from CO/CO2

Vol. 45 No. 1
SCIENCE IN CHINA (Series B)
February 2002
New process of low-temperature methanol synthesis from
CO/CO2/H2 based on dual-catalysis
ZENG Jianqing ()1, TSUBAKI Noritatsu2 & FUJIMOTO Kaoru2
1. Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China;
2. University of Tokyo, Tokyo 113-8656, Japan
Correspondence should be addressed to Zeng Jianqing (email: [email protected])
Received August 31, 2001
Abstract A new process of low-temperature methanol synthesis from CO/CO2/H2 based on
dual-catalysis has been developed. Some alcohols, especially 2-alcohol, were found to have high
catalytic promoting effect on the synthesis of methanol from CO hydrogenation. At 443 K and 5
MPa, the synthesis of methanol could process high effectively, resulting from the synergic catalysis
of Cu/ZnO solid catalyst and 2-alcohol solvent catalyst. The primary results showed that when
2-butanol was used as reaction solvent, the one-pass average yield and the selectivity of methanol,
in 40 h continuous reaction at temperature as low as 443 K and 5 MPa, were high up to 46.51%
and 98.94% respectively. The catalytic activity was stable and the reaction temperature was 80 K
or so lower than that in current industry synthesis process. This new process hopefully will become
a practical method for methanol synthesis at low temperature.
Keywords: dual-catalysis, methanol, low-temperature synthesis, new process.
Methanol is a versatile chemical and green fuel, being produced industrially by about 30 million tons per year around the world, at the temperature of 493—523 K and pressure of 5ü10 MPa,
using copper-zinc-based oxide catalyst. However, the efficiency of methanol synthesis is severely
limited by thermodynamics because methanol synthesis is an extremely exothermic reaction (H
= −91 kJ/mol). For example, at 573 K and 5 MPa, theoretic maximum CO conversion is around
20%. Therefore, developing a low-temperature process for methanol synthesis will greatly reduce
the production cost and utilize the thermodynamic advantage at low temperature.
BNL method realized this synthesis at 373ü403 K and 1ü5 MPa using very strong base
catalyst (NaH or a mixture of nickel acetate, t-amyl alcohol and sodium hydrid)[1]. However, a remarkable drawback of this process is that trace amount of carbon dioxide or water in the feed gas
or reaction system will deactivate the catalyst soon, which implies high cost coming from the
complete purification of the syngas from reformer, and reactivation of the deactivated catalyst.
Methanol synthesis from pure CO and H2 via the formation of methyl formate has been
widely studied, where carbonylation of methanol and hydrogenation of methyl formate were considered as two main steps of the reaction[2].
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NEW PROCESS OF LOW-TEMPERATURE METHANOL SYNTHESIS
107
CO + CH3OH = HCOOCH3
HCOOCH3 + 2H2 = 2CH3OH
--------------------------------------CO + 2H2 = CH3OH
However, similar to the BNL method, in this process CO2 and H2O act as poisons to the catalyst (RONa) and must be completely removed from syngas, making commercialization of
low-temperature methanol synthesis impossible.
It is well known that for methanol synthesis from CO/CO2/H2 over supported copper-based
catalysts formate was the key intermediate, which was further hydrogenated to produce methanol[3].
The hydrogenolysis of formate has two ways: direct hydrogenolysis and indirect one. In the former,
HCOO was hydrogenated one by one to form H2COO, H3COO and H3COH in sequence. In the
later, HCOO reacted at first with product methanol to give HCOOCH3 on the support of catalyst
and then HCOOCH3 migrates onto copper surface to undergo hydrogenolysis to methanol. Basic
support promotes the formation of HCOOCH3[2]:
HCOO(a) + CH3OH = HCOOCH3 + OH(a)
HCOOCH3 + 4H(a) = 2CH3OH
Obviously, when other alcohol (ROH) is used instead of CH3OH, methanol can be also produced
through the following two reactions:
HCOO (a) + ROH = HCOOR + OH (a)
HCOOR + 4H (a) = CH3OH + ROH
Herein ROH was called “catalytic solvent” because it not only was reaction medium but also participated in the reaction of methanol synthesis catalytically.
Fan et al.[4] investigated the synthesis of methanol from CO2/H2 firstly, using ethanol as reaction medium in a batch reactor and found high activity and selectivity for methanol at 443 K. Recently, we developed a new route of methanol synthesis from CO/CO2/H2 at 443 K, basing on the
dual-catalysis idea, and the overall reaction process was suggested as follows:
CO + H2O = CO2 + H2
CO2 + 1/2H2 + Cu = HCOOCu
HCOOCu + ROH = HCOOR + CuOH
HCOOR + 2H2 = ROH + CH3OH
CuOH + 1/2H2 = H2O + Cu
------------------------------------CO + 2H2 = CH3OH
where Cu stands for active site, ROH is catalytic solvent, and both CO2 and H2O are not only reactants but also products, transformed in cycle. Cu/ZnO are stable to CO2 and H2O under reaction
conditions.
108
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SCIENCE IN CHINA (Series B)
Vol. 45
Experimental
1.1
Catalyst preparation
The catalyst was prepared by the conventional coprecipitation method. An aqueous solution
containing copper and zinc nitrates (each 0.1 mol/L, Cu/Zn in molar ratio of 1) and an aqueous
solution of sodium carbonate (1.0 mol/L) were added slowly and simultaneously to an appropriate
amount of water under rapidly stirring. The precipitation temperature and pH value were maintained at 333—338 K and 8.3ü8.5, respectively. The resulting precipitate was filtrated and
washed with distilled water, followed by drying at 383 K for 24 h and calcinating at 623 K for 1 h.
This precursor was then reduced by a flow of 5% hydrogen in nitrogen at 473 K for 13 h and successively passivated by 2% oxygen diluted by argon. The catalyst had a surface area of 59.47 m2/g
from BET method.
1.2
Reaction and analysis
In the batch reaction experiment, a typical rustless steel batch reactor with inner volume of 80
mL and a stirrer was used. The stirring speed was carefully checked to eliminate the diffusion resistance between different phases. The standard reaction conditions were: catalyst used, 1.0 g; reaction temperature, 443 K; initial pressure, 3 MPa; alcohol solvent, 20 mL; stirring speed, 1250
r/min; reaction time, 2 h; feed gas, CO/CO2/H2/Ar = 31.90/5.08/60.08/2.94. At first a desired
amount of solvent and catalyst were added into the reactor. Then the reactor was closed and the air
inside the reactor was purged carefully by 2 MPa reactant gas for three times. Pressurized feed gas
was introduced and then the reaction took place at the desired temperature. After the reaction
ended, put the reactor into a cooling pool with ice-water and then collect the gas inside the reactor
with a bag. All products were confirmed on GC-MS and analyzed quantitatively by two gas
chromatographs with inner standard method, of which one was for gas product and the other for
liquid product. For no new gas matter formed after reaction, the analysis result of gas was used
only to calculate matter balance. The yields and selectivities of liquid products were calculated as
follows:
Yield of A (%) = 100 × mole number of A / total carbon mole number,
Selectivity of A (%) = 100 × mole number of A / total mole number of liquid products.
In the continuous reaction experiment, a flow-type semi-batch autoclave reactor with an inner
volume of 85 mL was employed. It was connected with a high-pressure gas flow controller system
and an auto-analysis system for post-reaction gas, as shown in fig. 1. During reaction, the effluent
gas released from the reactor went first through a cooler, then a collector in ice-water, and finally
an auto-analysis system, which consists of two gas chromatographs. One was Shimadzu GC-17A,
equipped with Propak Q for C1-C5 (FID), the other was Shimadzu GC-14B, equipped with active
carbon column for CO and CO2 (TCD). The standard reaction conditions were: catalyst used, 3.0 g;
reaction temperature, 443 K; reaction pressure, 5 MPa; reaction solvent, 20 mL; collecting solvent
(1-butanol), 15 mL; stirring speed, 1250 r/min; reaction time, 20—40 h; feed gas (CO/CO2/H2/Ar
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NEW PROCESS OF LOW-TEMPERATURE METHANOL SYNTHESIS
109
= 31.90/5.08/60.08/2.94), 20 mL/min. The post-reaction gas was not cycled. CO, CO2 and total
carbon conversions were calculated as follows:
CO-conv. (%) = 100h[RCO/Ar, 0 – RCO/Ar, 1] / RCO/Ar, 0,
CO2-conv. (%) = 100h[RCO2/Ar, 0 – RCO2/Ar, 1] / RCO2/Ar, 0,
Total carbon conv. (%) = CO-conv. h31.90/(31.90+5.08) + CO2-conv. h5.08/(31.90+5.08),
where RCO/Ar, 0 is the ratio of peak area of CO
to that of Ar in feed gas; RCO/Ar, 1 the ratio of
peak area of CO to that of Ar in post-reaction
gas; RCO2/Ar, 0 the ratio of peak area of CO2 to
that of Ar in feed gas; and RCO2/Ar, 1 the ratio
of peak area of CO2 to that of Ar in post-reaction gas.
The average yields of liquid products
(CH3OH, HCOOCH3, HCOOR), Pi yield,
were calculated from liquid analysis as described in batch experiment above. They
were calculated as follows:
Fig. 1. Scheme of continuous reaction apparatus for the
low-temperature synthesis of methanol from CO/CO2/H2.
1, Heat-electricity couple; 2, reactor; 3, catalyst and solvent;
4, stirrer; 5, cooler; 6, ice-water bath; 7, collector; 8, pressure
controller.
Pi yield = 100hPi mol number (C-mol)/total C-mol of feed gas in total reaction time.
The selectivities of the above liquid products, Pi selectivity, were calculated as follows:
Pi selectivity = 100hPi yield
∑ Pi yield.
2 Results and discussion
2.1 Batch reaction
The analysis results show that only CO and CO2 are the carbon-containing components in
post-reaction gas, and only CH3OH and HCOOR are the liquid products. So the total conversion is
defined to be the sum of the yields of both liquid products. Table 1 lists the reaction results from
using 13 kinds of alcohols as solvent separately under the same reaction conditions. For comparison, the results in the cases of cyclohexane as solvent or ethanol as solvent but no catalyst are also
listed in table 1. From table 1, no activity appears when cyclohexane is used as solvent or ethanol
used as solvent but no catalyst is used. However, when cyclohexanol is used instead of cyclohexane, methanol is the only product and its yield arrives at 6.18% after 2 h reaction. Benzyl alcohol
and ethylene glycol have also no activity for methanol synthesis. But the reason is not clear. Other
10 alcohols, especially 2-alcohols, all show high catalytic activity for methanol synthesis. For example, as shown in table 1, when 2-propanol, 2-butanol and 2-pentanol are used as solvents separately, the total yields of CH3OH and HCOOR are up to 23.46%, 21.48% and 11.81%, respectively.
These results show that many alcohols have high catalytic effects for methanol synthesis
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Table 1
SCIENCE IN CHINA (Series B)
Vol. 45
Effects of different alcohol solvents on the synthesis of methanol from CO/CO2/H2
Solvent
Yield of methanol
(%)
Yield of HCOOR
(%)
Total carbon conversion
(%)
Ratio of ROH to feed gas
carbona)
Cyclohexane
0
0
0
6.93
Ethanol, no catalyst
0
0
0
12.81
Ethanol
10.22
1.13
11.35
12.81
1-Propanol
9.27
0.16
9.43
9.94
2-Propanol
13.19
10.27
23.46
9.81
1-Butanol
8.97
0
8.97
8.16
2-Butanol
11.26
10.22
21.48
8.14
Iso-butanol
8.19
0
8.19
8.13
7.96
t-Butanol
5.83
0
5.83
1-Pentanol
7.74
0
7.74
6.91
2-Pentanol
3.72
8.09
11.81
6.91
Cyclohexanol
6.18
0
6.18
7.08
1-Hexanol
7.17
0
7.17
6.01
Ethylene glycol
0
0
0
13.40
Benzyl alcohol
0
0
0
7.22
Reaction conditions: temperature, 443 K; initial pressure, 3 MPa; catalyst, 1.00 g; alcohol solvent, 20 mL; stirring speed,
1250 r/min; feed gas, CO/CO2/H2/Ar = 31.90/5.08/60.08/2.94; reaction time, 2 h. a) Ratio of molar number of 20 mL alcohol to
total molar numbers of CO and CO2 in feed gas at 293 K and 3 MPa before reaction.
at low temperature during acting as solvent. Such catalytic effect is possibly produced through the
synergic effect of solvent and Cu/ZnO catalyst. Therefore, we call it “dual-catalysis”. For this
dual-catalysis, the mechanism of methanol synthesis reaction maybe changes, and the activity energy of the rate-determining step gets lower, leading to easy progress of the synthesis reaction.
For the same type of alcohol, higher carbon alcohol has lower catalytic activity, as shown in
the cases of five kinds of 1-alcohol from C2 to C6 in table 1. For alcohol with the same carbon
number but different structures, chain alcohol has higher activity than cyclo-alcohol, and alcohol
with little space obstacle has higher activity than that with larger space obstacle. For example,
1-hexanol > cyclohexanol; 1-butanol, 2-butanol > iso-butanol > t-butanol. Most interesting, the
catalytic activity of 2-alcohol is much higher than that of 1-alcohol. Although the reason for this
particular catalytic effect of 2-alcohol is not clear yet we can conclude that 2-alcohol is most helpful for HCOOR formation among all the tested alcohol. Additionally, 2-alcohol with more carbon
number has higher selectivity for HCOOR.
It should be noted that, for all alcohols, they have a large molar ratio of ROH to feed gas carbon, so the difference coming from the influences of molar number of different alcohol should be
little.
2.2 Continuous reaction
To see whether HCOOR could be hydrogenated to CH3OH and ROH easily in continuous reaction under stable H2 pressure we conducted some continuous reaction experiments for three
2-alcohols (2-propanol, 2-butanol and 2-pentanol) which present high selectivity of HCOOR in the
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NEW PROCESS OF LOW-TEMPERATURE METHANOL SYNTHESIS
111
batch tests. The analysis shows that only CO, CO2, H2 and Ar in the post-reaction gas, similar to
the batch test, and only CH3OH, HCOOCH3 and HCOOR as liquid products are detected.
HCOOCH3 is possibly resulted from the reactions of HCOO and CH3OH and/or of HCOOR and
CH3OH. The variations of the total carbon conversions with reaction time using 2-propanol and
2-pentanol as solvents separately are shown in fig. 2, where the reaction time is 20 h. The variation
of the total carbon conversion with reaction time using 2-butanol as solvent is shown in fig. 3,
where the reaction time is 40 h. As seen in figs. 2 and 3, not only the total carbon conversion is
high but also the catalytic activity is very stable when three 2-alcohols are used as reaction solvent.
For 2-propanol and 2-pentanol, their total carbon conversions increase with the time increasing
after 20 h reaction, and the difference between them in continuous reaction is much less than that
in batch reaction with respect to the total carbon conversion. When 2-butanol is used as solvent,
the total carbon conversion at the stable state is high up to 49% or so. Additionally, CO2 conversion is minus in the whole reaction time as shown in fig. 3, suggesting a rapid rate of CO conversion to CO2, particularly at the primary state of reaction.
Fig. 2. Variations of the total carbon conversions
with time using 2-propanol (□) and 2-pentanol (■) as
solvents in continuous synthesis of methanol. Reaction conditions: temperature, 443 K; pressure, 5 MPa;
catalyst, 3.00 g; reaction time, 20 h; stirring speed,
1250 r/min; feed gas, CO/CO2/H2/Ar = 31.90/5.08/
60.08/2.94, 20 mL/min.
Fig. 3. Variations of CO (○), CO2 (□) and the total
carbon (●) conversions with time using 2-butanol as
solvent in the continuous synthesis of methanol. Reaction
conditions: temperature, 443 K; pressure, 5 MPa; catalyst, 3.00 g; reaction time, 40 h; stirring speed, 1250
r/min; feed gas, CO/CO2/H2/Ar = 31.90/5.08/60.08/2.94,
20 mL/min.
It should be pointed out that the total carbon conversion gets to decrease slowly after 20 h reaction in the 40 h experiment, as shown in fig. 3. Such decrease might result from the product
accumulating because about 7.13 mL methanol is formed after 20 h reaction, i.e. the thickness of
liquid phase is increased by 1/3, leading to worse mass transfer and reaction efficiency. Therefore,
removing methanol from the reactor is very important and will accelerate the synthesis of methanol.
The yields and selectivities of all liquid products at 443 K are listed in table 2. CH3OH,
HCOOCH3 and HCOOR are the only three liquid products. The average total liquid yields for
2-propanol, 2-butanol and 2-pentanol solvent are high up to 43.26%, 47.18% and 42.76% respec-
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Vol. 45
tively. The selectivities for methanol in three cases are all higher than 98%, showing that the hydrogenolysis of HCOOR is easy under reaction conditions. These results demonstrate a deed catalytic effect of alcohol solvent in the new low-temperature process of methanol synthesis.
Table 2 Results of the continuous synthesis of methanol with three 2-alcohols as solvents from CO/CO2/H2
Reaction
solvent
Yield (%)
CH3OH
HCOOCH3
HCOOR
Total carbon
conversion (%)
Selectivity (%)
CH3OH
HCOOCH3
HCOOR
2-Propanol
42.71
0.13
0.29
43.26
99.03
0.30
0.67
2-Butanola)
46.51
0.17
0.33
47.18
98.94
0.36
0.70
2-Pentanol
41.94
0.12
0.58
42.76
99.34
0.28
1.36
Reaction conditions: temperature, 443 K; pressure, 5 MPa; catalyst, 3.00 g; stirring speed, 1250 r/min; feed gas,
CO/CO2/H2/Ar = 31.90/5.08/60.08/2.94, 20 mL/min; reaction time, 20 h. a) Reaction time, 40 h.
It should be pointed out that although the space-time yield of methanol in this primary study
is still low, it is very promising to obtain better results because () the present semi-continuous
reactor can be replaced by a continuous one or an integrated reaction-separation reactor. For the
latter, methanol formed during reaction can be removed simultaneously from the reactor, consequently leading to higher time-space yield of methanol; () the present catalyst can be modified
to have a higher catalytic activity. For example, incorporating of Cu-Zn catalyst and Cu-Cr hydrogenolysis catalyst might make up a more coordinating catalyst for methanol formation; and ()
although we have found that three 2-alcohols of 2-propanol, 2-butanol and 2-pentanol have high
catalytic effects, other alcohol solvents having higher catalytic activity could be found with the
research work going on.
3
Conclusion
In conclusion, our primary study has demonstrated that some alcohols, especially 2-alcohols,
had high catalytic promoting effect on the synthesis of methanol from CO hydrogenation in the
presence of Cu/ZnO solid catalyst. Owing to the synergic catalysis of Cu/ZnO solid catalyst and
2-alcohol solvent catalyst, the synthesis of methanol could process high effectively at 443 K and 5
MPa. Such dual-catalytic activity is stable and the reaction temperature is 80 K lower than that in
current industry synthesis process. This new process hopefully will become a practical method for
methanol synthesis at low temperature.
References
1.
2.
Brook Heaven National Laboratory, U.S.Patent, 4 614 749, 4 619 946, 4 623 634, 4 613 623, 1986; 4 935 395, 1990.
Fakley, M. E., Jennings, J. R., Spencer, M. S., Mechanisim of methanol synthesis from carbon monoxide and hydrogen on
copper catalysts, J. Catal., 1989, 118: 483ü486.
3.
Bowker, M., Hadden, R. A., Houghton, H. et al., The mechanisim of methanol synthesis on copper/zinc oxide/alumina
4.
Fan, L., Sakaiya, Y., Fujimoto, K., Low-temperature methanol synthesis from carbon dioxide and hydrogen via formic ester,
Appl. Catal., 1999, 180: 11—13.
catalysts, J. Catal., 1988, 109: 263ü273.