734
Chemistry Letters Vol.36, No.6 (2007)
Promotional Effect of Potassium Salt in Low-temperature Formate
and Methanol Synthesis from CO/CO2/H2 on Copper Catalyst
Tian-Sheng Zhao,1 Yoshiharu Yoneyama,4 Kaoru Fujimoto,2 Noriyuki Yamane,3
Kenichiro Fujimoto,3 and Noritatsu Tsubakiꢀ4
1Laboratory of Energy & Chemical Engineering, Ningxia University, Yinchuan 750021, P. R. China
2University of Kitakyushu, Wakamatsu, Kitakyushu 808-0135
3R & D lab., Nippon Steel Co., Futsu, Chiba 293-8511
4Department of Applied Chemistry, School of Engineering, University of Toyama,
3190 Gofuku, Toyama 930-8555
(Received February 1, 2007; CL-070129; E-mail: tsubaki@eng.u-toyama.ac.jp)
Alkyl formates can be formed from CO2-containing syngas
Table 1. Ethyl formate formation from CO/CO2/H2 in the
presence of alkali metal salts (0.25 mmol salt)
with C1–C4 alkyl alcohol solvents in the presence of potassium
carbonate, which changed to potassium formate as catalyst. The
formates can be in situ hydrogenolysized further to produce
methanol effectively over manganese oxide or magnesia-sup-
ported copper catalysts. These homogeneous and heterogeneous
catalysts constitute a novel system for methanol synthesis from
CO/CO2/H2 even at 443 K.
Alkali metal
Li2CO3 Na2CO3 Cs2CO3 K2CO3 KHCO3 HCOOK
salt
Yield of Ethyl
formate/%
0.26
1.48
10.6
4.10
3.1
2.94
alkyl alcohol solvents in the presence of potassium carbonate.
And then the obtained formates were further hydrogenolysized
in situ to produce methanol effectively over tailor-made copper
metal catalyst.
Table 1 shows the results of the formation of ethyl formate
from CO/CO2/H2 in the presence of various alkali salts with
ethanol as solvent.9 It is clear that ethyl formate was formed in
the presence of IA group alkali carbonates, and Cs2CO3 showed
the best activity; for different potassium salts, potassium carbon-
ate (K2CO3) exhibited the highest activity.
Methanol economy receives increasing attention as a
reasonable and practical alternative after oil and natural gas.1
The commercial methanol synthesis from synthesis gas uses
Cu/ZnO/Al2O3 catalysts under conditions of 523–573 K and
50–100 bar. The one-pass conversion of this exothermal reaction
at such high temperatures is 15–25% due to the limitation of
thermodynamics. This inevitably causes higher energy con-
sumption by recycling the unreacted gas.
In the presence of K2CO3, the corresponding alkyl formates
were produced from CO/CO2/H2 in C1–C4 alkyl alcohol sol-
vents as shown in Figure 1. Among them, methanol showed
the highest yield to the formation of methyl formate. As K2CO3
amount increased, the yield of ethyl formate increased and
reached almost equilibrium state as in Figure 2. It should be
noted that products other than the alkyl formates were not
observed. The total carbon amount (CO + CO2) in the reactor
was 32.20 mmol, and the converted carbon at 4% yield was
1.29 mmol, rather larger than the amount of K2CO3 (0.25 mmol,
Table 1). So K2CO3 including other species should be catalytic.
Figure 3 shows the 13C NMR spectra of CO/CO2/H2 con-
version in methanol solvent in the presence of K2CO3.10 By
comparison with standard spectra, KHCO3 (ꢀ 161 ppm) and
HCOOK (ꢀ 170 ppm) were found besides solvent CH3OH (ꢀ
Without thermodynamic limitation, methanol synthesis at
low temperature has been extensively studied. One typical previ-
ous process was operated in tetrahydrofuran at 373 K using the
Ni catalyst derived from NaH/tertiary-amyl alcohol/Ni(OAc)2.2
However, subsequent experiments demonstrated that the basic
catalyst was too sensitive to even low concentration of CO2,3
which means impossible for industrial application where feed-
stock gas from methane reformer or coal gasifier generally
contains trace acidic CO2 in addition to CO and H2. Another
low-temperature process was via a two-step pathway: methanol
carbonylation to methyl formate and successive hydrogenolysis
of methyl formate to methanol.4 Alkali methoxide/nickel
compound5 or alkali methoxide/copper compound6 catalysts
were developed for this process. However, the catalysts still
encountered deactivation by the reaction between alkoxides
with trace amounts of CO2 and H2O in feedstock gas.7
The present authors reported a new low-temperature process
for methanol synthesis where catalytically active alcoholic
solvents were employed in addition to solid Cu catalysts.8 Both
CO2 and H2O participated the reaction, and the reaction exhib-
ited stable conversion of CO/CO2/H2 at 423–473 K. The rate-
determining step for this methanol synthesis reaction was the
formation of alkyl formate. To increase the rate of the total reac-
tion at low temperature, we report in this letter the addition effect
of potassium salt to the catalytic system mentioned above, acting
as new catalyst for the first half part and synthesizing formate in
homogeneous phase, faster than in the solid Cu catalyst surface.8
Alkyl formates were firstly formed from CO/CO2/H2 in C1–C4
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
CH3OH
C2H5OH
n-C3H7OH
n-C4H9OH
Figure 1. Alkyl formate formation from CO/CO2/H2 and
alkyl alcohol solvents in the presence of K2CO3 (0.025 g).
Copyright Ó 2007 The Chemical Society of Japan