828
Chemistry Letters 2002
Novel Catalysts for Carbon Dioxide-induced Selective Conversion of Methane to C2 Hydrocarbons
Yingchun Cai, Lingjun Chou, Shuben Li,ꢀ Bing Zhang, and Jun Zhao
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, P. R. China
(Received April 15, 2002;CL-020327)
The combination of Mn with BaCO3 leads to active catalysts
for carbon dioxide-induced selective conversion of methane to
ethane and ethylene in the absence of oxygen.
CH4 þ CO2 ¼ 2CO þ 2H2
ð4Þ
The data processing method has been described in reference 4.
The effect of Mn/Ba ratio on the catalytic performance of
Mn–BaCO3 is shown in Table 1. As may be seen, MnO2 alone
exhibited high CH4 conversion of 9.8% with very low C2
selectivity of 6.3%. Itis very interesting that BaCO3 alone showed
no obvious catalytic effectiveness for the reaction. For the Mn-
BaCO3 catalysts, CH4 conversion decreased to some extent
compared with that of MnO2, but C2 selectivity and yield
increased dramaticaly. The fact that not only C2 selectivity but
also C2 yield for the Mn–Ba catalysts is higher than that for each
component showed that obvious synergistic interaction in C2
formation exists between MnO2 and BaCO3.
The development of new routes for effective utilization of
methane and carbon dioxide is of great interest in chemistry. In
last two decades, a large amount of research papers have been
published on the oxidative coupling of methane with O2 to
produce C2 hydrocarbons (ethane and ethylene) and a number of
catalysts have been found for this reaction.1;2 The inevitable
formation of CO2, however, seems to be one of the most serious
problems from a practical point of view.3 A novel approach is to
use CO2 as an oxidant instead of O2, CO will be the only by-
product in this case. Moreover, unlike O2, CO2 will not induce
gas-phase radical reactions, which result in the decrease in C2
selectivity, it thus can be expected that the development of active
catalyst achieves high selectivity to C2 hydrocarbons.
Table 1. Catalytic activity for the conversion of CH4 by CO2
Catalyst
CH4
Conv./%
C2H4
Sel./%
C2H6
Sel./%
C2
Yield/%
Recently, some workers have attemped the CO2-induced
selective conversion of methane to C2 hydrocarbon. The catalytic
effectiveness of more than 30 metal oxides have been reported,4;5
and a series of binary oxide catalysts have also been reported.6{9
Unfortunately, there are very few reports on the Mn-containing
catalysts, although Mn is one of the most extensively studied
components in oxidative coupling of methane. Recently, methane
conversion to C2 hydrocarbons with CO2 over unsupported MnO2
catalyst was reported, but only 0.1–0.4% C2 yield was obtained.10
The present paper reports a novel effective Mn-containing
catalyst, Mn–BaCO3.
BaCO3
0
—
—
—
2.6
4.0
4.1
3.7
3.4
3.0
0.6
Mn/Ba(0.2)
Mn/Ba(0.4)
Mn/Ba(0.6)
Mn/Ba(0.8)
Mn/Ba(1.0)
Mn/Ba(2.0)
MnO2
3.5
5.5
5.8
6.2
6.8
8.1
9.8
23.1
28.4
29.9
24.7
17.4
15.0
1.1
48.3
45.2
41.6
35.4
30.8
23.2
5.2
All the data were obtained after 3 h reaction under the standard
reaction conditions.
The catalysts with different Mn/Ba atomic ratios were
prepared by simultaneously adding the solutions with appropriate
concentration of Mn(NO3)2(A.R), Ba(NO3)2(A.R) to 1.1 times its
stoichiometric requirement of a well stirred 0.5 M aqueous
solution of K2CO3 maintained at 65 ꢁC, the resulting slurry (pH of
about 7.0) was filtered and washed several times with distilled
water, the product was calcined at 900 ꢁC after dried at 110 ꢁC
overnight, then the catalyst was crushed and sieved 20–40 mesh.
The granular catalyst was first pretreated with air in a Y-type
quartz reactor (I.D. 9.5 mm), followed by replacement with high
pure N2. Then, a mixture of CH4 and CO2 was introduced to the
reactor. The standard reaction conditions were as follows:
t ¼ 875 ꢁC, PðCH4Þ ¼ 30:3 kPa, PðCO2Þ ¼ 70:7 kPa, total flow
rate ¼ 100 cm3Á minÀ1, 3.0 g catalyst was used.
Figure 1 shows the temperature dependence of the catalytic
performance of Mn–BaCO3 catalyst with Mn/Ba ratio of 0.6. As
shown in Figure 1, CH4 conversion and C2 yield increased with
the increase of reaction temperature, C2 selectivity increased with
the reaction temperature at 6825 ꢁC, the increase in reaction
temperature decreased C2 selectivity when the temperature
exceeded 825 ꢁC. CH4 conversion of 5.8% and 8.0% was attained
at 875 ꢁC and 900 ꢁC respectively, while C2 selectivity was 71.5%
and 58.8%. The C2H4/C2H6 ratio in C2 hydrocarbons was not
shown in Figure 1, but the selectivity of C2H4 and C2H6 was
shown respectively, the results show that the C2H4/C2H6 ratio
increased with the increase of reaction temperature.
The change in reaction performance with time on stream is
shown in Figure 2, where the catalyst with Mn/Ba ratio of 0.6 is
used. As may be seen, conversion of CH4 and CO2 and selectivity
of C2 reached a steady state after reaction of ca. 1.5 h and not
change even after 20 h. Such the stable performance of the Mn–
BaCO3 catalyst after 1.5 h suggests that the C2 hydrocarbons are
from the reaction of CH4 with CO2, not with lattice oxygen atom.
To verify this point, the fresh catalyst and the catalyst after
reaction of 2 h, 10 h, 20 h was subjected to XRD measurement. As
shown in Table 2, the crystalline phase of the fresh catalyst was
After removal of H2O from theeffluent, C2H6, C2H4, CO, and
H2 were analyzed with an on-line gas chromatograph. The
following reactions were taken into account for data processing:
2CH4 þ CO2 ¼ C2H6 þ CO þ H2O
2CH4 þ 2CO2 ¼ C2H4 þ 2CO þ 2H2O
CH4 þ 3CO2 ¼ 4CO þ 2H2O
ð1Þ
ð2Þ
ð3Þ
Copyright Ó 2002 The Chemical Society of Japan