Ind. Eng. Chem. Res. 2000, 39, 1221-1227
1221
Direct Conversion of Methane and Carbon Dioxide to Higher Hydrocarbons Using Catalytic Dielectric-Barrier Discharges with Zeolites Baldur Eliasson, Chang-jun Liu,† and Ulrich Kogelschatz* ABB Corporate Research Ltd, 5405 Baden, Switzerland
Direct higher hydrocarbon formation from the greenhouse gases methane and carbon dioxide using a dielectric-barrier discharge (DBD) with zeolite catalysts is presented. This catalytic DBD can be operated at ambient conditions and leads to direct hydrocarbon formation. The products include alkanes, alkenes, oxygenates, and syngas (CO + H2). The product distribution depends on the pressure, the input power, the flow rate, the CH4/CO2 feed ratio, and the catalyst used. It is not sensitive to gas temperature in the range from room temperature to 150 °C. From the experiments it can be concluded that a cogeneration of syngas and higher hydrocarbons can be achieved using the catalytic DBD. The optimum CH4/CO2 ratio in the feed for such cogeneration is in the range 2/1 to 3/1. The energy efficiency of CO2 and CH4 conversion increases substantially at higher discharge powers. Introduction Among all greenhouse gases, CH4 and CO2 contribute most to the man-made greenhouse effect. Any success in the research and development of a feasible utilization of CH4 and CO2 will signify the attainment of double objectives of slowing down a buildup of greenhouse gases in the atmosphere and better carbon resource utilization. A possible utilization of CH4 and CO2 is the production of higher hydrocarbons (HHCs) or mixed alcohols via the syngas pathway:
the production and compression of syngas. Third, some undesirable carbon deposits cannot be avoided in the present catalyst design of CO2 reforming. Syngas production from CO2 reforming of CH4 so far is not competitive with the process of steam reforming of CH4.1 Since 1990, attempts of direct HHC formation from CH4 and CO2 have been reported.2-4 The ideal products from such direct HHC formation are C2 hydrocarbons, especially ethylene:
2CH4 + CO2 f CO + C2H6 + H2O, ∆G1073K ) 35 kJ/mol (4) C2H6 + CO2 f CO + C2H4 + H2O, ∆G1073K ≈ 0 kJ/mol
However, there exist considerable difficulties in such utilization of CH4 and CO2 in an economical way. We describe experiments investigating the combined action of catalysts and a nonequilibrium gas discharge, which may lead to alternative methods of syngas and hydrocarbon production from CH4 and CO2. Background The formation of CO + H2 (syngas) by reaction 1 is an endothermic reaction and requires a high temperature for a favored equilibrium. During the CO2 reforming of CH4 (1), all hydrogen atoms in the CH4 molecule are stripped and replaced by oxygen atoms. This is a very energy-intensive process. Subsequent reactions of syngas, like the F-T synthesis, need high-pressure operation for a reasonable conversion. Therefore, the produced syngas has to be compressed. This requires additional energy. For the commercialized setup of the HHC synthesis from syngas, >60% of the cost goes to * To whom correspondence should be addressed. Telephone: +41 56 486 81 67. Fax: +41 56 493 4569. E-mail:
[email protected]. † On leave from State Key Laboratory of C Chemical 1 Technology, Tianjin University, Tianjin 300072, People’s Republic of China.
(5)
At 1073 K, the equilibrium yields of ethane and ethylene are sufficiently high (13% and 57%, respectively).2 However, the experimental yield of C2 hydrocarbons over a metal oxide catalyst is
