Factors affecting the catalytic oligomerization of methane via microwave heating

Date of Completion

January 2003


Chemistry, Inorganic|Engineering, Materials Science




Catalytic microwave heating has been used as a method for the oligomerization of methane to higher hydrocarbons. Many catalysts were tested in this reaction. Nickel powder, raney nickel, iron powder and activated carbon were the most active and efficient catalysts for the production of higher hydrocarbons. When helium was used as a diluent gas and the applied power was optimized, the selectivities were controlled to the most desired products. In general, the most abundant products for all the experiments were C2s. Iron powder was active only at high power (1130 W). At these conditions acetylene was avoided and ethylene and ethane were produced in the same proportion. Activated carbon catalysts with helium as diluent led to a selectivity towards benzene up to 33%. ^ Some manganese oxides such as OMS-1, OMS-2 and MnO2 (dielectric constant, ε ≈ 104) were not active in these reactions. These data suggest that the dielectric constant is not the most important factor in the oligomerization of methane via microwave heating. Conversion and activities of these materials are not proportionally related to the surface area of the catalysts. ^ Higher catalytic activity was observed for Raney nickel than for regular nickel powder. The maximum conversion obtained was 24% at 400 W and 10 min of irradiation time. For regular nickel powder that conversion can be achieved only after 700 W of power and more than 20 min of reaction. BET surface area, Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy, and Temperature-Programmed Desorption and Reduction analysis were performed to characterize the catalyst before and after reaction. Deactivation of Raney nickel by fouling and sintering was observed after 500 W and/or 15 min of reaction. ^ The effect of microwave radiation frequency on activity and product distribution for methane oligomerization has been studied. Nickel, iron, and activated carbon catalysts were used in these studies. Experiments were done with pure methane and using He as diluent. Changes in product distribution due to changes in frequency have been observed, and might be related to different transverse magnetic modes at different frequencies. Different transient heating may occur at different values of frequency. ^