Frequency Effects in the Catalytic Oligomerization of Methane via Microwave Heating

Microwave radiation has been used to catalytically activate methane for oligomerization to higher hydrocarbons. The effect of frequency of microwave radiation on the microwave field distribution pattern and, consequently, on the product distribution for the methane reaction has been studied. Nickel catalysts, in the form of branched-type filaments, were used in these studies. Experiments were done with pure methane and using He as diluent. In general, as power increases, conversion of methane increases and selectivity toward C2s decreases. At 4.60 GHz, as power increases from 130 to 370-W, selectivities of ethylene and ethane decrease and selectivity of acetylene increases. As power increases, selectivity toward benzene also increases. These data differ from the case at low frequency (2.45 GHz), in which the presence of He helped to prevent the formation of acetylene at all power levels. Changes in product distribution due to changes in frequency have been observed. For the case of methane both with and without diluent, as frequency increases from 2.40 to 4.60 GHz at 370 W, selectivity toward benzene also increases. When He was used as a diluent gas, as frequency increases, selectivity toward ethylene and acetylene also increases. When pure methane was used, the opposite was observed. These changes in product distribution due to changes in frequency might be related to different transverse magnetic modes at different frequencies. Different transient heating patterns may occur at different values of frequency.