Effect of Dielectric Constant, Cavities in Series, and Cavities in Parallel on the Product Distribution of the Oligomerization of Methane via Microwave Plasmas

The use of microwave-induced plasmas as a method to oligomerize methane to higher hydrocarbons has been studied. The pressure range used was 10−20 Torr and the applied power was 60 W. The microwave power is coupled to the plasma by means of either an Evenson or a Beenakker cavity, the Beenakker being the most effective. We explored the effect of the presence of a dielectric material on the product distribution for this reaction. The values of the dielectric constants for these materials varied from 2.6 for Pb(Ac)2 to 10 000 for MnO2 relative to the vacuum. No direct correlation was found, but in some cases the selectivities toward C6s to C8s were enhanced. TiO2 and Li2CO3 increased the selectivities toward C6s. SnO2 was the best for selectivities to C7s and C8s. When a coating of Si/SiC on the reactor walls was present in the plasma zone, the selectivities toward C6s and C7s increased with respect to both materials (Si and SiC) by themselves. We also studied the effect of cavities in series and cavities in parallel on the oligomerization of methane with and without dielectric material in between the cavities. When methane and iodine are activated separately and then recombined, it seems that the oligomerization of methane is enhanced toward higher hydrocarbons. We found that when a dielectric material is placed in between and when the distance between the two cavities in series is the largest, the oligomerization of methane toward high molecular weight hydrocarbons is maximized.