The mechanism of isobutane cracking over amorphous and crystalline aluminosilicates

Abstract

The acid catalyzed cracking of isobutane over a silica-alumina catalyst and several zeolites was studied. The activity of these catalysts, for whatever reason, increased in the order silica-alumina < H-ZSM-5 < H-Y < H-M, i.e., the same order found in a closely related study of neopentane. The higher the activity the lower was the temperature required to achieve a given conversion, the higher was the paraffin-to-olefin ratios, and the greater was the isomerization of iso- to normal hydrocarbons. It was demonstrated that with all these catalysts two primary reactions occurred resulting in the formation of CH 4 and H 2 and concomitantly the t-butyl and isopropyl cations. The latter either desorbed as the corresponding olefins or underwent secondary reactions. So long as the latter were relatively unimportant the C 3 + C 4 hydrocarbons formed nearly equaled the isobutane reacted. However, the sum of the CH 4 and H 2 produced was generally less than this value. Evidently hydrogen transfer is more important in the conversion of isobutane than of neopentane. This chemistry is discussed.