Isomerization and cracking of 13C-labeled hexanes over H-mordenite : I. Labeling of the products and general reaction scheme

Abstract

The catalytic conversion of branched hexanes over H-mordenite at 170 °C has been studied with a constant hydrocarbon pressure in a flow system. Under these conditions, isomers are desorbed first from the catalyst and cracked products (isobutane and isopentane exclusively) appear later on in the gas phase. Extrapolation of product distribution at zero conversion gives evidence for the occurrence of a direct interconversion between 3-methylpentane and 2,3-dimethylbutane and shows that 2-methylpentane is the only isomer yielding cracked fragments as primary products. The use of 13C-labeled 3-methylpentanes demonstrates that isobutane, isopentane, and part of the hexanes result from the random association of 13C and 12C carbon atoms and that two types of isotopic isomer are formed. A detailed reaction scheme, involving the β-fission of a 2-methylpentenium-4 cation as initial step, is proposed to explain the formation of cracked products. It is suggested that sites of two different locations are involved in the reactions of hexanes on H-mordenite.