Isomerization of hexane by zeolite HZSM-5 - The effect of cyclic hydrocarbons

The main products of hexane (H) isomerization on HZSM-5 at 160 degrees C in liquid phase are the isomers 2-methylpentane (2MP) and 3-methylpentane (3MP), accompanied by cracking and disproportionation products, but no alkenes. 2MP and 3MP are not formed by a simple carbocationic isomerization of H, because the 2MP/3MP ratio, (6-7)/1 is higher than the equilibrium ratio of 2.1 and does not change with conversion between 1 and 10%. The same observation applies to the small amounts of 2,3-dimethylbutane formed. Methylcyclopentane (MCP) has a minor accelerating effect on the conversion of H only at ratios I:1. No hydride transfer catalysis operates. At the same ratio (I:1), cyclopentane (CP) has a somewhat greater effect than MCP on the conversion of H. The redistribution of the label in the conversion of H-u-d(4.3) containing 4-5% MCP and of H containing 5% MCP-1-d showed that the products were formed from alkenes, which are the major intermediates of the reaction, even though none of them is found in the products desorbed. Each alkene exchanges hydrogen with the catalyst several times before being desorbed. The direct H/D exchange between the alkene (or its mechanistic equivalent) and the alkane, if it occurs, is less important. Thus, the usual mechanistic representation for alkane conversion, borrowed from reactions in superacid solutions, does not apply to the reaction with the zeolite catalyst. (C) 2000 Elsevier Science B.V. All rights reserved.