Reactions of hexane isomers on H-mordenite at moderate temperatures in liquid and gas phases: the effect of cyclic hydrocarbons

Abstract

H-mordenite, a more active acid catalyst than HZSM-5, showed features of a carbocationic/cationoidic isomerization in the reaction of liquid 3-methylpentane, but not of liquid hexane. The carbocationic isomerization features were not seen in the gas phase, for either reactant. Thus, there are mechanistic differences between the reactions of alkanes in the liquid phase and in the vapor phase on the zeolite. The liquid-phase reaction of hexane in the presence of methylcyclopentane, cyclopentane, and deuteromethylcyclopentane showed the absence of hydride transfer catalysis. The label redistribution between deuterated and nondeuterated reactants showed that the products were formed from olefinic reaction intermediates, although none were found in the products desorbed. Each olefin exchanged repeatedly hydrogen with the catalyst before being hydrogenated and desorbed. The possibility that alkenyl cations retained on the catalyst play a role, as in trifluoromethanesulfonic acid, is discussed. As the space requirement of hydride transfer is smaller than that of β-cracking, the results speak against protonated alkanes (“carbonium” ions) in the catalysis on zeolites. The type of activation observed in superacid catalysts, where the standard carbocationic mechanism operates, does not occur in zeolites. The chemisorbed alkenes may be formed by one-electron oxidation or by dehydrogenation on tetracoordinated aluminum atoms, either in the lattice or in cages (extralattice aluminum).