Tellurium NaX zeolites: I. Deuterium tracer studies of cyclohexane dehydrogenation to benzene

Abstract

Deuterium tracers were used to explore the mechanisms of dehydrogenation of six-membered cyclic compounds to benzene over a Te NaX zeolite catalyst. Dehydrogenation of cyclohexane to cyclohexene (and probably to cyclohexadiene) precedes the formation of benzene. A large primary kinetic isotope effect ( k c 6h 12 k c 6D 12 ~ 2.5 at 400 ° C ) indicates that cleavage of the CH bonds in cyclohexane is the rate-determining reaction step. Although the presence of gaseous hydrogen is required to maintain catalyst activity, the dehydrogenation rates are the same in H 2 as they are in D 2. Only a limited amount of exchange occurs between D 2 and H atoms in the hydrocarbons. The exchange which is observed probably arises from double bond shift that occurs in the olefins by an addition/abstraction mechanism on a single Te atom containing one adsorbed D atom. Cyclohexane can be formed from cyclohexene by hydrogen transfer from another cyclohexene molecule much more readily than it can be formed by direct saturation with gaseous H 2. Both the dehydrogenation and isomerization sites are probably Te 0 surface atoms.