Dehydrocyclization of n-Paraffins Over Te-NaX Zeolite Catalysts

Abstract

Both 14C and deuterium tracers have been used to establish the reaction pathways and rate limiting steps in the formation of aromatics from n-paraffins and cycloparaffins over a shape-selective tellurium zeolite catalyst. The catalyst functions mainly as a dehydrogenation agent with linear mono-olefins and higher olefins being exclusive gas phase intermediates. Cyclization is likely a non-catalytic step involving ring closure of a linear hexatriene unit, followed by catalytic dehydrogenation of the cyclohexadiene nucleus to an aromatic. Alkyl cyclohexanes definitely are not intermediates in the aromatization of n-paraffins, although by themselves they can be dehydrogenated to aromatics in high yields over this catalyst. The absence of extensive H-D scrambling and the presence of a large primary kinetic isotope effect (kH/kD >2 at 400°C) when deuterated hydrocarbons are reacted simultaneously with similar undeuterated molecules strongly suggests that the slow step in the reaction is cleavage of one or more C-H bonds.