Catalytic hydrogenation of alkenes on acidic zeolites: Mechanistic connections to monomolecular alkane dehydrogenation reactions

Abstract

Brønsted acid sites in zeolites (H-FER, H-MFI, H-MOR) selectively hydrogenate alkenes in excess H 2 at high temperatures (>700 K) and at rates proportional to alkene and H 2 pressures. This kinetic behavior and the De Donder equations for non-equilibrium thermodynamics show that, even away from equilibrium, alkene hydrogenation and monomolecular alkane dehydrogenation occur on predominantly uncovered surfaces via microscopically reverse elementary steps, which involve kinetically-relevant (C–H–H) + carbonium-ion-like transition states in both directions. As a result, rate constants, activation energies and activation entropies for these two reactions are related by the thermodynamics of the overall stoichiometric gas-phase reaction. The ratios of rate constants for hydrogenation and dehydrogenation reactions do not depend on the identity or reactivity of active sites; thus, sites within different zeolite structures (or at different locations within a given zeolite) that favor alkane dehydrogenation reactions, because of their ability to stabilize the required transition states, also favor alkene hydrogenation reactions to the exact same extent. These concepts and conclusions also apply to monomolecular alkane cracking and bimolecular alkane–alkene reaction paths on Brønsted acids and, more generally, to any forward and reverse reactions that proceed via the same kinetically-relevant step on vacant surfaces in the two directions, even away from equilibrium. The evidence shown here for the sole involvement of Brønsted acids in the hydrogenation of alkoxides with H 2 is unprecedented in its mechanistic clarity and thermodynamic rigor. The scavenging of alkoxides via direct H-transfer from H 2 indicates that H 2 can be used to control the growth of chains and the formation of unreactive deposits in alkylation, oligomerization, cracking and other acid-catalyzed reactions.