Competitive pathways of methane activation on Zn2+-modified ZSM-5 zeolite: H/D hydrogen exchange with Brønsted acid sites versus dissociative adsorption to form Zn-methyl species

Abstract

To clarify the pathways of methane activation on Zn-modified high-silica zeolites, the kinetics of both dissociative adsorption of the alkane C–H bond to form Zn-methyl species and H/D hydrogen exchange between the alkane and Brønsted acid sites (BAS) have been analyzed for Zn²⁺/H-ZSM-5 containing exclusively Zn²⁺ cations (no ZnO species in the zeolite) and BAS. Analysis of the kinetics was performed by ¹H MAS NMR spectroscopy in situ at 410–540 K. In spite of the activation barrier for H/D hydrogen exchange (68 kJ mol⁻¹) being larger than that for Zn-methyl formation (46 kJ mol⁻¹), the rate of H/D hydrogen exchange has been found to be one order of magnitude higher than the rate of the formation of Zn-methyl species within the studied temperature range. This implies that Zn-methyl species cannot be involved in the reaction of H/D hydrogen exchange as the intermediate responsible for facilitation of this reaction due to the presence of Zn²⁺ cations in the zeolite (J. Catal., 2008, 253, 11). A new mechanism has been suggested for C–H bond activation in methane on the zeolite modified with Zn²⁺ cations. It includes first the formation of a transient molecular complex of methane with Zn²⁺ cations. The complex either is further involved in the reaction of H/D hydrogen exchange or evolves toward the formation of Zn-methyl species and BAS.