Methane Activation on In-Modified ZSM-5 Zeolite. H/D Hydrogen Exchange of the Alkane with Brønsted Acid Sites

Abstract

In relation to clarifying the pathway of methane activation on In-modified zeolites, a comparative analysis of kinetics of hydrogen (H/D) exchange between methane-d4 and Brønsted acid sites (BAS) for both the pure acid form zeolite (H-ZSM-5) and In-modified zeolites (In+/H-ZSM-5 and InO+/H-ZSM-5) has been performed. Monitoring of the kinetics has been carried out with 1H magic-angle spinning NMR spectroscopy in situ within the temperature range of 453–568 K. While the rate of exchange on In+/H-ZSM-5 is 1 order of magnitude larger than that on H-ZSM-5, the exchange occurs on InO+/H-ZSM-5 by 2 orders of magnitude faster than that on H-ZSM-5. Significant increase of the rate and decrease of the activation energy (Ea = 74 ± 6 kJ mol–1) and the temperature threshold (453 K) for the reaction of the exchange on InO+/H-ZSM-5 compared to the rate, activation energy, and temperature threshold (543 K) for the reaction on In+/H-ZSM-5 (Ea = 127 ± 27 kJ mol–1) and H-ZSM-5 (Ea = 118 ± 9 kJ mol–1) have been rationalized in terms of involvement of both InO+ and BAS in activation of methane molecules on the zeolite. Some transient intermediate complex of methane with the zeolite InO+ species and BAS has been assumed to be formed within the zeolite pore. This complex is involved either in the reaction of H/D exchange with BAS of the zeolite or evolves further to offer indium methyl species.