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

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-<i>d</i>₄ 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 (<i>E</i>_a = 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 (<i>E</i>_a = 127 ± 27 kJ mol^–1) and H-ZSM-5 (<i>E</i>_a = 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