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

In relation to clarifying the pathway of methane activation on In-modified zeolites, a comparative analysis of kinetics of hydrogen (HID) exchange between methane-d(4) and Bronsted 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 H-1 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 (E-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 (E-a = 127 +/- 27 kJ mol(-1)) and H-ZSM-5 (E-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 HID exchange with BAS of the zeolite or evolves further to offer indium methyl species.