H/D Exchange of n-Butane on In-Modified Zeolite BEA Investigated by ¹H Magic-Angle Spinning Nuclear Magnetic Resonance In Situ

Abstract

To establish the effect of indium species introduced in the zeolite on protolytic activation of the alkane by zeolite Brønsted acid sites (BASs), the kinetics of H/D hydrogen exchange between n-butane-d 10 and the BAS of indium-modified zeolite H-BEA has been monitored by 1H magic-angle spinning nuclear magnetic resonance spectroscopy in situ at 455–573 K. Regioselectivity in the methyl groups of the alkane, high rate, and low activation energy (52–64 kJ mol–1) of the H/D exchange in the presence of either InO+ or In+ cations in the zeolite indicate that the mechanisms of protolytic activation and the H/D exchange for n-butane differ from that on pure acid-form zeolites. The promoting effect of InO+ cations on the reaction kinetics is more essential than that of In+ cations. It is proposed that InO+ sites located in the vicinity of BASs provide fast exchange with the methyl groups of the alkane in the initial period of the kinetics. The BASs that are remote from InO+ sites are involved in the H/D exchange with the alkane at the final period of the kinetics by protium transfer from the remote BAS to BAS vicinal to InO+ sites. The transfer is realized by the H/D exchange between the remote BAS and BAS vicinal to InO+ sites with the assistance of a minor quantity of molecular hydrogen (H2), evolved at n-butane dehydrogenation and working as the vehicle for this transfer.