H/D Exchange of Isobutane on In-Modified BEA Zeolite Investigated by 1H MAS NMR

With the aim of discovering the effect of indium species on the protolytic activation of isobutane by Br & oslash;nsted acid sites (BAS), the kinetics of H/D hydrogen exchange between isobutane-d(10) and BAS of the InO+/H-BEA zeolite was monitored by H-1 MAS NMR spectroscopy. The exchange has been established to occur exclusively between methyl groups of isobutane-d(10) and BAS and demonstrates a significant induction period. Dehydrogenation of isobutane to isobutene is observed alongside the H/D exchange in the alkane. Kinetics of protium transfer from BAS to both deuterated isobutane and isobutene are interpreted in terms of the indirect cyclic mechanism of H/D exchange with isobutene as a key intermediate. It is inferred that indium sites perform isobutane dehydrogenation and strongly retain isobutene formed. This accounts for the prolonged induction period in the H/D exchange reaction kinetics. Indium sites located in the zeolite pores decelerate protium transfer from zeolite BAS to isobutane via an indirect H/D exchange mechanism and, contrary to the case with linear alkanes, are not able to facilitate the direct hydrogen exchange between BAS and methyl groups in isobutane.