# Mechanism and Kinetics of Propane Dehydrogenation
and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H‑MFI
with GaCl_{3}

journal contribution

posted on 26.12.2018, 00:00 by Neelay
M. Phadke, Erum Mansoor, Matthieu Bondil, Martin Head-Gordon, Alexis T. BellIn this study, the mechanism and
kinetics of C

_{3}H_{8}dehydrogenation and cracking are examined over Ga/H-MFI catalysts prepared via vapor-phase exchange of H-MFI with GaCl_{3}. The present study demonstrates that [GaH]^{2+}cations are the active centers for C_{3}H_{8}dehydrogenation and cracking, independent of the Ga/Al ratio. For identical reaction conditions, [GaH]^{2+}cations in Ga/H-MFI exhibit a turnover frequency for C_{3}H_{8}dehydrogenation that is 2 orders of magnitude higher and for C_{3}H_{8}cracking, that is 1 order of magnitude higher than the corresponding turnover frequencies over H-MFI. C_{3}H_{8}dehydrogenation and cracking exhibit first-order kinetics with respect to C_{3}H_{8}over H-MFI, but both reactions exhibit first-order kinetics over Ga/H-MFI only at very low C_{3}H_{8}partial pressures and zero-order kinetics at higher C_{3}H_{8}partial pressures. H_{2}inhibits both reactions over Ga/H-MFI. It is also found that the ratio of the rate of dehydrogenation to the rate of cracking over Ga/H-MFI is independent of C_{3}H_{8}and H_{2}partial pressures but weakly dependent on temperature. Measured activation enthalpies together with theoretical analysis are consistent with a mechanism in which both the dehydrogenation and cracking of C_{3}H_{8}proceed over Ga/H-MFI via reversible, heterolytic dissociation of C_{3}H_{8}at [GaH]^{2+}sites to form [C_{3}H_{7}-GaH]^{+}-H^{+}cation pairs. The rate-determining step for dehydrogenation is the β-hydride elimination of C_{3}H_{6}and H_{2}from the C_{3}H_{7}fragment. The rate-determining step for cracking is C–C bond attack of the same propyl fragment by the proximal Brønsted acid O–H group. H_{2}inhibits both dehydrogenation and cracking over Ga/H-MFI via reaction with [GaH]^{2+}cations to form [GaH_{2}]^{+}-H^{+}cation pairs.