Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCl₃

In this study, the mechanism and kinetics of C₃H₈ dehydrogenation and cracking are examined over Ga/H-MFI catalysts prepared via vapor-phase exchange of H-MFI with GaCl₃. The present study demonstrates that [GaH]²⁺ cations are the active centers for C₃H₈ 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 C₃H₈ dehydrogenation that is 2 orders of magnitude higher and for C₃H₈ cracking, that is 1 order of magnitude higher than the corresponding turnover frequencies over H-MFI. C₃H₈ dehydrogenation and cracking exhibit first-order kinetics with respect to C₃H₈ over H-MFI, but both reactions exhibit first-order kinetics over Ga/H-MFI only at very low C₃H₈ partial pressures and zero-order kinetics at higher C₃H₈ partial pressures. H₂ 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 C₃H₈ and H₂ 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 C₃H₈ proceed over Ga/H-MFI via reversible, heterolytic dissociation of C₃H₈ at [GaH]²⁺ sites to form [C₃H₇-GaH]⁺-H⁺ cation pairs. The rate-determining step for dehydrogenation is the β-hydride elimination of C₃H₆ and H₂ from the C₃H₇ 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. H₂ inhibits both dehydrogenation and cracking over Ga/H-MFI via reaction with [GaH]²⁺ cations to form [GaH₂]⁺-H⁺ cation pairs.