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

Abstract

In 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.