Ga+-Chabazite Zeolite: A Highly Selective Catalyst for Nonoxidative Propane Dehydrogenation.

ABSTRACT

Ga-chabazite zeolites (Ga-CHA) have been found to efficiently catalyze propane dehydrogenation with high propylene selectivity (96%). In situ Fourier transform infrared spectroscopy and pulse titrations are employed to determine that upon reduction, surface Ga2O3 is reduced and diffuses into the zeolite pores, displacing the Brønsted acid sites and forming extra-framework Ga+ sites. This isolated Ga+ site reacts reversibly with H2 to form GaHx (2034 cm-1) with an enthalpy of formation of ∼-51.2 kJ·mol-1, a result supported by density functional theory calculations. The initial C3H8 dehydrogenation rates decrease rapidly (40%) during the first 100 min and then decline slowly afterward, while the C3H6 selectivity is stable at ∼96%. The reduction in the reaction rate is correlated with the formation of polycyclic aromatics inside the zeolite (using UV-vis spectroscopy) indicating that the accumulation of polycyclic aromatics is the main cause of the deactivation. The carbon species formed can be easily oxidized at 600 °C with complete recovery of the PDH catalytic properties. The correlations between GaHx vs Ga/Al ratio and PDH rates vs Ga/Al ratio show that extra-framework Ga+ is the active center catalyzing propane dehydrogenation. The higher reaction rate on Ga+ than In+ in CHA zeolites, by a factor of 43, is the result of differences in the stabilization of the transition state due to the higher stability of Ga3+ vs In3+. The uniformity of the Ga+ sites in this material makes it an excellent model for the molecular understanding of metal cation-exchanged hydrocarbon interactions in zeolites.