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

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 Ga 2 O 3 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 H 2 to form GaH x (2034 cm -1 ) with an enthalpy of formation of ∼-51.2 kJ·mol -1 , a result supported by density functional theory calculations. The initial C 3 H 8 dehydrogenation rates decrease rapidly (40%) during the first 100 min and then decline slowly afterward, while the C 3 H 6 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 GaH x 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 Ga 3+ vs In 3+ . 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.