Understanding the Correlation between Ga Speciation and Propane Dehydrogenation Activity on Ga/H-ZSM‑5 Catalysts

H-ZSM-5 zeolite-supported Ga (Ga/H-ZSM-5) has been considered as a selective catalyst for nonoxidative propane dehydrogenation (PDH) for decades; however, the reaction mechanism remains a topic of considerable discussion. In particular, the correlation between various Ga species present on the catalyst at the reaction conditions and the PDH activity has yet to be established. In this work, intrinsic PDH rates and activation energies were determined on Ga⁺–H⁺ pair sites and isolated Ga⁺ sites on Ga/H-ZSM-5 samples with a wide range of Si/Al and Ga/Al ratios. The turnover frequency on Ga⁺–H⁺ pair sites in the PDH is higher than that of isolated Ga⁺ sites by a factor of ∼15. Experimental measurements combined with a dual-site model show the activation energy in the PDH on the Ga⁺–H⁺ pair sites and isolated Ga⁺ sites to be 90.8 ± 1.5 and 117 ± 4.7 kJ·mol^–1, respectively. These results demonstrate that Ga⁺–H⁺ pair sites are much more active in the PDH than isolated Ga⁺ sites. The activation energy of GaH_x decomposition to form H₂ was determined to be 40–60 kJ·mol^–1 higher than that of the PDH on Ga species, suggesting that the GaH_x decomposition is unlikely to be part of the PDH mechanism. Although both Brønsted acid and Ga sites interact with propane, Fourier transform infrared spectroscopy results provide strong evidence suggesting that the alkyl mechanism is more likely in the PDH on Ga/H-ZSM-5 catalysts.