The role of Ga 2O 3 and proton acidity on the dehydrogenating activity of Ga 2O 3-HZSM-5 catalysts: evidence of a bifunctional mechanism

Abstract

In this paper, we investigated the effects of the Ga loading amount and H2 treatment temperature for the reductive solid-state ion-exchange reaction on the generated Ga species in Ga-exchanged MFI zeolites (Ga-MFIs) as well as their catalysis for ethane dehydrogenation (EDH). For the formation of isolated Ga hydrides in the zeolites, [GaH]2+ ions were preferentially formed in the low-loading Ga-MFI (Ga/Al = 0.3) treated with H2 at 550 °C, corresponding to the conventional preparation conditions, (Ga-MFI-0.3(550)), while the high Ga loading (Ga/Al = 1.0) and high-temperature H2 treatment (800 °C) (Ga-MFI-1.0(800)) induced the formation of [GaH2]+ ions as the major Ga hydrides, as revealed by in situ Fourier transform infrared spectroscopy including the isotope experiment using D2. In the context of other Ga species, such as Ga+ cations and partially reduced Ga oxides (GaOX), Ga+ cations and GaOX coexist in Ga-MFI-0.3(550), as indicated by pyridine adsorption experiments. On the other hand, GaOX was hardly observed and a larger amount of Ga+ cations was formed in Ga-MFI-1.0(800). The remaining Brønsted acid sites (BASs) were also characterized by the NH3 adsorption experiment. In the EDH reaction, Ga-MFI-1.0(800) exhibited high selectivity owing to low coke formation, resulting in the highest durability among the series of Ga-MFIs tested. Under the optimized conditions, Ga-MFI-1.0(800) exhibited the highest C2H4 formation rate among previously reported Pt-free catalysts. Based on the combined results of characterization, catalyst tests, and kinetic studies, the high selectivity and durability of Ga-MFI-1.0(800) can be ascribed to the low amount of the remaining BASs by isolated Ga species ([GaH]2+, [GaH2]+ ions and Ga+ cations) as well as the major formation of [GaH2]+ ions among isolated Ga hydrides.