Propane Dehydrogenation on Ga2O3-Based Catalysts: Contrasting Performance with Coordination Environment and Acidity of Surface Sites

alpha-Ga2O3, beta-Ga2O3, and gamma-Ga2O3 as well as the silica-supported catalysts gamma-Ga2O3/SiO2, gamma-Ga2O3/SiO2, and Ga(NO3)(3)-derived Ga/SiO2 were prepared, characterized, and evaluated for propane dehydrogenation (PDH) at 550 degrees C. The coordination environment and acidity of surface sites in stand-alone and SiO2 -supported Ga2O3 catalysts were studied using FTIR, N-15 dynamic nuclear polarization surface-enhanced NMR spectroscopy (N-15 DNP SENS), and DFT modeling of the adsorbed pyridine probe molecule. The spectroscopic data suggest that the Lewis acidic surface Ga sites in gamma-Ga2O3 and beta-Ga2O3 (the latter obtained from colloidal nanocrystals of gamma-Ga2O3 via thermal treatment at 750 degrees C) are similar, except that beta-Ga2O3 contains a larger relative fraction of weak Ga-3+ Lewis acid sites. In contrast, alpha-Ga2O3 features mostly strong Lewis acid sites. This difference in surface sites parallels their difference in catalytic activities: i.e., weak Lewis acid surface sites are more abundant in beta-Ga2O3 relative to alpha-Ga2O3 and gamma-Ga2O3 and the increased relative abundance of weak Lewis acidity correlates with a higher initial catalytic activity in PDH, 0.41 > 0.28 > 0.14 mmol C(3)H(6)m(-2) (Ga2O3) h (1 )at 550 degrees C, for respectively beta-, alpha-, and gamma-Ga2O3 with initial propene selectivities of 86, 83, and 88%. Dispersion of gamma-Ga2O3 or beta-Ga2O3 on a silica support introduces strong as well as abundant weak Bronsted acidity to the catalysts, lowering the PDH selectivity. The gamma-Ga2O3/SiO2 catalyst was slightly more active than beta-Ga2O3/SiO2 in PDH (Ga normalized activity) with initial propene formation rates of 11 and 9 mol C3H6 mol (sel = 76 and 73%, respectively). However, these catalysts deactivated by ca. 55% within 100 min time on stream (TOS) due to coking. In contrast, Ga/SiO2, with mostly tetracoordinated surface Ga sites and abundant, strong Bronsted acid sites, gave a lower activity and selectivity in PDH (3.5 mol C3H6 mol Ga-1 h(-1) and 49%, respectively) but showed no deactivation with TOS. DFT calculations using a fully dehydroxylated oxygen-deficient model beta-Ga2O3 surface show that tetra- and pentacoordinated Ga Lewis acid sites bind pyridine more strongly than tricoordinated Ga sites and a higher relative fraction of strong Lewis acid sites correlates with increased coking. Overall, our results indicate that weakly Lewis acidic, tricoordinated Ga3+ sites are likely driving the superior PDH activity of beta-Ga2O3.

Automated summary

The study investigated the coordination environment and acidity of surface sites in different Ga2O3 catalysts for propane dehydrogenation, with spectroscopic data suggesting that weak Lewis acid surface sites in beta-Ga2O3 contribute to its superior catalytic activity compared to alpha-Ga2O3 and gamma-Ga2O3.