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.