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

α-Ga 2 O 3, β-Ga 2 O 3, and γ-Ga 2 O 3 as well as the silica-supported catalysts γ-Ga 2 O 3 /SiO 2, β-Ga 2 O 3 /SiO 2, and Ga(NO 3 ) 3 -derived Ga/SiO 2 were prepared, characterized, and evaluated for propane dehydrogenation (PDH) at 550 °C. The coordination environment and acidity of surface sites in stand-alone and SiO 2 -supported Ga 2 O 3 catalysts were studied using FTIR, 15 N dynamic nuclear polarization surface-enhanced NMR spectroscopy ( 15 N DNP SENS), and DFT modeling of the adsorbed pyridine probe molecule. The spectroscopic data suggest that the Lewis acidic surface Ga sites in γ-Ga 2 O 3 and β-Ga 2 O 3 (the latter obtained from colloidal nanocrystals of γ-Ga 2 O 3 via thermal treatment at 750 °C) are similar, except that β-Ga 2 O 3 contains a larger relative fraction of weak Ga 3+ Lewis acid sites. In contrast, α-Ga 2 O 3 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 β-Ga 2 O 3 relative to α-Ga 2 O 3 and γ-Ga 2 O 3 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 (Ga 2 O 3 ) h –1 at 550 °C, for respectively β-, α-, and γ-Ga 2 O 3 with initial propene selectivities of 86, 83, and 88%. Dispersion of γ-Ga 2 O 3 or β-Ga 2 O 3 on a silica support introduces strong as well as abundant weak Brønsted acidity to the catalysts, lowering the PDH selectivity. The γ-Ga 2 O 3 /SiO 2 catalyst was slightly more active than β-Ga 2 O 3 /SiO 2 in PDH (Ga normalized activity) with initial propene formation rates of 11 and 9 mol C 3 H 6 mol Ga –1 h –1 (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/SiO 2, with mostly tetracoordinated surface Ga sites and abundant, strong Brønsted acid sites, gave a lower activity and selectivity in PDH (3.5 mol C 3 H 6 mol Ga –1 h –1 and 49%, respectively) but showed no deactivation with TOS. DFT calculations using a fully dehydroxylated oxygen-deficient model β-Ga 2 O 3 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 Ga 3+ sites are likely driving the superior PDH activity of β-Ga 2 O 3 .