Ga₂O₃-based catalysts for the dehydrogenation of propane (PDH) feature hexa- and tetracoordinated Ga³⁺ sites, the latter are likely the active centers for PDH.[1] However, little is known about the distribution and nature of surface sites in Ga₂O₃-based PDH catalysts and the influence of surface acidity, structural defects and the support on the catalytic activity. Here, we report that the specific activity of silica-supported γ–Ga₂O₃ nanoparticles (γ–Ga₂O₃/SiO₂) in PDH is higher than that of β–Ga₂O₃/SiO₂ and a Ga(NO₃)₃-derived benchmark, Ga/SiO₂. We characterized surface sites by pyridine-FTIR and ¹⁵N DNP NMR and developed structure-activity relationships for this class of catalysts. γ–Ga₂O₃/SiO₂ is a more active but less selective catalyst than unsupported γ–Ga₂O₃ (Fig. 1, A, B). Surface acidity studies reveal mild Brønsted acid sites only for γ–Ga₂O₃/SiO₂. Such sites likely result from the interaction between γ–Ga₂O₃ and SiO₂ and may be associated with the high specific activity and lower selectivity of γ–Ga₂O₃/SiO₂. The defect-spinel structure of γ–Ga₂O₃ contains structural vacancies susceptible to cationic substitutions,[2] which could promote an interaction with the silica support, thus yielding mild Brønsted acidity with distinct catalytic performance. Interactions between Lewis acid sites and surface silanol groups generating Brønsted acidic sites have been described for zeolites and silica-aluminas,[3,4] but to our knowledge, they have not yet been characterized for interfacial sites in SiO₂-supported metal-oxide nanoparticles. In contrast to all other catalysts in this work, Ga/SiO₂, the benchmark featuring only tetracoordinated Ga sites, does not deposit any coke and does not deactivate on stream. ¹⁵N DNP NMR spectra (Fig. 1C) show features at ca. 235 ppm ascribed to Ga(IV) sites for all catalysts except α–Ga₂O₃. This suggests that higher-coordinated Ga sites could be associated with coke-related deactivation in PDH. Overall, we establish structure-activity relationships for Ga₂O₃-based PDH catalysts by elucidating the surface sites of silica-supported and unsupported polymorphs of Ga₂O₃, also providing insight for the interaction between γ–Ga₂O₃ and the silica support.

Figure 1. PDH specific activity (A) and selectivity (B) for Ga₂O₃ catalysts. ¹⁵N DNP NMR results, relating features at different chemical shifts with the pyridine-bound surface Ga sites (C).

[1] Fan, K.N., et al., J. Catal., 2008, 256, 293–300.
[2] Gupta, P, et al., J. Phys. Chem. C., 2018, 122, 22094−22105.
[3] Meriaudeau, P., et al., J. Mol. Catal., 1990, 59, L31 - L36.
[4] Hensen, J. M., et al., J Phys. Chem. C., 2012, 116, 21416–21429.