The Relation between Nature and Stability of H2-Dissociating Sites and Propene Selectivity in Silica-Supported (Ga,Al)2O3 Mixed Oxide Propane Dehydrogenation Catalysts

Colloidal solutions of gallia-alumina (Ga,Al)₂O_3(x:y) solid-solution nanoparticles with nominal atomic Ga : Al (x:y) ratios of 1 : 6, 1 : 3, 3 : 1, and 1:0 were used to prepare silica-supported catalysts for propane dehydrogenation (PDH). A comparison of the unsupported and silica-supported catalysts reveals that the dispersion on silica increases the Ga-normalized PDH rates for all catalysts, albeit with a notably lower propene selectivity for (Ga,Al)₂O_3(1:6)/SiO₂. Fourier transform infrared (FTIR) spectroscopy allows contrasting the H₂ dissociation sites in the calcined and H₂-treated (Ga,Al)₂O_3(x:y)/SiO₂, indicating a transformation of Ga³⁺ surface sites with Al (mainly) and Ga atoms in the second coordination sphere (Ga_(Al/Ga) sites) in the calcined (Ga,Al)₂O_3(1:6)/SiO₂ to predominantly Ga_(Ga/Si) surface sites in the H₂-treated material. The resulting sites are similarly unselective as in amorphous gallia on silica. H₂ produced during the PDH reaction can cause a similar transformation as H₂ pretreatment in (Ga,Al)₂O_3(1:6)/SiO₂, rapidly resulting in a notably lowered selectivity. The stable and selective Ga_(Al/Ga) surface sites in (Ga,Al)₂O_3(1:3)/SiO₂ yield a Ga−H band at ca. 1990 cm⁻¹ under H₂ dissociation conditions while the less selective surface sites, observed for the other Ga : Al ratios, give Ga−H bands at ca. 2040 and 2060 cm⁻¹.