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

Abstract

AbstractColloidal solutions of gallia‐alumina (Ga,Al)2O3(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)2O3(1:6)/SiO2. Fourier transform infrared (FTIR) spectroscopy allows contrasting the H2 dissociation sites in the calcined and H2‐treated (Ga,Al)2O3(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)2O3(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)2O3(1:6)/SiO2, rapidly resulting in a notably lowered selectivity. The stable and selective Ga(Al/Ga) surface sites in (Ga,Al)2O3(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.