Acid Catalysis over Crystalline Zr₃SO₉: Role of the Local Structure in Generating Acidity

Sulfated zirconia (ZrS) has been widely used as an acid catalyst in industrial processes for the isomerization of n-alkanes. Despite the excellent catalytic properties of this material, its catalytically active structure has been hardly identified to date. In this study, we investigated the crystal structure and acid properties of crystalline Zr3SO9, which was reported 30 years ago by Kato et al. Zr3SO9 was a hexagonal-shaped plate consisting of a (101) plane of the tetragonal ZrO2 phase with S species uniformly dispersed in the form of SO42–. Zr3SO9 exhibits an outstanding catalytic activity for acid reactions, which is superior to those of typical acid catalysts. The introduction of H2O into Zr3SO9 altered the nature of the acid sites, and almost all Lewis acid sites (LAS) were transformed into Brønsted acid sites (BAS). Spectroscopic analyses and density functional theory calculations revealed that the Zr sites adjacent to SO42– functioned as a LAS, while the H2O coordinated to this Zr site acted as a BAS. Moreover, Pt-loaded Zr3SO9 demonstrated a stable catalytic activity in the isomerization of n-butane under continuous gas-flow conditions, which was significantly higher than that of Pt-loaded ZrS that is employed as an industrial catalyst for this reaction.