Determination of acid structures on the surface of sulfated monoclinic and tetragonal zirconia through experimental and theoretical approaches

Sulfated zirconia (SZ) plays an important role in solid acid catalysts. To understand the origin of the super acidity over the surface of sulfated zirconia, pure monoclinic and tetragonal phase zirconia and their sulfated counterparts (M-SZ and T-SZ, respectively) were prepared, and the acid properties of these samples were characterized by 31P MAS NMR using TMP as the probe molecule. In comparison with zirconia, stronger Lewis acid (LA) and Brønsted acid (BA) sites were generated over the SZ samples after sulfation. The density ratios of the newly generated LA and strong BA sites were found to be ca. 2.0 : 1.0 on M-SZ and ca. 1.8 : 1.0 on T-SZ. Based on this quantitative relationship between the newly generated LA and strong BA sites, the possible acid structures on the SZ surface with different crystal phases are established. After structural optimization and theoretical calculation, the super acid sites originating from the bisulfate ions on the monoclinic (001) plane and the tetragonal (111) plane are determined.