Zirconia-supported heteropoly acids: Characterization and catalytic behavior in liquid-phase veratrole benzoylation

The liquid-phase benzoylation of veratrole with benzoic anhydride (BA) to 3,4-dimethoxy benzophenone was investigated using zirconia-supported silicotungstic acid (STA) as a catalyst. Catalysts with different STA loadings (5-25 wt%) and calcination temperatures (600-850 °C) were prepared by suspending zirconium oxyhydroxide in methanol solution of STA followed by drying and calcination. These catalysts were characterized by XRD, Raman spectroscopy, DTA, XPS, NH3-TPD, FTIR pyridine adsorption, and DRUV-vis spectroscopy. XRD results indicated that the presence of STA retarded the crystallization of zirconia and stabilized ZrO2 in tetragonal phase. Characterization of the catalyst with optimum STA loading (15%) by Raman spectroscopy showed the presence of zirconia-anchored mono-oxotungstate as the major tungsten species up to 750 °C. The catalysts showed both Brönsted and Lewis acidity, and 15% STA on zirconia calcined at 750 °C (15 SZ-750) had the highest Brönsted acidity and total acidity. The catalytic activity was found to depend on STA coverage, and the highest activity corresponded to the monolayer of silicotungstate on zirconia. Further, a comparison of the catalytic activity of 15 SZ-750 with zirconia-supported phosphotungstic acid (15%, calcined at 750 °C, 15 PZ-750) and silica-supported silicotungstic acid (15%, calcined at 300 °C, 15 SS-300) showed that silicotungstic acid and phosphotungstic acid supported on zirconia acted as efficient and stable solid acid catalysts, whereas silicotungstic acid supported on silica was leached into the reaction medium and catalyzed the reaction homogeneously. Under the reaction conditions of 120 °C and a veratrole-to-BA molar ratio of 5, the most active catalyst, 15 SZ-750, showed 99% BA conversion, and the deactivated catalyst could be regenerated by calcination without appreciable loss in activity.