Sulfated zirconia as an efficient catalyst for organic synthesis and transformation reactions

Abstract

The efficacy of sulfated zirconia catalyst was investigated towards various acid-catalyzed organic syntheses and transformation reactions in the liquid phase. The SO4 2−/ZrO2 efficiently catalyzes synthesis of 1,5-benzodiazepine derivatives, electrophilic substitution of indoles with aldehydes to afford the corresponding bis(indolyl)methanes, synthesis of 3,4-dihydropyrimidinones, synthesis of diaryl sulfoxides, and tetrahydropyranylation of alcohols and phenols. Various advantages associated with these protocols include, simple work-up procedure, solvent-free conditions, short reaction times, high product yields and easy recovery and reusability of the catalyst. The SO4 2−/ZrO2 catalyst was obtained by immersing a finely powdered hydrous Zr(OH)4 into 1MH2SO4 solution and subsequent drying and calcination at 923 K. The Zr(OH)4 was prepared from aqueous ZrOCl2·8H2O solution by hydrolysis with dilute ammonium hydroxide. The bulk and surface properties of the prepared catalysts were examined by X-ray powder diffraction, BET surface area, ammonia-TPD and Raman spectroscopy techniques. All characterization results revealed that the incorporated sulfate ions show a significant influence on the surface and bulk properties of the ZrO2. In particular, XRD and Raman results suggest that impregnated sulfate ions stabilize the metastable tetragonal phase of ZrO2 at ambient conditions. Ammonia-TPD and BET surface area results indicate that sulfated catalyst exhibits enhanced acid strength and specific surface area than that of unprompted ZrO2.