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 SO 4 2−/ZrO 2 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 SO 4 2−/ZrO 2 catalyst was obtained by immersing a finely powdered hydrous Zr(OH) 4 into 1 M H 2SO 4 solution and subsequent drying and calcination at 923 K. The Zr(OH) 4 was prepared from aqueous ZrOCl 2·8H 2O 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 ZrO 2. In particular, XRD and Raman results suggest that impregnated sulfate ions stabilize the metastable tetragonal phase of ZrO 2 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 ZrO 2.