Modified zirconia solid acid catalysts for organic synthesis and transformations

Article abstract of DOI:10.1016/j.molcata.2004.09.003

The sulfate, molybdate and tungstate promoted ZrO₂ catalysts were investigated by X-ray diffraction, NH₃-TPD, and Raman spectroscopy and were evaluated for various organic synthesis and transformation reactions. All catalysts exhibit good catalytic activity for synthesis of diphenylureas, coumarines and 1,5-benzodiazepines, acylation of alcohols, phenols and amines, and protection of carbonyl compounds. A series of sulfate, molybdate and tungstate promoted ZrO₂ catalysts were prepared by wet impregnation method. To incorporate these promoters to Zr(OH)₄, sulfuric acid, ammonium heptamolybdate, and ammonium metatungstate were used as precursors, respectively. Further, a Pt promoted Mo-ZrO₂ catalyst was also prepared separately by impregnating with hexachloroplatinic acid. The surface and bulk properties of various promoted ZrO₂ catalysts were investigated by means of X-ray powder diffraction, BET surface area, ammonia-TPD, and Raman spectroscopy techniques. The unpromoted ZrO₂ when calcined at 873 K exists in the crystalline form with monoclinic phase dominating over the tetragonal phase. Incorporation of various promoters into Zr(OH)₄ shows a strong influence on the bulk and the surface properties. Addition of promoters enhanced the tetragonal zirconia phase and the surface acidity. In the case of Pt/Mo-ZrO₂ catalyst, a complete tetragonal ZrO₂ phase is observed. The ammonia-TPD results indicate that the impregnated sulfate ions show a strong influence on the acidity of ZrO₂, which is followed by molybdate. The prepared catalysts were evaluated for various organic synthesis and transformation reactions in the liquid phase. All catalysts exhibit good catalytic activity for synthesis of diphenylureas, coumarines and 1,5-benzodiazepines, acylation of alcohols, phenols and amines, and protection of carbonyl compounds. In particular, the sulfate and molybdate promoted catalysts exhibited excellent catalytic activity.