Surface characterization and catalytic activity of sulfate-, molybdate- and tungstate-promoted Al 2O 3–ZrO 2 solid acid catalysts

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Alumina–zirconia mixed oxide-supported sulfate-, molybdate- and tungstate-promoted solid acid catalysts were synthesized and characterized by various techniques. The Al–Zr hydroxide gel was obtained by a co-precipitation method from their corresponding nitrate salts by hydrolysis with aqueous ammonia. To the Al–Zr-hydrous mixed oxide support, aqueous solutions of sulfuric acid, ammonium heptamolybdate and ammonium metatungstate were added and the mixtures were refluxed at 110 °C, followed by evaporation of the water, drying and calcination at 650–750 °C. The surface and bulk properties of the catalysts were examined by using X-ray diffraction, BET surface area, TGA/DTA, ammonia-TPD and XPS techniques. The XRD results reveal that Al 2O 3–ZrO 2 mixed oxide calcined at 650 °C is in amorphous or poorly crystalline state exhibiting broad diffraction lines due to tetragonal ZrO 2 phase. At 750 °C calcination, a better crystallization of the zirconia tetragonal phase is observed. The XRD results further indicate that incorporation of alumina into zirconia stabilizes the tetragonal phase. The TGA–DTA studies provide information that there are at least two types of sulfate species with different thermal stabilities in the case of SO 4 2−/Al 2O 3–ZrO 2 catalyst. The TPD study reveals that the SO 4 2−/Al 2O 3–ZrO 2 catalyst exhibits an ammonia desorption peak maximum at 600 °C indicating super-acidic nature of the catalyst. The XPS peak intensities and the corresponding binding energies indicate that sulfate ion interacts with the support more strongly than other promoters. All characterization results provide information that the impregnated sulfate ions show a relatively strong influence on the physicochemical properties of the Al 2O 3–ZrO 2 mixed oxide. The prepared catalysts were evaluated for acetylation of alcohols and amines with acetic anhydride in the liquid phase. In line with physicochemical characteristics, the SO 4 2−/Al 2O 3–ZrO 2 exhibits better product yields under very mild reaction conditions.