Characterization and evaluation of solid superacid catalyst SO₄^2-/SnO₂- Al₂O₃ prepared by aging at low temperature

The solid superacid catalysts SO₄^2-/SnO₂ and SO₄^2-/ SnO₂-Al₂O₃ were prepared by the method of aging at low temperature. The influences of introduction of aluminum on the surface structure, crystallite structure, specific areas, acidity, types of acid site and the thermal stability of the prepared catalysts were investigated by the means of Fourier transform infrared (FT-IR), powder X-ray diffraction (XRD), BET surface area analysis (BET), temperature-programmed desorption of ammonia (NH₃-TPD), infrared spectra of adsorbed ammonia (NH₃-IR) and thermogravimetry (TG), respectively. The results show that the introduction of aluminum inhibits the growth of SnO₂ crystallites, decreases the crystallite size and promotes higher specific areas of the catalyst. Comparing with SO₄^2-/SnO₂, in SO₄^2-/ SnO₂-Al₂O₃ the intensity of attached-on-surface sulfuric groups is increased, the amount of effective acid is enhanced and the strengths of Br?nsted and Lewis acid sites are improved. The TG results reveal that the aluminum could improve the sulphate binding capacity of SnO₂ surface. When SO₄^2-/SnO₂and SO₄^2-/SnO₂- Al₂O₃ were used separately as catalyst to synthesis butyl glycol acetate (BGA), after the catalysts were used for six times, the yields of BGA are 58.9% and 81.2%, respectively; thus indicating that the introduction of aluminum obviously prolongs the catalytic life of the SO₄^2-/SnO₂ type solid superacid catalyst.