Aluminum phosphate-based solid acid catalysts: Facile synthesis,characterization and their application in the esterification of propanoic acid with n-butanol

Novel solid acid catalysts synthesized from aluminum phosphate were prepared via a precipitation method and a subsequent sulfating treatment. Their catalytic performances for the esterification of propanoic acid with n-butanol were investigated. The as-prepared catalysts were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption–desorption, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption of ammonia (NH₃-TPD), infrared spectroscopy of adsorbed pyridine, and other techniques. Experimental results of esterification reactions indicated that the calcination temperature can significantly affect the catalytic performances and the catalyst calcined at 500 °C (SO₄²⁻/AlPO₄-500) exhibited the highest activity. The effects of different reaction conditions including reaction time, reaction temperature, catalyst amount and alcohol/acid molar ratio were studied in detail. The maximum propanoic acid conversion of 91% was achieved under optimum reaction conditions. Furthermore, the as-prepared SO₄²⁻/AlPO₄-500 catalysts were tested for their reusability in repeated reaction cycles and could be effectively regenerated by a simple reactivation method.