Esterification and Transesterification on Al2O3-Doped Sulfated Tin Oxide Solid Acid Catalysts

A series of Al2O3-doped (0.5%-3.0%, molar fraction) sulfated tin oxide catalysts were prepared by a co-precipitation method. The structures and textural properties of these catalysts were characterized by N-2 adsorption, thermogravimetric (TG) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Raman spectroscopy, and Al-27 magic. angle spinning nuclear magnetic resonance (MAS NMR). The number of acid sites on the catalysts was measured by the potentiometric titration of n-butylamine. Their catalytic performance for the esterification of lauric acid with methanol and the transesterification of triacetin with methanol was also investigated. The results showed that the addition of Al2O3 to sulfated tin oxide improved the catalytic activity markedly. The remarkable activities of the Al2O3-doped catalysts are caused by the larger number of acid sites. The SO42-/SnO2 catalyst doped with a 1.0% molar fraction of Al2O3 exhibited the highest activity. The lauric acid conversion was 92.7% after esterification for 6 h, and the triacetin conversion was 91.1% after transesterification for 8 h on this catalyst.