Preparation, characterization and catalytic activity of WO3 supported on sulfated tin oxide catalysts

Abstract

Solid acid catalysts of 15wt.%SO4/SnO2 loaded with 5–45wt.%WO3 were prepared, calcined at 400–800°C, and then characterized by TG–DTA, XRD, FT-IR spectra and nitrogen adsorption techniques. The surface acidity was determined by potentiometric titration with n-butylamine in acetonitrile and FT-IR spectra of chemisorbed pyridine.The presence of sulfate ions together with WO3 hindered the crystallization of tin oxide and the products at a lower temperature of ⩽500°C found amorphous. The crystallinity of tin oxide increased with raising the calcination temperature to provide nanocrystalline cassiterite structure, whereas, increasing WO3 loading up to 25wt.% decreases the process. Careful control of calcination temperature and WO3 loading was found to have a critical effect on the specific surface area and therefore on the resulted surface acidity.The surface acidity measurements indicate that the catalysts possess very strong acid sites that designate both of Brønsted and Lewis acid sites.Catalytic application toward esterification of propionic acid with n-butanol was tested and the effect of various parameters, i.e., reaction temperature, reactant molar ratio, weight of the catalyst, WO3 loading and calcination temperature was also investigated. The highest conversion level was found for products loaded with 25wt.%WO3 and calcined at 400°C. Although both of Brønsted and Lewis acid sites are responsible for the reaction, the former acid sites found more effective on catalyzing the esterification reaction.