The influence of sulfate contents on the surface properties of sulfate-modified tin(IV) oxide catalysts

Tin(IV) oxide catalysts, pure and surface-doped with different loading levels of sulfate, have been prepared and characterized by means of thermal analysis (TGA and DTA), X-ray powder diffraction (XRD), nitrogen adsorption at 77 K, FT-IR spectroscopy using KBr pellets and potentiometric titration using n-butylamine. The catalysts were prepared from aqueous solutions of ammonia and tin(IV) chloride, with production of an amorphous precipitate that calcined at 873 K for 3 h to give tin(IV) oxide (SnO2). The sulfation was carried out by impregnation of sulfuric acidic 0.5 M with tin(IV) hydroxide with different amounts of sulfate (6%, 12% and 18% SO4-2 by weight). Structural investigation of the catalysts by TGA, XRD and N2-sorption revealed that tin(IV) gel has at least two molecules of water to give the formula SnO2·2H2O and the addition of sulfate does not modify the crystalline structure of tin(IV) oxide (tetragonal phase) but decreases the crystallite size and, consequently, increase the specific surface area. The increase in loading level of sulfate resulted in increase in specific surface area of the catalysts. Acidity measurement by potentiometric titration using n-butylamine show that the addition of sulfate can increase the acidity of tin(IV) oxide and all sulfated tin(IV) oxide having strong acid sites. Moreover, FT-IR spectra expose that sulfated tin(IV) oxide has two different structures of sulfates.