Study of Some Physico –Chemical Properties of Tin Oxide SnO2 Loaded with Tungsten Oxide WO3

Abstract

In this research WO₃/SnO₂ catalyst was developed by calcination of the dry support SnO₂ at 300^o and 500^o C, then loaded with 15wt% WO₃, PWS3 and PWS5. The products were calcined in the range of 200-500^o C. The textural properties, surface acidity and catalytic activity of the catalysts were performed by FT-IR spectra, adsorption of nitrogen at 77K, potentiometric titration in nonaqueous media, and FT- IR of adsorbed pyridine. The catalytic performance was evaluated on the esterification of propionic acid (PA) with n-butanol (B) in liquid phase.

The results indicate that WO₃ binded to the surface of SnO₂, and the textural properties of the catalysts, i.e. surface area S, total pore volume (V_T), mean pore radius (r_a ), micropore volume (V_o) and pore size distribution, depends on the calcination temperature of both the support and the catalysts. The N₂ adsorption isotherms are of type IV with H1 hysteresis loop for PWS5 and PWS3-200indicating that these catalysts exhibit a narrow range of uniform cylindrical  mesopores, whereas the PWS3-300 and PWS3-500 catalysts show isotherms between type I and type IV which mean that these two catalysts contain pores of different type and shape. The PWS3-300 shows the highest surface area (92 m²/g). The prepared catalysts are superacids in which they possess very strong acid sites, i.e. E_i = 111– 270 mV, with high acidic densities for 200- products, i.e. >10x10¹⁷ N/m². The catalysts show good catalytic activity for esterification of PA with B. The percentage of conversion of PA related to reaction temperature (R.Te), catalyst weight(C.W.), molar ratio PA:B and other factors. The catalyst PWS3-300 shows the highest activity (58.73%) at the conditions: R.Te.=110^o C, PA:B=1:2, C.W.=0.2 g, and R.T.= 4h. The mechanism of the reaction obey Rideal-Iley mechanism in which PA molecules are adsorbed chemically to BrØnsted and Lewis  sites forming stable intermediate of PAH⁺ and carbocation which react with B from bulk liquid to form new unstable intermediate which desorbed from the surface and decomposed to produce the products (water and ester). The process of PA adsorption and formation of stable intermediate is the rate-controlling step of the reaction.