The Ti-V-Sb-Oxide system for toluene oxidation

S. LARRONDO, M. MATRANGA, R. TEJEDA, A. BARBARO, B. IRIGOYEN, N. AMADEO

Limerick, Irlanda

Congreso; 5th European Congress on Catálisis. EuropacatV; 2001

The VSbO4 obtained by solid-state reaction between vanadium (V) oxide and antimony (III) oxide has a rutilo-related phase with composition V1-ySb1-yO4 (0<y<0.1)[1]. Besides, it is known that VSbO4 solid have two redox couples, Sb3+/Sb5+ and V3+/V4+, that have a strong influence on the solid behaviour as a catalyst. In a previous work, we have found that antimony replacing by titanium has improved the catalytic performance in toluene oxidation [2]. The aim of the present work was to compare the effects, the vanadium or antimony replacement by titanium, produced in the structure and reactivity of vanadium-antimony oxides. The vapour-phase oxidation of toluene was used as a reaction test. Solids with nominal composition VSbO4(a), V0.8SbTi0.2O4(b) and Sb0.8Ti0.2O4(c), were prepared by solid-state reaction of mechanical mixtures of V2O5, Sb2O3 and TiO2. The solids were characterised by X-ray Diffraction (XRD), Temperature Programmed Reduction (TPR), N2 isothermal adsorption and catalytic tests. The catalytic tests were carried out in a fixed-bed isothermal reactor operating at atmospheric pressure. The composition of the effluent was analysed using on-line gas chromatography. XRD data have indicated that the three solids are monophasic and have a rutile structure similar to that reported for VSbO4 [1]. The absence of lines associated with titanium (IV) oxide and the changes in the lattice parameters are indicative of the incorporation of titanium within the rutile phase. Specific BET areas of 1 m2/g were obtained for the three samples. The TPR profiles have shown two peaks for samples (a) and (c), pointing out that there are two centres with different reduction capability in these solids. On the contrary, only one peak was observed in case of sample (b). Then, it is clear that the replacement of antimony or vanadium by titanium do strongly affects the redox properties. It is known that when Ti4+ replaces Sb5+, the V3+ is oxidised to V4+, on the other hand the replacement of V3+ by Ti4+ reduces the proportion of V4+ in the solid. Therefore, in sample (b) vanadium is in a reduced state and has lost the known ability of V4+ to incorporate oxygen into the catalyst structure [3]. Catalytic tests have also shown a strong influence of titanium doping on activity and selectivity. The activity increases as V/Sb ratio increases, indicating that vanadium participates in the activation process of toluene. Besides, at the same conversion level, sample (c) shows the largest benzaldehyde selectivity and sample (b) the smallest. In conclusion, the replacement of Sb by Ti improves the catalytic performance in selective oxidation because an efficient redox system is achieved. This solid provides enough vanadium sites for toluene activation and high mobility of oxygen. The opposite behaviour was found in case of Vanadium substitution.