Nature of vanadium species in SnO2 - V2O5-based catalysts. Chemistry of preparation, characterization, thermal stability and reactivity in ethane oxidative dehydrogenation over V-Sn mixed oxides

Tin-vanadium mixed oxides have been prepared either from V4+-Sn4+ solutions by coprecipitation, or by the solid-state reaction between SnO(OH)2 and V2O5, and characterized by means of chemical analysis, FTIR spectroscopy, EPR, X-ray diffraction and surface area measurements. Interaction between the hydroxy groups of the tin oxohydrate and the vanadium ions, reduction of V5+ ions to V4+ and stabilization inside the rutile structure led to the formation of a VxSn1-xO2 solid solution after calcination at 700 °C. A maximum amount of 10 atom% of vanadium entered the SnO2 lattice; at values of up to x = 0.02, V4+ was likely to be homogeneously dispersed, while higher amounts probably formed V4+ oxide clusters inside the rutile matrix. In addition, amorphous V5+ oxide was formed over the rutile surface, and at an overall vanadium content greater than 20-25 atom% crystalline V2O5 was also formed. In samples where x ≥ 0.02-0.03, the solid solution was not stable at temperatures greater than 700 °C, and some of the V4+ was released from the structure forming segregated amorphous V5+ oxide, while for x < 0.02 the solution was stable. The V-Sn mixed oxides were tested as catalysts for ethane oxidative dehydrogenation. The catalysts initially exhibited an unstable behaviour due to a reduction of the V5+ oxide in the reaction environment. Tin oxide activity was enhanced by the addition of V4+; for x = 0.018, also the selectivity to ethene at temperatures higher than 480 °C was significantly greater. In contrast, selectivity to ethene at low temperatures was lower for x > 0.018.