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

Abstract

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 V_xSn_1–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.