Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X = Nb, Sb, Te) mixed oxides

Mo–V–X (X = Nb, Sb and/or Te) mixed oxides have been prepared by hydrothermal synthesis and heat-treated in N₂ at 450 °C or 600 °C for 2 h. The calcination temperature and the presence or absence of Nb determines the nature of crystalline phases in the catalyst. Nb-containing catalysts heat-treated at 450 °C are mostly amorphous solids, while Nb-free catalysts heat-treated at 450 °C and samples treated at 600 °C clearly contain crystalline phases. TPR-H₂ experiments show higher H₂-consumption on catalysts with amorphous phases. Catalytic results in the oxidative dehydrogenation of ethane indicate that the selective production of the olefin is strongly related to the development of the orthorhombic Te₂M₂₀O₅₇ or (SbO)₂M₂₀O₅₆ (M = Mo, V, Nb) phase (the so-called M1 phase), which is mainly formed at 600 °C. This active and selective crystalline phase is characterized to show moderate reducibility and active centers enough for the selective oxidative activation of ethane with the minimum quantity possible of active centers for ethylene activation. In this sense, the best yield to ethylene has been achieved on a Mo–V–Te–Nb mixed oxide.