Influence of Nb Content on the Structure, Cationic and Valence Distribution and Catalytic Properties of MoVTe(Sb)NbO M1 Phase Used as Catalysts for the Oxidation of Light Alkanes

The influence of niobium Nb as a bulk constituent of phase-pure MoVTe(Sb)NbO mixed oxides, with an M1 structure has been studied. Sb and Te containing M1 phases with various amounts of Nb were prepared and characterized using several techniques such as V K-edge and Sb or Te L₁-edge Xanes spectroscopy. The mechanism through which the presence of niobium affects the cationic and valence distributions in the M1 phases has been determined. Various M1 phases were tested as catalysts for the oxidation of propane to acrylic acid, or the oxidative dehydrogenation of ethane, in order to study the influence niobium on the catalyst’s properties or stability. It is shown that Nb has no influence on the catalyst’s activity, and that it has a positive effect on its selectivity to acrylic acid for propane oxidation, and to ethylene for ethane oxidation. This effect is associated with a decrease in the surface acidity of the M1 phase, which promotes unselective oxidation of propene and with a decrease in the catalyst’s oxidation power, which inhibits the over-oxidation of acrylic acid or ethylene. Both of these effects are related to the increase in Mo⁵⁺ content of the phases, which appears to be a key parameter in the selectivity of the M1 phase. Although a possible influence of Nb on the crystallinity or texture of the catalysts was not observed, in the case of the preparation techniques used in the present study, an increase in stability was observed over long periods of time on stream in the case of Nb- containing phases.