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This paper presents an in situ study of the crystallographic phases formed during the thermal treatment of precursors of vanadium and molybdenum oxides, measured under synchrotron X-ray diffraction. The interest in the speciation of Mo x V y O z mixed oxides lies in the excellent catalytic performance of these materials for the selective conversion of glycerol to acrylic acid employing the oxydehydration reaction. The crystallographic structure of the active phases of Mo x V y O z directly influences on the nearby metal valence and, therefore, on the dynamic changes in metal oxidation states during the catalytic reaction. In the present study, the thermal treatment of a mixture of the precursors of Mo and V under oxidizing or inert atmospheres revealed the major formation of 61% of MoV 2 O 8 or 29% of Mo 4 V 6 O 25 , respectively, at a final temperature of 500 • C. The most active phase for acrylic acid formation was MoV 2 O 8 (3.5 times more active than the separate metal oxides), due to the instability of the phase with respect to framework oxygen at the reaction temperature. The cycle of reduction and oxidation of the vanadium in MoV 2 O 8 during the reaction caused pronounced dynamic formation of oxygen vacancies, resulting in 97% conversion of glycerol and 32% selectivity towards acrylic acid.

The one-step oxydehydration of glycerol to acrylic acid over molybdenum and vanadium mixed oxides was investigated. The Mo-V oxide catalysts were prepared by a simple hydrothermal method under different synthesis and calcination atmospheres and were characterized by in situ XRD, TPD-NH 3 , N 2 adsorption/ desorption, X-ray absorption near vanadium K-edge spectroscopy and thermogravimetry. The catalytic performance of the samples at different temperatures (290, 320 and 350°C) and under different gas flow compositions (20% O 2 in N 2 , 100% O 2 , or 100% N 2) revealed that the arrangement of the crystallographic structures of the active phases directly influenced the catalytic performance. It was found that the catalysts heattreated in oxidizing atmosphere gave superior catalytic results comparing with the catalysts heat-treated in inert atmosphere due to the equilibrium between the crystalline phases MoVO 5 and Mo 4.65 V 0.35 O 14 that contains V +4 and V +5. Catalytic oxydehydration at 320°C under a flow of 100% O 2 gave the best performance, achieving selectivity of 33.5% towards acrylic acid and 100% conversion of glycerol.

The ZSM-5 zeolite (MFI structure, Si/Al = 40) was treated using NaOH and either oxalic acid or HCl to obtain hierarchical materials with different characteristics, followed by impregnation with vanadium oxides (V 2 O 5) to generate redox-active sites. The impact of the multiple treatments on the efficiency and stability of the catalysts in the conversion of glycerol to acrolein and acrylic acid was investigated and correlated with catalyst porosity, acidity, and chemical composition. The treated and impregnated V 2 O 5 catalysts were subjected to XRD, 27 Al NMR, nitrogen physisorption, TPD-NH 3 , TG, and UV-Vis analyses, in order to associate the properties of the catalysts with their activities. The studies showed that the catalytic performance of the materials depended on the acidic and textural properties of the zeolites, which influenced both the dispersion of V 2 O 5 and its interaction with the acid sites of the supporting zeolites. All the catalysts provided conversion values exceeding 65%, even after 6 h on glycerol stream. The distribution of products strongly reflected the effects of pore formation, acid treatment with oxalic acid or HCl, and the presence of vanadium oxide. The effects of these modifications resulted in higher selectivity to acrolein and acrylic acid, a reduced rate of coke accumulation in the zeolite, and a longer catalyst lifetime.