T. V. Sviridova

Transformations of mixed molybdenum-vanadium oxides in the oxidation of hydrocarbons by molecular oxygen

Structural-morphological transformations of molybdenum-vanadium mixed oxide catalysts of varying composition in the oxidation of hydrocarbons are studied by X-ray diffraction analysis, electron microscopy, electron paramagnetic resonance technique, and Raman scattering spectroscopy. It is shown that the catalytic reactions, which are analyzed in detail for oxides of various compositions, cause a considerable restructuring of the mixed oxides.

Oxidation of benzene on a vanadium-molybdenum catalyst in the presence of thiophene

The catalytic oxidation of benzene by air oxygen on a vanadium-molybdenum mixed oxide (1 − x)V2O5 · xMoO3 (x = 0.25) over a temperature range of 200–320°C is studied. It is shown that the introduction of small amounts of thiophene into benzene inhibits the oxidation to maleic anhydride in this temperature range. It is established that the operation of the catalyst is accompanied by significant changes in its phase composition and morphology, with a few first operation cycles being characterized by a high conversion of benzene. A possible mechanism of the process is proposed.

Oxidation of dodecane on vanadium-molybdenum catalysts

The catalytic oxidation of dodecane with air oxygen on individual and mixed vanadium-molybdenum (1 − x)V2O5 · xMoO3 oxide is studied over a temperature range of 250 to 400°C. It is shown that oxidation of dodecane to organic acids at 250°C produces undecylic acid C11H26COOH and carbon oxides, as products of the subsequent oxidation of formic acid. The most effective catalyst is a mixed oxide containing 75 mol % MoO3 and 25 mol % V2O5. At 275–300°C, this catalyst provides the maximum yield of acids and a relatively low fraction of complete-oxidation products. Above 250°C, a mixture of acids and carbon oxides is formed, the yield of which increases with the temperature up to 300°C. As the temperature is increased still further, the yield of acids decreases due to their subsequent oxidation. The catalytic oxidation is accompanied by changes in the phase composition, morphology, and degree of crystallinity of the mixed-oxide catalysts. A possible mechanism of the catalytic process is considered.

Nanostructured vanadium-molybdenum mixed oxides prepared by the solvothermal method

The specific features of the structure, morphology, and valence composition of nanostructured molybdenum-vanadium mixed oxides prepared by copolycondensation of relevant oxoacids under solvothermal conditions are investigated. It is shown that the resultant oxide phases are mixtures of hexagonal molybdenum trioxide and a V2O5 xerogel-based solid solution in which some vanadium ions in the lattice are substituted by molybdenum ions. It is demonstrated that vanadyl centers in thus obtained mixed oxides exhibit a high degree of magnetic association. The parameters of the spin Hamiltonian of V(IV) ions were determined from EPR spectra.

Thermally induced transformations in nanostructured molybdenum-vanadium oxides synthesized by a solvothermal method

The phase composition and morphology of xMoO3: (1 − x)V2O5 (x = 0.25–0.75) mixed oxides synthesized by a solvothermal method and heat-treated at 400°C have been investigated. It has been demonstrated by EPR spectroscopy that this heat treatment changes the local concentration of paramagnetic V(IV) centers in the samples. The thermally induced changes in the morphology and phase composition of the oxides have been investigated by IR spectroscopy, X-ray diffraction, and electron microscopy. A probable effect of these factors on the catalytic properties of the nanostructured molybdenum-vanadium oxides obtained by solvothermal synthesis is discussed.

Molybdenum–vanadium mixed oxides synthesized by the hydrothermal method

The effect of pressure (5–20 MPa) on the process of hydrothermal synthesis of a molybdenum–vanadium mixed oxide from a molybdenum–vanadium mixed poly(oxyacid) is studied. It is shown that the synthesis product is a solid solution of isomorphically substituted vanadium pentoxide with a pressure-independent composition. With increasing pressure, the degree of crystallinity of the synthesized products increases, concentration of isolated V(IV) ions decreases, habitus of the crystals changes from prismatic to needlelike, and overall dispersity increases. This makes it possible to regard the pressure as a dispersing factor for the synthesis of nanostructured molybdenum–vanadium oxides.

Solid-state synthesis of molybdenum–vanadium mixed oxide of tubular morphology

The thermally stimulated processes in the disperse carbon–V2O5: MoO3 mixed oxide composite at 400°C were studied by electron microscopy, Raman spectroscopy, and EPR. The mixed oxide phase recrystallized to form tubulene-like structures in the form of rolled lamellae.