T. Yu. Kardash

Effect of preparation conditions on the phase composition of the MoVTe(Nb) oxide catalyst for the oxidative conversions of propane

The replacement of expensive propylene by propane, which requires the development of catalysts for the direct oxidation of propane into acrylonitrile, is an important and insufficiently studied problem. Multicomponent MomVnTekNbx oxide systems are promising in one-stage ammoxidation of propane to acrylonitrile. Despite considerable attention of various authors to the preparation methods for these catalysts, the reproducibility of their physicochemical and catalytic properties is low. To optimize the technology of catalyst synthesis, we studied the effect of drying method (evaporation or spray drying) for the aqueous suspension of the initial compounds on the formation of the Mo1V0.3Te0.23(Nb0.12) oxide catalyst. It is shown that the method of drying determines the chemical and phase composition of solid catalyst precursors and the phase composition of the final catalyst in high-temperature treatment. The use of spray drying provides the required physicochemical characteristics of the catalyst (the specific surface area and the phase composition) that determine the high activity and selectivity in the selective conversion of propane. These catalysts contain two crystalline phases: orthorhombic M1 and hexagonal M2 in an optimal ratio of 3: 1.

Heterogeneous catalytic oxidative conversion of ethane to ethylene

Multicomponent and multiphase oxide catalysts with a uniform composition Mo1V0.3Te0.23Nb12 but different phase contents were studied in the oxidative conversion of ethane to ethylene and characterized by X-ray diffraction analysis. The phase composition of the samples was varied by modifying such conditions of catalyst preparation as the pH of raw precursor, the drying of the wet precursor, and the conditions of solid precursor calcination. The content of the detected phase is determined via Rietveld refinement. The catalytic activity of MoVTeNb oxide catalysts studied in the oxidative conversion of ethane was found to be determined by the content of the orthorhombic M1 phase (the layered four-component compound (TeO)0.23(Mo,V,Nb)5O14).

Synthesis, crystal structure and thermal properties of [Co(NH3)5Cl]MO4 (M = Mo, W)

Crystal structures of [Co(NH3)5Cl]MoO4 and [Co(NH3)5Cl]WO4 complex salts are determined by single crystal X-ray diffraction. It is demonstrated for both salts that within the temperature range T = −123−20°C there is a negative thermal expansion (about 0.26%) towards the c axis of the orthorhombic unit cell (Pnma space group). Thermal properties of the salts are investigated. The phase composition of the products obtained on heating the salts in different gas atmospheres is studied.

Effect of thermal treatment conditions on the phase composition and structural characteristics of V-Mo-Nb-O catalysts

The formation of an active phase in V-Mo-Nb oxide catalysts for the selective oxidation and ammoxidation of ethane during thermal treatment in air and in helium was studied using high-temperature in situ and ex situ X-ray diffraction analysis, transmission electron microscopy, IR spectroscopy, and the differential dissolution method. It was found that, in thermal treatment below 500°C, the formation occurred through the same irreversible steps with the formation of a unidimensionally ordered layered compound with structure elements like Mo5O14 regardless of the calcination atmosphere. Above 500°C, the formation of crystalline phases took place; the composition and structure of these phases depended on the atmosphere of thermal treatment. The unidimensionally ordered V-Nb-Mo oxide with structure elements like Mo5O14 exhibited the best catalytic properties.

Effect of SiO2 on the physicochemical and catalytic properties of VMoTeNbО catalyst in oxidative conversion of ethane

Supported oxide catalysts of the overall composition V0.3Mo1Te0.23Nb0.12/n SiO2 (n = 0, 10, 25, 35, and 50 wt %) were tested in oxidative conversion of ethane to ethylene and were characterized by chemical analysis, X-ray diffraction, and high-resolution transmission electron microscopy. On introducing SiO2, coarse crystals of the active М1 phase become partially coated with layers of amorphous SiO2. The support does not influence the selectivity with respect to the reaction products. The catalysts with 10–25 wt % SiO2 content exhibit the highest activity owing to the presence of nanodomains of the M1 phase.

Structural features of promoted MoVTeNbO catalysts for the oxidative dehydrogenation of ethane

The morphology, crystal structure, and phase composition of MoVTeNbO catalysts modified with K, Ca, Zr, or Bi were studied, and the effects of promoters on their catalytic properties in the oxidative conversion reaction of ethane were examined. With the use of high-resolution transmission electron microscopy (HRTEM), it was found that Bi and K were inserted into the structure of the active phase M1 and potassium changed its morphology. Zr and Ca formed individual oxides and molybdates. Changes in the structure of the promoted models affected their catalytic properties.

Oxidative dehydrogenation of ethane on VMoTeNbО/Al–Si–O catalysts: Effect of the support on the physicochemical and catalytic properties

Supported oxide catalysts of the overall composition V0.3Mo1Te0.23Nb0.12/nAl–Si–O (n = 0, 10, 25, 35, 50, and 70 wt %) were tested in oxidative dehydrogenation of ethane and were characterized by chemical analysis, X-ray diffraction analysis, and transmission electron microscopy. The use of the Al–Si–O support in a wide range of its content (from 10 to 50 wt %) favors formation of nanodomains of the active М1 phase ensuring higher, compared to the bulk catalysts, activity in oxidative dehydrogenation of ethane. The formation of secondary phases of aluminum molybdate and vanadium–molybdenum double oxide, observed at the support content increased over 35 wt %, leads to worsening of the catalytic properties.

In situ high-temperature X-ray diffraction measurements: Application to the study of heterogeneous catalysts

When characterizing the phase composition and structure of catalysts, researchers are faced with the problem of reversible effects of temperature and gas medium during their preparation, activation, or operation. Therefore, particular attention is paid to the methods of their research in appropriate conditions, i.e., in situ high-temperature X-ray diffraction. The theoretical framework and experimental features of high-temperature X-ray diffraction techniques are examined. Examples are given of their effective application to the study of the structural features of oxide catalysts under close-to-real conditions.