V. M. Bondareva

Formation of active component of MoVTeNb oxide catalyst for selective oxidation and ammoxidation of propane and ethane

Abstract The effect of slurry pH on the formation of active component of MoVTeNbO catalyst for selective (amm)oxidation of ethane and propane has been studied. pH affects the nature and composition of the crude and dry precursors as well as chemical and phase composition of the final catalyst. The most effective catalyst is prepared at pH = 3.0, which is characterized by a maximum content of M1 phase.

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).

Development of the local and average structure of a V-Mo-Nb oxide catalyst with Mo5O14-like structure during synthesis from nanostructured precursors

Abstract A combination of X-ray and neutron PDF measurements with powder diffraction and EXAFS data was used to determine the structures of a V—Mo—Nb-oxide catalyst and its poorly crystallized precursors that exhibit the strongest catalytic activities. The crystalline material belongs to space group P-4 21m, a = 22.8, c = 4.002, and is build up of pentagonal MeO7 bipyramids surrounded by edge sharing Me-octahedrons (Me = Mo, V, Nb). In the average structure all MeO7 units are at the same z-level, while the local structure analysis shows systematic shifts along [001]. Samples synthesized at 300 °C and 400 °C exhibit a nanostructure, whose local structure predates the final crystalline structure. Initial nanoparticles are spherical and grow predominantly along the c-axis. The successful analysis required a reverse analysis that took the crystalline material as starting model for the samples synthesized at lower temperatures.

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.

Effect of pressure on the oxidative conversion of ethane on VMoTeNbO catalyst

Study of the catalytic properties of the VMoTeNbO catalyst in the oxidative conversion of ethane to ethylene at pressures of 0.1 to 2.1 MPa demonstrated that the pressure positively affects the conversion of ethane and favors formation of oxygen -containing products of deep and partial oxidation: carbon oxides and acetic acid, respectively. With increasing pressure, the yield of the product of oxidative dehydrogenation of ethane, i.e., ethylene, decreases.

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.

Structural, textural, and acid–base properties of layered Mg–Al oxides modified with a tungstate or phosphate and their activity and selectivity in gas-phase glycerol dehydration

The effect of the nature of the anion (WO2-4, [H2W12O40]6–, HPO2-4) introduced into the interlayer space of Mg–Al hydroxides with Mg/Al = 3 on their structural, textural, acid–base, and catalytic properties has been investigated. The synthesized materials are layered Mg–Al hydroxides with a hydrotalcite-like structure. Heat treatment of these hydroxides at 450–600°C yields a MgO-based solid solution, which turns into a mixture of MgO and MgAl2O4 phases on being heat-treated at 900°C. The samples modified with the [H2W12O40]6– and HPO2-4 anions additionally contain MgWO4 and Mg3(PO4)2 phases, respectively. The OH groups of the materials examined do not show pronounced Brønsted acidity. (NH4)2HPO4 introduced into the interlayer space of the Mg–Al hydroxide is the most effective modifier enhancing the surface acidity, while the highest basicity is attained by introducing the Na2WO4 modifier. Glycerol dehydration at 275–350°C, involving acid sites of the surface, yields a wide variety of products containing 2 to 12 carbon atoms per molecule. A correlation has been established between the amount of glycerol dehydration products (acrolein + acetol) and the number of strong Lewis acid sites (νCO = 2177–2195 cm–1). The presence of basic sites varying in strength and concentration on the catalyst surface is favorable for condensation, cyclization, and oligomerization reactions yielding a wide range of C4+ products.

Processing of Oil Refinery Gases: Oxidative Dehydrogenation of the Ethane–Ethylene Fraction

Oxidative transformations of the ethane–ethylene fraction of oil refinery gases, containing 20 vol % C2H4, on VMoTeNb oxide catalyst in the temperature interval 330–450°C were studied. Comparison with oxidative transformations of the individual components (oxidative dehydrogenation of C2H6 and oxidation of C2H4) shows that ethylene does not noticeably influence the ethane conversion, whereas ethane strongly suppresses the ethylene conversion. The maximal yield of ethylene from the ethane–ethylene fraction is close to that reached in oxidative dehydrogenation of ethane under similar conditions and amounts to 70–72%.

The evolution of the M1 local structure during preparation of VMoNbTeO catalysts for ethane oxidative dehydrogenation to ethylene

The so-called M1 phase (the common formula (TeO)x(Mo, V, Nb)5O14) is a very promising catalyst for ethane oxidative dehydrogenation (ODE). It shows 90% selectivity to ethylene at 78% ethane conversion (400 °C, contact time – 5.5 s). The active crystal structure is formed under certain synthetic conditions in VMoNbTe mixed oxides. This paper is devoted to the analysis of how the local and average structure of the M1 phase is developed during the synthesis and what happens at particular synthetic steps. The analysis of the local structure was performed using the EXAFS and pair distribution function (PDF) methods. The EXAFS analysis of the initial VMoTe water solution and VMoNbTe slurry showed that Anderson-type heteropoly anions are formed in the solution and are preserved after fast spray-drying of the slurry. Nb cations do not enter the structure of the polyanions, but form an extended hydrated oxide matrix, where distorted NbO6 and NbO7 polyhedrons are connected to each other. The hydrated oxide matrix with captured polyanions provides the compositional homogeneity of the precursor. The distances in the second coordination shell are redistributed after thermal treatment at 310 °C. After being heated at T > 350°, the local structure of the M1 phase is organized and pentagonal domains are formed. These domains consist of a NbO7 pentagonal bipyramid and five MeO6 adjacent octahedra (Me = Mo, V). In the first stages, the building blocks are stacked along the [001] direction. The crystallization process results in the connection of the pentagonal domains to the extended polygonal grid. The formation of the regular grid with TeOx containing channels is accompanied by the increase in ethane conversion and ethylene selectivity of the catalysts.