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The effect of gadolinium additives on the morphology, phase composition, and catalytic properties of MoVSbNbGdOx/SiO2 catalysts in the oxidative dehydrogenation of ethane to ethylene (ODE) is studied. It is shown that gadolinium concentration has a significant effect on the catalytic properties. At an optimum gadolinium content (Gd/Mo = 0.01–0.015), an increase in catalytic activity and ethylene selectivity is observed: at a temperature of 400°C, the ethylene yield achieves 72%, the ethane conversion is higher than 91%, and the ethylene selectivity is higher than 79%. According to X-ray diffraction analysis and scanning and transmission electron microscopy, the main component of the catalyst is the M1 phase stabilized both in the bulk and on the surface of SiO2 particles. The modification of the catalyst with gadolinium significantly affects the morphology of the M1 phase particles. At an optimum gadolinium content, heat treatment leads to the formation mostly of M1 phase particles with a needle-like morphology and the most developed [001] plane emerging on the surface, which exhibit the maximally high activity in ODE. The M1 phase particles with this morphology are mostly stabilized in the bulk of porous spherical SiO2 particles; therefore, they do not undergo strong agglomeration and sintering. A smaller portion of the M1 phase particles with a plate-like morphology, which exhibit a lower catalytic activity, as well as the M2 phase particles, are stabilized on the outer surface of the SiO2 particles. In the catalyst containing no Gd additives, M1 phase particles with a plate-like morphology are mostly formed; this factor leads to a decrease in the catalyst activity. The synthesized catalyst exhibits a long-term stable on-stream behavior in the reaction medium without a deterioration of the properties. [ABSTRACT FROM AUTHOR]

The oxidative dehydrogenation of ethane in the presence of CO2 was studied on SiO2-supported chromium oxide catalytic systems with different texture characteristics. The highest activity was displayed by the samples deposited on SiO2 with a high specific surface area. On the 5%CrОх/SiO2 catalyst, the ethylene selectivity was ~80% at ~50% conversion of ethane. The results of our physicochemical analysis showed that the catalyst activity depended on the content of Cr(VI) particles, whose formation depended on the nature of support. [ABSTRACT FROM AUTHOR]

Sodium-tungsten-manganese supported on silica (Na-W-Mn/SiO2) and hydroxyapatite (HAp) are representative catalysts for oxidative coupling of methane (OCM). In this work, the effect of the HAp doping in a Na-W-Mn/SiO2 catalysts on the OCM performance was studied. To enhance the ethylene selectivity of the Na-W-Mn/SiO2 catalyst, silica supports were coated with HAp containing hydroxyl and phosphate groups as oxygen species. A series of Na-W-Mn/xHAp_SiO2 (x=1, 3, 5 and 7) catalysts with the different HAp coating cycles were prepared through the alternative soaking method, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the amount of HAp doping was dependent on the HAp coating cycles. In addition, the change of oxygen species upon HAp doping was examined with X-ray photoelectron spectroscopy (XPS) and oxygen temperature-programmed desorption (O2-TPD) techniques. With HAp doping, the increase of oxygen species assigned to metal oxide responsible for selective oxidation of methane to ethylene was observed in O 1s XPS spectra. In addition, weakly bound oxygen species were observed with the introduction of HAp doping in O2-TPD profiles of prepared catalysts. The influence of these oxygen species on OCM catalytic performance was evaluated at an operating temperature of 775 °C and gas hourly space velocity of 18,000 ml/gcat·h. The amount of HAp doping provided reactive oxygen species for oxidative dehydrogenation of ethane, which resulted in as much as 120% increase in C2H4/C2H6 ratio over the Na-W-Mn/3HAp_SiO2 catalyst compared to the Na-W-Mn/SiO2 catalyst. [ABSTRACT FROM AUTHOR]

A multicomponent MoVSbNbCeOx/SiO2 oxide catalyst exhibiting high catalytic activity in the oxidative dehydrogenation of ethane to ethylene is synthesized by spray drying of a suspension of aqueous solutions of the precursors and the subsequent heat treatment in He at 350 and 600°C. At a temperature of 400–450°С in a wide ethane conversion range, the catalyst exhibits a fairly high activity and ethylene selectivity. In the presence of this catalyst, the maximum ethylene yield achieves 74% (ethane conversion of 91%, ethylene selectivity of 81.5%); this value significantly exceeds the yield in the presence of Sb-containing catalysts known from the literature. The catalyst exhibits a long-term stable on-stream behavior under reaction medium conditions without any change in the phase composition and a deterioration of the catalytic characteristics. It is shown that the catalytic properties of the synthesized catalyst are comparable to the properties of MoVTeNbOx, which is one of the best catalysts for this process. However, the use of a Te-containing catalyst is limited, because tellurium exhibits a high toxicity and volatility during the synthesis and use of the catalysts. According to X-ray diffraction analysis, the main components of the catalyst are M1 and M2 phases stabilized on a SiO2 surface. The HRTEM data show that a structural feature of the catalyst is the presence of an interface formed by the coherently intergrown crystals of the M1 and M2 phases. [ABSTRACT FROM AUTHOR]

Unmodified and sulfate-modified SBA-15-supported CrO catalysts were prepared by impregnation method. The physico-chemical properties of the supports and catalysts were determined by nitrogen adsorption/desorption, powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), laser-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (UV-DRS), inductively coupled plasma optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM) and temperature-programmed reduction (TPR) techniques. Oxidative dehydrogenation of ethane to ethylene (ODE) with CO as oxidant was carried out on these catalysts in a fixed-bed reactor at temperatures in the range of 600-700 °C and at atmospheric pressure. The changes in structural and textural properties because of sulfate modification were identified. Sulfate modification affected the nature of interaction of CrO species with the SBA-15 support. During the evaluation, it was observed that sulfate modification enhances ethane conversion and ethylene selectivity of the catalyst. Better dispersion of CrO and the increase in Cr/Cr ratio seem to be the reasons for the higher performance of the sulfate-modified catalysts compared to that of the unmodified catalyst. [ABSTRACT FROM AUTHOR]

A one-dimensional non-isothermal steady state model was developed to simulate the performance of three-reactor configurations for the oxidative dehydrogenation of ethane (ODHE) to ethylene. These configurations consist of side feeding reactor (SFR), conventional fixed bed reactor (CFBR) and membrane reactor (MR). The performance of these reactors was compared in the terms of CH conversion, CH and CO selectivity and temperature profiles. The use of sectional air injections on the wall of SFR with a limited number of injection points showed that the performance of reactor significantly improves and optimum pattern of oxygen consumption is also obtained. Moreover, our SFR with a liquid coolant medium operates in an effectively controlled temperature profile that is comparable with that of the MR, which is cooled by a coolant stream of air. Hence, an enhancement in the level of selectivity is obtained for the SFR configuration. Consequently, the side feeding procedure can decrease the high operating temperature problem and low ethylene selectivity in the ODHE process. According to obtained results, the SFR would be a proper alternative for both the MR and CFBR. [ABSTRACT FROM AUTHOR]

The oxidative dehydrogenation of ethane (ODE) into ethylene over a Mo-V-Nb-Te-O mixedoxide catalyst in a cyclic mode with alternate feeding of ethane and air has been investigated. The amount of oxide-phase oxygen involved in the reaction has been estimated by titrating the oxygen of the active phase of the catalyst with ethane. The reactivity of this oxygen increases with an increasing temperature. The amount active oxygen involved in ODE at 360-400°C is 0.2-0.6 mmol/g. [ABSTRACT FROM AUTHOR]

The operational stability of a mixed oxide catalyst of Mo-V-Te-Nb-O composition in the oxidative dehydrogenation of ethane (ratio of CH: O = 3: 1) is studied in a flow reactor at temperatures of 340-400°C, a pressure of 1 atm, and a WHSV of the feed mixture of 800 h. It is found that the selectivity toward ethylene is 98% at 340°C, but the conversion of ethane at this temperature is only 6%; when the temperature is raised to 400°C, the conversion of ethane is increased to 37%, while the selectivity toward ethylene is reduced to 85%. Using physical and chemical means (XPS, SEM), it is found that the lack of oxidant in the reaction mixture leads to irreversible changes in the catalyst, i.e., reduced selectivity and activity. Raising the reaction temperature to 400°C allows the reduction of tellurium by ethane, from the +6 oxidation state to the zerovalent state, with its subsequent sublimation and the destruction of the catalytically active and selective phase; in its characteristics, the catalyst becomes similar to the Mo-V-Nb-O system containing no tellurium. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It is shown that catalytic activity of mixed oxide catalyst of Mo-V-Te-Nb-O composition in oxidative dehydrogenation (OD) of ethane is determined to a substantial degree by the Nb-to-(CO) ratio in niobium-containing precursors. A pH value of 2.8 to 3.0 for a mixture is optimal when conducting the hydrothermal synthesis of a mixed oxide catalyst; this is achieved by using oxaloniobic acid as a niobium-containing precursor. It is determined that substituting antimony for tellurium results in a loss of catalyst activity during the OD of ethane. The optimum Te content in a catalyst is 0.17 mol %. [ABSTRACT FROM AUTHOR]