MoVTeNb Mixed Oxide Catalysts Research Articles

Physicochemical Properties of MoVTeNb Mixed Oxide Catalysts Synthesized using Different Vanadium Sources

The use of different vanadium sources in the synthesis of multi-metal MoVTeNb oxide catalysts has been investigated for their effect on the physicochemical properties of catalysts. Metal oxides were synthesized by slurry method assisted with a microwave irradiation. Vanadium pentoxide (V2O5), vanadyl sulphate (VOSO4) and ammonium metavanadate (NH4VO3) were used as the vanadium sources, respectively. X-ray diffraction (XRD) pattern showed the existence of orthorhombic (M1) phases in all catalysts. The catalyst prepared using V2O5 produced the highest formation of the phase. This was further supported by Inductive Couple Plasma-Atomic Emission Spectroscopy (ICP-AES), which showed that the V2O5 catalyst has the highest V: Mo ratio, mainly responsible for the high catalytic activity. Temperature Programmed Reduction in Hydrogen (H2-TPR) showed better reducibility for the catalyst when compared to the others. Temperature Programmed Reaction (TPRn) confirmed that the oxidants active for propane conversion into acrylic acid were originated from the lattice of the catalyst.

Roles of Enhancement of C−H Activation and Diminution of C−O Formation Within M1‐Phase Pores in Propane Selective Oxidation

AbstractPropane and propene oxidations on M1 phase MoVTeNb mixed oxide catalysts exhibit relatively high selectivity to acrolein and acrylic acid. We probe the ability of the reactant molecules to access the catalytic sites inside the heptagonal pores of these oxides and analyze elementary steps that limit selectivity. Measured propane/cyclohexane activation rate ratios on MoVTeNbO are nearly an order of magnitude higher than non‐microporous VOx/SiO2, which suggests significant contribution of M1 phase pores to propane activation because both molecules react via homologous rate‐limiting C−H activation. Density functional theory suggests that desired C3H8 dehydrogenation and C3H6 allylic oxidation to acrolein and acrylic acid are limited by C−H activation steps, while less valuable oxygenates form via steps limited by C−O bond formation. Calculated activation barriers for C−O formation are invariably higher than C−H activation when these activations occur inside the pores, suggesting that reactions restricted within the pores are highly selective to desired products. These results demonstrate the role of pore confinement and a framework to assess selectivity limitation in hydrocarbon oxidations involving a complex network of sequential and parallel steps.

Promoting effect of cerium on MoVTeNb mixed oxide catalyst for oxidative dehydrogenation of ethane to ethylene

Ce-incorporated MoVTeNbO catalysts were developed to enhance ethylene productivity of oxidative dehydrogenation of ethane (ODHE) to ethylene. Structural characterizations (XRD, TEM, STEM, Raman, and UV–vis DRS) and DFT calculations revealed that Ce atoms were incorporated into MoVTeNbO framework with maintaining its unique structure (M1 phase), which is active phase for ODHE. The reducibility of the catalysts was enhanced and both V5+ and the lattice oxygen species available to ODHE reaction were enriched by incorporation of Ce, confirmed by TPR, XPS, and pulse injection method, respectively. These improved properties enhanced the conversion of ethane while maintaining their excellent selectivity to ethylene for MoVTeNbCeO catalysts. It is noteworthy that 56.2% of ethane conversion and 95.4% of ethylene selectivity were retained for 200 h over MoVTeNbCeO-0.1 catalyst. Ethylene productivity was calculated to be 1.11 kgC2H4/kgcat h. The developed catalyst exhibits substantial level of ethylene productivity and stability having the possibility with low production of COx to make a step forward for industrialization of oxidative dehydrogenation of ethane.

Metal solution precursors: their role during the synthesis of MoVTeNb mixed oxide catalysts

Crystals of MoVTeNbOxM1 phase for the ODH of ethane to ethylene.

Controlled silylation of MoVTeNb mixed oxide catalyst for the selective oxidation of propane to acrylic acid

Acrylic acid is an important industrial chemical, and efficient catalysts for its direct preparation by propane oxidation are highly desirable. For this purpose, neutral silica networks were introduced on the surface of MoVTeNb mixed oxide catalysts by controlled silylation using a methyl silicate oligomer (MS-51). The modified catalysts gave ∼56.5% yield of acrylic acid with a selectivity of 77.1% in the oxidation of propane at 380°C. The catalysts were characterized by X-ray fluorescence, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller specific surface area, X-ray diffraction (Rietveld analysis), pyridine desorption, and scanning electron microscopy. MoVTeNb mixed oxide was found to be composed of 90.9% M1 phase and 2.3% M2 phase, and upon silylation, the surface was uniformly covered by a thin SiO2 layer with 0.14 molar ratio with respect to Mo and an estimated thickness of 2.4nm. The amount of acid sites decreased after the first three silylation cycles, but was not affected by repeated cycles. The results of the kinetic study based on the comparison of the simulated contribution of each side reaction were consistent with those of the model reactions using acrylic acid and other reactants: the controlled silylation effectively suppressed acrylic acid oxidation, especially after repeated silylation cycles, which is responsible for the superior performance of the silylated catalysts. Considering the relatively large size of acrylic acid compared to propane and the efficient propane activation by silica-covered catalysts, the controlled silylation was proposed to have two roles, by which further consecutive oxidation is prevented effectively to exhibit excellent performance in oxidation of propane: i) to block the unfavorable acidic sides, ii) to generate a silica layer with pore mouth openings on the surface of MoVTeNb mixed oxide, which allow the entrance of propane but inhibit re-entrance of the produced acrylic acid.

Partial Oxidation of Isobutane and Isobutene to Methacrolein Over a Novel Mo–V–Nb(–Te) Mixed Oxide Catalyst

Aiming at a catalyst with improved catalytic properties in the oxidation of isobutane and isobutene to methacrolein and methacrylic acid, a novel MoVNb(Te) mixed oxide catalyst was synthesized. Rietveld analysis revealed that the crystal structure is well distinguishable from the known MoVTeNb mixed oxide catalyst used in the (amm-)oxidation of C3 hydrocarbons. In the conversion of isobutane, activity as well as selectivities to methacrolein and methacrylic acid were low, and the dehydrogenation of isobutane to isobutene was found to be rate determining. However, the novel catalyst is highly suitable for the partial oxidation of isobutene. While selectivity to methacrolein is comparable to that reported for other catalysts, the activity is superior, resulting in very high space-time-yields at temperatures up to 420 °C. The novel mixed oxide is a Mars van Krevelen-type catalyst. Excess oxygen in the feed is required to ensure optimum performance.

Kinetic Study of Oxidative Dehydrogenation of Ethane over MoVTeNb Mixed-Oxide Catalyst

A MoVTeNb multimetallic mixed oxide was studied for the oxidative dehydrogenation of ethane, a promising alternative for catalytic ethylene production. Lab-scale steady-state experimental reaction data were obtained according to a 3k experimental design to investigate the simultaneous effect of temperature (400–480 °C) and space–time [23–70 gcat h (mol of ethane)−1]. A fixed-bed reactor at atmospheric pressure was employed, feeding a mixture of ethane, oxygen, and nitrogen. Ethane conversion varied from 17 to 85%, whereas selectivity for ethylene and COx varied from 94 to 76% and from 4.0 to 24%, respectively. These types of analyses are useful for determining the optimum reaction conditions to enhance the catalytic performance of the mixed oxides presented herein.

Reaction products and pathways in the selective oxidation of C 2–C 4 alkanes on MoVTeNb mixed oxide catalysts

The catalytic properties of MoVTeNbO catalysts during the selective oxidation of short chain alkanes and olefins (C 2–C 4) have been comparatively studied. The main reaction products have been: ethylene from ethane, acrylic acid from propane, maleic anhydride from n-butane and methacrolein from isobutane. FTIR studies of the adsorption of the main reaction products, i.e. olefins and aldehydes, over MoVTeNbO catalyst has been carried out. Accordingly, the reaction pathway is explained on the basis of the characteristics of the alkane fed, the stability and reactivity of both the intermediates and the reaction products, and the nature of the catalytic sites involved in each reaction.

Effect of diluent and reaction parameter on selective oxidation of propane over MoVTeNb catalyst using nanoflow catalytic reactor

The selective oxidation of propane to acrylic acid over an MoVTeNb mixed oxide catalyst, dried and calcined before reaction has been studied using high-throughput instrumentation, which is called nanoflow catalytic reactor. The effects of catalyst dilution on the catalytic performance of the MoVTeNb mixed oxide catalyst in selective oxidation of propane to acrylic acid were also investigated. The effects of some reaction parameters, such as gas hourly space velocity (GHSV) and reaction temperature, for selective oxidation of propane to acrylic acid over diluted MoVTeNb catalyst have also been studied. The configuration of the nanoflow is shown to be suitable for screen catalytic performance, and its operating conditions were mimicked closely to conventional laboratory as well as to industrial conditions. The results obtained provided very good reproducibility and it showed that preparation methods as well as reaction parameters can play significant roles in catalytic performance of these catalysts.

Analysis of structural transformations during the synthesis of a MoVTeNb mixed oxide catalyst

This work presents a detailed investigation of the preparation routine for the multi-metal oxide Mo 1V 0.30Te 0.23Nb 0.125O x used as catalyst for the selective oxidation of propane to acrylic acid. In situ Raman spectroscopy on the initial aqueous polyoxometalate solution prepared from ammonium heptamolybdate, ammonium metavanadate and hexaoxotelluric acid reveals the coexistence of Anderson-type anions [TeM 6O 24] n− , M = Mo, V; n ≥ 6 and protonated decavanadate species [H x V 10O 28] (6− x)− . Raman analysis showed that the monomeric motif of the Anderson-type tellurate is preserved after addition of the Nb precursor and the subsequent spray-drying process. Calcination of the X-ray amorphous spray-dried material in air at 548 K seems to be the essential step, leading to a re-arrangement of the tellurate building blocks, generating nanocrystalline precursors of the phases finally established during treatment in helium at 873 K.