Preparation and characterization of three pure magnesium vanadate phases as catalysts for selective oxidation of propane to propene

Three magnesium vanadate phases, i.e., MgV₂O₆ (metavanadate), -Mg₂V₂O₇ (pyrovana-date) and Mg_s V₂O₈ (orthovanadate), have been successfully prepared with high purity by the citrate method at a relatively low temperature (550°C). FT-IR, LRS, XRD and SEM techniques have been used to characterize these vanadate phases. The effect of calcination temperature has also been investigated. It was found that the particle size and morphology of the MgV₂O₆ phase, which is a function of calcination temperature, appear to have a strong effect on the infrared spectra. Furthermore, the catalytic properties of the three phases were examined in the oxidative dehydrogenation of propane. The propene selectivity follows the order: -Mg₂V₂O₇ > Mg₃ Vg₂O₈ > MgV₂O₆, which is consistent with their redox properties. This fact suggests that there is some correlation between the catalytic and redox properties of these magnesium vanadate phases.     

Aerogel VOx/MgO catalysts for oxidative dehydrogenation of propane

VO_x/MgO aerogel catalysts were synthesized using three different preparation methods: by mixing the aerogel MgO support with dry ammonium vanadate, by vanadium deposition from a precursor solution in toluene, and by hydrolysis of a mixture of vanadium and magnesium alkoxides followed by co-gelation and supercritical drying. The latter aerogel technique allowed us to synthesize mixed vanadium–magnesium hydroxides with the surface areas exceeding 1300 m²/g. The synthesized catalysts were studied by a number of physicochemical methods (XRD, Raman spectroscopy, XANES and TEM). A common feature of all synthesized samples is the lack of V₂O₅ phase. In all cases vanadium was found to be a part of a surface mixed V–Mg oxide (magnesium vanadate), its structure depending on the synthesis method. The VO_x/MgO mixed aerogel sample had the highest surface area 340 m²/g, showed higher catalytic activity and selectivity in oxidative dehydrogenation of propane compared to the catalysts prepared by impregnation and dry mixing. The addition of iodine vapor to the feed in 0.1–0.25 vol.% concentrations was found to increase to propylene yield by 40–70%.     

Oxidative Dehydrogenation of Propane on Calcium Hydroxyapatites Partially Substituted with Vanadate

Solid solutions of phosphate and vanadate calcium hydroxyapatites were synthesized and the catalytic activities for the oxidative dehydrogenation of propane to propylene on those catalysts were examined. Although the conversion of propane and the selectivity to propylene were 7.6 and 3.5% on calcium hydroxyapatite (CaHAp), the incorporation of vanadate to CaHAp by V/P=0.05 (atomic ratio) resulted in the enhancement of the conversion and the selectivity to 17.2 and 52.4%, respectively, corresponding to those on Mg₂V₂O₇ under the same reaction conditions (14.0 and 50.9%, respectively).     

Structural characterisation of a VMgO catalyst used in the oxidative dehydrogenation of propane

A VMgO catalyst (containing 14 wt% vanadium) that is used in the oxidative dehydrogenation of propane (ODHP) reaction has been examined in detail by in situ EXAFS, in situ XRD and HREM. These characterisation techniques have revealed that, as prepared, the catalyst is in effect a three-component system comprising discrete magnesium orthovanadate (Mg₃V₂O₈) particles, magnesium oxide and a disordered vanadium-containing overlayer supported on the MgO. When the catalyst is exposed to typical ODHP reaction conditions at $500^\circ {\text{C}}$ the in situ EXAFS indicates a change in vanadium oxidation state from 5+ to 3+. Under the same conditions, in situ XRD suggests that Mg3V2O8 transforms to a cubic spinel type structure with a lattice parameter of 8.42 Å. These changes are reversible on exposure to air at $500^\circ {\text{C}}$ . HREM shows that the overlayer on MgO changes from a disordered state to a weakly ordered structure after exposure to normal reaction conditions whilst pure propane (strongly reducing conditions) induces pronounced structural ordering of the overlayer. Image simulations have led us to the conclusion that the ordered layer comprises a cubic spinel (MgV₂O₄) phase in parallel epitaxy with the MgO support. The surface regions of the bulk Mg₃V₂O₈ particles are also found to undergo structural modification under typical reaction conditions decomposing to a mixture of MgO crystallites and MgV₂O₄; strong reduction causes a complete conversion to MgV₂O₄.     

Deposition and Characterization of Highly Oriented Mg3(VO4)2 Thin Film Catalysts. 2. Controlled Variation of Oxygen Content

Thin films of magnesium vanadates oriented to expose a single crystalline face, could potentially serve as ideal models for high surface area magnesium vanadate catalysts for oxidative dehydrogenation. The growth of oriented films of one particular magnesium vanadate phase, the orthovanadate (Mg₃(VO₄)₂), has been achieved by rf sputter deposition of the orthovanadate onto Au(111) surfaces. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy have been used to investigate the structure and composition of the films. The orthorhombic orthovanadate grows epitaxially with the (021) plane oriented parallel to the surface. By varying oxygen flow rates during deposition the stoichiometry of the films can be varied from fully oxidized to highly oxygen deficient. At very low oxygen flow rates or in the complete absence of oxygen, a reduced Mg₃V₂O₆ phase is formed. This reduced phase has a cubic structure and grows with the (100) plane parallel to the surface.     

Propane Oxidative Dehydrogenation on V–Sb/ZrO2 Catalysts

The catalytic properties of V–Sb/ZrO₂ and bulk Sb/V catalysts for the oxidative dehydrogenation of propane were studied. Samples were characterized by nitrogen adsorption, temperature-programmed reduction, temperature-programmed pyridine desorption and photoelectron spectroscopic techniques. Vanadia promotes the transition of tetragonal to monoclinic zirconia and the formation of ZrV₂O₇. Surface V and Sb oxide species do not appear to interact among them below monolayer coverage, but SbVO4 forms above monolayer. Simultaneously the excess of antimony forms α-Sb₂O₄. Activity and selectivity show no dependence on the acidity of the catalysts. However, there is a strong dependence of activity/selectivity on composition; surface vanadium species are active for propane oxidative dehydrogenation and the presence of Sb, affording rutile VSbO₄ phase makes the system selective to C₃H₆, this is believed to be related to the redox cycle involving dispersed V⁵⁺ species and lattice reduced vanadium site in the rutile VSbO₄ phase.     

Optimization of V2O5–MgO/TiO2 catalyst for the oxidative dehydrogenation of propane effect of magnesia loading and preparation procedure

The effect of magnesia loading and preparation procedure of vanadia on titania catalysts on the physicochemical characteristics and the performance in propane oxidative dehydrogenation were investigated. A series of magnesia promoted vanadia catalyst (5 wt% V₂O₅) with varying amounts of MgO (1.9--10 wt%) were synthesized by synchronous and sequential deposition on titania support. The catalysts were characterized using several techniques (BET, XRD, H₂-TPR and NH₃)-TPD). Both MgO loading and preparation procedure affect the catalyst surface properties and the behavior in the oxidative dehydrogenation of propane. Magnesia addition results in drastic increase in propene selectivity, while the effect on activity is negative. The activity is inversely related with the magnesia loading. Deposition of V₂O₅ on previously prepared MgO/TiO₂ presents a beneficial effect in the activity of the sample. The role of acidity and reducibility is explored. There is no correlation between reducibility and activity of the catalysts, whereas the acidity seems to influence the catalytic performance. Catalyst containing 5 wt% V₂O₅ and 1.9 wt% MgO prepared by sequential deposition of V₂O₅ on already doped with MgO titania exhibits the most interesting results.     

Catalytic and electrocatalytic oxidation of propane on V-Mg-O and V-Mg-O (Ag) catalysts

Vanadium magnesium oxide catalysts prepared in this work were found active in selective oxidation of propane to propene. A selectivity as high as 79% was obtained at 10% conversion (813 K). No oxygenated or C₂ products were detected and the catalysts were found to undergo no change in activity over many weeks of operation. Under electrochemical pumping of oxygen (EOP) towards the catalyst (with oxygen present in the feed gas), both conversion and selectivity were found to increase slightly as external current increased, indicating the effect of electrical current can be exhibited by an oxide catalyst. However, in the absence of oxygen in the feed gas, EOP could lead to an even higher selectivity: 84 and 86.9% respectively for a 24 V-Mg-O and a 24 V-Mg-O (Ag) (1/2) catalyst. The overall results obtained suggest that electrochemically supplied oxygen is more selective towards C₃H₆. Mechanisms of both catalytic and electrocatalytic oxidation of propane were tentatively suggested, with surface oxygen ion vacancy identified as active surface species and the rate determining step involving heterolytic splitting of the C₃H₈ molecule to form a surface bonded C₃H ₇ ^– ion and a surface hydroxyl ion. The higher selectivity towards C₃H₆ in case of EOP was explained on this basis. While mixing with Ag powder was found to improve significantly the electrocatalytic performance of vanadium magnesium oxide, its role appears to be non-chemical: it simply gives rise to a larger area of the gas/catalyst/Ag electrode interface.     

A Comparison of Catalytic Oxidative Dehydrogenation of Ethane Over Mixed V-Mg Oxides and Over Those Prepared via a Mesoporous V-Mg-O Pathway

The catalytic oxidative dehydrogenation of ethane was investigated in a fixed-bed tubular microreactor at 500, 550 and 600 °C and a space velocity of 35 027ml g^-1h^-1. Two kinds of V-Mg oxides catalysts containing various V/Mg atomic ratios were employed. One group of catalysts was prepared by the solid reaction between fine powders of vanadium pentoxide and magnesium nitrate and the other ones were obtained from mesostructured V-Mg-Os. For the former catalysts, it was found that the selectivity to ethene increased and the conversion of ethane passed through a maximum with increasing V/Mg atomic ratio. For the catalysts obtained from the mesoporous materials, an optimum V/Mg atomic ratio was found, for which the conversion of ethane and the selectivity to ethene were maxima. Compared with the mixed-oxide catalysts, those obtained from the mesoporous materials exhibited much higher yields to ethene. Several new phases, such as pyro-Mg₂V₂O₇, ortho-Mg₃(VO₄)₂ and meta-MgV₂O₆, formed between magnesia and vanadia, were identified by XRD in the mixed V-Mg oxide catalysts; they may be responsible for the catalytic activity. In the catalysts prepared from mesoporous V-Mg-O, a V₂O₃ phase, which may contain highly dispersed magnesium, was identified and suggested to be responsible for the higher catalytic performance.     

Ru Nanoparticles Entrapped in Mesopolymers for Efficient Liquid-phase Hydrogenation of Unsaturated Compounds

A chemical potential diagram of the V–Mg–O system was constructed for comparison in an in-situ experiment. A V–Mg–O catalyst used in the oxidative dehydrogenation of n-butane was prepared by the impregnation method and was characterized by in-situ X-ray diffraction (XRD). Mg₃V₂O₈ and MgO were detected on the in-situ XRD pattern under the oxygen pretreatment at 600 °C, and the in-situ XRD data under working conditions showed that Mg₃V₂O₈ is reduced to MgV₂O₄, having a cubic spinel structure with a lattice constant of a = 8.427 Å. The observed reduction of V⁵⁺ in Mg₃V₂O₈ to V³⁺ in MgV₂O₄ under the working conditions could be well understood through a chemical potential diagram.     

Structure and catalytic consequence of Mg‐modified VOx/Al2O3 catalysts for propane dehydrogenation

Supported VO_x catalysts are promising nonoxidative propane dehydrogenation (PDH) materials for their commercially attractive activity and propylene selectivity. However, they frequently suffer from rapid deactivation caused by coke deposition. This article describes the promoting role of magnesium on the stability of VO_x/Al₂O₃ catalysts for PDH. A series of VO_x/Al₂O₃ and Mg‐modified VO_x/Al₂O₃ catalysts were synthesized by an incipient wetness impregnation method. The catalysts were carefully characterized by Raman spectra, UV‐Vis spectra, STEM, TGA and in situ DRIFTS. We showed that the stability of a 12V/Al₂O₃ catalyst was significantly improved on addition of small amounts of MgO. Experimental evidences indicate that V₂O₅ nanoparticles emerge in the 12V/Al₂O₃ samples, and appropriate Mg addition helps dispersing the V₂O₅ nanoparticles into 2D VO_x species thus decreasing coke formation and improving stability in nonoxidative dehydrogenation of propane. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Redox Behaviors of Magnesium Vanadate Catalysts During the Oxidative Dehydrogenation of Propane

In order to examine the mobility of lattice oxygen in magnesium vanadates, these catalysts were employed for the oxidative dehydrogenation of propane in the absence of oxygen for 2.25h, followed by the addition of gaseous oxygen into the feedstream. Depending on the degree of the abstraction of lattice oxygen from these catalysts during the oxidation in the absence of the gaseous oxidant, oxygen in the effluent was detected at approximately 1.4 and 9min with Mg₃V₂O₈ and Mg₂V₂O₇ respectively, after the addition of gaseous oxygen under the present reaction conditions. However, no oxygen was detected with MgV₂O₆ even after 18.5min from the addition of gaseous oxygen. ⁵¹V MAS NMR was also employed for the observation of redox behaviors of vanadium species in these catalysts during the reaction.     

Catalytic Wet Oxidation of H<Subscript>2</Subscript>S to Sulfur on V/MgO Catalyst

The V/MgO catalysts with different V₂O₅ loadings were prepared by impregnating MgO with aqueous vanadyl sulfate solution. All of the catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). It was observed that the H₂S removal capacity with respect to vanadia content increased up to 6 wt%, and then decreased with further increase in vanadia loading. The prepared catalysts had BET surface areas of 11.3 ~ 95.9 m²/g and surface coverages of V₂O₅ of 0.1 ~ 2.97. The surface coverage calculation of V₂O₅ suggested that a vanadia addition up to a monomolecular layer on MgO support increased the H₂S removal capacity of V/MgO, but the further increase of VO _x surface coverage rather decreased that. Raman spectroscopy showed that the small domains of Mg₃(VO₄)₂ could be present on V/MgO with less than 6 wt% vanadia loading. The crystallites of bulk Mg₃(VO₄)₂ and Mg₂(V₂O₇) became evident on V/MgO catalysts with vanadia loading above 15 wt%, which were confirmed by a XRD. The TPR experiments showed that V/MgO catalysts with the loading below 6 wt% V₂O₅ were more reducible than those above 15 wt% V₂O₅. It indicated that tetrahedrally coordinated V⁵⁺ in well-dispersed Mg₃(VO₄)₂ domains could be the active species in the H₂S wet oxidation. The XPS studies indicated that the H₂S oxidation with V/MgO could proceed from the redox mechanism (V⁵⁺ V⁴⁺) and that V³⁺ formation, deep reduction, was responsible for the deactivation of V/MgO.     

Effect of MgO on structural,thermal and conducting properties of V2-xMgxO5-δ (x = 0.05–0.30) systems

MgO containing vanadate glasses of composition V_2-xMg_xO_5-δ (x?=?0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) have been prepared successfully by melt-quench technique. The structural properties of the as-quenched samples are explored with the help of X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Thermal properties and conductivity of the samples are investigated using differential thermal analysis (DTA) and impedance spectroscopy, respectively. Density, inter-ionic distance, and fragility index are found to decrease with the addition of MgO content. Higher content of MgO improves the glass formation tendency and reduces the thermal stability of the samples. The conductivity of the V_2-xMg_xO_5-δ samples decreases as the MgO concentration increases due to hindrance in the polaronic conduction of vanadium ions. The conductivity of the samples decreases to ? 10^?4 S?m^?1 for x?=?0.30 at 300?°C, while activation energy increases to 0.38?eV. These samples may find application in solid state batteries and fuel cells due to their higher conductivity.     

Correlation Between the Characteristics and Catalytic Performance of Ni–V–O Catalysts in Oxidative Dehydrogenation of Propane

Ni–V–O series catalysts for the oxidative dehydrogenation (ODH) of propane were prepared and characterized by BET, XRD, H₂-TPR, O₂-TPD-MS and electrical conductivity. At 425°C a C₃H₆ selectivity of 49.9% was observed on Ni_0.9V_0.1O _Y at a C₃H₈ conversion of 19.4%, and the obtained selectivity is almost two times higher than that over NiO at the roughly same conversion of C₃H₈. The mobile oxygen species created by the interaction of NiO and V₂O₅ has been found in the composite catalysts by O₂-TPD-MS and electrical conductivity studies, which seems to be responsible for the enhanced selectivity of the propane oxidative dehydrogenation.     

Scavenging effect on ionic conduction behaviour at the grain boundary of MgO partially stabilised zirconia

The scavenging effect of magnesium oxide (MgO) addition on electrical property of 9 mol-% MgO partially stabilised zirconia (Mg-PSZ) was investigated in terms of phase transformation and intergranular phase formation. The addition of MgO up to 5 mol-% caused a stabilisation of Mg-PSZ, which led to an increase in the cubic phase and a decrease in the monoclinic and tetragonal phases in Mg-PSZ. The Mg-PSZ with the addition of 5 mol-% of MgO also exhibited the maximum ionic conductivity (0.3915?S?cm^?1 at 1500°C) and forsterite (Mg₂SiO₄) was observed on the grain boundaries of Mg-PSZ. The intergranular phases, formed by reactions between the silicon in Mg-PSZ and MgO addition, reduced the grain boundary resistance, because the siliceous phase which is a hindrance for oxygen ion conduction was scavenged by the formation of Mg₂SiO₄.     

Dehydrogenation of propane with selective hydrogen combustion: A mechanistic study by transient analysis of products

The role of lattice and adsorbed oxygen species in propane dehydrogenation in a perovskite hollow fiber membrane reactor containing a Pt–Sn dehydrogenation catalyst was elucidated by transient analysis of products with a sub-millisecond time resolution. Propane is mainly dehydrogenated non-oxidatively to propene and hydrogen over the catalyst, while lattice oxygen of the perovskite oxidizes preferentially hydrogen to water. For achieving high propene selectivity at high propane conversions, the formation of gas phase O₂ on the shell side of the membrane reactor should be avoided. Otherwise, oxygen species adsorbed over the Pt–Sn catalyst participate in non-selective C₃H₈/C₃H₆ transformations to C₂H₄ and CO_x.     

Oxidative Dehydrogenation of Cyclohexane to Cyclohexene over Mg-V-O Catalysts

The oxidative dehydrogenation of cyclohexane was studied over Mg-V-O catalysts with different Mg/V atomic ratios. Catalysts were prepared via citric acid complexation and characterized by N₂-adsorption, XRD, FT-IR, Raman spectroscopy, NH₃-TPD and H₂-TPR techniques. Among the pure magnesium vanadates, Mg₃(VO₄)₂ has the isolated active sites, weakly basic surface and lower reducibility of the metal cations, and could be recognized as the catalytic active phase. Furthermore, a series of mechanically mixed catalysts were studied in the reaction in attempting to investigate the synergetic effect. The finding revealed synergetic effects in the conversion, in the yield, and in the selectivity were observed in Mg₃(VO₄)₂ mixing with a suitable amount of MgO and Mg₂V₂O₇ due to a solid solution of MgO in the Mg₃(VO₄)₂ phase and the remote control mechanism, respectively. Among the biphasic catalysts, (7/4)MgVO catalyst exhibited a better catalytic performance, on which a cyclohexene selectivity of 45.7% at cyclohexane conversion of 13.9% was obtained.     

An Investigation of the Reduction and Reoxidation of Isolated Vanadate Sites Supported on MCM-48

The reduction and subsequent reoxidation of isolated vanadate species supported on silica was investigated using temperature-programmed reduction and oxidation, along with in-situ XANES and Raman spectroscopy. Approximately 70–80% of the vanadium was reduced to V³⁺ after reduction in H₂ at temperatures up to 923 K. Upon reduction, the vanadyl oxygen was removed and the three remaining V–O bonds are lengthened by 0.2 Å. The vanadate species are rapidly reoxidized when exposed to O₂, with the amount of oxygen uptake matching well with the amount removed during reduction. In-situ Raman spectroscopy during reoxidation in ¹⁸O₂ showed that significant scrambling occurs between gas phase oxygen and surface oxygen species during the reoxidation of the vanadate species.     

Enhancement of the activities with feedstream doping by tetrachloromethane in the oxidative dehydrogenation of propane on α-magnesium pyrovanadate

The oxidative dehydrogenation of propane to propylene has been investigated on -magnesium pyrovanadate (Mg₂V₂O₇) at 723 K in the presence and absence of tetrachloromethane (TCM). Under the present conditions, the conversion of propane and the selectivity to propylene were 5.0 and 74.5%, respectively, in the absence of TCM while those were 14.0 and 70.2%, respectively, upon addition of a small amount of TCM (P(TCM) = 0.34 kPa) into the feedstream on the catalyst. The conversion of propane on Mg₂V₂O₇ without oxidant in the presence and absence of TCM revealed that a contribution of lattice oxygen in the catalyst to the oxidation was strongly controlled by the addition of TCM, resulting in the enhancement of the activity with TCM.