Vanadium Oxide Catalysts Research Articles

Kinetic parameter estimation for supported vanadium oxide catalysts for propane ODH reaction: Effect of loading and support

Kinetic parameters are estimated for a sequential Mars van Krevelan (MVK) reaction model occurring over several supported vanadium oxide (vanadia) catalysts involved in the propane oxidative dehydrogenation (ODH) reaction. The estimated kinetic parameters, pre-exponential factors and activation energies, are used to understand the effect of vanadia loading and oxide support. The pre-exponential factors and vanadia normalized pre-exponential factors vary with vanadia loading and oxide support. The monotonic increase in normalized pre-exponential factors with vanadia loading and the variation of pre-exponential factors with oxide support appears to be related to the change in acidity/basicity of the catalyst and the redox nature of the catalyst, respectively. The activation energy for propene degradation does not significantly change with catalyst; however, the activation energy for propane oxidation is different for the V 2O 5/Al 2O 3 catalyst. It appears that two important considerations are required for the development of an efficient propane ODH catalyst: a high rate constant associated with the propane oxidation reaction, and a high ratio of the rate constant for propene formation to degradation reaction. Based on the observations in the present study it is proposed that a higher TiO 2 support surface area will assist in increasing the propane oxidation activity and propene yield.

Oxidative dehydrogenation of ethane to ethylene over alumina-supported vanadium oxide catalysts: Relationship between molecular structures and chemical reactivity

The influence of vanadium oxide loading in the supported VO x /Al 2O 3 catalyst system upon the dehydrated surface vanadia molecular structure, surface acidic properties, reduction characteristics and the catalytic oxidative dehydrogenation (ODH) of ethane to ethylene was investigated. Characterization of the supported VO x /Al 2O 3 catalysts by XPS surface analysis and Raman spectroscopy revealed that vanadia was highly dispersed on the Al 2O 3 support as a two-dimensional surface VO x overlayer with monolayer surface coverage corresponding to ∼9 V/nm 2. Furthermore, Raman revealed that the extent of polymerization of surface VO x species increases with surface vanadia coverage in the sub-monolayer region. Pyridine chemisorption-IR studies revealed that the number of surface Brønsted acid sites increases with increasing surface VO x coverage and parallels the extent of polymerization in the sub-monolayer region. The reducibility of the surface VO x species was monitored by both H 2-TPR and in situ Raman spectroscopy and also revealed that the reducibility of the surface VO x species increases with surface VO x coverage and parallels the extent of polymerization in the sub-monolayer region. The fraction of monomeric and polymeric surface VO x species has been quantitatively calculated by a novel UV–Vis DRS method. The overall ethane ODH TOF value, however, is constant with surface vanadia coverage in the sub-monolayer region. The constant ethane TOF reveals that both isolated and polymeric surface VO x species possess essentially the same TOF value for ethane activation. The reducibility and Brønsted acidity of the surface VO x species, however, do affect the ethylene selectivity. The highest selectivity to ethylene was obtained at a surface vanadia density of ∼2.2 V/nm 2, which corresponds to a little more than ∼0.25 monolayer coverage. Below 2.2 V/nm 2, exposed Al support cations are responsible for converting ethylene to CO. Above 2.2 V/nm 2, the enhanced reducibility and surface Brønsted acidity appear to decrease the ethylene selectivity, which may also be due to higher conversion levels. Above monolayer coverage, crystalline V 2O 5 nanoparticles are also present and do not contribute to ethane activation, but are responsible for unselective conversion of ethylene to CO. The crystalline V 2O 5 nanoparticles also react with the Al 2O 3 support at elevated temperatures via a solid-state reaction to form crystalline AlVO 4, which suppresses ethylene combustion of the crystalline V 2O 5 nanoparticles. The molecular structure–chemical characteristics of the surface VO x species demonstrate that neither the terminal V O nor bridging V O V bonds influence the chemical properties of the supported VO x /Al 2O 3 catalysts, and that the bridging V O Al bond represents the catalytic active site for ethane activation.

Effects of alkali metal cations on the structures, physico-chemical properties and catalytic behaviors of silica-supported vanadium oxide catalysts for the selective oxidation of ethane and the complete oxidation of diesel soot

A comparative study on the effects of alkali metal on the structures, physico-chemical properties and catalytic behaviors of silica-supported vanadium catalysts for the selective oxidation of ethane and the complete oxidation of diesel soot was reported.

Dehydrogenation of isobutane in the presence of carbon dioxide over supported vanadium oxide catalysts

The dehydrogenation of isobutane to isobutene in the presence of carbon dioxide was carried out over supported vanadium oxide catalysts. The influence of the support on the catalytic performance was investigated. The isobutane conversion and isobutene selectivity in the presence of carbon dioxide were compared with the results obtained during the dehydrogenation reaction in the presence of helium (inert gas). The catalysts were characterized by temperature-programmed techniques (TPR, TPD-NH3, TPD-CO2).

Dispersion and reactivity of monolayer vanadium oxide catalysts supported on zirconia: The influence of molybdena addition

This investigation reports the influence of MoO 3 on the dispersion of vanadium oxide on zirconia support. Samples containing various MoO 3 loadings ranging from 0.5 to 4 wt.% were prepared by impregnation of previously prepared 6 wt.% V 2O 5/ZrO 2 with requisite amounts of ammonium heptamolybdate solution. Dispersion of vanadia was determined by oxygen chemisorption at 640 K and was found to decrease with the increase of molybdena loading. The calcined catalyst samples were characterized by specific BET surface area, XRD, FT-IR, Oxygen chemisorption and TPR of H 2 techniques. The XRD results show that the intensity of tetragonal phase decreases and monoclinic phase intensity increases with the addition of MoO 3 to V 2O 5/ZrO 2 catalyst. The TPR of H 2 results reveal that the reducibility of vanadia was found to decrease with the addition of molybdena to V 2O 5/ZrO 2 catalyst. The catalytic properties were evaluated during the ammoxidation of 3-picoline to nicotinonitrile. The activity in ammoxidation reaction did not change much with the addition of molybdenum oxide. However, the selectivity of nicotinonitrile was found to increase with molybdena loading, indicates that the added molybdena created additional sites for the ammoxidation of 3-picoline reaction. The addition of molybdenum oxide inhibited the interaction between vanadium and zirconia, leading to decrease in dispersion and reducibility of vanadia.

The combination of propane partial oxidation and of WGS reaction in a single catalytic bed, and the self-adapting catalytic properties of vanadium oxide catalyst

The gas-phase oxidation of propane catalyzed by unsupported vanadium oxide was studied under the reaction conditions that are usually employed for the oxidative dehydrogenation of alkanes. It was found that considerable amounts of H 2 are produced above the temperature at which oxygen is totally consumed. Hydrogen formed in part by propane dehydrogenation to propylene and by coke formation, in part by the water–gas-shift reaction, in the fraction of catalytic bed that operated in the absence of gas-phase oxygen. V 2O 5 catalyzed the oxidation of propane to propylene, CO, CO 2 and H 2O in the first part of the bed, whereas reduced vanadium oxides (VO 2, V 2O 3) formed in the anaerobic section of the reactor and catalyzed the reaction between CO and H 2O to produce H 2 and CO 2. It was also found that the selectivity to oxygenates (acetic acid mainly) and to propylene at low temperature was affected by the vanadium oxide type: while V 2O 5 was poorly selective to these compounds, higher selectivity was reached with V 2O 3. The latter oxide was also more active than V 2O 5 in propane oxidation. The role of ionic oxygen in V 2O 5 was studied by feeding propane in the absence of gas-phase oxygen and following the non-steady catalytic performance along with the increasing time-on-stream. Fully oxidized vanadium oxide was very active but non-selective to propylene; the progressive reduction of V 2O 5 led to a decrease of conversion and to an increase of the selectivity to the olefin, due to the higher contribution of dehydrogenation with respect to oxidative dehydrogenation.

Catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of saturated and unsaturated hydrocarbons

The catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of different organic substrates (cyclohexane, cyclohexene, 1-hexene) using tertbutylhydroperoxide (TBHP) and molecular oxygen as the oxidizing agents was studied. Titanium (1.9nm), vanadium (2.3nm) and niobium (1.6nm) oxide nanoparticles stabilized on silica were synthesized by the reduction of TiCl4·2THF, VCl3·3THF and NbCl4·2THF with K[BEt3H]. These materials were characterized by inductive coupled plasma-optical emission spectroscopy (ICP-OES), Fourier transformed infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analyses. The solids obtained were employed as heterogeneous catalysts. For 1-hexene and cyclohexene, the titanium oxide nanostructured material showed 100% selectivity towards the epoxidation product, with conversions above 50%. In the case of cyclohexane, the titanium oxide nanoparticles are 100% selective towards the desired oxidation products (cyclohexanol and cyclohexanone). The highest oxidation activity was achieved with the vanadium oxide nanoparticles for the three substrates studied: cyclohexane, cyclohexene and 1-hexene, TN 562, 878 and 1190mol of product/mol of metal, respectively, after 6h of reaction time at 80°C. Nevertheless, with this metal the selectivity is different to the one obtained for with the titanium oxide nanostructured system. The niobium catalyst was less active than the titanium and vanadium oxide catalysts, although it proved to be more selective towards the formation of alcohols.

Effect of Sol–gel Synthesis on Physical and Chemical Properties of V/SiO2 and V/MgO Catalysts

Catalytic properties of supported vanadium oxide catalysts are highly dependent on surface vanadia configuration and the nature of the support. In this study, the effects of using the same supports (silica and magnesia) from two different preparation procedures on the catalytic properties in methanol partial oxidation and on the vanadium surface structures were investigated. Both types of catalysts were prepared by the incipient-wetness method and characterized using BET, Raman spectroscopy, UV–Vis and H2-TPR to determine the structures of vanadium on the supports’ surface. Both aerogel-prepared V/SiO2 and V/MgO catalysts were more active than conventionally prepared samples in methanol partial oxidation. These differences in catalytic results are attributed to the configurations of active phase. It was also found that small vanadium phases were more active in methanol partial oxidation than larger phases.

On the umbrella model for supported vanadium oxide catalysts

An earlier-proposed model for the molecular structure of VO 4 species on alumina was tested and compared with the pyramid model on a silica support. The model can be described as a chemisorbed O support V O(O 2) species with a similar geometry as on alumina. Results of DFT calculations on this model are consistent with the experimental Raman and EXAFS data collected on low-loaded silica-supported vanadium oxide catalysts. The band observed at 915 cm −1 is assigned to an O O stretch vibration. The thermal motion of the bound O 2 molecule can explain the broadness of this band. These findings, in combination with our previous work, demonstrate that the umbrella model is a viable, internally consistent model for supported vanadium oxide catalysts at least at low loadings.

A new application of clay-supported vanadium oxide catalyst to selective hydroxylation of benzene to phenol

The highly selective direct hydroxylation of benzene with hydrogen peroxide to phenol was observed on a clay-supported vanadium oxide catalyst. Under mild reaction conditions at 313 K, high selectivity to phenol of 94% was realized on the clay-supported vanadium oxide catalyst at 14% conversion of benzene, comparable with the well-known TS-1 catalyst. On the other hand, vanadium on other supports and other metals on the clay support gave inferior catalytic performances. V–O–Al and V–O–Si bridges in the catalyst may be responsible for the reaction.

Influence of the acid–base/redox properties of TiO x-sepiolite supported vanadium oxide catalysts in the gas-phase selective oxidation of toluene

Catalytic behaviour in the selective oxidation of toluene of a series of vanadium systems supported on TiO x -coated sepiolite (6, 12 and 25 wt.% TiO 2) with a vanadia loading around the theoretical monolayer (10 wt.%) has been investigated. The surface acid–base/redox properties of the solids were also evaluated by using 2-propanol conversion and pyridine chemisorption. The reducibility of surface vanadia species was studied by H 2-TPR. Surface properties of vanadia species and, consequently, their catalytic behaviour were influenced by titania loading on sepiolite. Thus, the vanadium systems with the highest titania loading were the most active and selective in toluene oxidation. Furthermore, this behaviour seems to be mainly related to the density of the active sites capable of being reduced and producing propanone in the vanadium systems.

Elucidation of the molecular structure of hydrated vanadium oxide species by X-ray absorption spectroscopy: correlation between the V⋯V coordination number and distance and the point of zero charge of the support oxide

The effect of the point of zero charge (PZC) of the support oxide (Al(2)O(3), Nb(2)O(5), SiO(2) and ZrO(2)) on the molecular structure of hydrated vanadium oxide species has been investigated with EXAFS spectroscopy for low-loaded vanadium oxide catalysts. It was found that the degree of clustering (i.e., the V[dot dot dot]V coordination number) and the V...V distance increase with decreasing PZC of the support oxide; i.e., Al(2)O(3) (8.7) < ZrO(2) (7) < Nb(2)O(5) (3.3) < SiO(2) (2). Upon hydration the silica-supported vanadium oxide exhibited a clear alteration in the position of the oxygen atoms surrounding the central vanadium atom and the number of oxygen atoms around vanadium increased to five. In contrast, only minor changes in the molecular structure were detected for the alumina-, niobia- and zirconia-supported vanadium oxide catalysts. Based on a detailed analysis of the EXAFS data a semi-quantitative distribution of vanadium oxide species present on the surface of the different support oxides can be obtained, which is in good agreement with earlier characterization studies primarily making use of Raman spectroscopy.

Oxidative dehydrogenation of ethane in a fluidized bed membrane reactor

The performance of a pilot scale fluidized bed membrane reactor (FLBMR) was studied experimentally in comparison to the conventional operation as a fluidized bed reactor (FLBR) for the catalytic oxidative dehydrogenation of ethane using a γ-alumina supported vanadium oxide catalyst. For both reactor configurations, the influence of process parameters such as temperature and contact time was investigated. Further, the experimental data obtained were compared to previous experiments with a fixed-bed reactor (FBR) and a packed-bed membrane reactor (PBMR) operated with a similar catalyst. For identical overall feed rates, the distributed oxidant feeding in the FLBMR improves the selectivity to ethylene significantly. The beneficial effect of oxidant dosing over the membrane is most pronounced at high temperatures and long contact times. Under limiting oxidant supply the FLBMR and the PBMR show a similar performance, but under moderate oxygen excess the FLBMR outperforms the PBMR significantly. The maximum ethylene yield observed in the FLBMR was 37% compared to 35% for the PBMR. Beside a high productivity, for the FLBMR a broader favorable operation range with respect to the oxygen–hydrocarbon ratio was observed, what indicates a lower sensitivity against oscillations and disturbances in the reactant feed, corresponding to a higher safety of operation. Because of an excellent heat transfer characteristics, the fluidized bed membrane reactor concept is very promising for maximizing the yield of the desired intermediates also in large-scale plants, especially for strongly exothermic reactions.

Characterization and reactivity of Al 2O 3–ZrO 2 supported vanadium oxide catalysts

Vanadium oxide catalysts with V 2O 5 loadings ranging from 2.5 to 20 wt.% supported on Al 2O 3–ZrO 2 (1:1 wt.%) mixed oxide have been prepared by wet impregnation method. The calcined samples were characterized by X-ray diffraction (XRD), BET surface area, pulse oxygen chemisorption, electron spin resonance (ESR), temperature-programmed reduction (TPR) of H 2, X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectroscopy (UVDRS). The catalytic properties have been evaluated for vapor phase ammoxidation of toluene to benzonitrile. Dispersion of vanadia was determined by the pulse oxygen chemisorption method at 643 K. Vanadia is found to be present in a highly dispersed state at lower loadings and dispersion decreases steadily with increase of vanadia loading. The ESR spectra obtained under ambient conditions shows the presence of V 4+ in tetrahedral symmetry. The TPR results show a single reduction peak corresponding to V 5+→V 3+. XPS results reveal that vanadium is present in a fully oxidized state (+5) in all the samples. The intensity ratio V 2p 3/2/Al 2p 3/2 and V 2p 3/2/Zr 3d 5/2 is found to increase with increase in vanadia loading up to 12.5 wt.% and levels off at higher vanadia loadings. The UV–vis diffuse reflectance spectra indicate the existence of isolated and clusterized tetrahedral (Td) V 5+ species in all catalysts. Ammoxidation activity increases with vanadia loading up to 12.5 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic activity in ammoxidation of toluene to benzonitrile has been correlated to the oxygen chemisorption sites.

Dehydrogenation of propane over a silica-supported vanadium oxide catalyst

Dehydrogenation of propane over a silica-supported vanadium oxide catalyst

Oxidation of Methanol to Formaldehyde on Supported Vanadium Oxide Catalysts Compared to Gas Phase Molecules

The oxidation of methanol to formaldehyde on silica supported vanadium oxide is studied by density functional theory. For isolated vanadium oxide species silsesquioxane-type models are adopted. The first step is dissociative adsorption of methanol yielding CH3O(O=)V(O-)2 surface complexes. This makes the O=V(OCH3)3 molecule a suited model system. The rate-limiting oxidation step involves hydrogen transfer from the methoxy group to the vanadyl oxygen atom. The transition state is biradicaloid and needs to be treated by the broken-symmetry approach. The activation energies for O=V(OCH3)3 and the silsesquioxane surface model are 147 and 154 kJ/mol. In addition, the (O=V(OCH3)3)(2) dimer (a model for polymeric vanadium oxide species) and the O=V(OCH3)3(*+) radical cation are studied. For the latter the barrier is only 80 kJ/mol, indicating a strong effect of the charge on the energy profile of the reaction and questioning the significance of gas-phase cluster studies for understanding the activity of supported oxide catalysts.

Synthesis, characterization and catalytic activity of Nd 2O 3 supported V 2O 5 catalysts

A series of rare-earth neodymia supported vanadium oxide catalysts with various V 2O 5 loadings ranging from 3 to 15 wt.% were prepared by the wet impregnation method using ammonium metavanadate as the vanadium precursor. The nature of vanadia species formed on the support surface is characterized by a series of different physicochemical techniques like X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), BET surface area, diffuse reflectance UV–vis spectroscopy (DR UV–vis), thermal analysis (TG-DTG/DTA) and SEM. The acidity of the prepared systems were verified by the stepwise temperature programmed desorption of ammonia (NH 3-TPD) and found that the total acidity gets increased with the percentage of vanadia loading. XRD and FTIR results shows the presence of surface dispersed vanadyl species at lower loadings and the formation of higher vanadate species as the percentage composition of vanadia is increased above 9 wt.%. The low surface area of the support, calcination temperature and the percentage of vanadia loading are found to influence the formation of higher vanadia species. The catalytic activity of the V 2O 5-Nd 2O 3 catalysts was probed in the liquid phase hydroxylation of phenol and the result show that the present catalysts are active at lower vanadia concentrations.

Characterization of Vanadium and Titanium Oxide Supported SBA-15

Supported vanadium and titanium oxide catalysts were prepared by adsorption and subsequent calcination of the vanadyl and titanyl acetylacetonate complexes, respectively, on mesoporous SBA-15 by the molecular designed dispersion (MDD) method. Liquid and gas phase depositions at different temperatures were carried out with vanadyl acetylacetonate, and the different results together with those of titanyl acetylacetonate in the liquid phase deposition were discussed. The bonding mechanism, the influence of the metal interaction with the support material, and differences due to the way of deposition and the temperature were investigated by TGA, chemical analysis, FTIR, and Raman spectroscopy. Elevated dissolving temperatures in the liquid phase led to higher final loadings on the SBA-15 without the formation of clusters, even at high loadings. The decomposition of the anchored vanadium and titanium complexes, their thermal stability, and the conversion to the covalently bound VO(x) and TiO(x) species on SBA-15 were studied and investigated by in situ transmission IR spectroscopy. In general, the titanium complex is more reactive than the vanadium complex toward the surface of SBA-15 and has a higher thermal stability. The MDD method of the VO(acac)2 and TiO(acac)2 enables to create a dispersed surface of supported VO(x) and TiO(x), respectively. The structure configurations of VO(x) and TiO(x) oxide catalysts obtained at different metal loadings were studied by Raman spectroscopy. Pore size distributions, XRD, and N2 sorption confirmed the structural stability of these materials after grafting. VO(x)/SBA-15 and TiO(x)/SBA-15 samples, with different metal loadings, were also catalytically tested for the selective catalytic reduction (SCR) of NO with ammonia.

Diesel soot oxidation over supported vanadium oxide and K-promoted vanadium oxide catalysts

Several systems of vanadium oxide and K-promoted vanadium oxide catalysts which supported on different carries with the variations of V contents and K contents were prepared by incipient-wetness impregnation method. Their catalytic performances for diesel soot catalytic oxidation were investigated with temperature-programmed oxidation reaction (TPO). Spectroscopic techniques (FT-IR and UV–vis DRS) were utilized to determine the structure of VOx species for vanadia supported on γ-Al2O3, SiO2, TiO2, and ZrO2. Thermal analysis techniques, including thermogravimetrical (TG-DTA) analyses and temperature-programmed reduction (TPR), were used to characterize the thermal behavior and redox properties of K promoting vanadium oxide catalysts, respectively. The results showed that the structure of supported-vanadia catalysts depends on the support materials and also on the V loading. At high V loading, VOx species exists predominantly as polyvanadate species and V2O5. The catalytic activities of the catalysts for soot oxidation are correlated to their redox property and the mobility of surface atoms. The catalytic activity for soot oxidation orders as Ti>Zr>Si>Al, and the sequence for selectivity to CO2 formation (SCO2) is Ti>Zr>Si∼Al for the catalysts over different supports. Adding potassium into supported vanadia catalysts can improve the catalytic activity of soot oxidation. And when n(K):n(V):n(Ti)=4:4:100 the catalyst of K4V4/TiO2 gives the best activity for soot combustion, i.e., the lowest reaction temperature.

Modification of photocatalytic center for photo-epoxidation of propylene by rubidium ion addition to V 2O 5/SiO 2

The photo-oxidation of propylene with molecular oxygen was investigated over silica-supported vanadium oxide catalysts (VS) at 323 K. The addition of Rb ions to VS resulted in the great improvement of the formation rate of propylene oxide in the photo-oxidation of propylene (175 μmol g −1 h −1 for Rb-ion-modified VS, V 2O 5 loading 0.5 wt%, Rb/V = 1.5). The photocatalytic activity was significantly enhanced when the Rb/V ratio was greater than 1.0. UV–Vis diffuse reflectance and X-ray absorption spectra of a series of Rb-ion-modified VS catalysts indicated the presence of one-to-one interaction between a Rb ion and isolated VO 4 tetrahedra over silica.