Polymeric Vanadia Research Articles

Preparation and Catalytic Performance of Al2O3, TiO2 and SiO2 Supported Vanadium Based-Catalysts for C–H Activation

A series of Al2O3, SiO2 and TiO2-supported vanadium oxide catalysts with different vanadium loadings has been prepared by two different methods: wet impregnation using ammonium metavanadate as a vanadium precursor and grafting using VOCl3. Characterization of these catalysts by XRD, FTIR, TEM, SEM, EDX, DR and TGA techniques revealed that the structure of vanadium oxide VO x species depends on the preparation method, the type of the support and loadings of vanadium on the oxides surfaces. Monomeric vanadium oxide species are predominant at low vanadium loadings while polymeric vanadium oxide species increase with higher loadings. The catalytic activity of the prepared catalysts was evaluated for the liquid-phase oxidation of cyclohexane as a model reaction to obtain cyclohexylhydroperoxide, cyclohexanol and cyclohexanone with PCA as co-catalyst. The results show that the catalysts exhibit good cyclohexane conversion and remarkable selectivity to the target products and high turnover numbers (TON).

The role of surface vanadia species in butane dehydrogenation over VO x/Al 2O 3

A series of θ-Al 2O 3 supported VO x catalysts, of different vanadium loadings, have been characterised and employed for the selective dehydrogenation of n-butane. Characterisation of the unreacted catalysts has been carried out by solid-state NMR ( 51V MAS NMR, 27Al MAS NMR and 27Al 3Q-MAS NMR), and FT-IR spectroscopies, with reference to previously acquired Raman and UV–vis spectroscopy data. As vanadium loading increases, so does the domain size of the supported vanadate units with significant quantities of V 2O 5 observed at the highest loadings. The influence of calcination, pre-reduction, reaction and regeneration on the structure of the catalysts has been studied by NMR, FT-IR, EPR, microanalysis and TEOM. Calcination disperses crystalline vanadate units, and at high loadings AlVO 4 formation is observed. Pre-reduction reduces the vanadium oxidation state from 5+ to 3+, while regeneration results in the formation of highly crystalline V 5+ species. From these data it is possible to determine structure–activity relationships, with polymeric vanadia clusters favouring the formation of butenes and butadienes, while more isolated species are highly active towards the formation of polynuclear aromatic hydrocarbons retained on the catalyst surface post-reaction. These large polynuclear aromatic hydrocarbons are however not the principle cause of catalyst deactivation in this reaction.

Redox Isotherms for Vanadia Supported on Zirconia

Redox isotherms were measured for zirconia-supported vanadia between 10−2 and 10−28 atm at 748 K for two vanadia loadings, 2.9 and 5.8 V/nm2, corresponding to isolated VO4 species and monolayer, polymeric vanadates. The catalyst with isolated VO4 species, which is expected to have predominantly V–O–Zr linkages, had a redox isotherm that showed a well-defined step corresponding to one oxygen per V. By contrast, the redox isotherm for the catalyst with polymeric vanadates changed more gradually with $$ \hbox{P}_{{\rm O}_2} $$ and the change in the oxygen stoichiometry corresponded to 0.85 O/V. Comparison of these results to the redox isotherms for bulk vanadates suggests that oxidation of the isolated vanadates proceeds by a direct transition from V+3 ↔ V+5, while transitions from V+3 ↔ V+4 and V+4 ↔ V+5 are possible with the polyvanadates. Rate measurements for methanol and propane oxidation over the two supported vanadia catalysts and several bulk vanadates showed that specific rates for each reaction were similar on all of the samples, suggesting that that the V–O bond strength does not affect the rate determining step of these reactions.

Development of Silica/Vanadia/Titania Catalysts for Removal of Elemental Mercury from Coal-Combustion Flue Gas

SiO2/V2O5/TiO2 catalysts were synthesized for removing elemental mercury (Hg0) from simulated coal-combustion flue gas. Experiments were carried out in fixed-bed reactors using both pellet and powder catalysts. In contrast to the SiO2-TiO2 composites developed in previous studies, the V2O5 based catalysts do not need ultraviolet light activation and have higher Hg0 oxidation efficiencies. For Hg0 removal by SiO2-V2O5 catalysts, the optimal V2O5 loading was found between 5 and 8%, which may correspond to a maximum coverage of polymeric vanadates on the catalyst surface. Hg0 oxidation follows an Eley-Rideal mechanism where HCI, NO, and NO2 are first adsorbed on the V2O5 active sites and then react with gas-phase Hg0. HCI, NO, and NO2 promote Hg oxidation, while SO2 has an insignificant effect and water vapor inhibits Hgo oxidation. The SiO2-TiO2-V2O5 catalysts exhibit greater Hg0 oxidation efficiencies than SiO2-V2O5, may be because the V-O-Ti bonds are more active than the V-O-Si bonds. This superior oxidation capability is advantageous to power plants equipped with wet-scrubbers where oxidized Hg can be easily captured. The findings in this work revealed the importance of optimizing the composition and microstructures of SCR (selective catalytic reduction) catalysts for Hg0 oxidation in coal-combustion flue gas.

Thermal treatment study of vanadium-loaded hydrotalcites employing in situ DXAS

A comparative study was made in which two hydrotalcites were loaded with vanadium employing different procedures: (1) aqueous phase impregnation, with vanadyl sulfate solution in pH 6.5 or 9.5; (2) solid phase mixing, with vanadyl sulfate or vanadium pentoxide physically mixed up. TPR investigation revealed that vanadium-loaded hydrotalcites obtained by aqueous phase impregnation in two distinct pH, resulted in different reduction profiles. The vanadium-loaded hydrotalcites were characterized by FTIR, XRD and TGA–DTA analysis. In situ dispersive XAS (DXAS) spectra obtained during thermal treatment in air showed that, for the physical mixture of solids, vanadium is oxidized above 350 °C. For the samples prepared by aqueous phase impregnation, the effect of alkaline pH was to produce polymeric vanadium species deposited onto the HT in comparison with the isolated vanadium species in acidic conditions. The influence of both, the hydrotalcite interlayer anion type and the vanadium loading procedure, on the reducibility of vanadium species is also discussed.

Influence of vanadia content onto TiO 2–SiO 2 matrix for photocatalytic oxidation of trichloroethylene

Direct synthesis of vanadia onto titania–silica matrices as photocatalysts was achieved by using simple sol–gel method. This synthetic strategy revealed that the vanadia species could be loaded into TiO 2–SiO 2 matrices up to 18.5 wt%. Results from FTIR indicated that at low loading amounts of vanadia species (i.e. ⩽1), the formation of monolayer vanadia species onto the matrices was successfully fabricated; however, a polymeric vanadate could be formed with high loading of vanadia species. On such heterogeneous photocatalytic systems, the oxidation affinity of trichloroethylene was substantially affected by the loading amount and the degree of dispersion V 2O 5 particles onto the TiO 2–SiO 2 support matrices, indicating the exclusive effect of the V 2O 5 nanoparticles on this photocatalytic reaction. Clearly evident is that this polymeric vanadate was a relatively inactive photocatalysts for the oxidation of trichloroethylene.

Vanadium oxides supported on TiO2-Sepiolite and Sepiolite: Preparation, structural and acid characterization and catalytic behaviour in selective oxidation of toluene

The synthesis of vanadium oxide systems (5–20 wt% of V2O5) supported on TiO2-Sepiolite (with titania loading around the theoretical monolayer, 12 wt%) and on Sepiolite was carried out by the wet impregnation method. Materials were characterized by EDX, XRD, DR UV–vis, Raman, 29Si and 1H MAS NMR, TEM and nitrogen adsorption. Surface acid properties were evaluated through pyridine chemisorption. Their catalytic behaviour in the selective oxidation of toluene was studied. All solids exhibited the type IV isotherm, which corresponds to mesoporous solids. The decomposition of the Sepiolite into magnesium silicate and silica due to the effects of the vanadium incorporation process and calcination was not observed. The nature of the vanadium species depended on the support type and vanadium loading. Thus, in general, at the lowest vanadia loadings, isolated tetrahedral and polymetavanadate type species predominated. At higher loadings (15 wt% or above), vanadium oxides were also presented as crystalline V2O5 nanoparticles and, especially in %V/Sepc systems, as pyro- and meta-magnesium vanadates. On the other hand, in relation to the supports, the vanadium systems exhibited new acid sites whose density and strength both depended on the vanadia species structures. The catalytic performance of the vanadium oxide systems in the selective oxidation of toluene was also influenced by the structure and nature of vanadia species. Thus, the species formed on TiO2-coated Sepiolite support exhibited more activity in the formation of benzaldehyde (and benzoic acid) than the ones on Sepiolite. This formation is favoured by the polymeric vanadia species.

Asymmetric Addition of Trimethylsilyl Cyanide to Aldehydes Promoted by Chiral Polymeric Vanadium(V) Salen Complex as an Efficient and Recyclable Catalyst.

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Synthesis and Characterization of Polymeric V<SUB>2</SUB>O<SUB>5</SUB>/AlO(OH) with Nanopores on Alumina Support

Polymeric vanadium pentoxide gel was formed via the reaction of V2O5 powder with hydrogen peroxide. The polymeric vanadium pentoxide gel was then dispersed in alumina gel. Different vanadium loading composites were coated on alumina support and calcined at 500 degrees C for 1 hr. These composite layers were characterized using TGA, FT-IR, XRD, SEM, and Autosorb. It was found that the lamellar structure of polymerized vanadium pentoxide was retained in the inorganic matrix. Crystalline alumina in gamma phase was formed after calcinations. However, the vanadium-alumina mixed oxides are lack of the well defined PXRD peaks for polycrystalline V2O5. This is possibly because the vanadia species are highly dispersed in the alumina matrix or the vanadia species are dispersed as crystalline which is smaller than 4 nm. In addition, the imbedded polymeric vanadium oxide improved the specific area and average pore diameter of the composite layer.

Low temperature selective catalytic reduction of NO by NH3 over V2O5 supported on TiO2–SiO2–MoO3

The V2O5 catalysts supported on TiO2–SiO2–MoO3 (TSM) prepared by the coprecipitation method were investigated for the selective catalytic reduction (SCR) of NO by NH3 at low temperatures. The V2O5/TSM catalyst with 7–13 wt% SiO2 was found to exhibit a superior SCR activity and a good sulfur tolerance at low temperatures (<250 °C). The presence of highly active polymeric vanadates formed by the incorporation of MoO3 to TiO2–SiO2 and superior redox properties seems to enhance SCR activity, and furthermore the very lower SO2 oxidation activity due to the higher acidity leads to the remarkable improvement of sulfur tolerance.

Asymmetric addition of trimethylsilyl cyanide to aldehydes promoted by chiral polymeric vanadium(V) salen complex as an efficient and recyclable catalyst

The asymmetric addition of trimethylsilyl cyanide to various aldehydes catalyzed by efficient new vanadyl polymeric salen complexes having 12 repeating salen units was investigated at room temperature. An excellent yield of the trimethylsilylether of cyanohydrins (up to 98%) with high chiral induction (96%) in case of 2-methylbenzaldehyde was achieved in 18 h. The catalyst recovered four times with retention of its performance.

Nonhydrolytic vanadia-titania xerogels: Synthesis, characterization, and behavior in the selective catalytic reduction of NO by NH3

V2O5-TiO2 catalysts with V2O5 contents ranging from 6 to 18wt% were obtained by calcination at 773K of xerogels prepared by a nonhydrolytic sol–gel route from VOCl3, TiCl4, and iPr2O. These materials were characterized by X-ray diffraction, micro-FT-Raman spectroscopy, static 51V NMR, SEM, N2 physisorption, temperature-programmed desorption of ammonia (NH3-TPD), and temperature-programmed reduction by H2 (H2-TPR). In all cases mesoporous solids were obtained with specific surface areas up to 87m2g−1. Raman spectroscopy indicated the presence of the same vanadia species as in conventional vanadia-titania catalysts. Up to a critical V2O5 loading (between 10 and 12wt%), all the vanadium was highly dispersed in the form of monomeric vanadyl and polymeric vanadate species, whereas for higher loadings V2O5 crystallites also formed. Most of the vanadium species (80–90%) were accessible to reduction by H2. The catalytic properties of these materials were evaluated in the selective catalytic reduction of NO by NH3. The estimated turnover frequency ‘TOF’ at 473K (mole of NO converted per mole of V per hour) went through a maximum value (16.6h−1) at a V loading of 8wt%, corresponding to an estimated surface vanadia loading of about 10μmolm−2. The N2O selectivity increased upon V loading as expected. A compromise between activity and high N2 selectivity (98% at 623K) could be found with the material containing 6wt% V2O5.

Selective oxidation of hydrogen sulfide containing excess water and ammonia over vanadia–titania aerogel catalysts

A series of V 2O 5–TiO 2 aerogel catalysts were prepared by sol–gel method with subsequent supercritical drying with CO 2. The aerogel catalysts showed much higher surface areas and total pore volumes than V 2O 5–TiO 2 xerogel and impregnated V 2O 5–TiO 2 catalysts. Two species of surface vanadium in the aerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Aerogel catalysts showed very high conversion of H 2S without harmful emission of SO 2. Temperature programmed reduction (TPR), XRD and Raman analyses revealed that the high catalytic performance of the aerogel catalysts originated from their highly dispersed VO x species and high reducibility.

V2O5-WO3/TiO2-SiO2-SO42− catalysts: Influence of active components and supports on activities in the selective catalytic reduction of NO by NH3 and in the oxidation of SO2

The V2O5-WO3 catalysts loaded on the Ti-rich TiO2-SiO2-SO42− prepared by the coprecipitation method were investigated for the influences of the active components and the supports on the activities in SCR of NO by NH3 and in the oxidation of SO2 in comparison with a commercial V2O5-WO3/TiO2-SO42− catalyst. The physico-chemical properties of the catalysts were characterized by BET, XRD, IR, Raman, NH3-TPD, XPS and acidity measurements. The incorporation of sulfate to TiO2-SiO2 considerably increases the acidity of TiO2-SiO2, especially Brønsted acidity of the corresponding catalyst, resulting in the remarkable increase of the SCR activity and simultaneously, the addition of WO3 significantly enhances the SCR activity. The SCR activity changes depending on the calcination temperature of TiO2-SiO2-SO42−, which influences its acidity, surface area and titania crystallinity. The higher SCR activity was observed at high vanadia loadings above 2wt.% as compared with the V2O5-WO3/TiO2-SO42− catalyst. This seems to be due to the fact that polymeric vanadates on the catalyst, which cause the oxidation of NH3 to N2O, are almost absent as evidenced by Raman analysis. The SO2 oxidation activity is increased by elevating the calcination temperature of TiO2-SiO2-SO42−, which shifts the oxidation state of vanadium species to the higher state, and the activity is slightly enhanced by WO3 but not changed by sulfate. The tendency of the increase in the SO2 oxidation activity with increasing V2O5 loadings is significantly smaller than the V2O5-WO3/TiO2-SO42− catalyst. This was attributed to the lower oxidation state of vanadium species, almost the absence of polymeric vanadates responsible for the SO2 oxidation and also a more strongly inhibiting effect of ammonia on the oxidation of SO2.

Partial Oxidation of Methanol Over Highly Dispersed Vanadia Supported on Silica SBA-15

The partial oxidation of methanol to formaldehyde (FA) was studied over highly dispersed vanadia supported on mesoporous silica SBA-15 (VO x /SBA-15). VO x /SBA-15 catalysts were prepared by a novel grafting/ion-exchange method and characterized using UV–VIS- and Raman spectroscopy. The resulting surface vanadium oxide species (0–2.3 V/nm2), grafted on the inner pores of the SBA-15 silica matrix, consist of tetrahedrally coordinated monomeric and polymeric vanadia. The VO x /SBA-15 catalysts are active and highly selective for the production of FA between 300 and 400 °C. Comparison of the reactivity results with literature data reveals that a better catalytic performance can be obtained over vanadia supported on mesoporous silica in comparison with conventional silica samples with the same vanadium loading. Raman characterization of the catalyst after reaction at high conversion indicates that dispersed vanadia partly agglomerates into vanadia crystallites during methanol oxidation.

N 2O formation in the ammonia oxidation and in the SCR process with V 2O 5-WO 3 catalysts

Commercial catalysts based on vanadia and tungsta supported on titania are generally employed in the selective catalytic reduction (SCR) of nitrogen oxides present in the stack gases of power stations; similarly, they are active for the oxidation of traces of ammonia and the elimination of dioxins and furans. When these catalysts are used for gas purification in the presence of ammonia, they can produce nitrous oxide that is released to the atmosphere. In order to evaluate the formation of nitrous oxide during the total ammonia oxidation and SCR of nitrogen oxides with ammonia, a series of V 2O 5-WO 3/TiO 2 catalysts were prepared and tested under conditions close to those found in their industrial use. The results confirm that nitrous oxide formation mechanisms in both processes were clearly related to the active phase composition, increasing the formation of this undesired compound as the vanadia loading in the catalyst was raised. With the oxidation of trace ammonia, nitrous oxide is formed from the adsorption of two ammonia molecules over polymeric vanadyl species. For the SCR process, nitrous oxide is also formed by nitric oxide molecules adsorbed on adjacent centres of the polymeric vanadia.

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.

A Novel Series of Vanadium‐Sulfite Polyoxometalates: Synthesis, Structural, and Physical Studies

Reaction of NH4VO3 with sulfur dioxide affords the hexanuclear cluster (NH4)2(Et4N)[(V(IV)O)6(mu4-O)2(mu3-OH)2(mu3-SO3)4(H2O)2]Cl x H2O (1), and the decapentanuclear host-guest compound (Et4N)5{Cl subset [(VO)15(mu3-O)18(mu-O)3]} x 3 H2O (2). Sequential addition of magnesium oxide to an acidic aqueous solution of NH4VO3 (pH approximately 0) followed by (NH4)2SO3 resulted in the formation of either the non-oxo polymeric vanadium(IV) compound trans-(NH4)2[V(IV)(OH)2(mu-SO3)2] (3) or the polymeric oxovanadium(IV) sulfite (NH4)[V(IV)O(SO3)1.5(H2O)] x 2.5 H2O (4) at pH values of 6 and 4, respectively. The decameric vanadium(V) compound {Na4(mu-H2O)8(H2O)6}[Mg(H2O)6][V(V)10(O)8(mu6-O)2(mu3-O)14] x 3 H2O (5) was synthesised by treating an acidic aqueous solution of NH4VO3 with MgO and addition of NaOH to pH approximately 6. All the compounds were characterised by single-crystal X-ray structure analysis. The crystal structure of compound 1 revealed an unprecedented structural motif of a cubane unit [M4(mu4-O)2(mu3-OH)2] connected to two other metal atoms. Compound 3 comprises a rare example of a non-oxo vanadium(IV) species isolated from aqueous solution and in the presence of the reducing agent SO3(2-), while compound 4 represents a rare example of an open-framework species isolated at room temperature (20 degrees C). In addition to the synthesis and crystallographic studies, we report the IR and magnetic properties (for 1, 2 and 3) of these vanadium clusters as well as theoretical studies on compound 3.

Deep oxidation of light alkanes over titania-supported palladium/vanadium catalysts

The total oxidation of short-chain hydrocarbons has been studied using a titania-supported palladium catalyst modified with vanadium. A range of catalysts has been prepared by coimpregnation of the titania support with palladium (II) chloride and ammonium metavanadate. The addition of vanadium promoted the rates of oxidation at lower temperatures. Vanadium loadings between 0.5 and 3.0 wt% were investigated and the most active catalyst was 0.5% Pd1.5% V/TiO 2. The addition of vanadium decreased the palladium dispersion and the number of surface palladium sites. Microlaser Raman characterisation identified the supported vanadium species and the vanadium loading influenced these. The addition of palladium modified the vanadium species. In particular the presence of palladium increased the concentration of polymeric vanadium species. Temperature-programmed reduction studies showed that the addition of palladium to V/TiO 2 significantly increased the ease of catalyst reduction. Characterisation using electron paramagnetic resonance spectroscopy showed that the presence of palladium significantly increased the concentration of V 4+ species. It is proposed that the increased catalyst activity is related to the modified redox properties of the catalysts.

Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: role of vanadia species

Pure and K-doped vanadia/titania prepared by different methods have been studied in order to elucidate the role of vanadia species (monomeric, polymeric, bulk) in catalytic toluene partial oxidation. The ratio of different vanadia species was controlled by treating the catalysts in diluted HNO 3, which removes bulk vanadia and polymeric vanadia species, but not the monomeric ones, as was shown by FT-Raman spectroscopy and TPR in H 2. Monolayer vanadia species (monomeric and polymeric) are responsible for the catalytic activity and selectivity to benzaldehyde and benzoic acid independently on the catalyst preparation method. Bulk V 2O 5 and TiO 2 are considerably less active. Therefore, an increase of the vanadium concentration in the samples above the monolayer coverage results in a decrease of the specific rate in toluene oxidation due to the partial blockage of active monolayer species by bulk crystalline V 2O 5. Potassium diminishes the catalyst acidity resulting in a decrease of the total rate of toluene oxidation and suppression of deactivation. Deactivation due to coking is probably related to the Brønsted acid sites associated with the bridging oxygen in the polymeric species and bulk V 2O 5. Doping by K diminishes the amount of active monolayer vanadia leading to the formation of non-active K-doped monomeric vanadia species and KVO 3.