In-situ UV-Raman study on soot combustion over TiO2 or ZrO2-supported vanadium oxide catalysts

UV-Raman spectroscopy was used to study the molecular structures of TiO₂ or ZrO₂-supported vanadium oxide catalysts. The real time reaction status of soot combustion over these catalysts was detected by in-situ UV-Raman spectroscopy. The results indicate that TiO₂ undergoes a crystalline phase transformation from anatase to rutile phase with the increasing of reaction temperature. However, no obvious phase transformation process is observed for ZrO₂ support. The structures of supported vanadium oxides also depend on the V loading. The vanadium oxide species supported on TiO₂ or ZrO₂ attain monolayer saturation when V loading is equal to 4 (4 is the number of V atoms per 100 support metal ions). Interestingly, this loading ratio (V₄/TiO₂ and V₄/ZrO₂) gave the best catalytic activities for soot combustion reaction on both supports (TiO₂ and ZrO₂). The formation of surface oxygen complexes (SOC) is verified by in-situ UV Raman spectroscopy and the SOC mainly exist as carboxyl groups during soot combustion. The presence of NO in the reaction gas stream can promote the production of SOC. Supported by the National Natural Science Foundation of China (Grant Nos. 20473053, 20773163 and 20525621), the Beijing Natural Science Foundation (Grant No. 2062020), and the 863 Program of China (Grant No. 2006AA06Z346)     

In-situ UV-Raman study on soot combustion over TiO2 or ZrO2-supported vanadium oxide catalysts

UV-Raman spectroscopy was used to study the molecular structures of TiO₂ or ZrO₂-supported vanadium oxide catalysts. The real time reaction status of soot combustion over these catalysts was detected by in-situ UV-Raman spectroscopy. The results indicate that TiO₂ undergoes a crystalline phase transformation from anatase to rutile phase with the increasing of reaction temperature. However, no obvious phase transformation process is observed for ZrO₂ support. The structures of supported vanadium oxides also depend on the V loading. The vanadium oxide species supported on TiO₂ or ZrO₂ attain monolayer saturation when V loading is equal to 4 (4 is the number of V atoms per 100 support metal ions). Interestingly, this loading ratio (V₄/TiO₂ and V₄/ZrO₂) gave the best catalytic activities for soot combustion reaction on both supports (TiO₂ and ZrO₂). The formation of surface oxygen complexes (SOC) is verified by in-situ UV Raman spectroscopy and the SOC mainly exist as carboxyl groups during soot combustion. The presence of NO in the reaction gas stream can promote the production of SOC.     

Synthesis of vanadium-phosphorus oxide catalysts supported on pyrogenic silica and titanium dioxide

Various methods for the preparation of vanadium-phosphorus oxide (VPO) catalysts supported on aerosils A-300 and A-50 and TiO₂ were studied: a traditional method (in an organic solvent under varying the support addition time, the nature of the reducing agent, and the degree of reduction of vanadium oxide) and barothermal and mechanochemical syntheses. With the use of XRD analysis, it was found that the composition of the resulting VPO phase depends on the time of support addition to the synthesis and the temperature of thermal treatment. Conditions for the formation of a supported phase of VOHPO₄·0.5H₂O, the precursor of the active component (VO)₂P₂O₇, were determined. The presence of vanadium in an oxidation state of +4 was demonstrated using EPR and UV-VIS spectroscopy. The specific surface areas and pore structures of the synthesized catalysts were determined. The catalytic properties of samples in the reactions of n-butane oxidation in an excess of the hydrocarbon and oxidative ethane dehydrogenation were studied. It was found that, as compared with traditional bulk VPO catalysts, the use of the synthesized supported VPO catalysts made it possible to improve the process characteristics of n-butane oxidation and did not change these characteristics in the reaction of oxidative ethane dehydrogenation.     

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4 Nanorods

The electronic and structural properties of vanadium‐containing phases govern the formation of isolated active sites at the surface of these catalysts for selective alkane oxidation. This concept is not restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn the typical combustion catalyst manganese oxide into a selective catalyst for oxidative propane dehydrogenation. Nanostructured, crystalline MnWO₄ serves as the support that stabilizes a defect‐rich MnO_x surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction temperature. Terminating MnO_x zigzag chains on the (010) crystal planes are suspected to bear structurally site‐isolated oxygen defects that account for the unexpectedly good performance of the catalyst in propane activation.     

Supported MgO–V<Subscript>2</Subscript>O<Subscript>5</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for oxidative propane dehydration: Effect of the molar Mg : V ratio on the phase composition and catalytic properties of samples

The physicochemical properties of V₂O₅/Al₂O₃ and MgO–V₂O₅/Al₂O₃ supported catalysts (Mg : V = 1 : 1, 2 : 1, and 3 : 2) obtained by consecutive impregnation of the support with solutions of vanadium and magnesium precursors are studied using a complex of mutually complementary methods (XRD, Raman spectroscopy, UV–Vis spectrometry, and TPR-H₂). The effect of the formation of surface magnesium vanadates of various composition and structure on the catalytic properties of the supported vanadium oxide catalysts in the oxidative dehydrogenation of propane is studied. The introduction of magnesium in the samples and an increase in its content, accompanied by a change in the structure of the surface vanadium oxide phases from polymeric VO₆/VO₅ species to surface metavanadate species, magnesium metavanadate, and further to magnesium divanadate, significantly affects their catalytic properties in the reaction of the oxidative dehydrogenation of propane to propylene.     

Ethylenebis(5‐chlorosalicylideneiminato)vanadium dichloride immobilized on MgCl2‐based supports as a highly effective precursor for ethylene polymerization

Ethylenebis(5‐chlorosalicylideneiminato)vanadium dichloride supported on MgCl₂(THF)₂ or on the same carrier modified by Et_nAlCl_3?n, where n = 1–3, was used in ethylene polymerization in the presence of MAO or a common alkylaluminium compounds as a cocatalyst. The support type alter vanadium loading and also change the characteristic of the catalytic active sites. Et₂AlCl is the best activator for a catalyst which has been immobilized on a nonmodified support, whereas the systems which contain a carrier which has been modified by an organoaluminium compound reveal the highest activity in conjunction with MAO. That difference, together with different temperature effects on polymerization efficiency (i.e., decrease and increase of catalytic activity for increasing temperatures, respectively) suggest the formation of different types of active sites in the catalytic systems supported on modified and nonmodified magnesium carrier. However, all supported precatalysts possess a long lifetime, still being active towards ethylene polymerization after 2 h. All the systems yield wide MWD polyethylene, while bimodal MWD is found for some part of analyzed samples. Polyethylene with bimodal particle size distribution is formed with the system which contain modified carriers at higher temperatures. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3480–3489, 2009     

Vapour phase synthesis of isobutyraldehyde from methanol and ethanol over mixed oxide supported vanadium oxide catalysts

The vapour phase synthesis of isobutyraldehyde from methanol and ethanol in one step was investigated over titania-silica, titania-alumina, titania-zirconia, titania-silica-zirconia, and magnesia supported vanadium oxide catalysts at 623 K and under normal atmospheric pressure. Among various catalysts the titania-silica binary oxide supported vanadia provided higher yields than the other single or mixed oxide supported catalysts. The high conversion and product selectivity of V₂O₅/TiO₂-SiO₂ catalyst (20 wt% V₂O₅) was related to the better dispersion of vanadium oxide over titania-silica mixed oxide support in addition to other acid-base and redox characteristics. A reaction path for the formation of isobutyraldehyde from methanol and ethanol mixtures over these catalysts was described.     

Oxidative dehydrogenation of propane on the VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/CeZrO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> supported catalyst

The oxidative dehydrogenation of propane on a supported vanadium catalyst was studied (the support was a complex oxide system consisting of a ceria–zirconia solid solution deposited on γ-Al₂O₃ (CeZrO/γ-Al₂O₃)). A comparative analysis of the properties of the support and the catalyst prepared on its basis was performed. The support and catalyst were characterized by the BET method, scanning electron microscopy, X-ray diffraction analysis, and Raman spectroscopy. The catalytic properties of the catalyst and support were studied in propane oxidation at 450 and 500°C with pulse feeding of the reagent. The effect of propane on the support was found to improve the oxidative properties of the latter. This behavior of the support is related to the preparation procedure, which leads to the formation on its surface of the crystalline phase of the ceria–zirconia solid solution and amorphous ZrO₂ and Al₂O₃ phases and/or their solid solution. Similar processes occur with the catalyst support during the oxidative dehydrogenation, giving rise to additional active centers (CeVO₄).     

Catalytic properties of the VO<Subscript><Emphasis Type="Italic">х</Emphasis></Subscript>/Ce<Subscript>0.46</Subscript>Zr<Subscript>0.54</Subscript>O<Subscript>2</Subscript> oxide system in the oxidative dehydrogenation of propane

Ce_0.46Zr_0.54O₂ solid solution prepared using a cellulose template was employed as a carrier for vanadium catalysts of the oxidative dehydrogenation of propane. The properties of VO _х /Ce_0.46Zr_0.54O₂ catalyst (5 wt % vanadium) are compared with the properties of the neat support. The carrier and catalyst are studied by means of BET, SEM, DTA, XRD, and Raman spectroscopy. It is shown that the CeVO₄ phase responsible for the ODH process is formed upon interaction between vanadate ions and cerium ions on the surface of the solid solution. The catalytic properties of the catalyst and the support are studied in the propane oxidation reaction at temperatures of 450 and 500°C with pulse feeding of the reagent. It is found that the complete oxidation of propane occurs on the support with formation of CO₂ and H₂O. Three products (propene, CO₂, and H₂O) form in the presence of the vanadium catalyst. It is suggested that there are two types of catalytic centers on the catalyst’s surface. It is concluded that the centers responsible for the complete oxidation of propane are concentrated mainly on the carrier, while the centers responsible for propane ODH are on the CeVO₄.     

Preparation of rhodium catalysts on laminar and zeolitic structures by anchoring of organometallic rhodium

Rhodium catalysts supported on six different aluminosilicate structures were prepared by hydrogen reduction of a cationic organometallic rhodium complex anchored to the support. The precursor active phase was incorporated in acetone medium through ion exchange using [Rh(Me₂CO)_x(NBD)]ClO₄ as the metal precursor species, in which NBD is 2,5‐norbornadiene and (Me₂CO)_x is acetone. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts thus prepared. The supports were characterized by X‐ray diffraction, energy‐dispersive X‐ray analysis, volumetric adsorption and surface acidity. For the precursors and catalysts, the metal load was determined by UV–VIS spectra, the reduction temperature was determined by differential scanning calorimetry, and rhodium dispersion was measured by chemisorption. The structure of the materials used as supports had a great influence on the catalyst prepared. A higher metal content was achieved in the supports with laminar structures, whereas better dispersion was shown by the catalysts supported on zeolitic structures. Copyright © 2001 John Wiley & Sons, Ltd.     

Copolymerization of ethylene and propylene catalyzed by magnesium chloride supported vanadium/titanium bimetallic Ziegler-Natta catalysts

Magnesium chloride supported vanadium/titanium bimetallic Ziegler-Natta catalysts with di-i-butyl phthalate as internal donor for copolymerization of ethylene and propylene were prepared.The effects of reaction temperature, ethylene/propylene molar ratio,aluminium/vanadium（Al/V）molar ratio and titanium/vanadium molar ratio on the catalytic activity were investigated.The molecular weight,molecular weight distribution,sequence composition and crystallinity of the products were measured by gel permeation chromatography,¹³C-NMR and differential scanning calorimetry analysis, respectively.In comparison to the vanadium and titanium catalysts,the bimetallic catalyst showed higher catalytic activity and better copolymerization performance.The obtained ethylene/propylene copolymers have high molecular weight （10⁵）,broad molecular weight distribution,high propylene content with random or short blocked sequence structures （r_Er_P=1.919）,low melting temperatures and low crystallinities（X_c<20%）.     

Active component of supported vanadium catalysts in the selective oxidation of methanol

The structure of catalysts based on vanadium oxide supported on different oxides (SiO₂, γ-Al₂O₃, ZrO₂, and TiO₂) was investigated. Their catalytic properties in the selective oxidation of methanol in a temperature range of 100–250°C were studied. It was shown that the nature of the support determines the structure of the oxide forms of vanadium. The supporting of vanadium on SiO₂ and γ-Al₂O₃ leads to the preferred formation of crystalline V₂O₅; the surface monomeric and polymeric forms of VO_x are additionally formed on ZrO₂ and TiO₂. It was established that the crystalline V₂O₅ oxide is least active in the selective oxidation of methanol; the polymeric forms are more active than monomeric ones. The mechanism of the selective oxidation of methanol to dimethoxymethane and methyl formate on the vanadium oxide catalysts is considered.     

Bulk and surface structures of V2O5/ZrO2 catalysts for n-butane oxidative dehydrogenation

The present investigation focuses on the structural properties and reactivity of zirconia-supported vanadium oxide catalysts, prepared by equilibrium adsorption in basic (pH 10) or in acid (pH 2.7) conditions with vanadium content up to 6 wt.% (pH 10) and up to 11.6 wt.% (pH 2.7). The samples, heated at 823 K for 5 h in air, were characterized by X-ray diffraction, Raman spectroscopy and TPR, both as prepared and after leaching with an ammonia solution to remove species not anchored to the zirconia surface. Some representative samples were also tested for the n-butane oxidative dehydrogenation (ODH) reaction. Depending on vanadium content, various vanadium species were identified by Raman spectroscopy that reacted differently on exposure to H₂. At similar loading, the fraction of vanadium in a dispersed state and thus interacting with the support was higher in samples prepared at pH 10 than in those at pH 2.7. Samples prepared at pH 2.7 contained a higher fraction of large polymeric structures in addition to ZrV₂O₇ and V₂O₅.In line with literature data for propane ODH on similar catalysts, our catalytic results suggested that the active sites for the ODH reaction are associated with the V–O–V bonds of the polymeric exposed structures, whereas the Zr–O–V sites favour alkane combustion.     

Effect of calcination temperature on the properties of industrial V2O5-TiO2 (anatase) catalysts in methanol oxidation

The properties of the catalysts for partial oxidation of o-xylene depend on the structure of the supported vanadium sites. The structure itself is strongly dependent on the calcination temperature of the catalyst at which thermal deposition of the metal oxide on the oxide support takes place. We have investigated the effect of calcination temperature on the activity and selectivity of industrial V₂O₅-TiO₂ (anatase) supported catalysts designed for partial oxidation of o-xylene in their application to methanol oxidation.This revised version was published online in December 2005 with corrections to the Cover Date.     

Liquid Phase Selective Oxidation of Alcohols over VPO Catalysts Supported on Mesoporous Hexagonal Molecular Sieves (HMS)

The vanadium phosphorous oxide (VPO) catalysts, supported on mesoporous hexagonal molecular sieves (HMS) with different vanadium loadings, were prepared by precipitation method on organic phase. Techniques such as XRD, BET and SEM, were used for characterization of the catalyst. The bulk VPO catalyst contains vanadyl pyrophosphate phase ((VO)₂P₂O₇), and a small amount of VOPO₄. The high surface area, large pore volume and pore size of HMS in VPO/HMS samples, provide an excellent dispersion of same phase of VPO compound on the support surface. Oxidation of various alcohols was studied in the liquid phase over VPO/HMS catalyst, using tert‐butylhydroperoxide (TBHP) as an oxidant. The activity of VPO/HMS samples were considerably increased with respect to bulk VPO catalyst. At 90 °C, the obtained activities were 0.567 and 6.545 g_pro.g^?1_VPOh^?1 over the bulk VPO and 20 wt% VPO/HMS catalysts, respectively. The effects of substrates, reaction time, reaction temperature, solvents, catalyst recycling and leaching of VPO in liquid phase reaction were also investigated. The following order has been observed for the percentage of conversions of alcohols: Benzylic alcohol > Secondary alcohol ～ Primary alcohol. The kinetic of benzyl alcohol oxidation using excess TBHP over VPO/HMS catalyst was investigated at temperatures of 27, 60 and 90 °C, and followed a pseudo‐first order with respect to benzyl alcohol.     

The Influence of Sulfate‐Doping on the Nature of V Sites in VOx/TiO2 Catalysts

EPR, UV/Vis and FTIR spectroscopy as well as thermal analysis (TA/MS) were applied to study the influence of sulfate species present in the anatase support on the specific nature of VO_x species in supported VO_x/TiO₂ catalysts. Those sulfate species modify the local structure of the supported vanadyl species and lead to the formation of two types of VO²⁺ sites instead of only one type being formed on sulfate‐free anatase. EPR and FTIR spectroscopic measurements revealed that a part of the VO²⁺ species are directly bound to the surface sulfate species. By TA/MS it was found that SO₂ is released at lower temperature from VO_x/TiO₂ in comparison to the vanadium‐free support. The direct bonding between sulfate and VO_x species stabilizes the latter on the surface of VO_x/TiO₂ resulting in three effects: 1) a higher V site dispersion in comparison to sulfate‐free TiO₂, 2) a better resistance of surface vanadyls against diffusion into the bulk of the support and 3) a much faster reoxidation of reduced V sites than observed on sulfate‐free TiO₂.     

Acid base properties and catalytic activity of vanadium pentoxide supported on cerium oxide

The surface acidity and basicity of vanadium pentoxide supported on CeO₂ were determined using a set of Hammett indicators. The data have been correlated with the catalytic activity of these oxides towards liquid phase acylation of phenol.     

Ethylene polymerization with a supported vanadium catalyst obtained by the molecular deposition method

An active-phase monolayer has been deposited on SiO₂ using replacement of the surface OH groups by VOCl₃ vapour. The amount of vanadium fixed on the SiO₂ surface depends on the initial concentration of the silanol groups and ranges from 3.36 to 1.43%. In combination with diethyl aluminium chloride, the products are active catalysts for ethylene polymerization. The effects of the reaction conditions (the time of catalyst-complex formation, the catalyst life time and the temperature of polymerization) as well as the effect of the vanadium content, the A1:V ratio and the presence of diphenyl magnesium on the activity of the catalyst system have been investigated. The catalyst activity was found to depend strongly on the amount of vanadium fixed on the support surface. The maximum productivity obtained is about 22,000 gPE/g vanadium. Some basic characteristics of the synthesized polymer such as tensile strength, elongation at break, density and crystallization degree are given.     

Effect of vanadium over NiMo/sepiolite hydrotreating catalyst

Summary Hydrotreating (HDT) and hydrodesulfuration (HDS) using an FCC feed were carried out at 673-748 K and 50 MPa total pressure. The effect of vanadium impregnated on a NiMo catalyst supported on sepiolite for HDT and HDS reactions was studied. A commercial NiMo/Al₂O₃ catalyst was used as reference. The hydrotreating conversions (wt.% HDT) is defined here as the net hydrotreating conversion into products boiling below 653 K. The results were compared with an accelerated ageing test using the catalysts on a FCC feed, with vanadium in the form of naphthenate (2000 ppm of V) added to rapidly deactivate catalysts via metal deposition. The results indicate that vanadium affects more the catalyst supported on sepiolite that the commercial catalyst. Also, at our reaction conditions, the effect of vanadium on sepiolite catalyst is similar, to those used with vanadium impregnated on the catalyst or on the catalyst where the vanadium in naphthenate form was added to FCC feed.     

Homopolymerization and copolymerization of vinyl chloride over supported metalorganic catalysts

The homopolymerization of vinyl chloride and its copolymerization with ethylene over dibutyl ether–modified SiO₂-supported Ziegler–Natta catalysts based on titanium and vanadium chlorides have been studied. The supported metal complexes are sufficiently active in the polymerization of vinyl chloride. Their activity depends on the catalyst composition and conditions of formation of the catalyst on the surface of the support. The chain structure of the resulting polyvinyl chloride (PVC) has been studied by NMR spectroscopy. The thermal properties of the synthesized PVC have been investigated by differential scanning calorimetry. The PVC obtained possesses enhanced thermal stability owing to the specific features of its chain structure. Vinyl chloride polymerization over the supported metalorganic catalyst proceeds mainly via a free-radical mechanism. Process conditions have been found for conducting the copolymerization of vinyl chloride with ethylene over supported metal complexes resulting in the formation of true statistical copolymers, which is confirmed by IR and NMR spectroscopy.