Alumina-supported catalysts for propane oxidative dehydrogenation from mixed VXM(6−X)O19 (M=W, Mo) hexametalate precursors

γ-Al₂O₃ supported vanadium oxides were modified by tungsten and molybdenum oxides in order to improve dispersion and selectivity towards olefins in propane oxidative dehydrogenation (ODH). Both vanadium–tungsten and vanadium–molybdenum catalysts were obtained by adsorption of mixed isopolyanions (VW₅O₁₉⁵⁻, V₂W₄O₁₉⁴⁻, VMo₅O₁₉⁵⁻ and V₂Mo₄O₁₉⁴⁻) from aqueous solutions. The isopolyanion solutions were characterized by UV-Vis and ⁵¹V NMR spectroscopy. Vanadium, vanadium–tungsten and vanadium–molybdenum precursors and catalysts were also characterized by UV-Vis (diffuse reflectance) and solid state ⁵¹V NMR spectroscopy. An improved selectivity to propene in the presence of tungsten and molybdenum in VO_x/γ-Al₂O₃ was observed and attributed to dilution of vanadium by tungsten or molybdenum oxides on the γ-Al₂O₃ surface.     

Preparation, Characterisation and Catalytic Behaviour of a New TeVMoO Crystalline Phase

TeM_xMo_1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N₂. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb₄Mo₁₀O₃₁ and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.     

Immobilization and activation of vanadium(III) and titanium(III) single‐site catalysts for ethylene polymerization using MgCl2‐based supports

Significant increases in the activity of vanadium(III) amidinate catalysts for ethylene polymerization have been obtained by immobilization on a MgCl₂‐based support prepared by reaction of AlEt₃ with a MgCl₂/ethanol adduct. Catalyst immobilization and activation on this type of support prevents the rapid decay in activity observed under homogeneous polymerization conditions with unsupported catalysts. Stable polymerization activity is also observed with analogous titanium(III) complexes. Polyethylene with narrow molecular weight distribution and spherical particle morphology is obtained without reactor fouling. Copyright © 2005 Society of Chemical Industry     

Gas-phase selective oxidation of toluene on TiO2–sepiolite supported vanadium oxides: Influence of vanadium loading on conversion and product selectivities

Gas-phase selective oxidation of toluene has been carried out on vanadium oxide systems (5–20 wt.% of V₂O₅, equivalent to 0.4–1.7 theoretical monolayers) supported on TiO₂–sepiolite (with titania loading around the theoretical monolayer, 12 wt.%) and on sepiolite. A study has been made on both the influence of vanadia loading and of the support on the catalytic behaviour of the supported vanadium systems. The reducibility by H₂ TPR was also studied as well as the acid and basic/redox sites from the results of conversion of the 2-propanol test reaction of the solids. Benzaldehyde, benzoic acid and several coupling products were the main ones detected, attaining over 50% selectivity towards the benzaldehyde and benzoic acid products at a total conversion around 10%. The activity and selectivity to the selective products exhibited by vanadium systems supported on mixed support were superior to those exhibited by the systems supported on sepiolite and increased notably in both series with the increase in vanadium loading. The best catalytic behaviour exhibited by the vanadium systems supported on mixed support, which also exhibited the highest density of sites capable of being reduced (as well as their reducibility) and of those responsible for propanone formation, could be attributed not only to the different balance of the vanadia species existing in the two supports (monomeric + oligomeric/polymeric), but also to such other factors as the nature of the support and, concretely, its chemical composition.     

Preparation and characterisation of vanadium catalysts supported over alumina-pillared clays

Supported vanadium-containing catalysts were prepared by impregnation of Al-pillared clays with NaVO₃ precursor aqueous solutions. A montmorillonite and a saponite, both natural as well as previously pillared with Al₁₃-Keggin polycations, were used as supports, being impregnated with solutions of the precursor by means of the incipient wetness technique. The layered structure of the supports is maintained after impregnation, and even after calcination of the impregnated solids at 500 °C, although with a significant worsening of the textural properties of the solids, in particular, a loss of specific surface area. Three crystalline vanadium-containing phases were identified in the impregnated solids, namely, NaV₃O₈, AlVO₄ and V₂O₅ (shcherbinaite). The reduction of these phases to V³⁺ species was observed as wide processes in the 450–750 °C range, all of them centred at ca. 600 °C.     

利用正交试验确定钒渣焙烧的最佳工艺条件

介绍了钒分厂利用正交试验法进行试验方案的设计及试验结果的直观分析，确定出钒渣烙烧的最佳工艺条件是：钒渣粒径小于0．12mm，烙烧气中氧气体积分数大于10％，焙烧温度850℃，烙烧时间60min，碱比(碱与钒渣质量比)16％，盐比(盐与钒渣质量比)5％，从而改变了以往在生产和转渣时组织生产的被动局面，为公司创造了可观的经济与社会效益。     

真空碳热还原法制备高密度碳化钒

研究了采用真空碳热还原法由三氧化二钒制备碳化钒，并研究了碳化钒产物密度随实验条件的变化规律。找到了用于强化碳化钒产物密度的添加剂，研究结果表明，反应温度，添加剂是影响碳化钒产物密度的主要因素。反应时间对产物密度也有一定影响。研究结果同时表明，含铁化合物能有效地提高产物密度。     

Quantum chemical SINDO1 study of vanadium pentoxide

An improved set of parameters for vanadium in the semiempirical quantum chemical SCF MO method SINDO1 is presented. It is shown that both the geometries and heats of formation of a number of vanadium-containing compounds calculated by this method are in good agreement with available experimental data. Model clusters of increasing size are used for the study of geometric and energetic properties of vanadium pentoxide. Both hydrogen atom and proton adsorption on the (010) surface of vanadium pentoxide and a subsequent formation of different oxygen vacancies have been investigated. Based on these computational results the reactivities of V₂O₅-surface oxygen atoms for adsorption are discussed.     

含钒铁水中硅,钛含量对炼钢,炼铁工序指标的影响

利用生产实践数据，分析了铁水中Ｓｉ＋Ｔｉ含理对炼铁焦比、产量、生铁Ｓ以及提钒时的钒回收率、钒渣品位、白灰消耗等工序指标的影响，提出了适合于承钢铁水的Ｓｉ＋Ｔｉ含量。     

Vanadium species in new catalysts for the selective oxidation of methane to formaldehyde: Activation of the catalytic sites

New vanadium oxide supported on mesoporous silica catalysts for the oxidation of methane to formaldehyde were investigated by infrared and Raman spectroscopies to identify and characterize the molecular structure of the most active and selective catalytic sites. In situ and operando experiments have been conducted in order to understand the redox and hydroxylation/dehydroxylation processes of the vanadium species. (SiO)₂VO(OH) species were identified in these catalysts in reaction conditions and shown to undergo a deprotonation at 580 °C under vacuum, leading to a site giving a photoluminescence band at 550 nm attributed to reverse radiative decay from the excited triplet state:

(V⁴⁺–O⁻)^*  (V⁵⁺O²⁻). An activation mechanism of vanadium monomeric species with electrophilic oxygen species is proposed.