Effects of vanadium supported on ZrO2 and sulfolane on the synthesis of phenol by hydroxylation of benzene with oxygen and acetic acid on palladium catalyst

The catalyst system consisting of Pd, transition metal-modified ZrO₂ and acetic acid was found to catalyze the hydroxylation of benzene with molecular oxygen without hydrogen and phenol was formed. Of transition metals employed, only vanadium additive was found to be effective for improving the rate of phenol formation as well as the selectivity, while any other transition metals such as iron, molybdenym, tungsten and yttrium were not promotive. Support effects on vanadium were in the order: V/ZrO₂> V/Al₂O₃> V/SiO₂. The highest rate of phenol formation was obtained at 0.5wt%V/ZrO₂ catalyst. Phenol selectivity was dramatically improved by the addition of sulfolane, while benzene conversion and STY of phenol formation decreased. It is assumed that Pd(II) and Pd(IV) intermediates derived from acetic acid, oxygen and palladium acetate could play an important role in hydroxylation of benzene.     

One-pot Hydrothermal Synthesis of Mesoporous V-SBA-16 with a Function of the pH of the Initial Gel and its Improved Catalytic Performance for Benzene Hydroxylation

Abstract  

V-SBA-16 catalysts with uniform cubic mesoporous structure were prepared by direct hydrothermal method as a function of the pH of the initial gel and characterized by ICP, XRD, TEM, N₂ adsorption–desorption, DRUV—vis and Raman spectra. The pH of the initial gel in synthesis of V-SBA-16 show important effects on the maintenance of well ordered mesoporous structure, introduced vanadium content and the incorporation of vanadium into the network of SBA-16 type mesoporous material. The initial gel system with a pH value of 2.0 was found to be a suitable for incorporation of vanadium and retaining the mesostructure of SBA-16. The catalytic activities of V-SBA-16 catalysts were evaluated for the hydroxylation of benzene using molecular O₂ as the oxidant. The highest phenol yield of 30.4% with a selectivity of 90% and turnover number of 105 were obtained over the VS-2.0 (1.67) sample prepared at the initial gel system with pH value of 2.0, which is attributed to its high V content and uniform framework V species that highly dispersed on the well ordered SBA-16 type mesoporous materials.     

Effect of vanadia on the performance of NiO in vapor-phase oxidative decarboxylation of benzoic acid to phenol

A series of vanadia-loaded NiO catalysts was tested for the vapor-phase oxidative decarboxylation of benzoic acid to phenol at 673 K. NiO catalyzed the selective formation of phenol at low conversions of benzoic acid. However, the phenol yield declined with time on stream. Introduction of vanadium oxide to the catalyst resulted in pronounced improvements in the phenol yield and its stability, whereas V₂O₅ alone predominantly catalyzed the combustion of benzoic acid to carbon oxides. Under identical testing conditions, the highest selectivity to phenol (46.9%) was obtained with a 0.49 wt% V₂O₅ sample, while the highest phenol yield of 7.6% was obtained on a 3.7 wt% V₂O₅ sample.     

Partial oxidation of benzene catalyzed by vanadium chloride in novel reaction-extraction-regeneration system

This paper investigates the liquid-phase partial oxidation of benzene to phenol in a novel system consisting of reactor, extractor and regenerator. Since vanadium catalyst (V³⁺) is oxidized in the reactor and therefore deactivated, the regenerator with Pd or Pt catalyst and H₂ feed is employed to regenerate the deactivated vanadium. The V⁴⁺ ion can be reduced to V³⁺ and consequently the phenol production can be enhanced. Although the regenerator can regenerate vanadium catalyst and the reaction can proceed for over 100 h, some V⁴⁺ is still present. The feed position of benzene and catalyst solution have the influence on mixing condition in the reactor and interface area between benzene and catalyst solution. Counter current flow operation with the feeds of catalyst solution and benzene at the top and the bottom respectively shows the highest phenol production. The operating temperature of reactor, extractor and regenerator showed insignificant effect on phenol production rate.     

A REACTION-EXTRACTION-REGENERATION SYSTEM FOR HIGHLY SELECTIVE OXIDATION OF BENZENE TO PHENOL

The reaction-extraction-regeneration system for the liquid-phase oxidation of benzene to phenol in the benzene-water-oxygen system was investigated. Phenol was extracted in the extractor to reduce the concentration of phenol in the benzene phase. As vanadium catalyst was oxidized to inactive species after the oxidation reaction, the regenerator was installed in the system to reduce the oxidation state of vanadium catalyst from V⁴⁺ or VO²⁺ to the active V³⁺ under H₂ flow. The effects of various operating parameters including concentration of VCl₃ catalyst, O₂ and H₂ flow rates, benzene bubble size, pH, surface area of Pt regeneration catalyst, the metal species, and amount of ascorbic acid were investigated. Ascorbic acid was employed as a reducing agent for helping reduce the V⁴⁺ form to the active form and therefore improving the activity of vanadium catalyst. VCl₃ catalyst concentration of 10 mol/m³ with pH of 3–4 in the reactor and Pt surface area of 0.05 m² in the regenerator showed optimal conditions for the system.     

Direct hydroxylation of benzene to phenol on Cu–V bimetal modified HMS catalysts

A series of mesoporous Cu_x–V-HMS catalysts with different copper content were prepared by the co-synthesis method. The addition of copper improves the benzene adsorption ability of the catalyst and the redox property of vanadium species, which facilitate the benzene hydroxylation reaction. Among the catalysts studied, Cu_0.90–V-HMS exhibited the best catalytic activity with a phenol yield of 29.0% compared with 20.7% over V-HMS catalyst.     

Influence of temperature on hydroxylation of benzene to phenol using molecular oxygen catalyzed by V/SiO2

Effect of temperature on hydroxylation of benzene to phenol with molecular oxygen as an oxidant was studied over V/SiO₂ using different reductants. The V/SiO₂ with highly dispersed vanadium species was prepared by a sol–gel process and characterized by diffuse reflectance UV–Vis and ESR. This work shows that the onset temperature range of benzene hydroxylation and the temperature reaching the maximum phenol yield differ corresponding to each reductant. The reducing capacity of reductant can be shown by changing temperature and affects, in turn, benzene conversion, product selectivity, and the amount of leaching of V species.     

Room temperature direct oxidation of benzene to phenol using hydrogen peroxide in the presence of vanadium-substituted heteropolymolybdates

The liquid-phase direct catalytic oxidation of benzene to phenol was studied at room temperature using vanadium-substituted heteropolyacids as catalysts. Glacial acetic acid was employed as the solvent for the first time, while hydrogen peroxide was used as the oxidant. A yield of 26% and a selectivity of 91%, respectively, were obtained. The as-prepared phenol was separated by column chromatography and was characterized by infrared and mass spectrometry. The catalysts have been characterized by elemental analysis, thermal gravimetric analysis, infrared spectroscopy, UV–vis spectroscopy, X-ray diffraction, and ³¹P NMR and ⁵¹V NMR techniques. The effects of various reaction parameters, such as solvent, reaction temperature, reaction time and the amount of hydrogen peroxide used, were studied. The effects of different vanadium species on the catalytic performance were also studied. Glacial acetic acid was found to be the most suitable solvent among the solvents used in present work. An appropriate molar ratio of H₂O₂ to benzene of 1.7, and a favorable reaction time of 100 min were optimized. H₄PMo₁₁VO₄₀·13H₂O was found to be the most active in terms of turnover based on vanadium atom and the most stable catalyst.     

Oxidation of hydrocarbons with hydrogen peroxide catalyzed by maltolato vanadium complexes covalently bonded to silica gel

The bis(maltolato)oxo complexes of vanadium(V) and vanadium(IV) both in a soluble form and anchored to chemically modified silica gel, when used in combination with pyrazine-2-carboxylic acid (PCA), catalyze the oxidation of benzene to phenol and alkanes to the corresponding alkyl hydroperoxides with H₂O₂ at 40–50 °C. Heterogenizing the complexes leads to changes in regioselectivity and bond selectivity parameters of the alkane oxidation, and this opens new routes to the enhancement of the selectivity in oxidation processes.     

Naphthalene oxidation over vanadium-modified Pt catalysts supported on γ-Al2O3

Alumina-supported platinum catalysts modified by vanadium were synthesised and tested for the complete oxidation of naphthalene. The catalysts were characterised by BET, pulsed CO chemisorption, powder X-ray diffraction, laser Raman spectroscopy and temperature-programmed reduction. Whilst BET and CO chemisorption results showed that the addition of vanadium modified both the textural properties of the support and the distribution of Pt, XRD and TPR data suggested the presence of V₂O₅ on catalysts with higher V loadings. TPR data showed that the concentration of V₂O₅ and possibly some other vanadium species increased as the V loading increased. Only 0.5%V was found to promote the activity of the 0.5%Pt/γ-Al₂O₃ catalyst. The activity enhancement has been related to the presence of a more easily reducible vanadium species coupled with the enhanced number of surface Pt sites. On the other hand, the reduced activity demonstrated by catalysts with higher vanadium content (1 – 12%) has been attributed to the presence of crystalline V₂O₅.     

Preparation of high-quality FeV50 alloy by an improved SHS-EAH multi-stage process

As the most widely used vanadium intermediate alloy, ferrovanadium is mainly prepared by the self-propagating high-temperature synthesis (SHS) process with the advantages of short process and large heat release. However, there is a trade-off between the amount of reduction agent of Al and the product quality of ferrovanadium, which is caused by the thermodynamic equilibrium limitation of the reduction between Al and vanadium oxides. The reasonable adjustment of the trade-off has become an urgent problem to be solved for the preparation of high-quality ferrovanadium. In the present work, the thermodynamic equilibrium for the reduction limitation using Al as reducing agent has been thoroughly analyzed with the reference to the equilibrium phase diagram. It was found that the theoretical reduction limitation of total vanadium (T.V) in slag was decreased from 4.08 wt.% to wt. 0.86 wt.% when the Al content in alloy reached 2.0 wt.% at 1800 °C. The vanadium in slag was mainly presented in the form of MgV₃O₈ and coexisting with other phases of MgAl₂O₄, CaAl₂O₄, CaAl₄O₇, CaAl₁₂O₁₉. With the purpose of further decrease the high Al concentration in alloy and vanadium loss in slag, an improved SHS-EAH process was implemented by multi-stage gradient aluminum addition coefficient pattern, the average vanadium content in slag was reduced from 1.82 wt.% to 0.83 wt.%, while the aluminum content in FeV50 alloy was decreased from 1.5 wt.% to 0.4 wt.%. The SHS-EAH process was proved to be a better process with improved yield and quality for the preparation of ferrovanadium.     

Adsorption and Diffusion of VO2+ and VO2 + across Cation Membrane for All‐Vanadium Redox Flow Battery

A method based on a selectivity coefficient and the Nernst‐Planck equation is proposed to determine diffusion coefficients of vanadium ions across a cation exchange membrane in VO²⁺/H⁺ and VO₂ ⁺/H⁺ systems. This simplified method can be applied to high concentrations of vanadium ions. Three cation exchange membranes were studied. The logarithmic value of the selectivity coefficient was linearly dependent on the molar fraction of vanadium ions in solution. The diffusion coefficient of vanadium ions decreased with decreasing water content. The membrane with the lowest diffusion coefficient was selected as a battery separator and showed the lowest capacity loss of the studied membranes.     

Raman spectroscopy studies of concentrated vanadium redox battery positive electrolytes

Spectroscopic changes in highly concentrated vanadium(V)-sulfate solutions to be used in the vanadium redox battery are consistent with the presence of more than one V(V)-sulfate species. The results of Raman spectroscopy indicate that the major species in highly acidic conditions are VO₂SO₄ ^–, VO₂(SO₄)₂ ^3–, VO₂(HSO₄)₂ ^–, VO₃ ^–, V(V) dimers with V₂O₃ ⁴⁺ and V₂O₄ ²⁺ central units. The nature and amount of these species depends upon the V(V) and total sulfate concentrations as well as on S to V and H⁺ to V ratios in the positive half-cell electrolyte. V(V) forms V₂O₃ ⁴⁺, VO₂(SO₄)₂ ^3– and their copolymer species at higher total sulfate concentrations, which tends to stabilize the vanadium (V) positive electrolyte in the vanadium redox battery. The V(V) and V(IV) species show the least interaction with each other. Ageing of concentrated V(V) solutions at elevated temperature (50 °C) produces decomposition of species causing formation of V₂O₅ precipitates with a decrease in the amount of vanadium polymer.     

Effect of V2O5 Loading on Propane Combustion over Pt/V2O5–Al2O3 Catalysts

Propane combustion was studied on Pt(0.4%)/V₂O₅–Al₂O₃ catalysts containing up to 20% V₂O₅. The density, strength and nature of surface acid sites were determined by TPD of NH₃ and FTIR spectra of adsorbed pyridine. The sample acidity increased with the vanadium content, essentially because the addition of vanadium oxide generated Brønsted acid sites. The Pt dispersion as determined by H₂ chemisorption increased with increasing V₂O₅ loading. The sample activity for propane combustion was evaluated through both conversion versus temperature (light-off curves) and kinetically-controlled conversion versus time catalytic tests. The propane combustion turnover rate on Pt/V₂O₅–Al₂O₃ increased with the amount of vanadium, probably because the intrinsic Pt oxidation activity increases with the sample acidity.     

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).     

Direct incorporation of vanadium into three-dimensional KIT-6: 1. Optimization of synthesis conditions

Three dimensional (3-D) cubic KIT-6 with directly incorporated vanadium was hydrothermally synthesized by using Pluronic P123 and n-butanol as the structure-directing mixture, tetraethyl orthosilicate (TEOS) as the silica source and NH₄VO₃ as the vanadium source. The molar composition was varied in the range of 0.017 P123/0.08–2.4 V/1.0–2.0 TEOS/1.31–1.70 BuOH/1.83–3.00 HCl/195 H₂O. The orderness of mesopore structure was estimated by X-ray diffraction, N₂ adsorption, and TEM analysis. The effects of the amount of HCl, TEOS and BuOH on the structure of KIT-6 were discussed. The time and temperature for the synthesis of KIT-6 were also optimized. The amount of vanadium content influenced the framework structure and crystallinity of the Ia3d phase significantly.     

Synthesis,characterization and catalytic properties of two novel vanado-aluminosilicates with EU-1 and ZSM-22 structures

The synthesis in the presence of alkali ions of two novel vanadium containing zeolites V-Al-EU-1 and V-Al-ZSM-22 is reported. Both V⁴⁺- and V⁵⁺-ions are present in as-synthesized samples. Cyclic voltammograms of the samples reveal the presence of two types of V⁵⁺ in the calcined samples. A weak shoulder at 980 cm^–1 is observed in the IR spectra of the calcined samples. On extraction with acid a sharp band appears at 960 cm^–1. The acid washed samples are more active in the hydroxylation of phenol and the oxidation of toluene (with H₂O₂) than the calcined samples.     

Deactivation of VMgO x Catalysts by Coke in the Process of Isobutane Dehydrogenation with Carbon Dioxide

The process of isobutane dehydrogenation in the presence or absence of carbon dioxide was carried out over VMgO _x catalysts with different vanadium loading. The performed tests show that both the reaction atmosphere and physicochemical properties of the catalysts (related to vanadium content) have a great influence on the activity decrease and the carbonaceous deposit formation. Despite small ability of carbon dioxide to remove coke in the Boudouard reaction, the amounts of carbonaceous species deposited on the catalysts after the isobutane dehydrogenation under CO₂ atmosphere were even twice greater in comparison to those deposited in helium stream. Moreover, the rate of coke deposition during the dehydrogenation in the inert gas flow was only slightly dependent on the reaction time, in contrast to the process in carbon dioxide atmosphere. The results show that the coke formation on VMgO _x is enhanced predominantly by surface acidity of the catalysts, which grows with the vanadium content and the presence of CO₂ in the feed.     

Characterization and activity of V2O5/CeO2-MgO catalyst in the dehydrogenation of ethylbenzene to styrene

Supported vanadia catalyst was impregnated on CeO₂-MgO and characterized by N₂ adsorption, XRD, XPS, TPR and solid NMR. The vanadia species dispersed well on the support surface with vanadia content up to 10 wt%. At higher vanadia content, the non-active Mg₃(VO₄)₂ was formed. Vanadium oxides existing on the support surface as tetrahedral vanadium were observed. An ethylbenzene conversion of 43% and styrene selectivity of 91% were obtained with the 10V₂O₅/CeO₂-MgO catalyst.     

Synthesis of V-containing Keggin polyoxometalates: Versatile catalysts for the synthesis of fine chemicals?

Vanadium-containing polyoxometalates (POM): H₄PVMo₁₁O₄₀, H₅PV₂Mo₁₀O₄₀ and H₆PV₃Mo₉O₄₀ were synthesized by two distinct procedures and characterised by XRD, ³¹P MAS-NMR, FT-IR, BET, UV–vis and SEM. These vanadium-substituted POM were shown to be able to oxidize benzene into phenol with nearly 100% selectivity in the presence of H₂O₂. Furthermore, other aromatics were also oxidised to the corresponding phenols or ketones at appreciable degrees of conversion. However, the question of their stability toward leaching arises and has been investigated. While, there appears to be a correlation for liquid-phase reactions between the yields toward oxygenates and the vanadium content in the Keggin structure, we were also able to observe a decrease in the stability of these POM after successive catalytic runs. Furthermore, POM crystals having a regular size and shape, with peculiar orientation have been prepared and hence exhibited the highest catalytic activity.