Vanadium heteropolyacids

In phenol hydroxylation, remarkable selectivities to single products have been achieved using vanadium heteropolyacid catalysts.485 The use of the ZSM-5 titanium silicalite (TS-1)483 permits the oxidation of phenol to catechol and hydroquinone to be carried out on an industrial scale with a higher selectivity at a greater conversion of substrate that was not previously possible with strong acid catalysts. 

Herve et al. (57) investigated the thermal changes of structures by means of XRD and TG-DTA for Keggin-type heteropolyacids and proposed Scheme 2. Infrared spectroscopy of H4PMo, VO40 showed the release of vanadium atoms to form H3PM012O40 and vanadium phosphate species (55). Exposure to water vapor induces the decomposition of the latter (indicated by the disappearance of a band at ca. 1037-1030 cm -1) (58). 

One of the methods to synthesize heteropolyacids is the direct interaction of the oxides of ligand atoms (tungsten, molybdenum, vanadium) with oxides or acids of heteroatoms (phosphoms, silicon, etc.). However, because of low reactivity of oxides, these processes are slow and energy-consuming (heating for tens hours is required) quantitative yield is rarely achieved. 

Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield ftom isobutyric acid was successfully enhanced by the stabilization of the vanadium-substituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (V0)2P207 was reversibly oxidized to the Xi (8) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12-molybdates in solution showed 93% conversion on H2O2 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. 

Dehydrogenation to methyl methacrylate of the isobotyrate produced by the carbonyiation of propylene in the presence of methanol and a complex of rhodium, cobalt or palladium. The isobutyrate is converted with catalysts based on molybdenum, tungsten, vanadium, phosphomoiybdic acids and possibly heteropolyacids. It has been developed chiefly by Asahi, BASF, Eastman, Mitsubishi, Mobil, foray, etc. 

Heteropolysalts with composition I (NH4)3-xPMoi2O40, prepared hy precipitation, showed a good catalytic activity and selectivity in IBA oxydehydrogenation over the short-term. Obtained results were comparable to the best ones reported in literature for heteropolyacids containing vanadium atoms. The catal rtic performance was relatively stable even after tretaments that led to partial structural decompositions. The cationic composition affected the catalytic performance the best results were obtained with the (NH4)3PMoi2O40 sample, characterized by a low specific surface area. 

Red-ox type catalysts are mostly used in oxidation or related types of reactions. For instance, vanadium catalysts containing ions of different valence state are used in the oxidation of benzene to maleic anhydride. Bismuth molybdate catalyst can be used both for the oxidation or ammoxidation of propene. Anchored metal-complex catalysts combine the advantage of both homogeneous and heterogeneous catalysts, however in these catalysts the molecular character of the active sites is maintained. In the last generation of this type of catalysts, heteropolyacids are fixed first to the support and in the second step different metal-complexes are anchored to the heteropolyacid. In this way highly active and stable catalyst have been prepared for different reactions. ... 

Molybdivanadophosphate Phosphate concentration can be determined by reaction of the phosphate ions with vanadium molybdenum heteropolyacid (without reduction). 

The oxidative bromination of Cs-Cg olefins can be carried out using a system composed of catalytic thallium(iii) oxide (0.2 M of Tl(iii)) and a vanadium-based heteropolyacid cocatalyst (HPA-n = H3+ PMOi2 V 04o n = 2-8) in the presence of hydrogen bromide solution (1.0 M of Br ) and oxygen as co-oxidant. The HPA-n cocatalyst can promote oxidation of thal-lium(i) to thallium(iii) in the presence of bromide ions. For instance, the reaction of 1-hexene with the thallium(iii)/HPA-6/bromide system at 100 °C and 8 bar of oxygen led to a mixture of 1,2-dibromohexane and hex-enebromohydrin along with a small amount of 2-hexanone in different ratios. In the absence of thallium(m) species, the oxidation process proceeds slowly, presumably due to the HPA-6 anion-mediated oxidation of bromide to bromine. 

Ai, M. (1996). Comparison of Catalytic Properties for Partial Oxidation between Heteropolyacids and Phosphates of Vanadium and Iron, J. Mol Catal. A Chem., 114, pp. 3-13. 

Dong X, Wang D, Li K, Zhen Y, Hu H, Xue G. Vanadium-substituted heteropolyacids immobilized on amine-functionalized mesoporous MCM-41 a recyclable catalyst for selective oxidation of alcohols with H2O2. Mater Res Bull. 2014 57 210-220. 

This technology is still under development. It is carried out essentially on Fe-or Mo-modified, zeolites. Very few investigators reported their work on vanadium and heteropolyacid based catalysts. Investigations using CaO catalysts also appear in the literature. The monomeric and dimeric ferric ions were found to be active in ODH of ethane with nitrous oxide giving a product selectivity of 40-60% at 350°C on Fe-modified zeolites (ferrierite and MFI). On the other hand, Fe oxide nanoclusters resulted in overoxidation of ethane and/or ethylene to C, CO and CO2 [88]. Iron modified zeolites of different structures viz., ZSM-5, zeolite Y and mordenite were tested in the ethane and propane ODH. The nature of zeolite dictated the catalytic activity. The best... 

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