Vanadium heteropoly catalysts

Heteropoly catalysts have significant activities for the oxidation of isobutane into methacrolein and methacrylic acid. The yield increased up to 6% by vanadium substitution or salt formation, as follows. With Cs2.5Ni0.08H0.34+JrPV,Mo12 - O40, the highest conversion and selectivity were observed at x 1 (355). Increases in the reaction temperature to 613 K led to increased yields, up to 9.0%. A similar increase in the yield resulted from the substitution of As for P as a heteroatom or from the addition of various transition metals (106, 356). 

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. 

There are few examples of ketone oxidation. Copper(II) and its amine complexes are useful catalysts and vanadium heteropoly acids have been employed. Cyclohexanone can be converted to the 1,2-dione, which is cleaved and dioxygenated to adipic acid. 

Under microwave irradiation and applying MCM-41-immobilized nano-iron oxide higher activity is observed [148]. In this case also, primary aliphatic alcohols could be oxidized. The TON for the selective oxidation of 1-octanol to 1-octanal reached to 46 with 99% selectivity. Hou and coworkers reported in 2006 an iron coordination polymer [Fe(fcz)2Cl2]-2CH30H with fez = l-(2,4-difluorophenyl)-l,l-bis[(l//-l,2,4-triazol-l-yl)methyl]ethanol which catalyzed the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide as oxidant in 87% yield and up to 100% selectivity [149]. An alternative approach is based on the use of heteropoly acids, whereby the incorporation of vanadium and iron into a molybdo-phosphoric acid catalyst led to high yields for the oxidation of various alcohols (up to 94%) with molecular oxygen [150]. 

P oly 0 xome tall ate s, derived from both isopoly adds and heteropoly adds, are important homogeneous oxidation catalysts. The metals involved are vanadium, niobium, tantalum, molybdenum, and tungsten. The reactions involved are the oxidation of a wide range of organic compounds by hydrogen peroxide or organic hydroperoxide. 

Fructose, one of the most common ketohexoses, readily dehydrates to afford HMF in the presence of Br0nsted acids in polar solvents. A variety of aprotic polar solvents, including DMSO, DMF, N,N-dimethylacetamide (DMA), and sulfolane, are used for these liquid-phase reaction because of the solubility of carbohydrates. A variety of solid acids, such as ion-exchange resins [156], zeolites [157, 158], metal oxides, and heteropoly acid salts, have been examined for HMF production from fructose [159,160]. Niobic acid, niobium phosphate, vanadium phosphate, sulfated zirconia, Amberlyst-15, and acid-functionalized mesoporous silicas are also found to exhibit high catalytic activity for fructose dehydration [161-167]. Moreover, soHd acid catalysts have also been examined in ionic liquids [168-175]. 

Ge, H., Leng, Y, Zhou, C., et al. (2008). Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Phase Transfer Catalysts Cyclodextrins Complexes with Vanadium-Substituted Heteropoly Acids, Catal. Lett., 124, pp. 324—329. 

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