Supported multicomponent molybdates

Oxidation in the original Sohio process941,942 was carried out over a bismuth molybdate catalyst, which was later superseded by bismuth phosphomolybdate with various amounts of additional metal ions (Ce, Co, Ni), and multicomponent metal oxides based on Mo, Fe, and Bi supported on silica. 

More complex Bi-Mo oxide catalysts are claimed in patents, such as Bi-Mo-M +-M +-M+-X-Y-0, where M+ is an alkali metal, X=Sb, W, and Y=P, B, and so on. In multicomponent catalysts, the metal molybdates M +MoOd and M2 +(Mo04)3 serve as supports for the active phase, a -Bi2M03Oi2. Bulk migration of ions through lattice vacancies plays an important role in enhancing catalytic... 

Wolfs and Batist [121] have proposed a structure for these oxide mixtures comprising a core of Me Mo04 and Me 2(MoO)4 encapsulated inside a thin shell of bismuth molybdate. This model has been supported by recent transmission electron microscopy analysis in which a cross-section of a multicomponent bismuth molybdate catalyst was shown to comprise a surface layer of Bi2Mo30i2 supported on and encapsulating a core of Con/i2Fei/i2MoOx [107]. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

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