Bismuth molybdate catalyst multicomponent

Oxidation in the original Sohio process941,942 was carried out over a bismuth molybdate catalyst, which was later superseded by bismuth phosphomolybdate with various amounts of additional metal ions (Ce, Co, Ni), and multicomponent metal oxides based on Mo, Fe, and Bi supported on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

Multicomponent Bismuth Molybdate Catalyst A Highly Functionalized Catalyst System for the Selective Oxidation of Olefin... 

Multicomponent bismuth molybdate catalyst. h Heteropoly compound. 

In spite of the accumulated mechanistic investigations, it still seems difficult to explain why multicomponent bismuth molybdate catalysts show much better performances in both the oxidation and the ammoxidation of propylene and isobutylene. The catalytic activity has been increased almost 100 times compared to the simple binary oxide catalysts to result in the lowering of the reaction temperatures 60 80°C. The selectivities to the partially oxidized products have been also improved remarkably, corresponding to the improvements of the catalyst composition and reaction conditions. The reaction mechanism shown in Figs. 1 and 2 have been partly examined on the multicomponent bismuth molybdate catalysts. However, there has been no evidence to suggest different mechanisms on the multicomponent bismuth molybdate catalysts. 

II. Characterization and Working Mechanism of Multicomponent Bismuth Molybdate Catalysts... 

Typical Reaction Conditions for the Oxidation and Ammoxidation of Propylene on the Simple and Multicomponent Bismuth Molybdate Catalyst°... 

One typical way to improve the catalyst system was directed at the multi-component bismuth molybdate catalyst having scheelite structure (85), where metal cations other than molybdenum and bismuth usually have ionic radii larger than 0.9 A. It is important that the a-phase of bismuth molybdate has a distorted scheelite structure. Thus, metal molybdates of third and fourth metal elements having scheelite structure easily form mixed-metal scheelite crystals or solid solution with the a-phase of bismuth molybdates. Thus, the catalyst structure of the scheelite-type multicomponent bismuth molybdate is rather simple and composed of a single phase or double phases including many lattice vacancies. On the other hand, another type of multi-component bismuth molybdate is composed mainly of the metal cation additives having ionic radii smaller than 0.8 A. Different from the scheelite-type multicomponent bismuth molybdates, the latter catalyst system is never composed of a simple phase but is made up of many kinds of different crys-... 

Multicomponent Bismuth Molybdate Catalysts with Scheelite Structure... 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

Fig. 5. Composition of multicomponent bismuth molybdate catalyst having multiphase structure.

Molybdenum comprises usually 50% or a little more of the total metallic elements. Most of molybdenum atoms form (Mo04)2 anion and make metal molybdates with other metallic elements. Sometimes a little more than the stoichiometric amount of molybdenum to form metal molybdate is included, forming free molybdenum trioxide. Since small amounts of molybdenum are sublimed continuously from the catalyst system under the working conditions, free molybdenum trioxide is important in supplying the molybdenum element to the active catalyst system, especially in the industrial catalyst system. In contrast, bismuth occupies a smaller proportion, forming bismuth molybdates for the active site of the reaction, and too much bismuth decreases catalytic activity somewhat. The roles of alkali metal and two other additives are very complicated. Unfortunately, few reports refer to these elements, except patents. In this article, discussion is directed only at the fundamental structure of the multicomponent bismuth molybdate catalyst system with multiphase in the following paragraphs. 

Choosing divalent and trivalent cations and determining the composition is the most important in designing the multicomponent bismuth molybdate catalyst system. Catalytic activities of typical tri- and tetracomponent bismuth molybdate catalysts having multiphase structure were reported for the oxidation of propylene to form acrolein (35, 36, 40-43, 97, 98). A typical example of the activity test is shown in Fig. 6. Summarizing the results shown in Fig. 6 and reported previously (30, 43, 44), the following trends are generally found. 

Fig. 6. Catalytic activity of multicomponent bismuth molybdate catalysts in the oxidation of propylene (41).

Oxidation of propylene to form acrolein depends on the first order of propylene and is independent of oxygen on multicomponent bismuth molybdate catalysts under the usual reaction conditions. The observed kinetics is the same with simple bismuth molybdates and suggests that the oxidation of propylene proceeds via the similar reaction scheme reported for simple molybdates, the slow step being the abstraction of allylic hydrogen (9-15, 19, 20). However, the reaction sometimes depends on the partial pressure of oxygen under lower temperature and lower oxygen pressure (41, 42). 

Fig. 9. Arrhenius plots of the oxidation of propylene over various multicomponent bismuth molybdate catalysts.

Fig. 11. Comparison of the amount of l6Oi2a7,ice incorporated into the oxidation products over various multicomponent bismuth molybdate catalysts. Open columns, amount of whole 16Oia iCC in the catalyst shaded columns, amount of l6Oiaurce in the Bi2MoiO,2 phase solid columns, total amount of l60 incorporated into the oxidation products , oxygen conversion was 80% and the others were 60%. (a) Tricomponent system, Mo-Bi-M(II)-0, and tetra-component system, Mo-Bi-M(Il)-M (II)-) without M(III). (b) Tetracomponent system, Mo-Bi-M(II)-M(III)-0 (41).

Significant progress has been made in explaining the prominent catalytic performances of the multicomponent bismuth molybdate catalysts. Summing up the progress, we have seen the following ... 

The lower activation energy of the multicomponent bismuth molybdate catalyst suggests that some structural or electronic modification of the active component, Bi2(Mo04)3, is given by M(II) and M(III) molybdates, which also contribute to the enlargement of the surface area of bismuth molybdate. 

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