Bismuth molybdate catalyst metal addition

Modem day bismuth molybdate catalysts, in addition to A, —3.V i2.v hio04 phases, contain other compounds such as Bi2Mo06 and Bi2M02O9 as well as small amounts transition metal molybdates that not only increase conversion rates and selectivity but also increase catalyst hfetime and allow operation at lower temperatures. ... 

Oxidation in the original Sohio process941,942 was carried out over a bismuth molybdate catalyst, which was later superseded by bismuth phosphomolybdate with various amounts of additional metal ions (Ce, Co, Ni), and multicomponent metal oxides based on Mo, Fe, and Bi supported on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

The improvement of bismuth molybdate catalyst by the addition of various kinds of metal elements has been continued after the establishment of... 

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-... 

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. 

At this stage, it is still difficult to determine whether the conclusion is appropriate for the fundamental part of the multicomponent bismuth molybdate catalyst. Unfortunately, we have no available information on the number of active reaction sites on the catalyst system. In the heterogeneous catalysis, apparent activation energy does not necessarily correspond to the real energy barrier of the elementary slow step of the reaction. Multicomponent bismuth molybdate catalyst has been established industrially, whereas only parts of the fundamental structure and working mechanism have been elucidated. In addition, important roles of alkali metals and other additives such as lanthanides remain unknown. Apparently, further investigations should be done to clarify the complete working mechanism of the multicomponent bismuth molybdate catalyst. 

Propene oxidation to acrolein is carried out commercially over a range of bismuth molybdate catalysts to which are added 3-4 additional metal oxides to boost the activity. The final catalysts are mixtures of binary and ternary oxides and some solid solutions. One feature is the ability of lattice oxygen to transfer readily at the reaction temperature between the multiple phases that make up this catalyst and to the reacting propene. Another key feature is that the initial point of activation of the propene is one of the methyl C-H bonds with the production of a surface allyl intermediate, hence the term allylic oxidation. 

The addition of divalent metal cation, M(II) with ionic radius smaller than 0.8 A (Ni2+, Co2+, Fe2+, Mg2+, Mn2+, etc.), to the pure bismuth molybdate increases the specific surface area of the catalyst system, but the specific activity of the tricomponent system, Mo-Bi-M(II)-0 never exceeds that of pure bismuth molybdate. 

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