Bismuth molybdate catalyst specific activity

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. 

In conclusion, to make an excellent catalyst system, it is important to activate bismuth molybdate by both the divalent and trivalent metal cations with ionic radii smaller than 0.8 A at the same time. A part of the increasing activity of the Mo-Bi-M(II)-M(III)-0 system compared to the pure bismuth molybdate comes from the increase in surface area and the remains arise from the increase in specific activity. Semiquantitative evaluations of tri- and tetracomponent bismuth molybdates are listed in Table V in comparison with simple bismuth molybdate catalyst. 

Some remarks must be made about the role of oxygen coordination. Several authors have remarked that the coordination in catalytic oxides is of major importance. Mitchell and Trifiro (e.g. ref. 219) concluded that a bismuth molybdate catalyst is most active if the amount of tetrahedrally coordinated molybdenum is large in comparison with octahedrally coordinated molybdenum. However, V205 and Sb2Os are structures with specific octahedral coordination [142] and often the coordination is changed by reduction of the catalyst or by the support [203]. In a- and /3-cobalt molybdates the coordination differs, but the catalytic behaviour is really the same. The low temperature Bi2Mo06 (7 phase) has an octahedral coordination but is an effective catalyst. 

The specific activity of pure bismuth molybdate, a-phase Bi2(Mo04)3 or y-phase Bi2Mo06, is fairly high. However, owing to its low surface area, the activity per unit weight of the catalyst is not so prominent. 

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. 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

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