Bismuth molybdate active site

Bismuth iron molybdate, 27 207-209 X-ray diffraction, TlilW Bismuth molybdate, 27 184-187, 189, 191-194, 196, 199--204, 30 124-125 active site, 27 210-213 alumina supported, 27 203, 204 ammoxidation, 30 159 P phase, 27 201 catalyst... 

Finally, Arrhenius treatments of the catalytic data were examined for the HTAD synthesized substitutional series, Bi(2-2x) 2x 030i2, and the binary bismuth molybdate series where Bi/Mo ratios were varied fi-om pure Mo oxide to pure Bi oxide. The noteworthy aspect of the oxidation results is that in the most reactive regime of x = 0-5% atom fi-action Fe, before separate phase Fe3Mo30j2 begins to dominate the catalyst composition in the iron series, the apparent activation energies were all in the range of 19-20 kcal/mol. Furthermore, the activation energies for the pure Bi-Mo series were between 19-20 kcal/mol while the activities were considerable different. Thus, the chief difference in the reactivities in both series is in the preexponential factor, i.e. the number of active sites. 

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. 

Many studies have been devoted to the clarification of the selective oxidation mechanism and the nature of the active sites. In the following, the known bismuth molybdate phases and their significance will first be briefly reviewed, followed by a discussion of the mechanism and kinetics. 

Investigating a bismuth molybdate catalyst with Bi Mo = 1 oxidizing pentenes to isoprene, Watanabe and Echigoya [344] found that isomers of pentenes were less reactive than those of the corresponding butenes in flow experiments, but the reverse was true in pulse experiments. Heat of adsorption measurements make it clear that the active sites are not uni-... 

The occurrence of an almost constant, albeit rather low, activity level, which is reached after a number of pulses, signifies that a certain quasiequilibrium concentration of active sites is maintained by transport of bulk oxygen anions to the surface. Such a mobility of oxygen is particularly observed for bismuth molybdates and some related catalysts (see below). Typical examples of catalysts which completely loose their activity at a low degree of reduction are the antimonates this is primarily caused by the absence of anion mobility. 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

These observations suggest a reaction scheme for bismuth molybdate catalysts where the allylic species is formed initially at a bismuth center and then reacts further at a molybdenum site to produce acrolein. Thus, once the allylic complex is formed, the MoO polyhedra are highly active and selective for acrolein formation. This hypothesis was tested by investigating the oxidation of bromoallyl (C3HjsBr) over molybdenum oxide 116). Since the C—Br bond in bromoallyl is much weaker than the C—H bond in propylene, the ease of formation of the allylic species should be significantly enhanced with bromoallyl compared with propylene. If the initial propylene activation occurs on bismuth, then the reaction of bromoallyl over molybdenum oxide should approach the activity and selectivity of propylene over bismuth molybdate. This was the observed result, and the authors concluded that the bismuth site was responsible for the formation of the allylic intermediate. 

A somewhat different picture emerges from the adsorption studies of Matsuura and Schuit 117). They have attempted to elucidate the active adsorption sites on -y-bismuth molybdate by measuring the adsorption equilibria of butene, propylene, oxygen, water, butadiene, acrolein, and... 

Another communication has presented a comparison of the adsorption properties of the a, /3, and y phases of bismuth molybdate (120). The / phase exhibited two types of adsorption a slow, activated type for butadiene and a fast, weak type for butadiene and butene. The two types of adsorption were similar, but not completely identical, to the A and B sites on the y phase of bismuth molybdate. The slow and strong adsorption on the /3 phase was stronger than on the y phase and the weak adsorptions were single site instead of dual site. 

Scheme 5.5. Schematic of the active site for acrolein formation from propene over bismuth molybdate...

When the oxygen vacancies in the catalyst are located primarily near the surface, the reoxidation is very rapid. The activation energy for the reoxidation of these surface vacancies was found to be about 1-2 kcal/mol for the a and p phases of bismuth molybdate. Reoxidation of subsurface vacancies is much slower, however, and is limited by the ability of the catalyst to transport the oxygen from the surface O2 chemisorption sites to these subsurface vacancies. For catalysts having structures that facilitate this type of diffusion the reoxidation proceeds rapidly. 

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