Oxygen molybdate catalysts

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

Acrolein Production. Adams et al. [/. Catalysis, 3,379 (1964)] studied the catalytic oxidation of propylene on bismuth molybdate catalyst to form acrolein. With a feed of propylene and oxygen and reaction at 460°C, the following three reactions occur. 

The studies performed over promoted manganese molybdate catalysts have shown significant changes in catalytic behavior due to presence of the promoter. The preliminary results suggest that the pronounced differences observed in selectivity and activity may be related to the effect of the promoter cations on the reactivity of the lattice oxygen and the availability of adsorbed oxygen. 

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. 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).

The kinetics of the parallel formation of by-products is generally similar to the kinetics of the main reaction (i.e. rate independent of the oxygen pressure and first order with respect to propene), which presumes identical reaction steps. A study of the origin of acetaldehyde and formaldehyde carried out by Gorshkov et al. [144] is of interest in this connection. 14C-Labelled propene was oxidized over a bismuth molybdate catalyst at 460° C the results showed that acrolein, acetaldehyde and formaldehyde are formed via a symmetrical intermediate, presumably one and the same intermediate. The study, moreover, shows that acetaldehyde is exclusively formed from this intermediate, while formaldehyde may also be formed from the aldehyde group of acrolein and the methyl group of acetaldehyde. 

The hypothesis of a bifunctional mechanism involving allyl radical formation and oxygen incorporation on distinct sites is advocated by Haber et al. [147,152], This hypothesis is particularly based on experiments with Mo03, Bi203 and mechanical mixtures of these oxides, which are compared with bismuth molybdate catalysts. The reaction was carried out in cyclic operation (alternating feeds of oxygen and of propene diluted with nitrogen). The results are collected in Table 5. The authors con-... 

Selective oxidation of methanol is the industrial route to formaldehyde. In practice, two types of process are used, differing with respect to the catalyst and process conditions. Silver is a very active catalyst at 600— 700°C and requires a high methanol/oxygen ratio for a good selectivity, while iron molybdate catalysts are already active at 350° C and may be used with low methanol/oxygen ratios. 

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. 

A redox mechanism for oxidation catalysis was proposed by Mars and van Krevelen (34) for the oxidation of aromatics over V205. This mechanism introduced the concept that lattice oxygen of a reducible metal oxide could serve as a useful oxidizing agent for hydrocarbons. Moreover, it formed the basis for the early work at SOHIO which led to the development of the bismuth molybdate catalyst. Since that time there have been many reports which support the redox concept. 

Aykan (35) reported that ammoxidation of propylene occurred over a silica-supported bismuth molybdate catalyst in the absence of gas-phase oxygen, although the catalytic activity decreased rapidly with increasing catalyst reduction. The reduction process was followed by X-ray and it was found that phase changes which occurred in the catalyst and the decrease in catalytic activity corresponded quantitatively to the depletion of lattice oxygen. 

Peacock et al. (37) showed that a bismuth molybdate catalyst can be reduced with propylene and that the oxygen appearing in the gaseous products (acrolein, carbon dioxide, and water) can be quantitatively replaced in the lattice. The amount of oxygen removed during reduction corresponds to the participation of many sublayers of oxide ions. 

Sancier et al. (43) used oxygen-18 to examine the relative role of adsorbed versus lattice oxygen in propylene oxidation over a silica-supported bismuth molybdate catalyst as a function of temperature. At 400°C they observed the formation of predominantly acrolein[I60] rather than acrolein[I80], indicating significant participation of lattice oxygen. However, as the reaction temperature was decreased, the authors concluded that the role of adsorbed oxygen became more important. 

Christie et al. (45) and Pendleton and Taylor (46) have recently reported the results of propylene oxidation over bismuth molybdate and mixed oxides of tin and antimony and of uranium and antimony in the presence of gas-phase oxygen-18. Their work indicated that for each catalyst, the lattice was the only direct source of the oxygen in acrolein and that lattice and/or gas-phase oxygen is used in carbon dioxide formation. The oxygen anion mobility appeared to be greater in the bismuth molybdate catalyst than in the other two. 

Firsova et al. (136) also investigated a cobalt molybdate catalyst containing a small amount of Fe3+, after exposure to a reaction mixture of propylene and oxygen. The authors observed the valence change of Fe3+ to Fe2+ and the formation of a surface complex between the hydrocarbon and the iron (Fe—O—C—). In contrast to pure iron molybdate which also forms a surface Fe—O—C— complexes, the electronic transitions in the cobalt iron molybdate were reversible. The observed valence change showed that iron ions increase the electronic interaction between ions in the catalyst and the components of the reaction mixture. 

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