Bismuth molybdate redox mechanism

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... 

Mann and Ko [202] likewise examined the selective oxidation of isobutene on bismuth molybdate. With an integral flow reactor, the highest selectivity was obtained at over 30% conversions for an oxygen/olefin ratio of 2/1 and a W/F = 2.5 g h mol-1 (390°C). The data were correlated with a rather complicated Langmuir—Hinshelwood expression inconsistent with a redox mechanism. This was based on a rate-controlling step between adsorbed isobutene and adsorbed oxygen, and included an inhibiting effect of methacrolein by competitive adsorption with isobutene, viz. 

Ammosov and Sazonov [21,22,24] demonstrated that for iron antimonates the initial selectivities are lower than for bismuth molybdates due to a higher rate of the parallel combustion reaction. It is proved that both selective oxidation and combustion occur by a redox mechanism. In another publication [23], the same authors report the kinetics of the butene and butadiene combustion reactions. 

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. 

In addition to the capability of Raman spectroscopy in determining the coordination numbers and bond lengths of molybdate species in bismuth molybdate phases, Raman spectroscopy can also be used as an in situ probe for the bismuth molybdates under reaction conditions. In situ Raman studies have been earned out, for example, on the 6-Bi2Mo209 phase under redox conditions, where insights into the surface mechanism of the... 

Although catalytic oxidation of propylene has been found to be first order with respect to the olefin (5), a dependence on oxygen has also been reported (9, 54). Investigations of the participation of lattice oxygen in the oxidation process over mixed oxides, which were thought to contain antimony(V), antimony(III), and tin(IV), reported no support for the redox mechanism observed with bismuth molybdate. The matter of oxygen participation has also been considered by Christie et al. (53), who reported that the rates of... 

The structural coherence of these two phases allows for facile migration of lattice oxygen. Since the Fe-Co-Mo-0 phase is not selective for the propylene (amm)oxidation reaction, the promoting effect of the phase must be a result of its specialized function of reoxidizing the catalytically active Bi-Mo-0 phase. The criticality of specialization of functions in the complex Fe-Co-Bi-Mo-0 catalyst is further manifested in bismuth molybdate, which lacks the redox capability of Fe-Co-Mo-0 but which uniquely carries out the required steps of the surface reaction mechanism of selective alkene (amm)oxidation (see below). 

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