Bismuth molybdate, alkene oxidation

Light hydrocarbons consisting of oxygen or other heteroatoms are important intermediates in the chemical industry. Selective hydrocarbon oxidation of alkenes progressed dramatically with the discovery of bismuth molybdate mixed-metal-oxide catalysts because of their high selectivity and activity (>90%). These now form the basis of very important commercial multicomponent catalysts (which may contain mixed metal oxides) for the oxidation of propylene to acrolein and ammoxidation with ammonia to acrylonitrile and to propylene oxide. 

In summary, the available experimental evidence suggests that an adsorbed form of molecular oxygen is involved in partial oxidation while lattice oxygen is required for carbon dioxide production. This proposed mechanism is directly opposed to that generally accepted for propene oxidation over mixed oxide catalysts such as bismuth molybdate. In this case, lattice oxygen is responsible for acrolein formation while adsorbed oxygen results in complete combustion. This means that the fully oxidized phase is the selective catalyst while the reaction is first order with respect to alkene. 

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). 

With the supply of large amounts of propane in the 1950s the search began to find a system for its direct oxidation with molecular oxygen to yield acrolein. Attempts with cuprous oxide marked the beginning of the technical development of alkene oxidation in the gas phase by metal oxide catalysts [22]. But this system showed weak points in the conversion (20%) [23,24] and in the selectivity, with the consequence that most of the propane added had to be recycled and many side products had to be removed. The development and introduction of the bismuth molybdate/bismuth phosphomolybdate system (Sohio, 1957) as a catalyst [25-27] and the following application for propane... 

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