Mechanistic features of selective oxidation and ammoxidation catalysis

Abstract

Selectivity in the oxidation of olefins over heterogeneous catalysts, an important requirement for efficient use of feed resources, has been achieved by the use of catalysts which produce an allylic intermediate via dissociative chemisorption. The development of catalysts for ammoxidation and oxidation of propylene has resulted in substantial increases in the supply of important monomers such as acrylonitrile, resulting in the discovery of new applications in the fibres and plastics industry.

The best known and most studied ammoxidation and oxidation catalysts are those based on bismuth molybdate, which operate by a redox mechanism composed of a catalyst reduction cycle (olefin oxidation–selective product formation), and a catalyst reoxidation cycle (lattice-oxygen regeneration). Relative rates of reduction for a number of bismuth molybdate systems: multi-component > Bi₂Mo₂O₉ > Bi₂Mo₃O₁₂ > Bi₃FeMo₂O₁₂ > Bi₂MoO₆, and the requirement for partial reduction to attain maximum selectivity are consistent with a mechanism in which coordinately unsaturated metal ions in complex shear domains are responsible for selective oxidation. The reoxidation process, which follows the order of decreasing rates: Bi₂MoO₆ > Bi₂Mo₂O₉ > Bi₂Mo₃O₁₂ > Bi₃FeMo₂O₁₂≳ multicomponent system, is composed of a low-activation-energy surface reoxidation regime and a higher-activation-energy reoxidation regime involving bulk anion vacancies. The latter is strongly dependent on the structure of the catalyst. Substituent effects and allyl radical in situ studies have shown that selective oxidation occurs via rate-determining α-hydrogen-atom abstraction by oxygen associated with Bi centres to produce a radical-like π allyl molybdenum complex. For oxidation, subsequent reversible formation of a σ-O allyl molybdate (which can be formed in situ from allyl alcohol) is followed by a second hydrogen abstraction. This rate-determining step in the conversion of the σ species to acrolein is enhanced by the presence of bismuth in the catalyst. In ammoxidation, the analogous σ-N allyl molybdenum species is formed reversibly from the π complex after conversion of terminal MoO sites to MoNH via condensation with ammonia. The σ-N complex undergoes a second hydrogen abstraction to produce a reduced molybdenum 3-iminopropene complex, in a process of higher activation energy than the formation of acrolein from the corresponding σ-O complex. Reoxidation and a subsequent third hydrogen abstraction forms acrylonitrile and a reduced site. The reduced site is then reoxidized by lattice oxygen to complete the catalytic cycle.

From this detailed work, the necessary components for a selective and active oxidation catalyst are identified as a bifunctional site, composed of an active α-hydrogen abstraction (Bi) and a selective, O or NH inserting (Mo) species, and favourable solid-state and chemical properties which enhance the rapid donation of lattice oxygen and reconstitution of reduced sites by molecular oxygen.