Intermediates vanadium phosphate catalysts

The success of butane selective oxidation inevitably led to testing of vanadium phosphate catalysts for oxidation of other alkanes and alkenes. Pentane has been similarly transformed to phthalic anhydride in addition to MA (91-97). Phthalic anhydride is an important intermediate in the manufacture of plastics. Flowever, the investigation of vanadium... 

Benzylamine- [e. g. 45] or benzaldehyde-like [e. g. 46] species have been proposed as reaction intermediates. Ammonia, which must be inserted, is chemisorbed and activated by the catalyst [47-49] and does not act directly from the gas-phase. Recent investigations using transient experiments with % support this idea [50], As indicated for vanadium phosphate catalysts, NH4+ ions can act as potential insertion species in the ammoxidation cycle, suitable chemisorption sites (OH groups) will be formed during the catalytic reaction [51]. Otherwise, the formation of imido species (=NH) is often discussed as active site for the N-insertion of catalysts, containing, e. g., molybdenum instead of vanadium [1,52-54],... 

Mechanism of n-Butane to Maleic Anhydride over Vanadium Phosphate Catalysts. The oxidation of n-butane to maleic anhydride occurs via 14-electron oxidation. It involves the abstraction of eight hydrogen atoms, the insertion of three oxygen atoms, and a multistep polyfunctional reaction mechanism that occurs entirely on the adsorbed phase. No intermediates have been observed under standard continuous flow conditions, although mechanisms for this process have been proposed on the basis of a variety of experimental and theoretical findings. 

In this method, large amounts of acetonitrile, CH3CN, occur as a by-product. Catalysis is by vanadium oxide on AI2O3, or borophosphate, titanium phosphate, or bismuth phosphomolybdate on silica acid. The mechanism is still unestablished. On the one hand, acrolein does not appear to be an intermediate product, since it is not formed in the absence of ammonia. On the other hand, acrylonitrile can also be obtained from acrolein with ammonia and oxygen, and molybdenum oxide as catalyst. In another variation, the conversion is not made with ammonia and air, but with nitric oxide ... 

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