Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen

Terminally oxidized hydrocarbons are of considerable interest as potential feedstocks for the chemical and pharmaceutical industry, but the selective oxidation of only the terminal methyl groups in alkanes remains a challenging task. It is accomplished with high efficiency and selectivity by some enzymes; but inorganic catalysts, although inferior in overall performance under benign conditions, offer significant advantages from a processing standpoint. Controlled partial oxidation is easier to achieve with ‘sacrificial’ oxidants, such as hydrogen peroxide, alkyl hydroperoxides oriodosylbenzene, than with molecular oxygen or air. These sacrificial oxidants, themselves the product of oxidation reactions, have been used in catalytic systems involving tailored transition-metal complexes in either a homogeneous state, encapsulated in molecular sieves or anchored to the inner surfaces of porous siliceous supports. Here we report the design and performance of two aluminophosphate molecular sieves containing isolated, four-coordinated Co(III) or Mn(III) ions that are substituted into the framework and act, in concert with the surrounding framework structure, as regioselective catalysts for the oxidation of linear alkanes by molecular oxygen. The catalysts operate at temperatures between 373 K and 403 K through a classical free-radical chain-autoxidation mechanism. They are thus able to use molecular oxygen as oxidant, which, in combination with their good overall performance, raises the prospect of using this type of selective inorganic catalyst for industrial oxidation processes.

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