Designing a Solid Catalyst for the Selective Low-Temperature Oxidation of Cyclohexane to Cyclohexanone

摘要

The catalytic activation, especially partial oxidation, of alkanes constitutes one of the major challenges of present-day chemistry; and the conversion of cyclohexane to cyclohexanone is among the principal target reactions since the latter is used asfeedstock in several industrial processes including the production of nylon from epsilon-caprolactam and adipic acid. There have been numerous fundamental studies of the role of cobalt acetate as catalysts for such processes and it is now recognized that cobalt(III) acetate exists as a number of distinct species, both in solution and in the solid-state. Although stable alkylperoxo complexes of Co~(III) are formed in these systems there is also unmistakable evidence for free radicals such as RO_2 and ROin the reaction, the occurrence of which is symptomatic of autoox-idation. Homogeneous catalytic processes with Co(OAc)3 suffer from the generation of significant quantities of unwanted byproducts such as alkyl acetates, other ketones, cyclohexylidene diacetate, alkyl chlorides, and occasionally, cyclohexylmethanol. To confer greater stability upon the homogeneous Co~(III) acetate catalyst it is also necessary to employ, for reasons that are not entirely understood, either acetic acid or trifluoro- (ortrichloro-) acetic acid as solvent for the reactant mixture and bromides are often used as promoters.