The selective oxidation of cyclohexane (CyH) with molecular oxygen still constitutes a major challenge for chemists. [1] The synthesis of cyclohexanone (QO) is of special interest, [2] as it is a key chemical for the production of polyamides such as nylon 6 and nylon 6,6. It is generally recognized that the thermal reaction of CyH with O 2 proceeds via a complex radical chain mechanism, with a limit of 5 % on the conversion to prevent overoxidation of the desired products. [3][4][5] Currently, the industrial autoxidation of CyH is often carried out in the presence of small amounts of dissolved cobalt salts. [3,4] Cobalt ions are able to assist in the generation of new radicals from the primary chain-propagation product, cyclohexyl hydroperoxide (CyOOH), via a Haber-Weiss catalytic cycle [reactions (1) and ( 2)]. [1,[3][4][5][6][7][8] This catalytic decomposition is indeed much faster than thermal initiation through a bimolecular reaction of CyOOH with CyH or QO. [9] CyOOH
Chromium, however, is known to catalyze additionally the dehydration of CyOOH to QO, which results in a much more favorable product distribution. [3,4] Our recently developed insights into the mechanism of CyH autoxidation [10][11][12] prompted us to look at the opportunities for using Cr as a catalyst. The earlier mechanistic view, [3][4][5] which assumes that QO is responsible for the majority of by-products, is at odds with our results that stem from a combined experimental and theoretical approach to the subject.