Low-bromide containing MC catalyst for the autoxidation of para-xylene

The autoxidation of para-xylene (pX) was carried out with the usual MC catalyst, Co(OAc)(2)/Mn(OAc)(2)/Br-, in the presence of acid additives, but with significantly lower concentrations of bromide. However, the reaction came to a halt after only 15% of the theoretical uptake of oxygen had occurred, presumably due to the loss of inorganic bromide (Br-) in the form of benzylic bromides. But, a tiny amount of Zn(OAc)(2) restored the effectiveness of the catalyst, even with lower bromide, down to 0.28 mmol L-1. Autoxidation of pX occurs by a chain mechanism that involves formation of the dibromide radical which abstracts a hydrogen atom from para-xylene to form the carbon-centered radical ArCH2 center dot. However, excess dibromide radicals are formed during fast chain reactions, which react with ArCH2 center dot to produce undesired benzylic bromides, in parallel with its involvement in the oxidation of substrates. It is believed that Zn(OAc)(2) acts as a bromophilic agent and stores excess Br- in the form of monobromozinc(II), and minimized the loss of Br. (C) 2007 Elsevier B.V. All rights reserved.