Thermodynamic and Kinetic Studies to Elucidate the Amoco Co/Mn/Br Autoxidation (‘MC’) Catalyst

Abstract

An efficient, general,and high yield method to prepare aromatic acids is to pass dioxygen through an acetic acid solution of Co/Mn/Br salts containing alkylaromatic compounds. This method has been licensed world-wide to prepare terephthalic acid from p-xylene. We have used the reaction of m-chloroperbenzoic acid (MCPBA) with mixtures of Co(II) acetate/ Mn(II) acetate/bromide in acetic acid/water solutions to understand the functions of each catalyst component. The sequence of redox reactions that occurs is first the reaction of MCPBA with Co(II) to give Co(III); Co(III) then oxidizes Mn(II) to Mn(III) and finally, Mn(III) oxidizes bromide to bromine. Some of the functions of each component are 1) the cobalt rapidly reacts (very selectively) with the MCPBA (Mn and Br react slowly), 2) Mn lowers the steady state of Co(III) which significantly reduces solvent decomposition and also avoids Co(III) re-arranging into a less reactive form, and 3) bromine reacts rapidly with the methylaromatic compound to generate methylaromatic radicals (Co(III) and Mn(III) react slowly). The dimeric structure of Co(II) in acetic acid is partly responsible for the highly selective nature of the MCPBA oxidation of Co(II). The order of the redox reactions is the opposit to that expected from thermodynamics.