Formation and role of cobalt and manganese cluster complexes in the oxidation of p-xylene

The mono- and multinuclear metal complexes present during the aerial oxidat ion of p-xylene by the homogeneous catalyst systems viz. Co/Br, Mn/Br, Co/M n/Br, Co/Ce/Br, Co/Zr/Br, Co/Mn/Zr/Br, Co/Mn/Ce/Br, Ni/Mn/Br and Ni/Mn/Zr/B r in acetic acid solvent have been investigated by electronic and EPR spect roscopies. The reaction mixtures contain, in addition to Co(OAc)(2). 4H(2)O and Mn(OAc)(2). H2O, species like Co(OAc)Br, Co(OAc)(3), Co-3(O)(OAc)(x), Mn-3(O)(OAc)(x) and hetero-multinuclear complexes like Co2Mn(O)(OAc)(x) and CoMn2(O)(OAc)(x). While mononuclear Co(OAc);?, Mn(OAc), and Co(OAc)Br comp lexes predominate in the initial stages of the oxidation reaction, signific ant concentrations of multinuclear Co(III) and Mn(III) complexes are detect ed in the later stages. Zr(IV), when present, facilitates the oxidation of Mn(II) to Mn(III), a crucial step in the kinetic pathway of the oxidation r eaction. Our EPR results indicate the: presence of home-nuclear Mn-3(O)(OAc )(x) type clusters as the major species with a minor component of CoMn2(O)( OAc)(x) complex in catalyst systems with Co:Mn = 1:3(mol). When an excess o f cobalt catalyst is used (e.g. Co:Mn = 3:1) or when Zr and Br- are also pr esent in optimal concentrations, the heteronuclear cluster Co2Mn(O)(OAc)(x) predominates while Co-3(O)(OAc)(x) and CoMn2(O)(OAc)(x) occur as minor con stituents. The yield of terephthalic acid (TA) is enhanced at high concentr ations of cluster complexes like Co2Mn(O)(OAc)(x) and CoMn2(O)(OAc)(x). The combination Co/Mn/Zr (3:1:0.1mol), exhibits a high catalytic activity and I;electivity for terephthalic acid, especially in the presence of an optima l concentration of the bromide ion. (C) 2000 Elsevier Science B.V. All righ ts reserved.