Effect of water content on the kinetics of p-xylene liquid-phase catalytic oxidation to terephthalic acid

In a previous paper (Wang, Q.; Li, X.; Wang, L.; Cheng, Y.; Xie, G. Ind. Eng. Chem. Res. 2005, 44 (2), 261-266) a lumped kinetic scheme and a fractional kinetic model for the liquid-phase oxidation of p-xylene to terephthalic acid catalyzed by cobalt acetate and manganese acetate and promoted by hydrogen bromide was proposed and tested. Here, the effect of water content on the time evolution of the experimental product distribution and kinetic constants of the developed model were investigated. Experiments on four levels of initial water content were carried out in a semi-batch oxidation reactor where the gas and liquid phase were well mixed. The results show that for the first two fast steps of p-xylene and p-tolualdehyde oxidation, the rates decrease with the increase of water content, while for the latter two slower oxidation steps of p-toluic acid and 4-carboxybenzaldehyde, the rates increase with the increase of water content. There was an optimal water content for the reaction system. The mechanism was interpreted by the competition between the coordination effect, which had negative influence on the transfer rate of the electron, and the decrease of the redox potential of the cobalt ion, which had positive influence on the reaction rate.