Effects of guanidine on the liquid-phase catalytic oxidation of p-xylene to terephthalic acid

In previous papers [Wang, Q.; Li, X.; Wang, L.; Cheng, Y.; Xie, G. Ind. Eng. Chem. Res. 2005, 44 (2), 261-266; 2005, 44 (13), 4518-4522], a lumped kinetic scheme and a fractional kinetic model for the liquid-phase oxidation of p-xylene catalyzed by Co-Mn-Br was proposed and tested and the effect of water on the oxidation reaction kinetics was investigated. In this paper the effect of guanidine catalyst additive on the liquid-phase oxidation of p-xylene to terephthalic acid over Co-Mn-Br catalyst system was studied. Experiments on four levels of guanidine composition were carried out in a semibatch oxidation reactor where the gas and liquid phases were well mixed. The results show that the addition of guanidine apparently accelerates the oxidation of the second methyl group. However, for the oxidation of the first methyl group there is an optimal guanidine composition omega(0). When the composition of guanidine is lower than omega(0), the oxidation rates for the first methyl group increase with the increase of guanidine composition, whereas when the composition of guanidine is higher than omega(0), the oxidation rates decrease with the increase of guanidine composition. Moreover, the addition of guanidine inhibits the side reactions remarkably. A possible mechanism for interpretation of the effect of guanidine on the oxidation of p-xylene to terephthalic acid is presented.