1982 Volume 25 Issue 5 Pages 273-280
Retardation by trimellitic acid (TMA) and 4-methyl orthophthalic acid (MOA) of the main steps of the oxidation of pseudocumene with cobalt catalyst and methyl ethyl ketone activator in acetic acid was examined. The removal of these inhibitors from the oxidation system was also studied, and insoluble compounds containing TMA, MOA and cobalt were found formed by heat treatment of the filtrate of the reaction mixture.
The first order rate constants of dimethyl benzoic acid (DBA), 2-methyl terephthalic acid (MTA) and 4-methyl isophthalic acid (MIA) decreased linearly with increase in the TMA or in the MOA concentration.
Oxidation of DBA: k2=1.12(1-0.53[TMA]/[Co]) (Figs. 2, 3)
Oxidation of MTA and MIA: k3=1.80(1-1.43[TMA]/[Co]) (Fig 4)
It was confirmed that TMA coordinates with cobaltous ion and forms a 1:1 complex compound (Figs. 6, 7); however, its deactivation effect on the cobalt catalyst was different in each reaction. For instance, the deactivation effect on the rate constant of the oxidation of MTA was about three times as effective as that of DBA. The retardation effect of MOA per molecule is almost the same as that of TMA. The oxidation of PC is also retarded by the presence of TMA and MOA; however, there is no relationship between decrease in the rate constant and TMA and MOA concentrations (Fig. 1). In the continuous oxidation of PC, k2, the rate constant of the DBA→MPA step, is almost proportional to ([Co]-[TMA]) (Fig. 8).
The formation of insoluble compounds containing TMA, MOA and Co by heat treatment of the filtrate of the reaction mixture was observed. These insoluble compounds were thought to be of two kinds and the mole ratios of (TMA+MOA) to Co of the two kinds of compounds are close to 1 (Table 1). The rate of formation of these insoluble compounds was accelerated by the addition of a model precipitate as a nucleus of crystal and the amount of the precipitate approached maximum after treating at 140°C for 1hr. (Fig. 10). The amount of precipitate was proportional to the concentrations of Co and TMA, and the value of ([Co]× [TMA]) of the filtrate could be reduced to 0.05-0.08 (wt%)2 by this treatment (Figs. 11, 12).
Using a circulating system, including the steps for heat treatment of the filtrate of the reaction mixture and for separation of the precipitate formed, PC was continuously oxidized for a long time during which MPA was produced without a marked decrease in the rate constant of each step (Table 2, Figs. 15, 16).
