Electrochemical initiation in liquid-phase catalytic oxidation of alkylarenes with oxygen

Abstract

AROMATXC carboxylic acids (ACA) and their derivatives are widely used in polymer synthesis. The principal method of preparing ACA is by oxidation of alkylarenes by gases containing oxygen in acetic acid solution in the presence of cobalt, manganese and bromine salts. ACA obtained by this method contain some products of incomplete oxidation. Their presence in ACA is inadmi~ible since they markedly lower the quality of polymer prepared using ACA. A method of increasing the purity of ACA is by intensification of oxidation. All methods of intensification are based on the production in the solution to be oxidized of free radicals, which involve intermediate products in oxidation by chain extension, thus promoting full oxidation. We found it possible [1] to intensify oxidation of alkylarenes by passing direct current through the solution to be oxidized. The following electrodes were used: anode--smooth platinum, cathode--platinum plated platinum. To find optimum conditions of oxidation ensuring maximum phthalic acid yield, conditions of oxidation were optimized using the step-by-step simplex method [2]. The electrodes were polarized using P-5848 and P-5827M potentiostats. Anode potential was measured in relation to an EV'L-1M3 saturated chlorine-silver electrode. The temperature of oxidation was 110 ° in all experiments, d u r a t i o n 5 hr. Results of optimization of oxidation of o-xylene, 4-tert~butyl-o-xylene and 4-bromo-o-xylene are tabulated. These results suggest that passing direct current through the solution to be oxidized enables the purity of end products to be i n c ~ . Electrochemical oxidation of intermediate products t a~ng place independently of catalytic oxidation may be a possible cause of the effect observed. The rate of catalytic oxidation in this case while passing through direct current is equal to the total of rates of catalytic and electrochemical oxidation. Process rates were evaluated from the rate of consumption of o-xylene, of which the concentration was measured by GLC. I t was established that when [o-xylene]----1-5, [Co(OAc)s• 4HsO]----0.15, [NaBr]=0.05, [HsO]e----1-25mole/1. and at 70 ° the rate of catalytic oxidation of o-xyiene is 1.56 × 10-Zmole/1..miu. The rate of electro° chemical oxidation under the same conditions is 5× 10-'mole/l.-min. Tho