Para Xylene Oxidation in High Temperature Water

Nikhil Ramsubramaniam

 

Terephthalic acid is a monomer used to produce polyethylene terephthalate. The main commercial route to terephthalic acid is the partial oxidation of p-xylene in acetic acid as a solvent at approximately 200 °C and 15-30 atm with a combination of manganese, bromine, and cobalt as catalyst. Recent studies have considered the use of high temperature water (HTW) as a solvent instead of acetic acid as it is more environmentally benign. The synthesis of terephthalic acid has been carried out in water in both sub critical and supercritical conditions. The yields of terephthalic acid and other products have been studied in a catalytic environment using MnBr2 as the catalyst and hydrogen peroxide as the oxidant. The variation in the yield of terephthalic acid has been studied by varying factors like temperature, batch holding time, catalyst concentration, oxidant concentration and initial p-xylene concentration.The highest yield obtained was 49 ± 8 % at 300oC using MnBr2 as the catalyst and hydrogen peroxide as the oxidant. The hydrothermal stability of terephthalic acid in high temperature was also evaluated. Benzoic acid was the most stable product of decarboxylation and showed negligible degradation.  A decarboxylation pathway was also established and the various parameters involved in the rate equation were fitted according to experimental data. Finally an economic and environmental assessment of the HTW based process was done in order to study the feasibility of p-xylene oxidation as a continuous process. With a capital investment nearly equal to the traditional acetic acid based process coupled with reduced emissions this method seems to have a promising future.