A Green Oxidation Process of p-Xylene to Terephthalic Acid through the UV Irradiation-Enhanced Ozonation Process

The industrial oxidation of para-xylene (PX) to terephthalic acid (TPA) is the footstone of the polyester industry, but this traditional oxidation process utilizes harsh reaction conditions such as high temperatures, corrosive solvents, and catalysts. In this work, a green alternative process of PX oxidation was developed to obtain a high TPA yield of 84% when using pure acetonitrile as the solvent under conditions of ozone and UV irradiation. The key reaction conditions were optimized to avoid the occurrence of overoxidation, including solvent composition, temperatures, ozone, and UV irradiation. DFT calculations reveal that the aldehyde group on the benzene ring was generated through the dehydration of geminal diol formed by the consecutive insertion of two O(1D) into the same methyl group, while the carboxyl group was produced through the oxidation of the aldehyde group by ozone. When using ozone to catalyze PX oxidation to p-TA, the activity of the para-methyl group was not inhibited by the carboxyl groups, which is different from traditional PX oxidation using the Co/Mn/Br catalyst. Results show that the aldehyde group is easily oxidized by ozone alone to form a carboxyl group without light, which reveals the significant role of ozone for promoting the oxidation reaction. It was found that the presence of water helps to generate hydroxyl radicals that act on the benzene ring responsible for the ring opening of PX and other intermediates. The insights in this work may provide a green alternative approach for the large-scale production of TPA free of corrosive solvents and catalysts.