Liquid phase oxidation of p-xylene to terephthalic acid at medium-high temperatures: multiple benefits of CO2-expanded liquids

The Co/Mn/Br catalyzed oxidation of p-xylene to terephthalic acid (TPA) is demonstrated in CO2-expanded solvents at temperatures lower than those of the traditional Mid-Century (MC) process. As compared with the traditional air (N2/O2) oxidation system, the reaction with CO2/O2 mixture at 160 [degree]C and using an additional inert gas (N2 or CO2) pressure of 100 bar increases both the yield of TPA and the purity of solid TPA via a more efficient conversion of the intermediates, 4-carboxybenzaldehyde and p-toluic acid. At the same time, the amount of yellow colored by-products in the solid TPA product is also lessened, as determined by spectroscopic analysis. Equally important, the decomposition or burning of the solvent, acetic acid, monitored in terms of the yield of the gaseous products, CO and CO2, is reduced by ca. 20% based on labeled CO2 experiments. These findings broaden the versatility of this new class of reaction media in homogeneous catalytic oxidations by maximizing the utilization of feedstock carbon for desired products while simultaneously reducing carbon emissions.