Selective aerobic oxidation of para-xylene in sub- and supercritical water. Part 2. The discovery of better catalysts

An extensive and systematic study has been carried out on the catalytic effect of more than 20 elements on the aerobic oxidation of p-xylene to terephthalic acid in super- and subcritical water. Reactions have been performed in a continuous reactor under catalyst unsaturated conditions. Reaction product, by-products and intermediates have been quantified as well as the burn (the amount of CO2 originating from total oxidation of p-xylene). CuBr2 has been found to be a superior catalyst to MnBr2, which has been widely used in the literature for this reaction in water at high temperatures. At catalyst unsaturated conditions (i.e. with low concentrations of catalyst), MnBr2 gives a terephthalic acid yield of 36.1% whereas CuBr2 enhances this value to 55.6%. A strong synergistic effect has been found between CuBr2 and other metals and sources of bromide. Indeed, we show that Cu/Co/Br, Cu/Co/NH4/Br and other mixtures give better results than CuBr2 reaching a terephthalic acid yield of 70.5% for the four component catalyst. The compositions of the catalyst as well as the reactor temperature have been optimized and their effects on the analyzed compounds are discussed. A substantial amount of additional data is included in the electronic supplementary information.