CoBr2-MnBr2 containing catalysts for catalytic oxidation of p-xylene to terephthalic acid

Catalytic oxidation of p-xylene (PX) to terephthalic acid (TA) was studied with catalysts containing cobalt acetate, manganese acetate, CoBr2 and MnBr2. The catalysts contain neither highly corrosive hydrogen bromide nor other metal ions, and have the advantage of easy catalyst recovery. The effects of Br/Co atomic ratio, reaction time and temperature, PX concentration, oxygen pressure, and catalyst concentration on PX conversion and product/intermediate yields were investigated. The catalyst system had a suitable reaction temperature of 100 degrees C, which was much lower than the commercial process temperature (175-225 degrees C). The maximum product (TA) yield was 93.5%, obtained at a Br/Co atomic ratio of three. Higher Br concentration resulted in the lower TA yield, which was ascribed to the benzylic bromide formation. The synthesis of TA could be adequately described as four reaction steps in series (PX -> p-tolualdehyde -> p-toluic acid -> 4-carboxybenzaldehyde -> TA), with a pseudo-first-order rate equation for each step, and the third step was rate-limiting. The rate constant ratios (K-j/K-3, j = 1 -> 4) obtained at 100 degrees C were similar to the k(j)/k(3) values reported earlier for cobalt acetate/manganese acetate/ HBr catalysts in a range of 185-191 degrees C. (c) 2008 Elsevier B.V. All rights reserved.