Toward the development of a biocatalytic system for oxidation of p-xylene to terephthalic acid: oxidation of 1,4-benzenedimethanol

The oxidation of hydrocarbons is an important value-enhancing chemical transformation. Current inorganic catalysts often operate under harsh conditions and produce large amounts of heavy-metal waste. Enzymatic oxidations, on the other hand, operate under relatively mild conditions and produce little if any waste. In the present work, we describe several steps in the oxidative pathway leading from p-xylene to terephthalic acid that are catalyzed by enzymes. Chloroperoxidase (CPO) from Caldariomyces fumago was used to oxidize p-xylene. However, only one of the two aromatic methyl groups was oxidized. To examine the route from 1,4-benzenedimethanol (1,4-BDM) to terephthalic acid, we investigated numerous peroxidase and oxidase enzyme systems. A combination of two enzymes, CPO and xanthine oxidase (XO), was found to produce the highest yield of terephthalic acid from 1,4-BDM. Oxidation of 1,4-BDM to a mixture of predominantly terepthaldicarboxaldehyde, 4-carboxybenzaldehyde, and 4-hydroxymethylbenzaldehyde was carried out by CPO with the continuous addition of hydrogen peroxide as an oxidant. Subsequent addition of XO resulted in a 65% yield of terephthalic acid. A tandem system in which both CPO and XO were present enabled the initial H2O2 to be enzymatically regenerated an average of 1.6 times. However, much lower final yields (ca. 2%) of terephthalic acid were obtained.