Bio-based oxidation of pX into TPA

A NEW biocatalysis system could be a greener, more efficient alternative to the chemical manufacture of terephthalic acid (TPA) from p-xylene (pX), according to researchers at Korea Advanced Institute of Science and Technology (KAIST).

TPA is a large-volume commodity chemical used for manufacturing polyethylene terephthalate (PET), the products of which include plastic bottles and clothing fibres. Currently, TPA is chemically synthesised from pX through a series of oxidation reactions, with a typical yield of over 95 mol%.

Now, researchers at KAIST have published a promising bio-based method for producing large amounts of TPA from pX using a metabolically engineered Escherichia coli strain. Their technology presents several advantages over the conventional process, such as ambient reaction temperature and pressure, no use of heavy metals or other toxic chemicals and the removal of byproduct formation.

The team designed their synthesis pathway by incorporating the upper xylene degradation pathway of a Pseudomonas putida strain and the lower p-toluene sulphonate pathway of a Comamonas testosteroni strain. This successfully produced TPA from pX in small-scale cultures, forming p-toluate (pTA) as a major byproduct. This pathway was then optimised, to give a final strain of E. coli which produces more TPA and eliminates the pTA byproduct.

The researchers used the strain in an aqueous and organic two-phase fermentation system. The two-phase design was used to overcome problems such as volatility, insolubility and toxicity caused by feeding pX into the fermentor, where strong agitation and aeration were also needed to grow cells. Oleyl alcohol (OA) was used as the organic carrier of pX, due to its biocompatibility with E. coli, good phase stability and low cost, while the E. coli was contained in an aqueous culture medium. Fermentations took 30°C, and the researchers tested various conditions, including the quantity and rate of pX feeding, cell densities and oxygen control. The highest yield of TPA was 13.3 g from 8.8 g pX, giving a yield of 97 mol%.

Lead researcher Sang Yup Lee said: “Our research is meaningful in that it demonstrates the feasibility of the biotechnological production of bulk chemicals, and if reproducible when up-scaled, it will represent a breakthrough in hydrocarbon bioconversions.'

In their paper, the authors affirm that since pX can be derived from biomass (such as by using a zeolite catalyst), TPA can now be produced without fossil resources. They state that future research should include the development of a new pathway and engineered strain that can produce TPA by direct one-step fermentation from carbohydrates such as glucose.

Nature Communications: http://doi.org/b79p