Deconstruction of carbon-carbon bonded polymers for biological conversion through COOH-functionalization and Fenton chemistry

For carbonaceous waste materials that cannot be readily recycled, upcycling through deconstruction and subsequent biological conversion of the breakdown products to fuels, chemicals, or intermediates is a promising strategy. However, deconstructing certain important types of waste plastics such as polyolefins and polystyrene, and also lignin, requires cleaving carbon-carbon bonds. We show that carbon-carbon backbone bonds can be cleaved efficiently by a Fenton reaction in polymers that contain carboxylic acid (COOH) groups. For polyacrylic acid, a model water-soluble polymer, decrease in molecular weight from M_p = 28,000 g/mol to M_p < 700 g/mol occurred with H₂O₂ loading of only 2 molar equivalents to monomer at pH 4. This method was then applied to partially deconstruct two commercial oxidized polyethylene (oxPE) wax samples with much lower density of COOH groups. A modification of the method, in which dihydroxybenzene was included for a chelator-mediated Fenton (CMF) reaction, greatly increased the deconstruction efficiency. For a sample with acid number of 35 (or ∼ 1 COOH per 114 C atoms), 9.5 % +/- 0.6 % of the carbon was recovered as water-soluble species and an additional 13 % was recovered with subsequent acetone extraction. Mass spectrometry analysis of the water-soluble compounds revealed that most of the low molecular weight fragments form three series, where the members of each series differ by the number of CH₂ units. The series contains carboxylic acid and alcohol groups. A panel of microorganisms was tested for growth on this substrate as sole carbon source and several organisms showed promising growth as measured by optical density of cultures at 600 nm. Rhodococcus ruber and Brevundimonas vesicularis grew well on the PE breakdown products with no further carbon or nitrogen sources added whereas Bacillus megaterium, Yarrowia lipolytica MYA 2613, Sphingopyxis witflariensis, and Sphingomonas PWE1 along with Rhodococcus ruber and Brevundimonas vesicularis grew well on the PE breakdown products in the presence of tryptone. Several ideas are discussed to improve the overall conversion with this approach.