Bio-electrochemical conversion of plastic waste into value added chemical products

Abstract

Olefins are important feedstocks in the chemical industry that are currently exclusively produced from fossil resources. The electrochemical oxidative decarboxylation of carboxylic acids, such as succinic acid, can offer an environmentally friendly route to generate olefins provided the electrolysis uses renewable electricity and the acids are obtained from waste streams. We have developed two different approaches for the sustainable production of olefins from waste substrates. Firstly, a two-step process, combining a thermal pretreatment and an electrochemical conversion, that can convert polyolefin based plastic waste into ethene as main product. This approach allows to resynthesize polyethylene and offer a genuine chemical recycling route for plastics. The second conversion process is a bio-electrochemical approach allowing the conversion of polyester waste into olefins. In both processes, dicarboxylic acids are central intermediates that undergo electrochemical reactions forming ethene as value added final product. With spectroscopical and electroanalytical approaches as well as quantum mechanical calculations, insights into the reaction mechanism could be gained as well. Furthermore, different electrocatalysts were tested and material characteristics, beneficial for the olefin formation could be determined. [1] A range of electrochemical and spectroscopic approaches such as Koutecky–Levich analysis, rotating ring-disk electrode (RRDE) studies, Tafel analysis, quantum chemical calculations, electron paramagnetic resonance (EPR), and in situ infrared (IR) spectroscopy generated further insights into the reaction mechanism. As previously mentioned, different pre-treatment procedures (thermal and biocatalytic) were applied to make the insoluble plastic waste substrates accessible for electrochemistry. Finally, electrocatalytic tests in batch and flow cells were performed to convert the waste streams into alkenes. It was found that the nature of the carbon material significantly influences the reaction outcome and well- ordered, two-dimensional electrode materials are preferred, such as flat graphite. Furthermore, it could be shown that the hydrophobic or hydrophilic nature of the carbon plays and important role concerning stability of the electrodes. A distinct influence of the reaction medium was determined.[2] This is of high relevance for the combination with biochemical pretreatments of plastic wastes, as this is performed in buffered solutions. The influence of the buffer solutions on the electrochemical decarboxylation was investigated and MD simulations were used to understand the underlying mechanism and the molecular structure at the electrode surface. Finally, combined thermo-electrochemical and bio-electrochemical processes are demonstrated, allowing the conversion of plastic (polyolefins or polyesters) into ethene as value added product. The processes could also be demonstrated in a flow-setup, for continuous olefin production. Our results show that different types of plastic wastes can be converted in coupled thermal/bio- electrochemical processes into value added olefin products. This is of great significance as it enables chemical recycling prospects at low temperatures and mild conditions towards defined gaseous products. Furthermore, it opens the possibility to obtain olefins in a sustainable way, and therefore offering one of the few alternatives to the current fossil-based olefin production routes. References 1. Pichler, C.M.; Bhattacharjee, S.; Rahaman, M.; Uekert, T., Reisner, E. ACS Catal. 2021, 11, 15, 9159–9167. 2. Pichler, C.M.; Bhattacharjee, S.; Lam, E.; Su, L.; Collauto, A.; Roessler, M., Cobb, S.J.; Badiani, V.; Rahaman, M.; Reisner, E. ACS Catal. 2022, 12, 21, 13360–13371