Electrochemical recycling of polymeric materials

Abstract

Polymeric materials play a pivotal role in our modern world, offering a diverse range of applications. However, they have been designed with end-properties in mind over recyclability, leading to a crisis in their waste management. The recent emergence of electrochemical recycling methodologies for polymeric materials provides new perspectives on closing their life cycle, and to a larger extent, the plastic loop by transforming plastic waste into monomers, building blocks, or new polymers. In this context, we summarize electrochemical strategies developed for the recovery of building blocks, the functionalization of polymer chains as well as paired electrolysis and discuss how they can make an impact on plastic recycling, especially compared to traditional thermochemical approaches. Additionally, we explore potential directions that could revolutionize research in electrochemical plastic recycling, addressing associated challenges.

Fig. 1. Electrochemical recycling of polymeric materials. (a) Electrochemical depolymerization of multilayered polymeric materials. Selective depolymerization of the first polymer component (red polymer) is achieved at a given electrochemical potential. The second polymer component (blue polymer) is then depolymerized at a higher potential. (b) General scheme for closed-loop polymer recycling. Polymers are either selectively depolymerized into monomers (red spheres) or other building blocks (blue spheres). In the former case, monomers can be repolymerized back to (the same) polymer and in the latter case, the building blocks can be used for other applications. (c) Post-polymerization functionalization. Functional groups are electrochemically added to the side chain or the backbone of the polymer. (d) Parallel paired electrolysis. The electrochemical oxidative functionalization of the polymer is performed simultaneously with the reduction of a small molecule (purple sphere) to another value-added product (green sphere).

Fig. 2. Depolymerization of PET to TPA and EG. (a) Hydrolysis under basic conditions; (b) electrochemical approaches reported; (c) electrochemically generated pH gradient; (d) redox-mediated reductive depolymerization.

Fig. 3. Electrochemical depolymerization of POM. (a) Reaction conditions reported to generate formaldehyde, formic acid and 1,3,5-trioxane via electrochemical depolymerization of POM. (b) The dual role of HFIP in breaking the crystallinity of POM and in generating H⁺ during its oxidation to trigger POM depolymerization.

Fig. 4. Electrochemical depolymerization of biopolymers. (a) Cellulose; (b) chitosan; (c) lignin and the corresponding products obtained. For lignin, only a general scheme is provided, we refer the readers to these reviews for a more detailed description of the existing methodologies.

Fig. 5. Redox-mediated electrochemical depolymerization of poly(dithiothreitol). (a) Scheme of the reaction. (b) Proposed mechanism.

Fig. 6. Integrated tandem chemical/electrochemical depolymerization of polyethylene to ethylene and propylene.

Fig. 7. C–N containing polymers. (a) Schematic structure of polyurethane, polyamide and polyamine; (b) example of hydrogenative depolymerization of polyamide back to amine, aldehyde, or alcohol.

Fig. 8. Electrochemical PPF of cellulose. (a) Scheme of reaction conditions reported for the electrochemical PPF of cellulose; (b) example of proposed mechanism of TEMPO mediated electrochemical oxidation of cellulose.

Fig. 9. Electrochemical dehalogenation. (a) Electrochemical degradation of PVC; (b) tandem electrochemical dechlorination of PVC with concomitant chlorination of an electron-rich arene; (c) electrochemical defluorination of PVF; (d) schematic representation of the DEHP-mediated reductive dichlorination of PVC coupled with chlorination of an electron-rich arene. The abbreviations dPVC and dPVF refer to (partial) dechlorinated PVC and (partial) defluorinated PVF, respectively.

Fig. 10. Electrochemical PPF strategies for C–H bond functionalization. (a) Electrochemical oxidation of LDPE; (b) electrochemical azidation of PS; (c) electrochemical azidation of polynorbornene; (d) schematic representation of the Mn-mediated oxidative azidation of PS.

Fig. 11. Electrochemical reduction of PS followed by chemical reduction and epoxidation.

Fig. 12. Schematic representation of the most common paired-electrolysis reactions.

Fig. 13. Parallel paired electrolysis polymer recycling methodologies. (a) Electrochemical conversion of cellulose/CO₂ to formate; (b) electrochemical chitosan to acetate conversion with concomitant hydrogen production; (c) electrochemical conversion of EG/CO₂ to various products.

Fig. 14. Two examples of future challenges for electrochemical recycling of polymeric materials. (a) Electrochemical activation of terminated polymer chains. The blue ball with the yellow star represents the activated chain-end of the polymer; (b) introducing weak linkages within the polymer backbone. The double green boxes represent the cleavable unit.

Weizhe Zhang

Lars Killian

Arnaud Thevenon