Plastic waste residing in either landfill or the natural environment is one of the great environmental challenges of the 21st century. Whilst recycling rates are increasing traditional methods remain limited because the harsh remelting conditions reduce the quality of the material each time it’s recycled. There is therefore a clear opportunity to develop sustainable chemical recycling strategies to overcome such challenges, with catalysis central to such innovation. Now researchers at the University of Bath led by Professor Matthew Jones have developed a mild and rapid chemical recycling process for polycarbonates, a robust class of plastics commonly used in construction and engineering. Using a zinc-based catalyst and methanol, they were able to completely break down commercial poly(bisphenol A carbonate) (BPA-PC) beads within 20 minutes at room temperature. The waste can then be converted into its chemical constituents, namely bisphenol A (BPA) and dimethyl carbonate (DMC), helping to preserve product quality over an infinite number of cycles. Importantly, BPA recovery prevents leakage of a potentially damaging environmental pollutant, whilst DMC is a valuable green solvent and building block for other industrial chemicals. The new and simple method for upcycling plastic waste can be conducted at room temperature and the researchers hope the new process will help recycling become more economically viable. The original research article is now published in ChemSusChem, noting a major advantage which is enhanced process efficiency and milder conditions compared to previous methods.
The authors reported a series of ZnII-complexes bearing half-salan ligands and their application to the mild and selective degradation of several commercial polyesters and polycarbonates. Various strategies (e. g. alcoholysis, glycolysis and aminolysis) were used to obtain a diverse range of value-added chemicals. A completely circular upcycling approach to plastic waste is demonstrated through the production of several renewable poly(ester-amide)s based on a terephthalamide monomer derived from bottle-grade PET.
Promisingly, the catalyst is also tolerant to other commercial sources of BPA-PC (e.g. CD) and mixed waste feeds, increasing industrial relevance, whilst being amenable to other plastics (e.g. poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET)) at higher temperatures. The team has also demonstrated a completely circular approach to producing several renewable poly(ester-amide)s (PEAs) based on terephthalamide monomers derived from waste PET bottles. These materials have excellent thermal properties and could potentially be used in biomedical applications, for example drug delivery and tissue engineering.
The new method creates new opportunities for polycarbonate recycling under mild conditions, helping to promote a circular economy approach and keep carbon in the loop indefinitely. The reported method has only been demonstrated on a small scale, however, the research team is now working on catalyst optimisation and scaling up the process (300 mL).
In a nutshell, the new study demonstrated the successful preparation of a range of ZnII-complexes based on half-salan ligands in both the solution and solid-state. Catalyst versatility and robustness was demonstrated through the mild and selective degradation of various commercial polyesters and polycarbonates. A range of strategies (e. g. alcoholysis, glycolysis and aminolysis) were employed to obtain a diverse range of value-added chemicals with uses as green solvents and chemical building blocks. Such findings promise to stimulate new and exciting developments in both polymer design and recycling, enabling key challenges associated with a dynamic and evolving plastics economy to be addressed. Further work is ongoing to explore the optimization of such catalysts for chemical recycling applications.
Jack M. Payne, Muhammad Kamran, Matthew G. Davidson, Matthew D. Jones. Versatile Chemical Recycling Strategies: Value‐Added Chemicals from Polyester and Polycarbonate Waste. ChemSusChem, 2022; DOI: 10.1002/cssc.202200255