Producing Value‐Added Chemicals from Polyester and Polycarbonate Waste

Significance 

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.

Producing Value‐Added Chemicals from Polyester and Polycarbonate Waste - Advances in Engineering

About the author

Professor Matthew Jones
Professor, Department of Chemistry
Centre for Sustainable and Circular Technologies (CSCT)
University of Bath

Our lab research activities and interests within the group focus on several different aspects of the synthesis of homogeneous and heterogeneous catalysts for sustainable chemical transformations and green chemistry. Our work involves a major synthetic component, most of which is carried out using inert atmosphere techniques. Work utilises solution-state NMR (within the department), mass spectrometry, electron microscopy and X-ray crystallography to probe the structure of the homogeneous catalysts.

Production of biopolymers: In this area my group is developing new initiators for the production of polylactide (PLA), co-polymers and polymers from terpenes. PLA is a biodegradable and annually renewable polymer. We are pioneering new ligands and complexes for the production of isotactic PLA – this work has recently been published in Chemical Science 2015 and Chemical Communications 2014, 2016. These papers describe a new “self-correcting” method of the polymerisation of lactide and illustrate the subtle nature that the initiator has on selectivity and rate of polymerisation.

Catalytic upgrading of renewables: In this area we are interested in the conversion of ethanol into 1,3-butadiene (a monomer for the production of synthetic rubber). This is driven by the in-stability in the supply and the cost fluctuation of the monomer. There has been a lot of work in this area in the 1920’s, but with the bountiful supply of crude oil the “bio” route fell out of favour. This work has attracted industrial interest, (e.g. a patent has been filed WO2014180778A1) where we have developed a catalyst that is capable of producing butadiene with a selectivity in excess of 70%. There are still significant challenges posed by this research. For example, the selectivity towards ethylene and diethyl ether are relatively high. We are working on new catalysts (understanding how the acid/base properties affect this) to minimise these unwanted side reactions.

Reference

Jack M. Payne, Muhammad Kamran, Matthew G. Davidson, Matthew D. Jones. Versatile Chemical Recycling Strategies: ValueAdded Chemicals from Polyester and Polycarbonate WasteChemSusChem, 2022; DOI: 10.1002/cssc.202200255

Go To ChemSusChem

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