From Plastics to Rings and Back Again: Catalytic Recycling of Waste Oxygenated Plastics

点击翻译按钮获取中文摘要

Despite the exponential increase in plastic usage in the past century, little attention has been paid to dealing with the huge amount of plastic left at the end of life. A solution to the growing plastic waste is desperately needed. The majority of plastics are derived from petrochemicals and whilst mechanical recycling is well developed, it is typically associated with high energy costs and material degradation. Chemical recycling has the potential to create a circular plastic economy as the original starting materials can be recovered under mild conditions, allowing for their reincorporation into new plastics and avoiding the need for new raw materials.Plastics are made up from long, chain-like molecules called polymers. These in turn are built up from a series of monomers. The order in which the monomers are connected, how they are connected and the individual structure of each monomer determines the properties of the polymer and which types of plastic they are used for. Chemical recycling breaks down the polymers in the plastic to their starting monomers which can then be reused.Chemical recycling faces many challenges. Many commercial plastics are incompatible with chemical recycling due to their chemical make-up and therefore alternative plastics which can be chemically recycled are needed to replace these. Polymers which contain oxygen linkers, such as polycarbonates, are particularly promising as they can be broken down much more readily. Polycarbonates can be produced by combining carbon dioxide with an epoxide. Epoxides are three-membered rings which contain an oxygen atom. Altering the epoxide used affects the properties of the resulting polycarbonate. Whilst previous research has shown that various polycarbonates can be broken down to recover the starting epoxide, the selectivity towards these starting epoxides is a major issue as a cyclic carbonate product is often formed instead. Catalysts are key tools in overcoming this challenge as they can help to favour the epoxide formation as well as speed up the decomposition reaction. Although the catalysed decomposition of several different polycarbonates to their starting epoxides has been successfully demonstrated, for polycarbonates to be adopted on an industrial scale their properties must be competitive with current commercial polymers. Poly(cyclohexene carbonate) (PCHC) and poly(propylene carbonate) (PPC) are two types of polycarbonate which have the greatest potential to replace current commercial polymers. Unfortunately the chemical recycling of these polycarbonates has proven more difficult due to selectivity issues. Recent work by the Williams group has developed a new and highly effective catalyst to chemically recycle PCHC to its epoxide1 but the chemical recycling of PPC remains a challenge.This project aims to develop catalysts for the chemical recycling of PPC back to its starting epoxide. Using PCHC as a case study, this project will investigate the mechanism behind the catalysed decomposition of polycarbonates and how changing the shape and structure of the polycarbonate affect its rate of decomposition. This information will be used to inform future catalyst and polymer designs. In addition, the chemical recycling of other types of oxygenated polymers, specifically poly(esters-alt-ethers), will be explored.The ultimate goal is to develop effective methods of chemically recycling polymers to enable a sustainable circular plastic economy. This project falls within the EPSCR 'Manufacturing the Future' research area.