BY ANTHONY KING
A German team has reported a new route to recycling polystyrene waste. This plastic is widely used in packaging and construction, yet suffers a low recycling rate of 1% and represents around one-third of the content of landfills around the world.
A group at the Friedrich Wöhler Research Institute for Sustainable Chemistry in Göttingen has now developed an electrocatalytic method for the breakdown of polystyrenes (Angewandte Chemie, DOI: 10.1002/anie.202412689. ‘Polystyrene accumulates in nature because it consists of strong carbon-carbon and carbon-hydrogen bonds,’ says Lutz Achermann, who led the research.
Key to the process is an iron(III)-based catalyst that degrades polystyrenes under oxygen atmosphere. This selectively cleaves carbon-hydrogen bonds using what is called a hydrogen atom transfer, which involves the transfer of a proton and electron.
The process targets a carbon atom directly bonded to the benzene ring, while leaving the aromatic system. ‘We selectively cleave at this benzylic position and maintain the core structure of the aromatic system so that we can give rise to benzoic acid,’ says Achermann.
Relatively high fractions of monomeric benzoyl products are generated in his process, important because they are starting materials for chemical processes, as well as some short polymer chains.
Real-life polystyrene waste yielded 8.7% benzoic acid using the catalyst. To show that the method could be used to degrade plastic using renewable energy, a constant current electrolysis was powered by solar energy, generating hydrogen and benzoyl compounds. ‘We wanted to use the protons and electrons from electricity directly. For instance in the UK, ideally we could use wind energy,’ Achermann explains.
‘Importantly, we show that the iron electrocatalyst is compatible with even waste material and impurities in the waste material,’ he adds. The paper reports on a batch process, but the group is now working on a flow setup with a constant feed of polymer waste.
The main products are benzoic acid and benzaldehyde. Benzoic acid is a starting material for a variety of chemical syntheses, for example, in the production of scents and preservatives, the researchers note.
‘The upcycling of polymers is important because it can reduce plastics waste and simultaneously produce feedstock chemicals,’ says Patrick Holland, a chemist at Yale University, US. The advance in this paper is exciting because polystyrene can be broken down into smaller pieces, he adds.
‘However, it gives a mixture of products, and the primary product (benzoic acid) is not the most useful material. Also, scale-up would be challenging for this process, because the electrochemical reaction requires solvent and electrolyte,’ says Holland.
He flags a recent paper from a Princeton lab that reports a process to break down polystyrene, but this does not need an electrode or electrolyte and generates styrene as the product, ‘which is a much more useful as a feedstock chemical’ (ACS Central Science, DOI: 10.1021/acscentsci.4c01317).
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