Thermochemical Recycling: Transforming Polystyrene Waste into High-Quality Chemicals

2023-08-08 08:44:00

An American research team has succeeded in obtaining high-quality basic chemicals from polystyrene in just two process steps, so that it can be fed back into the material cycle.

Packaging and insulation materials in particular could be returned to a real chemical recycling economy with the thermochemical recycling approach. (Image: Wiley-VCH)

Guoliang Liu and his co-authors from Virginia Tech in Blacksburg (USA) explain that the new “degradation upcycling” technique (“deg-up” for short) can be used to produce a library of high-quality aromatic chemicals from polystyrene waste. This works in two stages: First, polystyrene is broken down into benzene, which is then chemically modified in the same reactor in a second reaction step.

The new upcycling process aims to recycle large-volume polystyrene waste into high-quality chemicals and make it available for other processes. Foams and insulating materials in particular are hardly suitable for profitable mechanical recycling through sorting, shredding and transport to produce new workpieces. The deg-up process presented here, on the other hand, is robust, tolerates contamination and is suitable as a platform for chemical upcycling of large-volume polystyrene waste.

There are various ways of breaking down polystyrene into its components, but most of them require expensive catalysts or are very energy-intensive. Liu and his team have now developed a thermochemical process that uses inexpensive aluminum chloride as a catalyst and runs at a moderate 80 °C. Another advantage is the intelligent use of the solvent benzene. “Only the benzene that is broken down from the polymer is converted into the desired chemical product. Unused solvent is recycled into the processing of more polymer feedstock,” says Liu.

This is how the proof was presented

To prove the strategy’s viability, the team dissolved polystyrene waste, such as packaging foam or plastic tableware, in benzene and heated the mixture in a reactor under the exclusion of air using aluminum chloride as the sole reagent. The liquid product, mainly benzene, could be converted directly into higher value chemicals in high yield and selectivity.

For example, by adding the reagent acetyl chloride, the team obtained acetophenone, which is an important basic chemical for the cosmetics and pharmaceuticals industries. A similar reagent, oxalyl chloride, gave rise to benzophenone, a component of sunscreens and an important plastics additive. Sulfur-containing aromatic products – some of which are high-performance solvents for the plastics industry – can also be obtained from the polystyrene waste with a high level of selectivity.

Plastics recycling: The big overview

(Bild: Bits and Splits – stock.adobe.com)

Do you want to know everything about plastic recycling? It is clear that sustainability does not end with the actual product: Products must be recycled according to their material characteristics and cycles closed. But which processes are actually used when recycling plastics? Are there limits to recycling? And what actually is downcycling and upcycling? You can find out everything you need to know about it here.