A research team from the Dalian Institute of Chemical Physics in China has developed a new process that uses sunlight to convert polystyrene waste and excess sulfur into valuable organic products. They published their findings in the Journal of the American Chemical Society.
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The chemical industry produces over 20 million tons of polystyrene annually for food packaging, electronics and insulation, but less than 1% of the material is recycled. Current recycling methods require large amounts of energy and are inefficient.
The new method uses excess elemental sulfur, a byproduct of crude oil and natural gas refining. An estimated 70 million tons of elemental sulfur is produced each year. This recycling method could find a climate-friendly use for this waste.
Sulfur’s dual role
The scientists combined excess elemental sulfur with polystyrene waste, driving the process with solar energy, to generate organic products. The organic products produced included 2,4-diphenylthiophene, used in semiconductor materials, and 1,3,5-triphenylbenzene, a versatile organic building block. The method was used to recycle various waste products, including spoons, cups, food packaging, disposable test tubes and culture plates.
Mechanistic studies found that sunlight causes a photothermal effect in the sulfur, raising its temperature and generating sulfur radicals. These radicals abstract hydrogen atoms from the polystyrene backbone, causing chain scission and producing sulfur-containing and unsaturated olefin intermediates.
Hydrated sulfur radicals then react with these intermediates through further transformation steps to produce the final compounds, with H2S generated as a byproduct at a yield of 58% relative to the sulfur used. The process consistently produces 2,4-diphenylthiophene and 1,3,5-triphenylbenzene in an approximately 2:1 ratio, a selectivity governed by the intrinsic chemistry of the reaction rather than external conditions such as sulfur dosage or light intensity. The full conversion takes as little as two minutes under solvent-free ambient air conditions.
From waste to valuable products
Up to 40% of the final reaction mixture is partially degraded polystyrene by weight. However, the team found that this could be used to fully depolymerize polystyrene or act as a UV-blocking additive in polystyrene films.
Gram-scale transformation has demonstrated a yield of 21% 2,4-diphenylthiophene and 10% of 1,3,5-triphenylbenzene, slightly lower than those achieved at lab scale. The team notes that the process is potentially compatible with large-scale existing photothermal equipment.
The team is now working on scaling up the process and extending it to other common plastics such as polyethylene, polypropylene and polyvinyl chloride.
