Mechanochemical recycling converts polystyrene to a commodity chemical

A solvent-free ball-milling process can break down waste polystyrene into benzene and aliphatic hydrocarbons in as little as 15 min (Angew. Chem., Int. Ed. 2025, DOI: 10.1002/anie.202512687). By also including benzoic anhydride in the reaction vessel, Junpeng Wang and his team at the University of Akron converted the freshly generated benzene directly to benzophenone, creating a one-pot method to valorize consumer waste into a useful chemical feedstock.

Polystyrene is one of the most recognizable and widely used plastics, with applications as diverse as packaging, construction, and medical equipment. However, the high cost of current recycling processes, combined with the low value of recovered material, means polystyrene is also one of the world’s least recycled plastics. In Spain, for example, 80% ends up in landfills and the remainder is incinerated.

Meanwhile, mechanochemistry, which drives chemical reactions by stimulating reactants via mechanical forces, is an inherently cost- and energy-efficient method. This simple process has already attracted interest as a strategy to handle plastic waste, including per- and polyfluoroalkyl substances (PFAS) and polyethylene. But rather than destroying or depolymerizing the plastic, Wang’s team focused on degrading polystyrene into simple benzene units, which they upcycled into valuable commodity chemicals.

The researchers began by grinding plastic pellets with an aluminum trichloride catalyst in a six-ball mill, driving a reaction known as a Friedel-Crafts dealkylation to form benzene and long-chain aliphatic hydrocarbons. Within 15 min, the reaction had degraded more than 90% of the starting polystyrene, a substantial time and energy savings compared with previous recycling methods.

With a route to benzene in hand, the team next explored coupling this degradation process with a second mechanochemical reaction, settling on a Friedel-Crafts acylation to generate benzophenone. The shared aluminum trichloride catalyst makes this sequence ideally suited to a one-pot procedure, and after an initial degradation period, benzoic anhydride was added directly to the ball mill and milled for a further 3 h. Initial iterations of the combined process produced a disappointing 11% yield, significantly less than the steps in isolation, but troubleshooting experiments quickly identified the cause of these low yields.

The heat generated by the milling process was sufficient to evaporate a substantial quantity of the benzene recovered from the polystyrene, leaving little available to react in the second step. Sealing the grinding apparatus with parafilm and incorporating a supramolecular trap to retain this volatile substrate immediately boosted the yield to 39%. Subsequent tests with real postconsumer waste—including Styrofoam boxes, spoons, and cups—produced even higher yields of benzophenone, demonstrating that the process is also compatible with typical plastic additives and contaminants.

Duncan Browne, a mechanochemist at University College London, found this an intriguing approach to polystyrene waste but suspects the volatility of benzene may make this process challenging to scale. “The key is that you want to know how much heat is generated from this process, and that’s something you would need to get a handle on before you go to too large a scale,” he says. “You’d need a chemical engineer to help you calculate those numbers and understand if you can do this safely [and] competitively with the current best practice.”

Despite this practical reservation, Browne believes this is nonetheless a significant finding. “Valorization is inherently sustainable, and mechanochemistry is also sustainable. So you’re almost amplifying sustainability by bringing these ideas together,” he says. “It’s a really nice demonstration of the potential of mechanochemistry as a solution to this type of high-impact problem.”