Cheap catalysts recycle polystyrene into valuable products

Polystyrene, the polymer commonly used to make foam packaging, insulation, and food containers, is notoriously difficult to recycle. Instead of turning the material into new polystyrene products, breaking it down chemically into more valuable substances could be a more cost-effective alternative. So far, though, chemical recycling has been energy-intensive and expensive. Two groups of researchers have now independently come up with simple, low-cost processes that use light to drive the catalytic breakdown of polystyrene into commodity chemicals.

One group uses an acid as a catalyst and violet-blue light to cleave the strong carbon-carbon and carbon-hydrogen bonds in polystyrene (J. Am. Chem. Soc. 2022, DOI: 10.1021/jacs.2c01410). The other uses white light to trigger an iron chloride catalyst to break the bonds (J. Am. Chem. Soc. 2022, DOI: 10.1021/jacs.2c01411).

Older methods of chemically cracking polystyrene’s bonds tend to be complex, use harsh chemicals and high temperatures, and “almost inevitably produce a soup of many compounds that are difficult to separate, so the value is decreased,” says Jianliang Xiao, a chemist at the University of Liverpool who led the acid catalyst study.

The two teams set out to create simple processes that generate just a few products that can be easily harvested from the reaction mixture. Both reactions use oxygen, light-emitting diodes (LEDs), and readily available catalysts to generate benzoic acid—which can be worth about twice as much as polystyrene by weight, according to market research firm ChemAnalyst.

Xiao says that his graduate student Zhiliang Huang made the “surprising discovery” that acids work better than previously studied metal catalysts to cleave bonds in polystyrene. When irradiated with a high-power blue-violet LED, the acid reacts with the polystyrene to form reactive oxygen species, which then cause a chain reaction that breaks up polystyrene’s strong bonds. The resulting formic acid and benzoic acid were easy to separate from the reaction mixture.

Meanwhile, Cornell University chemist Erin E. Stache and her graduate student Sewon Oh chose iron chloride as a catalyst because chlorine radicals are known to break strong C–H bonds. Under white-light irradiation and oxygen-rich air, those broken C–H bonds created peroxy radicals, which in turn cleave C–C bonds, degrade the polymer, and make benzoic acid.

Although the processes are similar, each has its advantages. The acid-catalyzed process gives a benzoic acid yield of about 50% and even higher yields of formic acid as a side product, while the iron catalyst achieves 23% benzoic acid yield. However, the acid catalyst is double the price of the iron chloride, Stache says. And Xiao adds that Stache’s team’s use of white light might reduce cost and energy use.

In tests, the methods worked for a range of commercial polystyrene products, including disposable cup lids, food containers, and polystyrene foam. Both teams demonstrated that their recycling processes could work in a flow reactor, which would be needed for commercialization.

The downside of the techniques is their use of light, says Frank Leibfarth, a chemist at the University of North Carolina at Chapel Hill. “Scaling up photochemistry is challenging, especially on the scale that commodity polymers are produced,” he says. “The development of a method that works thermally would have a higher potential for making it to market.”

Still, the upcycling of polystyrene into defined products with a clear market using inexpensive, earth-abundant catalysts is an important advance, Leibfarth says. Tackling the challenge of plastic waste will require many technologies, and these approaches to turn polystyrene into benzoic acid “could be one piece of that puzzle.”