The Stache Lab at Cornell, led by Prof. Erin Stache, chemistry and chemical biology, has discovered an efficient way to decompose non-biodegradable plastic polystyrene — a material commonly used in everyday products like Styrofoam and food packaging — into simpler products such as benzoic acid — a chemical widely used for commercial and research purposes. Their experimental breakthrough, which can be efficiently scaled up, could prove to be an effective method for upcycling polystyrene waste.
Polystyrene makes up almost a third of the waste in landfills worldwide. The large-scale accumulation of polystyrene waste in the environment has led to a great deal of unsustainability, as it remains present in the environment almost indefinitely. Polystyrene also releases toxins like styrene and benzene, which react with nitrogen to form ground-level ozone — a hazardous pollutant. Additionally, as a floatable material, it has amassed along coastlines across the world as the main component of marine debris.
“There’s so much accumulation of plastic, but we don’t know how to get rid of them,” said Stache Lab researcher and lead author Sewon Oh grad. “They end up going to the ocean, landfills or even outside in the playground.”
Breaking down polystyrene into products like benzoic acid helps to reduce the amount of plastic waste by transforming it into simpler useful products.
There are already varied efforts to find ways to decompose polystyrene and reduce the amount of waste accumulation, but many entail complex reaction procedures that would be unfeasible on a larger scale.
For example, polystyrene can be heated with nitrogen and converted to a smaller molecule, which is then used to make new polystyrene — establishing a circular economy for the material. However, this method proves to be inefficient as it requires heating to a high temperature of 400 degrees celsius, and the amount of polystyrene recovered by the end is small compared to the initial sample.
To address this issue, the Stache Lab set out to find an efficient process to decompose polystyrene. They found that using light, oxygen and a small amount of an iron chloride catalyst could successfully decompose polystyrene to produce benzoyl products, such as benzoic acid. The group’s findings were published in the Journal of the American Chemical Society in a paper titled, “Chemical Upcycling of Commercial Polystyrene via Catalyst-Controlled Photooxidation.”
“We wanted to make sure that we break [down] polystyrene very efficiently, but also very cost-efficiently,” Oh said.
In line with this aim, the novel process developed by the Lab is mild and inexpensive. After screening different types of light, the group found that white light worked better than others. Since white light present in ambient sunlight, it makes the reaction feasible to perform from even a windowsill.
Oxygen – required for the reaction – is present in abundance and can be easily replenished. Likewise, the iron-based catalysts used to drive the reaction are also widely available.
Oh is currently working on finding ways in which the other products of the reaction could be utilized. For instance, some of the resulting polymers could be reused to make other types of plastics.
In the experimental trials, a range of commercial polystyrene samples ranging from white coffee cup lids, Styrofoam and a transparent food packaging lid, all degraded to yield benzoic acid. This was despite the presence of composite and insoluble material on these samples, affirming their applicability in the degradation of polystyrene waste.
The one commercial product that did not degrade as efficiently was a black coffee cup lid. The group hypothesized that this was because the black dyes inhibit light penetration since black dye would typically absorb all wavelengths of light. This implies that different colors in many commercial plastic products could prove to be a hindrance in future upcycling efforts.
However, according to Stache, the researchers have also found a solution to this problem .ince all dyes absorb certain wavelengths of light, developing catalysts that operate outside those wavelengths would allow the light hitting the sample to activate the catalysts and not be absorbed by the dyes.
In the pursuit of efficient waste management, this process would have to be successfully scaled up. After performing the initial trials with small samples, the group moved to larger samples weighing one or more grams, which were more representative of commercial polystyrene products.
“One of things we’re looking at is making this large-scale process even more efficient — trying to engineer different systems that will use very little energy,” Stache said.
The researchers are building on this project by moving forward in several directions, such as trying to develop catalysts that could work with longer wavelengths of light, like red and green. Simultaneously, the success of the experimental trials with polystyrene has led them to try and extend this to other commercial plastics.
The novel process developed by the Stache Lab could become a viable method of upcycling polystyrene waste, owing to its efficient and cost-effective nature. By sticking to simple abundant reactants, the group has managed to make an important stride to effectively manage plastic waste.