US chemists develop method to break down plastic waste using air moisture

Chemists at Northwestern University, US, have developed a cost-effective method to break down plastic waste by harnessing moisture from the air. They describe this solvent-free approach as offering a sustainable alternative to current plastic packaging recycling methods.

The research, published in Green Chemistry, employed a molybdenum catalyst and activated carbon — both abundant, inexpensive, and non-toxic materials to break apart the bonds in PET, commonly found in F&B packaging.

Yosi Kratish, research assistant professor at Northwestern University and the study’s co-corresponding author, tells Packaging Insights: “This technology originated from my earlier work, which utilized the same catalyst but with hydrogen gas. What’s particularly exciting about our research is that we harnessed moisture from air to break down the plastics, achieving an exceptionally clean and selective process.”

“By recovering the monomers, which are the basic building blocks of PET, we can recycle or even upcycle them into more valuable materials.”

Kratish claims there is a desperate need for better technologies that can process different types of plastic waste.

“Most of the technologies that we have today melt down plastic bottles and downcycle them into lower-quality products,” he says.

Recycling plastic

The process developed by the research team involves adding PET to the catalyst and activated carbon and applying heat to the mixture. Polyester plastics are large molecules with repeating units linked by chemical bonds. Within a short period, the bonds within the plastic broke apart. 

The next step is to expose the material to air. Utilizing the trace amounts of moisture present in the air, the material converts into terephthalic acid (TPA), a highly valuable precursor to polyesters.

PET recycling is not fully circular at the moment.Kratish says the biggest challenge was addressing safety concerns related to the use of explosive gas.

“With additional research, we found that we can replace hydrogen gas with ambient moisture, significantly improving safety. A surprising outcome was that the reaction rate also accelerated considerably,” he explains.

“Another major challenge involved equipment limitations — since we are working with a heterogeneous solid catalyst, a highly viscous PET polymer melt, and moisture from the gas phase, effective mixing in this three-phase system was critical. At our current scale, we have successfully addressed this challenge.”

Key advantages

When improperly disposed of, plastic accumulates in landfills or degrades into microplastics and nanoplastics, which contaminate waterways and can pose environmental and health risks.

According to the scientists, current plastic recycling methods require extreme high temperatures, intense energy inputs, and solvents, resulting in toxic byproducts. Furthermore, these methods often rely on expensive catalysts, such as platinum and palladium. The separation of recycled materials from solvents is another challenge, adding to the complexity and cost of recycling.

“A key advantage of our catalyst is its selectivity for polymers such as PET. It can also effectively depolymerize other polyester-based polymers,” highlights Kratish.

Another advantage of this process is its speed and efficiency. Within just four hours, 94% of the potential TPA is recovered. Additionally, the only byproduct generated is acetaldehyde, a valuable industrial chemical that is easy to remove. The catalyst is durable and can be reused multiple times without losing its effectiveness.

“Looking ahead, we envision a system capable of processing mixed plastic waste, such as textiles that often contain multiple polymer fibers. By customizing catalysts to selectively depolymerize one polymer at a time, we can significantly reduce the purification steps required to achieve fiber- and food-grade monomers,” Kratish continues.

“This selective approach is crucial for making chemical recycling of mixed plastic waste more efficient and commercially viable.”

Future prospects

The research team tested their process on real-world materials, including plastic bottles, clothing, and mixed plastic waste. The method proved equally effective across different samples, breaking down colored plastics into pure, colorless TPA.

“We have developed the capability to produce large quantities of the catalyst, but scaling up requires transitioning to larger-scale processing equipment to demonstrate feasibility. We are actively seeking partners and funding to facilitate this transition,” says Kratish.

“To advance commercialization, I recently incorporated NylaNova, a company dedicated to developing and scaling our innovative plastic recycling processes. Our primary focus is on nylon and PET recycling processes which operate in a similar manner. That is, no solvent mild conditions, and the monomer is removed during the reaction and isolated.”