Scientists use air, inexpensive catalyst to break down plastic

Using moisture from the air, Northwestern University chemists in Evanston, Illinois, have developed a new method for breaking down end-of-life plastics.

The university says the nontoxic, environmentally friendly, solvent-free process uses an inexpensive catalyst to break apart the bonds in polyethylene terephthalate (PET), the most common plastic in the polyester family. The researchers then expose the material to ambient air, leveraging the trace amounts of moisture in the air to convert PET into monomers, the crucial building blocks for plastics. The researchers say the monomers could be recycled into new PET products or other, more valuable materials.

Described as safer, cleaner, cheaper and more sustainable than current plastic recycling methods, the new technique could offer a path toward creating a circular economy for plastics, Northwestern says.

The study, “Thermodynamically leveraged solventless aerobic deconstruction of polyethylene-terephthalate plastics over a single-site molybdenum-dioxo catalyst,” was published in Green Chemistry, a journal of the Royal Society of Chemistry. The research was supported by the U.S. Department of Energy.

“The U.S. is the No. 1 plastic polluter per capita, and we only recycle 5 percent of those plastics,” says Northwestern’s Yosi Kratish, the study’s co-corresponding author. “There is a dire 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. 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.”

“Our study offers a sustainable and efficient solution to one of the world’s most pressing environmental challenges: plastic waste,” adds Naveen Malik, the study’s first author. “Unlike traditional recycling methods, which often produce harmful byproducts like waste salts and require significant energy or chemical inputs, our approach uses a solvent-free process that relies on trace moisture from ambient air. This makes it not only environmentally friendly but also highly practical for real-world applications.”

Kratish, a research assistant professor of chemistry at Northwestern’s Weinberg College of Arts and Sciences, co-led the study with Tobin J. Marks, the Charles E. and Emma H. Morrison Professor of Chemistry at Weinberg and a professor of materials science and engineering at Northwestern’s McCormick School of Engineering. At the time of the research, Malik was a postdoctoral fellow in Marks’ laboratory; now he is a research assistant professor at the SRM Institute of Science and Technology in India.

In previous work, Marks’ group at Northwestern developed catalytic processes that do not require solvents. In the new study, the team again devised a solvent-free process.

“Using solvents has many disadvantages,” Kratish says. “They can be expensive, and you have to heat them up to high temperatures. Then, after the reaction, you are left with a soup of materials that you have to sort to recover the monomers. Instead of using solvents, we used water vapor from air. It’s a much more elegant way to tackle plastic recycling issues.”

The researchers used a molybdenum catalyst and activated carbon, both inexpensive, abundant and nontoxic materials, they say. To initiate the process, they added PET to the catalyst and activated carbon and heated the mixture. Polyester plastics are large molecules with repeating units linked together with chemical bonds. After a short time, the chemical bonds within the plastic broke apart.

They then exposed the material to air. With the moisture from air, the material turned into terephthalic acid (TPA), the precursor to polyesters. The only byproduct was acetaldehyde, a valuable, easy-to-remove industrial chemical, they say.

“Air contains a significant amount of moisture, making it a readily available and sustainable resource for chemical reactions,” Malik explains. “On average, even in relatively dry conditions, the atmosphere holds about 10,000 to15,000 cubic kilometers of water. Leveraging air moisture allows us to eliminate bulk solvents, reduce energy input and avoid the use of aggressive chemicals, making the process cleaner and more environmentally friendly.”

“When we added extra water, it stopped working because it was too much water,” Kratish says. “It’s a fine balance. But it turns out the amount of water in air was just the right amount.”

The process is fast and effective, the researchers say. In just four hours, 94 percent of the possible TPA was recovered. The catalyst also is durable and recyclable, meaning it can be used again without losing effectiveness. And the method works with mixed plastics, selectively recycling only polyesters. With its selective nature, the process bypasses the need to sort the plastics before applying the catalyst, they add.

When the team tested the process on plastic bottles, shirts and mixed plastic scrap, it proved just as effective, they say, even breaking down colored plastics into pure, colorless TPA.

The researchers plan to increase the scale of the process for industrial use. By optimizing the process for large-scale applications, they aim to ensure it can handle vast quantities of plastic.

“Our technology has the potential to significantly reduce plastic pollution, lower the environmental footprint of plastics and contribute to a circular economy where materials are reused rather than discarded,” Malik says. “It’s a tangible step toward a cleaner, greener future, and it demonstrates how innovative chemistry can address global challenges in a way that aligns with nature.”