How scientists are using air to recycle plastic

A team of chemists has recently introduced a non-toxic, eco-friendly, solvent-free approach to recycling plastic by using moisture found in ambient air. 

In a study published in the journal Green Chemistry, the experts describe how a low-cost catalyst splits apart the bonds in polyethylene terephthalate (PET) – the most common polyester plastic. 

After that, the fragmented plastic is exposed to ordinary air, converting it into monomers that could be reconstituted into new PET products or higher-value materials. The team’s findings raise the potential for a more cost-effective, sustainable path toward creating a circular economy for plastics.

Yosi Kratish is a research assistant professor of chemistry in the Weinberg College of Arts and Sciences at Northwestern University.

“The U.S. is the number one plastic polluter per capita, and we only recycle 5% of those plastics. There is a dire need for better technologies that can process different types of plastic waste,” said Kratish.

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

Using moisture to recycle plastic

According to Kratish, what is particularly exciting about the research is that the team 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.”

Naveen Malik, the paper’s first author, noted that the study offers a sustainable and efficient solution to one of the world’s most pressing environmental challenges: plastic waste.

“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,” said Malik. 

Kratish co-led the study with Tobin J. Marks, a longtime chemistry professor at Weinberg and a professor of materials science and engineering at Northwestern’s McCormick School of Engineering. 

Critical need for plastic solutions

PET plastics, commonly used in items such as food packaging and beverage bottles, make up approximately 12% of all plastics globally. 

Their persistence in the environment underscores a major dilemma: PET is resistant to decomposition and therefore liable to end up in landfills, fragment into microplastics or nanoplastics that seep into water sources, or accumulate in the natural environment.

Scientists worldwide are seeking new approaches to manage or recycle plastic, but existing processes often employ harsh conditions – extreme temperatures, high energy demands, strong solvents – that generate toxic byproducts. 

The required catalysts can also be expensive or hazardous, compounding waste problems. Furthermore, once the recycling reaction finishes, researchers face a tough job of extracting useful materials from the leftover solvents, adding cost and complexity.

Solvent-free method for recycling 

In prior work, Marks’ group at Northwestern pioneered catalytic methods that operate without solvents. This new paper builds on that achievement, once again using a solvent-free reaction approach.

“Using solvents has many disadvantages,” Kratish said. “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.”

Simple chemistry to recycle plastic

To test this new process, the team combined PET with a low-cost, non-toxic molybdenum catalyst plus activated carbon, then heated the mixture. Through this step, the large molecules of PET began to break down. 

Next, the scientists introduced the fragmented plastic to ambient air, which contains minimal levels of water vapor, triggering a conversion of plastic remnants into monomers known as terephthalic acid (TPA) – the valuable precursor for making new polyesters. 

The process yielded just one byproduct, acetaldehyde, which is considered relatively straightforward to remove in industrial contexts.

“Air contains a significant amount of moisture, making it a readily available and sustainable resource for chemical reactions,” Malik noted. 

“On average, even in relatively dry conditions, the atmosphere holds about 10,000 to 15,000 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.”

Tasting the recycling technology

In preliminary trials, 94% of the TPA was recovered in merely four hours, demonstrating both speed and efficiency. The catalyst was shown to be robust and reusable, making it a practical choice for large-scale applications. 

Because the technique selectively breaks down only polyester plastics, other types of plastic or non-plastic contaminants in a mixed batch are effectively bypassed.

When tested on actual plastic bottles, shirts, and mixed plastic waste, the process functioned similarly well, even eliminating dyes so that the recovered TPA turned out colorless.

Scaling up the new method

The scientists’ next move involves scaling up the method to meet industrial demand and handle bigger loads of discarded plastic.

By optimizing factors such as reaction time, catalyst concentration, and temperature, they hope to create an economical solution for converting waste plastics into building blocks that can then be turned into new products.

“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,” said Malik.

“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.”

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