Nylon-6, which is found in fishing nets, carpet, and clothing, is a major contributor to plastic pollution, including ocean pollution. Chemists have now developed a new catalyst that rapidly, cleanly, and completely degrades Nylon-6 in a matter of minutes – all while producing no harmful byproducts. Even better, the process does not require toxic solvents, costly materials, or extreme conditions, making it suitable for everyday use. The new process recovered 99% of the polymer’s building blocks in experiments, which can then be upcycled into higher-value products.
Many people are familiar with the disturbing images of wildlife entangled in abandoned fishing nets, including sea turtles, dolphins, and seals.
The main problem with Nylon-6, the plastic found in these nets, carpets, and clothing, is that it is too strong and durable to degrade on its own. As a result, once in the environment, it lingers for thousands of years, polluting waterways, destroying coral reefs, and strangling birds and sea life.
Northwestern University chemists have developed a new catalyst that rapidly, cleanly, and completely degrades Nylon-6 in a matter of minutes – all while producing no harmful byproducts. Even better, the process does not require toxic solvents, costly materials, or extreme conditions, making it suitable for everyday use.
Not only could this new catalyst play an important role in environmental remediation, it also could perform the first step in upcycling Nylon-6 wastes into higher-value products. The research is published in the journal Chem.
Our research represents a significant step forward in the field of polymer recycling and sustainable materials management. The novel approach addresses a critical gap in current recycling technologies, providing a practical and efficient solution to the problem of nylon waste.
Liwei Ye
“The whole world is aware of the plastic problem,” said Northwestern’s Tobin Marks, the study’s senior author. “Plastic is a part of our society; we use so much of it. But the problem is: What do we do when we’re finished with it? Ideally, we wouldn’t burn it or put it into landfills. We would recycle it. We’re developing catalysts that deconstruct these polymers, returning them to their original form, so they can be reused.”
Marks is the Charles E. and Emma H. Morrison Professor of Chemistry and Vladimir N. Ipatieff Professor of Catalytic Chemistry at Northwestern’s Weinberg College of Arts and Sciences and a professor of materials science and engineering at Northwestern’s McCormick School of Engineering. He also is a faculty affiliate at the Paula M. Trienens Institute for Sustainability and Energy. Northwestern co-authors include Linda J. Broadbelt, the Sarah Rebecca Roland Professor of Chemical and Biological Engineering and senior associate dean of McCormick, and Yosi Kratish, a research assistant professor in Marks’ group.
A deadly difficulty
Nylon-6 is found in a wide range of materials that most people use every day, from clothing to carpet to seat belts. However, when people dispose of these materials, they end up in landfills or, worse, are released into the environment, including the ocean. The World Wildlife Federation estimates that up to 1 million pounds of fishing gear is abandoned in the ocean each year, with Nylon-6 fishing nets accounting for at least 46% of the Great Pacific Garbage Patch.
“Fishing nets lose quality after a couple of years of use,” said Liwei Ye, the paper’s lead first author who is a postdoctoral fellow in Marks’ laboratory. “They become so water-logged that it’s difficult to pull them out of the ocean. And they are so cheap to replace that people just leave them in the water and buy new ones.”
“There is a lot of garbage in the ocean,” Marks added. “Cardboard and food waste biodegrade. Metals sink to the bottom. Then we are left with the plastics.”
The greenest solvent is no solvent
Currently, the only way to dispose of Nylon-6 is to bury it in landfills. When Nylon-6 is burned, it emits toxic pollutants such as nitrogen oxides, which have been linked to a variety of health problems including premature death, or carbon dioxide, a notoriously potent greenhouse gas.
Although other laboratories have investigated catalysts to degrade Nylon-6, those catalysts require extreme conditions (such as temperatures as high as 350 degrees Celsius), high-pressure steam (which is both expensive and inefficient in terms of energy), and/or toxic solvents that only contribute to pollution.
“You can dissolve plastics in acid, but then you are left with dirty water,” Marks said. “What do you do with that? The goal is always to use a green solvent. And what type of solvent is greener than no solvent at all?”
Recovering building blocks for upcycling
To avoid these problems, the researchers turned to a novel catalyst developed in Marks’ laboratory. The catalyst uses yttrium (a cheap metal found on Earth) and lanthanide ions. When the researchers heated Nylon-6 samples to melting temperatures and then applied the catalyst without a solvent, the plastic disintegrated, returning to its original building blocks with no byproducts.
“You can think of a polymer like a necklace or a string of pearls,” Mr. Marks said. “Each pearl is a monomer in this analogy. These monomers serve as the foundation. We devised a method to dismantle the necklace while retaining the pearls.”
In experiments, Marks and his team were able to recover 99% of plastics’ original monomers. In principle, those monomers then could be upcycled into higher-value products, which are currently in high demand for their strength and durability.
“Recycled nylon is actually worth more money than regular nylon,” Marks said. “Many high-end fashion brands use recycled nylon in clothes.”
Efficiently targeting Nylon-6
In addition to recovering a high monomer yield, the catalyst is highly selective, acting only on Nylon-6 polymers and not on surrounding materials. This means that the industry could apply the catalyst to large amounts of unsorted waste and target Nylon-6 specifically.
“If you don’t have a catalyst that’s selective, then how do you separate the nylon from the rest of the waste?” Marks stated. “You’d have to hire people to sort through all of the waste to remove the nylon. That is both expensive and inefficient. However, if the catalyst only degrades the nylon and leaves everything else alone, it is extremely efficient.”
Recycling these monomers also avoids the need to produce more plastics from scratch. “These monomers are produced from crude oil, so they have a huge carbon footprint,” Ye said. “That’s just not sustainable.”
What’s next?
Marks and his team have already received interest from potential industrial partners after filing a patent for the new process. They hope that their catalysts can be used on a large scale to help solve the global plastic problem.
“Our research represents a significant step forward in the field of polymer recycling and sustainable materials management,” he said. “The novel approach addresses a critical gap in current recycling technologies, providing a practical and efficient solution to the problem of nylon waste. We believe it has implications for reducing plastics’ environmental impact and contributing to a circular economy.”
