Chemists at Northwestern University have developed a new catalyst that could simplify the recycling of polyolefin plastics, which are widely used in single-use items such as trash bags, condiment bottles, milk jugs, and shampoo bottles. The research introduces a nickel-based catalyst that can selectively break down these plastics without requiring them to be sorted by type beforehand.
The process targets polyethylenes and polypropylenes, which make up nearly two-thirds of global plastic consumption. When applied to unsorted waste, the catalyst converts solid plastics into liquid oils and waxes. These products can then be upcycled into higher-value materials like lubricants, fuels, and candles. The catalyst remains effective even when the plastic waste is contaminated with polyvinyl chloride (PVC), a substance that typically renders plastics unrecyclable.
“One of the biggest hurdles in plastic recycling has always been the necessity of meticulously sorting plastic waste by type,” said Northwestern’s Tobin Marks, the study’s senior author. “Our new catalyst could bypass this costly and labor-intensive step for common polyolefin plastics, making recycling more efficient, practical and economically viable than current strategies.”
“When people think of plastic, they likely are thinking about polyolefins,” said Northwestern’s Yosi Kratish, a co-corresponding author on the paper. “Basically, almost everything in your refrigerator is polyolefin based — squeeze bottles for condiments and salad dressings, milk jugs, plastic wrap, trash bags, disposable utensils, juice cartons and much more. These plastics have a very short lifetime, so they are mostly single-use. If we don’t have an efficient way to recycle them, then they end up in landfills and in the environment, where they linger for decades before degrading into harmful microplastics.”
Globally each year more than 220 million tons of polyolefin products are produced. However, according to a 2023 report in Nature journal (https://www.nature.com/articles/s41586-023-05952-2), only between less than 1% to 10% of these plastics are recycled worldwide due to their durable composition.
Kratish explained that designing catalysts for these materials is challenging because all bonds within polyolefins are strong carbon-carbon links: “When we design catalysts, we target weak spots,” Kratish said. “But polyolefins don’t have any weak links. Every bond is incredibly strong and chemically unreactive.”
Current recycling methods require meticulous sorting or involve high temperatures—upwards of 400 to 700 degrees Celsius—to degrade the material but consume significant energy resources.
“Everything can be burned, of course,” Kratish said. “If you apply enough energy you can convert anything to carbon dioxide and water. But we wanted to find an elegant way to add the minimum amount of energy to derive the maximum value product.”
The team focused on hydrogenolysis—a process using hydrogen gas with a catalyst—to break down polymers into smaller hydrocarbons at lower temperatures than traditional approaches using expensive noble metals like platinum or palladium.
“The polyolefin production scale is huge but the global noble metal reserves are very limited,” said Qingheng Lai from Marks’ group at Northwestern University. “We cannot use the entire metal supply for chemistry. And even if we did there still would not be enough to address the plastic problem. That’s why we’re interested in Earth-abundant metals.”
By developing a single-site molecular nickel catalyst synthesized from readily available compounds rather than nanoparticles with multiple reaction sites found elsewhere in industry applications—the researchers achieved selective breakdown while operating at lower temperature (100 degrees less) and pressure (half as much hydrogen gas) compared with other nickel-based systems.
“Compared to other nickel-based catalysts our process uses a single-site catalyst that operates at a temperature 100 degrees lower and at half the hydrogen gas pressure,” Kratish said. “We also use 10 times less catalyst loading and our activity is 10 times greater. So we are winning across all categories.”
The new method also showed resilience against PVC contamination; surprisingly PVC improved catalytic performance rather than deactivating it—a challenge that previously made mixed-plastic recycling difficult.
“Adding PVC to a recycling mixture has always been forbidden,” Kratish said. “But apparently it makes our process even better. That is crazy It’s definitely not something anybody expected.”
The research was published September 2nd in Nature Chemistry under support from U.S Department of Energy award DE-SC0024448 along with The Dow Chemical Company.