Researchers in the United States have developed a nickel-based single-site catalyst that could reshape polyolefin recycling, making the process more efficient, selective, and cost-effective compared to current methods.
Polyolefin recycling: new catalysts set to revolutionize the industry
Breakthroughs in polyolefin recycling, one of the most widely used (and produced) plastic families in the world, are emerging from the United States. A team of researchers has developed a highly selective new catalyst for these polymers that could simplify, or even eliminate, the costly process of sorting mixed plastic waste.
“One of the biggest obstacles to plastic recycling has always been the need to meticulously separate waste by type,” said Tobin Marks of Northwestern University, senior author of the study. “Our new catalyst could bypass this expensive and labor-intensive step for common polyolefin plastics, making polyolefin recycling more efficient, practical, and economically viable than current strategies.”
What polyolefins are
This family of plastics includes low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP), mostly fossil-based polymers whose multiple carbon-carbon bonds give them high chemical inertia and stability.
Industrial-scale recycling is still largely based on mechanical treatments. Waste is shredded into flakes, melted, and subjected to downcycling to form low-quality plastic pellets. This route has another drawback: different plastics have varying properties and melting points, requiring meticulous sorting before actual processing.
An alternative is thermal depolymerization, which breaks C–C bonds to produce gas and liquid mixtures. However, the process requires temperatures above 400 °C, high pressures, or costly catalysts. In addition, it remains poorly selective and expensive.
Chemical recycling, the most promising path
Against this backdrop, chemical recycling is gaining ground as the most promising, though still immature, industrial solution. The approach converts post-consumer plastic waste into high-purity small-molecule precursors that can be used for repolymerization. Research focuses largely on developing catalysts and catalytic processes that can facilitate the breaking of carbon-carbon bonds.
This is precisely where Northwestern University’s work comes in. Marks and his team turned to hydrogenolysis, a process that uses gaseous hydrogen and a catalyst to break down polyolefin polymers into smaller, useful monomers. While hydrogenolysis is not new, existing approaches typically require extremely high temperatures and expensive noble-metal catalysts such as platinum and palladium.
A new nickel-based catalyst
The Northwestern team synthesized a single-site molecular catalyst using more affordable cationic nickel. The single-site design enables the catalyst to act like a highly specialized scalpel, selectively cutting carbon-carbon bonds, instead of breaking down the entire plastic structure indiscriminately.
As a result, the catalyst allows selective breakdown of branched polyolefins (such as isotactic polypropylene) when mixed with unbranched ones, effectively separating them chemically. “Compared to other nickel-based catalysts, our process uses a single-site catalyst that operates 100 degrees lower in temperature and at half the hydrogen gas pressure,” said Kratish. “We also use ten times less catalyst, with activity ten times higher. In every category, we’re outperforming the alternatives.”
Read the article “Stable single-site organonickel catalyst preferentially hydrogenolyses branched polyolefin C–C bonds” in Nature Chemistry.
