The age of pre-sorting mixed plastic waste may soon be over. The secret weapon? A cheap catalyst made from nickel that targets one of our most problematic polymers. The findings are detailed in a study published September 2 in the journal Nature Chemistry.
Even after decades of worldwide efforts, recycling plastic remains much easier said than done. The frustrating reality is largely thanks to a group of polymers called polyolefins. Humans manufacture roughly 220 million tons of polyolefin-based products every year, most of which are single-use items like condiment bottles, milk jugs, plastic wrap, trash bags, and juice cartons.
“Basically, almost everything in your refrigerator is polyolefin based,” Northwestern University chemist and study co-author Yosi Kratish said in a statement.
Plastics are typically broken down using catalysts–compounds capable of exploiting weak chemical bonds to kickstart decomposition in the materials that otherwise take hundreds or thousands of years to deteriorate.
We annually recycle less than 10 percent of polyolefin products, resulting in mountains of waste destined for either landfills or industrial furnaces. That’s because while other plastics are typically broken down with catalysts, polyolefins are a different story. These resilient polymers resist eroding due to tiny molecules linked by notoriously tough carbon-carbon bonds.
“Polyolefins don’t have any weak links. Every bond is incredibly strong and chemically unreactive,” said Kratish.
Our current solutions aren’t “solutions” so much as stopgaps. Polyolefin products can be shredded, melted, and downcycled into low-quality plastic pellets, but even then there are caveats. Human-assisted separating is still necessary, and even the smallest amounts of food residue or non-plastic material can compromise an entire batch. Meanwhile, burning polyolefins requires temperatures as high as 1,292 degrees Fahrenheit.
“Everything can be burned, of course,” said Kratish. “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.”
A potential solution may reside in hydrogenolysis, a process in which a combination of hydrogen gas and a catalyst deconstruct polyolefin plastic into actually useful hydrocarbons. Existing hydrogenolysis options also involve high temperatures and expensive, noble metal-derived catalysts, but Kratish’s team found a workaround.
Unlike rare earth metals like palladium and platinum, engineers discovered that a synthesized alternative called cationic nickel is cheap, abundant, and easy to amass. Other nickel-based catalysts include multiple reaction sites. Cationic nickel’s single-site variant allows it to function more like a precise laser or sharp knife. Instead of breaking down all of a plastic’s structure, this option specifically targets those resilient carbon-carbon bonds at a much lower temperature and with half the hydrogen gas pressure. The new catalyst is so stable that it holds up to infamous, contaminant-heavy plastics like PVC.
“Adding PVC to a recycling mixture has always been forbidden. But apparently, it makes our process even better,” Kratish said. “That is crazy. It’s definitely not something anybody expected.”
If proven to be scalable and efficient, the new catalyst could largely eradicate the need for painstaking plastic pre-sorting while also drastically reducing the amount of microplastics released into the environment every day.