Among the major challenges facing efforts to chemically recycle common waste plastics is that many methods require a pure stream of a single type of polymer to be effective. Now, researchers at Northwestern University (NU; Evanston, Ill.; www.northwestern.edu), led by Tobin Marks and Yosi Kratish, devised a catalyst that could allow plastics-recycling processes that don’t require meticulous sorting of polymer types.
The NU team was searching for ways to break down unsorted plastic waste into useful mixtures of compounds using the lowest possible energy, and at the same time, using catalyst materials that are earth-abundant. They focused on hydrogenolysis, a process that uses hydrogen gas and a catalyst to break down polyolefins into useful hydrocarbons. Current hydrogenolysis approaches typically require high temperatures and catalysts made from metals like platinum and palladium.
The researchers developed a single-site organonickel catalyst, supported on sulfated alumina (AlS/NiIIH). The catalyst’s single-site design, with precisely defined and isolated active sites, exhibits unique selectivity — preferentially cleaving branched polyolefin carbon-carbon bonds, rather than indiscriminately breaking down the plastic’s entire structure. As a result, the catalyst allows for the selective breakdown of branched polyolefins (such as isotactic polypropylene) when they are mixed with unbranched polyolefins. This can effectively separate them chemically.
And, in an unexpected finding, the AlS/NiIIH catalyst remains highly selective and active for hydrogenolysis of iPP that has been admixed with polyvinyl chloride (PVC), a notoriously difficult polymer to recycle, the NU team pointed out.
“Compared to other nickel-based catalysts, our process . . . operates at a temperature 100 degrees lower and at half the hydrogen gas pressure,” Kratish says. “We also use 10 times less catalyst loading, and our activity is 10 times greater.” Further, spent catalyst can be repeatedly regenerated by treatment with triethylaluminum.
These results, published in a recent issue of Nature Chemistry, “highlight the potential of nickel-based systems for the selective upcycling of complex plastic waste streams,” the researchers say.