Chemists crack catalytic chemical recycling of polyethylene

Chemists at the University of California, Berkeley, have found a way to turn polyethylene (PE) plastics into propene, which can then be used to make new materials. This method of chemical recycling is one of the few to target the commonly used PE and one of the first to completely break down the plastic into a single product. Organic chemist John Hartwig and his group used three catalysts to transform PE into alkenes and then create propene monomers (Science 2022, DOI: 10.1126/science.add1088). The reaction makes few side products, yields over 80% propene, and works under mild conditions. The team used an iridium catalyst to remove hydrogens from PE, creating alkenes, and then a ruthenium catalyst to transform these to terminal alkenes. The Palladium catalyst works together with a ruthenium catalyst in a process called isomerizing ethenolysis to produce propene and another alkene with one less carbon in its chain. That alkene reenters the ethenolysis cycle to produce another propene. “So if you have polyethylene that we cut into chains of 100 carbons, it does that [cycle] 100 times to get down to the end of the chain,” Hartwig says. PE is particularly tricky to take apart, he says, because it’s composed of unstrained carbon-carbon single bonds, which require extreme temperatures to break. This new method proceeds at 200 °C. Though not yet ready for commercial use, the method shows that chemists can transform hard-to-react waste PE into something useful via common organic reactions.