Milk cartons, toys, grocery bags, bubble wrap, machine parts, and even artificial hips. Every year, humanity makes more products from polyethylene than any other plastic, about 100 million metric tons in all. We also throw a lot of it away. Polyethylenes make up about 60% of the plastics in landfills worldwide, where they degrade slowly if at all. Now, researchers report that they’ve repurposed a pair of existing catalysts to break down a wide array of polyethylenes, converting them into liquid fuels and other valuable chemicals. Faster and more durable versions of the catalysts are in the works, and they could spur recycling efforts that prevent millions of tons of the plastics from clogging our landfills and swirling around the world’s oceans.
As the name suggests, polyethylenes are made of many copies of ethylene, a simple hydrocarbon building block with two carbon atoms surrounded by four hydrogens. Catalysts connect millions of these ethylenes into long chains, which can be linear or branching, affecting the rigidity, toughness, and density of the finished products. In most cases, the final polyethylenes are inert and stubbornly resistant to breakdown.
That durability stems from a simple fact: All the links between atoms are single bonds, which are highly stable and difficult to break, explains Zhibin Guan, a chemist at the University of California, Irvine, who helped lead the new effort. To change that, Guan and his colleagues teamed up with researchers led by chemist Zheng Huang at the Chinese Academy of Sciences in Shanghai to repurpose two existing catalysts. These catalysts, developed by University of North Carolina, Chapel Hill, chemist Maurice Brookhart and colleagues, are normally used to link short hydrocarbons, called alkanes, together into longer—and more valuable—hydrocarbon chains, such as those found in diesel fuel.
When the two catalysts are added to a batch of short alkanes, the first catalyst strips hydrogen atoms off adjacent carbon atoms in single alkane molecules. The newly free chemical handles bond to each other, forming a double bond between the neighboring carbon atoms. The double bonds create a weak link in the short alkane chains—a vulnerability that the second catalyst exploits to split the alkane chain. Split alkanes then react with each other, forming a soup of very short alkanes and medium-length alkanes. The latter typically contain 10 to 12 carbons—the perfect ingredients for diesel fuel.
Guan and Huang wondered whether the same process could work in reverse to break apart the very long polyethylenes, which can contain up to millions of carbons. To find out, they mixed polyethylene waste such as garbage bags with short liquid alkanes and then added in the two catalysts. Again, the first one stripped off hydrogens from adjacent carbon atoms in both the long polyethylene chains and short alkanes to form double bonds; the second split the molecules and randomly stitched split molecules back together. The result, which the researchers report today in Science Advances, is that they continue to break down the long chains until they reach the size of chains found in fuels and other valuable hydrocarbons.
“This is very innovative and a clever application of these catalysts,” Brookhart says. But he cautions that the process still needs work to be commercially viable. For starters, the catalysts break down polyethylene slowly, over the course of a day or more. They are also expensive and decompose after breaking apart just a few thousand polymer chains, far less than the millions carried out by most commercial catalysts. Guan and his colleagues are working to overcome those problems, in the hopes that they can one day extract new value from the millions of tons of plastic waste that we discard every year.