Polyethylenes, a type of plastic, are used in nearly everything, and 80 million tons are produced each year. Unfortunately, this also means that we throw a ton of it away, where it ends up in landfills or the ocean. Polyethylenes degrade very slowly or not all and represent a huge environmental problem. Now, scientists may have found a solution. In a newly published paper in Science Advances, a team of researchers delve into a chemical process that can break these polyethylenes down. They used a pair of catalysts to degrade the plastics, allowing them to be converted to fuel and other useful chemicals.
Polyethylenes are difficult to break due to the nature of their chemical bonds. They are relatively inert and consist of hydrocarbon chains. The single bonds holding them together are stable and almost impossible to break. Knowing this, the researchers decided to try two known catalysts. These catalysts are currently used to bond short hydrocarbons together, creating fuel. One catalyst removes the hydrogen atoms off the alkane, forcing the carbons to double bond. Double bonds are weaker and easier to break than single bonds so a second catalyst can then break the chain apart completely. These short alkane chains can now react with each other, forming longer chains used for diesel fuel.
The authors decided to try using the same catalysts on polyethylenes. They took polyethylene waste, such as bottles and plastic bags, and added short liquid alkanes to the mix. Then, they added the catalysts. The first catalyst removed hydrogen atoms, forcing the polyethylene chains and liquid alkanes to form double bonds. The second catalyst then bonded the polyethylenes and short alkanes together. The long polyethylene chains continued to be broken down until they resembled the hydrocarbons used for fuel.
While this research shows significant potential for reducing plastic waste and converting polyethylenes to useful materials, it’s not practical on a commercial scale just yet. The catalysts are expensive to purchase and work slowly, taking over a day to break down polyethylenes. They also need to be replaced frequently, they get “used up” after breaking down a few thousand polyethylene chains. Still, this technique shows a lot of promise for converting plastics into fuel and other valuable chemicals.
Jia, X., C. Qin, T. Friedberger, Z. Guan, and Z. Huang. Efficient and Selective Degradation of Polyethylenes into Liquid Fuels and Waxes under Mild Conditions. Science Advances (2016).