Plastics upcycling with a zirconia-based catalyst

Researchers from the Institute for Cooperative Upcycling of Plastics (iCOUP), led by the US Department of Energy’s Ames Laboratory, have developed a new type of catalyst to break down polyolefin plastics into new, useful products. Described in the journal Nature Catalysis, the catalyst is made only of earth-abundant materials, which can break carbon–carbon (CC) bonds in aliphatic hydrocarbons.

Aliphatic hydrocarbons are organic compounds made up of only hydrogen and carbon. Polyolefin plastics are aliphatic hydrocarbon materials composed of long chains of carbon atoms linked together to form strong materials. These materials are a big part of the plastic waste crisis, with study co-leader Wenyu Huang noting that more than half of produced plastics so far are polyolefin based.

Polyolefin plastics are used everywhere in the modern world, including in shrink wrap and other packaging products, containers for liquids such as detergents or milk, fibres in waterproof clothing, dental floss and electronics. They are also some of the most difficult plastics to recycle, according to study co-leader Andreas Heyden, and so new approaches are needed. One such promising alternative to recycling is known as upcycling, which involves the chemical transformation of the materials into higher value products.

One way to upcycle polyolefins is a chemical process called hydrogenolysis. During this process, a catalyst splits chains of molecules by cutting CC bonds and adding hydrogen. According to study co-leader and iCOUP Director Aaron Sadow, catalysts that are used for hydrogenolysis are typically based on precious metals, such as platinum. Platinum is used in many types of catalytic transformations due to its high effectiveness, but it is also expensive due to its low abundance in the Earth’s crust.

“We thought we’d be able to use earth-abundant elements to create much cheaper catalytic materials, and by assembling these elements in a certain way we might achieve a high selectivity and still very good activity,” Heyden said.

The team discovered that zirconia, an earth-abundant metal oxide, can cut CC bonds in aliphatic hydrocarbon polymers at about the same speed of precious metal catalysts. Sadow noted, “We were surprised that we could do hydrogenolysis of CC bonds using zirconium oxide as the catalyst. The conventional paradigm is that zirconia is not very reactive on its own.”

The key lay in the structure of the catalyst, which was designed by Huang and his group. Huang explained, “In this architecture, ultrasmall zirconia nanoparticles are embedded between two plates of mesoporous silica. The two silica plates are fused with the zirconia embedded in the middle, like a sandwich. The pores in the silica provide access to the zirconia, while the sandwich-like structure protects the zirconia nanoparticles from sintering or crystallisation, which would make them less effective.”

Heyden’s team was in charge of modelling the reaction and understanding where and how the active site works under reaction conditions; he explained, “For that we do both quantum chemical modelling of the catalyst and the chemical reactions together with some classical chemical reactor modelling. And here we really saw the importance of that amorphous zirconia structure.”

According to Sadow, the idea to study zirconia in hydrogenolysis was based on previous pioneering research of polymer depolymerisation using zirconium hydrides studied in the late 1990s. “Harnessing zirconium hydrides for hydrogenolysis is really nice chemistry,” he said. “The problem is those zirconium organometallic species are really air and water sensitive. So they have to be handled under the cleanest of conditions. Typically polymer waste is not pure and isn’t supplied as a clean and perfectly dry starting material. Using a zirconium hydride catalyst, you’d have to really worry about impurities that inhibit the chemistry.”

The team’s new zirconia material is simply heated under vacuum before the reactions, and it stays active during the hydrogenolysis process. Sadow noted, “Zirconium oxide is easily handled in air and then activated. It doesn’t require any kind of really specialised conditions, which was also exciting.

“Being able to take an air-exposed metal oxide, heat it with an alkane and generate an organometallic is a really powerful reaction that enables this kind of hydrogenolysis process. It potentially could enable lots of interesting catalytic transformations of hydrocarbons that were previously not considered.”

Image caption: Cartoon representation of the zirconia catalyst. The teal shows the mesoporous silica plates; the red represents the zirconia nanoparticles between the two sheets. The polymer chains enter the pores, contact the zirconia nanoparticles and are cut into shorter chains.