New zirconia-based catalyst can make plastics upcycling more sustainable

A novel catalyst breaks down polyolefin plastics into new, useful products. This project is part of a new strategy to reduce the amount of plastic waste and its impact on our environment, as well as to recover value lost by throwing away plastic. The catalyst was developed by a team from the Institute for Cooperative Upcycling of Plastic (iCOUP), a US Department of Energy, Energy Frontier Research Center. The effort was led by Aaron Sadow, director of iCOUP, scientist at Ames National Laboratory and professor at Iowa State University; Andreas Heyden, professor at the University of South Carolina; and Wenyu Huang, a scientist at Ames Lab and a professor at Iowa State. The new catalyst consists only of earth-abundant materials that they have shown 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 bonded together to form strong materials. These materials are a big part of the plastic waste crisis. Wenyu Huang said, “More than half of the plastics produced to date are based on polyolefins.”

Polyolefin plastics are used everywhere in the modern world, including in shrink wrap and other packaging products, containers for liquids like detergent or milk, fibers in waterproof clothing, dental floss, and electronics. However, as Andreas Heyden explained, polyolefins are among the most difficult plastics to recycle and new approaches are needed. One such promising alternative to recycling is so-called upcycling. This approach involves the chemical conversion of the materials into higher value products.

One way to recycle polyolefins is through a chemical process called hydrogenolysis. A catalyst splits molecular chains by breaking CC bonds and adding hydrogen. According to Aaron Sadow, catalysts used for hydrogenolysis are typically based on precious metals such as platinum. Platinum is expensive due to its low abundance in the earth’s crust and is used in many types of catalytic conversions due to its effectiveness.

To address both sustainability and economic challenges, Heyden said, “We thought we might be able to take elements that are abundant on Earth to make much cheaper catalytic materials, and by assembling those elements together into one.” certain way we could achieve high selectivity and still very good action.”

The team discovered that zirconia, an earth-abundant metal oxide, can cleave C-C bonds in aliphatic hydrocarbon polymers at about the same rate as precious metal catalysts. “We were surprised that we could perform C-C bond hydrogenolysis using zirconia as a catalyst. The traditional paradigm is that zirconia alone is not very reactive,” Sadow said.

The key to success is the structure of the catalyst, designed by Wenyu Huang and his group. “In this architecture, ultra-small zirconia nanoparticles are embedded between two sheets of mesoporous silica. The two silica plates are fused, with the zirconia embedded in the middle, like a sandwich,” Huang said. “The pores in the silica provide access to the zirconia, while the sandwich-like structure protects the zirconia nanoparticles from sintering or crystallization, which would make them less effective.”

Heyden’s team was responsible for modeling the reaction and understanding where and how the active site works under reaction conditions. “For this, we perform both a quantum chemical modeling of the catalyst and the chemical reactions together with some classical modeling of chemical reactors,” he explained. “And that’s where we really saw the importance of this amorphous zirconia structure.”

According to Sadow, the idea of ​​studying zirconium oxide in hydrogenolysis was based on earlier pioneering research on polymer depolymerization using zirconium hydrides, investigated in the late 1990s. “Using zirconium hydrides for hydrogenolysis is really beautiful chemistry,” he said. “The problem is that these organometallic zirconium species are really air and water sensitive. They must therefore be handled under the cleanest conditions. Typically, polymer waste is not pure and is not supplied as a clean and perfectly dry starting material. Using zirconium hydride catalyst, you would have to be really concerned about impurities inhibiting the chemistry.”

The new zirconia material the team developed is simply heated under vacuum prior to reactions and remains active throughout the hydrogenolysis process. “Zirconium oxide is easy to handle in air and then activate. It doesn’t require any really special conditions, which was also exciting,” said Sadow. “Being able to take a metal oxide exposed to air, heat it with an alkane and create an organometallic is a really powerful reaction that enables this type of hydrogenolysis process. It could potentially enable many interesting catalytic conversions of hydrocarbons that have not previously been considered.”

This research is further discussed in the article “Ultrasmall amorphous zirconia nanoparticles catalyze polyolefin hydrogenolysis” by Shaojiang Chen, Akalanka Tennakoon, Kyung-Eun You, Alexander L. Paterson, Ryan Yappert, Selim Alayoglu, Lingzhe Fang, Xun Wu, Tommy Yunpu Zhao, Michelle P Lapak, Mukunth Saravanan, Ryan A Hackler, Yi-Yu Wang, Long Qi, Massimiliano Delferro, Tao Li, Byeongdu Lee, Baron Peters, Kenneth R Poeppelmeier, Salai C Ammal, Clifford R Bowers, Frédéric A Perras, Andreas Heyden, Aaron D. Sadow, and Wenyu Huang, and published in the natural catalysis.