Plastic pollution is one of the main hazards our planet is currently facing, endangering a variety of ecosystems and contributing to climate change. Despite increased efforts, very little plastic is currently recycled, due to a range of economic and practical reasons. However, petroleum-based plastic waste presents an untapped source of carbon-based chemicals which could serve as the building blocks for useful durable materials and fuels.
A research team led by the Pacific Northwest National Laboratory (PNNL) has now discovered an innovative method to convert plastics more efficiently to valuable chemicals, through a process called “upcycling.” This technique is cheaper than previous ones and produces less methane.
“The key discovery we report is the very low metal load,” said study senior author Janos Szanyi, a chemist at PNNL. “This makes the catalyst much cheaper.” “It was very interesting to us that there had been nothing previously published showing this result,” added first author Linxiao Chen, a postdoctoral fellow at the same laboratory. “This research shows the opportunity to develop effective, selective, and versatile catalysts for plastic upcycling.”
Scientists have already known that adding hydrogen to difficult-to-recycle plastics such as polypropylene and polyethylene presents a promising strategy to convert plastic waste into value-added small hydrocarbons, through a process called hydrogenolysis. Now, the PNNL scientists discovered that reducing the amount of the precious metal ruthenium could improve the polymer upcycling efficiency and selectivity, by causing the structure to shift from an orderly array of particles to disordered rafts of atoms. Moreover, they showed that single atoms – or very small clusters of atoms – are more effective catalysts during this process.
In order to make the method practical to use with mixed plastic recycling streams, the researchers are now exploring how the presence of chlorine affects the efficiency of the chemical conversion.
“We are looking into more demanding extraction conditions,” said study co-author Oliver Y. Gutiérrez, an expert in industrial applications for catalysis at PNNL. “When you don’t have a clean plastic source, in an industrial upcycling process, you have chlorine from polyvinylchloride and other sources. Chlorine can contaminate the plastic upcycling reaction. We want to understand what effect chlorine has on our system.”