Turning plastic waste into valuable chemicals with single-atom catalysts - Nanotechnology - Nanomaterials

Pathways for toluene production from crude oil and PS waste. Credit: Wang et al.

The rapid accumulation of plastic waste is currently posing significant risks for both human health and the environment on Earth. A possible solution to this problem would be to recycle plastic waste, breaking it into smaller molecules that can be used to produce valuable chemicals.

Researchers at Nanjing Forestry University and Tsinghua University recently introduced a new approach to convert polystyrene (PS), a plastic widely used to pack some foods and other products, into toluene, a hydrocarbon that is of value in industrial and manufacturing settings. Their proposed strategy, outlined in a paper published in Nature Nanotechnology, entails heating polystyrene waste in hydrogen and breaking it down into smaller vapor molecules, a process known as hydro-pyrolysis.

Life-cycle and techno-economic analyses performed by the team showed that the newly introduced process could reduce the carbon footprint of toluene production by 53%, producing toluene at an estimated cost of $0.61/kg, which is below the current industry benchmark.

"This work is based on our earlier development of a two-stage fixed-bed reactor and was inspired by the unique activity and stability of single-atom catalysts in plastic hydrogenolysis," Dr. Zedong Zhang, co-author of the paper, told Phys.org.

"We found that tuning Ru active centers at the atomic level enables control of product selectivity and keeps the high reactivity in polystyrene conversion. The main objective of this study was to demonstrate that single-atom catalysts can overcome the traditional yield–selectivity trade-off and offer new routes for plastic upcycling."

Credit: Wang et al.

Breaking down waste into smaller molecules

The team's proposed strategy for converting plastic waste into smaller molecules relies on a single reactor system that implements two separate steps. The first step, known as pyrolysis, consists in heating up solid PS to high temperatures, until it breaks apart.

The hot vapors derived from this step subsequently flow into a second part of the reactor that contains a catalyst and hydrogen gas. Here, a reaction known as hydrogenolysis takes place, via which hydrogen cuts chemical bonds in the vapor molecules and shapes them into the desired chemical (i.e., toluene).

"In our experiments, solid PS was converted into vapor intermediates in the first reactor (475℃, 0.4MPa) and then selectively transformed into toluene over a Ru single-atom catalyst in the second reactor (275℃, 0.4MPa)," explained Dr. Zhang.

"In collaboration with Dr. Jia Wang and Prof. Jianchun Jiang, we validated this approach through Lab-scale catalytic tests and scale-up experiments, including higher feed amounts and larger fixed-bed reactors."

Further improvement and possible applications

In initial tests, the approach developed by these researchers enabled the conversion of PS into toluene with a selectivity of over 99% and a toluene yield of 83.5%. Notably, the method also led to 53% less carbon emissions compared to current strategies to produce toluene.

"We believe that this tandem catalytic hydrogenolysis strategy offers a promising route for upcycling waste plastics into petrochemical feedstocks," said Dr. Zhang.

"Beyond the current study, we are working toward continuous solid feeding to enable longer operation times and larger-scale plastic conversion. More broadly, the unique properties of single-atom catalysts in plastic hydrogenolysis could help reshape how catalytic plastic conversion is designed."

In the future, the reactor designed by the researchers could be improved and adapted to enable the conversion of PS into other chemicals or fuels. If deployed in real-world settings, it could eventually contribute to the reduction of plastic pollution on Earth.

"Our future research will focus on developing single-atom catalysts with enhanced selectivity for plastic hydrogenolysis, with the goal of better controlling product distribution," added Dr. Zhang. "At the same time, we aim to further explore scalable tandem fixed-bed reactor designs to support longer operation times and higher plastic throughput."

by Ingrid Fadelli, Phys.org

edited by Gaby Clark, reviewed by Robert Egan 

Source: Turning plastic waste into valuable chemicals with single-atom catalysts