Development of Transition Metal Hydride Mechanocatalysts for Hydrogenolysis of Poly(ethylene)

Plastic products provide many benefits in daily life from shopping bags to water bottles to food packaging. Most of the plastic is not recycled and instead accumulates in landfills or leaks into rivers and oceans. Polyethylene is the common type of plastic in packaging. It consists of long chains of tightly bonded carbon atoms. Recovering it as small, reusable molecules requires breaking many of these strong bonds. Because polyethylene is a solid, it is hard to bring it into close enough contact with a catalyst that could help with these reactions. Today's recycling methods require high temperatures or added chemicals to force the reaction, which adds costs and waste. This project will use mechanical forces in ball mills to deconstruct polyethylene under milder conditions with the help of metal catalysts. Grinding will supply the energy needed to press the plastic and catalyst particles together and break carbon bonds at room temperature without solvents. The team will study how different metal catalysts change during grinding and use that knowledge to develop better catalysts. This work will support a practical, low-energy way to turn waste polyethylene into useful hydrocarbon mixtures for fuel and chemical manufacturing. It will give industry an economically attractive alternative to landfilling or burning waste plastics. The project will support NSF's mission to advance science and national prosperity by developing novel manufacturing methods to convert solid raw materials into valuable products. The project will also provide hand-on research training for graduate and undergraduate students. This project will have three technical goals. First, the researchers will determine how transition metal hydrides form and react under mechanochemical conditions. Second, they will build a predictive kinetic model of the resulting hydrogenolysis reaction. Third, they will use these findings to design an improved generation of hydride mechanocatalysts. To characterize the hydride species formed during ball milling, the researchers will use x-ray diffraction, x-ray photoelectron spectroscopy, inelastic neutron scattering, and temperature-programmed desorption. These techniques will measure how readily surface and lattice hydrogen take part in the reaction during milling. Parallel reactivity studies will focus on the evolution of decomposition products from polyethylene and on eicosane, a well-defined model alkane. These studies will extract rate constants linking catalyst structure and milling energy to process performance metrics. They will also identify the conditions that lead to carbide and coke formation. By correlating hydride stability, defect density, and hydrogen transport with catalytic performance, the project will establish structure-performance relationships for hydrogenolysis catalysts. These relationships will advance the fundamentals of catalysis for this reaction. The results will also provide broader design principles for chemical engineering and surface chemistry. These principles could apply to other mechanically driven catalytic processes, such as biomass depolymerization. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.