Recycling plastic could be a whole lot easier if it worked like children’s building blocks—snapped apart into simpler pieces. Unlike conventional recycling, which often requires temperatures exceeding 500 °C and results in a lot of unusable waste, a new sustainable approach could break down landfill-bound plastics into valuable chemicals, leading to less waste and more useful products.
Jason Lim, Head of the Sustainable Supramolecular Materials Group at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE), said that metal-organic frameworks (MOFs) could help make this vision a reality. MOFs are highly customisable materials made of metal ions and organic molecules, structured like a sponge with adjustable pores that can trap and break down other molecules.
Polyolefins—plastics like polyethylene and polypropylene—are among the toughest materials to recycle due to their strong, unreactive chemical bonds. To tackle this issue, Lim and colleagues turned to a zirconium-based MOF called UiO-66. Their study thereof was a collaborative effort that included Enyi Ye, A*STAR IMRE Deputy Department Head of Advanced Biomaterials; A*STAR IMRE colleagues Jerry Heng, Tristan Tan and Xin Li; Lili Zhang from the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2); and researchers at Kyoto University, Wakayama University and JEOL Ltd. in Japan.
UiO-66, deliberately engineered to possess unique ‘coordinatively unsaturated sites’, showed promise as a catalyst, facilitating the breakdown of polyolefin plastics under milder conditions.
“Our study is the first demonstration that UiO-66 containing unsaturated sites can be effective catalysts for polyolefin pyrolysis,” Lim explained. “Our experiments show that the product distributions obtained from our UiO-66 MOF catalysts are different from zeolites, which offers product complementarity.” This distinct framework structure of the MOF prevents aggregation of the catalytically active zirconium-oxo nodes, which can otherwise reduce efficiency.
In experiments, UiO-66 proved highly effective at breaking down polyolefins into smaller hydrocarbons, reducing the formation of less valuable solid residues by nearly 50 percent. Unlike traditional zeolite catalysts which can produce aromatic compounds, these UiO-66 catalysts produced a higher yield of useful aliphatic alkanes and olefins. The team showed for the first time that polyolefins were able to penetrate the MOF structure, making full use of its internal surface for efficient catalytic reactions. Additionally, the MOF was thermally stable enough to withstand the pyrolysis temperatures and be reused.
“By breaking down the polyolefins into oligomers through catalytic pyrolysis, we retain the inherent carbon content of these polymers for different uses,” said Lim. “Some include being fed back as feedstock into the chemical industry, as well as being used as fuel.”
Lim and his team plan to explore new MOF designs and modifications to further reduce energy demands and optimise yields towards scalable, eco-friendly solutions to industrial plastic waste.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2).
