Newswise – Polyethylene terephthalate (PET) is one of the most commonly used plastics. Discarded PET mostly ends up in landfills or in the environment because the recycling rate remains low. A team of researchers has now reported in the journal Angewandte Chemie on a zirconium-based metal-organic framework that catalyzes the breakdown of PET into its monomers. These can then be reused to produce high-quality PET, enabling the development of a circular economy.
Almost 70 million tons of PET are produced annually. PET is processed into products such as fibers, beverage bottles and food packaging. Although PET can be melted down and reused, the high temperatures used result in lower quality recycled products, limiting this strategy to only a few cycles. Chemically breaking down PET into its monomers would provide feedstock for the re-manufacture of high-quality PET products, but requires large volumes of solvents and reagents, high pressures, and expensive removal of troublesome by-products. Additives and colored products could also complicate matters. The alternative would be a catalytic process.
A team led by Omar K. Farha from Northwestern University in Evanston, USA, has now reported a catalyst that converts PET waste into the building blocks at 260 °C in yields of up to terephthalic acid (TA) and monomethyl terephthalate (MMT). Terephthalic acid (TA) and monomethyl terephthalate (MMT) breaks down 98%. The catalyst belongs to a class of materials known as metal-organic frameworks (MOFs), which are porous structures with metals at the nodes and organic molecules connecting them. Researchers chose UiO-66, a well-known zirconium-based MOF that can be easily fabricated on a large scale. UiO-66 consists of clusters with six zirconium atoms as nodes connected by six terephthalic acid molecules as linkers.
Extensive structural analysis revealed that, surprisingly, upon degradation of PET, UiO-66 rearranges into another form with the same composition: MIL-140A, a framework of seven-coordinate zirconia chains linked to six other chains by terephthalic acid bridges. This rearrangement causes only a small decrease in catalytic activity.
Detailed mechanistic studies indicate that the primary pathway for PET deconstruction is β-cleavage. This reaction also plays an important role in thermolysis, but takes place at a much lower temperature in the presence of the catalyst. Under hydrogen, degradation also takes place via hydrogenolysis. Neither polyethylene or polypropylene additives nor PET die-off interfered with the degradation process.
These results illustrate the potential of established MOFs as a new class of polymer-degrading catalysts to overcome long-standing challenges related to plastic waste.
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About the author
Omar K. Farha is the Dow Chemical Company Professor and the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern University. His current research spans various areas of chemistry and materials science, ranging from energy to defense-related challenges. In particular, his research focuses on the rational design of metal-organic frameworks (MOFs) for applications in sensing, catalysis, storage, separation, and water purification. His research has been recognized with several awards and honors.
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