Step toward Circular Economy?

Catalytic deconstruction of PET with zirconium metal-organic framework

Polyethylene terephthalate (PET) is one of the most common
plastics. Discarded PET most often ends up in landfills or in the
environment because the rate of recycling remains low. In the journal Angewandte
Chemie, a research team has now reported a
zirconium-based metal-organic framework material that catalyzes the
degradation of PET into its monomers. These can then be reused to make
high-value PET, allowing for development of a circular economy.

Step toward a Circular Economy? - Catalytic deconstruction of PET with zirconium metal-organic framework

© Wiley-VCH, re-use with credit to ‘Angewandte Chemie’ and a link to the original article.

Almost 70 million tons of PET are produced annually. PET is
processed to make products like fibers, drink bottles, and food
packaging. Although PET can be melted down and reused, the high
temperatures employed result in a lower quality of recycled products,
limiting this strategy to only a few cycles. Chemical deconstruction of
PET into its monomers would provide starting material for the renewed
production of high-quality PET products but it requires large amounts of
solvents and reagents, high pressures, and expensive separation of
troublesome byproducts. Additives and dyed products could also make
things difficult. The alternative would be a catalytic process.

A team led by Omar K. Farha at
Northwestern University in Evanston, USA has now reported a catalyst that
breaks PET waste down at 260 °C into the building blocks terephthalic
acid (TA) and mono-methyl terephthalate (MMT) in yields up to 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 linking
them. The researchers chose to use UiO-66, a well-known zirconium-based MOF that
can easily be produced on a large scale. UiO-66 consists of clusters
containing six zirconium atoms as nodes, which are connected by six
terephthalic acid molecules as linkers.

Extensive structural analyses showed that during
deconstruction of PET, UiO-66 surprisingly rearranges into a different
form with the same composition: MIL-140A, which is a framework of
seven-coordinate zirconium oxide chains that are linked to six other
chains by terephthalic acid bridges. This rearrangement only causes a
slight decrease in the catalytic activity.

Detailed mechanistic studies indicate that the primary
pathway for the PET deconstruction is a β-scission. This reaction also
plays an important role in thermolysis but proceeds at a significantly
lower temperature in the presence of the catalyst. Under hydrogen, the
deconstruction also proceeds via hydrogenolysis. Neither polyethylene or
polypropylene additives nor dying of the PET disrupted the
deconstruction process.

These results illustrate the potential of established MOFs
as a new class of polymer-deconstructing catalysts to overcome
longstanding challenges related to plastic waste.

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About the Author

Omar K. Farha is the Dow Chemical Company Professor and
Charles E. and Emma H. Morrison Professor in Chemistry at Northwestern
University. His current research spans diverse areas of chemistry
and materials science ranging from energy to defense-related challenges.
Specifically, his research focuses on the rational design of
metal-organic frameworks (MOFs) for applications in sensing, catalysis,
storage, separations, and water purification. His research
was recognized by several awards and honors.

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