Metal-Organic Frameworks Used to Break Down Plastic into Its Component Parts

Imagine if a plastic bottle had a circular life cycle, where a used plastic bottle could be returned to its original constituent parts, ready to be remade into a new plastic bottle rather than being dumped in a landfill. 

Metal-Organic Frameworks Used to Break Down Plastic into Its Component Parts.

Image Credit: Northwestern University

A research team from Northwestern University is the first to show that a material known as a metal-organic framework (MOF) is a stable and selective catalyst for degrading polyester-based plastic into its constituent parts.

Just three things are required: hydrogen, plastic and the catalyst. A key advantage is that one of the constituent parts the plastic is degraded into is terephthalic acid, a chemical used to create plastic. With the Northwestern researchers’ technique, it is not required to return all the way back to oil and the costly and energy-demanding manufacture and separation of xylenes.

“We can do a lot better than starting from scratch when making plastic bottles,” said Omar Farha, a professor of chemistry in Northwestern University’s Weinberg College of Arts and Sciences. He is the study’s corresponding author. “Our process is much cleaner.”

Details of the research have been recently published in the journal Angewandte Chemie.

The scientists selected a zirconium-based MOF known as UiO-66 as it is easy to create, inexpensive and scalable. Yufang Wu, the first author of the study and a visiting graduate student on Farha’s team, used the plastic that was most convenient: the plastic water bottles her colleagues had thrown away in the laboratory. She cut the bottles up, heated the pieces of plastic and applied the catalyst.

The MOF performed even better than we anticipated. We found the catalyst to be very selective and robust. Neither the color of the plastic bottle or the different plastic the bottle caps were made from affected the efficiency of the catalyst. And the method doesn’t require organic solvents, which is a plus.

Omar Farha, Professor of Chemistry, Weinberg College of Arts and Sciences, Northwestern University

What Are MOFs?

A group of nano-sized materials, MOFs have been extensively explored because of their well-ordered structures. Farha has analyzed MOFs for more than 10 years and earlier demonstrated they can be employed to break down toxic nerve agents.

In the present study, Farha stated that MOFs act in a similar manner — disintegrating an ester bond to break down polyethylene terephthalate (PET). This plastic is one of the most widespread consumer plastics around the world.

We’ve been using zirconium MOFs to degrade nerve agents for years. The team then wondered if these MOFs could also degrade plastic even though the reactions and mechanism are different. That curiosity led to our recent findings. This research helps to address long-standing challenges associated with plastic waste and opens up new areas and applications for MOFs.

Omar Farha, Professor of Chemistry, Weinberg College of Arts and Sciences, Northwestern University

Envision Tinkertoys

MOFs are composed of organic molecules and metal ions or clusters that self-assemble to create multidimensional, extremely crystalline, porous frameworks. To visualize the MOF’s structure, Farha stated, imagine a set of Tinkertoys wherein the metal ions or clusters are the square or circular nodes and the organic molecules are the rods keeping the nodes together.

Besides being easy to create, inexpensive and scalable, another benefit of UiO-66 is that the organic linker, terephthalic acid (TA), of the MOF is the product that is obtained when degrading plastic.

Structural characterization research exposed that while the degradation takes place, UiO-66 experiences a stimulating conversion into another zirconium-based MOF known as MIL-140A. This MOF also displayed significant catalytic activity in the PET degradation process.

Farha also works at Northwestern’s International Institute for Nanotechnology.

The research was aided by the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center subsidized by the Department of Energy, Office of Basic Energy Sciences (DE-SC0012702), and the National Science Foundation’s Materials Research Science and Engineering Centers program (Grant no: NSF DMR-1720139).

Journal Reference:

Wu, Y., et al. (2022) Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework. Angewandte Chemie.

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