Thursday, December 08, 2022

Method efficiently breaks plastic bottles into component parts: Researchers first demonstrate use of metal-organic framework to degrade plastic

What if the life cycle of a plastic bottle is circular? Where a used plastic bottle was returned to its original components, ready to be made into a new plastic bottle rather than possibly ending up in a landfill.

A Northwestern University research team is the first to demonstrate that a material called a metal-organic framework (MOF) is a stable and selective catalyst for breaking down polyester-based plastics into its constituent parts.

Only three things are needed: plastic, hydrogen and a catalyst. A significant bonus is that one of the components that breaks down plastics is terephthalic acid, a chemical used to produce plastics. With the northwestern method, it is not necessary to go all the way back to the expensive and energy-intensive production and separation of oil and xylene.

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

The work was recently published in the journal applied Chemistry.

The researchers have called the zirconium-based MOF UIO-66 because it is easy to manufacture, scalable and cheap. Yufang Wu, the study’s first author and a visiting graduate student in Farha’s group, used the plastic that was easiest: plastic water bottles discarded by his colleagues in the lab. He cut them, heated the plastic and applied the catalyst.

Farah said, “MoF performed even better than we expected.” “We found the catalyst to be very selective and robust. Neither the color of the plastic bottle or the different plastic bottle caps affect the efficiency of the catalyst. And the method does not require organic solvents, which is a plus.” ”

MOFs previously used on nerve agents

A class of nano-sized materials, MOFs have been widely investigated because of their highly ordered structures. Farha has studied MOFs for more than a decade and has previously shown that they can be used to destroy toxic nerve agents. In the current study, Farha said, MOFs act in much the same way — breaking an ester bond to degrade polyethylene terephthalate (PET). This plastic is one of the most popular consumer plastics worldwide.

“We’ve been using zirconium MOFs to degrade nerve agents for years,” Farha said. “The team then wondered whether these MOFs could also degrade plastic, even though the reactions and mechanisms differ. That curiosity led to our recent findings.”

“This research helps address long-standing challenges associated with plastic waste and opens up new areas and applications for MOFs,” Farha said.

More on MOF

MOFs are composed of organic molecules and metal ions or clusters that self-assemble to form multifunctional, highly crystalline, porous structures. To illustrate the structure of a MOF, Farha said, imagine a set of tinkertoys in which metal ions or clusters are circular or square nodes and organic molecules are rods holding the nodes together.

In addition to being easy to make, scalable, and inexpensive, another advantage of UiO-66 is that the MOF’s organic linker, terephthalic acid (TA), is what you get when breaking down plastic.

Structural characterization studies have shown that during the degradation process, UIO-66 undergoes an interesting transformation into another zirconium-based MOF called MIL-140A. This MOF also showed great catalytic activity towards PET degradation.

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