What if the life cycle of the plastic bottle was circular? Where a used plastic bottle has been returned to its original components, ready to be turned into a new plastic bottle instead of possibly ending up in a landfill.
A research team from Northwestern University is the first to demonstrate that a material called metalorganic framework (MOF) is a stable and selective catalyst for breaking down polyester-based plastic into its component parts.
Only three things are needed: the plastic, the hydrogen and the catalyst. An important benefit is that one of the components of plastic is terephthalic acid, a chemical used to produce plastic. With the Northwestern method, there is no need to go back to petroleum and the expensive and energy-intensive production and separation of xylenes.
“We can do a lot better than starting from scratch to make plastic bottles,” said Omar Farhachemistry teacher at Weinberg College of Arts and Sciences. He is the corresponding author of the study. “Our process is much cleaner.”
The work was recently published in the journal Angewandte Chemie.
The researchers chose a zirconium-based MOF called UiO-66 because it is easy to fabricate, scalable, and inexpensive. Yufang Wu, the study’s first author and visiting graduate student in Farha’s group, used the most practical plastic: the plastic water bottles his lab colleagues had thrown away. She chopped them up, heated the plastic, and applied the catalyst.
“MOF did even better than expected,” Farha said. “We found the catalyst to be very selective and robust. Neither the color of the plastic bottle nor the different plastics from which the bottle caps were made affected the effectiveness of the catalyst. And the method does not require organic solvents, which is a plus.
What are MOFs?
A class of nano-sized materials, MOFs have been widely studied due to their highly ordered structures. Farha has studied MOFs for over a decade and has previously shown that they can be used to destroy nerve agents. In the current study, Farha said, the 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 in the world.
“We’ve been using zirconium MOFs to degrade nerve agents for years,” Farha said. “The team then wondered if these MOFs could also degrade plastic even if the reactions and the mechanism are different. This curiosity has led to our recent discoveries.
“This research helps address long-standing challenges associated with plastic waste and opens up new areas and applications for MOFs,” Farha said.
MOFs consist of organic molecules and metal ions or aggregates that self-assemble to form highly crystalline multidimensional porous frameworks. To imagine the structure of a MOF, Farha said, imagine a set of Tinkertoys in which the metal ions or clusters are the circular or square nodes and the organic molecules are the rods that hold the nodes together.
Besides being easy to manufacture, scalable, and inexpensive, another benefit of UiO-66 is that the MOF’s organic linker, terephthalic acid (TA), is what you get when plastic breaks down. .
Structural characterization studies revealed that during the degradation process, UiO-66 undergoes an interesting transformation into another zirconium-based MOF called MIL-140A. This MOF also showed a great catalytic activity towards the degradation of PET.
Farha is also a Fellow of Northwestern’s International Institute for Nanotechnology.
The research was supported by the Inorganometallic Catalyst Design Center, an Energy Frontiers Research Center funded by the Department of Energy, Office of Basic Energy Sciences (DE-SC0012702) and the National Science Foundation’s Materials Engineering and Research Centers. program (NSF grant number DMR-1720139).
The title of the article is “Catalytic degradation of polyethylene terephthalate using a metal-organic zirconium-based phase transition framework”.