A Northwestern University research team has become the first to demonstrate that a material called a metal-organic framework (MOF) is a stable and selective catalyst for breaking down polyester-based plastic into its component parts.
Only three things are needed: plastic, hydrogen and the catalyst. An important bonus is that one of the component parts the plastic is broken down into is terephthalic acid, a chemical used to produce plastic. It is therefore unnecessary to go all the way back to oil and the expensive and energy-intensive production 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 the Weinberg College of Arts and Sciences. He is the corresponding author of the study. “Our process is much cleaner.”
The work was published recently in the journal Angewandte Chemie.
Metal-organic frameworks are a relatively new class of porous, crystal materials that have found application in a diversity of industrial areas in recent years. Composed of metal ions and organic ligands, which act as linkers in the network structure, these materials self-assemble to form multidimensional frameworks and have highly tuneable properties.
In this study, entitled Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework, a zirconium-based MOF called UiO-66 was chosen because it is easy to make, scalable and inexpensive. Yufang Wu, the study’s first author and a visiting graduate student in Farha’s group, used the plastic that was most handy: the plastic water bottles her colleagues in the lab had discarded. She chopped them up, heated the plastic and applied the catalyst.
“The MOF performed even better than we anticipated,” Farha said. “We found the catalyst to be very selective and robust. Neither the colour of the plastic bottle nor 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.”
Farha has studied MOFs for more than a decade and previously showed 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 PET.
Structural characterisation 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 great catalytic activity toward PET degradation.
Farha also is a member of Northwestern’s International Institute for Nanotechnology.
The research was supported by the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the Department of Energy, Office of Basic Energy Sciences and the National Science Foundation’s Materials Research Science and Engineering Centers program.