Magnesium complex plucks fluorine from polytetrafluoroethylene, then gives it away

A magnesium complex can remove fluorine atoms from solid microparticles of polytetrafluoroethylene (PTFE) at room temperature, and then donate the fluorine atoms to other compounds (J. Am. Chem. Soc. 2023, DOI: 10.1021/jacs.3c02526).

“I haven’t seen any previous report describing this kind of work, especially at room temperature, that directly cuts the fluorine off the PTFE polymer solid,” says Jinyong Liu, an environmental engineer at University of California, Riverside, who was not involved in the research.

While just a proof of concept, the researchers hope their experiment will inspire others to try harvesting fluorine from waste PTFE and other fluoropolymers. “The main thing is kind of showing that it could be done, and drawing this connection,” says Mark R. Crimmin, a chemist at Imperial College London.

Fluoropolymers are produced for various purposes in huge volumes every year. Their strong carbon-fluorine bonds make them useful but they are also highly persistent, contributing to the problem of accumulating per- and poly-fluoroalkyl substances (PFAS).

In 2011, while Crimmin and graduate student Daniel J. Sheldon were studying aluminum complexes, they labeled their aromatic ligands with ¹⁹F atoms so they could monitor the complexes via nuclear magnetic resonance. Before long, the researchers realized that the fluorine atoms were migrating from carbon to aluminum. “We had some sort of C-F activation going on,” Crimmin says. They began exploring how they could work with the challenging carbon-fluorine bonds.

PTFE, the most widely used fluoropolymer, became an obvious target. The researchers sought out main group metal complexes similar to the aluminum ones they studied. The complexes, Crimmins says, “have this signature push-pull effect, where they will give up a pair of electrons, but they also want to take the fluoride, and this makes them very, very reactive to these types of bond activations.”

When Sheldon took a compound first reported by Monash University researchers containing two weakly bound magnesium centers and reacted it with micron-sized particles of PTFE, a magnesium fluoride complex formed. This contained two fluorine atoms in a four-membered ring between the two magnesium centers. “There is an excess of C-F bonds, so the reaction can go to completion,” says Sheldon. From the magnesium-fluoride complexes, he then prepared a few fluorinated compounds with practical uses.

Crimmin says that one limitation of the work is that PTFE is not fully defluorinated. He expects that only amorphous regions on the surface reacted, while other crystalline regions remain fluorinated. It is also unlikely that the magnesium complex can be prepared in kilogram or ton scales, Crimmin says. “This is maybe a kind of good general strategy for approaching PTFE waste. But this system won’t be the system that is scaled up.”

Because other sources of fluorine such as inorganic fluoride salts are cheap, the idea of preparing fluorinated compounds this way appears impractical to Liu. But as someone who studies the defluorination of various small-molecule PFAS, he says attacking solid PTFE at room-temperature is “amazing”. Most PFAS defluorination efforts have focused on small, soluble molecules that contain some type of functional group, such as the carboxylic acids, he says. Liu believes that once partially defluorinated, PTFE could be vulnerable to further chemical or even microbial degradation.

Liu hopes that this study will help environmental scientists become more aware of the solutions that organometallic and coordination chemistry could offer for addressing environmental problems.

Crimmin’s team is currently designing new ligands to improve the magnesium complexes’ ability to donate the fluorine atoms and testing this defluorination procedure for polyvinylidenedifluoride (PVDF), an up-and-coming fluoropolymer widely used in electric vehicles.