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CU �鶹ӰԺ chemist Niels Damrauer and his research colleagues use visible light to break environmentally persistent carbon-fluorine bonds in PFAS

The strength of the bond between carbon and fluorine can be both a positive and a negative. Because of its seeming unbreakablility, food doesn’t stick to Teflon-coated frying pans and water rolls off rain jackets rather than soaking in.

However, these bonds are also what put the “forever” in “forever chemicals,” the common name for the thousands of compounds that are perfluoroalkyl and polyfluoroalkyl substances (PFAS). PFAS are so commercially abundant that they can be found in everything from candy wrappers to home electronics and guitar strings—to say nothing of their presence in industrial products.

 

Niels Damrauer, a CU �鶹ӰԺ professor of chemistry, and his research colleagues are using visible light to break environmentally persistent carbon-fluorine bonds in PFAS.

The C-F bond is so difficult to break that the products containing it could linger in the environment for thousands of years. And when these molecules linger in a human body, they are associated with increased risk for cancer, thyroid disease, asthma and a host of other adverse health outcomes.

“There are a lot of interesting things about those bonds,” says Niels Damrauer, a �鶹ӰԺ professor of chemistry and fellow in the Renewable and Sustainable Energy Institute. “(The C-F bond) is very unnatural. There are a lot of chemical bonds in the world that natural systems have evolved to be able to destroy, but C-F bonds are uncommon in nature, so there aren’t bacteria that have evolved to break those down.”

Instead of long-used methods of breaking or activating chemical bonds, Damrauer and his research colleagues have looked to light. , the scientists detail an important finding in their ongoing research, showing how a light-driven catalyst can efficiently reduce C-F bonds.

“What we’re really trying to do is figure out sustainable ways of making transformations,” Damrauer explains. “We’re asking, ‘Can we change chemical reactivity through light absorption that we wouldn’t necessarily be able to achieve without it?’ For example, you can break down PFAS at thousands of degrees, but that’s not sustainable. We’re using light to do this, a reagent that’s very abundant and that’s sustainable.”

A foundation of spectroscopy

An important foundation for this research is spectroscopy, which can use light to study chemical reactions that are initiated with light, as well as the properties of molecules that have absorbed light. As a spectroscopist, Damrauer does this in a number of ways on a variety of time scales: “We can put light into molecules and study what they do in trillionths of a second, or we can follow the paths of molecules once they have absorbed light and what they do with the excess energy.”

Damrauer and his colleagues, including those in his research group, frequently work in photoredox catalysis, a branch of photochemistry that studies the giving and taking of electrons as a way to initiate chemical reactions.

“The idea is that in some molecules, absorption of light changes their properties in terms of how they give up electrons or take in electrons from the environment,” Damrauer explains. “That giving and taking—giving an electron is called reduction and taking is called oxidation—so that if you can put light in and cause molecules to be good reducers or good oxidizers, it changes some things you can do. We create situations where we catalyze transformations and cause a chemical reaction to occur.”

Damrauer and his research colleague Garret Miyake, formerly of the CU �鶹ӰԺ Department of Chemistry and now at Colorado State University, have collaborated for many years to understand molecules that give up electrons—the process of reduction—after absorbing light.

 

Using light as a reagent to activate carbon-fluorine bonds, rather than heat or precious metal-based catalysts, is a much more sustainable solution, says CU �鶹ӰԺ researcher Niels Damrauer.

Several years ago, Miyake and his research group discovered a catalyst to reduce benzene, a molecule that’s notoriously difficult to reduce, once it had absorbed light. Damrauer and his graduate students Arindam Sau and Nick Pompetti worked with Miyake and his postdoc and students to understand why and how this catalyst worked, and they began looking at whether this and similar catalysts could activate the C-F bond—either breaking it or remaking it in useful products. This team also worked with Rob Paton, a computational chemist at CSU, and his group.

They found that within the scope of their study, the C-F bond in molecules irradiated with visible light—which could, in principle, be derived from the sun—and catalyzed in a system they developed could be activated. They found that several PFAS compounds could then be converted into defluorinated products, essentially breaking the C-F bond and “representing a mild reaction methodology for breaking down these persistent chemicals,” they note in the study.

Making better catalysts

A key element of the study is that the C-F bond is “activated,” meaning it could be broken—in the case of PFAS—or remade. “C-F bonds are precursors to molecules you might want to make in chemistry, like pharmaceuticals or other materials,” Damrauer says. “They’re a building block people don’t use very much because that bond is so strong. But if we can activate that bond and can use it to make molecules, then from a pharmaceutical perspective this system might already be practical.”

While the environmental persistence of PFAS is a serious public health and policy concern, “organofluorines [containing C-F bonds] have a tremendous impact in medicinal, agrochemical and materials sciences as fluorine incorporation results in structures imparting specific beneficial attributes,” Damrauer and his colleagues write.

By pursuing systems that mitigate the negative aspects of C-F bonds and harness the positive, and using the abundant resources of visible light and organic molecules, Damrauer says he hopes this research is a significant step toward sustainably producing products that use light as a reagent rather than heat or precious metal-based catalysts.

While the catalytic process the researchers developed is not yet at a level that it could be used on PFAS in the environment at a large scale, “this fundamental understanding is really important,” Damrauer says. “It allows us to evolve what we do next. While the current iteration isn’t good enough for practical application, we’re working to make better and better catalysts.”

Xin Liu, Arindam Sau, Alexander R. Green, Mihai V. Popescu, Nicholas F. Pompetti, Yingzi Li, Yucheng Zhao, Robert S. Paton and Garret M. Miyake also contributed to this research.

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