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Taming Halogens

Researchers in Berlin’s Dahlem district are working on the materials of the future: Chemistry professor Sebastian Hasenstab-Riedel studies innovative compounds involving fluorine, chlorine, bromine, and iodine.

May 15, 2019

The first one is widespread in toothpaste, where it is used to fight cavities – fluoride. The second one is something most of us consume too much of, in the form of table salt – chloride. Chlorine atoms are also present in countless kinds of plastics, in coolants, and in propellants.

Chemistry professor Sebastian Hasenstab-Riedel studies innovative compounds involving fluorine, chlorine, bromine, and iodine.

Chemistry professor Sebastian Hasenstab-Riedel studies innovative compounds involving fluorine, chlorine, bromine, and iodine.
Image Credit: Miriam Klingl

The third one, bromine, can calm or irritate – depending on whether it is part of a medication or tear gas. And the fourth is known as a wound disinfectant. Professor Sebastian Hasenstab-Riedel doesn’t have much to do with these kinds of “common,” everyday uses of these four highly reactive elements from the halogens group, though. Working in his lab, he teases the unusual out of fluorine, chlorine, bromine, and iodine.

A chemist, Hasenstab-Riedel works on innovative halogenating agents: substances that transfer a fluorine or chlorine atom to a different molecule during a chemical reaction, for example. And these agents are in high demand. “When you consider that about 55 percent of all of the products of the chemical industry have ‘seen’ a chlorine atom at some point in the course of their synthesis or still contain it in the end, you get a sense of the huge volumes of chlorine that are used,” Hasenstab-Riedel says.

About seven percent of all of the electricity used in the German state of North Rhine-Westphalia goes into production of chlorine gas (Cl2), which is produced from aqueous sodium chloride solutions through electrolysis. But chlorine gas is poisonous. It was even used as a chemical weapon in World War I. Practically every swimming pool has a pressurized cylinder of chlorine gas somewhere, but storing chlorine is risky. Leaks are a frequent occurrence. With this in mind, Hasenstab-Riedel develops compounds known as “polyhalides,” which bind halogens in liquid or solid form. “These are readily dosable halogen storage systems that contain negatively charged ions (anions) with three, nine, 11 or even 13 halogen atoms and can release them again under certain reaction conditions.”

“While the unfluoridated active ingredient has to be taken daily, just one tablet a week of the fluoridated version is enough”

Together with a leading German producer of chlorine and plastics, Hasenstab-Riedel has applied for a patent for polychlorides. Halogens are used to produce PVC, polyurethanes, and other types of plastic, but that isn’t all. Many medications also contain one or more chlorine atoms, and some, like anesthetics, contain bromine. Others contain fluorine, while still others contain multiple different halogens in the same molecule.

A single halogen atom can change the effect of a drug such as a diabetes medication. “While the unfluoridated active ingredient has to be taken daily, just one tablet a week of the fluoridated version is enough,” Hasenstab-Riedel explains.

After apprenticing as a chemical lab technician at Siemens and Degussa in Hanau in the 1990s and then studying chemistry at the universities in Siegen and Würzburg, Hasenstab-Riedel started out with theoretical calculations of molecules. And that included molecules containing halogen atoms.

After earning his doctorate, Hasenstab-Riedel went to the University of Helsinki, where he studied metal fluorides using matrix isolation spectroscopy, a method that allows researchers to study even highly reactive molecules. Together with an absolutely inert noble gas like neon (the matrix), the fluorides are deposited onto a mirror that has been cooled to a temperature as low as negative 269 degrees Celsius. With the sample “fixed” in this way, the structure and composition of the molecules can be determined using UV/Vis or infrared spectroscopy.

His First Trick: Crystalline Polybromides

Hasenstab-Riedel moved deep into preparative fluorochemistry during a research stay in Canada. Now, his own field of research was clear: theory, synthesis, and analysis of innovative halogen compounds. He wrote his professorial thesis at the University of Freiburg in 2013 and joined Freie Universität Berlin as a professor later that same year. Crystalline polybromides were his first coup. “Polyiodides were already known, but there wasn’t anything comparable with bromine and chlorine.”

By now the team has synthesized a broad range of polybromides. These compounds have two major advantages. First, the bromine atoms are now more firmly bonded in chemical terms, making them easier to handle than elemental bromine (Br2). Second, they are an excellent conductor of electricity, so they are interesting for their potential to store energy as well. Together with colleagues in Freiburg and at the Fraunhofer Institute for Solar Energy Systems (ISE), Hasenstab-Riedel and his team are now working on ultra-long-lasting redox flow batteries, also known as wet cell batteries.

Then he moved on to polyfluorides. These compounds exist only at negative 275 degrees Celsius, which means they are primarily of academic interest. That left only the urgently needed polychlorides. “We didn’t really get the hang of it until two years ago,” Hasenstab-Riedel says. “We use this method to obtain a kind of ‘liquid chlorine’ and even solid substances that can now be used for chlorination reactions instead of chlorine gas.”

Alongside halogen storage molecules, Hasenstab-Riedel concentrates on substances that can be used as insulating gases in high-energy switching systems. “So far, sulfur hexafluoride (SF6) has been used for that. It has good insulating properties and prevents or quickly extinguishes arcing – but unfortunately, it’s a highly pronounced greenhouse gas.” In that regard, SF6 is 23,500 times more harmful than CO2.

Searching for a Perfect All-purpose Solution

“We’re researching alternatives that are significantly better for the environment,” Hasenstab-Riedel says. And that’s no easy task; after all, they are looking for a single solution that is perfect for any purpose. It has to be a gas that doesn’t condense even at very low temperatures like those found in Siberia. It must not be used up or break down, and it should be non-toxic and under no circumstances interfere with the metal components in the switch boxes. “We’ve tried out a few things, and we’re making good progress toward finding a replacement for SF6.” Hasenstab-Riedel offers only vague indications of the kind of molecule he means, since competition is fierce.

Together with a producer of SF6, he now holds seven patents on substance classes and new synthesis methods for SF6substitutes. “If we succeed in developing a significantly better substitute, it would be a huge gain in terms of protecting the climate,” he says. In addition to his research and teaching activities, Hasenstab-Riedel and his colleague Professor Thomas Braun of Humboldt-Universität zu Berlin initiated a new collaborative research center titled “Fluorine-Specific Interactions: Fundamentals and Functions.” He also obtained millions of euros in grant funding from the European Research Council (ERC).

Hasenstab-Riedel is a busy man. But he still makes time to regularly bring things to a boil in the inorganic chemistry lecture hall in the chemistry building on Fabeckstraße during the Long Night of the Sciences. Amid bangs, bubbling, hissing and flashing, he shows everyone what experimental research can be: not just a lot of work, but also a great pleasure.

The original German version was published on February 23, 2019, in the university's supplement of the daily newspaper Der Tagesspiegel.