Novel crystal puts greenhouse gas in a bind

Emissions of greenhouse gases contribute significantly to global warming. Carbon dioxide (CO2) is the best-known greenhouse gas, but several others also play a role, including the fluorine-containing gases known as per- or polyfluorinated hydrocarbons (PFCs). Researchers at the Institute of Organic Chemistry of Heidelberg University in Germany, led by Michael Mastalerz, recently developed new crystalline materials that can selectively adsorb molecules containing carbon-fluorine bonds. The Heidelberg researchers hope that these porous crystals, which they report in a paper in Advanced Materials, may be useful for the targeted binding and recovery of PFCs. PFCs are organic compounds of various lengths, in which the hydrogen atoms of alkanes are partly or fully replaced by fluorine atoms. These atoms are chemically highly stable, but are not ubiquitous in nature. They are used mainly for etching processes in the semiconductor industry, in eye surgery, and in medical diagnostics as contrast enhancers for certain ultrasound examinations. “Unlike CO2, which is integrated in natural material cycles, PFCs accumulate in the atmosphere and stay there for several thousands of years before breaking down,” says Mastalerz. Compared to carbon dioxide, PFCs thus have a much greater global warming potential – the impact of one PFC molecule is equivalent to 5000 to 10,000 CO2 molecules. According to the researchers, that makes PFCs a permanent problem that is not only contributing to global warming but accelerating it as well. With his research group, Mastalerz has now developed a new type of crystalline material that can adsorb PFCs highly selectively, by binding them to its interior surface. These porous crystals are based on shape-persistent organic cage compounds that carry fluorine-containing side chains on the interconnected struts. These side chains operate according to the ‘like attracts like’ principle, via fluorine-fluorine interactions with the PFC molecules, ensuring they are deposited on the inner surface of the material. In their experiments, the Heidelberg researchers proved that the crystals they developed bind certain fluorine-containing gases such as octafluoropropane and octafluorocyclobutane approximately 1500 to 4000 times more strongly than dinitrogen, the main component of air. According to Mastalerz, these numbers represent extraordinarily high selectivities for binding such PFCs. Currently Mastalerz and his team are working on further increasing the selectivity of the crystals and extending the process to other fluorinated gases, such as those used in medical anaesthesia. “I see enormous potential for development in this area,” says Mastalerz. He hopes that this crystalline adsorbent can be used for recovering PFCs at their point of use.