Selecting Safer Halocarbons

A theoretical approach to screening the global warming potential of fluorinated hydrocarbons could help industrial chemists design safer molecules before they even hit the atmosphere. 

Perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs) have served as replacements for ozone-destroying chlorofluorocarbons in refrigerators, air conditioners, and other industrial applications since the Montreal Protocol called for chlorofluorocarbons' phase-out in 1989. Unfortunately PFCs and HFCs have well-documented atmospheric problems of their own, acting as potent greenhouse gases with long atmospheric lifetimes.

Chemists are beginning to tease apart the fundamental properties that contribute to a molecule's ability to absorb and trap infrared radiation, and therefore, its global warming potential. The number of highly polar C-F bonds, as well as the relative positions in the molecule of CF₃ groups with respect to oxygen atoms or oxygen-containing groups, are two examples of such properties.

Now Partha P. Bera, a chemist at the National Aeronautics and Space Administration's Ames Research Center at Moffett Field, Calif., and colleagues, have used modeling methods called ab initio electronic structure calculations to evaluate the radiative efficiency of a series of perfluorinated compounds (J. Phys. Chem. A 2009, 113, 12694). Radiative efficiency, along with properties like toxicity and atmospheric lifetime, are important considerations for industrial applications. The worst offenders, they found, were perfluoroethers and hydrofluoroethers, which readily absorb IR radiation, due to, among other things, their very polar C-O bonds and  to the increased polarity of C-F bonds whose carbon is also bonded to an oxygen (Proc. Nat. Acad. Sci., DOI: 10.1073/pnas.0913590107).

This strategy could be used to look at the global warming potentials of other classes of molecules

Paul Blowers, a chemical engineering professor at the University of Arizona, Tucson, has mixed feelings about the work. "The authors are dead-on that this type of evaluation should be done up front before chemicals are introduced into wide-scale use," he tells C&EN. This, Blowers says, is 'critically important for making better choices at the bench scale before we scale up to large manufacturing issues.'

However, he notes, chemists need to be aware that moving fluorine atoms to different positions in the molecule in order to change IR absorption characteristics, as suggested by the authors, could also have dramatic effects on the useful physical properties of the molecule.

He points out his lab's recent analysis  of the global warming potentials of various HFC refrigerants. Although hydrofluoroethers in general have favorable characteristics,  only one of these molecules would be an effective coolant (Env. Sci. & Tech. 2010, 44 ,1526). "This demonstrates you can't always just move molecularity around and still have something that works," he warns.