A Raman spectroscopy study of the separation of hydrofluorocarbons using zeolites

Hydrofluorocarbons (HFCs), such as HFC-134a (CF3CFH2), are replacing chlorofluorocarbons (CFCs) in many applications because of their low ozone depletion potentials. The more complex chemical syntheses of HFCs, however, may not provide the molecular species desired with sufficient purity, leading to the need to separate reaction products. An important example is provided by the separation of the isomers HFC-134a and HFC-134 (CF2HCF2H) over the faujisitic zeolite NaX (Na86Al86Si106O384) An understanding of the fundamental mechanism controlling this separation involves the unusual aspect that HFC-134 can exist as either a trans or gauche conformer; a key factor is that the trans conformer is nonpolar whereas the gauche conformer is highly polar. Here we report the results of a Raman scattering study which show the trans-gauche population ratio for HFC-134 adsorbed on NaX is decreased as much as 20-fold compared to the gas phase. Furthermore, the conformational ratio changes with HFC loading, the polar gauche conformer being more highly favored at low loading where the HFC-134a/HFC-134 separation factor is maximum. These observations thus demonstrate that the efficiency of this HFC separation is directly related to the HFC-134 trans-gauche population distribution on the zeolite surface.