Double Resonance NMR and Molecular Simulations of Hydrofluorocarbon Binding on Faujasite Zeolites NaX and NaY: the Importance of Hydrogen Bonding in Controlling Adsorption Geometries

Abstract

Double resonance NMR experiments have been used to study the binding of the asymmetric hydrofluorocarbons (HFCs) CF3CFH2 (HFC-134a), CF3CF2H (HFC-125) and CF2HCFH2 (HFC-143) on zeolites NaX and NaY. By exploiting the very large differences in 19F chemical shifts for the −CF3-xHx end groups of each molecule, individual 19F → 23Na cross-polarization (CP) build-up curves involving polarization transfer from different parts of the molecule have been obtained. CP efficiencies in the order CF3≪CF2H<CFH2 were found, indicating that the hydrogen-containing groups are bound more strongly to the zeolite framework. This effect is most pronounced for the lowest-sodium content zeolite studied (NaY: Si/Al = 7.6), and increases with HFC loading. Both the 1H NMR resonances and 1H → 27Al and 1H → 17O CP MAS NMR experiments are consistent with H-bonding interactions with the zeolite framework. Molecular dynamics and docking calculations for HFC-134 and 134a on model NaY and NaX zeolites revealed the importance of both H-bonding and Na−F interactions in determining the low-energy sorption sites. Stable binding sites for HFC-134a in NaY were found by docking to involve only CFH2−Na(SII) contacts, in qualitative agreement with the CP results. The simulations indicate that Na-binding to the groups with more H atoms is favored by the possibility of achieving shorter Na−F distances and maximizing the number of H-bonds with the framework. The higher sodium content zeolite containing both SII and SIII cations gives minimum energy binding sites involving multiple Na−F and H-bonding contacts. For HFC-134 (gauche conformer), the global energy minimum actually involves several H-bonds but no short Na−F contacts.