Kwang H. Lim, Fabien Jousse, Scott M. Auerbach and Clare P. Grey,
``Double Resonance NMR and Molecular Simulations of Hydrofluorocarbon
Binding on Faujasite Zeolites NaX and NaY: The Importance of Hydrogen
Bonding in Controlling Adsorption Geometries,''
J. Phys. Chem. B 105, 9918-9929 (2001).
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 -CF(3-x)Hx 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< 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.
Prof SM Auerbach
16 October 2001