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

Summary: Double Resonance NMR and Molecular Simulations of Hydrofluorocarbon Binding on
Faujasite Zeolites NaX and NaY: the Importance of Hydrogen Bonding in Controlling
Adsorption Geometries
Kwang Hun Lim,,| Fabien Jousse,, Scott M. Auerbach,, and Clare P. Grey*,
Department of Chemistry, SUNY Stony Brook, Stony Brook, New York 11794-3400, Department of Chemistry,
UniVersity of Massachusetts, Amherst, Massachusetts 01003, and Department of Chemical Engineering,
UniVersity of Massachusetts, Amherst, Massachusetts 01003.
ReceiVed: December 31, 2000; In Final Form: May 8, 2001
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 f 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 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 f 27Al and 1H f 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

  

Source: Auerbach, Scott M. - Department of Chemistry, University of Massachusetts at Amherst

 

Collections: Chemistry