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Molecular Dynamics Studies of Hydrofluorocarbons in Faujasite-type Zeolites: Modeling Guest-Induced Cation Migration in Dry Zeolites
 

Summary: Molecular Dynamics Studies of Hydrofluorocarbons in Faujasite-type Zeolites: Modeling
Guest-Induced Cation Migration in Dry Zeolites
Eugenio Jaramillo, Clare P. Grey, and Scott M. Auerbach*,,
Departments of Chemistry and Chemical Engineering, UniVersity of Massachusetts,
Amherst, Massachusetts 01003, and Department of Chemistry, SUNY Stony Brook,
Stony Brook, New York 11794
ReceiVed: May 24, 2001; In Final Form: September 14, 2001
We have developed and applied a new force field for simultaneously modeling the dynamics of
hydrofluorocarbons (HFCs) and exchangeable Na cations in faujasite-type zeolites. Our aim is to account for
(i) the zeolite's capacity of separating HFC isomers, (ii) the experimentally observed unusual cation migration
in Na-Y from the -cages into the supercages upon the adsorption of HFCs, and (iii) the abnormal trans/
gauche ratio in these systems. Energy minimizations and molecular dynamics simulations performed with
this force field give excellent agreement with experimental data on heats of adsorption, guest-host distances,
infrared spectra, and conformer ratios for different coverages of HFC-134 (CF2H-CF2H) and HFC-134a
(CH2F-CF3) in Na-X (Si:Al ) 1.2) and Na-Y (Si:Al ) 2.4). The force field also accounts partially for the
observed cation migration at intermediate loadings and fully at high loadings. The extent of migration is
found to be influenced by the competition among Na-O, Na-F, and Na-Na interactions. The Na-O
interaction disfavors migration because Na(I) sites are found to be energetically more favorable than Na(III)
sites; the Na-F attraction obviously favors migration; and surprisingly, the Na-Na repulsion also favors
migration because moving Na cations into supercages leads to better cation dispersion. This migration occurs

  

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

 

Collections: Chemistry