Neutron and Raman spectroscopies of 134 and 134a hydrofluorocarbons encaged in Na-X zeolite
- National Inst. of Standards and Technology, Gaithersburg, MD (United States)
- E.I. Du Pont de Nemours and Co., Wilmington, DE (United States)
There is an impetus to develop replacement refrigerants for chlorofluorocarbons (CFCs) due to increasing concerns for the damage that the ozone-depleting CFCs present to the environment. Economically viable synthesis and separation routes remain a critical issue in the pathway to commercialization of many of the most promising new refrigerants. Key to the development of advanced production methods is the role of surface chemistry in both the synthesis and separation steps. Although an enormous effort has been given to the development of environmentally benign refrigerants such as hydrofluorocarbons (HFCs), there is still much to be learned about their properties, chemistry, and interactions with surfaces of solids. Inelastic neutron scattering methods were used in conjunction with Raman spectroscopy to probe the vibrational density of states of the hydrofluorocarbons (HFCs) 134 (HF{sub 2}C-CF{sub 2}H) and 134a (F{sub 3}C-CFH{sub 2}) adsorbed in the cages of dehydrated Na-X zeolite. A comparison of the vibrational spectra of the encaged HFC species with those of their gas-phase analogs indicates that the HFCs adsorb nondissociatively at room temperature and are most likely associated with Na cations in the supercages at the S{sub III} sites. Guest-host interactions are manifested by adsorption-induced perturbations of the gas-phase torsional and C-H stretching vibrations and the presence of additional features presumably due to low-energy whole-molecule vibrations and adsorbate-coupled zeolite framework vibrations. Moreover, although the 134 trans conformer is favored by 5 kJ/mole in the gas phase at 300 K, the gauche conformer seems to be more prevalent in the zeolite at this temperature and below. This suggests that a sizeable fraction of the Na-X adsorption sites provides a stabilizing configuration for the otherwise higher-energy gauche conformation, perhaps due to hydrogen-bonding interactions with the zeolite framework.
- OSTI ID:
- 225320
- Report Number(s):
- CONF-941144--; ISBN 1-55899-278-2
- Country of Publication:
- United States
- Language:
- English
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