Integral Absorption Coefficients of C-H Stretching Bands in IR Spectra of Ethane Adsorbed by Cationic Forms of Y Zeolite

An attempt to measure absolute intensities of IR C-H stretching bands of ethane specifically adsorbed by Na+, Ca2+, or Mg2+ cations in Y zeolite was carried out via simultaneous transmission IR and volumetric measurements at very low pressures using self-supporting wafers. For wafers with a thickness of 7-9 mg/ cm2 quantitative measurements of absorption coefficients of C-H stretching bands were possible. More pronounced shifts of C-H bands toward lower frequencies indicated that the perturbation of the corresponding bonds in ethane upon adsorption by the bivalent Ca2+ or Mg2+ ions was much stronger than by the monovalent Na+ ions. In addition, the integral extinction coefficient of the C-H stretching bands of ethane adsorbed by Na+ ions was only about 53% of that of gas-phase ethane. For adsorption on the bivalent Ca2+ or Mg2+ ions the coefficients exceeded those of the free ethane molecule, indicating strong polarization of the C-H bonds during the vibration. Bands representing the fully symmetric C-H stretching vibrations, which are IR inactive for free ethane, were perturbed most strongly upon adsorption. Their intensities reached up to 60% of the total intensity in the C-H stretching region. In contrast, the intensities of the other C-H bands were several times lower than for free ethane. These trends indicate anisotropy in the polarizability of the adsorbed molecules. In addition, the absorption coefficients indicate a weaker adsorption and polarization of ethane by Mg2+ than by Ca2+ ions in Y zeolite. Because of their smaller ionic radius, the Mg2+ cations are more strongly shielded by the surrounding oxygen atoms of the zeolite framework than the Ca2+ cations, and therefore, their polarizing ability is decreased