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quantitative IR spectroscopy; absorption coefficient; polarization of chemical bonds; chemical activation of adsorbed alkanes; zeolites; ethane
Intensities of IR stretching bands as a criterion of polarization and activation of adsorbed molecules in heterogeneous acid-base catalysis
Abstract:
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.
