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IR spectroscopy; integral molar extinction coefficients; propene; adsorption; zeolites
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 integral molar extinction coefficients (IMEC) of C-H and C=C stretching bands of propene specifically adsorbed by Na+, Ca2+ and Mg2+ cations in Y zeolite was carried out via parallel IR spectroscopic and volumetric measurements. Propene was adsorbed at low pressures (< 0.5 mbar) on self-supporting wafers pressed from the zeolite powders. The IMEC of the C=C-stretching vibrations of adsorbed propene increased for these cations in the following sequence: Na < Ca < Mg. For Ca2+ and Mg2+ cations the IMEC values exceeded those for free propene. Simultaneously, the IMEC of the C-H stretching band decreased relative to that of the C=C band. These observations indicate that the C=C bond is more strongly polarized and thus more activated during interaction with bivalent than with monovalent cations. This conclusion is confirmed by oligomerization of propene on ZnY zeolite, which proceeds at room temperature. Intensities of IR bands thus provide a sensitive criterion for the activation of double bonds arising from their polarization by cations.
