Intensities of combination IR bands as an indication of the concerted mechanism of proton transfer from acidic hydroxyl groups in zeolites to the ethylene hydrogen-bonded by protons

Abstract

Adsorption of ethylene by sodium forms of mordenite and Y zeolite is reversible and results in no chemical activation of olefin. Therefore, diffuse reflectance infrared Fourier transform spectra of ethylene adsorbed by these zeolites do not differ much from those of liquefied or frozen C 2H 4. In contrast, adsorption of ethylene by HY and HMOR results in strong hydrogen bonding with acidic hydroxyl groups and subsequent oligomerization of ethylene at room temperature. The hydrogen bonding strongly perturbs OH frequencies but exerts only a weak influence on the frequencies and intensities of the fundamental C H stretching vibrations of ethylene. In contrast, intensities of the infrared (IR) bands from combinations of the double-bond stretching vibrations with the bending vibrations of CH 2 groups increase significantly. According to previously published quantum chemical calculations, these combinations contribute most significantly to the reaction coordinates of protons added to adsorbed ethylene. Therefore, the results obtained allow formulation of a new spectral criterion of the reactivity index for concerted proton transfer to adsorbed ethylene; the IR bands corresponding to combination frequencies of most strongly polarized chemical bonds involved in this elementary step have unusually high intensities. It is also suggested that this criterion is of more general significance and also can be applied to other elementary steps of acid or acid–base catalytic reactions when chemical activation of adsorbed molecules results from simultaneous polarization of several chemical bonds.