IR Spectroscopic Study of Ethylene Adsorption and Oligomerization on the Hydrogen Form of Mordenite

Diffuse-reflectance IR spectroscopy is used to study adsorption and oligomerization of ethylene on the hydrogen form of mordenite at room temperature. Ethylene adsorbs on bridging acid hydroxyl groups of the zeolite and forms -complexes with a firm hydrogen bond. The interaction with hydroxyl groups most strongly excites composite vibrations in adsorbed molecules. These vibrations are a combination of the stretching vibration of a double bond and the deformational vibrations of the CH₂ group. A conjecture is drawn that these composite vibrations correspond to the reaction coordinate of ethylene transformation to the ethoxy groups. Their further reactions with weakly adsorbed molecules result in ethylene oligomerization. A linear oligomer is formed, grafted on the zeolite surface and filling the pores of zeolites.