Vibrational spectra of molecules on zeolites. Part 1.—Acetylene on A-type zeolites

Abstract

Raman and infrared spectra of acetylene on alkali and alkaline earth cation exchanged A-type zeolites are reported. A complete set of five fundamentals has been observed for adsorbed acetylene on NaA and CaA. Adsorption induced frequency shifts exhibiting an inverse polarizing power dependence have been explained in terms of a model involving the interplay of several effects: adsorbate–adsorbate interaction, cation–adsorbate attraction and oxide–adsorbate repulsion. The importance of the last of these factors is a consequence of shielding of the cation by the oxide framework and of the inability of acetylene to penetrate the cation sites. Asymmetry of the line profile of v₂ of acetylene is considered to reflect cation site effects. Normal coordinate analyses for adsorbed acetylene confirm that frequency displacements on adsorption follow force constant changes.