Adsorption of benzene and toluene on Y zeolites studied by infrared spectroscopy

Abstract

The adsorption of benzene and toluene molecules on NaY, NaHY, dehydroxylated zeolites and NaHY zeolite containing preadsorbed pyridine was studied by i.r. spectroscopy. The frequencies of the i.r. bands of adsorbed benzene and toluene are similar to those found for dissolved hydrocarbons, although the intensity of some bands is several times lower than in dissolved hydrocarbons, indicating that the adsorption of aromatic hydrocarbon molecules in the zeolite cavities results in a certain distortion of the electron cloud of the molecule. The first molecules of benzene or toluene to be adsorbed are bonded to Na⁺ ions, the next adsorbed molecules forming hydrogen bonds with the OH groups vibrating at 3640 cm^–1. The frequency shift of the i.r. band of the OH groups may be taken as a measure of their acid strength. The acid strength of high-frequency OH groups decreases with the amount of pyridine adsorbed.