Surface acidity of solid acids. Ultraviolet spectroscopic investigation of adsorbed pyridine

The ultraviolet spectra of adsorbed pyridine has been studied on several acidic solids (SiO2, η-Al2O3, NaX). The adsorbed pyridine had the π-π∗ band slightly shifted to longer wavelength from that of pyridine vapor. The bands of protonated pyridine and those of hydrogen bonded pyridine showed characteristic vibrational structure with a peak maximum at 256 nm. Pyridine coordinated on η-alumina gave a broad band at 258 nm. In the case of NaX zeolite, three kinds of adsorbed species were found. The adsorbed states are discussed.