An infrared study of pyridine adsorbed on acidic solids. Characterization of surface acidity

The infrared spectrum in the 1400 to 1700 cm−1 region has been determined for pyridine adsorbed on acidic solids. The spectrum of pyridine coordinately bonded to the surface is markedly different from that of the pyridinium ion. This permits the differentiation of acid type on the surface of acidic solids. From the frequency shift of one of the bands of coordinately bonded pyridine over that found in the liquid phase, and from the relative retention of the band upon evacuation and heating, a very rough estimate of the strength of surface Lewis sites can be inferred. It is shown that a silica surface hydrogen bonds pyridine only. Alumina has considerable strong Lewis acidity but no proton acidity while a cracking catalyst has both. The effect on the acidity of sodium poisoning a cracking catalyst is discussed.