2,6-Dimethylpyridine as a probe of the strength of Bronsted acid sites: study on zeolites. Application to alumina

The positions of nu(8a) and nu*(NH) bands in the spectrum of protonated 2,6-dimethylpyridine vary with the strength of Bronsted acidity: the higher the nu*( NH) wavenumber and the lower the nu(8a) wavenumber, the stronger the acidity. The results obtained with 2,6-dimethylpyridine adsorption correlate with those obtained by CO adsorption experiments on a series of faujasite zeolites (LiHNaY, KHNaY, HY, HYSA, HNaX). These relations were extended to gamma-Al2O3 having weak Bronsted acidity, not detected by pyridine and hardly detected by CO. The number (0.1 per nm(2)) and the strength (corresponding to Delta nu(OH) by CO = 155 cm(-1)) of the most acidic OH groups of Al2O3, as well as the position of the corresponding nu(OH) band (<3700 cm(-1)) are deduced from 2,6-dimethylpyridine adsorption experiments.