Use of pyridine as a probe for the determination, by IR spectroscopy, of the Brønsted acid strength of M I HNaY zeolites

The acidity of a series of MHNaY zeolites (M = Li, K, Rb, Cs) has been studied by CO adsorption at low temperature and pyridine desorption at increasing temperature, both followed by IR spectroscopy. It clearly appeared that the Brønsted acidity decreased in the following order: LiHNaY > KHNaY > RbHNaY CsHNaY. Attention was paid to the comparison of the IR spectra of pyridinium species persisting after evacuation at 473 K. The wavenumber of some combination bands and their relative intensity, notably in the 3300–3150 and 2350–1900 cm–1 ranges, were found to be sensitive to the acid strength of the involved hydroxy groups, showing the pyridine can be used to determine not only the nature and the number of zeolite acid sites but also the relative strength of the Brønsted acid sites.