USE OF PYRIDINE AS A PROBE FOR THE DETERMINATION, BY IR SPECTROSCOPY,OF THE BRONSTED 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 a t increasing temperature, both followed by IR spectroscopy. It clearly appeared that the Bronsted acidity decreased in the following order: LiHNaY > KHNaY > RbHNaY greater than or equal to CsHNaY. Attention was paid to the comparison of the IR spectra of pyridinium species persis ting after evacuation at 473 K. The wavenumber of some combination ban ds and their relative intensity, notably in the 3300-3150 and 2350-190 0 cm(-1) ranges, were found to be sensitive to the acid strength of th e involved hydroxy groups, showing the pyridine can be used to determi ne not only the nature and the number of zeolite acid sites but also t he relative strength of the Bronsted acid sites.