BRONSTED ACID STRENGTH OF ZEOLITES STUDIED BY H-1-NMR - SCALING, INFLUENCE OF DEFECTS

Many industrial chemical reactions, particularly in petrochemistry, re quire acidic catalysts (alone or associated with a hydrogenating phase ) capable of activating strong covalent bonds (C-C, C-H). However, it is important that the acidity (density and strength of the acid sites) be adapted to the desired reaction; hence the need to know the streng th of the acid sites of a catalyst before its use in a given reaction. The acid strength of an OH group depends not only on the ionic charac ter of the bond but also, and often predominantly, on its polarizabili ty. It must therefore be studied in the presence of a proton acceptor. We have shown that by studying the dipolar interactions by H-1 NMR at 4 K it is possible to characterize the different species in the equil ibria solid-OH + H2O = solid-OH...OH2 = solid-O- + H3O+ and, consequen tly, to define a broad Bronsted acidity scale going practically from n on-acidic solids to superacids. We apply here this technique to the st udy of various zeolites. (C) 1998 Elsevier Science B.V. All rights res erved.