On the Use of CHClF₂ as a Probe of Basic Sites in Zeolites: The Host−Guest Interactions Investigated by Multinuclear NMR

The use of chlorodifluoromethane (CHClF2) as a probe molecule of zeolites basicity has been investigated by using infrared and multinuclear NMR spectroscopies and a series of alkali-exchanged faujasite with different Si/Al ratios (X and Y) and compensating cations of different nature. The 1H NMR peak of adsorbed CHClF2 shifts to low fields and the CH stretching frequency (νCH) shifts to low wavenumbers as the zeolite basicity determined by the Sanderson method increases. Poorer linear correlation is observed for the νCH band suggesting the occurrence of extra interactions of the fluorine atoms of the adsorbed chlorodifluoromethane with the nonframework cations. This interaction is evidenced for the first time by the spectroscopic modification of the probe molecule itself; 19F NMR chemical shifts and 1J(C,F) spin−spin coupling constants are determined by the nature of the extraframework atoms and not by the framework basicity. The occurrence of the interactions between the fluoride atoms and the compensating cation does not allow quantifying the number of sites of similar basicity.