Proton affinities of Brsnsted hydroxy groups in H-forms of zeolites and SAPOs are determined by an FTIR study of the low-temperature adsorption of carbon monoxide. This method reveals the range of heterogeneity of Brsnsted hydroxy groups associated with unmodified zeolites where only one band interacting with carbon monoxide is evident in the infrared spectrum. Three types of acid hydroxy groups associated with different IR bands are detected in a modified zeolite (US-Y), (i) strongly acidic [3599 cm-', proton affinity (€,J = 1112 kJ mol-'1, (ii) medium acidic (3627 cm-', E,, = 1142 kJ mol-') and (iii) weakly acidic (ca. 3740 and 3675 cm-', ED, = 1192 f 29 kJ mol-I).

Hydrogen is used as a probe molecule for characterisation of Brsnsted acidity in zeolites. Hydrogen adsorption is monitored volumetrically (physisorption of hydrogen) and by FTlR (OH--H2 complex formation), simultaneously. The physisorption of hydrogen is a function of the pore size of a zeolite whereas the OH. * .H2 interaction reflects the strength of the acid sites. Hydrogen complexes observed by FTlR are characterised by four parameters which depend upon acid strength: shifts in the position and the increase in absorbance of the hydroxy and the H , bands. For the zeolites studied the behaviour of these parameters is consistent and shows the following order of acidity: SAPO-37 < H-Y < H-EMT < H-ZSM-5 < H-MOR. Hydrogen is also compared with carbon monoxide, another probe widely used for acidity determination.

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