Brønsted acid sites in zeolites. FTIR study of molecular hydrogen as a probe for acidity testing

Hydrogen is used as a probe molecule for characterisation of Brønsted acidity in zeolites. Hydrogen adsorption is monitored volumetrically (physisorption of hydrogen) and by FTIR (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 FTIR are characterised by four parameters which depend upon acid strength: shifts in the position and the increase in absorbance of the hydroxy and the H2 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