Are the active sites of protonic zeolites generated by the cavities?

The surface hydroxyl groups and the surface acidity of silica, silicalite, silica–aluminas, silicated aluminas, aluminated silicas and silicalite, and of H-MFI and H-FER zeolites have been investigated by IR spectroscopy. CO, pyridine and lutidine have been used as molecular basic probes. The data suggest that bridging hydroxyl groups SiOHAl are fully stable structures only in the cavities of zeolites; they are associated to strong IR absorption bands at 3630–3500 cm−1 well correlated with the framework Al content. These species do not exist at the external surface of zeolites and on silicated aluminas. Extremely small bands near 3610 cm−1 may be found on some silica–alumina samples (mostly prepared in organic media) and on aluminated silicas. These bands certainly correspond to very few OH groups and impurities (such as bicarbonates) might contribute to their formation. On the other hand, in disordered mesoporous or microporous amorphous materials, zeolite-like pores may accidentally form and host bridging sites. Acidity-enhanced terminal silanols seem to represent the predominant Brønsted acid sites in non-zeolitic materials based on combinations of silica and alumina.