The combined use of acetonitrile and adamantane-carbonitrile for FTIR spectroscopic characterisation of acidity in zeolites.

Acetonitrile and adamantane-carbonitrile form hydrogen-bonded adducts with both silanols and Brønsted acid sites of zeolites. The corresponding changes in the O-H stretching vibration render the zeolite surface IR-readable. The combined use of both probe molecules helps to differentiate between internal and external OH groups of medium pore zeolites, since acetonitrile can enter the zeolite channels while adamantane-carbonitrile cannot. The method was demonstrated by analysing the interaction of both probe molecules with H-ZSM-5 samples which were synthesized under different conditions. It was shown that a sample prepared in the presence of a template agent (tetrapropylammonium bromide) had a proportion of internal silanols much higher than that found for an analogous H-ZSM-5 sample synthesized by a template-free route. Bridged Si(OH)Al groups (Brønsted acid sites) were found to be located mainly at the internal surface of the zeolites, although the sample prepared using the template agent showed some evidence for the presence of a small proportion of these acid sites at the external surface.