Perturbation of surface species on zeolite HY by Xe observed by FT-IR spectroscopy

The perturbation of surface species of zeolite HY by Xe has been studied with the use of in situ FT-IR spectroscopy. The Bronsted acid sites, characterized by a band at 3638 cm^-1, are located in the zeolite supercages, as suggested previously, while Bronsted acid sites attributed to a 3545-cm^-1 band are located in the small zeolite cages and are inaccessible to the Xe molecules. The Bronsted acid sites in the supercages are found to have a weaker acidity than those in the small cages. The data also indicate that pyridine molecules are more strongly bonded to the supercage Bronsted acid sites than are the NH₃ molecules. This work is an example of the usefulness of liquid-nitrogen-temperature FT-IR techniques, coupled with multiple-scan data acquisition methods, for studying weak interactions between Xe and zeolite surface species in detail. This work also shows how FT-IR spectroscopy can complement Xe-129 NMR spectroscopy in studies of the nature of zeolite surface species using Xe as a probe molecule.