COMPARATIVE IR-SPECTROSCOPIC STUDY OF LOW-TEMPERATURE H2 AND CO ADSORPTION ON NA ZEOLITES

Extraframework cation sites in the sodium forms of the zeolites ZSM-5, mordenite, Linde-4A and faujasite-type X and Y have been investigated by using low-temperature adsorption of dihydrogen and carbon monoxide as IR spectroscopic probes. The extent of H-H and C-O bond polarizati on was found to be dependent not only on the cation electrostatic fiel d, but also on the neighbouring oxygen atoms of the zeolite framework. The influence of these oxygen atoms is most keenly felt by adsorbed m olecular hydrogen, but they also affect the IR frequency shift of the stretching vibration of adsorbed carbon monoxide. The Si : Al ratio of the zeolite framework modulates the basic strength of the oxygen atom s, and this was found to be reflected in the IR stretching frequency o f both adsorbed molecules, H2 and CO.