FIRST OBSERVATION OF VIBRATION-ROTATION DRIFT SPECTRA OF PARA-HYDROGEN AND ORTHO-HYDROGEN ADSORBED AT 77 K ON LIX, NAX AND CSX ZEOLITES

A comparison of the DRIFT spectra of conventional hydrogen (with a rat io of o-H-2 to p-H-2 equal to 3) and those of the para-enriched hydrog en adsorbed at 77 K on LiX and CsX zeolites, confirms the earlier assi gnment of the weak high-frequency bands to the vibration-rotation tran sitions. The results obtained indicate almost free rotation of the hyd rogen adsorbed on CsX, but substantially hindered rotation for hydroge n adsorption on the LiX form. However, the adsorption isotherms for Li X, NaX and CsX zeolites are almost identical indicating close lying va lues of heats of adsorption. To explain this discrepancy, it is sugges ted that the interaction of the adsorbed hydrogen with the cations pro vides only a part of the adsorption energy, to which the interaction w ith the basic oxygen of the zeolite framework also substantially contr ibutes. The frequency of the oscillations of the adsorbed hydrogen rel ative to the adsorption sites, as obtained from the positions of the s atellites in the DRIFT spectra, is used to discuss the motion of the h ydrogen inside the zeolite micropores.