First observation of vibration-rotation drift spectra of para- and ortho-hydrogen adsorbed at 77 K on LiX, NaX and CsX zeolites

A comparison of the DRIFT spectra of conventional hydrogen (with a ratio of o-H(2) to p-H(2) equal to 3) and those of the para-enriched hydrogen adsorbed at 77 K on LiX and CsX zeolites, confirms the earlier assignment of the weak high-frequency bands to the vibration-rotation transitions. The results obtained indicate almost free rotation of the hydrogen adsorbed on CsX, but substantially hindered rotation for hydrogen adsorption on the LiX form. However, the adsorption isotherms for LiX, NaX and CsX zeolites are almost identical indicating close lying values of heats of adsorption. To explain this discrepancy, it is suggested that the interaction of the adsorbed hydrogen with the cations provides only a part of the adsorption energy, to which the interaction with the basic oxygen of the zeolite framework also substantially contributes. The frequency of the oscillations of the adsorbed hydrogen relative to the adsorption sites, as obtained from the positions of the satellites in the DRIFT spectra, is used to discuss the motion of the hydrogen inside the zeolite micropores.