Far-infrared study of cation motion in dry and solvated mono- and divalent cation containing zeolites X and Y (original) (raw)

1977, The Journal of Physical Chemistry

Publication costs assisted by Brown University and the Union Carbide Corporation Far-infrared ion motion bands have been observed and assigned in the spectra of dry synthetic zeolites X and Y containing Li' , Na+, K+, Rb' , Cs+, Ag+, Ca2+, Sr2+, and Ba2+ cations. The site I' and site I1 cation vibrational bands overlap and form the strongest feature in the spectra of samples exchanged with monovalent ions. The site I cation band appears at lower frequency than the site I1 envelope in these samples, but in divalent ion exchanged zeolites the opposite order occurs. A very low frequency site I11 cation band, typical of monovalent X zeolites, has been observed in CsY. For a given cation, the frequency on X is higher than on Y due to the higher framework charge of X zeolites. The vibrational frequencies also follow an approximate m-1/2 dependence for the two types of cation and the two forms of the zeolite. Solvation of the monovalent zeolites with H20, THF, Me2S0, pyridine, and CH2C12 results in the appearance of a new band at higher frequency than the ion-framework modes which concurrently diminish in intensity, especially the site I11 band. The high frequency band is due to ion motion in a solvation shell which is unsymmetric at low hydration levels and at all solvation levels with organic adsorbates. In addition to a cation mass dependence, the ion-solvent frequencies are also dependent on the adsorbent's effective dielectric constant which is greater in X than Y zeolites. Increasing solvation shifts the ion-framework vibrations to lower frequency, but this shift with the organic adsorbates is primarily an indirect effect due to solvent delocalizing the framework charge. The implications of the cation vibrational frequencies for the mechanism of ionic conduction are considered in terms of a simple free-ion model. Ion transport in Y zeolites which have a large number of vacant cation sites so that each jump may be considered an independent event is adequately explained by this model, but ion transport in X zeolites is best interpreted in terms of cooperative effects.