Mobility of water in Linde type A synthetic zeolites: an inelastic neutron scatteringstudy

Abstract

In the present work the effects produced by cation substitution on the dynamics of water inLinde type A (LTA) synthetic zeolites were investigated by means of inelastic neutronspectroscopy (INS). In particular, we performed measurements on fully hydrated Na-A andMg-exchanged-A zeolites. The collected INS spectra showed two broad bandsdue to the vibrational (water–cation and hydrogen bond stretch) and librational(rock, twist and wag) modes of water. To quantitatively assign these variousmodes, the spectra were decomposed into Gaussian components and compared withthe INS spectra of ice and of water in other zeolites. The observed shifts of theband positions for different temperatures were discussed. In particular, a slightshift of the three librational modes towards higher frequencies with increasingtemperature was revealed for both samples. The hydrogen bond stretching andlibrational modes of water in Na-A and Mg50-A zeolites shift to lower frequencies withrespect to ice. Furthermore, in the case of Mg50-A zeolite, we observed a shiftupwards in frequency of librational bands with respect to Na-A zeolite. The obtainedresults were discussed in relation to our previous elastic and quasi-elastic neutronscattering (ENS and QENS respectively) measurements on the same samples.