Hydration dynamics in zeolite A - An X-ray diffraction and infrared spectroscopic study

The structural evolution of sodium zeolite A with increasing hydration has been studied by powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The lattice parameter and cell volume, determined from time resolved XRD patterns during hydration at ambient conditions, display contraction followed by expansion, suggestive of a hydration-driven flexibility transition, with a two phase region separating hydrated zeolite A from its dehydrated form. Details of bonding dynamics of water molecules with sodium ions and characteristic building units of zeolite Na-A were studied using time resolved FTIR as the sample underwent hydration. The O-H stretching vibration shifts from 3450 cm⁻¹ representing chemisorption to 3350 cm⁻¹ indicating physisorption with increasing hydration. The FTIR data strongly suggest that both water and sodium ions change their locations and bonding as a function of water content. The structural and dynamic observations support our earlier calorimetric studies of the enthalpy of water absorption, pointing to a phase transition related to framework flexibility in response to hydration and to variable water content in both the less hydrated and the more hydrated phase.