Crystal Structure of Tubular Na−LTA Zeolite Membrane Used for a Vapor Permeation Process: Unusual Distribution of Adsorbed Water Molecules

Abstract

The crystal structure of two tubular zeolite Na−LTA membranes—one as-prepared membrane and a membrane used in vapor permeation (VP) processes of bioethanol—was analyzed using Rietveld refinement with parallel-beam powder X-ray diffraction data. The crystal structure of the as-prepared membrane was consistent with that of typical Na−LTA powder. After the VP process, an unusual relative peak intensity ratio between 200 and 220 reflections was observed in the membrane. The number of adsorbed water molecules in the β-cage was reduced to half. Furthermore, detachment of sodium cations at the six-membered rings was confirmed. The electron density distributions of these Na−LTA zeolite membranes were analyzed by combining the Rietveld method and the maximum entropy method (MEM). MEM electron density images revealed that the adsorbed water molecules and Na+ cations were bonded, forming a hydrated cluster, and that the guest atoms or molecules were highly disordered.