Grazing Incidence X-ray Diffraction Analysis of Zeolite NaA Membranes on Porous Alumina Tubes

Abstract

Zeolite NaA-type membranes hydrothermally synthesized on porous alumina tubes, for dehydration process, were characterized by grazing incidence 2θ scan X-ray diffraction analysis (GIXRD). The fine structure of the membrane was studied fractionally for surface layer and for materials embedded in the porous alumina tube. The thickness of the surface layer on the porous alumina tube in the membranes used in this study was approximately 2–3 μm as determined from transmission electron microscopy with focused ion beam thin-layer specimen preparation technique (FIB-TEM). To discuss the effects of the membrane surface morphology on the GIXRD measurements, CaA-type membrane prepared by ion exchange from the NaA-type membrane and surface-damaged NaA-type membrane prepared by water leaching were also studied. For the original NaA-type membrane, 2θ scan GIXRD patterns could be clearly measured at X-ray incidence angles (α) ranging from 0.1 to 2.0 deg in increments of 0.1 deg. The surface layers of the 2–3 μm on the porous alumina tube correspond to the α values up to ca. 0.2 deg. For the CaA-type and the surface-damaged NaA-type membranes, however, diffraction patterns from the surface layer could not be successfully detected and the others were somewhat broad. For all the three samples, diffraction intensities of both zeolite and alumina increased with depth (X-ray incidence angle, α) in the porous alumina tube region. The depth profile analysis of the membranes based on the GIXRD first revealed that amount of zeolite crystal embedded in the porous alumina tube is much larger than that in the surface layer. Thus, the 2θ scan GIXRD is a useful method to study zeolite crystal growth mechanism around (both inside and outside) the porous alumina support during hydrothermal synthesis and to study water permeation behavior in the dehydration process.