Characterization of Zeolite NaA Membrane by FTIR-ATR and Its Application to the Rapid Evaluation of Dehydration Performance

Abstract

A zeolite NaA (LTA) membrane supported by an alumina porous support tube was characterized by Fourier Transform Infrared Attenuated Total Reflectance method (FTIR-ATR) with a diamond prism as the waveguide. A method using the FTIR-ATR was developed to estimate rapidly the EtOH/H₂O pervaporation (PV) performance of the membrane. The Si-O asymmetric stretching vibration region of LTA membrane spectra synthesized hydrothermally on seeded alumina substrates showed a bimodal peak (830 - 1200 cm^-1). The two peaks were assigned to a surface LTA directly derived from the seed crystal (1012 cm^-1), and to LTA and/or amorphous substances embedded in the alumina porous support (930 cm^-1). The spectrum from LTA membrane synthesized on nonseeded alumina substrate, however, showed a single broad peak similar to the powder-formed one. These results indicate that the Si-O spectral shape of the LTA membrane is influenced strongly by the synthesis method. Also, the FTIR-ATR of the LTA membrane can detect the Si-O peaks as part of the depth information. It was first shown that the relative ratio (930 cm^-1/1012 cm^-1) of the two Si-O peaks from the LTA membranes on seeded alumina substrates closely relates to the water selectivity (α) in the PV of EtOH/H₂O mixture; the α increases exponentially with the peak ratio. This result suggests that the differences in the vertical distribution of LTA crystal and amorphous material strongly affect the dehydration performance in the EtOH/H₂O PV, that is, the amorphous-like material embedded in the alumina porous support plays an important role. The relative peak ratio measurement can be used for the rapid evaluation of the dehydration performance of the membrane.