FTIR-ATR study of the surface of a tubular zeolite NaA membrane ultrasonically reacted with water and acetic acid.

For the dehydration process of biomass alcohol using a zeolite NaA (LTA) membrane supported by an alumina porous support tube, it is very important to understand damage of the surface crystal, which may be caused by acetic acid. Therefore, the surface structure and the chemical composition of the LTA membrane that reacted with water or acetic acid (pH 3-4) were investigated by the Fourier Transform Infrared Attenuated Total Reflectance method (FTIR-ATR), with a diamond prism as the waveguide, scanning electron microscope and an energy-dispersive X-ray analysis. For leaching experiments using water and acetic acid, ultrasonication was applied to promote the leaching rate and to surely prepare damaged membranes as a model experiment. The Si-O asymmetric stretching vibration spectra of the original LTA membranes showed a bimodal peak. LTA membranes after water leaching also showed the same peak. However, for the LTA membrane reacted with acetic acid of pH 3, damage of the surface LTA crystal and the loss of sodium by dissolution were clearly observed. Also, its Si-O spectral shape was broad, which suggests amorphous-like substances secondarily formed on the membrane surface. Sodium acetate was not detected for all LTA membranes reacted with acetic acid. To evaluate surface-damaged and sodium-loss membranes, FTIR-ATR can sensitively detect a Si-O spectral change corresponding to the surface structure and the chemical composition of the LTA membrane.