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Syntheses of self-supported tubular zeolite a membranes

Gücüyener, Canan
Zeolites are microporous hydrated aluminosilicate crystals containing alkali and/or alkali earth metal cations in their frameworks. Due to their molecular size pores, they can separate molecules according to their size and shape. Zeolites are mostly used in ion exchange, adsorption processes and catalytic applications. The hydrophilic/hydrophobic character of zeolites also makes them favorable materials for adsorption based separations. Recently the potential of zeolite/ceramic composite membranes have been shown in the separation of liquid and gas mixtures. Self-supported zeolite membranes with asymmetric structure can be an alternative to the composite zeolite membranes. Because asymmetric structure may eliminate the problems originated from the differences in thermal expansion coefficients of zeolites and ceramics. In this study tubular zeolite A membranes were prepared on binderless zeolite A supports. The supports were perepared by hydrothermal conversion of amorphous aluminosilicate tubes into zeolite A. The amorphous aluminosilicate powder, which was obtained by filtering the homogenous hydrogel with a composition of 2.5Na2O:1Al2O3:1.7SiO2:150H2O, was mixed with an organic binder (HEC-Hydroxyethyl Cellulose) and water to obtain the paste. The paste was then extruded through a home-made extruder into bars and tubes. These extrudates were dried at room temperature for 24 hours, calcined at 600oC for 2 hours to remove organic binder and finally synthesized at 80oC for 72 hours in hydrothermal conditions to convert amorphous aluminosilicate to zeolite. The effect of composition of the synthesis solution on the crystallinity and morphology of zeolite A tubes and bars were investigated. The crystallization field of zeolite A bars has been established and shown on a ternary phase diagram. Tubes were mechanically stable, typically had a crystallinity over 90% and a macroporosity of 35%. The tubes were composed of highly intergrown crystals of zeolite A. The average particle size was 3.5 m. The asymmetric membranes were synthesized by growing zeolite A films on binderless zeolite A supports with a geometry of disk, bar and tube. Continuous zeolite A films can only be obtained when the supports were saturated with water prior to synthesis. The film thicknesses were approximately 5 m on disks and approximately 10 m on tubes. A method was proposed to prepare self-supported tubular zeolite A membranes in this study.