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Preparation and characterization of highly oriented films and membranes of zeolite A
Zeolite NaA films and membranes have been prepared using both in situ and seeded growth preparation processes. Films prepared using in situ preparations have shown this technique to be unsuitable for further development due to its inability to control the film microstructure, poor reproducibility, and dissolution of the substrate resulting in amorphous material incorporated in the film. Seeded growth, however, shows the ability to prepare highly oriented zeolite NaA films, the first zeolite films reported with this high degree of orientation. ^ For the seeded growth preparation, nanometer sized zeolite particles are used in suspension to cast seed films. These films are prepared using dip coating, film casting, and electrostatic deposition. The seed films show a high degree of orientation with the [h00] planes of the seed crystals aligned parallel to the substrate surface. A higher degree of orientation where the particles are deposited in a hexagonal packed array can be achieved using dip coating with extremely slow withdrawal rates (∼1 cm/hr). These seed films are then subjected to a secondary growth process to eliminate the interzeolitic pores and form continuous zeolite layers. This has been achieved with clear solutions or gels resulting in continuous films 0.5 to 7μm thick with a high degree of orientation. The regrowth mechanism was investigated and results indicate that the growth of zeolite A films proceeds by multiple processes including epitaxial growth of the seeds and deposition of particles from solution. ^ The membranes have been used for alcohol/water pervaporation. The membranes are highly selective for water and show selectivities >3,100 for water using (90/10) ethanol/water feed systems. In permeation measurements, these membranes show no selectivities other than Knudsen for permanent gases. Unlike Knudsen diffusion, these membranes show increasing permeation with increasing temperature. This indicates the probability of small defects in the films around 10Å. The defects shown by the gas permeation measurements indicate that cracks have formed in the membranes, possibly upon drying. It is believed that these are caused by the contraction of the zeolite NaA structure upon removal of water, which gives a 0.16% contraction in the dimensions of the unit cell. ^
Engineering, Chemical|Engineering, Materials Science
Boudreau, Laura Catherine, "Preparation and characterization of highly oriented films and membranes of zeolite A" (1999). Doctoral Dissertations Available from Proquest. AAI9932296.