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4.1 Conclusions NaX and ZSM-5 zeolite films were grown on ceramic cordierite monoliths using different synthesis procedures and the adsorption properties of these structured adsorbents were studied. Different synthesis solutions influence the morphology, porosity, and adsorption prop- erties of the structured adsorbents. Zeolite films could be grown in synthesis gels, however a large amount of sediments were deposited on the films, which were shown to be a dis- advantage in adsorption processes due to increased diffusion paths. Less sediments were deposited on films grown in clear synthesis mixtures. The duration of the hydrothermal synthesis was also found to influence the zeolite film growth and the physical properties of the adsorbents. When growing NaX films in one long hydrothermal treatment, the zeolite films consisted of small and not well intergrown crystals, and sediments. In particular, NaX films grown in the clear solution consisted of small and badly intergrown crystals, and in addition hydroxysodalite and crystals grew in the film. ZSM-5 and NaX films with a very low amount of sediments and other crystalline phases could be grown using a multiple step synthesis procedure employing clear synthesis mixtures. The best structured NaX adsorbents displayed very sharp breakthrough fronts, which indicated a close to ideal flow through the adsorbent with virtually no axial dispersion and very low mass transfer resistance. The effect of Si/Al and Na/Al ratios in ZSM-5 films was found to have a great influence on the NOx adsorption properties of the films. An increase in Al and Na content in the films causes an increase in NO2 adsorption capacity at low temperature (30◦ C), due to an increased number of weak adsorption sites. At higher temperature, the amount of NO2 adsorbed was nearly independent of the Si/Al ratio of the ZSM-5 films, indicating that an increase in Al and Na concentration in the films increases only the physisorption of NO2. The thicker ZSM-5 films showed stronger mass transfer resistance than the thinner 37 Chapter 4 ConclusionsPDF Image | Structured Zeolite Adsorbents for PSA Applications
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