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3.1. General properties of structured adsorbents 23 All these results suggest that clear solutions are more suitable than gels for growth of thin NaX or ZSM-5 zeolite films free from sediments on the walls of ceramic cordierite monolith supports. 3.1.2 Selection of synthesis procedure, Papers A-C Figure 3.4 (a) shows a top-view SEM image of a NaX film grown in the clear solution in one long hydrothermal treatment with a duration of 4 h. The film consists of small and not well intergrown crystals with a size ranging between 170 and 1000 nm. In addition, hydroxysodalite (HS) crystals with a typical ball-of-yarn habit and a diameter of about 2000 nm are observed. The pores in HS zeolite are too small to allow any CO2 adsorption [52]. Phase changes are common in the crystallization system of zeolite NaX upon longer synthesis duration [11], this is probably the reason why HS crystals are observed in this sample. The NaX film has started to dissolve and HS crystals are formed as reported previously. Since the zeolite coating consists of small, not well intergrown crystals, it is difficult to define a film thickness. However, the NaX coating has an average thickness of about 0.4 μm, as shown in Figure 3.4 (b). In addition, the weight gain of the NaX film sample grown in 4 h is about half of the one grown with a multiple step synthesis procedure, as will be described later in the thesis and in more detail in paper B, while the film thickness of the former sample is about 3 times lower, suggesting also presence of sediments after one long hydrothermal treatment in the clear solution. To limit the formation of HS crystals and sediments, and to obtain a better inter- grown film, a multiple step synthesis procedure was developed, where the clear synthesis solution was replaced every 1 h 20 min. Figure 3.4 (c) shows a top-view image of the film grown in 5 steps of 1 h 20 min; the film appears better intergrown than the film grown in one long hydrothermal treatment, and the number of HS crystals embedded in the zeolite film was significantly reduced. As discussed above, the film has an aver- age thickness of about 1.5 μm, see Figure 3.4 (d), and again appears to be well intergrown. By varying the number of steps in the multiple step synthesis procedure it was possible to control the thickness of the zeolite films. Figures 3.5 (a, b) show cross sectional SEM- images of ZSM-5 films grown in the clear synthesis solution in 6 and 13 steps of 48 h, with film thicknesses of about 0.8 and 1.9 μm, respectively. The results presented above demonstrate that a multiple step synthesis procedure using a clear solution is effective for preparation of zeolite films with: • A low amount of sediments in case of NaX and ZSM-5 films • Controlled thickness in case of NaX and ZSM-5 films • A low amount of other crystalline phases (i.e HS crystals) in the case of NaX films • Well intergrown films in case of NaX films.PDF Image | Structured Zeolite Adsorbents for PSA Applications
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