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cordierite monolith is shown in Figure 1 (b), which illustrates macropores with various size between the grains in the cordierite monolith. Figure 1 (c) shows a monolayer of 80 nm NaX seeds crystals adsorbed onto the surface of the support. The seeds cover the entire external surface of the cordierite monolith walls. A few aggregates of seed crystals with a size between 200 nm and 400 nm are also observed. Figure 2 (a) shows a top-view SEM image of sample G9. The film appears to be very well intergrown and typical NaX crystals with octahedral habit and with a size ranging between about 600 and 2000 nm are observed. A SEM image of sample C4 grown in the clear solution by a single hydrothermal treatment for 4 h is shown in Figure 2 (c). The film is less intergrown and comprised of NaX crystals with a size ranging between about 170 and 1000 nm and in addition, hydroxysodalite (HS) crystals with a typical ball-of-yarn habit and a diameter of about 2000 nm are observed. These hydroxysodalite crystals will not contribute to the CO2 or N2 adsorption capacity of the film due to the very small pore size (2.2 – 2.8 Å) [39] of this zeolite. Zeolite HS likely forms as a result of the decreased hydrothermal stability of the NaX crystals during a longer hydrothermal treatment as phase changes are common in the crystallization system of zeolite NaX upon longer synthesis durations [40]. However, no HS crystals formed on the G9 sample, which was prepared from a quite different synthesis mixture, i.e. a gel. To limit the deposition of sediments, formation of HS crystals, and obtain a better intergrown film in the clear solution, a multiple step synthesis procedure was developed where the clear synthesis solution was replaced every 1 h 20 min. A NaX film was grown in 5 steps of 1 h 20 min, see Figure 2 (e). This film (sample C5*1h20min) appears more well intergrown than the film in sample C4, but not as intergrown as the film in sample G9, and the number and size of HS crystals embedded in the zeolite film was significantly reduced compared to the one grown in one longer hydrothermal treatment (sample C4). The average thickness of the NaX film sample grown in the gel (sample G9) is about 1.1 m, as shown in Figure 2 (b) and reported in Table 1. Figure 2 (d) shows a cross- 72PDF Image | Structured Zeolite Adsorbents for PSA Applications
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