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A. Mosca et al. / Microporous and Mesoporous Materials 120 (2009) 195–205 199 Fig. 2. Side view images of the MFI film samples grown in the clear solution (a) in 6 steps of 48 h (C48-6), (b) in 13 steps of 48 h (C48-13), (c) in 9 steps of 96 h (C96-9) and (d) in the gel. Magnification of inset in (b) is 3.5 times larger than Fig. 2b. in this case when the synthesis mixture was replaced as seldom as every 96 h. Aluminium leaching from the support should influence samples C48-6 and C48-13 to a lesser extent, since the synthesis mixture was replaced more frequently, i.e. after 48 h in this case. In concert with previous work [20] the film thickness of the film grown in the gel was about 1.9 lm, as shown in Fig. 2d. The crys- tals in this film are more irregular and the growth mode is difficult to evaluate. Fig. 3a–c show top-view SEM images of the films grown in the clear solution. The films are quite similar; all are smooth and dense and cover the entire outer surface of the monoliths. Cracks and per- haps open grain boundaries with a width between about 100 nm to about 10 nm (resolution of the microscope on these samples) are observed. The cracks and open grain boundaries are thus mostly within the mesopore range. Fig. 3a shows some crystals on top of the film, probably originating from the bulk of the synthesis mix- ture due to sedimentation as reported before [15]. However, such crystals can only be observed at certain locations on the sample and this image was intentionally recorded at one such location. Top view images of the MFI film grown in the gel are shown in Fig. 4a and b. Also in this case, the film covers the entire monolith surface. The morphology of the crystals is completely different to the films grown in the clear solution and no cracks are observed. Well facetted crystals are observed in the film top-layer as re- ported before [20]. Although most of the film is smooth and free of sediments as illustrated in Fig. 4a, large agglomerates of crystals formed in the bulk of the synthesis solution have deposited by sed- imentation in some areas, as shown in Fig. 4b. The zeolite loading was equal to 0.06, 0.10 and 0.14 gzeolite/gsam- ple for the samples grown in the clear solution, see Table 1. Both the zeolite loading and film thickness increase with an additional number of hydrothermal treatments, as expected. However, higher zeolite loading should correspond to higher film thickness. Although the film thickness is the same, the zeolite loading of the sample C96-9 is 40% higher than C48-13, probably due to the presence of more sediments on top of the former film. As zeolite crystals in the bulk solution grow larger during hydrothermal treatment for 96 h, these crystals will sediment faster and more sediments should be deposited on top of the film. When using the gel, the zeolite loading was as much as 0.16 g/g (Table 1), which indicates the presence of sediments on top of the zeolite film, as observed by SEM (Fig. 4 b), whereas the average film thick- ness is equal to 1.9 lm as for samples C48-13 and C96-9. The N2 adsorption–desorption isotherms (cm3/g sample) for the film samples are shown in Fig. 5a. Larger adsorbed volume per gram sample is a result of a higher zeolite loading. As expected, the micropore volume per gram sample increases with increasing zeolite loading, as shown by Fig. 5 and Table 1. For all film samples, an upper hysteresis loop that closes at a relative pressure p/p0 of 0.45 (the loop is almost closing for sample C96-9) is observed. This loop is caused by capillary condensation, likely in the intercrystal- line mesopores in the form of grain boundaries and cracks in the zeolite films in concert with SEM observations. In addition, a lower hysteresis loop closing at p/p0 of about 0.1 is observed only for the samples C48-6 and C48-13, which have the lowest Al concentra- tion. Voogd et al. [23] measured the nitrogen sorption isotherms on ZSM-5 powder samples with varying Al content. A hysteresis loop was observed between p/p0 of 0.1 and 0.2 for the samples with highest Si/Al ratio in their work, i.e. Si/Al = 49. The author assigned this loop to a liquid-to-solid like transition of nitrogen. Since a low- er hysteresis loop was observed for samples C48-6 and C48-13, the N2 sorption data indicates that these samples have a Si/Al ratio above 49, in agreement with ICP-AES data for discrete crystals DC48 and XPS data for sample C48-6Q (see Tables 2 and 3). In addi- tion, epitaxial growth was observed by SEM for samples C48-6 and C48-13, which indicates that the surface Si/Al ratio is above 23 for these films in concert with N2 sorption, ICP-AES and XPS results. No hysteresis loop was observed for samples C96-9 and G, whichPDF Image | Structured Zeolite Adsorbents for PSA Applications
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