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200 A. Mosca et al. / Microporous and Mesoporous Materials 120 (2009) 195–205 Fig. 3. Top view images of the MFI film samples C48-6 (a), C48-13 (b), and C96-9 (c). Fig. 4. Top view images showing (a) the MFI film grown in the gel (G) and (b) the sediments on top of the zeolite coating. Magnification of inset in (b) is 20 times larger than Fig. 4b. indicates that these samples should have a Si/Al ratio below 49. This is in agreement with ICP-AES data for discrete crystals DCG and XPS data for sample G (see Tables 2 and 3). However, both ICP-AES for discrete crystals DC96 and XPS data for sample C96- 9Q indicate much higher Si/Al ratios than 49. Again, the Si/Al ratios for sample C96-9 may be lower than for sample C96-9Q, due to leaching of aluminium from the support and incorporation in the film of the former sample. Nevertheless, all data indicates that the Si/Al ratio is decreasing in the order C48-6 = C48-13 > C96- 9 > G. Both ICP-AES data on discrete crystals and N2 sorption data on films indicate that sample C96-9 has lower Si/Al ratio than sam- ples C48-6 and C48-13. In addition, sandwich growth was observed by SEM for the upper part of sample C96-9, which indicates that the surface Si/Al ratio is below 23 for this film in concert with N2 sorption data. The isotherm for sample C96-9 has a greater slope than the other samples in the p/p0 region below 0.4. This indicates presence of a higher amount of smaller mesopores in the sample C96-9, which will be discussed below. Fig. 5b shows the volume adsorbed per gram zeolite of the films grown in the clear solution and the reference powder DC72 (please note that the y-scales for the films are shifted, since data would overlap otherwise). For all samples, the volume adsorbed increased rapidly up to about 100 cm3 per g zeolite as expected for ZSM-5 crystals [23]. The isotherm of the reference powder crystals DC72 is typical for microporous materials and at higher pressures, only a very small increase in adsorbed volume is observed, which is expected for a microporous sample of large crystals without much mesopores be- tween the crystals. In addition, no hysteresis loop is observed, indi- cating the absence of mesopores, in contrast to the film samples. The micropore volume per gram sample increases with increasing zeolite loading, see Table 1. This is expected since zeolite loading is determined using the BET-method and the micropore volume using a t-plot and the BET-surface area is correlated to the t-plot [45]. The micropore volume per gram zeolite was aboutPDF Image | Structured Zeolite Adsorbents for PSA Applications
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