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8 Introduction with pore openings of 0.53 × 0.56 nm, and zig-zag pores running in the a-direction with pore openings of 0.51 × 0.55 nm, see Figure 1.5. Two zeolite types have the same MFI framework, namely silicalite-1 and ZSM-5, and differ only in the Si/Al ratio. There is no formal convention about the Si/Al ratio of the two zeolites; however, silicalite-1 has a very high Si/Al ratio (basically no Al in the structure), and ZSM-5 has a Si/Al ratio from several hundreds to 10 [11, 10]. The FAU framework also has a three-dimensional pore structure with pores running perpendicular to each other in the a-, b-, and c-direction, see Figure 1.5. The pore diameter is 0.7 nm and there is also a cavity with a diameter of 1.2 nm where the channels intersect. Two zeolites have the same FAU framework, namely zeolite X and zeolite Y, and the difference between these two zeolites is only the Si/Al ratio, which varies between 1 and 1.5 for zeolite X, and between 1.5 and 5.6 for zeolite Y [11, 10]. Figure 1.5: Schematic drawing illustrating the MFI framework in the b-direction (a) and the FAU framework (b). Three commonly used zeolites and some of their current major industrial applications are listed in Table 1.1. Zeolites are one of the main components in detergents, where their ion-exchange properties are used to soften the water. Zeolites are very useful as solid-acid catalysts, as summarized in Table 1.1, due to the high activity and selectivity they provide. The most important application for zeolite catalysts is in the oil refining industry [17] for hydrocarbon cracking in the fluid catalytic cracking (FCC) process using ultra-stable Y (USY) zeolite. Zeolite ZSM-5 is also used as catalyst in acid-catalyzed reactions such as xylene isomerisation [10, 18]. 1.3.2 Gas separation applications for zeolites Beyond industrial gas drying, zeolites are excellent adsorbents due to selectivity with respect to specific components in a gas mixture [20, 21]. The most important applications for zeolite adsorbents are oxygen enrichment from air (low silica zeolite X) [22, 23, 24, 25], CO2 removal from natural gas (zeolite X) [1, 26], or H2 removal from refinery coke gas (zeolite A) in carbon-zeolite layered beds to increase hydrogen purity (>99.9%) [1, 12, 27].PDF Image | Structured Zeolite Adsorbents for PSA Applications
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