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nm and are therefore completely microporous. The porosity of a zeolite provides a much larger total surface area compared to its external surface, which produces a high adsorption capacity solid where adsorption only occurs in the micropores. Molecular characteristics specific to a zeolite (i.e. pore size, shape, and properties like polarity) control how adsorption and desorption (opposite of adsorption) occur within a zeolite. The main building blocks of zeolites consist of SiO2 and Al3O2 units that connect tetrahedrally through oxygen atoms. Several of these units form larger secondary units that serve as building blocks for the zeolite structure. These secondary units, shown in Figure 2.1, illustrate silicon and aluminum atoms at the apices with lines representing oxygen bridges between them. More secondary units exist, but the units in Figure 2.1 represent the units used to build the more common zeolite types A, X, and Y. These secondary units are linked in 3-D space to create a porous crystalline structure. For example, shown in Figure 2.2 is a sodalite unit formed from S4R and S6R units (Figure 2.1). Eight sodalite units form the eight-membered oxygen ring of type A zeolites and connect to the final crystal (Figure 2.3) through D4R units (Figure 2.1). Type X and Y zeolites are formed by ten sodalite units connected by D6R units (Figure 2.1) organized in 3-D space to form a twelve-membered oxygen ring as the smallest pore diameter (Figure 2.3). These oxygen rings are responsible for providing entry for adsorbate molecules into the cavity of the zeolite. They determine pore size and what molecules can enter the structure. Since zeolites are 3-dimensional structures, accurately representing them on a 2-D surface is difficult. Figure 2.3 is one of the better illustrations in texts and literature.9 By comparison, a single nitrogen molecule is approximately 3.6 Å. 8PDF Image | LIMITS OF SMALL SCALE PRESSURE SWING ADSORPTION
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