Pore shape, general properties

It should be emphasized that the active sites located on the external surface, often in small amounts compared to the inner sites (<1% for crystallites of 1 //m), play a catalytic role. Generally, this leads to a selectivity decrease, the external surface lacking the shape selective properties of the inner pores.. However, recent results show that reactions which can occur only on the external surface of zeolites or just within the pore mouth are very selective, suggesting a shape selective influence of external surface depending on the nature of the substrate (Table 1.2). 

In some ways zeolites are similar in their properties in that they also provide size and shape selectivity because of the presence of pores within the three-dimensional structure. However in the zeolites these pores are generally much larger so that larger molecules can be incorporated into the structure. Zeolites are crystalline aluminosilicates which occur both in nature and as synthetic structures [204]. The zeolite framework is made up of aluminum and silicon atoms tetrahedrally coordinated by oxygen, giving a framework stoichiometry of MO2. For each aluminum atom within the framework there is a formal charge of —1 which is compensated by counterbalancing cations within pores in the structure. Typically these may be cations such as Na" ", Ca ", NH4, or HsO". ... 

While the sintering parameters have a significant effect on the final membrane pore size distribution and overall porosity, the initial properties of the ceramic particles also strongly influence the properties of the final membrane. In general, the pore size of porous solids is determined mainly by the particle size and the pore shape is governed by the shape of the starting powder (Ishizaki et al., 1998) although additional relationships are summarised in Table 8.2. 

Correspondingly, the values of nF characterize the connectivity of the lattice of particles or skeleton of a PS and the values of Zc characterize the interconnectivity of the lattice of pores of the same PS. Connectivity of PS is the major topological attribute, which in the general case does not depend on the shape and size of the PS s individual supramolecular elements, although the latter characterize the major geometrical properties of PS [8], Appropriately, the classification of PSs by the degree of interconnectivity with allocation of various types of integrity is possible. 

The most important property of adsorbent materials, the property that is decisive for the adsorbent s usage, is the pore structure. The total number of pores, their shape, and size determine the adsorption capacity and even the dynamic adsorption rate of the material. Generally, pores are divided into macro-, rneso- and micropores. According to IUPAC, pores are classified as shown in Table 2.2. 

The irregularly shaped pores between soil particles contain both air and water (Fig. 9-9). The soil pores, or voids, vary from just under 40% to about 60% of the soil by volume. Thus a soil whose pores are completely filled with water contains 40to60% water by volume. In the vicinity of most roots, moist soil contains 8 to 30% water by volume the rest of the pore space is filled with air. Therefore, the pores provide many air-liquid interfaces (Fig. 9-9) where surface tension effects can lead to a negative hydrostatic pressure in the soil water (Chapter 2, Sections 2.2G and 2.4E). Such a negative P is generally the main contributor to the water potential in the soil, especially as the soil dries. The thermal properties of soil are discussed in Chapter 7 (Section 7.5), so here we focus on soil water relations. 

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