Micro-meso-macroporous zeolitic materials

The strategy of this method is to utilize the inherent porosity of bulky substrates in the construction of hierarchical stractures by incorporating additional pore systems. Diatoms are unicellular algae whose walls are composed of silica with an internal pore diameter at submicron to micron scales. Zeolitization of diatoms, in which zeolite nanoparticles are dispersed on the surface of diatoms followed by a hydrothermal conversation of a portion of the diatom silicas into zeolites, resulted in the formation of a micro/mesoporous composite material. Similarly, wood has also been used as a substrate to prepare meso/macroporous composites and meso/macroporous zeolites. After the synthesis, wood is removed by calcination. ... 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

Nevertheless, for practical applications very rough recommendations about adsorption isotherms to be expected in experiments or to be used in industrial processes can be given. They are summarized in Table 7.3 below where sorbent materials simply are classified according to their pore spectrum as narrow (zeolites, molecular sieves, etc.) or wide (activated carbons, silica, etc.). In the first case we mainly expect microporous sorbent materials, whereas in the second case micro-, meso, and macropores as well can be included in the sorbent. The adsorptive molecules are classified by their polarity as either non-polar like (He, Ar, N2, O2, CH4) or polar like (CO, NO,... 

However, the equation is invalid, for instance, for microporous materials and combinations of micro-, meso-, and macropores, such as zeolites, activated charcoals, and several others. Therefore, anew standard isotherm was proposed, known as f-plot. In this case the adsorption isotherms can be represented by a single curve. 

Porous materials are elassified into three different categories based on the size of pores rpore, that is, microporous for rpore < 20 A, mesoporous for 20 A< Tpore < 500 A, and macroporous for rpote > 500 A. Here I will discuss micro- and meso-porous materials. Microporous crystals (hereafter called zeolites) with more than 150 different framework-type stmctures have been reported. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

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