Pores structural classification

In order to elucidate the pore structure of Csx, the adsorption-desorption isotherm of N2 was first measured. Tsrical results are given in Figure 4. H3PW12O40 exhibited a Type II isotherm (according to the lUPAC classification... 

The pore structure of a solid can contribute to the disintegration, dissolution, adsorption, and diffusion of a drug material [26,27]. Because of this, porosity and pore size distribution measurements have been used extensively to study tablets [28-30], granules [31,32], and excipients [33]. The following classification system of pore sizes has been developed based on the average pore radii [6] ... 

Given the complexity of the pore structure in high-surface-area catalysts, six types of adsorption isotherms have been identified according to a classification advanced by IUPAC 145-481. Out of these six, only four are usually found in catalysis ... 

Although both the laboratory and industrial scale materials science of catalysts requires an integrated approach as already mentioned above, it is customary to classify the characterization methods by their objects and experimental tools used. I will use the object classification and direct the introductory comments to analysis, primarily elemental and molecular surface analysis, determination of geometric structure, approaches toward the determination of electronic structure, characterization by chemisorption and reaction studies, determination of pore structure, morphology, and texture, and, finally, the role of theory in interpreting the often complex characterization data as well as predicting reaction paths. 

The structural classification of synthetic ion channels and pores differentiates between (macro)molecules and different classes of supramolecules [2]. Unimolec-ular synthetic ion channels and pores are usually hollow, often helical (macro)mol-ecules that are long enough to span common lipid bilayer membranes (2-4 nm). 

A classification of pores based on pore sizes was proposed by the International Union for Pure and Applied Chemistry (lUPAC). As illustrated in Fig. 1, pores are usually classified into three classes macropores (>50 nm), mesopores (2-50 nm) and micropores (<2 nm) [1], Micropores can be further divided into supermicropores (with a size of 0.7-2 nm) and ultramicropores (<0.7 nm in size), Since nanotechnology attracted the attention of many scientists recently, the pore structure has been required to be controlled closely, a part of which will be explained in Section 5. Wlten scientists wanted to express that they are controlling pores in the nanometer scale, some of them preferred to call the smallest pores nano-sized pores, instead of micro/mesopores. 

Zeolites can be classified in many ways. Two convenient methods are on the basis of pore size and chemical composition, that is, the Si/Al ratio. The pore diameter is determined by the size of the free apertures in the structure, which is dependent on the number of T atoms (T = Si or Al) that form the aperture. Table 10.1 summarizes some examples of zeolites based on pore size classification. It should be noted that the values typically reported in the literature are determined by crystallographic studies. While these numbers are good guides, it is important to note that the actual pore size depends on many factors, including temperature, firamework composition, and the type of extra-framework cations present in the zeolite. These factors can lead to subtle changes in effective pore sizes and subsequently large changes in material properties (adsorption/reactivity). 

ACs are the most commonly used form of porous carbons for a long time. Typically, they refer to coal and petroleum pitch as well as coconut sheUs-based AC. In most cases, ACs are processed to be filled with rich micropores that increase the surface area available for gas sorption and separation. For this category, to get a definite classification on the basis of pore structure is difficult due to their countless products as well as their complex pore features. Based on the physical characteristics, they can be widely classified into the following types powdered, granular, extruded, bead ACs, etc. For the pore structure of ACs, actually, all the three types of pores (micropore, mesopore, and macropore) are included in one product (Fig. 2.1), with a wide pore size distribution [1, 2]. Up to now, many kinds of ACs have been well commercialized in gas sorption/separation including CO2 capture. For example, the BPL type with specific area of 1,141 m g is able to adsorb 7 mmol g CO2 under the conditions of 25 °C and 35 bar, while under the same conditions MAXSORB-activated carbon with specific area of 3,250 g can capture up to 25 mmol g [3]. 

Mesoporous Aluminophosphates. The structure classification of amorphous mesoporous aluminophosphates is based on the pore arrangement that can be hexagonal, cubic, or disordered. Aluminum species in these materials are usually present as both four- and six-coordinated species, whereas phosphate units being always tetrahedral. The Al/P ratio differs from unity due to the uncompleted condensation of the mesoporous framework (16). 

Figure 1 shows nitrogen sorption isotherms measured at 77 K. Both samples show hysteresis loops of type H2 according to the lUPAC classification (Rouquerol et al. 1994), revealing developed mesoporosity with a substantial disorder in the pore structure. With the use of model assumptions, the sorption isotherms can be converted to the corresponding PSDs, some of which are exemplified in Fig. 2. 

The definition of the different types of pores is based on their width, which represents the distance between the walls of a slit-shaped pore or the radius of a cylindrical pore. This classification, which is not entirely arbitrary, is now widely accepted and used. It takes into account differences in the behaviour of molecules adsorbed in micropores and in mesopores. It appears that for pore widths exceeding 1.5-2.0 nm, the gaseous adsorbate condenses in a liquid-like state and a meniscus is formed. As a consequence, a hysteresis loop appears on desorption and its interpretation can lead to the distribution of the mesopores in the adsorbent [23]. The limit between mesopores and macropores at 50 nm is more artificial, and corresponds to the practical limit of the method for pore-size determination based on the analysis of the hysteresis loop. As a rule, the porous structure of the usual types of activated carbons is tridisperse, i.e. they contain micropores, mesopores and macropores. Micropores are of the greatest significance for adsorption owing to their very large specific surface area, and their large specific volume. At least 90-95% of the total surface area of an activated carbon can correspond to micropores. 

Pore Structure and Classification of Porous Coordination Polymers... 

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