Pore size IUPAC classification

Table 16-4 shows the IUPAC classification of pores by size. Micropores are small enough that a molecule is attracted to both of the opposing walls forming the pore. The potential energy functions for these walls superimpose to create a deep well, and strong adsorption results. Hysteresis is generally not observed. (However, water vapor adsorbed in the micropores of activated carbon shows a large hysteresis loop, and the desorption branch is sometimes used with the Kelvin equation to determine the pore size distribution.) Capillary condensation occurs in mesopores and a hysteresis loop is typically found. Macropores form important paths for molecules to diffuse into a par-... 

The average pore size of PS structures covers four orders of magnitude, from nanometers to tens of micrometers. The pore size, or more precisely the pore width d, is defined as the distance between two opposite walls of the pore. It so happens that the different size regimes of PS characterized by different pore morphologies and different formation mechanisms closely match the classification of porous media, as laid down in the IUPAC convention [Iu2]. Therefore the PS structures discussed in the next three chapters will be ordered according to the pore diameters as mostly microporous (d<2 nm), mostly mesoporous (2 nm50 rim). Note that the term nanoporous is sometimes used in the literature for the microporous size regime. 

The sorption isotherms can be grouped into five types, according to the classification of Brunauer, Emmet and Teller. l,2 3A However, we prefer a classification, based on the pore size of the adsorbent.5 The IUPAC classification6 of pores is given in table 2.1. 

The upper limit of 2.0 nm for the micropore width was put forward as part of the IUPAC classification of pore size [4]. It now seems likely that there arc two different microporc filling mechanisms, which may operate at p/p° below the onset of capillary condensation the first, occurring at low p/p°, involves the entry... 

It is recommended therefore that attention should be directed towards the mechanism of pore filling rather than to the specification of the necessarily rather arbitrary limits of pore size. Until further progress has been made it is undesirable to modify the original IUPAC classification or to introduce any new terms (e.g. ultra-pores or ultramicropores). 

Although some of the isotherms in Figures 9.18-9.22 are more complex than others, they are all essentially Type I in the IUPAC classification. The five carbons are evidently predominantly microporous, but with different ranges of pore size. Before any attempt is made to assess the pore size distribution of each carbon, it is worth examining the significance of the various characteristic features of the isotherms. 

In a typical amorphous adsorbent the distribution of pore size may be very wide, spanning the range from a few nanometers to perhaps one micrometer. Since different phenomena dominate the adsorptive behavior in different pore size ranges, IUPAC has suggested the following classification ... 

Pore size is also related to surface area and thus to adsorbent capacity, particularly for gas-phase adsorption. Because the total surface area of a given mass of adsorbent increases with decreasing pore size, only materials containing micropores and small mesopores (nanometer diameters) have sufficient capacity to be useful as practical adsorbents for gas-phase applications. Micropore diameters are less than 2 mn mesopore diameters are between 2 and 50 nm and macropores diameters are greater than 50 mn, by IUPAC classification (40). 

According to the IUPAC classification, porous solids can be arranged in three main categories, depending on their pore size micropore (<2nm), mesopore (2 50nm), and macropore (>50nm). Here, the prefix meso- describes a state between the micro-and the macro-. Mesopore sizes are in the nanometer region therefore, nanoporous is frequently used in its place in the literature. Sometimes the term ultra-micropore is used for pore sizes smaller than 0.7 nm. The pore sizes discussed here represent the diameter or the width of the pore, not the radius. 

Figure 3.3 Schematic representation of the pore size distribution of different siiicate-based materials and their classification according to the iUPAC nomenclature.

In addition to the classification described above, the real metric scale in form of nanometer, micrometer, and millimeter is often used particularly by materials scientists to characterize the pore size. To prevent misunderstandings with the IUPAC classification, we propose to use in these cases the expression always with the suffix meter, viz. nanometer, micrometer, and millimeter (sized) pores. 

Table 9.2 illustrates a convenient classification of porous solids by the characteristic sizes of their pores. This classification was originally proposed by Dubinin [114], and in 1972 it was officially adopted by International Union of Pure and Applied Chemistry (IUPAC) [7,53,58],... 

Until now we have mainly treated adsorption onto non-porous surfaces. In reality, most industrial and many natural materials are porous Textiles, paper, bricks, sand, porous rocks, food products, zeolites etc. We start our discussion with a classification of pores according to their size, which is recommended by IUPAC ... 

We have discussed previously diffusion in dense crystalline materials. Now, we study the transport of molecules in porous media. According to the classification scheme proposed by the International Union of Applied Chemistry (IUPAC), pores are divided into three categories on the basis of size macropores (more than 50 nm), mesopores (from 2 to 50 nm), and micropores (less than 2 nm) [74,75],... 

Note that when designating the fabricated porous structures in this way, we do not follow classification recommended for structural characterization of porous inorganic materials by the International Union of Pure and Applied Chemistry (IUPAC) [6], but simply proceed from the actual size of the pores. Classification of porous structures fabricated in SiC cannot yet be considered as established [7,8]. 

The classification of pores according to their size, proposed by Dubinin, has been adopted by IUPAC (127, 128) the classifications are macropores (d > 2000-4000 A), mesopores (30-32 A < d < 2000-4000 A), supermicropores (12-14 A < d < 30-32 A), and micropores (d < 12-14 A). This classification system has proved useful and is used in the study of amorphous silica having different structural characteristics (4). 

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