Pore size classification

In the context of physisorptlon, the following pore size classification is recommended by lUPAC (see sec. 1.9a) ... 

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). 

Figure 8.1 Pore size classification and corresponding nitrogen isotherm of porous materials.

The pore size classification given here is referred to pore diameter or more arbitrarily pore width. 

Fig. 1 Size of different biomoleeules and biologieal entities versus pore size classification...

FIGURE 9.8 New pore size classification as compared with the current lUPAC nomenclature. 

Given these drawbacks, the present author considered that it was timely to consider a new pore size classification that would be consistent with SI and related terms, and that would replace (rather than be used in parallel with) the current lUPAC scheme. This new classification is presented below. 

The new pore size classification is sumarised in Fig. 1. It is based on a logarithmic scale (to base 10), and all pores of interest are considered to be no smaller than 0.1 nm and no larger than 0.1 m (though in principle the new scheme could be extended beyond these boundaries). The three main pore size ranges are ... 

Fig. 1 New pore size classification compared with the current lUPAC scheme [2]. Figures in bold in the new scheme refer to the reference points for each main pore size class.

A new pore size classification is proposed diat unlike the current lUPAC scheme [2] is consistent with SI prefixes [1], and terms based on fiiese such as nanotechnology [3], is decoupled fiom any physico-chemical processes occurring in or associated with pores, and is not biased towards small pores. While the new scheme covers pores in the size range 0.1 nm to 0.1 m, it can in principle be easily extended beyond these boundaries. However, the new scheme is more complicated than the current lUPAC one, especially regarding sub-divisions of the main pore size classes. Also, the term micropore occurs in both schemes, which makes them incompatible, at least over the micropore size range as defined in the new classification (0.1 to 100 pm). 

Among the above categories, the pore size classification is most widely used because it is directly related to the properties of impurities to be removed. In Fig. 13.1 [1], the filtration spectrum presents the relationships among pore size, impurity materials, and filtration technology in varying filter media. In Sect. 13.3.1, a description of those filtration membranes made of electrospun nanofibers is summarized. 

TABLE 2.2 Pore-Size Classification After a Compilation of Mann et al. (1977)... 

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