Classification of Pore Sizes

The individual pores in heterogeneous catalysts and in other porous technical substances may vary greatly, both in size and in shape. An important quantity is the width of the pores, e.g. the diameter of a cylindrical pore, or the distance between the sides of a slit-shaped pore. A classification of pores according to their average width was originally proposed by Dubinin [4] and has now been officially adopted by the International Union of Pure and Applied Chemistry [5]. 

Widths of micropores range from 0.3 to 2 nm. Mesoporous substances have pore sizes from 2 nm up to 50 nm. Macropores run from pore widths of 50 nm up to about 105 nm. 

Surface areas and pore size distributions of mesoporous materials are most easily studied by nitrogen adsorption and nitrogen capillary condensation. The most appropriate method for the study of macroporosity is mercury porosimetry [6,7], a technique which will not be treated here. 

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Classification of pore sizes micropores, mesopores and macropores... 

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... 

To satisfy the first requirement, the porous solid must have small pore size with a reasonable porosity. This suggests that a good solid must have a combination of two pore ranges the micropore range and the macropore range. The classification of pore size as recommended by lUPAC (Sing et al., 1985) is often used to delineate the range of pore size... 

A new classification of pore sizes is proposed. It is based on prefixes defined by the Bureau International des Poids et Mesures under Le Systeme International d Unit s (SI) [1] (in particular nano-, micro- and milli-), unlike the current classification scheme defined by the International Union of Pure and Applied Chemistry (lUPAC) [2]. Thus the new classification is also consistent with other common scientific terms based on SI prefixes such as nanotechnology [3]. Further advantages are that unlike the lUPAC scheme - which is derived from physical adsorption phenomena in pores narrower than 50 nm - the new classification is entirely decoupled from any physieo-ehemieal system or process and is not biased towards small pores. However, the proposed new scheme is more complicated than the current lUPAC one, especially regarding suh-divisions of the main pore size classes. Also, the term micropore occurs in both schemes, which makes them incompatihle, at least over the micropore size range as defined in the new classification. 

The current classification of pore size recommended by lUPAC is as follows [2] ... 

Mays TJ (2007) A new classification of pore sizes. In Llewellyn P, RodrlguezReinoso F, Rouqerol J, Seaton N (eds) Studies in surface science and catalysis, vol 160. Elsevier, Amsterdam, pp 57-62... 

The ubiquity of porous materials has led to confusion in the usage of such terms as micropore, macropore, total pore volume and internal area. In the lUPAC classification of pore size, the micropore width is taken to not exceed about 2 nm (20 ), the mesopore width to be in the range 2-50 nm and the macropore width to be above about 50 nm (0.05 pm). In recent years these definitions have served us well, especially in the context of gas adsorption and mercury porosimetry, but it is becoming increasingly clear that some refinements are required and that account should be taken of pore shape. 

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. Siace different phenomena dominate the adsorptive behavior ia different pore size ranges, lUPAC has suggested the foUowiag classification ... 

Pores are classified into two types open pores, which connect to the outside of the material, and the closed pores, which are totally within the material. Penetrating pores are kind of open pores these have at least two openings located on two sides of a porous material. Penetrating pores are permeable for fluid, and therefore are important in applications such as filters. Many porous materials have been used in many applications. They are classified by many different criteria such as pore size, pore shape, materials and production methods. Classification by pore size and by pore shape is useful while considering the applications of porous materials. The classification of porous materials by pore size (according to Schaefer30) differentiates between microporous pores (pore diameter < 2 nm), mesoporous pores (2 nm < pore diameter <50 nm) and macroporous pores (pore diameter > 50 nm). 

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 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] ... 

The classification of pores according to their size (Rodriguez-Reinoso and Linares-Solano, 1989)... 

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 ... 

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. 

A characteristic feature associated with pore condensation is the occurrence of sorption hysteresis, i.e pore evaporation occurs usually at a lower p/po compared to the condensation process. The details of this hysteresis loop depend on the thermodynamic state of the pore fluid and on the texture of adsorbents, i.e. the presence of a pore network. An empirical classification of common types of sorption hysteresis, which reflects a widely accepted correlation between the shape of the hysteresis loop and the geometry and texture of the mesoporous adsorbent was published by lUPAC [10]. However, detailed effects of these various factors on the hysteresis loop are not fully understood. In the literature mainly two models are discussed, which both contribute to the understanding of sorption hysteresis [8] (i) single pore model. hysteresis is considered as an intrinsic property of the phase transition in a single pore, reflecting the existence of metastable gas-states, (ii) neiM ork model hysteresis is explained as a consequence of the interconnectivity of a real porous network with a wide distribution of pore sizes. 

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