Pore size IUPAC definitions

Fig. 7. Size scale associated with soil mineral particles, organic components, pores and aggregations of mineral and organic components (Baldock 2002). The definitions of pore size have used those developed by IUPAC (micropores < 2 nm, mesopores 2-50 nm and macropores > 50 nm). Alternatively, the pore sizes corresponding to the lower ( /m = - 1500 kPa) and upper ( /m = - 100 kPa) limits of water availability to plants may be used to define the boundaries between the different classes of pore size. /m is soil water metric potential.

Pores are found in many solids and the term porosity is often used quite arbitrarily to describe many different properties of such materials. Occasionally, it is used to indicate the mere presence of pores in a material, sometimes as a measure for the size of the pores, and often as a measure for the amount of pores present in a material. The latter is closest to its physical definition. The porosity of a material is defined as the ratio between the pore volume of a particle and its total volume (pore volume + volume of solid) [1]. A certain porosity is a common feature of most heterogeneous catalysts. The pores are either formed by voids between small aggregated particles (textural porosity) or they are intrinsic structural features of the materials (structural porosity). According to the IUPAC notation, porous materials are classified with respect to their sizes into three groups microporous, mesoporous, and macroporous materials [2], Microporous materials have pores with diameters < 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters > 50 nm. Nowadays, some authors use the term nanoporosity which, however, has no clear definition but is typically used in combination with nanotechnology and nanochemistry for materials with pore sizes in the nanometer range, i.e., 0.1 to 100 nm. Nanoporous could thus mean everything from microporous to macroporous. 

According to the IUPAC definition, porous materials ate divided into three different classes, depending on their pore sizes. Mesoporous materials are described as materials whose pore diameters lie in the range between 2 and 50 nm. Solids with a pore diameter below 2 nm or above 50 nm belong to the class of micro- and macroporous materials, respectively. 

In fact, the pore sizes of some mesoporous materials discussed in this chapter are slightly smaller than 2 nm. However, we still call them mesoporous materials even though they are beyond that official IUPAC definition. 

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