Microporous materials, IUPAC

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. 

Substrate Accessibility Porous materials are divided according to their pore size. This can be measured using adsorption techniques (the principles of adsorption are outlined below). According to the International Union of Pure and Applied Chemistry (IUPAC), microporous materials have pores under 2 nm in diameter, mesoporous materials have pores of 2-50 nm diameter, and any material with an average pore diameter above 50 nm is macroporous [74]. The pores must be sufficiently large for substrates to enter and products to exit. Pores come in different shapes and sizes (Figure 4.14). Some materials have large pores with narrow pore mouths, known as... 

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

The terminology is not yet homogeneous. The use of the prefix nano spread out in the 1990s. Until then, the common term used to be mesoscopic structures, which continues to be used. According to a definition by IUPAC of 1985, the following classification applies to porous materials microporous, < 2 nm pore diameter mesoporous, 2-50 nm macroporous, > 50 nm. 

Materials with uniform pore structures offer a wide range of applications, including catalysis, adsorption, and separation. These materials have the benefit ofboth specific pore systems and intrinsic chemical properties [1-3]. The pores in the materials are able to host guest species and provide a pathway for molecule transportation. The skeletal pore walls provide an active and/or affinity surface to associate with guest molecules. According to the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified into three main categories based on the diameters of their pores, that is, microporous, mesoporous, and macroporous... 

The shape of the isotherms of nitrogen ad/desorption (type IV, IUPAC) for SBA-15 and its replicas indicates the presence of mesopores and a small fraction of micropores. The isotherms for all the N-doped replicas of SBA-15 (Fig. la) are of the shape similar to that of the isotherm for a typical CMK-3 material [4]. The mesopores are also similarly large (ca. 4 nm, Fig. lc). However, the sorption capacities differ notably, being 1.1 and 1.40-1.76 cm3/g for CMK-3 and CMK-3N, respectively. 

The smallest pores that can be formed electrochemically in silicon have radii of < 1 nm and are therefore truly microporous. However, confinement effects proposed to be responsible for micropore formation extend well into the lower mesoporous regime and in addition are largely determined by skeleton size, not by pore size. Therefore the IUPAC convention of pore size will not be applied strictly and all PS properties that are dominated by quantum size effects, for example the optical properties, will be discussed in Chapter 7, independently of actual pore size. Furthermore, it is useful in some cases to compare the properties of different pore size regimes. Meso PS, for example, has roughly the same internal surface area as micro PS but shows only negligible confinement effects. It is therefore perfectly standard to decide whether observations at micro PS samples are surface-related or QC-related. As a result, a few properties of microporous silicon will be discussed in the section about mesoporous materials, and vice versa. Properties of PS common to all size regimes, e.g. growth rate, porosity or dissolution valence, will be discussed in this chapter. 

The most important property of adsorbent materials, the property that is decisive for the adsorbent s usage, is the pore structure. The total number of pores, their shape, and size determine the adsorption capacity and even the dynamic adsorption rate of the material. Generally, pores are divided into macro-, rneso- and micropores. According to IUPAC, pores are classified as shown in Table 2.2. 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

Relatively straightforward is the definition of nanoscopic voids. Nanopores and nanocavities are elongated voids or voids of any shape, and nanomaterials can incorporate especially nanopores in an ordered or disordered way. The former is of crucial importance for many of the hybrid materials discussed in the book (e.g., in Chapters 16 or 18). Nanochannel is also frequently used instead of nanopore, often in biological or biochemical contexts. Besides nanoporous, the term mesoporous is often found in hybrid materials research. Interestingly, the IUPAC has defined the terms mesoporous (pores with diameters between 2 and 50 nm), microporous (pores with diameters <2 nm) and macroporous (pores with diameters >50 nm), yet has not given a definition of nanoporous in the IUPAC Recommendations on the Nomenclature of Structural and Compositional Characteristics of Ordered Microporous and... 

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],... 

The adsorbents normally applied are in general porous. The classification of the different pore widths of porous adsorbents was carried out by the International Union of Pure and Applied Chemistry (IUPAC) [1], IUPAC classified these materials as microporous, with pore diameters between 0.3 and 2nm, mesoporous with pore diameters between 2 and 50nm, and macroporous with pore diameters greater than 50 nm [1], The pore width, Dp, is defined as equal to the diameter in the case of cylindricalshaped pores, and as the distance between opposite walls in the case of slit-shaped pores. 

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). 

L.B. McCusker, F. Liebau, and G. Engelhardt, Nomenclature of Structural and Compositional Characteristics of Ordered Microporous and Mesoporous Materials with Inorganic Hosts (IUPAC Recommendations 2001). Microporous Mesoporous Mater., 2003, 58, 3-13. 

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