Standards porous solids

Many essential transformations of carbohydrates regularly employ toxic and corrosive reagents, such as Lewis acids and strong mineral acids. Hence, the use of zeolites and related porous solids as catalysts in such reactions affords a practical and environmentally compatible alternative to the standard protocols. 

Some of the principal terms and properties associated with adsorption, powders and porous solids are defined in Tables 1.1, 1.2 and 1.3. These definitions are consistent with those proposed by the International Union of Pure and Applied Chemistry (IUPAC) (see Sing el al. 1985 Haber, 1991 Rouquerol et al., 1994) and by the British Standards Institution (1958, 1992) and other official organizations (see Robens and Krebs, 1991). 

The physisorption isotherm on a mesoporous or macroporous adsorbent follows the same monolayer-multilayer path as on the corresponding non-porous surface until the secondary process of capillary condensation occurs. In the case of a macro-porous solid, the deviation from the standard monolayer-multilayer isotherm does not take place until very high relative pressures are attained (with nitrogen adsorption at 77 K, this would be at p)p° > 0.99). 

The most straightforward form of as-plot is Type 11(a) in Figure 6.1, which is for a typical Type II isotherm with a moderate value of C ( 100). The extensive range of linearity and the zero intercept are the result of unrestricted monolayer-multilayer adsorption on a non-porous solid of very similar surface structure to that of the reference material. In this case the shapes of the experimental and standard isotherms are virtually identical and therefore the slope of the as-plot is directly proportional to the ratio of the surface areas, a(S)/aref. Thus, if the value of aKl is already known, it is a simple matter to calculate atest, which we denote a(S) to indicate it is calculated by the as-method. 

Stoeckli and Kraehenbuehl [1984] have proposed a relationship between the heat of immersion and the micropoious volume of a porous solid, applicable to materials having a wide range of external surface areas. This allows a rapid determination of the pore size distribution below 0.8 to 1 nm. The technique, however, requires a non-porous standard material of surface composition similar to the membrane material. 

Gas sorption porosimetry is a standard method for the characterization of the pore size distribution (PSD) of porous solids. To interpret the experimental isotherm and obtain the adsorbent PSD, one must adopt a model for the pore structure, and a theory that estimates the adsorption that will occur in pores of a particular size. If the porous solid is represented as an array of independent, noninterconnected pores of uniform geometry and identical surface chemistry, then the excess adsorption, /JP), at bulk gas pressure P is given by the adsorption integral equation... 

The measurement of the heat of immersion of a "dry" material in different liquids can permit a rapid and accurate determination of the surface area and pore size distribution below 10 A. The enthalpy change is related to the extent of the solid surface, to the presence of micropores and to the chemical and structural nature of the surface. The technique has been mainly applied to carbons [64]. The immersion liquid is usually water for hydrophilic oxides like mineral oxides, or an organic liquid (benzene, n-hexane) for hydrophobic solids like carbons. One of the limitations of this technique is that the specific enthalpy of immersion of the open surface must be determined with a non-porous standard material of surface composition similar to the porous solid studied. The non-microporous part of the surface area can be determined by prefilling the micropores with an absorbate prior to immersion. Information on the size of micropores can be obtained from the kinetics and enthalpy of immersion into a set of liquids with increasing molecular size [5]. 

From a thermodynamic point of view, the variation of standard free enthalpy associated to the electron transfer process represented by Equation (2.1), tAG °, can be related with the variation of such tliermodynamic quantity for the electron transfer process for species in solution phase, lAG °. and for the transfer of the oxidized, IaG, and reduced, forms of the electroactive species and the electrolyte cations, from the solution phase to the porous solid. The corresponding Bom-Haber-type cycle is shown in... 

In spite of its limitations, the linear standard B.E.T. equation (Equation 627) is widely used to determine the surface areas of adsorbents. Specific surface area is often correlated with rates of dissolution and other rate-related phenomena, such as catalyst activity, electrostatic properties of powders, light scattering, opacity, sintering properties, glazing, moisture retention, shelf-life and many other properties that can influence the processing and behavior of powders and porous solids in industry. Therefore, surface area measurement is probably the most widely used means of characterizing porous materials. Since the surface area corresponds to the roughness of the particle exterior and its porous interior, gas adsorption is the preferred technique. 

The methods of computer simulation of adsorption (and diffusion) in micro-porous solids were described in Chapter 4 a summary is given in Table 4.1. These techniques are now sufficiently well developed for physisorption that thermodynamic properties can be predicted routinely for relatively simple adsorbates, once the structural details of the host are known. Molecular mechanics using standard forcelields are very successful for zeolitic systems, which take into account dispersive interactions satisfactorily, but it is also possible to use higher level calculations. 

In the gas-liquid-solid MSR, catalyst is incorporated either as a packed bed or as a coating. In the packed bed reactors, standard porous catalysts are incorporated and the fluid streams are brought into contact. In the falling film MSR, the catalyst is incorporated as thin nonporous films or as particles in alumina-coated walls [4]. 

The practice In the chromatographic analysis of porous solids 1s Identical to the ordinary column calibration procedure using a series of fully characterized standard polymers. In fact, many of the works cited above aimed at the evaluation of the packing materials for SEC. The disadvantage of the SEC method Is that It cannot be applied to those materials unsuitable for packing in a chromatography column. Gel materials of fibrous or bulky form, or of insufficient rigidity, cannot form a stable gel bed In a column, and only the static solute exclusion method can be applied to these materials. 

The capillary condensation phenomenon was discovered by Zsigismody [139], who investigated the uptake of water vapour by silica materials. Zsigismody proved that the condensation of physicosorbed vapours can occur in narrow pores below the standard saturated vapour pressure. The main condition for the capillary condensation existence is the presence of liquid meniscus in the adsorbent capillaries. As it is known, the decrease of saturated vapour pressure takes place over the concave meniscus. For cylindrical pores, with the pore width in the range 2-50 nm, i.e., for the mesopores, this phenomenon is relatively well described by the Kelvin equation [14]. This equation is still widely applied for the pore size analysis, but its main limitations remain unresolved. Capillary condensation is always preceded by mono- and/or multilayer adsorption on the pore walls. It means that this phenomenon plays an important, but secondary role in comparison with the physical adsorption of gases by porous solids. Consequently, the true pore width can be assessed if the adsorbed layer thickness is known. 

Standardization of sorption measurements and reference materials for dispersed and porous solids... 

Standards for characterization of the surface structure of dispersed or porous solids... 

German DIN standards on the characterization of dispersed or porous solids are collected in ref. [6]. The most comprehensive description of the adsorption method is found in an lUPAC recommendation (7). Nowadays national standards are being harmonized either in the framework of the European Communities or at the international level. A list of standardization committees working in this field is appended (Table 4). Different methods of particle counting and characterization are collected in a VDI manual [8j. 

Standardization of Sorption Measurements and Reference Materials for Dispersed and Porous Solids, Chap. 3 of A. Dabrowski (ed.) Adsorption and its Application in Industry and Environmental Protection, Vol. 1 Application in Industry,... 

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