Nitrogen sorption isotherms, porosity

Comment. In many cases the density of the solid phase can well be approximated by using the respective literature values for the given chemical composition. For nonmacroporous samples with well-accessible porosity, the total pore volume and thus the backbone density can also be determined from the macroscopic density of the (degassed ) sample and the respective nitrogen sorption isotherm (see Chap. 21.6). 

In view of possible commercial applications of rice husk silicas, e.g. as catalyst supports, their surface areas and porosities are important properties. Nitrogen sorption measurements of the three differently treated rice husk silicas show that these are porous materials with moderately large surface areas. The surface areas are, in detail 73 m /g for the calcined material, 75 mVg in the case of the material oxidized by Fenton s reagent and 51 m /g for the rice husks treated with Caro s acid. As the shapes of the ad- and desorption isotherms reveal (Fig. 3), pores ranging from micro- up to macropores are present. 

To confirm the permanent porosity of the sample, a gas sorption isotherm is necessary, which is commonly carried out by nitrogen at 77 K. Argon with smaller size and spherical molecular shape can measure pore surface area and pore volume more precisely. When the pore size is too small to be entered by nitrogen, carbon dioxide and hydrogen with smaller kinetic diameters can be used instead. The gas sorption behavior is determined by the adsorbate, the temperature and pressure, and the structural characteristics of adsorbent such as pore size and shape, surface area, and pore volume. By virtue of the diversified and unique structures, MOFs have demonstrated many interesting properties potentially useful for practical applications. 

Sorption Analysis. Specific surface areas and porosity can be calculated from the adsorption isotherm of nitrogen at — 196 °C. The method of Brunauer, Emmett, and Teller [4.29] is generally accepted for the evaluation of specific surface areas (BET surface area in square meters per gram). The two-parameter equation is applicable to carbon black. The BET surface area comprises the outer surface area as well as the surface area of the pores. 

Both types of molecular sieves, MCM-36 and MCM-41, demonstrate large BET surface area and high static sorption capacity (see Table 2). Considerable qualitative differences are observed in N2 isotherms, which are shown in Figure 3. The nitrogen isotherm for MCM-41, prepared with cetyltrimethylammonium cation, is type IV [9] and shows the characteristic reversible steep capillary condensation at p/p0 = -0.4 corresponding to the pore opening -40 A [1]. MCM-36 also shows the type IV isotherm with almost linear and reversible uptake increase up to - p/p0 = 0.5, followed by a hysteresis loop. This profile of adsorption/desorption is typical for layered materials with slit-like porosity generated between layers [9],... 

In a study of the porosity of alumina-pillared montmorillonites (Al-PILCs), Zhu et al. (1995) have obtained values of the mean slit-width of 0.8-0.9 nm from the volume/surface ratio. In this case, the nitrogen adsorption values were in agreement with the corresponding dm values of c. 0.8 nm. However, effective micropore volumes obtained from the nitrogen isotherms and from water sorption data were significantly different and it was suggested that the density of the sorbed water was lower than that of liquid water. 

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