Pore shape, general properties adsorbents

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. 

The characterization of colloids depends on the purposes for which the information is sought because the total description would be an enormous task. Among the properties to be considered are the nature and/or distributions of purity, crystallinity, defects, size, shape, surface area, pores, adsorbed surface films, internal and surface stresses, stabUity and state of agglomeration [1, 2]. In general, the same broad characterization considerations apply whether the dispersed species are of colloidal size or nanosize (i.e. microscale or nanoscale), although clearly such things as imaging are much more difficult at the nanoscale. Some discussions of this can be found in References [3, 4]. 

The preparation conditions determine the special properties of silica gel. Physicochemical properties such as hardness and polarity of silica gel are related to particle size (fim), size distribution, shape, surface area (mVg), pore system [size (A) and distribution], and presence or absence of additives—that is, binders, contaminants, or indicators. The surfaee area of silica gel adsorbent for TLC is typically 300 ta600 m /g, pore volume is —0.75 ml/g, and pore diameters range from 40 to 80 A (most often 60 A or 6 nm). Adsorbent with larger surface area (smaller particle size) will generally give better resolution but a slower development time. The usual binder for commercial TLC adsorbent powders is 5-20% gypsum (silica gel G). Precoated plates usually have organic binders such as polyesters or polyvinyl alcohol. 

Nitrogen Capillary Condensation. Application of the Kelvin equation generally assumes circular pores but in reality for non-circular shapes the Kelvin equation evaluates a volume-surface capillary ratio. In view of the uncertainty of the Kelvin equation in terms of the variation observed between the adsorbed and the bulk physical properties of nitrogen, together with the inconsistency of t-curves and BET coefficients excessive refinement of the pore size distribution model has little warranty (13,17) and thus the modelless treatment was chosen (18). 

Another important physical property of the ash zeolites is their pore radius Rp. This parameter helps in studying the adsorption properties of zeolites as an adsorbent. Rp can be correlated with the specific surface area SSAbet, which can be determined by nitrogen adsorption technique (i.e., by employing BET method and the relationship, Rp = 2 VpISSAbet> where Vp is the pore volume) [44]. The pores are assumed to be cylindrical in shape for natural zeolites Clinoptilolite and Mordenite, for which SSAbet generally lies between 11-16 m and 115-120 m /g, respectively. The trend depicted in Fig. 2.4 exhibits an initial increase in Rp with an increase in SSAbet, up to 20 m /g, beyond which it decreases sharply [8]. This trend violates the inverse relationship between the two parameters as mentioned above. 

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