Crystal adsorption capacity

The crystals of 2 1 swelling clays are typically smaller than either kaolinite or fine-grained mica and thus have higher adsorption capacity and cation... 

As it has been discussed in previous section, from a practical point of view1, and also for comparison between adsorbents, hydrogen adsorption capacities should be reported in a volumetric basis, which makes necessary to know the sample density. Unfortunately, papers reporting hydrogen adsorption capacities of MOFs in volumetric basis use the crystal density of the materials, which is not realistic for this application because it does not include the inter-particle space. Crystal densities of MOFs can vary between 0.2 and 1.3 g cm 3 36 39, and similar to what happens with tap and packing densities of carbon materials, crystal densities of MOFs decreases when porosity increases. Therefore, as in the case of carbon materials (see Figure 5) a maximum is observed when the hydrogen uptake in volumetric basis is plotted versus the porosity of the MOFs samples, and a compromise between density and porosity is necessary from a practical point of view. 

Liu etal. [32] reported the characteristics and reactivity of highly ordered mesoporous carbon-titania hybrid materials synthesized via organic-inorganic-amphiphilic coassembly followed by in situ crystallization. In the degradation of Rhodamine B these materials also show enhanced properties due to the dispersion/stabilization of small titania nanocrystals and the adsorptive capacity of the nanocarbon. 

Tablel Nature, relative xenon adsorption capacities and relative crystailinities of the intermediate phases obtained at different stages of the SAPO-37 crystallization.

The properties of zeolites, most notably their stability, adsorptive capacity and catalytic activity, are strongly dependent on the precise location of Si and A1 in the emionic framework. This is one of the most challenging and debated problems in silicate crystal chemistry. 

New applications of zeolite adsorption developed recently for separation and purification processes are reviewed. Major commercial processes are discussed in areas of hydrocarbon separation, drying gases and liquids, separation and purification of industrial streams, pollution control, and nonregenerative applications. Special emphasis is placed on important commercial processes and potentially important applications. Important properties of zeolite adsorbents for these applications are adsorption capacity and selectivity, adsorption and desorption rate, physical strength and attrition resistance, low catalytic activity, thermal-hydrothermal and chemical stabilityy and particle size and shape. Apparent bulk density is important because it is related to adsorptive capacity per unit volume and to the rate of adsorption-desorption. However, more important factors controlling the raJtes are crystal size and macropore size distribution. 

The porous volumes measured by N2 adsorption are listed in Table 3. After the boronation, the total porous volumes (Vt) of the samples increase, corresponding to the increase of benzene adsorption capacity mentioned above. This should be resulted from the following aspects (1) The average mass of zeolite crystallite decrease and the number of crystal particles in unit weight of sample increases after the boronation owing to a limited introduction of trivalent atoms and Na+cations as counterions, as well as a severe dissolution of silicon. Thus, the total porous volume (mL/g) and the adsorption capacity increase. (2) The transformation of pore size occurs during the boronation. As shown in Table 3, the mesoporous volumes increase and the microporous volumes decrease after the boronation, meaning that some micropores are developed into mesopores due to the removal of silicon from the framework. This is also one of the important reasons why the total porous volumes as well as the adsorption capacities increase after the boronation. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

In order to utilize the absorption properties or the synthetic zeolite crystals in processes, the commercial materials arc prepared as pelleted aggregates combining a high percentage of the crystalline zeolite with an inert binder. The formation of these aggregates introduces macro pores in the pellet which may result in some capillary condensation at high adsorhate concentrations. In commercial materials, the inacropores contribute diffusion paths. However, the main pan of the adsorption capacity is contained in the voids within the crystals. 

Alumina, silica and many other metal oxides are insulators. However, recent experiments indicate that the surfaces of these insulators are mainly ionic (Masel, 1996). The pristine or freshly cleaved surfaces of single crystals of these oxides (cleaved under ultrahigh vacuum) are fairly inert and do not have significant adsorption capacities for even polar molecules such as CO and S02 (Masel, 1996 Henrich and Cox, 1994). However, the surface chemistry and adsorption properties are dominated by defects on real surfaces. For example, oxide vacancies on alumina expose the unsaturated aluminum atoms, which are electron acceptors, or Lewis acid sites. 

In principle, hydrolitic exchange (35, 36) also might influence the adsorption capacity however, such an assumption is difficult to bring into agreement with the data of x-ray crystal studies of calcium and... 

It is assumed that the crystallinity z of the samples is directly proportional to their adsorption capacity at identical conditions (i.e. n-hexane adsorption at 20 C and latm). Therefore, if we assume crystals with a mean radius 7q of the untreated material the crystallinity will be related to the shrinking mean particle radius by... 

From these three examples and the disparity of the studied systems, it is difficult to draw conclusions. One could conclude that for a given metal and adsorbate the adsorption capacity peak shifts in the same way as the pzc in the range of potential and that its shape is simpler for single-crystal faces than for a polycrystalline electrode. This is not always the case—for instance, for adsorption of pyridine on gold faces the capacity peaks are not distributed in the same range of potentials as the pzc s and the capacity peak... 

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