Selection of the adsorbent

For an equiUbrium-based separation, a convenient measure of the intrinsic selectivity of the adsorbent is provided by the separation factor which is defined by analogy with the relative volatility as... 

Nowadays, SPE has been shown to be a very powerful and robust alternative to the traditional extraction methods stated above [29,30], It offers the advantages of very low solvent consumption, speed, and ease of handling, and combines the concentration and (partial) clean-up into one on-line step. Various types of commercially available stationary phases enable the application of distinct separation mechanisms, thereby increasing the selectivity of the adsorbent to the analytes. 

Capillary Condensation. In a porous adsorbent the region of multilayer physical adsorption merges gradually with the capillary condensation regime, leading to upward curvature of the equilibrium isotherm at higher relative pressure. In the capillary condensation region the intrinsic selectivity of the adsorbent is lost. 

The studies of adsorbed hematin complex structure using the Mossbauer spectroscopy method made the selection of the adsorbed hematin structure by the author [94] more justified than in the case of the adsorbed method. Therefore, based on structural data the difference in catalytic activities of diluted and concentrated adsorptive hematin layers is justified in this work. 

The chemisorption bond between CO or NO and Pt is mainly attributed to the interaction of a electron of the adsorbate with the d hole of Pt, whereas the site selection of the adsorbate is assisted by the interaction between the adsorbate 2tt and the Pt 5d orbital. The shape of the d orbital is important for this site selection. The five d orbitals in the Oh space group are generally classified into eg (dx2 y2 and d3,2 r2) and r2g (dxy, dyz, and dM) orbitals with respect to the cubic coordinate. Of the two states of Pt, t2g has more d hole character [90], and furthermore, only the f2g state is upward-shifted and the eg is preferentially filled as a result of s-d hybridization [89]. The d band filling in the eg state due to the s-d hybridization for the alloy is supported by the band structure of this alloy in the Y-L direction observed by angle-resolved photoemission spectroscopy using synchrotron radiation [88]. 

Given the adsorbed-phase composition Xj from the solution of Equations (13)-(15), the selectivity of the adsorbent for component i relative to component j is... 

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

From the experimental data [44], the selectivity of the adsorbents was calculated as the relative retention volume of the separated species. It can be seen in Table 9 that Cs-CPM adsorbent possesses better selectivity than the Ca-CPM one with respect to the separation of the isomers of xylene and ethylbenzene with like properties. The results enabled us to conclude that the separation of the isomers of m- and p-cresol up to the purity degree of 99% can be achieved using the columns with the theoretical plates number (TPN) not exceeding 200, and that for the o- and m-isomers - employing the packed intermediate efficiency columns with TPN of 2000-2500. To separate the same pairs of the species using the Ca-CPM, the capillary columns with the TPN of 20000 were reqnired. 

Specific Site Poisoning- Crucial to our understanding of the activity of mixed oxide catalysis or oxides on more refractory supports is the ability to measure the surface area of the active oxide. Two methods come readily to mind selective chemisorption and selective poisoning. In each case the critical question will be the selectivity of the adsorbent and considerable further research is still required. As an example, can N2O decomposition be employed for V+4 analysis as it is for reduced copper ... 

Table 3.4 The selectivity of the adsorbents. The selectivity is obtained from the slopes of plots of the van der Waals volume of solutes and their molecular interaction energy values. BA represents the basic adsorbent, MA represents the methylated adsorbent, OH represents the hydroxylated adsorbent. Cyclic HCs represents cyclic hydrocarbons.

Chitosan, a widely used natural biopolymer, has been studied for the adsorption of various metal ions from dilute solutions. Unfortunately, the inherent properties of chitosan, such as hydrophilicity and metal-binding capability, are often insufficient to meet the requirements of a number of applications. To improve these properties, both chemical and physical modifications of chitosan are required. Thus, Aliquat 336-functionalized chitosan as adsorbent was prepared. In fact, the new chitosan adsorbent can also be described as P-SIL containing quaternary ammonium ionic liquid [23]. Unlike the previous reported structure, the Aliquat 336-functionalized chitosan, which was prepared by acid/base neutralization reaction, consists largely of cations and anions (Fig. 5.12). The structure was so flexible that the adsorption ability could be controlled precisely. Moreover, incorporation of Aliquat 336 into the chitosan backbone could significantly enhance its metal ions extraction ability. It has been shown to have much improved affinity for Pb " than pure chitosan. This may be explained in that the new strategy doesn t reduce the original amino active sites besides, the synergistic effect between cation and anion also contributes to the enhancement of adsorption capabilities. On the other hand, the effort to increase selectivity of the adsorbent for one metal ion over others is to make the adsorbent sterically efficient with that metal ion only. The new chitosan-... 

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