Adsorbents selection criteria

Adsorbent selection criteria for any application generally include the following main attributes capacity, selectivity, regenerability, kinetics, durability, and cost. These attributes represent combinations of properties that are strongly affected by the pertinent conditions. For example, the first four govern how much adsorbent is necessary for a particular application, and the last two affect the annual cost. Furthermore, the first three are tied to the equilibrium characteristics, about which much can be said (see Section 14.3). Likewise, kinetics is covered in more detail in Section 14.4. Both will be discussed briefly here, however. Finally, adsorbent cost obviously depends on both its price and lifetime, which can depend on its resistance to attrition, degradation, fouling, and so on. 

MenU F, Lucat C, Debich A (1994) The thick-film route to selective gas sensors. Sens Actuators B 25 415- 20 Miller JB (2001) Catalytic sensors for monitoring explosive atmospheres. IEEE Sensors J l(l) 88-93 Park J-H, Yang RT (2005) Simple criterion for adsorbent selection for gas purification by pressure swing adsorption processes. Ind Eng Chem Res 44 1914-1921... 

Selectivity for C02 it represents the C02 uptake ratio to the adsorption of any other gas (typically nitrogen for post-combustion capture, and methane for natural gas). It is an essential evaluation criterion, and affects the purity of the adsorbed gas, which will significantly influence the sequestration of C02. The simplest method to estimate the selectivity factor is to use single-component adsorption isotherms of C02 and nitrogen. 

Different charge-compensating cations in zeolite L have been tested for their promotional effect in n-hexane aromatization. Apparently, high basicity of the alkaline and alkaline earth promoter favors n-hexane aromatization. Basicity and selectivity both increase from Li and Cs 331) and from Mg to Ba (22,25). Bezouhanova et al. studied the FTIR bands of linearly adsorbed CO in the range of 2060-2075 cm . One band at 2075 cm", which is also found on unsupported Pt, is attributed to extrazeolite Pt particles, a second band shifts from 2060 cm" for Li to lower wavenumbers with K and Rb 331). Another criterion, used by Larsen and Haller, is the measured rate of competitive hydrogenation of benzene and toluene, which has been found to correlate with the zeolite basicity (25). As described in a previous section, this method had previously been used by Tri el al. to probe for the electron deficiency of Pt particles in acidic zeolites 332). The rate data are analyzed in terms of a Langmuir-Hinshelwood model and the ratio of the adsorption coefficients of toluene and benzene, A, /b, is determined. It was found to decrease from 8.6 for Pt/Si02, and 5.4 for Pt/MgL, to 4.4 for Pt/BaL. As direct electron transfer from the cations to neutral Pt particles is unlikely, an interaction of Pt with the zeolite framework or with... 

As shown in Table V, 2,3-dimethylbutane is the major reaction product in every case except for the cis isomer on platinum. Since both mechanisms A and B (nonselective and selective) compete on platinum films (86, 87), and probably as well on nickel and rhodium films, the excess of 2,3-dimethylbutane over the amounts expected by Mechanism A may be assigned to Mechanism B, involving the selective rupture of —CHj—CHj— bonds. a,a,, -Tetraadsorbed species were suggested to be associated with Mechanism B (89), whose contribution increases with different metals in the same order, Pt < Ni < Rh, as that for multiple exchange of methane (90). Since multiple exchange of methane is usually taken as a criterion for the ability of a metal to form adsorbed methylene, this result is in fair agreement with the proposed a,a,J ,)S-tetraadsorbed precursors (Scheme 34). The latter were... 

Absorption capacity of relevant adsorbates is also a good criterion but the accuracy is rather poor (10%). Moreover blocking of pores or channels mouths may also preclude the characterization by limiting the access of adsorbates to the pores. On the other hand such a limitation may be used with profit by measuring the rate of diffusion of such adsorbates which is obviously related to catalytic properties in case of shape selective reaction. Note also at this stage that such a diffusion rate also greatly depends on the crystallite size (46). 

However, another criterion that needs to be considered for the selection of a suitable ISPR method is the mode of contact between the microorganisms and the separation phase that removes the product from the vicinity of the cell. Direct contact between the microorganism and a water-immiscible solvent (phase toxicity) or solid adsorbent material can have inhibitory effects on the cell [31,33]. Therefore, this direct contact limits the choice of separative aids. In addition, stability and robustness of a process is reduced if the cells are in direct... 

This leads us to the selection of an appropriate pressure range for each adsorbent with the problem that the above "linearity criterion", which is essential, is unfortunately not self-sufficient, since several portions of the BET plot can fulfil this requirement. One can indeed consider, in Fig 1, several relative pressure ranges in which the plot is reasonably linear for instance 0.01-0.2, 0.02-0.05 and 0.05-0.15, the latter being the most commonly used. The resulting monolayer contents are 4.0, 4.5 and 5.2 mmol.g, respectively, i.e. a variation of up to 30% Hence the need of other criteria leading to the objective choice of a single linear portion of the BET plot. Those two selected hereafter are aimed to provide such a single choice ... 

However, acidity or basicity of a gas-phase adsorbate is not a sole criterion for its choice as a probe molecule. Firstly, the strength of an acidic or basic probe should be distinguished accordingly to its acid- or base-dissociation constant Ka or Kb). In addition, very important feature of probe molecule is its radius. If there is a need to locate all active sites in the structure of microporous solid material, the radius of probe molecule has to be smaller then the diameter of pore(s) opening(s). In other words, probe molecules have to be of appropriate size, so the entrance in the micropores of the solid and the access of adsorbate to each active site become possible. For example, ammonia, which is frequently used to reveal the acidic property of solids, is selected as a probe due to its basicity and due to the size of the molecule. Its molecule is smaller than the diameter of the pores in the zeolites structures, and also in many other solids. The other probe often used for investigation of solids acidity is pyridine however, the application of other chemical species is also possible. 

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