Sorbents pore characteristics

From the discussion so far it follows that sorption as well as diffusion play a role. Their relative importance depends on such sorbent material characteristics as pore size, sorption strength, and gas properties such as molecular size and shape, concentration, etc. 

They found that the average pore size and the total pore volume are not adequate measures to predict the CO2 uptake of microporous carbon sorbents, the pore volume of micropores strongly governs the amount of adsorbed CO2 [161]. Neither high surface area CDC after chemical activation (surface area 3,101 m g ) nor high pore volume nano-TiC-CDC (Vtotai 1-61 cm g ) correspond with the highest CO2 adsorption capacity. At ambient pressure, the CO2 uptake closely follows a linear correlation with the volume of pores smaller or equal to a diameter of 1.5 mn. Pores smaller than 0.5 mn contribute to the amount of adsorbed CO2, but the best correlation is found for pore volume smaller than 0.8 mn (Fig. 2.29). The correlation between the amount of adsorbed CO2 at low partial pressures and volume of smaller pores is the basis for the well-known application of CO2 sorption as a method to calculate the pore characteristics of microporous materials. Subatmospheric pressures are of particular interest for industrial applications, where partial pressure of CO2 is below 1 bar, and here, the best prediction of the CO2 uptake capacity at 0.1 bar would be based on the volume of pores smaller or equal to a diameter of 0.5 nm (Fig. 2.29). This correlation can be used to design better CO2 sorbents and CCS devices. 

With the possible exception of pore size and distribution, these characteristics can affect both physical and chemical sorption mechanisms. In chemical sorption, the overriding factor is the surface groups on the sorbent. However, the sorbent substrate itself can affect the ability of the coated sorbent to collect a substance, especially if the desired reaction does not occur rapidly. 

The silica surface (on both silica gel and MCM-41) provides a better substrate due to the lack of Lewis acid sites (unlike y-Al203), and consequently the Ag atoms in these sorbents are more capable of forming jr-complexation bonds with olefins. Although the effect of the physical characteristics of a substrate such as surface area and pore size would have on adsorption is clear, the effect of the electronic properties needs to be studied further. 

In the case of gel permeation or size-exclusion HPLC (HP-SEC), selectivity arises from differential migration of the biomolecules as they permeate by diffusion from the bulk mobile phase to within the pore chambers of the stationary phase. Ideally, the stationary phase in HP-SEC has been so prepared that the surface itself has no chemical interaction with the biosolutes, with the extent of retardation simply mediated by the physical nature of the pores, their connectivity, and their tortuosity. In this regard, HP-SEC contrasts with the other modes of HPLC, where the surfaces of the stationary phase have been deliberately modified by chemical procedures by (usually) low molecular weight compounds to enable selective retardation of the biosolutes by adsorptive processes. Ideally, the surface of an interactive HPLC sorbent enables separation to occur by only one retention process, i.e., the stationary phase functions as a monomodal sorbent. In practice with porous materials, this is rarely achieved with the consequence that most adsorption HPLC sorbents exhibit multimodal characteristics. The retention behavior and selectivity of the chromatographic system will thus depend on the nature and magnitude of the complex interplay of intermolecular forces... 

Fortunately, the effects of most mobile-phase characteristics such as the nature and concentration of organic solvent or ionic additives the temperature, the pH, or the bioactivity and the relative retentiveness of a particular polypeptide or protein can be ascertained very readily from very small-scale batch test tube pilot experiments. Similarly, the influence of some sorbent variables, such as the effect of ligand composition, particle sizes, or pore diameter distribution can be ascertained from small-scale batch experiments. However, it is clear that the isothermal binding behavior of many polypeptides or proteins in static batch systems can vary significantly from what is observed in dynamic systems as usually practiced in a packed or expanded bed in column chromatographic systems. This behavior is not only related to issues of different accessibility of the polypeptides or proteins to the stationary phase surface area and hence different loading capacities, but also involves the complex relationships between diffusion kinetics and adsorption kinetics in the overall mass transport phenomenon. Thus, the more subtle effects associated with the influence of feedstock loading concentration on the... 

Another recourse is to develop artificial sorbents with more favorable pore structure. Figure 45 compares the desulfurization characteristics of... 

However, HPLC sorbents also show SEC behavior to some extent dependent on the pore size of the stationary phase relative to the molar size of the solute. Copolymers with the same composition but diflFerent molar masses will in general have somewhat diflFerent retention characteristics. This may lead to copolymer HPLC fractions with heterogeneous chemical compositions and may contain some chains with different molar mass and comonomer content. 

V Pore volume of sorbent filled at P and T Vq Total pore volume of sorbent a Polarizability r Sorbate concentration Sorption potential q Initial sorption potential Dipole moment < ) Intermolecular potential Vq Characteristic vibrational frequency... 

The other posibility for characterizing structural properties of inorganic sorbents deals with the thermogravimetric analysis. By means of this analysis the pore size distribution, pore volume and specific surface area of adsorbents and catalysts which are fundamental adsorption characteristics may be obtained. 

In zeolites, porosils and AlPO s of known structures we can be rather clear about window dimensions, shapes, volumes and connectivities of pores - in contrast with microporous but amorphous sorbents. Because of the growing diversification of porous crystals of known structures it may be timely to review spatial characteristics of their pores. The pores are polyhedral their faces cure polygonal and, where shared with other polyhedra, may provide windows through which molecule diffusion can occur. 

Table 3.2 shows a compiled listing of some common HPLC sorbents and their characteristics (surface area and pore size). Note that the retentive... 

As in other chromatographic techniques overall performance in TLC is inextricably linked to the physical characteristics of the sorbent, in particular its particle and pore size and the particle size distribution. Normal TLC plates, i.e. those spread with 5 40 pm particle size distribution are capable of giving 1000-2000 theoretical plates per 5 cm migration. High performance... 

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