Pores size selectivity

Discussing pore size selection of columns leads directly to the issue of using single porosity columns or so-called linear or mixed-bed columns, which contain mixtures of different pore sizes in a single column (3,19). Both types of columns have advantages and disadvantages, as shown in Table 9.6. 

As the pore diameter increases in size (s decreases) relative to molecular or colloidal dimensions, less restrictions are imposed on the motions of contained species. Thus the exclusion effect gradually subsides as the pore size increases and consequently K-+1. For the separation of two molecules of different size, it is important to pick a pore diameter that will substantially exclude one species but not another. Pore size selection is thus of utmost importance in membrane science and in choosing a support for size exclusion chromatography (SEC). Aspects of pore size optimization in SEC based on the above partitioning theory have been developed [28]. 

Membrane pore size rating refers to the size of a specific particle to be retained by the filter with a specific degree of efficiency. Pore size will affect the flow rate, back pressure, and life of the filter. Membrane pore size is outlined in Table 9 and selected on the basis of the applications requirements. The size and amount of particles to be filtered from the sample can also affect the pore size selection. Most applications in the isolation of drug impurities and degradants would either require prefiltration to remove large clumps of insoluble excipients or a filtration/clarification step prior to FIPLC, and would be chosen on these bases.71... 

In membranes, the flux, or productivity, increases with increasing membrane pore size. Selectivity increases when the pore size is much smaller than the molecular diameter of the larger species in a binary mixture, but slightly larger than that of the smaller species. Explain the tradeoff between productivity and selectivity in terms of membrane pore sizes. 

As already described in Section S.2.2.3, the pore size selected defines the available surface. 

MOF windows. MOF Cu(L-asp)bpeo.s displays a porous 3D structure with saturated mononuclear copper nodes. Its monoprotonation leads to a new MOF of formula Cu(l-asp)bpeo.5(HCl)(H20) (Figure 9a). Unlike homogeneous aspartate complexes, protonation takes place at one of the carboxylic groups of aspartate, which is still bound to copper, generating a particularly acidic Brpnsted acid inside the pores. Methanolysis of propylene oxide and (2,3)-epoxybutane using this material shows moderate yields, but turnover numbers (TONs) drop to zero with the bulkier (2,3-epoxypropyl)-benzene, indicative of pore size selectivity. 

Scheme 11.6 Pore-size-selective functionalization of acrylate-based ROMP-derIvatIzed monoliths for the Immobilization of Pd-based nanoparticles.

Pore-size-selective functionalization of monoliths An approach to Pd(0)-mediated, ligand-free coupling reactions was developed by Bandari et al Electron-beam-tri ered free radical polymerization-derived monolithic supports prepared from a mixture of glyddyl methacrylate and TMPTA in 2-propanol, 1-dodecanol, and toluene were prepared in a way that porous monolithic matrix that was characterized by large (convective) pores in the 30 pm range as well as of pores <600 nm, formed. The epoxy groups present within the... 

Scheme 32 Pore-size-selective functionaiization of monoiithic supports.

The selectivity showed a complicated dependence upon the pore size (Figure 8.17). 

The size selectivity of a particular packing is not infinite, but is limited to a moderate range. All solutes significantly smaller than the pores move through the column s entire volume and elute simultaneously, with a retention volume, Vj, of... 

Smaller pore size microfilters in single-pass systems which have pore sizes small enough to remove some vimses by size exclusion have been examined (26,37,38). Minimum levels of vims removal can be estabhshed for these systems if fluid and process conditions are employed which minimize removal of viral particles by mechanisms other than size selection. 

Zeohte 5A is used because its pores can size-selectively adsorb straight-chain molecules while excluding branched and cycHc species. The normal hydrocarbon fraction has better than 95% purity, and the higher octane isomer fraction contains less than 2% normal hydrocarbons (64). 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

In shape-selective catalysis, the pore size of the zeoHte is important. For example, the ZSM-5 framework contains 10-membered rings with 0.6-nm pore size. This material is used in xylene isomerization, ethylbenzene synthesis, dewaxing of lubricatius oils and light fuel oil, ie, diesel and jet fuel, and the conversion of methanol to Hquid hydrocarbon fuels (21). 

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