Molecular sieving efFect

Adsorption. Although several types of microporous soHds are used as adsorbents for the separation of vapor or Hquid mixtures, the distribution of pore diameters does not enable separations based on the molecular-sieve effect. The most important molecular-sieve effects are shown by crystalline zeoHtes, which selectively adsorb or reject molecules based on differences in molecular size, shape, and other properties such as polarity. The sieve effect may be total or partial. 

Species separated by molecular sieving effects when kinetic diameters fall iato different zeoHte aperture size categories (standard molecular sieve diameters = 300, 400, 500, 800, 1000,1300 pm. 

Generally for dilute solutions (<1 2% of feed), unless molecular sieving effect also present. 

Separation of isomers is an application where zeolite membranes could be specifically interesting because of their well-defined pores that lead to molecular sieving effects. An application that is often considered is the xylene isomerization and related reactions. 

Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-fi ee. 

A reliable chromatographic method has been developed for the quantitative aneilysis of hydrophobic impurities in water-soluble polymeric dyes. The method utilizes both the molecular sieve effect of normal gel permeation chromatography and solute-column packing interaction, modified by solvent composition. This method eliminates the need to extract the impurities from the polymeric dye with 100 extraction efficiency, as would be required for an ordinary liquid chromatographic analysis. 

Kumar, R., Sithambaram, S. and Suib,S.L. (2009) Cyclohexane oxidation catalyzed by manganese oxide octahedral molecular sieves - effect of acidity of the catalyst. Journal of Catalysis, 262,304—313. Sithambaram, S., Kumar, R., Son, Y. and Suib, S.L. (2008) Tandem catalysis direct catalytic synthesis of imines from alcohols using manganese octahedral molecular sieves. Journal of Catalysis, 253, 269-277. 

Molecular sieve dryers, 10 613 Molecular-sieve effects, 16 821 Molecular sieve membranes, 15 813t Molecular sieve products commercial, 16 838-839t manufacturing processes for, 16 831 Molecular sieves, 16 811-853. See also Carbon molecular sieves Zeolite entries... 

Thus, there are two limitations of the pycnometric technique mentioned possible adsorption of guest molecules and a molecular sieving effect. It is noteworthy that some PSs, e.g., with a core-shell structure, can include some void volume that can be inaccessible to the guest molecules. In this case, the measured excluded volume will be the sum of the true volume of the solid phase and the volume of inaccessible pores. One should not absolutely equalize the true density and the density measured by a pycnometric technique (the pycnometric density) because of the three factors mentioned earlier. Conventionally, presenting the results of measurements one should define the conditions of a pycnometric experiment (at least the type of guest and temperature). For example, the definition p shows that the density was measured at 298 K using helium as a probe gas. Unfortunately, use of He as a pycnometric fluid is not a panacea since adsorption of He cannot be absolutely excluded by some PSs (e.g., carbons) even at 293 K (see van der Plas in Ref. [2]). Nevertheless, in most practically important cases the values of the true and pycnometric densities are very close [2,7],... 

Convective Absorption. Compounds with molecular radii of less than 4 nm can pass through pores in the gut membrane. The membrane exhibits a molecular sieving effect. 

Zonal techniques are the most frequently used form of electrophoresis and involve the application of a sample as a small zone to a relatively large area of inert supporting medium which enables the subsequent detection of the separated sample zones. A wide range of supporting media have been developed either to eliminate difficulties caused by some media (e.g. the adsorptive effects of paper) or to offer additional features (e.g. the molecular sieving effects of polyacrylamide gel). 

Which of the following electrophoretic media show molecular sieving effects ... 

Jhung, S.H., Lee, J.H., and Chang, J. (2008) Crystal size control of transition metal ion-incorporated aluminophos-phate molecular sieves effect of ramping rate in the syntheses. Micropor. Mesopor. Mater., 112, 178-185. 

The most widely recognized molecular sieving effect occurs with K exchange. With K exchange (monovalent) the cation site that in its native Na form sits in a pore window partially occluding it. 

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