Zeolite pore sizes

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

Li, Y., Chung, T.S., Cao, C., and Kulprathipanja, S. (2005) The effects of polymer chain rigidification, zeolite pore size and pore blockage on polyethersul-fone (PES)-zeolite A mixed matrix membranes./. Memhr. Sci., 260, 45-55. 

In aqueous solutions, the high negative surface charge of a zeolite must be neutralized by binding counterions, such as Na+, K+, and Ca +. The distribution of zeolite pore size can be modified, such that small molecules are included in the pores those too large to diffuse into the pores are excluded. Structure types are named by a three-letter lUPAC code, based in part on the name of the zeolite first used to identify the specific type. They are also classified by pore size, framework density, and/or symmetry. 

Figure 2. Typical zeolite pore sizes compared to the diameter of various molecules. (Adapted from ref. 1.)...

Another example of secondary shape selectivity is shown by John and co-workers (77,73). They found that the hydroisomerization/hydroeracking of n-hexane over Pt/H-mordenite is significantly inhibited by the presence of benzene. They also found a correlation between the aromatic size relative to zeolite pore size on the inhibition of the hexane reaction and the changes in isomer selectivities. Figure 4 illustrates the relation between the various aromatics co-fed and the n-hexane isomerization rates on H-mordenite. From Figure 4, it is shown that as the kinetic diameter of the aromatics is increased, the isomer formation rate appears to pass through a minimum. This result can be explained by considering the size of the zeolite pore and the kinetic... 

The term molecular sieve describes a material having pores that closely match the dimensions of a specific molecule. The best-known molecular sieves are composites of microcrystalline zeolites embedded in an inert clay binder. Zeolites are composed of regular clusters of tetrahedral aluminosilicates, with varying percentages of bound cations and water molecules, whose crystal structures incorporate small molecule-sized cavities. Because zeolite pore size is different for each of the numerous different crystal structures in this family, the size-selective nature can be tailored for specific applicatimis. Studies of the transport of liquid and gaseous organic species in molecular sieves indicate that the diffusion rate and equilibrium concentration of sorbed analyte are sensitive functions of their molecular dimensions, as well as zeolite pore size and shsqre [110]. 

Copper ions exchanged microporous molecular sieves, in particular Cu-ZSM-5, are active catalysts for the selective catalytic reduction of NO and N2O with hydrocarbons in the presence of O2 (HC-SCR). It has been reported that the catalytic activity may be controlled by intra-crystalline diffiisivity and by geometry-limited diffusion depending on the hydrocarbon molecular size and the zeolite pore size [1]. Therefore, it is of interest to prepare Cu-Al-MCM-41 mesoporous molecular sieves and to compare their activity with that of Cu-ZSM-5. 

Separation due to molecular sieving alone can only take place if the membrane is defect free and there is no adsorption of the permeating species on the zeolite crystals (i.e., at high temperatures) or the components in the mixture adsorb with similar heats of adsorption. For those mixtures, the shape and size of the components compared to the zeolite-pore sizes direct the separation toward molecular sieving. For example, the separation of H2/SF6 or N2/SF6 on MFl zeolite membranes at high temperatures where SFe does not adsorb significantly. In fact, the latter separation is commonly used to evaluate the membrane quality. Methane/f-octane represents another example of this mechanism since f-octane cannot fit into the MFl stmcture and the heat of adsorption of methane is very low in the zeolite [2]. 

Residue-forming reactions of 1-hexene on H-USY and (Na)H-USY zeolites have been examined at 393-673K, Except for the effects of differing zeolite pore sizes, the reaction pathway... 

The kinetic model reproduces satisfectorily experimental results. Deactivation experiments seems to indicate that the mechanism of deactivation changes with the nature of the contaminant used. When a strong poison for active acidic sites like pyridine is used, the catalyst gets totally deactivated when its concentration is over 250 ppm. In this case, the deactivation is fester than with CS2, hut not as fest as an acid base reaction should be. The behavior can be explained assiuning that the pyridine reaction with acidic sites is a diffesion controlled phenomenon enhanced by its molecular size, which is very near to the zeolite pore size. The presence of a mixed mechanism of deactivation and inhibition is also evident. 

In the dehydrated 5A zeolite, four of the eight 6-membered rings are occupied by Na+ ions whereas the other four are occupied by Ca2+ cations. The Ca2+ cations are located on the plane of the 6-ring, and the Na+ cation is about 0.4 A above the 6-ring plane, pointing toward the a-cage. This phenomenon indicates that, under there circumstances, the variation of the zeolite pore size depends on the exchange of Na+ by Ca2+. 

The CVD technique can be effective for zeolite pore-size adjustment, and the zeolites after modification through this technique exhibit distinctly enhanced shape-selective adsorption separation and catalytic performances. However, this technique requires vacuum apparatus and the monetary investment for this technique is large. In addition,... 

The separation effect for the mixture of 1,2,4-trimethylbenzene and 1,3,5-trimethyl-benzene is similar, and as the amount of Si(OCH3)4 used increases, the pore size of NaY zeolite is narrowed gradually, and the adsorption selectivity of the zeolite for 1,2,4-trimethylbenzene increases rapidly. When the Si(OCH3)4 amount used is 0.15 mL/g, the adsorption capacity of the zeolite for 1,2,4-trimethylbenzene remains almost unchanged, but the adsorption for 1,3,5-trimethylbenzene is very small, and the 1,2,4-trimethyl benzene adsorption selectivity is exceeds 90%, achieving an ideal separation effect as shown in Figure 6.24.[64] Comparison of the two systems reveals that to achieve an ideal separation effect, more modifier is required that is, the zeolite pore size should be narrower for separation of trimethylbenzene mixture because the size of the 1,3,5-trimethylbenzene molecule is smaller that that of 1-methylnaphthalene. 

Y.H. Yue, Y. Tang, and Z. Gao, Zeolite Pore Size Engineering by Chemical Liquid Deposition, Progress in Zeolite and Microporous Materials. Stud. Surf. Sci. Catal., Part C, 1996, 105, 2059-2065. 

Entry Zeolite pore sizes intersection volume (A3)... 

FIGURE 12.2-1 Molecular dimensions and zeolite pore size. Chart shows effective pore sizes of various zeolites over temperatures of 77-420 K. (From Raf, 12.1-8, copyright John Wiley Sons. Inc., repriated with permission,)... 

The silica pore size is up to 30 times higher (catalyst F) than the zeolitic pore size. Catalysts D, E and F both differ in pore size and acidity. The effect of these characteristic data is not clear. An optimum seems to exist for catalyst D. However, it cannot be excluded that different manufacturing procedures may result in different catalytic performances. 

Molecule Kinetic diameter [nm] Zeolite, pore size [nm] ... 

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