Molecules kinetic diameters

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

Gas molecule Kinetic diameter (A) Lennard-Jones diameter (A)... 

It is seen from the data in Table 3 that the (Si/Al)s ratio is higher than the bulk value (2.33). This suggests an aluminium depleted surface region. The low values observed for the ratio (N/AI)s reflect the fact that only part of the BrOnsted acid sites are accessible to pyridine. The pyridine molecule kinetic diameter of 5.9 A does not allow it to enter the sodalite cages with 2.2 A openings. Thus only the acid sites protruding in the supercages can chemisorb pyridine. The number of these molecules is estimated to be 24 per unit cell [41] and since the Y zeolite with a Si/Al ratio of 2.33 has 57 A1 atoms per unit cell, the maximum N/Al ratio is 0.42. This value is reasonably close to the 0.38 value obtained after calcination at 300°C. 

Figure 19 Pore apertures of DOH (6-membered ring. 0.28 nm), zeolite-A (LTA, 8-membered ring, 0.41 nm), silicalite-1 (MFI, 10-membered ring, 0.52 X 0.55 nm), and a zeolite-X or -Y (FAU, 12-membered ring, 0.74 nm) and a methane molecule (kinetic diameter = 0.38 nm).

The transport mechanisms through zeolite membranes depend on different variables such as operation conditions (especially temperature and pressure), membrane pore size distribution, characteristics of the pore surface of the zeohtic-channel network (hydrophilicity/hydrophobicity ratio), as well as the characteristics of the crystal boundaries and the characteristics of the permeating molecules (kinetic diameter, molecular weight, vapor pressure, heat of adsorption), and their interactions in the mixture. 

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

Potential length constant or collision diameter of the molecule. Kinetic diameter (Breck, 1974). 

Fig. 7. Molecular dimension and 2eohte pore si2e. Chart showing a correlation between effective pore si2e of various 2eohtes over temperatures of 77—420 K (-----------------) with the kinetic diameters of various molecules (1). M—A is a cation—2eohte A system. M—X is a cation—2eohte X system.

The pore size of Cs2.2 and Cs2.1 cannot be determined by the N2 adsorption, so that their pore sizes were estimated from the adsorption of molecules having different molecular size. Table 3 compares the adsorption capacities of Csx for various molecules measured by a microbalance connected directly to an ultrahigh vacuum system [18]. As for the adsorption of benzene (kinetic diameter = 5.9 A [25]) and neopentane (kinetic diameter = 6.2 A [25]), the ratios of the adsorption capacity between Cs2.2 and Cs2.5 were similar to the ratio for N2 adsorption. Of interest are the results of 1,3,5-trimethylbenzene (kinetic diameter = 7.5 A [25]) and triisopropylbenzene (kinetic diameter = 8.5 A [25]). Both adsorbed significantly on Cs2.5, but httle on Cs2.2, indicating that the pore size of Cs2.2 is in the range of 6.2 -7.5 A and that of Cs2.5 is larger than 8.5 A in diameter. In the case of Cs2.1, both benzene and neopentane adsorbed only a little. Hence the pore size of Cs2.1 is less than 5.9 A. These results demonstrate that the pore structure can be controlled by the substitution for H+ by Cs+. 

Molecule (cross section/A ) Kinetic diameter/A Pressure /Torr (P/PO) Temp. /K Amoimt of adsorption /pmol g-sohd Cs2.1 Cs2.2 Cs2.5 Ratio ... 

Figure 3.26. Kinetic diameters of some important organic molecules. For reference the pore dimensions of some common zeolites are shown (Van de Graaf et al., 1998).

The acidic and adsorptive properties of the samples in gas phase were evaluated in a microcalorimeter of Tian-Calvet type (C80, Setaram) linked to a volumetric line. For the estimation of the acidic properties, NH3 (pKa = 9.24, proton affinity in gas phase = 857.7 kJ.mol-1, kinetic diameter = 0.375 nm) and pyridine (pKa = 5.19, proton affinity in gas phase = 922.2 kJ.mol-1, kinetic diameter = 0.533 nm) were chosen as basic probe molecules. Different VOC s such as propionaldehyde, 2-butanone and acetonitrile were used in gas phase in order to check the adsorption capacities of the samples. 

Knowing the framework type of a material, the size of molecules that can be adsorbed can be estimated. Kinetic diameters for various molecules [5-9] are given in Table 2.2. Thus neopentane (kinetic diameter of 0.62 nm) is expected to be adsorbed by NaX zeolite (FAU structure type) which has channels defined by 12-... 

As mentioned above, many different probe molecules have been used to measure the acidity of zeolites. These molecules vary over a wide range of proton affinities, size (kinetic diameter) and shape. Table 4.4 lists these properties for several of the more commonly used probes. 

Larger probe molecules such as alkyl substituted pyridines and hindered amines have been used to probe only the external acidity of zeolites. The large kinetic diameter of these molecules prevents them from entering the pores of the zeolite... 

Figure 6.2 illustrates the separation of n-Csis and non-n-Cs/is in CaA molecular sieves or 5A. The separation mechanism is obvious when the kinetic diameter of the molecules and molecular sieve pore size opening are compared. n-Csjc have kinetic diameters of less than 4.4 A which can diffuse freely into the 4.7 A pores of the CaA molecular sieve, while non-n-Cs/ have kinetic diameters of 6.2A. A commercial example of shape-selective adsorption is the UOP Molex process, which uses CaA molecular sieves to separate Cio-C n-paraffins from non- -parafHns (aromatics, branched, naphthenes). 

Figure 10.1 Kinetic diameter of common industrial molecules shown relative to the pore sizes of common zeolite structures shaded areas represent the range of effective pore diameters for each group of zeolites.

A good example for reactant shape selectivity includes the use of catalysts with ERI framework type for selective cracking of linear alkanes, while excluding branched alkanes with relatively large kinetic diameters from the active sites within the narrow 8-MR zeolite channels [61, 62]. Here molecular sieving occurs both because of the low Henry coefficient for branched alkanes and because of the intracrystalline diffusion limitations that develop from slow diffusivities for branched alkane feed molecules. 

The influence of interchannel dlffusional limitation may be estimated by a comparison of catalyst para-selectivities to separate product molecules, the diameters of which are different. As is known, the kinetic diameter of molecules increases in the sequence para-XYI para-selectivities to dial-... 

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