Kinetic diameters of 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-... 

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.

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).

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-... 

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.

The styrene oxide isomerization is known to be an easy reaction due to the carbonium stabilization by the aromatic nucleus. In the case of H-ZSM-5, taking into account the respective size of this medium-pore zeolite (5.5A) and the kinetic diameter of the styrene oxide molecule (5.9A), it was assumed that the weak external acidic sites are active enough to catalyze the reaction (ref. 16). If this were the case for all zeolites, no shape-selectivity could be obtained for any epoxide rearrangement. Nevertheless, for large-pore zeolites, the contribution of all the acidic sites cannot be excluded. 

The probe molecules used in the single diffusion are p-xylene and o-xylene. The kinetic diameter of these probe molecules (o are om = 5.8 A for p-xylene, and om = 7.0 A for o-xylene [11],... 

However, zeolite-like materials with ultralarge pores such as Cloverite (20 T, 0.6 x 1.32 nm), VPI-5 (18 T, 1.27 nm), AIPO4-8 (14 T, 0.79 x 0.87 nm) were recently synthesized. A comparison between the pore openings of zeolites and the kinetic diameter of guest molecules shows clearly that zeolites can be used for molecular sieving. It should however be stressed that these values are temperature-dependant temperature increases both the flexibility of the guest molecules and the breathing motions of the host zeolite pore mouth and framework. 

For paraxylene separation, both kinds of selectivity can be observed. In the MFI structure, the aperture of the pores is sufficiently close to the dimensions of the molecules to make shape selectivity appear. However, the kinetic diameters of paraxylene and of ethylbenzene are identical, so that the selectivity is not effective for these two components. Moreover, the capacity of MFI zeolites is weak compared to other structures. More open structures which provide the opportunity to use equilibrium selectivity are preferred. The problem is that the selectivity is mainly due to interactions between the zeolite and the aromatic ring which are identical for all the xylenes. It will be shown in the following sections that this problem can be solved by using chosen FAU zeolites. 

As noted above, one of the most important characteristics of zeolites is their ability to discriminate between molecules solely on the basis of their size. This feature, which they share with enzymes, is a consequence of them having pore dimensions close to the kinetic diameter of many simple organic molecules. Hence, zeolites and zeotypes have sometimes been referred to as mineral enzymes. This so-called shape selectivity can be conveniently divided into three categories substrate selectivity, product selectivity and transition state selectivity. Examples of each type are shown in Fig. 2.11. 

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. 

It has recently found [22] with single crystal structure analysis, that even molecules as large as naphthalene, with a kinetic diameter of 3.8 7.4 A, can be accomodated in the pores of the MFI-framework, see Table 3. 

In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems Table 6). 

DD3R is the only 8-membered all-silica stmcture known. The template used for syntheazing DD3R (1-adamantanamine) can be removed completely. DD3R s capability of adsorbing small gases is comparable to other zeolites with 8-membered pore apertures like zeolite A [36].Thus, its ellipsoidal pore size of 4.4 x 3.6 A, which matches the kinetic diameters of most small gases closely, makes separations between branched and linear molecules possible. 

Both molecules are adsorbed, at the temperature applied, at a much lower rate than methane or ethane. The kinetic diameters of propane and butane are larger than that of ethane, due to the curvature of a molecule with three or more carbon atoms, and matches the pore size of the adsorbent more closely, restraining the entering of the pore system. 

Figure 1 Kinetic diameters of several molecules compared to the free apertures of the pore (bold lines) and the adsorption cutoff diameter (thin lines) of zeolite 4A, ZSM-5, and zeolite X or Y. (From Refs. 1, 2, 30.)...

Figure 11 Transition from Knudsen diffusion to configurational diffusion in ZSM-5 and 5A as a function of the ratio between the minimum kinetic diameter of the molecule and the maximum channel diameter of the zeolite at 300 K. The ratio between the length and the kinetic diameter of the diffusing molecule was taken to be 1.25. (Adapted from Ref. 37.)...

The permeability of polymers decreases with the increase in their glass transition temperature. This dependence becomes more pronounced with increasing kinetic diameter of penetrant molecules (Figure 9.9). 

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