Zeolites aperture dimensions

Molecular dynamics has been used to study dyamics of zeolite frameworks, particularly fluctuations in zeolite aperture dimensions, whereby the zeolite framework can breathe substantially depending on the topology. For example, some O—O distances across the window were found to vary by as... 

Table 2. Zeolites and Their Pore (Aperture) Dimensions ...

FIGURE 10.5 Molecules admitted to zeolites according to molecular dimensions and zeolite aperture sizes. 

The classification is shown in an abbreviated form in Table III with an example to illustrate each group. Included is information on the void fraction, expressed as cc per cc of dehydrated zeolite as derived from the water content for the fully hydrated zeolite, the framework density expressed in grams/cc, the approximate dimensions of the main channels (as derived from the structure of the hydrated zeolite), and the type of channel system, 1 for one-dimensional, etc. In many instances, of course, the dimensions of the apertures change substantially with dehydration. Consequently, the aperture dimensions are not necessarily consistent with the adsorption properties of the dehydrated zeolite. We need considerable information on the structures of dehydrated zeolites. In such a... 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

Zeolites are aluminosilicates enclosing channels and cages of molecular dimensions (see Section 1.5). We have discussed the structure of zeolites derived from sodalite units (Fig. 1.22), which result in large cavities (cages) connected by apertures (pores) of... 

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. 

A zeolitic structure can be described in various crystallographic terms. For many systems it is now possible to specify the following structural features the SBUs, the framework density, the coordination sequences, the unit cell dimensions and composition, the direction of the channels and the aperture (window) dimensions (Atlas of Zeolite Structure Types, 1992 Thomas et al., 1997). The framework density, FD, is defined as the number of T atoms per 1000 A1 (i.e. per 1 nm3) of the structure. 

The pore structure of type Y is described, for example, by Breck and Flanigen (9). It consists essentially of a three-dimensional array of large cavities with a diameter of about 12 A interconnected by pore apertures with diameters of about 8 A. These dimensions vary slightly with the Si/Al ratio in the zeolite and the number and kinds of cations present. 

The pores of zeolites are of molecular dimensions (Fig. 1). In principle molecules can be excluded from the pores if their diameter is larger than the pore apertures. The zeolite framework is, however, not a rigid structure. Especially at higher temperatures, the pores become more flexible. As a result, molecules with larger diameters than the dimensions of the pores will be able to penetrate the channels. The maximum diameter of molecules that are able to adsorb within the zeolite is called the adsorption cutoff diameter. This diameter is about 0.95 nm for zeolite X and Y [30], 0.65 nm for ZSM-5 [30], and 0.4 nm for zeolite-4 A [1] at 3(X) K. This means that, in principle, all molecules listed in Fig. 1 can permeate through a zeolitic membrane made from zeolite X, Y or ZSM-5, except for the large amines. Complete exclusion of molecules from the pores will therefore occur only when using a zeolite-A membrane. 

Molecular species may selectively penetrate into the zeolitic framework, and for this reason zeolites are often referred to as molecular sieves. This selectivity is governed by several factors such as aperture size, shape ( circular or elliptic ), available volume controlled by the degree of dehydration, and naturally the dimensions of the guest molecule. Permanent guests are water molecules but many others species can readily be accommodated The pore size can be... 

Intrinsic material selection aspects can guide construction of mixed matrix membranes with desirable performance characteristics. The molecular sieving phase must accurately discriminate between the size and shape differences of spherocylindrical O2 and N2 molecules. Silicalite, commonly used in reported mixed matrix membrane studies, is a hydrophobic zeolite possessing channels with dimensions between 5.2 and 5.8 A. Likewise, zeolite 13X possesses an aperture of 10 A (P). Clearly, these materials are not molecular sieves for O2 and N2 molecules with lengths of 3.75 and 4.07 A, respectively. Rather, an effective molecular sieve would possess an aperture size between the molecular dimensions of the two molecules. Zeolite 4A possesses an eight sided aperture with an effective aperture size of 3.8 A and is appropriate for this application. 

As discussed in Chapter 2, the class of adsorbents known as zeolites form crystalline structures containing apertures (referred to as windows) of molecular dimensions through which molecules of adsorbate smaller than the aperture may enter the well-defined internal ehannels leading to the larger cavities within the crystal where the sites for adsorption are located. Diffusion into zeolites is therefore relatively slow because of the restricted access. Diffusion coefficients Dc associated with zeolite crystal structures have magnitudes in the range 10to 10m s. ... 

Pt(NH3) ions since the molecular dimensions of these metal complexes exceed the pore apertures. Platinum was incorporated during the synthesis of the zeo-hte by mixing solutions of K2PtCl4 with solutions of sodium metasiUcate and sodium aluminate in the required amounts. More recently, Davis et al. [129,130] prepared intrazeoHtic 2-5 nm ruthenium particles in NaA and CaA zeoHtes by addition of [Ru(NH3)5Cl] CI2 to the hydrothermal synthesis mix of zeolite A. This technique of metal loading has never been widely used to prepare metal clusters in zeolites but is frequently employed to substitute lattice aluminum in zeoHtes or aluminophosphates by metal ions. 

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