Pore apertures

Zeolite IZA structure code Typical unit cell composition Si02/Al203 range by synthesis Dimensionality of channel system Pore apertures (nm)... 

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

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. 

Fig. 17.2. (Opposite) Immuno-gold localization of a T. muris-derived IFN-y homologue to the bacillary band and cuticular pore. (A) Transmission electron micrograph of bacillary band showing the pore chamber (PC), pore aperture (PA) and lamellar apparatus (LA) (x22,000). (B) High-power localization of antibody staining (black dots) to the lamellar apparatus (LA) and pore chamber (PC) (x36,000). (Courtesy ofF. Bughdadi.)...

The selective oxidations of the terminal positions of -alkanes are an example of substrate-shape selectivity. Product-shape selectivity has been used to enhance the selectivity of the type IIaRH oxidation of cyclohexane [66-68], For example, oxidation of cyclohexane at 373 K for 8 hr using FeAlPO-31 (pore aperture 5.4 A) as a catalyst resulted in 2.5% conversion to a mixture which contained 55.3% of adipic acid and 37.3% of a mixture of cyclohexanol and cyclohexanone [68]. In contrast, oxidation under identical conditions using FeAlPO-5 (pore aperture 7.3 A) resulted in only 9.2% of adipic acid and 89.5%... 

Fig. 8A shows that the selectivity for ketone and alcohol formation is quite similar for both molecular sieves. Fig. 8B indicates that regioselectivity exists for both molecular sieves, possibly due to the encaged nature of the complex. However, lower values of the C2/C3 and C2/C4 ratios are obtained in VPI-5 compared to zeolite Y, pointing to the existence of shape selectivity. The molecular graphics analysis, which enabled quantification of the free pore apertures, shows that the difference in selectivity can hardly be caused by differences in the zeolitic environment. The enhanced constraint observed for FePcY should then be related to the saddle-type deformation of the complex. 

Figure 9.39 Schematic representations of metal 4,4 -bipyridyl porous networks. Lines represent the bipyridyl ligand except for vertical lines in (c), which represent Ag - Ag bonds (2.977 A long), and horizontal lines in (d) which represent Cu---Cu. The chemical formula, framework dimensionality, structure type, degree of interpenetration and pore aperture are listed under each representation. (Reproduced with permission from Reference 38).

Figure 19.12 Control of the pore aperture by interaction of molecular gates with guests of different size and shape.

In the case of gas-phase hydrocarbon reactions, coke retention occurs for two main reasons 1771 (1) the condensation under liquid or even solid state of coke molecules on the catalyst is generally observed at low temperature (<473 K) coke molecules are therefore not sufficiently mobile or volatile to be eliminated from the catalyst under operating conditions and (2) the steric blockage (trapping) within the pores that often occurs at high temperatures (>623 K), when the size of the product molecules formed within the pores becomes intermediate between the size of the cages or channels and that of the pore apertures. 

The formation of coke requires therefore the possibility for (a) reactant(s) to undergo bimolecular reactions and for the reaction products to be retained in or on the zeolite. This retention occurs either because the products are not volatile enough to be eliminated from the zeolite under the operating conditions or because their size is greater than the pore aperture (hence a steric blockage in the cavities or at channel intersections). Obviously the first mode of retention concerns not only the coke molecules deposited within the micropores but also those deposited on the outer surface of the crystallites. 

Figure 1 Schematic representation of the pore structure of various zeolites with large pore apertures FAU (Y), MOR, OFF, with average pore apertures MF1 (ZSM5), with small apertures ERI.

Separation of molecules with different sizes can be achieved by a proper choice of zeolites (nature of the zeolite and adjustment of the pore architecture, especially the pore size). The simplest forms of shape selectivity come from the impossibility of certain molecules in a reactant mixture entering the zeolite pores (reactant shape selectivity) or of certain product molecules (formed inside the pore network) exiting from these pores (product shape selectivity). In practice, reactant and product shape selectivities are observed not only when the size of molecules is larger than the size of the pore apertures (size exclusion) but also when their diffusion rate is significantly lower than that of the other molecules. Differences of diffiisivities by 2 orders of magnitude are required to produce significant selectivities between reactant species (35). 

The mechanism of gas mixture separation depends on the type of adsorbent. Two different mechanisms are distinguished. The first mechanism is based on the kinetically controlled gas diffusion, caused by constrictions of the pore apertures. Here the diameters of pores are in the same range as those of the gas molecules. The particles having smaller diameter can penetrate much quicker into the pores than larger molecules. In the second separation mechanism, the pore system is sufficiently wide to enable fest diffusion, while the separation is caused by the selective adsorption dependent upon different van der Waals forces of the gas species [5]. 

CHEMTr.cn. (Aug. 1973) 498). and is described mathematically in a unique manner. For molecules exceeding the size ot the pore aperture, the interior of the pore is inaccessible. 

Metal aluminophosphates (MeAPO) contain framework metal (Me), aluminum, and phosphorus. When the metal is divalent (e.g., Zn +, Co +, and Mg +) and substitutes for aluminum, a negatively charged framework results, with H+, for example, serving to compensate the charge. Many aluminophosphate molecular sieves have been synthesized. SAPO-11 and MeAPO-11 have interesting catalytic properties. Their structures have onedimensional 10-ring channels. The 10-ring pore aperture is elliptical with dimensions 0.39 x 0.63 nm. Table 1 is a summary of the characteristics of the molecular sieves which have been used for the skeletal isomerization of n-butenes. 

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. 

Type Y zeolite has a much more open pore structure than most other zeolites, but counterdilfusion through its pore structure of small aromatic hydrocarbons can still be quite slow and depends strongly upon the nature of cations present. Counterdilfusion in the type Y zeolite appears to be modeled best as diffusion over periodic high energy barriers which are the pore apertures joining the supercages. The activation energy is... 

J. R. Katzer No, we have not studied counterdiffusion in the type X zeolite, but I am now in the process of doing so. The effect appears to be produced by the cations within the pore structure and resting near the pore apertures. Olson (Olson, D. H., J. Phys. Chem. 1968, 72, 4366) indicated that structural variations occurred in the CaX form which careful examination shows could reduce the pore aperture. However, he has recently indicated (Olson, D. H., personal communication, 1970) that the pore aperture remains essentially the same upon exchange from the... 

Pore apertures of eight-membered oxygen ring systems... 

Zeolite type Pore aperture (A) based on 0-ring Pore aperture (A) including cation Pore form... 

Pore apertures of ten-membered oxygen ring zeolites... 

As shown in Table 3 for the straight channels of MFI, p-xylene, of which the kinetic diameter is actually larger than the pore aperture diameter, can be accomodated by deforming the pore from circular to ellipsoidal. This phenomenon should be taken into account when studying the separation of xylenes with MFI-frameworks in a membrane configuration. 

In particular in the case of naphthalene, the pore apertures deform from a circular to a highly ellipsoidal shape upon adsorption and permeation. Next, this peristaltic behavior of the pore deformation is completed by the partial return of the ellipsoidal into a more circular aperture as the molecule is stabilized between the apertures. 

Small pore (6- and 8-ring) zeolite-based membranes might be used in industrial processes involving hydrogen, in air separation or in separation of linear and branched alkanes,. plying small pore apertures might lead to high separation/selectivity. 

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). 

---