Zeolites molecular sieve effects

Separation of isomers is an application where zeolite membranes could be specifically interesting because of their well-defined pores that lead to molecular sieving effects. An application that is often considered is the xylene isomerization and related reactions. 

Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-fi ee. 

In the above discussion, we have presumed that the tortuosity factor t is characteristic of the pore structure but not of the diffusing molecules. However, when the size of the diffusing molecule begins to approach the dimensions of the pore, one expects the solid to exert a retarding influence on the flux and this effect may also be incorporated in the tortuosity factor. This situation is likely to be significant in dealing with catalysis by zeolites (molecular sieves). 

Zeolite molecular sieves are widely used as solid acid catalysts or catalyst components in areas ranging from petroleum refining to the synthesis of intermediates and fine chemicals (112,113). An important reason for their widespread use is the flexibility they oflFer regarding the tailoring of the concentration and nature of catalytically active sites and their immediate environments. We note that discrimination between chemical and structural aspects works well at a conceptual level, but one faces quite severe limitations as soon as one tries to separate the contributions of the two effects. The complexity arises because the chemical properties of a particular molecular sieve are connected with its framework density. 

Molecular sieve dryers, 10 613 Molecular-sieve effects, 16 821 Molecular sieve membranes, 15 813t Molecular sieve products commercial, 16 838-839t manufacturing processes for, 16 831 Molecular sieves, 16 811-853. See also Carbon molecular sieves Zeolite entries... 

Species separated by molecular sieving effects when kinetic diameters fall into different zeolite aperture size categories (standard molecular sieve... 

Molecular sieve effects and their influence on catalytic selectivity offer important possibilities. Chen (48) showed that for a given reaction synthetic offretite, with its 12-membered rings of oxygen ions, exhibited no selectivity where the presence of small amounts of erionite (3%) resulted in an effective blocking of the large openings and the creation of selectivity. This emphasizes the possible influence of impurities on the practical uses of zeolite catalysts. 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

The dependence of Dt on gas molecular size has been found to be He > C02 > Ar > N2 > CH in four different polymers 23 25>26,39). This trend correlates smoothly with the minimum effective gas molecular diameter deduced from molecular sieving effects in zeolites 39). The corresponding trend of D2 is not so clearcut,... 

Adsorption. Although several types of microporous solids are useful as adsorbents for the separation of vapor or liquid mixtures, the distribution of pore diameters does not enable separations based on the molecular-sieve effect. The most important molecular-sieve effects are shown by crystalline zeolites. The sieve effect ntay be total or partial. 

Quite apart from this molecular sieving effect, zeolites are also effective in selectively sorbing particular components from a mixture of molecules all individually capable of penetrating the entire zeolite. Some liquid phase sorption equilibria studies have been reported for both the small-pore 5A molecular sieve (1 ) and the large-pore faujasite NaY zeolite (2). With the recent synthesis of intermediate pore sTze zeolites such as ZSM-5 and ZSM-11(3), a study of the selective sorption properties of these zeolites was initiated. 

Another method for altering the molecular sieving effect of a zeolite is the preadsorption of polar molecules. If small amounts of polar molecules, such as water or ammonia, are preadsorbed in a dehydrated zeolite, the adsorption of a second absorbate can be drastically reduced. It is assumed that the strong interactions... 

One of the most common low-temperature methods for removing impurities involves the use of selective solid adsorbents. Such materials as silica gel, carbon, and synthetic zeolites (molecular sieves) are widely used as adsorbents because of their extremely large effective surface areas. Most of the gels and carbons have pores of various sizes in a given sample, but the synthetic zeolites can be manufactured with closely controlled pore size openings ranging from 0.4 to 1.3 nm. This additional selectivity is useful because it permits separation of gases on the basis of molecular size. 

Molecular sieving Fig. 4(e) where, due to steric hindrance, only small molecules will diffuse through the membrane, seems to be a useful principle for achieving good separations. To ensure this molecular sieving effect, ultramicroporous membranes have to be prepared. Moreover, such membranes should not only be defect free but must also present a very narrow pore size distribution to avoid any other (less selective) permeation mechanisms defect-free zeolite membranes appear to be good candidates for this type of separation. 

Thus it is evident that zeolites offer considerable potential for steering reaction selectivity on the basis of differences in molecular shape. These possibilities extend far beyond the more familiar molecular sieve effects where bulky molecules are simply excluded entry into the zeolite cavities due to pore diameter restrictions. 

The question of which lattice components to include in the interpretive analysis reduces to the experimental problem of comparing systems of similar geometry but which contain different species in the lattice—e.g., Ge or Th in substitution for Si (1) S or F in substitution for O or OH, and say 2Na+ for lCa+2. The effect of change of cation is well known. In limiting cases it modifies the Molecular Sieve effect in zeolites. For host substituents, the availability of suitable systems is limited. The present work offers a comparison between a normal aluminosilicate and the same species after subjection to a substitution of halogen for oxygen and hydroxyl. 

Obviously the pore size determines which molecules can access the acidic sites inside the zeolite framework (molecular sieving effect) and is responsible for the shape selectivity observed with these materials (see later). The catalytic activity is also influenced by the acid strength of these sites which is determined by the Si/Al ratio (see above). The latter can be increased by post-synthesis removal of A1 atoms. Dealumination can be achieved by treatment with a... 

Gas phase adsorption on solids (silica gels, activated charcoal, synthetic zeolites, molecular sieves) is also practised industrially. It is more suitable for the purihcation of effluents that display some complexity of composition. Its effectiveness depends on many parameters, particularly the volatility and polarity of the feedstock components. 

Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties a uniform intra-crystalline microporosity that provides aceess to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. 

In many studies the separation factor, which is indicative of the membrane s ability to separate two gases in a mixture, is predominantly governed by Knudsen diffusion. Knudsen diffusion is useful in gas separation mostly when two gases are significantly different in their molecular weights. In other cases, more effective uansport mechanisms are required. The pore size of the membrane needs to be smaller so that molecular sieving effects become operative. Some new membrane materials such as zeolites and other molecular sieve materials and membrane modifications by the sol-gel and chemical vapor deposition techniques are all in the horizon. Alternatively, it is desirable to tailor the gas-membrane interaction for promoting such transport mechanisms as surface diffusion or capillary condensation. 

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