Zeolite molecular sieving properties

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

Three different ways in which a zeolite membrane can contribute to a better sensor performance can be distinguished (i) the add-on selective adsorption or molecular sieving layer to the sensor improves selectivity and sensitivity, (ii) the zeolite layer acts as active sensing material and adds the selective adsorption and molecular sieving properties to this, and (iii) the zeohte membrane adds a catalytically active layer to the sensor, improving the selectivity by specific reactions. 

Cancrinites are one of the rarest members of the feldspathoid group, classified as such due to its low silicon content. However, cancrinite is also classified as a zeolite, due to its open pore structure, which confers molecular sieve properties [1], Likewise, variable sodium carbonate and NaOH concentrations in the hydrothermal synthesis of cancrinite could direct the synthesis of the intermediate phase or the disordered cancrinite formation [2], The intermediate phase is described as a phase between cancrinite and sodalite [3], The disordered cancrinite is an intermediate phase which is much closer to the cancrinite structure than sodalite structure [2],... 

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. 

Considering all we know up to now, the specific properties of zeolites can be summarized as follows. Zeolites are aluminosilicates with defined microporous channels or cages. They have excellent ion-exchange properties and can thus be used as water softeners and to remove heavy metal cations from solutions. Furthermore, zeolites have molecular sieve properties, making them very useful for gas separation and adsorption processes, e.g., they can be used as desiccants or for separation of product gas streams in chemical processes. Protonated zeolites are efficient solid-state acids, which are used in catalysis and metal-impregnated zeolites are useful catalysts as well. 

The characteristics of aluminophosphate molecular sieves include a univariant framework composition with Al/P = 1, a high degree of structural diversity and a wide range of pore sizes and volumes, exceeding the pore sizes known previously in zeolite molecular sieves with the VPI-5 18-membered ring material. They are neutral frameworks and therefore have nil ion-exchange capacity or acidic catalytic properties. Their surface selectivity is mildly hydrophilic. They exhibit excellent thermal and hydrothermal stability, up to 1000 °C (thermal) and 600 °C (steam). 

Barter, R.M. (1948) Synthesis of a zeolitic mineral with chabazite-like sorptive properties. /. Chem. Soc., 127 Barter, R.M. and Riley, D.W. (1948) Sorptive and molecular sieve properties of a new zeolitic mineral. /. Chem. Soc., 133. 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

Mixed-matrix membranes containing dispersed zeolites in a continuous polymer matrix may retain polymer processabibty and improved selectivity for separation appHcations due to the superior molecular sieving property of the zeolite materials. 

Brcck, D. R. (1974). Zeolite Molecular Sieves. Wiley-Interscience, New York. Brenner, S. S. (1958). Growth and properties of whiskers. Science 128 569-575. Brindley, G. W. (1980). Order-disorder in clay mineral structures, pp. 125-195. In Brindley, G. W. and G. Brown, eds. Crystal Structures of Clay Minerals... 

At the earlier conferences on molecular sieves, in London in 1967 and in Worcester, Mass. (U.S.) in 1970, attention was focused exclusively on the zeolites. In an etymological sense (separation of molecules according to size by selective diffusion through pores of appropriate diameter) the field of molecular sieves must not be restricted to the tectosilicates with porous framework. This point is developed by R. M. Barrer in Chapter 1, where he gives a broad review of those compounds which can exhibit molecular sieve properties. 

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 surface for adsorption is essentially entirely internal due to the channels and cavities which uniformly penetrate the entire volume of the adsorbent. The molecular sieving properties of the zeolites are uniquely determined by their pore diameters, the magnitude of which determines what size molecules are totally excluded from the interior of the zeolite. 

Furthermore, the implantation of various boron-nitrogen compounds inside the zeolite framework can reduce, in a controlled way, the effective pore size of zeolites. When NH3 is added to the boranated zeolite (before hydrolysis reaction), at room temperature, the formation of amine-boranes can be detected, which changes the molecular sieving properties of the zeolite. 

The adsorptive separation is achieved by one of the three mechanisms steric, kinetic, or equilibrium effect. The steric effect derives from the molecular sieving property of zeolites. In this case only small and properly shaped molecules can diffuse into the adsorbent, whereas other molecules are totally excluded. Kinetic separation is achieved by virtue of the differences in diffusion rates of different molecules. A large majority of processes operate through the equilibrium adsorption of mixture and hence are called equilibrium separation processes. 

It should be reminded at this juncture (see Chapter 1) that only a fraction of the zeolite structures are used commercially and that with few exceptions (SAPO-11 for instance), almost no metallosilicates have found a large scale use. As far as the catalytic and adsorption properties are concerned, zeolite molecular sieves are further classified according to their pore openings, defined by the number of 02 anions delineating their pore mouths. The most useful zeolites are those containing 12, 10 or 8 of these Cr and are commonly referred to as 12-, 10-, and 8- ring structures. The zeolites FAU and MFI are the major components of the FCC catalysts. 

The earliest applications of zeolites utilized the molecular sieving properties of small pore zeolites, e.g. zeolite A, in separation and purification processes such as drying and linear/branched alkane separation [33]. In 1962 Mobil Oil introduced the use of synthetic zeolite X, an FCC (fluid catalytic cracking) catalyst in oil refining. In the late sixties the W. R. Grace company introduced the "ultra-... 

These stereo-selectivity properties do not only apply to the case of zeolite catalysts but also in the case of zeolite molecular sieves. 

Protonic zeolites find industrial applications as acid catalysts in several hydrocarbon conversion reactions. The excellent activity of these materials is due to two main properties a strong Bronsted acidity of bridging Si—(OH)-Al sites (Scheme 3.4, right) generated by the presence of aluminum inside the silicate framework and shape selectivity effects due to the molecular sieving properties associated with the well defined crystal pore sizes, where at least some of the catalytically active sites are located. 

There are a number of materials that exhibit molecular sieving properties. The best known among them are zeolites. Others include some porous glass, fine-pore silicas, montmorillonite and molecular sieve carbons. 

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