Molecular sieving properties

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

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. 

In reconstitution experiments, the self-assembly of the pore-forming protein a-hemolysin of Staphylococcus aureus (aHL) [181-183] was examined in plain and S-layer-supported lipid bilayers. Staphylococcal aHL formed lytic pores when added to the lipid-exposed side of the DPhPC bilayer with or without an attached S-layer from B coagulans E38/vl. The assembly of aHL pores was slower at S-layer-supported compared to unsupported folded membranes. No assembly could be detected upon adding aHL monomers to the S-layer face of the composite membrane. Therefore, the intrinsic molecular sieving properties of the S-layer lattice did not allow passage of aHL monomers through the S-layer pores to the lipid bilayer [142]. 

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],... 

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. 

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. 

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. 

Coal is microporous, with certain partial molecular sieve properties. (A microporous solid herein refers to that which contains pores with diameters of a few tens of A. or less.) Micropores can be considered as entities capable of sorbing foreign molecules, and it is known that additivity of their sorption potential fields enhances the sorption owing to dispersion interactions. As the pores become progressively narrower, the vapor adsorption isotherm (Figure 1) in the initial region up to point B becomes progressively steeper (toward the... 

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. 

Study of the molecular sieve properties and their possible application to develop highly selective catalysts. 

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. 

Cooper, B. J. Platinum-Carbon Catalysts with Molecular Sieve Properties. Shape Selectivity in Hydrogenation Catalysis. Platinum Metals Rev, 14, 133 (1970). 

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. 

Boehm, H.P. and Wamecke, H.H. Structural and molecular sieve properties of carbons prepared from metal carbides. In Proceedings of the 12th Biennial Conference on Carbon, pp. 149-150. Oxford Pergamon, 1975. 

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. 

If the catalyst is placed inside the membrane tubes and also the separative layer is coated at the inside of the tube, any compounds such as potassiumoxide from the catalyst might react with the silica separative layer to form keatite. This will destroy the molecular sieving properties of the silica toplayer, see paragraph 3.1. Additionally there is a risk that catalyst loading will damage the membrane layer. 

A common and well-known method to prepare silica membranes with molecular sieving properties is sol-gel coating [3-5], With this technique, microporous silica layers with a pore-size of about 0.5 nm are dip-coated on top of supported y-alumina membranes. The supports are porous a-alumina disks with pore diameters in the range from 100-200 nm. On top of these macroporous supports a 3 pm thick mesoporous y-alumina layer is coated, with a pore size of 3 nm. 

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-... 

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