Sieving selectivity

Of the many reagents, both heterogeneous and homogeneous, that can facilitate chemical reactions, the cycloamyloses stand out. Reactions can be catalyzed with many species such as hydronium ions, hydroxide ions, general acids, general bases, nucleophiles, and electrophiles. More effective catalysis can sometimes be achieved by combinations of catalytic species as in multiple catalysis, intramolecular catalysis, and catalysis by com-plexation. Only the latter catalysis can show the real attributes of an efficient catalytic system, namely speed and selectivity. In analogy to molecular sieves, selectivity can be attained by stereospecific complexation and speed can be likewise attained if the stereochemistry within the complex is correct. The cycloamyloses, of any simple chemical compound, come the closest to these goals. 

Novel manufacturing technologies are needed to address modem separation and reaction challenges. ZeoUtes possess molecular sieving, selective sorption properties and catalytic activity, as well as enhanced thermal and chemical stabiUty which make them good candidates for these applications. Future focus needs to address the processing methods and theoretical understanding of these materials so their potential for commercial appUcation can be realized. 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

Very often the liquids to be processed may be contaminated with substances detrimental to some types of zeolites consequently a complete knowledge of the process stream composition and physical properties must be available before preliminary sieve selection can be made. In the absence of prior knowledge of separation factors, competitive co-adsorption, environmental stability, regeneration techniques, or irreversible zeolite contamination, zeoli te contamination, zeolite specification must be proceded by time-con-... 

Figure 11. Molecular sieve selectivity for supercritical-fluid—molecular-sieve extraction (O) 2,2,4-trimethylpentane (9) 2,3-dimethylbutane...

C and 5 kg /cm2 pressure (see Molecular sieves). Selectivity for toluene and xylenes peaks at 550°C but continues with increasing temperature for benzene. The Cydar process (Fig. 6) devdoped joindy by BP and UOP uses a spherical, proprietary zeolite catalyst with a nonnoble metallic promoter to convert C3 or C4 paraffins to aromatics. The drawback to the process economics is the production of fud gas, a low value by-product. BP operated a... 

Diffusion, absorption, and mass transport properties, of course, also define microporous molecular sieve selectivity. No other physical proper-... 

Table III. Batch Runs, Effect of Water Content on Molecular Sieve Selectivity in Adsorption of 2,7-DMN from 257 —265 C Heart-Cut Isomers, 100 C, 2 Hrs.

It has been reported in the literature that the sieving selectivity is highest when analyte and sieving molecules are of similar size (contour length) [27-29]. This could be proved by the following approach. The slopes of the cali-... 

Kawabuchi, Y., Kawano, S., and Mochida, I. (1996). Molecular sieving selectivity of active carbons and active carbon fibers improved by chemical vapour deposition of benzene. Carbon, 34, 711—17. 

Solvent effects in heterogeneous catalysis are examined in terms of physical or chemical modifications to control the chemo-, regio- and stereoselectivity of a reaction. The main factors affecting selectivity are reactant solubility, polarity, reactivity or acido-basicity of solvents and competitive chemisorption of products and solvents In the special case of molecular sieves, selectivity control of a reaction by competitive adsorption, diffusion or shape selectivity and confinement catalysis are also examined. 

Moreira, R. H. Jose, and A. Rodrigues, Modification of pore size in activated carbon by polymer deposition and its effects on molecular sieve selectivity. Carbon. 2001, 39(15), 2269-2276. 

Zeolites are used as detergent builders, adsorbents, and catalysts. In the past decade, we saw the development of a variety of zeoiite membranes, and a number of investigators reported on the preparation of such membranes and their applications to a variety of separation systems. These research activities are motivated by features common to inorganic membranes, such as thermal resistance and resistance to organic solvents, and features unique to zeolite materials, such as molecular sieving, selective adsorption, and catalytic activity.In this article, the discussion will be restricted to zeolite membranes for use in separation and catalysis. First, an overview is presented on recent progress in zeolite membranes, followed by a discussion of our research activities. 

Table 1 Zeolites, zeotypes, and mesoporous moleeular sieves, selected examples, and their characteristic features...

Partide-Size Anal is- Particle-size distribution is normaUy measured by conducting a screen analysis" (sieve analysis) of a representative sample on 20-cm-diameter sieves. The sample size recommended for testir on these sieves varies according to the size range of the material. The sieves selected for use are stacked, with sieve apertures increasing in size from bottom to top in the stack. The sample is placed on the uppermost sieve of the stack, and the stack is placed on... 

By these means it is possible to prepare carbon sieves with effective micropore diameters ranging from about 4 to 9 A. The micropore size distribution of such sieves is much narrower than in a typical activated carbon and the porosity and therefore the adsorptive capacity are generally very much smaller, as may be seen from Figure 1.2. The ability to modify the effective pore size by adjusting the conditions of the manufacturing process makes it relatively easy to tailor a carbon sieve to achieve a particular separation. However, it is difficult to achieve absolute reproducibility between different batches, and the existence of a distribution of pore size, even if narrow, means that the molecular sieving selectivity of a carbon sieve seldom approaches the almost perfect separation achievable under fav orable circumstances with a zeolite sieve. Nevertheless, the kinetic selectivities which may be attained with a well-prepared carbon sieve are remarkably high. 

Porous Microporous Zeolites, silica, Vycor May act as molecular sieve, selective in several mixtures... 

The pores Geometry of windows Selectivity Molecular sieves Selectivity of reactants Selectivity of products Selectivity of isomers Separation... 

A number of application areas of MOFs in catalysis proposed on the basis of the elaborated synthetic principles are being successfully developed at present they include the heterogenization of the conventional homogeneous catalysts [80] stabilization in MOF of catalytically active nanosized particles, which are unstable otherwise [80] encapsulation of catalysts in the molecular framework [81] the combination of catalysis with chemical separation [82], postsynthesis introduction of catalytic metal sites [9,83,84] and catalysis with molecular sieve selectivity [55,80,83,85]. 

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