Microporous molecular sieves, formation

To better understand why and how microporous molecular sieves (aluminosilicates and aluminophosphates) are highly suitable catalysis for the skeletal isomerization of n-butenes, it is important to discuss the mechanisms of the reactions that control the formation of the desired isobutylene and to evaluate the relative importance of secondary reactions that may... 

The main difference between the synthesis of MCM-41 mesoporous material and traditional synthesis of zeolite or silica molecular sieve is the use of different templates. An individual organic molecule or metal cation is used for the traditional synthesis of silica microporous molecular sieve. For example, the typical template for ZSM-5 synthesis is tetrapropylammonium ion the crystal is formed through the condensation of silicate species around the template molecule, while for the formation of MCM-41, the typical template is the assembly of large molecules containing one hydrophobic chain with more than 10 carbons. 

Figure 9.7 Simplified representation of the formation of microporous molecular sieves using a small single molecule organic stmcture-directing agent. (Adapted and reprinted with permission from T. J. Barton et al, Chem. Mater. 1999, 11, 2633. Copyright 1999 American Chemical Society.)...

Series of Special Issues cover various aspects of microporous molecular sieves, metal-organic frameworks and ordered mesoporous materials. Synthesis principles, templating, formation mechanisms, characterization methods, functionalization strategies, and applications are discussed in excellent and comprehensive review articles and more specific research reports. 

There are, however, two limitations associated with preparation and application of zeolite based catalysts. First, hydrothermal syntheses Umit the extent to which zeolites can be tailored with respect to intended appUcation. Many recipes involving metals that are interesting in terms of catalysis lead to disruption of the balance needed for template-directed pore formation rather than phase separation that produces macroscopic domains of zeoUte and metal oxide without incorporating the metal into the zeohte. When this happens, the benefits of catalysis in confined chambers are lost. Second, hydrothermal synthesis of zeoHtic, silicate based soHds is also currently Hmited to microporous materials. While the wonderfully useful molecular sieving abihty is derived precisely from this property, it also Hmits the sizes of substrates that can access catalyst sites as weU as mass transfer rates of substrates and products to and from internal active sites. 

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

PORE. I A minute cavity in epidermal tissue as in skin, leaves, or leather, having a capillary channel to the surface that permits transport of water vapor from within outward but not the reverse. 2. A void of interstice between particles of a solid such as sand minerals or powdered metals, that permits passage of liquids or gases through the material in either direction. I11 some structures, such as gaseous diffusion barriers and molecular sieves, the pores ate of molecular dimensions, i.e 4-10 A units. Such microporous structures are useful for filtration and molecular separation purposes in various industrial operations. 3. A cell in a spongy structure made by gas formation (foamed plastic) that absorbs water on immersion but releases it when stressed. 

Whereas the acetylation of phenyl ethers over zeolite catalysts leads to the desired products, acetylation of 2-MN occurs generally at the very activated C-l position with formation of l-acetyl-2-methoxynaphthalene (l-AMN). A selectivity for l-AMN close to 100% can be obtained over silicoaluminate MCM-41 mesoporous molecular sieves[22] and FAU zeolites,133 341 whereas with other large pore zeolites with smaller pore size (BEA, MTW, ITQ-7), 2-AMN (and a small amount of l-acetyl-7-methoxynaphthalene, 3-AMN) also appears as a primary product. Average pore size zeolites, such as MFI, are much less active than large pore zeolites. These differences were related to shape selectivity effects and a great deal of research work was carried out over BEA zeolites in order to specify the origin of this shape selectivity the difference is either in the location for the formation of the bulkier (l-AMN) and linear (2-AMN) isomers (only on the outer surface for l-AMN, preferentially within the micropores for 2-AMN)[19 21 24 28 381 or more simply in the rates of desorption from the zeolite micropores.126 32 33 351... 

Bein and coworkers investigated the formation of poly(methyl methacrylate) (PMMA) through polymerization of methyl methacrylate (MMA) in channels of microporous crystals including Na-Y, mordenite, zeolite / , and ZSM-5, and mesoporous molecular sieves such as MCM-41 and MCM-48.[83] MMA in zeolites can also polymerize as acrylonitrile does, and with an increase in the host channel size the polymerization degree is enhanced. Electron microscopic observation indicates that the polymerization reaction proceeds mainly inside the zeolite channels, because almost no polymers are observable on the external surface of the zeolite particles. This is further confirmed by the fact that the polymer/zeolite composite lacks the characteristic glass transition temperature for a bulk polymer. 

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