Microporous crystalline materials preparation

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

As the shortcomings of the traditional preparative methods outlined above became apparent, it was realized that alternative procedures were required to produce uniform or tailor-made adsorbents and shape-selective catalysts. As we saw in Chapter 11, one major route was opened up by the Linde synthesis in 1956 of the crystalline molecular sieve zeolite A. The search for new microporous crystalline materials has continued unremittingly and has resulted in the synthesis of novel zeolitic structures including the aluminophosphates, which are featured in this chapter. 

Even though crystalline microporous materials include those with pore size between 10 and 20 A (called extra-large pore materials), few of them have a pore size within this range. This limits the applications of microporous materials to small molecules. There has always been a desire to increase the pore size of a crystalline material to more than 10 A while maintaining adequate thermal or hydrothermal stability required for various applications. Recent advances in chalcogenide and metal-organic framework materials have shown much promise for the preparation of extra-large pore materials. 

More promising for reactive separations involving gas phase reactions appears to be the development and use in such applications of microporous zeolite and carbon molecular sieve (Itoh and Haraya [2.25] Strano and Foley [2.26]) membranes. Zeolites are crystalline microporous aluminosilicate materials, with a regular three-dimensional pore structure, which are relatively stable to high temperatures, and are currently used as catalysts or catalyst supports for a number of high temperature reactions. One of the earliest mentions of the preparation of zeolite membranes is by Mobil workers (Haag and Tsikoyiannis... 

A series of ZSM-5 zeolites synthesized using carbon black Black Pearls 2000 as secondary template was compared with ZSM-5 prepared by confined space synthesis and carbon-free ZSM-5. By secondary templating approach it is possible to prepare highly crystalline materials with mesopore diameter of 12 ran corresponding to the size of carbon black particles. Mesopore volume and mesopore surface area of obtained materials increase with increasing amount of carbon black in the reaction mixture. Observed decrease in micropore volume can be attributed to the increasing amount of Si04 tetrahedra on the mesopore surface. In the case of the sample prepared by eonfined space... 

Since many properties of crystalline oxides, e.g., acidity, hydrothermal stability, etc., are the essential features exploited in commercial applications of these oxides, it is not unexpected that the ordered, mesoporous materials have not yet found much commerical use. The large void volumes, pore sizes and surface areas of the ordered, mesoporous materials provide advantages over microporous solids in certain areas of application but issues such as stability remain. Thus, if crystalline, extra-large pore solids could be prepared in the pore size and void volume ranges of the mesoporous materials, they would be immediately commercialized. The question remains as to why crystalline materials of this size range have not been synthesized. Navrotsky et al. have shown that pure silica, ordered, mesoporous silicas are energetically very close to pure silica, crystalline... 

However, the preparation of latex particles may be perceived as having reached a level at which the potential for a fundamental breakthrough in the final materials per se is rather limited. Pioneering efforts may instead be expected in the development of polymeric microcompartmentalized materials. This development, in a limited form, may be exemplified by the work of Gan and colleagues [28], who polymerized organic monomers solubilized in bicontinuous microemulsions and obtained microporous organic polymers. This area is, of course, of future interest, but the problem of lack of correlation between the microemulsion colloidal structure and the microstructure of the final material may result in a focus on the polymerization of liquid crystalline material where even complex systems [29,30] have been shown to retain their microstructure after polymerization. This area of polymerization has been further developed and systematized by Antonietti [31,32], Antonietti et al. [33], and Fendler [34]. 

Schematic representation of the pore size ranges of microporous and mesoporous solids. Whereas crystalline materials have well-defined pore dimensions, those solids without atomic order on the long range (particularly the mesoporous solids) can be prepared with pore sizes that can vary widely, depending on synthetic and post-synthetic treatment conditions.

Irrespectively of the iron content, the applied synthesis procedure yielded highly crystalline microporous products i.e. the Fe-ZSM-22 zeolite. No contamination with other microporous phases or unreacted amorphous material was detected. The SEM analysis revealed that size and morphology of the crystals depended on the Si/Fe ratio. The ZSM-22 samples poor in Fe (Si/Fe=150) consisted of rice-like isolated crystals up to 5 p. On the other hand the preparation with a high iron content (Fe=27, 36) consisted of agglomerates of very small (<0.5 p) poorly defined crystals. The incorporation of Fe3+ into the framework positions was confirmed by XRD - an increase of the unit cell parameters with the increase in the number of the Fe atoms introduced into the framework was observed, and by IR - the Si-OH-Fe band at 3620 cm 1 appeared in the spectra of activated Fe-TON samples. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

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