Material and shape selection

Chapter 13 Design Material and shape selection 389 Table I 3.2 Analogues between beam torsion, soap bubbles and two-dimensional steady heat flow... 

In view of this wide diversity of interest, it is pertinent to ask whether such a broadly based symposium is likely to be useful from a scientific standpoint. The first lUPAC Symposium on the Characterization of Porous Solids was held in 1987 (i.e. COPS I, Elsevier, 1988) and it was evident then that there was a need for further systematic work. In his introductory paper, Everett drew attention to some of the outstanding problems including the important requirement of predicting technological performance from the results of characterization measurements. This became all the more urgent with the development of advanced materials and shape selective catalysts which require the application of sophisticated characterization techniques. 

If L is a charac teristic dimension of a ciystal of selected material and shape, the rate of growth of a ciystal face that is perpendicular to L is, by definition,... 

Zeolites ZSM-5 and ZSM-11 are the most commercially important end-members of a continuous series of intermediate structures belonging to the so-called pentasil family (4,5). The first preparation of ZSM-5 was described in 1972 (6) and since thbn, a number of elaborate synthesis recipes have been reported in the patent literature. Because of the unique and fascinating activity and (shape) selectivity of this material for a variety of catalytic reactions currently processed in chemical industries, increasing attention has been devoted to a better understanding of the various mechanisms that govern the synthesis of ZSM-5 (7-33). 

Owing to the possibility of tuning (1) their acidic and basic properties, (2) their surface hydrophilicity, and (3) their adsorption and shape-selectivity properties, catalytic activity of zeolites was investigated in the production of HMF from carbohydrates. Whatever the hexose used as starting material, acidic pillared montmorillonites and faujasite were poorly selective towards HMF, yielding levu-linic and formic acids as the main products [81-83]. 

Since the last review by Venuto in 1968,[1] there has been a continuous interest in the application of microporous and mesoporous materials as catalysts in the synthesis of bulk and fine chemicals.[211 Indeed, their acidic and basic properties can be combined with their structural properties in order to take advantage of their adsorption and shape selectivity properties, the latter being an advantageous feature of zeolites compared with other heterogeneous catalysts. Another important aspect... 

Our work on the alkylation of meta-diisopropylbenzene with propene over the acid form of various 12-member ring zeolites and molecular sieves shows that these catalysts can reveal shape selective behavior (39). As the effective size of the voids increases, the ratio of the formed 1,3,5- to 1,2,4-triisopropylbenzene increases e.g., mordenite and zeolite Y give 1.1 and 2.5, respectively. Additionally, an amorphous Si02/Al203 catalyst yields a ratio of 3.5. Thus, the smaller 12-ring materials show shape selective behavior. Based on these results, extra-large pore materials such as VPI-5 may show some shape selectivity for this reaction, if acid sites can be incorporated into the material. 

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. 

The pore structure, such as pore geometry and pore size, of mesoporous materials is one of the key parameters for practical applications, especially those dependent on size and shape selectivity and ready access to porosity. Considerable efforts have been devoted to control the pore structure of mesoporous materials. 

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. 

Zeolite catalysts are frequently applied after treatments that tend to increase their stability and also to further enhance surface acidity and shape selectivity effects. These treatments, such as steam dealumination, can cause a decrease in the framework A1 content and the release of aluminum-containing species from the framework. This can contribute to the stability of the framework, but extraframework species can also contain additional catalyticaUy active acid sites. These particles can also narrow the size of the zeolite charmels or of their mouths, so improving the shape selectivity effects. Extra-framework material (EF) can also... 

In addition to the importance of the M41S materials for size- and shape-selective applications, these materials have been also regarded as a suitable mesoporous model adsorbent for testing theoretical predictions of pore condensation. Pore condensation represents a first order phase transition from a gas-like state to a liquid-like state of a pore fluid in presence of a bulk fluid reservoir, which occurs at a pressure p less than the saturation pressure po at gas-liquid coexistence of the bulk fluid [6,7]. In this sense pore condensation can be regarded as a shifted gas-liquid bulk phase transition due to confinement of a fluid to a pore. Recent work has shown that in fact the complete phase diagram of the confined fluid is shifted to lower temperature and higher mean density as compared with the bulk coexistence curve [e.g., 8,9]. 

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