Zeolite generation

Extra framework A1 in HY-type zeolites (generated during calcination and steaming) could directly react with VO+2 cations and form stable complexes thus increasing the crystal tolerance to V. 

Recently, we reported [5] on the calculation of the long-term stability of high-alumina zeolites in gas-desulphurization and gas-drying processes. The present contribution describes the chemical behaviour of different aluminosilicates under hydrothermal conditions and, especially, the prediction of the long-term stability of high-silica zeolites generated by the dealu-mination of faujasites. 

Pore size is not the only parameter that affects adsorption. Molecules containing polar groups such as -OH, >CO, -NH2 and polarizable groups such as >CO=CO< and C6H5- strongly interact with the surface of zeolites the cations in the zeolites generate... 

A particularly important reaction in zeolites generates acidic OH groups attached to framework cations which are directly responsible for acidic behaviour. Their acidic features include the nature of the sites, their strength and distribution in strength, their concentration and are characterized by many methods (9) such as IR spectroscopy, TPD, NMR, microcalorimetry, Hammett s indicators, etc. 

For silica hinder, Choudhary and his co-workers[1,2] showed in the case of H-gallosilicate that inter-crystalline and intra-crystalline acidity decreased appreciably, resulting in a decrease in total acidity. As in the case of alumina binder, Sousa-Aguiar et al.[7] found a similar interaction of silica binder with ultra-stable Y zeolite, generating an acidic silica-alumina compound. 

In order to take benefit of the strong geometrical constraints of the zeolite voids, so as to improve the basic properties, basic moities insertion inside the pores was achieved by several authors [2-4,9,12,13]. The direct incorporation of alkali hydroxide during their synthesis by treatment of aqueous or methanolic solution [13] can lead to loss of crystallinity by Al-0 solvolysis. This phenomena can be avoided by using a mild method consisting of a thermal decomposition of neutral alkali salts previously occluded inside the zeolite void cavities [9]. In this respect. Martens et al. prepared supported sodium clusters on zeolites generated by impregnation of sodium azide followed by its thermal decomposition [2-4]. 

It is well known, e.g. by the work of Sachtler et al., that ion-exchange of noble metals into zeolites generates Bronsted acidity via two routes ... 

Figure B3.3.14. Template molecule in a zeolite cage. The CFIA stmcture (periodic in the calculation but only a fragment shown here) is drawn by omitting the oxygens which are positioned approximately halfway along the lines shown coimecting the tetrahedral silicon atoms. The molecule shown is 4-piperidinopiperidine, which was generated from the dicyclohexane motif suggested by computer. Thanks are due to D W Lewis and C R A Catlow for this figure. For fiirther details see [225].

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Our own earlier work on the chlorination of toluene had been subject to similar constraints. In this case, chlorination with ferf-butyl hypochlorite had proved to be advantageous. In the presence of silica gel as catalyst the yield of chlorotoluenes was quantitative but the regioselectivity was more or less statistical (ref. 8). However, the use of proton-exchanged zeolite X allowed the production of chlorotoluenes with a para-selectivity of more than 90 % (Fig. 4) (ref. 9). No HCl is generated in this process since the by-product is tert-butanol, and there is no inhibition of the catalyst. Indeed, the catalyst can be reused if necessary. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

It must be noted that sometimes calcination is beneficial to create active species. Notable examples are the Sn-beta speciation [176] and generation of extra-framework Al-Lewis sites in beta zeolite for organic transformations... 

Surface science studies have generated much insight into how hydrocarbons react on the surfaces of platinum single crystals. We refer to Somorjai [G.A. Somor-jai. Introduction to Surface Chemistry and Catalysis (1994), Wiley, New York] for a detailed overview. Also, the reactions of hydrocarbons on acidic sites of alumina or on zeolites have been studied in great detail [H. van Bekkum, E.M. Flanigan and J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice (1991), Elsevier, Amsterdam],... 

R=phenyl) within the zeolite framework. The Biosym docking package " was used to generate a random set of ten energetically favourable configurations... 

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