Zeolite , new

The foregoing discussion has focused on the most important commercial molecular sieves, zeolites. New directions in the preparation of framework stmctures of different chemical composition and of large-pore molecular sieves have also appeared. 

As was shown here in some examples, the field of catalysis over zeolites, although marnre, is still very much alive. The chemists who work with the synthesis zeolites continue to be very creative, the focus now being placed on the synthesis of materials that can catalyze reactions other than the acidic ones and/or reactions of bulkier molecules, that is, synthesis of zeolites with larger micropores or with a very large external surface, such as nanosize and delaminated zeolites. New concepts related to the mode of action of zeolite catalysts continue to emerge, as shown here with the shape selectivity of the external surface. These concepts are particularly useful to scientifically design selective and stable catalysts. 

How might you contribute The mechanisms of reactions that take place inside zeolites are difficult to study and little is known about them. A knowledge of the mechanisms is needed to model the nature of the active sites within zeolites and predict useful new types of zeolites. New catalysts can then be designed to create custom polymers, control pollution, and synthesize medicines. 

Bourbigot, S., Le Bras, M., Tremillon, J.-M., Breant, P., and Delobel, R. 1996. Zeolites New synergistic agents for intumescent thermoplastic formulations—Criteria for the choice of the zeolite. Fire Mater. 20 145-158. 

Modern metallosiloxane chemistry mainly deals with three types of silanols silanetriols,11 incompletely condensed silasesquioxanes,15 and tri(tert-butyl) ester of orthosilicic acid.16 Research on these compounds has contributed to the development of model compounds for zeolites, new homogeneous catalysts, and low-temperature pathways to metal—silica materials. 

The calculation methods for pore distribution in the microporous domain are still the subject of numerous disputes with various opposing schools of thought , particularly with regard to the nature of the adsorbed phase in micropores. In fact, the adsorbate-adsorbent interactions in these types of solids are such that the adsorbate no longer has the properties of the liquid phase, particularly in terms of density, rendering the capillary condensation theory and Kelvin s equation inadequate. The micropore domain (0.1 to several nm) corresponds to molecular sizes and is thus especially important for current preoccupations (zeolites, new specialised aluminas). Unfortunately, current routine techniques are insufficient to cover this domain both in terms of the accuracy of measurement (very low pressure and temperature gas-solid isotherms) and their geometrical interpretation (insufficiency of semi-empirical models such as BET, BJH, Horvath-Kawazoe, Dubinin Radushkevich. etc.). 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

It was thus necessary to move to 12-membered ring zeolites (e.g., Y, mordenite, ZSM-12, Omega, Beta and MCM-22), before obtaining good results with liquid-phase operations (Figure 2.30). Based on these zeolites, new commercial processes or... 

Engelhardt G and Michel D 1987 High Resolution Solid State NMR of Silicates and Zeolites (New York Wiley)... 

P-ll - Isodimorphism of templates in zeolites. New crystal chemistry of analcime and its analogues... 

T2 Tsisihsvili, G.V., Andronikasvili, T.G., Kirov, G.N., Filizova, L.D. Zeolites, New York Ellis... 

Onyestyak, G., Kallo, D., in Natural Zeolites 93 Occurrence, Properties, Use, Irrt. Corrf. on Natural Zeolites, New York Brockport, 1995, p. 437. 

Figure 6 Chemical shifts of the building units in aluminosilicates. Reproduced with permission of John Wiley Sons from Engelhardt G and Michel D (1987) High Resolution Solid-State NMR of Solids and Zeolites. New York John Wiley Sons.

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