High-silica material, direct synthesis

Microporous materials are typified by natural and synthetic zeolites that are crystalline 3D aluminosilicates with open channels or cages. Synthetic and structural concepts of zeolites have to a large extent shaped the development of microporous materials during the past 50 years. For example, the use of organic structure-directing agents in the synthesis of high-silica zeolites and their all-silica polymorphs contributed to... 

One-step direct synthesis of the highly stable mesoporous silica-based material is also possible. MMS-H[209] has a structure analogous to that of MCM-48 but which contains zeolite building units. A mixture of CTAB and Brij30 was used as template for the mesopores. The use of TPAOH without the assistance of NaOH helps to introduce zeolite secondary building units, as well as the direct formation of acidity after removal of the template. This material was also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities. 

Pure silica end-members may be considered as special cases of aluminosilicate zeolites. They may be prepared directly from hydrothermal synthesis and in some cases from aluminosilicates by post-synthetic treatment. For example, the pure silica analogue of ZSM-5 (Silicalite-1) is readily prepared by direct synthesis, whereas purely siliceous zeolite Y can only be obtained by postsynthetic treatment (Chapter 6). The microstructures present in these solids depend on the way in which they are prepared. For direct preparation routes the presence or absence of fluoride as a mineraliser in the preparation (see Chapter 5) determines whether the framework is prepared defect-free or with high concentrations of terminal silanol (SiOH) hydroxyls, where silicon is attached to three bridging oxygen atoms and a hydroxyl group. Post-crystallisation preparation of pure silica zeolites can be achieved by treatment of appropriate starting materials with silicon tetrachloride or by removal of aluminium from the aluminosilicate framework by heating the ammonium form in steam (Chapter 6). 

The dimensions and accessibility of pores of zeohtes and microporous solids are confined to the subnanometer scale (<1.5 run), which hmits their applications when processing bulky molecules. Mesoporous materials with pore sizes ranging from 2 to 50 nm overcome these limitations. In contrast with microporous zeolites, these materials lack atomic ordering (crystallinity) in their silica walls as these are usually amorphous. The attractive properties of ordered mesoporous materials include well-defined pore system high surface area and pore sizes narrow pore size distribution tunable up to 100 nm existence of micropores in the amorphous wall (for thicker wall materials) existence of various wall (framework) compositions obtained from direct synthesis, or posttreatment or modification high thermal and hydrothermal stabilities if properly prepared or treated and various controllable regular morphologies on different scales from nanometers to micrometers. 

Probably the first report on the use of a metaUocene in zeolite synthesis dates back more than ten years [124]. In the past few years, however, essentially two groups reported on the influence (i.e. structure directing properties) of cobalt metallocene compounds in the synthesis of zeolites and zeolite-like materials [e.g. 125-128]. In none of these studies had a new framework topology been synthesized. However, very recently Balkus and coworkers reported the synthesis of the first fourteen-membered ring zeolite, which they called UTD-1 (University of Texas at Dallas No. 1) [129-132]. UTD-1 was synthesized with a quite unusual template, viz. with bis(pentamethyl-cyclopentadienyl)cobalt(III) hydroxide [129-132]. Both the essentially pure-silica version and high-silica derivatives have been prepared. A typical synthesis of the all-silica version com-... 

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