Direct hydrothermal synthetic routes

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). 

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