Zeolite synthesis windows

Figure 7.5. Compositional synthesis windows for the Na20-Al203-Si02-H20 system at 100°C and 90-98 mol% H2O. Source of Si02 is (a) sodium silicate and (b) colloidal silica. The area enclosing a letter represents the composition that yields the corresponding phase, while the + marks the typical composition of the product. A, X, and Y = zeolites types A, X, and Y B = zeolite P R = chabazite S = gmelinite and HS = hydroxysodalite (from Breck and Flanigen, 1968, with permission).

Synthesis of zeolite Y in the presence of Gd(III) complexes of 18-crown-6 resulted not only in the encapsulation of the complex but the complex also served as a template for EMT polytype zeolite Y (Fig. 22b) (86). Feijen et al. described how the two different polytypes (the cubic FAU and the hexagonal EMT) can be formed (87). In the absence of an organic template, the FAU structure will form. If Na" "-18-crown-6 is present, it can be absorbed on the surface of the growing zeolite layer. This will influence the interconnection of the layers and, therefore, in the presence of this crown ether, the formation of the EMT framework may be favored. The difference between the pore window sizes is that in the EMT there are two different types 7.3 x 7.3 A in the [001] direction and 7.5 x 6.5 A perpendicular to the [001] direction. (The FAU has pore windows with 7.4 x 7.4 A in the [111] direction.)... 

Of the zeolitic materials, AlPO s cut a conspicuous figure because of their structural diversity and the incorporation of other elements into their frameworks. The recently developed VPI-5 (refs. 2, 3) announced the feasibility of synthesis of micoporous structures with windows comprising rings of over 12-T. All AlPO s, SAPO s and MAPO s form a family of microporous structures constructed by or essentially by A1(I) and P(V). Some of them are isostructural with zeolites but a majority have novel structures. The primary building units (PBU) centred by P(V) are invariantly PO4 whereas those centred by A1(I) are AlO in most cases and AIO5 or even AlOs in a few cases. So far all AlPO s, SAPO s and MAPO s have been synthesized exclusively in the presence of amines or... 

Mesoporous silicas can possess intercage windows within the microporous region, but their stability and acid strength are low compared to those of zeolites. One current approach to this particular problem is the use of partially crystallised zeolite nuclei as starting materials in the synthesis of mesoporous solids, in an attempt to prepare a solid with structural features of both microporous and mesoporous solids. Such solids are said to possess hierarchical porosity, and are discussed further in Chapter 10. 

One of the most elegant synthetic approaches to using the internal cavities of zeolites as nanometre size reactors is that of the ship-in-a-bottle synthesis of metal complexes within zeolite cages, which are then too large to escape through the cage windows. The term was initially coined by Herron to describe metal complexes, such as those with salen-(bis(salicylidene)ethylendiamine-) " or phthalocyanine (Scheme 6.9) that were formed in the supercages of faujasitic zeolites. Zeolites X and Y are most commonly used, but the fully... 

In ship-in-a-bottle syntheses, metal ions, metal complexes, or small metal clusters in zeolite cages can be converted in reactions with CO, or with CO + H2 or CO + H2O, to give (larger) metal carbonyl clusters that fit within the cages but are too large to pass through the pore windows. [3-5, 30] Similar reactions for metal cluster synthesis takes place in solution [31, 32] and on surfaces [33-37]. They are called reductive carbonylation reactions and are discussed below. 

Figure 4-8. Synthesis of iridium carbonyl clusters in neutral solutions and on the nearly neutral surface of amorphous y-AljOs. The chemistry is very similar to that occurring in the cages of NaY zeolite (Fig. 4-7). [3, 5] Whereas the clusters can be readily extracted from the surface of y-AljOj, under the same conditions they cannot be extracted from the zeolite because they are too large to fit through the cage windows and are thus trapped in the supercages.

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