Pentasil zeolites frameworks

Titanium in the framework of pentasil zeolites induces oxygenation activity with diluted hydrogen peroxide as oxidant, thus constituting a new catalytic system. 

Zeolites can be hydrophilic or hydrophobic due to the different Si/Al ratios within the zeolite framework. Organic molecules rely on H-bonding, electrostatic, and 77-cation interactions for effective zeolite absorption, and these interactions will clearly be influenced by the number of cation sites present. As expected, the more Si present, the more hydrophobic the zeolite and, therefore, the greater the ability of these materials to interact with hydrophobic organic molecules or to exclude hydrophilic molecules, such as water. Zeolites X/Y have a Si/Al content at or close to 1 and are highly hydrophilic absorbants. Pentasil zeolite ZSM-5, which... 

The effect of zeolites on the photochemical reaction pathway of organic molecules has been studied recently. Turro et al. (40, 41) have shown that the photochemistry of methyl benzyl ketones (ACOB) in the presence of pentasil zeolites follows a different pathway depending on the location of adsorbed ketones. Figure 5 illustrates the photochemical reaction pathways in the gas phase and on ZSM-5. Para-ACOB is readily adsorbed by the pentasil framework, so that the radicals formed upon... 

Table VI. Concentrations of hydroxyl species and framework aluminium atoms in the hydrogen (H) or sodium (Na) form of pentasil zeolites synthesized with different organic templates and without organic templates...

Figure 2.5 Some chains that occur in several zeolite framework types, (a) Double zig-zag chain (b) double sawtooth chain (c) double crankshaft chain (d) narsarsukite chain (e) Pentasil chain. Reproduced with permission from [15]. Copyright (2001) Elsevier...

A number of structural features (cages, channels, chains, sheets) arc common to several different zeolite framework types, so designations such as a-cavity and (i-cage, pentasil unit, crankshaft and double crankshaft chain, and 4.82 sheet or net have crept into common usage. To help the reader, some of these subunits are shown in Figs. 1, 2 and 3. In these drawings, oxygen atoms have been omitted for clarity. 

The framework of terranovaite (Fig. 1), characterized by a pentasil chain, can be described by the interconnection of the polyhedral subunits 4 6, 4 5 and 5 6. The 4 6 unit has been found in laumontite and boggsite the 4 5 unit has been found in brewsterite, heulandite group zeolites and in synthetic SSZ-23 and SSZ-33 the 5 6 unit has been found in gottardiite, boggsite and in synthetic EU-1. The net of terranovaite projected onto the be plane (Fig. 1) is equivalent to that of many other pentasil zeolites (ferrierite, boggsite, ZSM-5, ZSM-11, theta-1), while the net projected onto the ab plane is equivalent to that of AIPO4-41 [9]. 

These cyclization reactions are suppressed if a phosphorus-modified B-pentasil zeolite is used, as shown by comparison with the conventional silica-based catalyst. Only the silica carrier is replaced by a zeolitic carrier. The amount of cyclic compounds is reduced from 6.7 mol % to 1.4 mol %, and the yield of the desired product adipodinitrile is about 10 % higher [20]. This is evidently due to the restricted transition state shape selectivity of the pentasil zeolite the steric constrains of the zeolite framework prevent the formation of the transition state of the cyclic compounds. The reactions on the outer surface are responsible for the 1,4 mol-% yield of cyclic compounds. 

FIGURE 1.9. Schematic diagram of one layer of a pentasil zeolite structure showing how the framework is built up from the D5R units. (From ref. 27, reprinted with permission.)... 

As for Ti-containing molecular sieves, the incorporation of V has been mainly attempted in pentasil-type frameworks (MFI and MFI/MEL), while little information is available about the possibility to synthesize vanadosilicates with the framework topology of other zeolites. Table 6 summarizes the data reported in the literature. Hydrothermal synthesis is the route used in most cases. 

Zeolite ZSM-5, as a member of the family of pentasil zeolites, has aroused tremendous interest after its first discovery by the research group of Mobile Company in the year 1972 [1]. With its adjustable framework A1 content (from 0 to about 8A1 per unit cell), two dimensional micropore channels (0.55 nm x 0.54 nm Fig. la), sinusoidal pore geometry along c axis (Fig.lb) and easy insertion of hetero-T atoms, this material plays an important role in many of crucial catalytic processes such as hydro-cracking, de-waxing, alkylation, etc., [2-5] as well as in separation of organic compounds with different sizes and shapes [6]. In the case when zeolite ZSM-5 was used as catalyst, most of reactions are diffusion-controlled [7]. This means that the product distribution largely depend on the nature and location of active sites in the crystalline framework of catalyst. Thus, the increase of the... 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

One coordination of bare Mn2+ ions was reported by Kevan et al. in (A1)MCM-41 [4], Because Co2+ spectra of type a and (3 are very different from the spectrum of tetrahedral Co2+ ion, none Co2+ ions were incorporated into framework position. Thus, the discrepancy in the number of reported cationic sites in Ref. [4] and in this work should reflect different metal loading in molecular sieve or differences in its chemical composition (Si/Al ratio). As it was already mentioned, population of transition metals in individa) cationic sites depends on the metal loading. The effect of the Si/Al ratio was not studied for MCM-41 matrix, but is well known for pentasil containing zeolites [1]. 

It is necessary to mention that observed VIS spectra of the Co those reported for pentasil containing zeolites (with deformed six-member rings present in the framework) and different from the spectra of the Co2+ ions located in A, X and Y zeolites [1,5,8,9], It indicates that deformed six-member rings are present in the structure of MCM-41, but the confirmation of this suggestion requires further detailed study. 

Most industrial shape selective catalytic processes today use medium-pore zeolites from the "pentasil" femily. (The name refers to the five-membered rings in their framework and to their high silicon content.) ZSM-5 is by far the most important member of this family. It has high acid catalytic activity and it is very stable The silica/alumina ratio in ZSM-S varies from the teens to the thousands. High silica/alumina ratios give hydrophobidty, high acid strength, and thermal, hydrothermal, and acid stability. 

Aluminum-27 NMR spectra show that after crystallization, all the TOA-containing zeolites exhibit a well resolved resonance at <50 ppm, corresponding to framework Al(IV) atoms. However, following NH.- exchange and calcination in air at 500°C, a new band appears at abdut 0 ppm due to Al(VI) resulting from aluminum removed from the crystal lattice. In general, calculated Si/Al ratio from silicon-29 NMR data are in reasonable agreement with chemical analysis results. Thus, all the aluminum atoms in these siliceous mordenite and Al-rich pentasils are believed to be in tetrahedral coordination and incorporated into the zeolite lattice. 

For analysis of the state of iron were employed EPR, FTTR, and Mossbauer spectroscopies. For structural interpretation of these results the concept of divalent transition metal cation siting was used as recently established for pentasil ring zeolites in a wide range of metal concentrations and Si/Al compositions. With help of UV-Vis and FTIR this approach evidenced three zeolite coordination of divalent cations in similar six-membered rings of framework local structures. Three cationic fiiamework sites were thus suggested, denoted as a, p and y. (For details see [7-11]). 

Nowadays, the term zeolite includes all microporous solids based on silica and exhibiting crystalline walls, as well as materials where a fraction of Si atoms has been substituted by another element, T, such as a trivalent (T = Al, Fe, B, Ga,. ..) or a tctravalent (T = Ti, Ge,...) metal. Crystalline microporous phosphates are known as zeotypes or as related microporous solids (14, 54). At present, there are 179 confirmed zeoHtc framework types. For the structure types, three-letter codes are used, which were adopted from the name of the first material reported with a specific stmcturc. As an example, FAU is given for the structure of faujasite and its synthetic equivalents X and Y, and MFl for the stracture of ZSM-5 or silicalite-1 (105). Figure 9.11 shows prominent examples of zeolite firameworks, for example, FAU, LTA, and MFI types (pentasil). 

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