EXTRAFRAMEWORK SPECIES

There are two ways to introduce heteroatoms into zeolites. One is the introduction of heteroatoms as extraframework species and the other as framework species. 

The other way to introduce heterometals is their isomorphous substitution for Si in the framework, in a similar manner to the isomorphous substitution of Al. The heteroatoms should be tetrahedral (T) atoms. In hydrothermal synthesis, the type and amount of T atom, other than Si, that may be incorporated into the zeolite framework are restricted due to solubility and specific chemical behavior of the T-atom precursors in the synthesis mixture. Breck has reviewed the early literature where Ga, P and Ge ions were potentially incorporated into a few zeolite structures via a primary synthesis route [9]. However, until the late 1970s, exchangeable cations and other extraframework species had been the primary focus of researchers. 

A large variety of extraframework species, cationic or neutral, monomeric to polymeric were identified in dealuminated samples9 Al3+, A10+, Al(OH)2+, Al(OH)2+,... 

Most data agree that when the A1 content is relatively low, the amount of Bronsted sites in zeolites actually strictly depends on A1 concentrahon, according to theory. The ratio between catalytically active sites and A1 ions ranges apparently from 80 to 100% for highly siliceous extraframework-species-free zeolites. 

Zeolite catalysts are frequently applied after treatments that tend to increase their stability and also to further enhance surface acidity and shape selectivity effects. These treatments, such as steam dealumination, can cause a decrease in the framework A1 content and the release of aluminum-containing species from the framework. This can contribute to the stability of the framework, but extraframework species can also contain additional catalyticaUy active acid sites. These particles can also narrow the size of the zeolite charmels or of their mouths, so improving the shape selectivity effects. Extra-framework material (EF) can also... 

Hydrothermally dealuminated PER and sample that were subsequently acid treated exhibited better selectivities for isobutylene formation than an untreated PER catalyst (27). Furthermore, hydrothermally dealuminated PER exhibited a lower activity than untreated PER but higher selectivity for isobutylene 30,62,66). A subsequent acid treatment (with 5% HCl solution) further decreased the conversion and increased the isobutylene selectivity. The hydrothermal treatment created mesoporosity by A1 extraction. The A1 extraframework species were located in the mesopores and/or in the micropores. The HCl treatment removed part of the extraframework Al, leaving part in the micropores. The elimination of extraframework A1 from the mesopores was evidently beneficial for isobutylene selectivity. Evidently, the active sites associated with extraframework Al located in large voids are nonselective in contrast, extraframework Al located in the micropores (and not removed by acid treatment) does not contribute to catalytic activity. The steamed and acid-washed ferrierite exhibits excellent isobutylene selectivity and catalytic stability 30). 

The structures and properties of heteroatom zeolites vary due to the introduction of heteroatoms in the zeolite frameworks. When treated in high-temperature water vapor, zeolites Y, mordenite, and ZSM-5 are dealuminated to be ultra-stabilized, whereas heteroatom zeolites undergo demetallation that is, the heteroatoms are removed from the frameworks to form extraframework species, and the resulting zeolites become catalytically active for special reactions. For instance, when treated in water vapor at... 

The inorganic support taken as model is an ultrastable Y zeolite (USY) prepared by steam calcination at 1023 K from an ammonium exchanged NaY (SK40 Union Carbide), followed by treatment with a 1 N citric acid solution at 333 K for 30 min for removing extraframework species. After this, the zeolite was thoroughly washed and dried at 403 K for 6... 

The main feature of a zeolite structure is its framework type, which describes the arrangement of the cages, the dimensionality of the channel system and the approximate size of the pore openings. A few framework types, selected for their industrial relevance and/or to illustrate some of the more common structural nomenclature, have been presented. However, there are many more, and for more information about a specific framework type, the reader is referred to the relevant references in the Atlas and the Collection. To fully understand the properties of a real zeolitic material though, not only the framework type, but also the composition and true geometry of the framework, the location and nature of the extraframework species, and the number and type of defects must be investigated. 

The complete absence of the band in the range of 300-450 nm in Fe-silicalite (873 K) indicates that the majority of Fe species in this catalyst is uniform and well isolated. Previous characterization showed that a fraction of iron in this sample remains in the framework, but this fraction is small compared to the extraframework species. The absence of iron clustering in Fe-silicalite (873 K) has been corroborated by voltammetry and HRTEM, where no iron oxide nanoparticles were observed. Increasing the steam-... 

The preparation method of iron-zeolites has been recomized as critical in order to obtain reproducible catalysts with a desired performance." A distribution of iron species is normally obtained upon activation of catalysts by available methods. Suppressing clustering of iron species into iron oxide is convenient, since these species are proven inactive at low temperatures in the various reactions catalyzed by Fe-zeolites. " Steam activation of isomorphously substituted FeMFI zeolites enables a certain control of the degree of iron clustering, and thus on the relative amount of certain species in the final catalyst, as compared toother methods. A rather unique achievement has been attained here with Fe-silicalite (873 K), in view of the remarkable uniform nature of extraframework species in isolated positions. A minor association of iron species is present... 

This study aims to provide a fast method to evaluate the iron incorporation degree in the walls of iron-containing mesoporous materials. Both Fe-MCM-41 systems and Fe+Al-MCM-41 systems were prepared with CieTMABr as surfactant, in hydrothermal conditions. The colour of the iron containing MCM-41 samples is determined just by coloured chromophores and, taking into account that the intratetrahedral Fe04 is not, it can be established a connection between colour, respectively luminescence (L), and the concentration of iron chromophores, in fact the extraframework species. An incorporation degree of iron into MCM-41 walls and additionally, an extraframework iron percent would be evaluated by this method. 

Samples No. 2-6 were almost spherically shaped. The formulae of samples No. 1 and 2 are idealized, i.e., both samples possess significant amounts of Al-containing extraframework species ( true Lewis sites)... 

Synthesis of molecular sieves with T sites isomorphously substituted by other elements is frequently accompanied by the formation of more or less oxidic extraframework species as by-products (see Sect. 4.1.2.2). In this section the intended preparation of finely divided oxides or oxide clusters within the voids of the framework will be discussed. 

Lewis acidity of zeolites to extraframework species, neglecting the evidence of their presence also at the external surface of the framework. 

The decrease of the total adsorption capacity of steamed HY zeolites with respect to that of the parent zeolite is due to the obstruction of the secondary network by the aluminic species created during the extraction of the framework aluminium atoms. An acid leaching solubilizes the extraframework species and so on increases widely the adsorption capacity. Electronic microscopy and nitrogen adsorption are used to study quantitatively the formation of the secondary network and to characterize the relative extent of the steps of calcination and acid leaching (ref. 9). 

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