Non-framework ions

With this addition, the structures of a wide variety of zeolites, both naturally occurring minerals and synthetic zeolites tailored to act as catalysts, can be modelled and then used to answer questions such as which position will the non-framework ions and molecules occupy and how do ions travel through the structure ... 

The powder XRD patterns of calcined and ion-exchanged MAPO-36 are shown in Fig.l. The XRD patterns of calcined MAPO-36 coincided with already reported data [7]. The XRD patterns of ion-exchanged MAPO-36 revealed absence of structural degradation. Further, there are no patterns corresponding to non-framework metal oxide. 

When the two-step process is repeated on the same material, the thermal treatment following the chemical dealumina t ion results in further expulsion of aluminum from the framework into zeolite cages or channels. The solubilization of non-framework aluminum during the first chemical treatment appears to facilitate further framework dealumination during the subsequent thermal treatment due to the altered steric and electrostatic parameters in the zeolite channels. The newly formed non-framework aluminum species can be readily solubilized by acid treatment. This cyclic method has allowed the almost total removal of aluminum from mordenite (5). 

Calcined and steamed FAU samples also have complex hydroxyl IR spectra. Figure 4.25 shows the difference between an ammonium ion-exchanged FAU before and after steaming and calcination. The very simple, easily interpretable hydroxyl spectrum of the ammonium exchanged FAU sample is transformed into a complex series of overlapping hydroxyl bands due to contributions from framework and non-framework aluminum atoms in the zeolite resulting from the hydrothermal treatment conditions [101]. 

When USY zeolite is reacted with a lower concentration and lower adding speed of nitric acid solution, only the non - framework aluminium species located in the pores are extracted and H+ ions could not attack the zeolite skeleton, The non - framework aluminium content of USY zeolites decreases with increasing the concentration and adding speed of acid solution. ... 

The basic unit of a zeolitic structure is the TO4 tetrahedron, where T is normally a silicon or aluminium atom/ion (or phosphorus in an aluminophosphate). In this section we deal with the aluminosilicate zeolites, which have the general formula M4/ [(A102)J/Si02) ].mH20. The zeolite framework is composed of [(AlO (SiOj) ] and M is a non-framework, exchangeable cation. 

It has also been shown that hydride abstraction on Lewis acid sites to generate carbenium ion may occur if non-framework aluminum species exist in the zeolite [10]. However, for the current experiments the contributions from Lewis acid sites of H-ZSM-5 seem negligible, since no non-framework aluminum species were identified by 27A1 NMR. 

Tt is interesting to note that while aqueous ammonium ion exchange of the calcined Nd,NaY zeolite results in the removal of the non-framework aluminum, as seen in the Al NMR spectrum, the si NMR spectrum remains practically unchanged (Figure lB,c). The persistence of the broad resonance and associated SSB suggests that the paramagnetic cation is irreversibly trapped in the smaller cages. 

Mesoporous Fe-MFI zeolites have been successfixlly prepared by treatment of isomorphously substituted Fe-silicalite and ion-exchanged Fe-ZSM-5 in alkaline medium. Iron in framework positions directs the silicon extraction towards mesoporosity development, whereas iron in non-framework positions inhibits silicon dissolution and limits mesoporosity development. 

Alkaline treatment of Fe-MFI zeolites prepared via ion exchange or isomorphous substitution leads to combined micro- and mesoporous Fe-MFI structures. The preparation method highly determines the impact of the alkaline treatment. Iron in framework positions (iron sihcahte) directs the silicon extraction towards controlled mesopore formation similarly to framework aluminium in ZSM-5. Iron in non-framework positions inhibits leaching of silicon to the filtrate. Higher concentrations of non-framework iron, as obtained in ion-exchanged Fe-ZSM-5 catalysts, lead to a lower mesoporosity development. TTie iron nanoparticles appear to remain unchanged upon Eilkaline treatment and speculatively isolated and oligonuclear iron species are held responsible for the restricted silicon dissolution and mesoporosity development. 

Theoretical studies on zeolites have concentrated mainly on three aspects - the preferred sites for non-framework cations and for the aluminium framework ions, the acidity for different... 

Gallium has been successfully introduced into munerous zeolite frameworks (Beta, MFI, offretite, faujasite,. ..). The Ga " " ions in zeolites can occupy tetrahedral framework sites (T) and non-framework cationic positions. The isomorphous substitution of gallium into aluminosilicate zeolites results in modified acidity and subsequently modified catalytic activity such as enhanced selectivity towards aromatic hydrocarbons. 

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