Intra-zeolite location

The well-known BE difference between XPS signals of metal ions in their oxides and in zeolites [33,35] has frequently been used to prove the intra-zeoHte location of species. Higher BE values observed with zeolites have been attributed to the influence of intra-crystalHne potentials, in some cases also to final-state effects (La, Cu, vide infra). In view of the considerable variation in the Na Is BE with the fi amework Al content [7-9,11,12] it may be expected that the binding energies of other metal ions will also depend on the Al/Si ratio. This has recently been confirmed for VO + ions [106], but a systematic study of this problem is not yet available. The BE of metal ions in zeoHtes are sometimes close to those in the corresponding hydroxides (e.g., Ni +, Cu +, Co +), so that discrimination between extra- and intra-zeolite locations on the basis of XPS binding energies alone is not safe. Thus, it has been found that the Cu 2p BE of Cu(II) oxide species well... 

Satellite structure maybe another spectral feature suited to demonstrate the intra-zeolite location of an element SateUites arise from interactions between imoccupied atomic levels and the valence band in the final-state wave fimction. 

For Cu-based systems (e.g., CuCl/Na-ZSM-5 [96]), the Cu Auger parameter (vide supra) is a powerful tool to prove intra-zeolite guest location. Special effects were reported for Co phthalocyanine, where an inequivalence of the N atoms, which is not observed in the XPS of solid phthalocyanine, can be detected when the molecule is dispersed in a zeolite [202]. With Ru and Os phthalocyanine (but not with the Co-, Ni-, and Fe-based complexes), the oxidation state of the metal and, consequently, its BE, has been reported to be higher in intra-zeolite locations than in the bulk solids [200,202]. 

Photoemission techniques offer a variety of tools for the discrimination between extra- and intra-crystalline location of components introduced (e.g. metal ions, MI ). Nevertheless, due to the specific limitations of these tools, detailed studies, preferably combining several of them, are often required to derive sound conclusions, in particular if both extra- and intra-zeolite species are present. Apart from quality assessment after ion-exchange steps, photoemission has been increasingly applied to describe mobility phenomena, e.g., the preparation of zeolite catalysts by solid-state reactions (soHd-state ion exchange [86-89],reductive dispersion (Ga203 into H-ZSM-5 [90-93]),chemical transport [94-97]), the penetration of metal poisons (Ni,V) into FCC catalysts [98-101] and the redistribution of active catalytic components in zeoHte crystals under reaction conditions [102-105]. Much of the earlier work in this field has been reviewed by Shpiro et al. [33,35]. 

Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties a uniform intra-crystalline microporosity that provides aceess to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. 

Xenon NMR spectroscopy was used to characterize xenon in Ca, Mg, Ni, Ag, La, Ce and Rb - exchanged Y and X zeolites. We report here some examples concerning the location of these cations and their inter - and - intra crystallite diffusion... 

Since the first " Xe NMR study of xenon adsorbed on a zeolite, this technique has been shown to be of interest for the investigation of the distribution and the size of supported metal particles, the quantitative distribution of phases chemisorbed on these particles, the dimensions of the void spaces of zeolites, the detection of structure defects, the location of cations and the effect of electric fields they create [i,2 ]. We report here some typical applications related to the study of intra-and inter-crystallite diffusion of cations in faujasite zeolites. 

In general, the surface of pure silicate mesostructures is weakly acidic. It is found that the incorporation of metal ions into the framework can introduce acidic and ion-exchange functionality and catalytically active sites. Various metal ions, such as Al +, Ti " ", V +, Ga +, and Fe +, have been incorporated into S BA-15 to enhance its catalytic performance. In contrast to zeolites, which have crystalline structures, the incorporation of metal ions in mesoporous silicates caimot be strictly defined as intra- or extra-framework incorporation since these ions are highly dispersed on the framework. A wide range of compositions with different coordination numbers and chemical environments can contribute to amorphous framework structures. For example, both tetrahedrally and octahedrally coordinated aluminum in S BA-15 are involved in the formation of the amorphous pore walls, and may be defined as intraframework Al. The former may exist inside the pore walls, while the latter may be located on the pore surface. 

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