EXTRAFRAMEWORK PHASES

Figure 2 Model for the Bronsted sites in the supercage of a dealuminated HY, depending on the nature and location of the extraframework aluminic phase (the drawing of the extraframework phase is only schematic). In the center is the unperturbed Bronsted site.

The second aspect which makes the interest of the study of zeolites by XPS is that it allows to understand many unexpected aspects of this spectroscopy, in particular when applied to solids. Zeolites in many respects are materials which may be regarded as model solids and not only because of their regular pore structure. Indeed by varying the Si/Al ratio, the nature of counter cations, the ion exchange level, the occlusion of extraframework phases, such properties as the density and strength of acidic or basic sites may be varied in a predictable fashion. All these variations are reflected in the XPS spectra yielding data... 

Progressive CO adsorption has been studied by FTIR spectroscopy on two series of acid-leached steamed HEMT and HY zeolites with various Si/Alp ratios. Acidity of structural OH groups of steamed hexagonal faujasites was determined and compared with the results already obtained with the cubic series. Moreover, quantitative estimation of Brdnsted acidity of extraframework phase (numbers and strength) was carried out and allowed to complete the comparison between the two structural varieties. Only minor differences were detectable. 

Spectroscopic methods have been devoted to the characterization of zeolite structure, of surface sites or species (hydroxyls, acid and basic sites, cations), of extraframework phases (oxides and hydroxides, supported metals), and of adsorbed phases (gaseous or liquids). The paper will consider examples of each type. 

It was also observed that no new hydroxyls bands are formed upon heating of SAPO-37 which might have revealed the presence of an extraframework phase (43). 

Although its main use is still the identification of crystalline phases, X-ray diffraction is also the most used technique for the determination of the location of extraframework cations. XRD is well suited to perform structural characterisation of dehydrated zeolites since the framework is highly crystallised and the extraframework cations are often heavy elements. 

Another advantage of XRD is the fact that it can be used in conditions close to those encountered during their use (in situ characterisation). However, if the system is complex, i.e. if there are different extraframework cations or if an adsorbed phase is present, XRD determination is no longer unambiguous [1], Indeed, for X-rays, the intensity diffracted by an element is proportional to its number of electrons which may be the same for two different elements (e.g. Rb+ and Sr23). This is also the reason why the localisation of lighter elements (H, C,. ..) is... 

The oxidation state of Cu ions in zeolite Y depends on the pretreatment procedure. After activation for more than 8 hours at 675 K phases similar to CuO occur. Admitting hydrogen at 575 K, a reduction to Cu(I) but not to Cu(0) metallic clusters is observed. Probably a reaction with extraframework oxygen takes place. However, a reduction to rather small metallic copper clusters is observed after co-addition of water vapour and hydrogen. This demonstrates the important role of strong ligands like water in the reduction mechanism, enabling a controlled formation of copper clusters. 

In Fig. 5c the dependence of the lratio upon temperature treatment is illustrated. The following can be noticed i) unlike the previous case, the ratio is not going to zero as temperature approaches 1573K this can be explained with the formation of an amorphous phase still containing substitutional titanium. ii) like the m-xylene adsorption capacity, the 970/ 550 ratio decreases already at temperature lower than 1173K this means that substitutional Ti is going into extraframework position before the occurrence of dramatic changes in sample crystallinity... 

---