Extra-framework species

Characterization of two different framework titanium quantification of extra-framework species in TS-1 silicalites. 

The connectivity (topology) of the zeolite framework is characteristic for a given zeolite type, whereas the composition of the framework and the type of extra-framework species can vary. Each zeolite structure type is denoted by a three-letter code [4], As an example, Faujasite-type zeolites have the structure type FAU. The pores and cages of the different zeolites are thus formed by modifications of the TO4 connectivity of the zeolite framework. 

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

Limitations or blockage of the access of reactant molecules to the active sites by carbonaceous deposits ( coke ) and by species resulting from catalyst degradation (e.g. extra-framework species resulting from dealumination). 

Silico-alumina zeolites are an important class of catalyst, serving both as solid acids and as supports in bifunctional catalysts. The acidity of the zeolite can be modified by substituting a heteroatom for the Si or A1 atoms in the zeolitic framework. Whenever a framework substitution is attempted, the first question is always whether the heteroatom is indeed in the framework, or instead exists as an extra-framework species. Then, if it can be demonstrated that the heteroatom is in the framework, the question arises as to the exact crystallographic site in the lattice where the substitution has occurred. Detailed knowledge of the site (the so-called T-site in a zeolite) is needed for a complete characterization of the catalyst. 

Microporous materials are formed with hydrated inorganic cations or organic species located within cavities of the extended inorganic or inorganic-organic hybrid host framework. Extra-framework organic species are usually protonated amines, quaternary ammonium cations, or neutral solvent molecules. Dehydration (or desolvation) and calcination are two methods frequently used to remove extra-framework species and generate microporosity. 

Similar to zeolites, chalcogenides could be prepared with either inorganic or organic cations as extra-framework species. A family of hydrated sulfides and selenides were made recently. These materials, denoted as ICF-m, were prepared in aqueous solutions from simple inorganic salts. One of the most... 

Finally, in deriving structural information from the features of d-d spectra of TMls, it must be considered that, because of the Laporte selection rule, ion sites with octahedral symmetry can contribute to the spectra only to a very limited extent, and so can escape spectroscopic detection. However, this behavior can be turned into a tool to monitor the distribution of TMIs in sites with different structure as a function of loading, as in the case of CoAPO zeotype materials. In this case, the attainment of a plateau level of the intensity of the d-d bands due to Co ions with tetrahedral symmetry that became inserted in the framework indicated the formation of extra-framework species, containing d-d silent octahedral Co sites with increasing loading (Figure 2.16) [77]. 

Figure 10.17 Example of the agreement between experimental and calculated diagrams of a stabilise zeolite requiring the incorporation into the model of cationic extra framework species.

According to the XRD pattern all samples are well crystallized and show the typical feature of the MFI structure. Its largely pure formation is confirmed by the results of n-hexane adsorption. The values of the micropore volume (at p/ps = 0.5) are fairly close to the theoretical ones calculated for an ideal MFI-structure (0.19 cm /g, see Table 1). Table 1 gives the Si/Me ratios of the fnunework as further characteristic data. An equal concentration of Me in the lattice have been strived for. However, the results of the chemically determined Me concentration and the ammonium ion exchange capacity disagree especially for the In-Sil. It is less pronounced for Fe-Sil. Therefore the creation of extra-framework species in In-Sil and Fe-Sil has to be considered which do not contribute to the Bronsted acidity but to other kinds of acidic sites. This is in agreement with the results of the TPD measurements. 

Figure 1 shows the relative concentrations of Lewis (L) and Bronsted (B) acid sites, calculated from IR spectra of ad-sorpted pyridine [22, 23], as a function of the number of framework aluminium atoms per unit cell, Nai. When Nai decreases and therefore NEFAL increases, [L]/[B] increases, meaning that the increase of extra-framework species corresponds to an increase of Lewis acidity. 

The channels and cages of a zeolite framework are usually filled with extra-framework species such as exchangeable cations, which balance the negative charge of the framework, removable water molecules, and/or organic species. These may come from the synthesis mixture or they may be the result of a post-synthesis treatment. Whatever their origin, it is often of interest to know where they are located. 

First, whatever the treatments the NH /Al ratios are always much smaller than 1.0 in the (HT), (HTA1) solids, to be compared with the 0.80 value found for the (IS) Y zeolites (Tables I, II). This indicates, in agreement with previous results (18,19), that a limited amount of extra-framework species possess a strong acidic character. ... 

In a very rough approach, the amorphous phase Si/Al ratios y have been chosen so that the zeolitic fraction (1-z) is equal to the XRD crystallinity of the solids. It appears, from these calculations, that y varies from 0 to about 2 for (HT) and (HTA1) solids, (Table I) which is in qualitative agreement with STEM results (31). Concerning the (HTA2) solids, y can reach very large values, (>20), which is consistent with the fact that the severe acid leaching extracts most of the extra-framework species. With these values for y, only approximate NH /Al ratios can be calculated (Table I). 

ID solids. The ID solids which are nearly exempt from structural defects and extra-framework species present, in agreement with (13, 14), the two structural h.f. and l.f. bands with a very weak silanol band (figure 1C). 

When zeolites are dealuminated by steam-calcination part of the framework A1 is extracted and generates extra-framework species (EFAL) that can be cationic, anionic or neutral. Some of these EFAL species can act as Lewis acid sites [19] or can influence the Brpnsted acidity, by either neutralizing Brpnsted acid sites by cation exchange, or by increasing the acidity by a polarization effect and/or by withdrawing electron density from lattice oxygens [20-22]. However, under mild steaming the A1 can also become partially, and reversibly, disconnected from the lattice [23]. This opens the way to Lewis acid catalysis by the A1 [24]. 

Yields and kinetics depend on the type and number of Ti species and the crystal size of the catalyst used. Ti distribution between lattice (selective) and extra-lattice (unselective) sites is, in turn, closely linked to synthesis and characterization procedures, both of which require special thoroughness [4]. Inadequate characterization and, therefore, the impossibility of clear assessment of siting of Ti in the catalyst, is a frequent obstacle to a correct evaluation of the literature, especially early publications. These considerations are of general value, but are central to the hydroxylation of phenol where extra-framework species are a major source of hydrogen peroxide decomposition and radical chain oxidations. The hydroxylation of phenol was indeed proposed by three different groups as an additional test to assess the purity of TS-1 [2, 9, 11]. Van der Pool et al. estimated from Weisz... 

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