Zeolites extra-framework species

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]. 

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. 

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... 

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.

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). 

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]. 

In zeolites, tetrahedral framework aluminium can be distinguished from non-tetrahedral extra-framework species by means of Al MAS NMR. However, some AlPOs are known to contain 5- or 6-coordinated Al in the framework, which complicates a quantitative determination [4—6]. Quantitative methods for the monitoring of substituting metals Me are, therefore, required. In the case of transition metal ions, possible changes in the oxidation state must also be taken into account, since the charge n of [Me02] building units should directly affect the number of Bronsted acidic sites. 

Negative framework charge may also be neutralized by extra-framework species, such as metal ions, which position themselves in the internal voids of the zeolite. Two important properties which arise from the presence of these ions... 

The location of extra-framework species such as metal cations and adsorbed organic molecules presents a number of problems distinct from those associated with the framework structure itself. In the case of metal cations there is often considerable disorder, resulting in the partial occupancy of a number of unique sites. Measurements of these positions will involve the detailed analysis of accurate diffraction data, and the problem becomes more difficult when the loading of cations is low, as in the case of high silica zeolites. For adsorbates, such as catalytic reactants and organic template molecules which are used in zeolite syntheses, there are additional factors such as conformational disorder and, at finite temperatures, the dynamic processes of diffusion and rotation. 

A combined Monte Carlo and energy minimisation method has been developed to model zeolitic materials with low and medium Si/Al and with a variety of extra-framework species. We present results for Na- and H-Mordenites with Si/Al of 5 and 11. The A1 and cation distributions obtained are in reasonably good agreement with experimental studies. Furthermore, our calculated vibrational spectra are in excellent agreement with experiment, which has allowed us to re-interpret the de-convolution and assignment of the various acid sites. 

Measurements of adsorption of pyridine into acid zeolites were severely impeded by the strong interaction of the adsorbate molecules with the adsorption sites, that is, centers of Bronsted and/or Lewis acid types, i.e., acidic OH groups and only threefold coordinated framework Al or Al-containing extra-framework species, respectively ( true Lewis sites see Vol. 4, Chap. 1,... 

The number of probe molecules suitable for studying the basicity of zeolites by application of IR spectroscopy is much more limited than in the case of acidity. The sites of basicity are most likely oxygen atoms of the framework or basic extra-framework species such as CS2O or alkali metal clusters (cf. Volume 3, Chapters 5 and 6 of the present series). 

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