Framework cation siting

The typical unit cell content of zeolite L is (K,Na)gAi9Si27072.nH20 and its Si/AI ratio varies in the range of 2.6 - 3.5 [1-4]. Takaishi recently determined the distribution of Al atoms in the framework of zeolite L ly analyzing Si-MAS-NMR spectra. He thereby deduced five kinds of extra-framework cation sites as shown in Fig. 1., and estimated the relative strengths of their cation affinities [5]. 

Transition metals exchanged into Y-zeolite offer a basis upon which to build an understanding of the important parameters involved in designing zeolitic redox catalysts. Y-zeolite was chosen for this study because it is the most thoroughly characterized catalytic zeolite. Thus, one can address such questions as what non-framework cation sites are occupied, whether the cations move between sites, whether interactions between the cations themselves are important and how these factors relate to the kinetics of catalytic reactions. 

According to the most recent structure refinement [25,26], there are two types of cation sites in phillipsite type I site is on the mirror plane (010) and is usually populated totally or partially by large-size cations type II site, whose occupancy does not exceed 50%, is located near the intersection of the two sets of channels and is usually populated by smaller cations, but can host also large cations. There are two other positions, close to site II, labelled sites IT and 11" [25,26], which are usually empty, unless the number of available cations is larger than that sites I and II may accommodate. The great selectivity of phillipsite for Ba suggests that this cation, in substantial accordance with data reported previously [25], can occupy every extra-framework cationic sites. 

The unselectivity showed by phillipsite and chabazite for Co is hardly explainable as a difficulty of access to sites, in fact the size of this cation should enable it to easily enter most sites of both zeolites. This behaviour can be therefore ascribed either to the high hydration energy of Co " (Ehyd = -1996 kj/mole), which would favour its stay in solution [29-30] and/or to the difficulty of this cation to assume the right coordination of the extra framework cationic sites of the two zeolites. Accordingly, as an example, Co might occupy more or less forcedly... 

Fig. 16. (a) Extra-framework cation sites in X- and Y-type zeolites, (b) Far- infrared spectrum of Na-Y with band assignments to cation sites according to [232]. (c) Experimental IR spectrum in comparison to simulated spectra calculated according to the shell model and occupancy of different cation sites, (d) Experimental spectrum in comparison to power spectra simulated by MD at occupancy of different cation sites (parts c and d from [79] with permission)... 

In specific examples, the details of framework geometry can be used to establish the contents of individual framework cation sites (T sites) when the mixed cations that are present at the site possess different numbers of electrons... 

The predicted location of extra-framework cation sites in zeolites can also be studied by MC methods, although the problem is further complicated by the disorder in both framework aluminium location (and associated charge) and partial occupancy of cation sites. In this case a large number of possible extraframework cation site distributions has to be considered, and a model assumed for the location of framework charge. 

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