Zeolites extra-framework cation sites

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]. 

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)... 

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. 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

The incorporation of Al atoms into the framework of zeolites occurs in a tetrahedral oxygen coordination and leads to negative framework charges. These framework charges are compensated by protons in acidic hydroxyl groups or by extra-framework cations such as Li , Na, Cs", Mg ", etc. Accordingly, these surface sites are responsible for the chemical behavior of zeolites in separation processes and in catalysis (199,200). 

A1r Separation Properties. Self-bound LSX adsorbents have an enhanced ability to selectively adsorb nitrogen from air. For thermodynamically driven adsorption processes, the quantity of a gas adsorbed by a zeolite at a given pressure and temperature Is a function of Its the affinity for the cationic adsorption sites as well as the quantity of sites available for Interaction. Electronic charge balance dictates that the LSX will have the maximum number of cationic sites available for direct Interaction with weakly Interacting adsorbates. The electric field within the zeolite cavity 1s dependent on both structure and the charge density of the extra-framework cation. Small polyvalent cations 1n the dehydrated/dehydroxylated state, especially calcium, show high selectivity for N2 from a1r.(l2)... 

An additional interaction is observed if altuninum containing extra-framework cations or Lewis acid sites are present. Due to the nuclear spin of Al (f= 2)> each nitrogen hyperfine line is split into six lines, some of which may overlap. An example of such interaction is shown in Fig. 20 for NO-treated H-ZSM-5. Witzel et al. have used this property to determine the density and acidity of Lewis sites in H-ZSM-5,H-Mordenite and H-Y zeohtes [94-95]. The results obtained are in good agreement with those of more conventional methods and, consequently, ESR spectroscopy with NO as probe is a valuable tool for studying Lewis acidity in zeolites. It is important to stress that ESR spectroscopy of adsorbed NO is only... 

The ability of crystalline microporous solids to adsorb molecules selectively on the basis of their size is a consequence of the well-defined pore windows that limit access of molecules to the internal surface area. The size and shape of these windows are determined mainly by the structure of the framework, which is specific to each type of material. For low silica zeolites it can often be modified by ion exchange, where the cations can adopt sites that partially block pore windows. The Atlas of Zeolite Framework Types gives the dimensions of the free diameters of these pores for each structure type, taking into account the radii of the framework oxygen atoms (taken as 1.35 A) and neglecting the effect of extra-framework cations. 

The interactions between the extra-framework cations Na, K, and Ca and the framework of heulandite-type zeolites have been studied by using an atomistic simulation technique [98C1]. The calculations showed that the Ml (= A2) position is the most favored position for Na K, and Ca ions. Substitution of A1 at the T2 site was favored for Na cations, which is in agreement with experiments. 

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