Cation sites in zeolites

Vibrational dynamics of small molecules adsorbed on cation sites in zeolite channel systems IR and DFT investigation... 

The interaction of CO and acetonitrile with extra-framework metal-cation sites in zeolites was investigated at the periodic DFT level and using IR spectroscopy. The stability and IR spectra of adsorption complexes formed in M+-zcolitcs can be understood in detail only when both, (i) the interaction of the adsorbed molecule with the metal cation and (ii) the interaction of the opposite end of the molecule (the hydrocarbon part of acetonitrile or the oxygen atom of CO) with the zeolite are considered. These effects, which can be classified as the effect from the bottom and the effect from the top, respectively, are critically analyzed and discussed. 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Evidence of heterogeneous dual cation sites in zeolites by combined IR andDFT investigation... 

Fig. 1. Cation sites in zeolite L. A AI atom, A AI atom located on a hidden site,0 site A,. site 6, (f site B", 0 site C, site D, 0 site D , site E...

In recent years Seff and co-workers (9) have extensively studied cation siting in zeolite A using single-crystal X-ray diffraction techniques. In favorable cases these workers have also been able to obtain detailed information on the interactions between cations and absorbate molecules. Two examples are shown in Fig. 4, where the adsorption complexes formed when acetylene and NO are adsorbed in Co(II)A have been resolved. In the former case it is proposed that a weak complex is formed via an induced dipole interaction with the polarizable 7i-orbitals of the acetylene molecule. For the NO complex there is good evidence for electron transfer resulting in a complex between CO(III) and NO. In both cases the organic molecules... 

The infrared spectrum of adsorbed nitrogen can also be used to probe cation sites in zeolites. Zecchina et al [34] compared vibrational frequencies of CO and N2 adsorbed at low temperatures in mordenite containing different alkali metal cations. In both cases the vibrational frequencies could be correlated with (Rx + Rm) > where Rx is the cation radius and Rm the radius of the adsorbed molecule, suggesting a simple electrostatic field explanation for the frequency shifts between different cations. The appearance of a band due to N2 interacting with a particular zeolite cation will also mean that that particular cation is located in sites accessible to the N2 molecule. 

Cation Exchange. Barrer and Villiger proposed five cation sites in zeolite L these are indicated in Figure 3. Site A is located in the middle of the hexagonal prisms, site B in the centre of the cancrinite cages, site C midway between centres... 

A more recent approach on the interpretations of Cu + spectra to specific cation sites in zeolites is based on the use of ab initio calculations [39-41]. The structure of appropriate model clusters of cation sites (e.g. six-ring and five-ring sites) are calculated and optimized with density fimctional theory. In a next step, the electronic spectra andg-values of the models are calculated. The first results of this approach look very promising and its rigorous application will certainly lead to a better insight into Ae coordination of transition metal ions in zeolites. An... 

Table 3. Nomenclature, number per imit cell (u.c.), symmetry and location of nonframework cation sites in zeolites LTA and FAU ...

Figure 2.6 Position and nomenclature of the most important cation sites in zeolites X and Y (FAU type). SI is in the D6R, SI and SIT in the sodalite cage, SII in the 6MR between the sodalite cage and the supercage and Sill is within the supercage.

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. 

A great deal is known about ion exchange in zeolites, because of their importance as detergent builders - Zeolite A is the most widely used. Most of the cation sites in zeolite A are accessible in or from the large a-cages (Figure 6.7)... 

Figure 6.7 Cation sites in zeolite A cations can adopt sites at the 6MRs of the sodalite cages, in the 8MRs and above 4MRs in the a-cages.

The cation sites in zeolites X and Y are given in Table 7.4. Sites I, T, and IF are not exposed and are not available for interaction with adsorbate molecules, with the possible exception of water, which could fit in the 6-oxygen ring (with an opening of 2.8 A). Thus, nearly one-half of the cations are not available for adsorption. Sites II and III are exposed to the cavity and are associated with, respectively, the 6- and 4-oxygen rings. Alkali metal cations occupy both of these sites but the alkaline earth metal cations tend to occupy site II only. 

The aluminium species in activated zeolites have been studied using NMR spectroscopy. Aluminium and iron in the framework of zeolites have been analysed quantitatively by Al NMR spectroscopy. The templating of high-silica zeolites by THF has been studied by C NMR spectroscopy. O NMR spectroscopy has been used to study the atomic multipoles in Fl-form aluminosilicate frameworks. A comparison of fast amplitude modulation shifted-echo pulse sequences for O 3Q- and 5Q-MAS NMR experiments has been carried out for oxygen sites in zeolites. By using a combination of the Monte-Carlo technique with Si NMR spectroscopy, cation sites in zeolites have been located. " The growth of zeolites on kaolin microspheres has been investigated by NMR spectroscopy. F1 MAS spin echo NMR spectroscopy,... 

Table 5. Cs+ population of the cation sites in zeolite Cs-Y prepared by SSIE results of Cs MAS NMR measurements (after [36])...

The species complexed with excess NO molecules to form mobile nitrosyl complexes were redistributed on ST cation sites in zeolite cages. 

---