Zeolite site symmetry

Figure 5 shows DOS from self-consistent field, full multiple scattering calculations as implemented in the FEFF8 code33 on the supported Ptio cluster illustrated. The support is mimicked by the 3 water molecules as shown the O atoms represent the O atoms in the support, and the H atoms effectively terminate the cluster and represent the Si or A1 atoms such as that in a zeolite. The DOS are shown from calculations with and without a core-hole on the 3 different site symmetries (surface, edge, and center). 

In cases where there is a low concentration of cation of interest, if the cations are highly disordered in the zeolite framework, or if good crystalline samples are unavailable, atom specific or environment-specific spectroscopic probes may be preferable to determine local structures about the cation in the zeolite. NMR (4), IR ( 5, 6) ESR (7-10), optical (9,10), MSssbauer effect (11-15), and x-ray absorption studies (2,16,17,18) have been used to determine cation microenvironments. In particular, it has been shown that EXAFS (Extended X-ray Absorption Fine Structure) of the cation can often be used to give direct structure information about cation environments in zeolites, but EXAFS techniques, while giving radial distances and relative coordination numbers, are insensitive to site symmetry and cannot, in general, give both coordination numbers and relative site populations. Clearly it is desirable to use complementary spectroscopic techniques to fully elucidate the microenvironments in dilute, polycrystalline zeolite systems. 

The most reasonable qualitative explanation of the change in the emission spectra is the above-mentioned resemblance to solution or solid state behavior which correlates well with the crystallinity of the uranyl-exchanged zeolites as determined by X-ray powder diffraction. However, there are differences between all of the zeolites which could be an indication of site symmetry and coordination in the lattice. The aluminosilicate lattice is preserved after ion-exhange except for zeolites A and ZSM-5. 

P. J. Ilutta to all possible low-symmetry local structures that may occur in zeolite sites [7J. Specific reported examples are the d -> d term diagrams applied to observed optical spectra of transition-ion complexes with olefins, dioxygen, mono-oxygen, and a number of other intrazeolite sorption assemblies. Predictive diagrams have been calculated for the specific locations of the Mn(ll) ions in zeolites, which have been analyzed for magnetic properties [8] and structure [9],... 

The crystallographic sites of nonframework cations in the structures of zeolite A (LTA) and zeolites X and Y (FAU) are indicated in Fig. 22 where their coordination to the framework is shown in the lower part. Their nomenclature, sites per unit cell, site symmetry and location are summarized in Table 3. Note that the SI position in LTA is equivalent to site SII in FAU and will be further designated as SIA. 

Zeolite Site Number of sites/u.c. Site symmetry Location... 

The spectrum of Mn2+ in zeolites has been used to study the bonding and cation sites in these crystalline materials. This is a 3d5 ion hence, one would expect a zero-field splitting effect. A detailed analysis of this system was carried out by Nicula et al. (170). When the symmetry of the environment is less than cubic, the resonance field for transitions other than those between the + and — electron spin states varies rapidly with orientation, and that portion of the spectrum is spread over several hundred gauss. The energies of the levels are given by the equation... 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Only fibrous zeolites (natrolite, mesolite, scolecite, edingtonite) are in this group. In their topological symmetry there are two T sites, one having a multiplicity 1/4 of the other (from now on indicated as Tl and T2 respectively). These tetrahedra are connected to construct a building block formed by five 4-membered rings (see Fig. la). In the case of order, Tl is occupied by Si (and will be called Sil from now on), whereas T2 splits into two symmetrically independent tetrahedra, one occupied by Si, the other by A1 (called Si2 and A1 respectively). 

The Si-Al order/disorder in this zeolite closely resembles that found in bikitaite. Its Si/Al ratio, given by the chemical analysis, is very near 3. In the topological symmetry there are 4 T sites one is occupied by Si (Al-content 6 %), the others by Si 2/3 and A1 1/3 [62]. Alberti and Vezzalini [63] pointed out that the real symmetry of brewsterite is iower than P2i/m, (probably P2i). The order in brewsterite could therefore be higher than that found in the structure refinements, with 6 Si-rich and 2 Al-rich sites in the structure. 

Schematic illustration of ordering of Na(2) ions in Na-A zeolite (a) Fm3c symmetry. The four equivalent off-center sites associated with an 8-ring are shown as shaded circles. The 0(1) and 0(2) atoms of the ring are shown as open circles joined by the cross-hatched circles. The sites represented by open circles are now unoccupied. Examples of complete ordering are shown by the full circles in...

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