Zeolite coordination numbers

The normal crystalline zeolites contain water molecules which are coordinated to the exchangeable cations. These structures can be dehydrated by heating under vacuum, and in these circumstances, the cations move position at the same time, frequently settling on sites with a lower coordination number. The dehydrated zeolites are extremely good drying agents, absorbing water to get back to the preferred hydrated condition. 

ESR work confirmed that copper tetraammine complex is formed when ammonia is adsorbed on CuY zeolites at room temperature (18). The coordination of ammonia to metal cations may take an important role in the reactions involving ammonia, although the coordination number of ammonia may be lower at higher temperature. 

Moreover, pore and channel size and shape can be tuned by using different alkaline-earth cations (Mg2+, Ca2+, Sr2+, Ba2+) having different sizes and coordination numbers. The heat of water adsorption ranges from —46 to —52 kj mol-1, values similar to those of silica-containing zeolites. As recently reported, the properties of these materials can be further differentiated through the incorporation of transition (i.e. Co2+) or alkaline cations (i.e. K+) into the channels of barium-linked materials composed of metal-assembled cages. 

Figure 3 Plot of AU and the area ratio R = AVBc/AVBb as a function of the Pt-Pt coordination number N, as obtained from the EXAFS analysis. Also shown is the estimated dispersion, cluster diameter and number of atoms/particle as estimated assuming spherical clusters and FCC packing. Data are shown for the acidic supports (LTL[K/A1=0.63] zeolite and CI-AI2O3, SiOi) and for the basic supports (LTL [K/A1=1.25] zeolite and K-AI2O3, K-Si02).

What emerges from this detailed EXAFS analysis is, first, that the tin is indeed substituted into the zeolitic framework. In many cases this kind of information is all that can be obtained from such an analysis—a first-shell fit in which the bond lengths and coordination numbers are consistent with a framework species versus a nonframework one. Flowever, in this example it was possible to analyze higher-shell data, up to a distance of 5 A, and thereby to determine the site in the zeolite framework where the tin is substituted. It is believed that the unique selectivity of this catalyst in Baeyer-Villiger oxidation reactions is a consequence of the occupation of specific crystallographically well-defined sites by tin in the framework of the zeolite in a spatially uniform manner. 

FIGURE 49 NO reduction rate at 573 Kin the presence of 0.5% H2 for propane-SCR on various silver-form zeolites as a function of the Ag-Ag coordination number characterizing Ag s+ clusters (Shibata et al., 2004). Reprinted from (Shibata et al., 2004), Copyright 2004, with permission from Elsevier. 

The authors showed that during treatment in helium at 773 K, all of the Cu(H) was reduced to Cu(I). They concluded that mononuclear Cu(II) entered the zeolite during the ion-exchange procedure and that a mixture of Cu(I) monomers and Cu(I) pairs were formed during the dehydration in helium. During NO decomposition at 773 K, Cu(I) was partly oxidized to Cu(II). Concomitantly, an increase in the Cu-O coordination number from 2.3 to 3.4 was observed (Table 7). With additional information from... 

Whereas the surfaces discussed so far have been generated from the bulk by a simple cut, leading to a decrease in the coordination number of the surface atoms, catalytically important acidic surfaces can also be generated in microporous or layered materials by isomorphous substitution of lattice cations. This occurs in zeolites and smectite clays. Zeolites and clays can be considered as aluminosilicates. Their lattice compositions can vary significantly. In zeolites the Al3+ ion can be substituted by many other trivalent cations. Si4+ can be partially substituted by Ti4+ or Ge4+. 

Cu isotopes both with nuclear spin I-3/2. The nucle r g-factors of these two isotopes are sufficiently close that no resolution of the two isotopes is typically seen in zeolite matrices. No Jahn-Teller effects have been observed for Cu2+ in zeolites. The spin-lattice relaxation time of cupric ion is sufficiently long that it can be easily observed by GSR at room temperature and below. Thus cupric ion exchanged zeolites have been extensively studied (5,17-26) by ESR, but ESR alone has not typically given unambiguous information about the water coordination of cupric ion or the specific location of cupric ion in the zeolite lattice. This situation can be substantially improved by using electron spin echo modulation spectrometry. The modulation analysis is carried out as described in the previous sections. The number of coordinated deuterated water molecules is determined from deuterium modulation in three pulse electron spin echo spectra. The location in the zeolite lattice is determined partly from aluminum modulation and more quantitatively from cesium modulation. The symmetry of the various copper species is determined from the water coordination number and the characteristics of the ESR spectra. 

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. 

Bond valence sum (BVS) analysis, developed by Brown (43) to calculate metal oxidation states in materials such as high-temperature superconductors and zeolites, has recently been shown by Thorp (44) to be predictive for metalloenzymes and model compounds. On the basis of crystallographic data, the empirical parameters r0 and B are determined. These values can then be used to calculate oxidation states from known coordination environments or coordination numbers from known oxidation states and bond lengths. The requisite equations are... 

Sulfate formation and coke deposition were often excluded as reasons for deactivation, using EXAFS spectroscopy. The formation of larger clusters in the zeolite, which can also lead to destruction of the host structure, can be observed by an increase in the coordination number of the Cu—Cu scattering contribution to the EXAFS. 

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