Cation distribution, dehydrated

Siting of metal ions (Zn2+, Co2+, Cu+) in the cationic sites of (A1)MCM-41 matrix has been investigated employing UV-VIS-NIR DR spectroscopy and UV-VIS emission spectroscopy. Four types of cationic sites were identified in dehydrated (A1)MCM-41. Divalent (Zn2+or Co2+) ions are accommodated only in two sites. Cu+ ions in reduced, Cu2+ ion exchanged (A1)MCM-41, occupy four types of cationic sites. Two sites are accessible for divalent cations, other two only for monovalent cations Distribution of cations among sites depends on the metal ion loading in molecular sieve. 

NH3 is similar to H2O in that they both possess large dipole moments and are both small molecules. The presence of NH3 in a zeolite is chemically similar to the presence of H2O in a zeolite. Therefore, the hydrated cation distribution in zeolites is probably more typical of NH3 adsorption in zeolites than the dehydrated cation distribution. According to Breck (18), for hydrated zeolite X, cations are found in sites SI, SI, SII, and SIV. Of these sites, SI, SII, and SIV would all be adsorption lattice solution sites. The cationic and anionic lattice solution sites (in the supercavity of NaX) are illustrated in Figure 8. For NH3, the subscript J1 will refer to SII sites, the subscript J2 will refer to SI sites, and J3 will refer to SIV sites. The anionic sites are two and are (l) in the center U-membered ring of the connecting frame and (2) near the center of the 0(2)—0(1)—0(l) triad of oxygen atoms. For NH3, the subscript il will refer to the first anionic site the subscript i2 will refer to the second anionic site. 

Sodium-23 MASNMR measurements have been used to examine the extent to which this method can be used to determine the cation distribution in hydrated and dehydrated Y-zeolites. Results have been obtained on Na-Y and series of partially exchanged (NH, Na)-Y, (Ca,Na)-Y and (La,Na)-Y zeolites which demonstrate that the sodium cations in the supercages can be distinguished from those in the smaller sodalite cages and hexagonal prisms. For the hydrated Y zeolites, spectral simulation with symmetric lines allows the cation distribution to be determined quantitatively. 

Daw et al. [61] used microscopy and X-ray diffraction to investigate the dehydroxylation and formation of enstatite during the heating of talc. Dehydration proceeds inhomogeneously with the formation of bubbles, possibly initiated at dislocations. The orientation of the first-formed enstatite was more random than that subsequently attained, in which the product was preferentially aligned in three possible directions with respect to the reactant, controlled by oxygen lattice preservation and cation distribution. 

Por the hydrated state, we can reasonably expect that the site I and site II cations are completely solvated with (three) water molecules. The interaction enerqy w can therefore he incorporated into e-p ejj and ejjj, such that the above equations formally reduce to those of the dehydrated state. Van Dun et. al. (2) found that as a function of the Al content, the cation distributions in dehydrated Na-exchanged FAU-type zeolites could he accurately predicted if it was assumed that the enerqy level differences relative to site I varied linearly with the framework negative charae. At around 48 Al/unit cell, the site preference chanqes from II > I > I (low Al content) to I > II > I (high Al content). 

In Y and faujasite, there should be about 29 divalent cations which could be entirely accommodated in sites I and II. The crystal of dehydrated Ca-exchanged faujasite (8) was shown free of other cations by microprobe analysis and was severely dehydrated at 475 °C before being sealed in its capillary. The cation distribution is close to the theoretical suggestion, but there are 2.6 Ca atoms in site I. Bennett and Smith pointed out that the 14.2 Ca in I and 2.6 Ca in I are consistent with no sharing of a polyhedral face, since 14.2 + 2.6/2 yields 15.5, which is less than 16. Dempsey and Olson (27) suggested that presence of water molecules draws cations from I and II into V such that n (I) + 0.5n (I ) = 16. There are insufficient data to test rigorously the detailed accuracy of this equation. 

Following the above studies, Bennett and Smith (9, 11) showed that stricter dehydration caused La atoms in La-exchanged faujasite to move into I. Furthermore, the same dehydrated crystal of La-faujasite had essentially equal occupancy factors at 25° and 420°C, thereby ruling out any control of cation distribution from a pure temperature variation. Although not strictly proven from x-ray data, correlation of the above results with those obtained by many authors—see particularly Rabo et al. (53)—from infrared methods shows beyond reasonable doubt that the positions of cations depend strongly on even small quantities of residual molecules. It is possible that 1 water molecule is sufficient to bridge between 2 or more La atoms in I. Hence, for 19 La atoms per unit cell of fully-exchanged Y-zeolite, only about 10 water molecules are needed, compared with 260 in the hydrated specimen. 

Table IV shows the cation distributions estimated for several forms of X dehydrated under various conditions. For trivalent cations (.—30 per unit cell), sites I and II are most suitable electrostatically, but the sample of La-X calcined at unspecified temperature (probably 350°C) showed all the La atoms on I, while site II was occupied by electron density explainable by 32 water molecules. This remarkable distribution yields a very stable chemical complex with each La bonded to 3 03 and 3 HoO at 2.5A and with each H20 bonded to either 2 or 3 La atoms. The H20 also is bonded weakly to 3 02 atoms of a free 6-ring.

Lin and Chao (1991) studied the variation of the cation distribution with the lanthanum exchange level in hydrated La,Na-Y zeolites by two-dimensional 23Na nutation NMR spectra in which the chemical shift and the quadrupole interaction could be separated. They proposed that the mobility of both water molecules and sodium ions can be reduced by lowering the temperature of the sample, and the migration of lanthanum ions from supercages to small cages causes the redistribution of sodium after dehydration... 

IR framework spectra were used as a diagnostic tool by Occelli et al. [260] in detecting the presence of offretite (via a band at 600-610 cm ) and erionite (bands at 410-425,550-610,655-685 cm ) in mixtures of these two structures. Roessner et al. [261 ] considered, in their IR spectroscopic work on the cation distribution in dehydrated calcium-exchanged erionite, also the framework vibrations of Ca-erionite besides OD vibrations, CO adsorption and DRIFT spectroscopy in the NIR region. They were able to show that the Ca + cations were selectively located in the supercages in front of the six-membered rings. Similar to the features encountered with Y-type zeolites and mordenite (vide supra), also with offretite a sufficiently linear relationship was found between the wave-numbers of the asymmetric and symmetric T-0 vibrations and the number of framework Al atoms per unit cell [262]. 

As a conclusion of this section, it can be said that the method used has to be carefully chosen according to the sample studied and/or the expected results. Conventional XRD may be sufficient to localise a single cation species in a dehydrated zeolite whereas for bicationic zeolites more elaborate techniques like anomalous XRD or MAS and MQMAS NMR may be necessary. If the focus of the study is more on the influence of adsorbed molecules on the distribution of the cations, neutron scattering may be needed to complete the work. Finally, highly dealuminated zeolites may be difficult to study with diffraction techniques, in this case NMR techniques may be the best available option. 

The shape selectivity of zeolites is influenced by the location and distribution of charge-compensating cations. The charge-compensating ions other than protons are all quadrupolar. and Li NMR spectra of dehydrated LiX-1.0 identified three crystallographically distinct sites [221]. In the case NaX with Si/Al ratio of 1.23, six distinct sodium sites were identified using fast Na NMR, DOR and nutation techniques [222]. Na MQMAS has been extensively studied for zeolites X and Y [155]. Other cations like Cs and La in zeolites have also been investigated [155,... 

The distribution of cations in a hydrated zeolite is mainly controlled by their sizes and can be described by a statistical model. In the dehydrated state, most of the cations are located on the intraframework sites their occupancies are governed by mutual repulsions and cation—framework interactions [1]. By which, the environments of the framework silicon atoms and their corresponding ssi NMR spectra are affected [2,3]. The chemical shift and lineshape of Si NMR have been found to depend on the nature and the distribution of cations in the small sodalite and double hexagonal prism (D6R) cavities of the dehydrated Y zeolites [3] The irreversible migration of La3 ions from the supercages to the small sodalite and/or D6R cavities by... 

It is then oxidized to afford phenol through cation 95. The pH of the reaction medium has a strong effect on conversion and product distribution since 94 may participate in an acid-catalyzed competitive dehydration to yield a radical cation (96). Compound 96 may be reduced to the starting material. If there is no other oxidant present, dimerization may also occur. This is the case when benzene undergoes radiolysis in aqueous solution.739,741... 

The exchangeable monovalent cations have a marked influence on the framework vibrations of hydrated Linde A and X. For some vibrational modes the frequency shifts appear to give a quantitative measure of the interaction between cations and lattice. A regularity is found for Li+, Na+, Ag+, K+, and T1+ exchanged forms which implies a similar distribution of cation sites for both zeolites. It is further deduced that in the Cs+ and Rb+ exchanged forms there is only a relatively weak interaction between the cations and the zeolite framework. This technique can be readily extended to study cation siting in other zeolites in both hydrated and dehydrated forms. 

Catalytic activity of metal ions coordinated to the framework of mesoporous molecular sieves atracted attention at first. Recently, catalytic activity of the metal ions incorporated into extraffamework positions of the MCM-41 in various reactions was also reported [3], But, the site geometry, coordination and distribution of the metal ions in the extraframework sites of the MCM-41 host matrix are not understood, and only a few papers have dealt with this problem. For dehydrated Mn-(A1)MCM-41, only one type of single cation was reported [4],... 

The authors of ref. 15 explain the observed strong quadrupolar effects by a distortion of A104 tetrahedra on dehydration, possibly caused by the closeness of the bare cation. They justify the postulate of a distribution of 27A1 chemical shifts with an observation of several aluminous species with distinct 7j values and chemical shifts. More work is needed to elucidate these important effects fully. 

These revolutionary ideas lead to further neutron measurements of the structure of zeolite A, which have confirmed the correctness of the traditional 4 0 ordering scheme.58 59 Neutron diffraction traces for several samples of a dehydrated Na zeolite A with Si to A1 ratios of 1.03, 1.09 and 1.12 failed to show any rhombohedral distortions similar to those reported in ref. 57, and in each case the data was consistent with a cubic structure.58 Neutron diffraction experiments on a T1 exchanged sample of the same Na zeolite that had shown the rhombohedral distortion in ref. 57 showed that the crystals now had cubic symmetry59 60 and therefore the distortion that had been measured for the Na zeolite A must be very sensitive to the identity of the exchangeable cations. Profile refinement of this neutron data56 57 also showed a pronounced bimodal distribution of the bond lengths as would be predicted by the 4 0 model. In conclusion it appears that the chemical shifts observed in the n.m.r. experiments can be influenced by factors such as local strain, as well as by the local environment of each Si atom. 

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