Zeolite hydrated

Diffraction patterns and FTIR spectra of skeletal vibrations of the ZSM-5 and ferrierite zeolites indicated high crystallinity of the analyzed samples. The strong band with a chemical shift of about 55 ppm in the 27Al MAS NMR spectra of hydrated zeolites indicated the presence of more than 97 % Al in the framework in tetrahedral coordination the very low intensity of the peak at 0 ppm indicated less than 3 % rel. of Al in octahedral coordination. 

After Breck (1974) as presented in Flanigen (1977), pp. 23-24, Table 2.2. ""Density and channel-way dimensions are based on unit cell of hydrated zeolite. 

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... 

The influence of exchangeable monovalent cations on the framework vibrations for the hydrated zeolites Linde A and X has been investigated. An approximately linear relationship is found between the frequency of some absorption bands and the inverse of the sum of the cation and framework oxygen ionic radii. It is proposed that the shift in framework vibrations is largely caused by those cations which are strongly interacting with the zeolite framework. Thus the linear relationship indicates that these monovalent cations are all similarly sited in the zeolite lattice. This is consistent with the presently available x-ray analyses on some of these zeolites. Since Rb + and Cs + are only partially exchangeable in both Linde A and Linde X, these cations deviate from this linear relationship. 

Electrical Properties of Hydrated and Partially Hydrated Zeolites X and Y... 

Procedure. Partially Hydrated Zeolites. Partially hydrated zeolites are made from samples previously dehydrated by evacuation at 400°C in the conductivity cell, by adsorbing known amounts of water. For comparison, adsorption isotherms were determined independently at the same temperature and pressure. After each adsorption of water, the pellet is allowed to equilibrate for 3 hours. Capacity and conductivity are measured at several frequencies in the range 200-107 Hfc. Regular, checks are made on this equilibrium period with overnight and weekend equilibration times. No appreciable changes of conductivity and capacity were observed after 3 hours. 

Hydrated Zeolites. The zeolitic pellets are hydrated by equilibration at atmospheric moisture content. The cell is immersed in liquid air, and a minimum equilibrium temperature of — 120°C was obtained. At that temperature the conductivity and capacity of the samples are measured over the frequency range 200-107 Hz. After eliminating the cooling liquid, the temperature rises slowly (0.5°C/min). Measurements are performed continuously in the same frequency range during the. temperature rise up to room temperature. The results are near-equilibrium values, and the errors are assumed to be the same over the complete temperature range. The same procedure was applied by Mamy for dielectric measurements on montmorillonite 11). 

Hydrated Zeolites. Figure 3 gives a typical plot of the conductivity vs. the reciprocal temperature for hydrated NaF86.5. The other samples behave qualitatively in the same way. Conduction and dielectric absorption are superposed. The position of the maximum of dielectric absorption is frequency dependent it shifts to higher temperatures with increasing frequency. In some favorable cases a second conduction phenomenon is observed on the low temperature side of the relaxation phenomenon (Figure 3). Because of a lack of reproducibility we cannot interpret it. 

Partially Hydrated Zeolites. Absorption I in X-type zeolites is clearly the same as that found in dehydrated X-type zeolites (8). The... 

The AS-values increased with respect to those of the dehydrated samples (8), especially for the samples with high H20/cation ratios. This means that there is a distortion of the cationic hydration shell or a partial dehydration of the cations during migration (18). In other words, water around the migrating cations cannot be regarded water of hydration. In that respect, hydrated zeolites resemble concentrated cationic solutions (19). 

These findings are in broad agreement with the wide-line measurements by Genser (158) who from the second moment of the 27A1 line in hydrated zeolite Y calculated vQ = 390 kHz, but was not able to observe a signal in dehydrated zeolite. Gabuda et al. (137) observed increased values of vQ after dehydration of zeolites. For the hydrated and dehydrated analcime the values were 270 and 390 kHz, respectively for Na-X, 165 and 285 kHz and for Na-A, 75 and 165 kHz. 

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. 

This is also true of nitrogen sorption in hydrated zeolites. 

Of particular interest in the present study is the apparent existence of an activated sorption mechanism for nitrogen in partially hydrated zeolite NaA, a further study of which is planned. The less definitely established irreversibly sorbed nitrogen on dehydrated zeolites NaA and NaX, but not on CaA and NaY, is in need of confirmation, and of further characterization. A practical utility of the results is the quantifying of nitrogen uptake from air, which persists even with appreciable sorbed water. This amount can be approximated from the isotherms as the capacity at 0.8 atm. inasmuch as the contribution due to oxygen is very small. Conversely, the isotherms can be used to assess the state of hydration of a zeolite, with the aid of a measurement of the amount of sorbed nitrogen, either by mass loss on evacuation at room temperature in a balance, or by collection of gas evolved on immersion of a sample in water. 

Compositions are expressed in terms of the full unit cell content. Two compositions are provided. The CHEMICAL COMPOSITION is the nominal material composition provided in the original reference, and is usually obtained from chemical analysis. The REFINED COMPOSITION is derived from the structure refinement. Because of the complexities of structure refinements, the chemical and refined compositions do not always concur. When available, refinements of hydrated zeolites were used to calculate the patterns. For synthetic zeolites, if the zeolite had been synthesized in the presence of organic material, those refinements of the uncalcined products that contained the occluded organic molecules were chosen. The sample locality is given in the case of natural zeolites. 

In the case of hydrated zeolites at high filling factors, the zeolite secondary porosity is occupied by water molecules, since the secondary porosity is formed by the space between zeolite crystallites [118], This water allows the electric connection between the zeolite crystallites, and then allows extended charge transport, that is, intra- and intercrystallite conductions [38,112,119],... 

FIGURE 8.13 I-Vcharacteristic curves of (a) dehydrated zeolite and (b) hydrated zeolite at 300K. 

Therefore, hydrated zeolites conduct through intra- and intercrystalline cationic conduction where the material acts as a solid electrolyte, with the help of the water adsorbed in the primary and secondary porosities of the zeolite [112,119], The conduction process is facilitated by oxidation-reduction reactions in the electrodes, initiated by OFF anions generated by an ion-exchange reaction between the protons generated by water dissociation and the Na+ extra-framework cations included in the zeolite. These reactions cause charge injection, which controls the conduction process [112],... 

A fully hydrated zeolite was characterized by a clean doublet at 2090-2030 cm-. Evacuation of excess water restored the low frequency components of the doublet. Thus the presence of one or more water molecules as ligands of the monovalent rhodium significantly altered its back-donating potential towards the H of the CO molecule and modified the bond angle as well (table 2). 

Photoinduced ammonium ion release from [Rh(NH3)5I]2+ absorbed into zeolite Y was studied to monitor the effects of an ion s environment on the photochemical behavior. The same photoreaction occurs in both media, but the quantum yield is reduced by almost a factor of 5 (to 0.18) in hydrated zeolite Y. This was presumed to be due to the steric hindrance of the zeolite walls, which hinder the access of solvent water to the photoexchange site. Consistent with this model, the quantum yield decreases still further (to 0.13) in partially dehydrated zeolite Y.728... 

At this time, the locations of cations in zeolites have been determined primarily by X-ray diffraction (XRD) techniques. Unfortunately, this method has the drawback of being able to locate only the most stationary cations in zeolites. In some studies of hydrated zeolites, less than 50% of the total cation population can be accounted for. A higher percentage of the cations can be located in dehydrated samples, but the effect of the dehydration step on the location of the cations is generally not well known. NMR measurements, on the other hand, are most sensitive to mobile cations and cations in high symmetry sites. 

Figure 17 The water clathrate in the large cage of hydrated zeolite A. It is composed of 20 H2O molecules arranged in a pentagonal dodecahedron...

The sites adopted by cations in these structures are complex and will not be considered here but, broadly, an increase in Si/Al (decrease in framework charge) depopulates Sites I and T in the hydrated zeolites in favor of positions in the large cage. Removal of water causes an increase in population of the Sites I and T regardless of framework charge. Details of individual cation sitings in ion-exchanged forms of X and Y are available, with water molecule positions, where known. 

Most zeolites have three-dimensional structures with cavities connected to charmels that exhibit ion sieve properties. If the pore volume of the cavity is large enough, the hydration stripped cations exchanged can be rehydrated in the cavity to simulate their hydration in the solution. The large cations (Rb, Cs, organic cations) cannot enter the channels or windows of the cavities of zeolites with small pores. Table 3 lists the pore openings of some hydrated zeolites [133]. 

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