Formation zeolite

The consideration that many zeolite types exist, each with many tunable properties (e.g., pore size and alumina content), leads not only to a wealth of options but also to a high level of complexity. Owing to this complexity and limited understanding of zeolite formation and permeation behavior, a lot of experimental effort is required in this field, slowing down developments toward successful application. 

Liu s description of the kinetics of the zeolite formation process can be formulated in terms of the following equation. 

The fourth lifetime components, r4, have values in the range of 2.7 - 5.0 ns (shown in Table 1 and 2), which, using equation (1), gives 0.34 - 0.47 nm for radii of the holes that are listed in Table 3 and 4. These lifetimes have also lower intensities than r3, suggesting the presence of the larger voids already in the initial phase of zeolite formation, but in a smaller extent. Here the correlation to the influence of cation sizes cannot be established. 

Microporous nanoparticles with ordered zeolitic structure such as Ti-Beta are used for incorporation into walls or deposition into pores of mesoporous materials to form the micro/mesoporous composite materials [1-3], Microporous particles need to be small enough to be successfully incorporated in the composite structure. This means that the zeolite synthesis has to be stopped as soon as the particles exhibit ordered zeolitic structure. To study the growth of Ti-Beta particles we used 29Si solid-state and liquid-state NMR spectroscopy combined with x-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HRTEM). With these techniques we monitored zeolite formation from the initial precursor gel to the final Ti-Beta product. 

Water adsorption on high silica zeolites. Formation of hydroxonium ions and hydrogen-bonded adducts... 

Framework infrared has also been used to look for the formation of a zeolite during synthesis. Since many of the secondary building units can be detected in the infrared spectrum, it is possible to see zeolite formation at very early stages in the synthesis. In fact, zeolite formation can be detected in the infrared before crystallinity is observed by X-ray diffraction. Most of the reported work has been done by sampling the zeolite synthesis at various stages, isolating the soUds and measuring infrared spectra of the dried samples. 

Donahoe, R. J. Liou, J. G. 1985. An experimental study on the process of zeolite formation. Geochi-mica et Cosmochimica Acta, 49, 2349-2360. 

A wide variety of zeolites are known to form in saline lakes where the species present is dependent upon the chemistry of the solutions. Rapid zeolite formation is aided by the existence of the volcanic glass and high water salinities. Potassium feldspar occurs with the common alkali zeolites (Hay and Moiola, 1963 Hay, 1964 Hay, 1966 Sheppard and Gude, 1969, 1971), however, albite is not evident as a diagenetic mineral in saline lakes. 

AIELLO (R.), COLELLA (C.) and SERSALE (R.), 1970. Zeolite formation from synthetic and natural glasses. Arne. Chem. Soc. 2nd Int. Zeolite Conf., 48-58. 

The reaction process was first described quantitatively by Kerr (S). During kinetic investigations on the formation of zeolite A he found that the rate of zeolite formation is always proportional to the amount of zeolite... 

Zeolite Formation in the System K20-Na20-Al20,-Si02-H20... 

Figure 2. Relative significance of the zeolite formation factors...

Zeolite formation depends on reaction conditions 2-4). It is generally believed that most zeolites are formed as metastable phases. According to Barrer (3), the course of the synthesis, beginning with the type of starting material, determines the structure of the zeolite formed. The studies of Zhdanov 2, 5) on the composition of liquid and solid phases of hydrogels indicate that the kind and composition of the zeolite formed depend on the hydrogel composition and that the results of crystallization of aluminosilicate gels obtained in the same way are reproducible. 

A shift in the silicate equilibrium towards DnR species upon substitution of large organic cations such as tetraalkylammonium (TAA) for the alkali. These silicates are still present in solution at 90 °C, i.e., close to zeolite formation temperatures. 

Basic Silicate Solutions Dynamics. Exchange reactions between silicates as well as zeolite formation involve condensation and hydrolysis reactions between dissolved silicate species. Therefore, we have extensively studied the dynamics of basic silicate solutions in order to obtain better knowledge of the properties of possible zeolite precursor species. Our first results were published earlier (11). Here we have again used selective excitation Si-NMR experiments, applying DANTE-type (13) pulse sequences to saturate a particular Si resonance belonging to a particular Si site. The rate of transfer of magnetization from this saturated site to other sites is then a measure of the chemical exchange rate between the two sites. 

Second, the amount of all small silicates present in solution seems to be constant throughout the duration of the synthesis. This finding is in line with the fast mutual exchange observed in silicate solutions (vide supra). Moreover, it indicates that if the zeolite is indeed grown from dissolved small silicates, the equilibrium between the polymeric and the smaller silicates is rapid enough to keep the amount of smaller silicates in solution at a constant level. The zeolite formation stops when the solution is no longer supersaturated with silica, which in this case means after about 10 days (see Table I). 

Kinetics of Zeolite Formation. Since the D5R synthesis mechanism, if operative, is clearly disturbed by DMSO (vide supra), we also carried out a number of time-dependent syntheses in the absence of DMSO in the hope of being able to relate the kinetics with the D5R concentration present. Starting from a molar composition of 40 Si02 1 A OjX 5 NaOH 1000 H20 X at T - 190 °C, various organics and/or seed crystals (X) were added and the solid products were characterized after various synthesis times. Properties of the products obtained when the highest crystallinities were reached are summarized in Table II. 

The mechanism of formation of zeolites is very complex, stemming from the diversity of chemical reactions, including various polymerization and depolymerization equilibria, nucleation and crystal growth processes. The physical and chemical nature of the reactants, which typically involve a source of aluminum and silicon along with hydroxides and salts determine the formation of zeolites. Physical effects such as aging, stirring, and temperature also play an important role. These effects lead to the complexity of zeolite formation, but are also responsible for the large number of frameworks that can be synthesized and the rich chemistry associated with this area. Cl. 21... 

Effect of Temperature. The temperature of a silicate solution also affects the polymerization of silicate anions in the solution. The distribution of silicate anions in an organic quaternary ammonium silicate solution at a fixed N/Si ratio and SiC concentration varies with the temperature of the solution (7,8,13,14,16). Ray and Plaisted (8) reported the temperature dependence of the distribution of silicate anions in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 2/3 and a SiC>2 concentration of 1.0 mol dm. The amount of the cubic octamer in the solution decreases with increasing temperature, and the cubic octamer practically disappears above 50 °C, indicating that the cubic octamer is unstable at higher temperatures. However, Groenen et al. (14) found that the cubic octamer remained in a significant concentration even at 85 °C, which was close to the temperature of actual zeolite formation, in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 1.0 and a Si02 concentration of 1.3 mol dm-. ... 

We have presented evidence that the interaction of molecules or ions occluded in the micropores between themselves and with the zeolite wall plays a decisive role in the thermodynamics of zeolite formation. 

Organic molecule properties favoring zeolite formation... 

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