Mechanism of zeolitization

Zhdanov (1) describes the mechanism of zeolite crystallization in terms of a quasiequilibrium between the solid and liquid phase in gels and emphasizes that the formation and growth of nuclei occurs in the liquid phase. 

While silicate and aluminate solutions have been extensively studied using 29Si and 27A1 NMR (see Sections III,B and III,I, respectively), there is only a handful of publications that investigate mixed (Si,Al)-bearing solutions by NMR spectroscopy with a view to elucidating the mechanism of zeolite synthesis. There is thus a considerable scope for further work in this important area. 

An extensive study was conducted on the effect of chemical and structural aspects of zeolites on the fire performance of the intumescent system, ammonium polyphosphate-pentaerythritol (APP-PER), where a marked improvement of the fire-retardant properties within different polymeric matrices has been observed.56 58 The synergistic mechanism of zeolite 4A with the intumescent materials was investigated using solid-state NMR. Chemical analysis combined with cross-polarization dipolar-decoupled magic-angle spinning NMR revealed that the materials resulting from the thermal treatment of the APP-PER and APP-PER/4A systems were formed by carbonaceous and phosphocarbonaceous species, and that the zeolite enhances the stability of the phosphocarbonaceous species. 

Coking Mechanism of Zeolite for Supercritical Fluid Alkylation of Benzene... 

To obtain the coking mechanism of zeolite catalyst for SCFP alkylation of benzene, two kinds of the zeolite used in LP and SCFP alkylation processes were analyzed by using the conventional catalyst analysis methods. Fresh zeolite is also analyzed for comparison. 

In the early days of XPS the first applications of the technique to zeolites were dealing with the determination of Si/Al ratios calculated using equation (23) and their comparison with bulk values [14-16]. This of course allowed to detect important compositional gradients in the surface region, a piece of information which is related to the mechanism of zeolite synthesis and which is technically important in order to monitor the concentration of Brdnsted acid sites on the external surface of the zeolite crystals. The measurement of... 

These calculations yield, subject to some simplifying assumptions, relative T-site alumimun substitution energies computed (1) for the thermodynamic equilibrium state, (2) at zero K and (3) for models devoid of non-firamework species. Framework zeolites, metastable structures, are produced under luetic control and if, as indicated by the most recent calculations, the relative T-site substitution energies for the (Cerent sites are not grossly disparate, the actual distributions in reed materitds will be determined by the particular conditions of synthesis. As the molecular-level mechanisms of zeolite sjmthesis remain obscure, we especially need some experimental indicator of which sites are actually adopted by aluminum in real MFI-framework materials. 

Kumakiri I. Preparation and permeation mechanism of zeolite membranes. PhD Dissertation, The University of Tokyo, Japan, 2000. Pera-Titus M, Llorens J, Cunill F, MaUada R, and Santamaria J. Preparation of zeolite NaA membranes on the inner side of tubular supports by means of a controlled seeding technique. Catal Today, 2005, 104 281-287. 

The study of alkaline silicate and aluminosilicate solutions is of fundamental importance for a better understanding of the mechanism of zeolite synthesis (1,2). Real progress has been realized durinq the last fifteen years thanks to the use of new techniques such as the trimethylsilylation (3,4) of polysilicic acids (followed by a chromatographic separation of the derivatives) and, even more, 29Si and 27Ai NMR spectroscopy (5-11). The latter techniques have enabled the identification of about twenty types of oligomers in silicate solutions and in some cases estimates of their respective concentrations. The investigation of alkaline aluminosilicate solutions is... 

Studies on the crystallization process and formation mechanism of zeolites are very important not only because of their theoretical significance but also due to practical... 

The proposition of different views on the formation mechanism of zeolites is due to the complexity of the formation of zeolites. The crystallization system of zeolites has solid and liquid components. The solid phase contains amorphous gel and zeolite crystals, while the liquid phase contains silicate ions with various polymeric states, aluminate, and aluminosilicate ions with various structures and polymeric states. The procedure for the synthesis of zeolites is not complicated, but the crystallization mechanism involved is... 

In the past 10 years, many characterization techniques have been used in the in situ study of the formation mechanism of zeolite from clear solution. For example, Honssian et al. studied the TPA-silicalite-1 system by using the small-angle X-ray scattering technique [34] Carlsson et al. performed a modeling study on the crystallization of silicalite-I from a liquid-phase system [35] Smaihi and colleagues applied in situ 27A1-,... 

Condensation Reaction between Silicate and Aluminate Ions in Solution Along with the ongoing study of the crystallization mechanism of zeolites, it becomes increasingly necessary to study the polymerization state of aluminosilicate ions in solution and the condensation reaction between silicate and aluminate ions. The polymerization state of individual silicate and aluminate ions has been extensively studied, which has been discussed in Section 5.1.1. However, studies on the... 

M. Smaihi, S. Kallus, and J.D.F. Ramsay, In-situ NMR Study of Mechanism of Zeolite A Formation. Stud. Surf. Sci. Catal., 2001,135 (Zeolites and Mesoporous Materials at the Dawn of the 21st Century), 271-278. 

Mintova, S, Olson, N.H., Valtchev. V., Bein, T. 1999. Mechanism of zeolite A nanocrystal growth from coUoids at room temperature. Science 283. 958-960... 

There have been several attempts to understand the synthesis mechanism of zeolites but still a complete understanding is yet to emerge. The causes for formation of different topology of silica lattice, the mechanism of incorporation of A1 in place of Si and the variations in the stability of zeolite lattices are certain intriguing questions questions remaining unanswered. There are two widely accepted proposals for the synthesis ... 

Density functional methods is an attractive alternative to Hartree Fock method due to its computational efficiency. There are density functional methods with different correlation functions [42-48]. Recently several calculations [49-53] were done on zeolites of varying cluster models with density functional methods. Periodic density functional method similar to periodic Hartree Fock is also being used recently [54]. Thus the quantum chemical technique has been at the center stage among the other computational methods in improving our understanding of stmeture and reactivity of zeolites. We apply this powerful technique to study the mechanism of zeolite synthesis. 

Zeolite properties are being studied by nearly every type of modern scientific discipline, and they are being utilized in many new chemical engineering processes. Important advances include detailed basic information on cations in zeolites, more understanding of the mechanism of zeolite formation, the formation and character of structural defects and hydroxyl groups, the role of zeolite structure in adsorption and catalysis, and the increasing technology of the use of molecular sieve zeolites in catalysis and adsorption. 

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