Mechanism of zeolite synthesis

Zeolite synthesis occurs at temperatures between 100 and 200°C in a basic medimn under hydrothermal conditions. Usually high concentrations of sUica and almnina sources are used and synthesis occurs from a state where initially a gel is formed. Upon heating, silicate and aluminate species dissolve from the gel and chemistry relevant to crystallization occurs in the solvent phase. Nucleation itself is assisted by nucleation sites offered by the gel material. 

The most detailed picture of zeolite synthesis available to us is for siliceous silicalite. We will discuss this here in an attempt to illustrate molecular recognition of template, evolutionary recombination of reaction intermediate oligomers and self replication. Detailed information became possible once homogeneous conditions for zeolite formation in silicate synthesis were found in the absence of gel formation. For a concise and excellent review on general mechanistic aspects of zeolite synthesis we refer to Cundy an Cox l 

A basic silicate solution consists of a mixture of many different monomers, illustrated in Fig. 8.13. Monomers, dimers, trimers and higher oligomers are formed, in ratios that 

The clusters in Figs. 8.14 and 8.15 have been assembled from oligomers as shown in Fig. 8.13, by a process which is driven by the specific stabilizing interaction of the tetrapropylammonium cation and silicate cluster. The Sisa cluster apparently has an optimum interaction between the tetrapropyl ion and the silicalite cage. It is the result of self assembly 

Interestingly, the local environments of the tetrapropylammonimn ion in the Siss precursor and of tetrapropy ammonium in silicalite are slightly different. The Siss environment resembles that of a channel, whereas the tetrapropylammonium ion occupies a channel cross-section in silicalite. The need to reorient the tetrapropylammonium ions when the zeolite crystal is formed may necessitate the intermediate formation of nanoslabs. The dimensions of these nanoslabs depend on the reaction conditions. Their size is typically 4 X 4 X 1.3 nm or 4 x 2 x 1.3 run. The number of 33 cluster rows varies in this example by a factor 2. 

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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. 

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... 

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... 

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. 

Much is now known about the general mechanism of zeolite synthesis, and recent microscopic evidence has shed light on both nucleation (TEM of... 

Murayama, N., Yamamoto, H., Shibata, J. Mechanism of zeolite synthesis from coal fly ash by alkali hydrothermal reaction. Inter. J. Miner. Process. 64, 1-17 (2002)... 

Murayama et al. [3] have explained the mechanism of zeolite synthesis from coal fly ash by its hydrothermal reaction with alkali. They have observed that alkaline medium type (viz., NaOH, Na2C03 and KOH) affects the mechanism of crystallization of zeolites. The authors have employed synthesis matrix bearing a specific solid/slurry ratio (i.e., 100 g/400 cm ), with the slurry being an aqueous mixmre of two different alkalis (viz., NaOH and Na2C03, NaOH and KOH and Na2C03 and KOH) to investigate the effect of the presence of different cations and/or anions on the alkali activation of fly ash. It has been reported that zeohtes, P and Chabazite, are the main crystals present in the synthesized product. The OH in the alkali solution remarkably contributes to the dissolution of Si" and Al " from coal fly ash, whereas Na" makes a contribution to the crystaUization of zeolite P, which has the tendency to capture K" in the cation exchange process. 

Future applications will also be considered, in particular with respect to the use of these techniques combined with the NMR of other nuclei (Na, Al, Si) for the investigation of zeolite synthesis mechanisms (12-14). 

Spectroscopic Studies of Zeolite Synthesis Evidence for a Solid-State Mechanism... 

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. 

The structure work consists of synthesizing the faujasite material and characterizing it by X-ray diffraction. The mechanism of the synthesis has been studied and an investigation has been made of the nature of the replacement of the sodium ion by the ammonium ion and of the details of the process of the decomposition of the ammonium ion into the protonic zeolite (Bronsted acid) and decationated zeolite (Lewis acid). (We shall call the material in which the cation has been displaced by a proton and then heated to remove the proton as water, the decationated material.)... 

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... 

B.D. McNicol, G.T. Pott, and K.R. Loos. Spectroscopic Studies of Zeolite Synthesis Evidence for a Solid-state Mechanism, in Molecular Series . Adv. Chem. Ser., 1973, 121, 152-161. 

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 ... 

For the first time we report the synthesis of macrocycles like calixpyrrole, cyclotriveratrylene (CTV), cyclotetraveratrylene (CTTV), porphyrine ete over molecular sieve as a catalyst. Calixpyrroles are synthesized from pyrrole and a ketone like acetone over MCM-41 under reflux conditions using suitable solvent. In case of MCM-41 cyclic calixpyrroles were obtained. On the other hand due to shape selectivity in case of Y zeolite linear di-, tri- and tetra- polypyrroles were obtained and cyclic tetramers were not observed. The mechanism of the synthesis of calixpyrrole is either by the dimerization of dimer with simultaneous cyclization to cyclic tetramer or cyclization of linear tetramer via recoil phenomenon. 

By contrast, the mechanisms of the synthesis of ordered mesoporous solids are very dilferent from those of silicate and phosphate frameworks. For control over the synthesis of mesoporous solids, the key is to understand the interactions of micellar surfactants with condensable inorganic framework-building units. Synthetic routes are also being developed to prepare mesoporous silicates made up of nanoparticles of zeolites, with the aim of combining advantages of microporous and mesoporous solids. 

Bodart, R, B. Nagy, J., Gabelica, Z. and Derouane, E.G. (1986) Rictors governing the synthesis of zeolites from aluminosilicate hydrogels a comparative study of the crystallization mechanisms of zeolites Y, mordenite and ZSM 5 ,/. Chim, Phys.-Chim. Biol 83(11-12), 777-790. 

Inorganic systems are much less sophisticated than the immunoresponse system, but similar in that, in principle, information on the transition state through the correct choice of template can be incorporated into the catalytic system such as in the case of zeolite synthesis. The mechanism of the zeofite synthesis reaction, discussed in the next, section has combinatorial evolutionary characteristics. 

The objective of this chapter is to review the open literature on molecular sieve zeohte synthesis, highlighting information regarding the fundamental mechanisms of zeolite crystallization in hydrothermal systems. The text, therefore, focuses on the three primary mechanistic steps in the crystallization process nucleation of new populations of zeolite crystals, growth of existing populations of crystals, and the role played by existing zeolite crystal mass in the subsequent nucleation of new crystals or the growth of zeolite crystals in the system. 

Consequently, Chapter 1 written by R. W. Thompson gives a modern account of our present understanding of zeolite synthesis. The fundamental mechanisms of zeolite crystallization (primary and secondary nucleation and growth) in hydrothermal systems are highlighted. 

Silicalite membrane useful for separation of n-Hexane and branched hydrocarbon (MP, DMB) at 100 - 300°C. 2 step hydrothermal synthesis (Evolutionary selection mechanism) of zeolite membrane has improved to create perfect zeolite layer. Hexane nano-permporosimetry technique determined pore size range 1 to 20 nm. The instrument was also found to be useful tool to confirm the membrane quality. 

As it was described above the mechanism of zeolite crystallization is a very complex one based on several steps each depending on both thermodynamic and kinetic factors. It is not a big surprise that many of them govern zeolite-a synthesis. The main physicochemical factors are listed below ... 

The traditional way to seek new zeolites was primarily based on a trial-and-error synthesis process. With the growing needs for applications of zeolite materials, the rational synthesis of zeolites with desired structures and functions has become the objective of synthetic chemists. However, the rational synthesis of zeolites with desired structures is still a great challenge due to the unclear formation mechanism of zeolite crystals. With the advancement of understanding the synthesis chemistry and strucmre chemistry of zeolites in recent... 

Abstract Though, naturally occurring and chemically synthesized (pure grade) zeolites have been used for various industrial applications in the past, their increasing demand for several novel applications (viz., as adsorbent or absorbent for waste water decontamination, soil remediation as fertilizers, aqua-culture purification, etc.) warrants their enhanced production. With this in view, several researchers have attempted to synthesize zeolites from the fly ash, an abundantly available industrial by-product, as described in this chapter. Furthermore, different methods employed for synthesis of fly ash zeolites, the mechanism of zeolites formation and potential fields of their appUcations have also been included herein. 

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