Mechanism of zeolites formation

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. 

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. 

In order to reduce the total time of synthesis ( 30 min), it has been reported to employ two stage synthesis processes, an initial microwave heating for around 30 min. followed by the conventional synthesis method [8, 40, 41]. Although, the purity of zeolite has been of much concern, the final yield used to contain unreacted (read residual) fly ash, which can affect the characteristics of synthesized zeolite. In fact, the presence of considerable quantity of unreacted fly ash residue in the final yield can be attributed to the factors, which are being discussed in the next chapter The mechanism of zeolite formation . 

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. 

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... 

The mechanism of HS formation is less obvious. The stabilizing effect of TMA on high silica sodalite units(22) may have some bearing. The destabilization of D4R and D6R at high Na/Si ratios in solution (high pH ) has been observed(16). HS forms as higher pH than the other zeolites studied here. This may lead to a mechanism Involving SnR. 

Gasoline composition from hexadecane cracking over calcined and steamed AFS and USY zeolites can be represented by general correlations as shown in Figure 5. As these correlations are unique to zeolite Y, they indicate that the Y zeolite framework topology plays an important role in the mechanism of product formation. The method of dealumination and subsequent steam treatment lead to various PONA compositions however, these compositions result from a... 

The formation of hydrocarbons from methanol catalyzed by zeolite H-MFI has been investigated extensively 60,61). As with many hydrocarbon conversions, the catalytic activity of the methanol-to-hydrocarbons reaction decreases over time as a result of the buildup of high-molecular-weight carbonaceous deposits (coke). UV Raman spectroscopy was employed to characterize the accumulation and chemical nature of deposited hydrocarbons as a function of time and reaction temperature with both methanol and dimethyl ether as reactants and with zeolite MFI of various Si/Al atomic ratios as catalysts the first account of this work reported results for a zeolite MFI with low acid content (Si/Al = 90) (62). Both polyolefin and a cyclopentadienyl species were observed as intermediates during the formation of polyaromatic retained hydrocarbons. These observations strongly confirm the mechanism of coke formation proposed by Schulz and Wei (63) involving aromatic formation via a five-membered ring... 

Researchers have attempted to elucidate the mechanism for zeolite formation for over 50 years. Two directions of experimental work have been examined ... 

The manufacture of the industrially important zeolite types A, X and Y is generally carried out by mixing sodium aluminate and sodium silicate solutions, whereupon a sodium aluminosilicate get is formed. In this gel Si02- and Al203-containing compounds pass into the liquid phase, from which the zeolites are formed by cry.stallization. As the zeolite growth components are removed from the solution more gel dissolves. The reaction mechanism for zeolite formation is presently not yet fully understood. There is experimental evidence that, depending upon the reaction conditions, different mechanisms are possible. 

Photolyses of 3-ch 1 oro-3-pheny 1 -3//-dia/irine (63) were carried out within CyDs and FAU zeolites to foster the ring expansion of the carbene intermediate, chloro(phenyl)carbene (64). However, benzaldehyde (PhCHO) was formed instead of products derived from l-chloro-l,2,4,6-cycloheptatetraene (65). Since CyD innermolecular products were also formed, control experiments in relevant solvents were performed. The true structure of the innermolecular product that was formed in alkaline buffer, and then observed via FAB MS and RP HPLC, has not yet been determined. The ability of the basic medium to preserve the innermolecular product and reduce the amount of PhCHO formed seems to indicate that the latter stems from the former, e.g., O-H insertion product 82. However, the exact mechanism of PhCHO formation within the CyD ICs could not be pinpointed. 

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-,... 

R.R. Xu and X.S. Liu, The Mechanism of the Formation and the Crystal-Growth of Molecular-Sieve-Zeolite. 9. The Liquid-Phase Mechanism of the Formation of Zeolite K-... 

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. 

Carbenic mechanisms. Venuto and Landis [10] were the first to address the question of mechanism of hydrocarbon formation from methanol over zeolites, in this case zeolite X [11]. These workers proposed a scheme involving a-elimina-tion of water and polymerization of the resultant methylcarbenes to olefins. Swabb and Gates [12], elaborating on Venuto-Landis, proposed that concerted action of acid and basic sites in the zeolite (mordenite) facilitates a-elimina-tion of water from methanol. According to Salvador and Kladnig [13], carbenes are generated through decomposition of surface methoxyls (a-el imination of silanol) formed initially upon chemisorption of methanol on the zeolite (zeolite Y). Hydrocarbons are assumed to form, in the latter two schemes, also by carbene polymerization. 

ON THE MECHANISM OF HYDROCARBON FORMATION FROM METHANOL OVER PROTONATED ZEOLITES... 

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 ... 

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