Metastable zeolites

ZSM-20 is a highly metastable zeolite. Its preparation necessitates the use of very specific and drastic conditions low synthesis temperatures, adequate nucleation period and a careful selection of the ingredient nature and composition. A severe and simultaneous control of all the synthesis parameters is indispensible to obtain pure ZSM-20 in high yield and reproducible conditions. 

In particular, the Si/Al ratio of the FAU-polytype is higher than that characterizing zeolite Y but lower than that measured for the more siliceous ZSM-20. We conclude that under our temperature and compositional conditions, Aerosil provokes the formation of such a structure that accomodates the appropriate amount of A1 to get stabilized. At 170°C, the same gel yields the thermodynamically more stable zeolite Beta. This observation again goes in line with the general idea that the development of nuclei of metastable zeolites is favoured by a low temperature aging /nucleation of gels (51). 

Stability and high CEC of the residue, as well (refer Fig. 6.12). From Fig. 6.15 the residue of Step-3 can be assumed to be enriched in the more thermally stable form of zeolites FAZ at temperature lower than 600 °C. Mineral phase transition beyond this temperature in the TG-DTA curves can be attributed to conversion of Quartz and Mullite impurities to metastable zeolites. 

Since zeolites are metastable crystallization products tliey are subject to Ostwald s mle which states tliat metastable phases are initially foniied and gradually transfonii into tlie tlieniiodynaniically most stable product. The least stable zeolitic phase (tliat witli tlie lowest framework density) is tlierefore foniied first and consumed with furtlier syntliesis time at tlie expense of a more stable phase due to a continuous crystallization/redissolution equilibrium. 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

Pure silica zeolites or molecular sieves are metastable with regards to the thermodynamic stable polymorph at ambient conditions, a-quartz. However, they are... 

In their experiments with perdeuterioisobutane on various zeolites, Engel-hardt and Hall (36) found the carbenium ions to be metastable reaction intermediates. The lifetime of an intermediate was concluded to depend on the acid strength. 

Figure 1 shows the crystallization kinetics of ZSM-48. A good agreement is shown between the crystallinity evaluated by X-ray and adsorption of n-hexane. These kinetic curves confirm the metastability of ZSM-48 zeolite. Indeed the conversion of ZSM-48 into cristobalite, a dense and stable phase, occurs for long reaction times. The difference between the two curves at start reaction times is due to the presence of hydrated silica (Aerosil) that also adsorbs n-hexane. 

Note that dense silica polymorphs or silica-rich zeolites (ZSM-5, KZ-2, ZSM-22) are formed at the expense of metastable ZSM-48 for long reactions times in monoamine and diamine bearing systems (see above), indicating that excess of Al in the initial hydrogel does not play a particular role in the formation of these phases. In contrast, a higher Al content in the... 

As has been pointed out in the first part of this section, it is common to find two or more high-silica zeolites associated. This could be considered an example of metastable association but, equally, this could be considered an indication of a tendency toward a stable assemblage... 

Do these results also suggest that five-coordinate carbonium ions are not essential to explain alkane cracking The evidence is mixed. Kazansky and van Santen (132) reported low-level calculations and found a metastable carbonium ion (CH3-H-CH1) formed from ethane and a zeolite Brpnsted site, but this species was so high in energy that it did not appear to be thermally accessible. More extensive work by van Santen (133) shows, however, that the transition states leading from this species do not relate to ethane cracking Blaszkowski, Nascimento, and van Santen (134) found other transition states for ethane cracking (Fig. 26) that are similar to carbenium ions albeit with stabilization from the lattice. 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

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. 

For example, zeolites X and P are formed from the same reactants at 100°C and 150°C, respectively. Such phenomena are usually discussed in terms of successive transformation, with a metastable phase forming at first and converting successively to the more stable phase. 

Zeolite grown CNTs grow by thermal decomposition of template molecules within zeolite channels 0.45 Monodisperse diameter distribution, oriented tubes. CNTs metastable outside the channels... 

The importance of recognizing and dealing with zeolite synthesis as a kinetic process that involves the isolation of metastable phases is pointed out in this book in a variety of ways. An examination of the extensive scientific and patent literature on zeolite synthesis rapidly convinces one that a lack of understanding of this point has been a major bottleneck in the characterization of zeolite chemical and physical properties. The zeolite properties are defined not only by synthesis parameters, but also by treatment following synthesis for example, most synthesis treatment of zeolites with fluorine can be used to modify hydrophobicity drastically and increase catalytic activity for n-butane cracking. 

The synthesis of zeolite A, mixtures of A and X, and zeolite X using batch compositions not previously reported are described. The synthesis regions defined by triangular coordinates demonstrate that any of these materials may be made in the same area. The results are described in terms of the time required to initiate crystallization at a given reaction temperature. Control of the factors which can influence the crystallization time are discussed in terms of "time table selectors" and "species selectors . Once a metastable species has preferentially crystallized, it can transform to a more stable phase. For example, when synthesis conditions are chosen to produce zeolite A, the rate of hydroxysodalite formation is dependent on five variables. These variables and their effect on the conversion of zeolite A to hydroxysodalite are described mathematically. 

If one assumes that these different crystal species are merely metastable intermediates in the continuum HS-A-X-Y, the Si02 content of the precipitated amorphous precursor would specify the starting position in the continuum. The reaction environment would then dictate which species, with Si02/Al203 ratio to that of the precursor, will crystallize. Whenever the temperature is sufficient, the free energy relationship between these zeolites will permit any of them to form. 

Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na3KAl4Si40i6) or kalsilite (KAlSiOj (Y4), are of possible interest in relation to the formation of calcium aluminate cements. 

Potentially therefore we may be able to generate an unlimited number of possible zeolitic frameworks. Of these, only a portion is likely to be of interest as having desirable properties, with an even smaller fraction being amenable to synthesis in any given composition. It is this last problem, the feasibility of hypothetical frameworks, which is the key question in any analysis of such structures. The answer is not a simple one, since the factors that govern the synthesis of such materials are not fully understood. Zeolites are metastable materials, as shown by the calculated lattice energies reported in Table 1. Aside from this thermodynamic constraint, the precise identity... 

Gel dissolution Zeolite nucleation Crystal growth of the zeolite nuclei Dissolution/recrystallization of metastable phases (Oswald s Law of successive transformations)... 

Phillipsite is the most abundant zeolite in the surface sediments of the Pacific (Boles, 1977 Kastner and Stonecipher, 1978). Although it may be locally abundant (>50 wt.% on a carbonate-free basis, Bonatti (1963)), its etched surface, and the pattern of its decreasing abundance with the burial depth, suggest that it is a metastable phase under deep-sea conditions (Kastner, 1979). The primary mechanism of formation is thought to be alteration of basaltic glass, but it may also form by reaction of biogenic silica and dissolved AP+ (Arrhenius, 1963). Phillipsite is commonly found in association with authigenic smectite, and the combined formation of the two minerals may be represented as... 

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. 

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