Zeolites nucleation

The general behaviour of the alkali cations in presence of aluminosilicate gels and their influence on zeolite nucleation and crystallization rates has been discussed in detail elsewhere (26). Their specific role in the formation of the various (M)ZSM-5 zeolites can be depicted on the basis of the above described observations. ... 

Since some effects, observed during the crystallization of zeolites from gels, as for instance, the autocatalytic nature of zeolite nucleation (2,3,12, Katovic, A. Subotic, B. mit, I. Despotovic, Lj. A. Zeolites, in press) cannot be readily explained only by the classical approaches to the nucleation processes in the liquid phase (2,6,10,12), the objective of this work is to analyse the crystallization kinetics of different types of zeolites by the kinetic equation derived on the basis of Zhdanov s idea on autocatalytic nucleation (2), in order to explain the influence of autocatalytic nucleation on zeolite crystallization processes. 

The NMR and X-ray diffraction data are only consistent with substitution of boron into the framework structure of the mordenite. Although we prepared boron substituted mordenite directly from modified gels, direct synthesis has severe limitations. The solution chemistry of the substituting element can interfere with zeolite nucleation and crystallization, as... 

During the first restructuration of the silicate or Si-rich aluminosilicate complexes prior to the zeolitic nucleation, the HM++ entities play a structure stabilizing role. By their particular linear shape, they favour the formation of channel systems. Simultaneously, they can also neutralize one AIO2- negative center of... 

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

Zeolite formation in nature follows pathways which are familiar in laboratory synthesis. Zeolite nucleation, crystallisation and crystal growth lake place as a result of slow... 

It has therefore been concluded that (i) the most common process of zeolite nucleation relies on a primary nucleation mechanism and (ii) the most probable primary nucleation mode is heterogeneous and centred upon the amorphous phase of the reaction mixture (which for most clear solution syntheses is colloidal in nature). 

SUMMARIES of POSTER PRESENTATIONS 02 - Zeolite Nucleation and Growth (Monday)... 

P-32 - Influence of the thermal treatment of the aluminosilicate gel precursor on the zeolite nucleation... 

NOx reduction by propene NOx reduction with hydrocarbons NOx sorption, metal zeolites NOx storage, metal zeolites Noble metal zeolite Noble metals/MCM-4I Nonasil, synthesis Non-ionic surfactant Nu-86 synthesis NU-88, cracking catalyst NU-88, hydroconversion catalyst NU-88, pore topology nuclear waste Nucleation agent, zeolitic Nucleation and growth Nucleation, FAU Nucleation, LTA Nucleation, zeolites Nucleophilic Substitution... 

A key current limitation in the discovery of new zeolites is the lack of fundamental understanding of the zeolite nucleation and crystallization process. Therefore, correlations between structure-directing templates and the resulting zeolite material have to be achieved [3],... 

On September 25-30, 1988 in Los Angeles, California the first ACS Symposium on zeolite synthesis emphasized the importance that gel chemistiy, zeolite nucleation, crystal growth, crystallization kinetics, and structure-directing phenomena have in understanding zeolite (and molecular sieve) synthesis. The objectives of a similar ACS Symposium held in New York on August 25-30, 1990 where expanded to include papers on pillared clay synthesis and on the synthesis of other microporous materials that could be used in catalyst preparation. About 90% of all the chemical processes in the U.S. are based on catalysis and today catalysts have become indispensable to petroleum refining, an industry that in 1990 had sales of 140 billion (U.S. Dept, of Commerce U.S. Industrial Outlook, 1991). 

Zeolite synthesis can also be approached from classical nucleation-crystalli-zation theory [105]. A viable nucleus is estimated to have a size between one and eight unit cells, depending on the structure type and the experimental synthesis conditions [106]. However, because of crystallization from high surface area systems, the energetics of zeolite nucleation can be significantly different from that of more dense phases [107]. Advanced techniques such as HRTEM and cryo-TEM are the basis of the experimental information about zeolite nucleation [108]. 

In order to try to obtain information at the molecular level characterizing how zeolites nucleate and grow, more study of the chemistry of solutions containing dissolved silicate, aluminate, and aluminosilicate species is essential. It is important to know what anionic species are present as a function of temperature, pH, and concentrations of dissolved Si, Al, or other elements. Such work, carried out with the best available physical methods, is already in progress. 

It is often the case that the first zeolite to appear is not stable when left for a longer time at the reaction temperature in contact with its mother liquor. The first-formed crystals slowly dissolve a new type of zeolite nucleates and grows, thus consuming the first. Indeed this process may be repeated more than once, so that the second zeolite is consumed at a later time by yet another. Because the second zeolite usually has a different morphology and crystal size, rather than being a pseudomorph of the first, the transformation of the first into the second can reasonably be ascribed to a solution mechanism rather than to a solid-state rearrangement of the lattice of the first zeolite to give that of the second. 

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