Zeolite synthesis nucleation

Zeolite synthesis proceeds via nucleation, which is a consequence of local fluctuations, small in extent but considerable in degree of departure from the mean for the solution, followed by spontaneous growth of nuclei exceeding a critical size. 

The explanation of some experimentally observed features of zeolite synthesis follows from the treatment. A distinction is made between zeolitic stabilisers and nucleation templates. 

The use of a law typical of the solid state to describe the growth of zeolites was probably suggested by the presence of an amorphous gel in the synthesis medium. The rearrangement of this phase to form a zeolite network has been a favored theory until researchers showed that zeolites can nucleate and grow directly from solutions free of suspended solids (9). 

From the above results we can propose the following mechanism for hexagonal MCM-41 synthesis. At low crystallization temperature or short crystallization time a fibrous agglomerate structure is often observed by SEM on intermediate samples. The 100 and 200 reflections are not detected by XRD and the value of the specific surface area is low. This reflects the initial step of synthesis which is generally referred to the nucleation step in zeolite synthesis. After this step, the 100 and 200 reflections are present on the XRD diffraction pattern. The value of the specific surface area is between 700 and 900 m /g. The fibrous agglomerate structure disappears and crystals of MCM-41 appear. This corresponds to the crystallization step. Finally if both the synthesis temperature and time are continuously raised, a triphasic mixture MCM-41, MCM-50 and amorphous phase is identified by XRD. The... 

C.G. Pope, Nucleation and Growth Theory in Zeolite Synthesis. Microporous Mesoporous... 

Zeolites are a subclass of microporous materials in which the crystalline inorganic framework is composed of four-coordinated species interconnected by two-coordinated species. Traditionally these materials are aluminosilicates however, many different compositions have been synthesized. The templates used in the synthesis of microporous materials are typically small ionic or neutral molecular species. The function of the template in the synthesis of microporous materials is little understood, and there are at least four different modes by which an additive can operate in a zeolite synthesis a) It may act as a space filler occupying the voids in the structure, thereby energetically stabilizing less dense inorganic framework b) the additive may control the equilibria in the synthesis mixture, such as solution pH or complexation equilibria c) it may preorganize the solution species to favor the nucleation of a specific structure d) it may act as a true template determining the size and the shape of the voids in the structure. 

The statement is frequently made that one zeolite synthesis is faster than another but the measurement criteria may be extremely loosely defined. Often, there is no distinction between the induction time and the growth period (section 6), so that it is impossible to tell whether a reaction is slow because it takes a long time to nucleate or because the crystals grow slowly in the given circumstances. A comparison of some hypothetical synthesis reactions is shown in Table 2. The data are derived from a computer simulation of zeolite growth based on a very simple kinetic crystal growth model [50,73], i.e. 

It seems reasonable to suppose that the construction of the nucleus of a zeolite crystal may involve a more complex assembly process than is necessary for simpler substances whose unit cells are smaller and contain far fewer atoms, although there should be no difference in basic principles. The nature of the nucleation process in zeolite systems is also complicated by the nature of zeolite synthesis sols. These contain observable solid phases (amorphous gel, crystals) and components (cations, anions) in true solution. However, there is frequently also a colloidal component, invisible to the naked eye, which itself may be amorphous or crystalline. 

Nucleation processes in zeolite synthesis revealed through the use of different temperature-time profiles... 

Wu, M.G. Deem, M.W. Monte Carlo study of the nucleation process during zeolite synthesis. J. Chem. Phys. 2002. 116.2125-2137. 

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

In this rather new field of zeolite synthesis [5], preliminary information on precursors in solution has been obtained by measuring the free fluoride ion concentration with a specific fluoride anion electrode. The mean number N of F bonded to one Si or other T atom (T = Al, Fe, Ga) has been computed and results are summarized in Table 2. No information on the exact nature of the species in solution is available discussed in the case of OH" type synthesis. Such a study has the potential of revealing some interesting information given the slower nucleation/crystal growth processes, the smaller number of metastable phases observed and the crystallization of almost defect-free zeolites in this medium [5]. F NMR could be added to the other spectroscopic characterization techniques for such solutions. 

Initially, the number of nuclei formed per unit time is expected to increase. As nuleation and growth is assumed to consume the same precursor species, both processes at a given stage will start to compete. The nucleation rate will therefore pass through a maximum in time and then decline, in zeolite synthesis many experimental S-shaped crystallization curves have been observed (16,18). Zdhanov and Samulevich advanced a method for analysis of the nucleation and growth part of such curves (25). 

SAXS measurements are normally performed in situ and therefore require access to synchrotron radiation [21, 23, 27, 46]. SAXS is probably one of the most used in situ techniques for the study of zeolite synthesis. In situ cells, similar to those for WAXS or XRD measurements, can easily cope with elevated synthesis temperatures and pressures. It has been used to determine the activation energy of nucleation, where two independent studies show that it is around 70-85 kj mol [47, 48]. Also, three distinct particle sizes were observed in the reaction mixtures, being primary units of approximately 2.8 nm, their aggregates, and the actual zeolite crystals [47]. Other mechanistic studies include the effect of the precursor molecule, the chemical nature of which appears to be important in... 

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). 

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