Crystallisation of zeolites

The clear aluminosilicate solutions from which Ueda et al (4 ) studied crystallisation of zeolites Y, S and P were based on the composition range 1ONaO.(0.35-0.55)A1 0 (22-28)SiO. (250-300)HO. Figure 1... 

We can go beyond the case of fast-ion conductors and apply these principles to understand the role of templating species in inorganic and organic synthesis. A number of examples of zeolite frameworks have been shown in Chapter 2 to follow closely IPMS. The crystallisation of zeolites invariably requires the presence of templating species. A wide variety of templates have been used, from sodium ions, tetra-alkyl ammonium ions to crown ethers. 

Serrano, van Grieken 2001 Heterogeneous events in the crystallisation of zeolites [391... 

The s)mthesis of zeolites is traditionally performed by crystallisation from a sol-gel mixture comprising reagents such as silica, sodium aluminate, sodium hydroxide and water. Another key component of the sol-gel mixture is a base whose main role is to regulate the pH of the mixture. If an organic base is used then a templating effect may also be observed... 

The powder X-ray diffractograms of the composites present the characteristic peaks of the encapsulated zeolite [9]. It is interesting to observe that no characteristic peak of chitosan was present in the XRD patterns of composites. It is clear that the presence of the zeolites has prevented the crystallisation of chitosan when the gel has been dried. This phenomenon has been already observed when the zeolite content reaches a threshold value and has been attributed to a strong interaction between zeolite and chitosan [10],... 

In the X-ray powder diffraction patterns of the composites, the disappearance of the broad band centered at 22 °20, typical of amorphous silica, indicates that the zeolitisation of the mineral fraction of the parent composite was complete. In no diffraction pattern any sign of crystallised chitosan could be found. The two methods in which the silica-polymer beads were extracted from the aluminate solution after impregnation (methods A and C) allowed the formation of the expected zeolite X, with traces of gismondine in the case of the method C. The method B, in which excess aluminate solution was present during the hydrothermal treatment, resulted in the formation of zeolite A. 

Figure 1. Crystallisation fields of zeolites Y, S and P at 100°C from clear aluminosilicate solutions. In the cross-hatched area gel and solution co-exist. (Reproduced with permission from Ref. 4. Copyright 1984 Butterworths.)...

For crystallisation of Y the optimum composition of solution was 10Na 0.0. 45A1 0. 26Si02.270H2<). Faujasite (zeolite Y) appeared after 12 hours, and the crystals were of rather constant composition, having, over all the experiments, ratios SiO Al O between 5.1 and 5.6. Because these ratios are so much greater in the parent solutions the A1 has been selectively removed from solution and its concentration therein drops much faster than those of Na and Si. 

Nucleation and crystallisation kinetics generally follow S-shaped crystallisation curves as shown for zeolite A in Figs. 8.28 and 8.29. This means that a rather long incubation time or nucleation period precedes the crystallisation (compare Figs. 8.28b for zeolite A and 8.29b for ZSM5). The general trend in the kinetics of zeolite (ZSM5) synthesis can be summarised as follows ... 

Finally, Vroon et al. [82,97] reported the synthesis of continuous porous films of ZSM5 on top of y-alumina supported membranes (pore diameter 4 nm) by slip-casting with a zeolite crystal suspension. The porous zeolite layers (thickness 1-2.5 pm) consist of densely packed zeolite crystals with a diameter of 70-80 nm and with micropores in the zeolite and mesopores (diameter 8-24 nm) between the zeolite particles. This zeolite layer can be used as a support for further processing, e.g., pore filling of the mesopores or deposition of catalysts. First experiments by Vroon et al. to fill the mesopores by in situ crystallisation of MFI in the pores did not result in gas-tight membranes... 

J.M. Lachman, Method of crystallisation of a zeolite on the surface of a monolithic ceramic substrate. U.S. patent 4.800.187,1989. 

Samples of zeolite NU-85 were prepared by crystallisation from reaction mixtures of molar composition. 

The starting point for this work was the preparation of zeolite EU-1 [7], that is, crystallisation from a reaction mixture of (molar) composition ... 

The thermal behaviour of zeolites has thoroughly been investigated. When heated, a zeolite powder undergoes a series of physical and chemical changes, which include water loss, decomposition and gas evolution, phase transition, structure breakdown, re-crystallisation, melting, and others [75J. The thermal characterisation of natural zeolites has been carried out by various techniques and the relevant data may be found in several publications [44,76-78]. 

This short review of zeolite and zeotype synthesis is written for those who are relatively new to the field. It aims to present an overall introduction to some fundamental aspects of the subject and to indicate where further information can be found. An account of experimental practice is followed by a summary of mathematical modelling procedures. Observations from crystallisation studies then introduce basic principles of the synthesis process. 

STUDIES OF ZEOLITE AND ZEOTYPE CRYSTALLISATION A STEP-BY-STEP DESCRIPTION OF THE SYNTHESIS PROCESS... 

The procedure of Zhdanov and Samulevich enables the calculation of isothermal nucleation rate profiles from determinations of growth rate and crystal size distribution [16,82]. Originally implemented in analyses of zeolite Na-A [83] and Na-X [82] crystallisation, the method has subsequently been applied to other zeolite systems, including silicalite [84,85]. If it is supposed that all the crystals in a batch have the same (known) growth rate behaviour, the total growth time of each crystal can be calculated. Assuming also that the nuclcation point for each crystal can be obtained by linear extrapolation to zero time, the nucleation profile for the whole batch can be determined from their final sizes. 

In a typical zeolite synthesis, the first definite evidence for a successful reaction is the appearance of crystals of the product. As noted above (section 6.1), this signal for the end of the induction period is dependent upon the method of detection most commonly a combination of visual inspection or microscopy with X-ray diffraction. Thereafter, crystal growth can be monitored by the same techniques and the resulting S-shaped growth curve of bulk crystallinity against time is by far the most commonly reported measurement of zeolite crystallisation kinetics (fig- 2). 

Throughout the history of the synthesis of zeolite-like materials, there has been much debate about the location of the structure-forming activity [28,36,39,50]. At one extreme, there is the possibility of solution-mediated crystallisation. In this view, the reactants dissolve (to a greater or lesser extent) in the reaction mixture to afford active species, from which the product is formed as it crystallises from the solution. In the opposite view, the solution is seen as more or less inert with the product being formed within the gel phase by a process described as a solid state transformation . Whereas the solution-mediated route is well known in the science of crystallisation, the alternative is a somewhat shadowy phenomenon for which no chcmically-specific mechanism has ever been published. 

One of the most fundamental basis of the hydrothermal synthesis of zeolites is the mineralizing role of water, which is greatly cissisted by the free OH concentration in the solution / hydrogel. Apart from this basic requirement of mineralizability, other factors like. Si / A1 molar ratio, pH of the gel, aging at lower temperature, crystallization temperature and time etc., influence the type and quality of the crystalline material in rather specific ways [1]. It is clear that the enhancement of the crystallisation rate is not much dependent of the choice of counter cation (H, Na or K) of a particular oxyanion promoter, at least for high silica... 

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