Zeolite synthesis crystallisation

Control of pH is critical in the determination of the Si/Al ratio. As the pH increases, the ability of the silicate to condense decreases because of a decrease in the amount of Si-CCspecies relative to Si-OH. The anionic form is necessary in order for the initial nucleophilic attack to take place. In contrast, the condensation rate of Al(OH) 4 remains constant and so aluminium-rich zeolites crystallise preferentially at high pH and vice versa. Zeolite synthesis also depends on a wide range of experimental parameters, including concentrations and degree of supersaturation, the source of the framework materials, solvent... 

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... 

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). 

The main issue in zeolite synthesis is the control of the specific phase that crystallises, i.e. the phase selectivity of the crystallisation. Nobody can tell for sure whether a new given set of conditions (chemical composition, temperature, time, preparation, stirring and other synthesis parameters)... 

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. 

The synthesis and characterisation of silicalite-1 membranes on porous alumina ceramic supports have been described here. The growth of the silicalite-1 membrane could be optimised by controlling the hydrothermal synthesis conditions. It has been shown that by controlling the synthesis conditions it is possible to optimise the growth and structure of silicalite-1 membranes. Thus at lower synthesis temperatures (150 °C), the growth of silicalite inside the macro-pores of the ceramic support is favoured. At higher temperatures (190 °C), thick, well crystallised zeolite layers develop from the surface of the support. A more stable membrane is... 

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... 

Jia/Noble and coworkers [87,88] reported the successful synthesis of silicalite membranes on y-alumina composite supports using an interesting modification of the in situ crystallisation method. The support consisted of a short a-alumina tube coated on the inside with a 5 pm thick y-alumina film with an average pore diameter of 5 nm, commercially available from US Filter. The precursor solution was put into the support tube after plugging both ends with teflon and the filled tube was then placed in a teflon-lined autoclave. Hydrothermal treatment was carried out at 180°C for 12 h. After removal from the autoclave and washing the formed zeolite layer with water, the procedure was repeated with the tube inverted from its previous orientation to obtain a uniform coating. As reported by Vroon et al. [82,84,98], Jia/Noble [88] also concluded that at least two synthesis steps are necessary to obtain defect-free membranes. 

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... 

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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