Hydrothermal Synthesis

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Hydrothermal Synthesis

Many materials are made hydrothermally, that is using aqueous solutions as a crystallization medium. Zeolites/catalysts are extremely important members of this class. Typical synthesis conditions might be 80-250°C and 1 to 20  bars; as such one might consider these conditions to be a hybrid of the low end of high temperature and high pressure. A zeolite synthesis might consist typically of loading alumina and silica gels together with a templating agent into a hydrothermal bomb which is then sealed. As the temperature around the bomb is raised an autogeneous pressure is developed inside the bomb, that is the pressure at which the liquid and its gaseous form co-exist for that temperature (for example at 180°C, water and steam co-exist at about 10 bars pressure). These conditions are ideal for crystallisation of alumino-silicate zeolites; the purpose of the template included in the mix is to provide nucleating centres of about the right size and shape around which the zeolite framework will form in the desired way. In view of the industrial importance of zeolites, it is obviously valuable to have a time-resolved in-situ non-ambient diffraction technique that can conveniently track a zeolite synthesising under these conditions: the rate of crystallisation of different zeolite species, as a function of gel, template, temperature, provides valuable chemical engineering information. This has therefore not surprisingly become a popular type of study. After the initial use of neutron diffraction in the late 1980's to study in-situ zeolite synthesis (Polak et al., J.Appl.Cryst. 23, 258 (1990)) it was soon realised (Munn et al., Phase Trans. 39, 129 (1992); Chem. Phys. Lett. 196, 267 (1992)) that similar results could be obtained using the energetic intense X-rays from a synchrotron. This has been performed using both the angle-dispersive and energy-dispersive modes, examples of which are discussed next.

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