An in situ energy-dispersive X-ray diffraction study of the hydrothermal crystallization of zeolite A. 1. Influence of reaction conditions and transformation into sodalite

The hydrothermal crystallization of sodium zeolite A from amorphous alumino silicate gels at 80-120 degreesC has been studied by time-resolved in situ energy-dispersive X-ray diffraction. This has allowed the course of crystal lization to be followed in greater detail than previously possible. Quantit ative crystallization curves have been obtained, and these kinetic data are analyzed using the Avrami-Erofe'ev nucleation growth model to allow a simp le means of determining the crystallization rate. We observe that the choic e of silica starting material can affect the course of reaction; the use of fumed silica gives rise to a distinctive two-stage growth curve. This unus ual crystallization behavior is also dependent on both NaOH concentration a nd temperature. At the highest NaOH concentrations and when the amount of w ater is low, zeolite A is only present for a short time and hydroxosodalite is the sole product on continued heating. We discuss our new in situ obser vations in relation to previous quenching studies of zeolite crystallizatio ns, and their implications in understanding zeolite formation mechanism.