New insights on the autocatalytic nucleation in zeolite A synthesis

The course of hydrothermal crystallisation of aluminosilicates (zeolites) as well as their particulate properties can be affected by many factors, which influence processes at molecular level, especially nucleation events. Using kinetic data (rate of crystallisation, rate of crystal growth) and crystal size distributions of zeolite A at the end of crystallisation, the kinetics of nucleation and distribution of nuclei within the gel matrix were determined. Zeolite A was obtained by hydrothermal treatment (at 80 °C) of aluminosilicate hydrogels (3.05 Na 2 O × Al 2 O 3 × 1.91 SiO 2 × 238.1 H 2 O) aged at 25 °C for various times, t a = 0 to 209 h. We have found that the distribution of nuclei in gel matrix considerably changes with the time of hydrogel ageing. The nuclei formed during the precipitation of hydrogel are non-homogeneously distributed within the gel matrix and concentrated closer to the centres of the gel's particles, while the new nuclei are formed at the surface and sub-surface regions of gel's particles. The appearance of large number of nuclei, at the moment when gel is almost completely dissolved, was observed for the first time. The influence of gel ageing on crystallisation course as well as the observed effects of crystals particulate properties are discussed. On the grounds of obtained data, the definition of the term "autocatalytic nucleation" was extended. The objective of this work is the demonstration of the changes of the distribution of the nuclei (location and number) within gel matrix with ageing time of the hydrogel, as well as clarification of the "autocatalytic nucleation" theory on the pathway of zeolite A crystallisation.