Role of germanium on the nucleation and growth of zeolite A from clear solutions as studied by in situ small-angle X-ray scattering, wide-angle X-ray scattering and dynamic light scattering

Abstract

Understanding the formation processes of zeolites is essential if a more “rational” design principle, leading to more catalytically useful materials, is to be achieved. Optically clear “model” solutions allow for significant insight, while systematic variation of selected experimental conditions can reveal additional detail of these complex processes. In this work we have studied the synthesis of zeolite A from clear solutions and compared the effect of isomorphous germanium substitution on this process. To follow the entire process, a combination of in situ time-resolved small-angle X-ray scattering, wide-angle X-ray scattering, and dynamic light scattering was used. These techniques indicate a mechanism similar to that observed for silicalite-1 with the formation of amorphous nanoparticles that become increasingly stable until growth continues by addition to aggregates. Additionally, by comparison between the unsubstituted and germanium-substituted materials, we have identified the effect of metal substitution and have shown how it has a significant influence over the entire formation process, from the formation and stabilization of the initial nanoparticles to the type and rate of crystallization and the final crystal morphology. This study therefore demonstrates the importance of metal substitution effects for controlling the rational design of zeolites and indicates that a greater understanding of these effects is required to fully understand these systems.