Multi-Scale Modelling of Proto-Zeolitic Solutions

A number of aspects of the pre-nucleation zeolite synthesis solution are considered. Various environmental and structural effects on the Si NMR chemical shift of silicon nuclei are investigated, in order to ascertain the necessary computational model for a systematic study of oligomeric silicate species identified or postulated to be present during this phase of zeolite crystal growth. It is demonstrated that, using a model with the oligomer in a fully protonated state and including an implicit representation of the solvent, a reasonably inexpensive model can provide good agreement with experimental results. The systematic study of 59 oligomers reveals several cases for re-assignment of experimentally observed peaks, as well as providing assistance in cases where full assignment has proved impossible from experiment. The effects of structure directing agents (SDAs), commonly used in zeolite synthesis, are investigated. Using ab initio molecular dynamics (AIMD) to simulate the SDA in the presence of water and a specifically designed computer code, it is demonstrated how SDAs lead to the formation of various rings of water in their hydration layer. Furthermore, different properties of the SDAs are shown to induce the formation of different distributions of the various sizes of rings. It is found that certain SDAs result in the formation of clusters of water rings in a network which is isomorphic with some of the zeolite frameworks for which they are know to direct. Finally a new inter-atomic potential is developed for modelling silicate clusters, in order to allow longer simulations of larger systems than are accessible using AIMD methods. This potential is then used to simulate two cage-like silicate oligomers surrounded by water. In these simulations layers of ordered water, similar to those found at zeolite crystal surfaces, are found. These findings have implications for the understanding of the aggregation of oligomeric species prior to nucleation. This work was generously supported by the Engineering and Physical Science Research Council. The final chapter was also made possible by a Junior Research Fellowship from the Thomas Young Centre. The simulation presented in chapter 4 were performed on the HPCx supercomputer and UCL’s Legion supercomputer.