Understanding ordered silica: Linking topology and energetics

Traditionally compounds in crystal chemistry are described in terms of their composition and geometry (i.e. bond lengths and angles). While this geometrical description is powerful, it has an intrinsic weakness in that it is hard to describe structures in terms of features (e.g. pores) important to their application in the chemical industry (e.g. catalysis, adsorption). An alternative method of describing structures in terms of their underlying topology (i.e. atomic connectivity) tries to overcome this weakness. The work described in this thesis mainly focuses on expanding this topological description to a set of calculable descriptors that allow for facile comparison between structures. The first part of the thesis describes the development of topological descriptors, based on the face-size distribution, and its application to rationalise the energetics of the all-siliceous zeolites. It is shown that decomposing framework materials into a space-filling set of face-sharing polyhedral cages, and analysing the face-size distribution obtained, leads to powerful insights into the dependence of the thermodynamic viability of zeolite- and more general framework material synthesis upon changes in structural properties such as pore-size and framework density. The second part of the thesis focuses on silica nanoclusters and a description of their possible topologies. It is shown that silica and related oxides can in principle be synthesized in the form of fully coordinated nanoclusters with structures reminiscent of those of carbon buckyballs (fullerenes). Furthermore, it is demonstrated that such fully coordinated nanoclusters are interesting computational model systems (surface defects, infrared spectra) for dehydrated silica surfaces.Applied Science