Adjusting framework ionicity to favour crystallisation of zeolites with strained structural units. Periodic quantum chemical studies

Porosity in silicates is reached against the thermodynamic trend that favours the formation of the most dense phases. Porous silica materials, like Si-zeolites, are, therefore, metastable systems that take and keep their shape upon crystallisation and further calcination owing to a compromise between rigidity and flexibility of the various structural units. It is shown that, in the synthesis of porous silicates, one of the key effects that drive crystallisation is the framework ionicity in the as-made phase. This favours the formation of structural units which in the pure silica form are remarkably strained and barely stable. In this Perspective article, we employ ab initio quantum chemical results to analyse the influence of the polarisation of T-O bonds and, therefore, framework ionicity, in the synthesis of two types of zeolites: frameworks with cations more electropositive than Si and pure silica materials crystallised in the fluoride medium. It is shown that the enhancement of the structural flexibility driven by electronic effects in these cases favours the formation of structures with strained sub-units that in other synthesis conditions could not be reached.