The fluoride-based route to all-silica molecular sieves; a strategy for synthesis of new materials based upon close-packing of guest–host products

Abstract

This study surveys the use of a range of structure-directing agents (SDA) in zeolite synthesis experiments under the conditions using HF. The studies made are all for systems containing only silica as the inorganic component. Key points to emerge from the study are a reinforcement of the concept of more open-framework host structures forming when the reactions are more concentrated (lower H 2 O/SiO 2 ratios). This discovery had been reported previously from the research group at ITQ in Valencia, Spain. Our studies show that some novel all-silica compositions have been achieved using this route for synthesis. Two new relationships we explore are whether the use of fluoride anion in synthesis has different nucleation selectivities as a function of dilution, and whether guest molecules can achieve tighter packing in the host structures using the fluoride route as contrasted with the hydroxide route. We explore this with MAS NMR studies. We find that some large organo-cations, which produce no products in alkaline media, give interesting host structure in the fluoride reactions. .