A Simple Molecular Model for Hydrated Silicate Ionic Liquids, a Realistic Zeolite Precursor

TL;DR

This study develops a simple molecular model for hydrated silicate ionic liquids that elucidates the role of alkali cations in zeolite formation, combining simulations and experiments to advance understanding of early nucleation processes.

Contribution

It introduces a molecular model capturing the influence of alkali cations on HSIL structure, aiding in understanding zeolite nucleation.

Findings

Cation charge density and concentration affect HSIL structure.

Water content causes transition from glassy network to solvated ion pairs.

Cations facilitate silicate monomer condensation for zeolite formation.

Abstract

Despite the widespread use of zeolites in chemical industry, their formation process is not fully understood due to the complex and heterogeneous structure of traditional synthesis media. Hydrated silicate ionic liquids (HSILs) have been proposed as an alternative. They are truly homogeneous and transparent mixtures with low viscosity, facilitating experimental characterization. Interestingly, their homogeneous nature and simple speciation brings realistic molecular models of a zeolite growth liquid within reach for the first time. In this work, a simple molecular model is developed that gives insight into the crucial role of the alkali cations (sodium, potassium, rubidium and cesium). Thereby molecular dynamics simulations are combined with experimental measurements to demonstrate that the HSIL liquid structure strongly depends on the charge density and concentration of the alkali cation. As the water content increases, it transitions from a glassy network with fast ion exchange to an aqueous solution containing long-lasting, solvated ion pairs. Furthermore, simulations reveal that the cation is capable of bringing several silicate monomers together in the glassy network, displaying perfect orientations for condensation reactions that underlie zeolite formation. This work is an important step towards the development of molecular models that can fully describe the early nucleation process of zeolites in combination with experiments.