Unstable and Metastable Mesophases Can Assist in the Nucleation of Porous Crystals

Abstract

Porous crystals─including zeolites, metal–organic frameworks, and clathrates─are widely used as catalysts and molecular sieves and in energy applications. These materials are synthesized from solution through an intermediate amorphous phase. However, the structural gap between the amorphous and porous crystal phases can lead to large nucleation barriers and difficulties in the control of crystal polymorphs. Previous reports indicate that porous mesophases can facilitate the amorphous to porous crystal transformation, directing the crystallization toward specific crystal structures. To date, it is not known how mesophase stability and synthesis temperature impact the facilitation. Here, we use molecular simulations and nucleation theory to investigate the crystallization of a zeolite in a family of models with tunable mesophase stability. We find that the nucleation mechanism evolves from non-classical to classical as the mesophase stability approaches that of the crystal. The simulations reveal that even an unstable mesophase can facilitate porous crystal nucleation through the formation of a transient fluctuation within which the crystal is originated. We conclude that tapping into the medium-range order of mesophases that have ordered pores without crystalline tiling is promising to increase the crystallization rates of porous crystals while directing the synthesis toward specific polymorphs.