A Molecular Dynamics Model for ZIF-8 Formation

Metal-organic frameworks are a class of highly porous materials suitable for applications in energy storage and conversion, catalysis, and separations. Despite the large number of different MOF structures that have been discovered experimentally, little is known about their formation mechanisms. Here we present a simple computational model for ZIF-8 that utilizes an implicit solvent and coarse-grained interactions to elucidate the early stages of crystallization. Utilizing molecular dynamics we find that ZIF-8 nucleation is a multi-step process that is preceded by a metastable liquid-like phase that undergoes rearrangement and an increase in coordination number subsequent to the nucleation event. Furthermore, our model predicts the existence of a short lived gyroid-like structure that forms prior to ZIF-8 nucleation. Our simulation rationalize recent spectroscopic results that suggested the formation of a population of non-crystalline clusters prior to ZIF-8 growth.