Computer Simulation of the Growth of a Metal–Organic Framework Proto-Crystal at Constant Chemical Potential

Designing metal–organic frameworks (MOFs) synthesis protocols is currently largely driven by trial-and-error, since we lack a fundamental understanding of the molecular-level mechanisms that underlie their self-assembly processes. Previous works have studied the nucleation of MOFs, but their growth has never been studied by means of computer simulations, which provide molecular-level detail. In this work, we combine constant chemical potential simulations with a particle insertion method to model the growth of the ZIF-8 MOF at varying synthesis temperatures and reactant concentrations on the higher end of those used in the laboratory. Nonclassical growth mechanisms triggered by oligomer attachments were detected. At the lower concentrations explored, the formed layers exhibit a relatively structured density profile but contain defective sites characterized by the presence of 3-, 5-, and 7-membered rings, typical of amorphous phases. Higher concentrations lead to the formation of inhomogeneous amorphous phases. Compared to the amorphous intermediate species obtained at the nucleation part of the self-assembly process, larger-sized rings are more common in the grown layer. Moreover, these are favored by increasing reactant concentration and temperature, as is the degree of deviation with respect to the original crystal structure. We computed growth rates for the steady-state regime, and their nonlinear tendency with respect to concentration suggests that oligomer formation, which is more predominant at high concentrations, plays an important role in the growth mechanisms.