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

TL;DR

This study uses computer simulations to explore the molecular mechanisms of MOF growth, revealing non-classical growth pathways, defect formation, and the influence of temperature and concentration on growth dynamics.

Contribution

It introduces a novel simulation approach combining constant chemical potential and particle insertion to model MOF growth at the molecular level.

Findings

Non-classical growth mechanisms involve oligomer attachments.

Defective sites with rings of 3, 5, and 7 members are formed during growth.

Growth rate depends non-linearly on concentration, suggesting adsorption-controlled process.

Abstract

Designing metal-organic frameworks (MOFs) synthesis protocols is currently largely driven by trial-and-error, since we lack 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 concentrations of the reactants. Non-classical growth mechanisms triggered by oligomer attachments were detected, with a higher predominance in the most concentrated setups. The newly formed layers preserve the pore-like density profile of the seed crystal but contain defective sites characterized by the presence of 3, 5 and 7 membered rings, typical of amorphous phases. Compared to the amorphous intermediate species obtained at the nucleation part of the self-assembly process previously investigated in our group [Chem. Mater., doi: 10.1021/acs.chemmater.5c02028, 2025], 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 the non-linear tendency with respect to concentration leads us to hypothesize that in these conditions the growth is controlled by the adsorption rather than by the diffusion processes.