Computer Simulation of the Early Stages of Self-Assembly and Thermal Decomposition of ZIF-8

TL;DR

This study uses advanced molecular simulations to elucidate the early nucleation and decomposition mechanisms of ZIF-8, providing insights into MOF synthesis and a validated force field for future modeling.

Contribution

Developed and validated a novel force field and simulation methodology for modeling the early stages of ZIF-8 formation and decomposition.

Findings

Rapid Zn-N connectivity increase leads to amorphous aggregates.

Transient Zn(MIm) complexes observed with picosecond lifetimes.

Formation of vitreous phase during crystal decomposition.

Abstract

We employ all-atom well-tempered metadynamics simulations to study the mechanistic details of both the early stages of nucleation and crystal decomposition for the benchmark metal-organic framework ZIF-8. To do so, we developed and validated a force field that reliably models the modes of coordination bonds via a Morse potential functional form and employs cationic and anionic dummy atoms to capture coordination symmetry. We also explored a set of physically relevant collective variables and carefully selected an appropriate subset for our problem at hand. After a rapid increase of the Zn-N connectivity, we observe the evaporation of small clusters in favor of a few large clusters, that lead to the formation of an amorphous highly-connected aggregate. Zn(MIm)42- and Zn(MIm)3- complexes are observed, with lifetimes in the order of a few picoseconds, while larger structures, such as 4-, 5- and 6-membered rings, have substantially longer lifetimes of a few nanoseconds. The free ligands act as ``templating agents'' for the formation of the sodalite cages. ZIF-8 crystal decomposition results in the formation of a vitreous phase. Our findings contribute to a fundamental understanding of MOF's synthesis that paves the way to controlling synthesis products. Furthermore, our developed force field and methodology can be applied to model solution processes that require coordination bond reactivity for other ZIFs besides ZIF-8.