Understanding Metal–Organic Framework Nucleation from a Solution with Evolving Graphs

Abstract

A mechanistic understanding of metal−organic framework (MOF) synthesis and scale-up remains underexplored due to the complex nature of the interactions of their building blocks. In this work, we investigate the collective assembly of building units at the early stages of MOF nucleation, using MIL-101(Cr) as a prototypical example. Using large-scale molecular dynamics simulations, we observe that the choice of solvent (water and N,N-dimethylformamide), the introduction of ions (Na + and F − ) and the relative populations of MIL-101(Cr) half-secondary building unit (half-SBU) isomers have a strong influence on the cluster formation process. Additionally, the shape, size, nucleation and growth rates, crystallinity, and short and long-range order largely vary depending on the synthesis conditions. We evaluate these properties as they naturally emerge when interpreting the self-assembly of MOF nuclei as the time evolution of an undirected graph. Solution-induced conformational complexity and ionic concentration have a dramatic effect on the morphology of clusters emerging during assembly. While pure solvents lead to the rapid formation of a small number of large clusters, the presence of ions in aqueous solutions results in smaller clusters and slower nucleation. This diversity is captured by the key features of the graph representation. Principle component analysis on graph properties reveals that only a small number of molecular descriptors is needed to deconvolute MOF self-assembly. Descriptors such as the average coordination number between half-SBUs and fractal dimension are of particulalr interest as they can be can be followed experimentally by techniques like by time-resolved spectroscopy. Ultimately, graph theory emerges as an approach that can be used to understand complex processes revealing molecular descriptors accessible by both simulation and experiment.

Understanding and controlling the nucleation and growth of metal–organic frameworks

“…For example, a pure aqueous solvent has been shown to facilitate fast nucleation and produce larger clusters compared to an aqueous solution with ions. 33 Additionally, free ions, such as Na + and F À , in solution can dictate the extent of defects in a crystal structure. 93 This occurs as a result of the ions interacting with the MOF building units during growth, preventing ligands from rotating and consequently forming defects.…”

Coumarin-functionalized metal–organic frameworks: adsorbents with photo-responsive active sites for adsorptive desulfurization

“…2A), all of the samples show five distinct peaks at 2 θ of 2.77°, 3.30°, 5.10°, 8.40°, and 9.00°, corresponding to (311), (511), (531), (882), and (911) crystal plane diffractions of MIL-101(Cr), respectively. 15 This indicates that the crystal structure is well preserved after modification. All samples exhibit N 2 adsorption–desorption isotherms of type I that are consistent with MIL-101(Cr) (Fig.…”

Theoretical and computational approaches to study crystal nucleation from solution

“…[64,46,125] For example, Demichelis et al [126] performed metadynamics simulations (described in section 4.2) to confirm the relative stability of calcium phosphate complexes as the first associates to form in solution and thought to be directly involved in mineral nucleation from experiments. [127] Simulations have also shed light on structural building units that act as precursors in the nucleation of metal-organic frameworks [128,129,130] and the complexes that assemble to form inorganic functional materials.…”