Abstract
Metal-organic framework (MOF) crystallization is governed by molecular assembly processes in the pre-nucleation stage. Yet, unravelling these pre-nucleation pathways and rationalizing their impact on crystal formation poses a great challenge since probing molecular-scale assemblies and macroscopic particles simultaneously is very complex. Herein, we present a multimodal, integrated approach to monitor MOF nucleation across multiple length scales by combining in situ optical spectroscopy, mass spectrometry, and molecular simulations. This approach allows tracing initial metal-organic complexes in solution and their assembly into oligomeric nuclei and simultaneously probing particle formation. During Co-ZIF-67 nucleation, a metal-organic pool forms with a variety of complexes caused by ligand exchange and symmetry reduction reactions. We discriminate complexes capable of initiating nucleation from growth species required for oligomerization into frameworks. Co 4 -nuclei are observed, which grow into particles following autocatalytic kinetics. The geometric and compositional variability of metal-organic pool species clarifies long-debated amorphous zeolitic imidazolate framework (ZIF)-particle nucleation and non-classic pathways of MOF crystallization. Simultaneous probing of metal-organic complexes, oligomeric nuclei, and MOF particles Metal-organic pool forms as pre-equilibrium with geometry diverse complexes “Nucleation” and “growth” complexes are discriminated, which assemble into nuclei Stable nuclei are observed, growing into MOF particles with autocatalytic kinetics The molecular pathway that governs metal-organic framework formation is challenging to grasp. Filez et al. establish a multi-scale characterization approach combined with theory to link the somewhat different worlds of “molecular” metal-organic ligand assembly and MOF “particle” crystallization.
Highlights
Metal-organic frameworks (MOFs) display unparalleled chemical diversity and functionality in their building blocks, providing a unique toolbox to design new application-tailored materials.[1,2,3] Despite this enormous potential, MOF synthesis mostly presents a laborious, case-by-case endeavor[4,5,6,7] based on trial-and-error experimentation instead of predictive synthesis
SUMMARY Metal-organic framework (MOF) crystallization is governed by molecular assembly processes in the pre-nucleation stage
Unravelling these pre-nucleation pathways and rationalizing their impact on crystal formation poses a great challenge since probing molecularscale assemblies and macroscopic particles simultaneously is very complex
Summary
Metal-organic frameworks (MOFs) display unparalleled chemical diversity and functionality in their building blocks, providing a unique toolbox to design new application-tailored materials.[1,2,3] Despite this enormous potential, MOF synthesis mostly presents a laborious, case-by-case endeavor[4,5,6,7] based on trial-and-error experimentation instead of predictive synthesis. To speed-up MOF discovery, high-throughput synthesis[8] is often applied to efficiently screen a reaction parameter grid. Such ‘‘brute force’’ methods typically yield only a small subset of computationally[9] and rationally[10] predicted design space. Even when successful MOF synthesis recipes are found, these are not generally transferrable to other MOFs, even within structurally similar families—not suitable for scale-up—since slightly altering the synthesis parameters impacts nucleation conditions and product formation.[5] a major leap forward toward predictive synthesis necessitates an in-depth understanding of the general principles underlying MOF crystallization, rather than a case-by-case empirical optimization of synthesis recipes
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