Assessing Crystallisation Kinetics of Zr Metal-Organic Frameworks through Turbidity Measurements to Inform Rapid Microwave-Assisted Synthesis.

Sarah L Griffin,Joop H Ter Horst,Maria L Briuglia,Ross S Forgan

doi:10.1002/chem.202000993

Sarah L Griffin, Joop H Ter Horst + Show 2 more

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https://doi.org/10.1002/chem.202000993

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Abstract

Controlling the crystallisation of metal‐organic frameworks (MOFs), network solids of metal ions or clusters connected by organic ligands, is often hindered by the significant number of synthetic variables inherent to their synthesis. Coordination modulation, the addition of monotopic competing ligands to solvothermal syntheses, can allow tuning of physical properties (particle size, porosity, surface chemistry), enhance crystallinity, and select desired phases, by modifying the kinetics of self‐assembly, but its mechanism(s) are poorly understood. Herein, turbidity measurements were used to assess the effects of modulation on the solvothermal synthesis of the prototypical Zr terephthalate MOF UiO‐66 and the knowledge gained was applied to its rapid microwave synthesis. The studied experimental parameters—temperature, reagent concentration, reagent aging, metal precursor, water content, and modulator addition—all influence the time taken for onset of nucleation, and subsequently allow microwave synthesis of UiO‐66 in as little as one minute. The simple, low cost turbidity measurements align closely with previously reported in situ synchrotron X‐ray diffraction studies, proving their simplicity and utility for probing the nucleation of complex materials while offering significant insights to the synthetic chemist.

Highlights

Despite the growth in interest and vast number of metal-organic frameworks (MOFs)—coordination polymers wherein organic ligands connect metal ions or clusters into network solids—synthesised over recent years, there has been relatively little investigation into the crystallisation process of these materials
An alternative is the use of coordination modulation—the addition of monotopic ligands, known as modulators, into synthetic mixtures to compete with the linkers for metal coordination, allowing fine control over a number of physical properties such as size, morphology, defectivity and porosity in MOFs linked by high valent metals—a technique ubiquitous in the self-assembly of Zr MOFs.[12,13,14,15]
Experimental runs proceeded through the reported UiO-66 solvothermal synthesis,[16] consisting of a 1:1 ratio of ZrCl4 and BDC in N,N-dimethylformamide (DMF, unpurified ACS grade unless otherwise stated) at a range of concentrations, divided www.chemeurj.org

Summary

Introduction

Despite the growth in interest and vast number of metal-organic frameworks (MOFs)—coordination polymers wherein organic ligands connect metal ions or clusters into network solids—synthesised over recent years, there has been relatively little investigation into the crystallisation process of these materials. As a result, understanding of the reaction and crystallisation mechanisms remains limited, with MOF synthesis often proceeding more through trial and error than by design.[1,2] Understanding how these materials assemble would allow a rational approach towards MOF design and synthesis, increasing the ability to generate highly specialised materials of desired size, topology and functionality.[3,4,5]. An in situ synchrotron energy dispersive X-ray diffraction (EDXRD) study into the formation of the isoreticular Zr-fum analogue, [Zr6O4(OH)4(fumarate)6], in water showed that the addition of increasing equivalents of formic acid modulator

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