Collaborative Research: ISS: Assessing the Effect of Microgravity on Growth and Properties of Metal-Organic Framework (MOF) Crystals

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Metal-organic frameworks (MOFs) are a unique class of materials consisting of a central metal ion coordinated to organic ligands that extend into two or three dimensions. MOFs have generated scientific excitement due to their extremely tunable chemical, electrical and mechanical properties with over 20,000 different structures reported and studied within the last decade. Yet, difficulties in realizing uniform, large-scale MOF crystals have hindered mass adoption, and perhaps, the viability of target applications such as hydrogen storage, membrane separation, and water splitting. In this work, MOF crystals will be grown in gravity (on Earth) and microgravity (on the International Space Station, ISS). Through experimentation in microgravity on the ISS, large, uniform MOF crystals may be grown when gravity-driven processes, such as sedimentation and buoyancy-driven convection, are removed. This collaborative award also aims to use data collected on the ISS to create analysis-based curriculum for K-12 and community college students to stimulate active student engagement with the project. Three types of MOF crystals will be grown in prolonged microgravity via an ISS-compatible, room temperature aqueous mixing process and probed upon return to Earth. This award seeks to better understand MOF crystal growth in a diffusion-controlled growth environment and identify potential advantages of growing MOFs in microgravity. It is hypothesized that long-term microgravity synthesis will produce MOF crystals that are (1) lower in defect quantity, (2) larger in size, and (3) more uniform in size and morphology compared to Earth-grown MOF crystals. The root of this hypothesis lies in solutal convection, a phenomenon that exists in isothermal, solution crystal growth on Earth. Upon crystal nucleation, MOF crystals grow by depleting the surrounding solution of the dissolved metal ion and organic linker. This process leaves behind a less-dense liquid consisting of simply aqueous solvent, compared to the denser, bulk medium with dissolved solutes. This density gradient induces convection, specifically solutal convection, under isothermal conditions. The movement and turbulency prompted by convection generates conditions for additional points of nucleation—creating countless crystallites with uneven growth rates resulting in numerous defect-prone crystals of small size (nano- to micro-scale) that are non-uniform in size and morphology. The absence of gravity alleviates the detrimental solutal convection phenomenon.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.