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

金属-有机骨架（M0 F）是一类独特的材料，其由中心金属离子与延伸到二维或三维的有机配体配位组成。MOFs由于其极其可调的化学，电学和机械特性而引起了科学界的兴奋，在过去十年中报道和研究了超过20，000种不同的结构。然而，实现均匀的大规模MOF晶体的困难阻碍了大规模采用，并且可能阻碍了目标应用如储氢、膜分离和水分解的可行性。在这项工作中，MOF晶体将在重力（地球上）和微重力（国际空间站，ISS）下生长。通过在国际空间站上进行微重力实验，当重力驱动的过程（如沉降和浮力驱动的对流）被去除时，可以生长出大而均匀的MOF晶体。该合作奖项还旨在利用ISS收集的数据为K-12和社区大学学生创建基于分析的课程，以激励学生积极参与该项目。三种类型的MOF晶体将通过ISS兼容的室温水溶液混合过程在长时间的微重力下生长，并在返回地球时进行探测。该奖项旨在更好地了解在扩散控制生长环境中的MOF晶体生长，并确定在微重力下生长MOF的潜在优势。据推测，长期微重力合成将产生与地球生长的MOF晶体相比（1）缺陷量更低，（2）尺寸更大，（3）尺寸和形态更均匀的MOF晶体。这一假设的根源在于溶质对流，这是一种存在于地球上等温溶液晶体生长中的现象。在晶体成核时，MOF晶体通过耗尽周围溶液中的溶解的金属离子和有机连接剂而生长。与具有溶解的溶质的密度较大的散装介质相比，该过程留下由简单的水性溶剂组成的密度较小的液体。在等温条件下，这种密度梯度引起对流，特别是溶质对流。由对流引起的运动和对流产生了额外的成核点的条件-产生了无数的具有不均匀生长速率的微晶，从而产生了许多尺寸和形态不均匀的小尺寸（纳米至微米尺度）的缺陷倾向晶体。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.