Ho Ching-Hwa, Valentine Mason L, Chen Zhijie, Xie Haomiao, Farha Omar, Xiong Wei, Paesani Francesco
Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA, 92093, USA.
Department of Chemistry and International Institute of Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208, USA.
Commun Chem. 2023 Apr 15;6(1):70. doi: 10.1038/s42004-023-00870-0.
Metal-organic frameworks (MOFs) are a class of materials with diverse chemical and structural properties, and have been shown to effectively adsorb various types of guest molecules. The mechanism of water adsorption in NU-1500-Cr, a high-performance atmospheric water harvesting MOF, is investigated using a combination of molecular dynamics simulations and infrared spectroscopy. Calculations of thermodynamic and dynamical properties of water as a function of relative humidity allow for following the adsorption process from the initial hydration stage to complete filling of the MOF pores. Initial hydration begins at the water molecules that saturate the open Cr sites of the framework, which is then followed by the formation of water chains that extend along the channels connecting the hexagonal pores of the framework. Water present in these channels gradually coalesces and fills the hexagonal pores sequentially after the channels are completely hydrated. The development of hydrogen-bond networks inside the MOF pores as a function of relative humidity is characterized at the molecular level using experimental and computational infrared spectroscopy. A detailed analysis of the OH-stretch vibrational band indicates that the low-frequency tail stems from strongly polarized hydrogen-bonded water molecules, suggesting the presence of some structural disorder in the experimental samples. Strategies for designing efficient water harvesting MOFs are also proposed based on the mechanism of water adsorption in NU-1500-Cr.
金属有机框架材料（MOFs）是一类具有多样化学和结构性质的材料，并且已被证明能有效吸附各类客体分子。使用分子动力学模拟和红外光谱相结合的方法，研究了高性能大气取水MOF材料NU-1500-Cr中的水吸附机制。计算水的热力学和动力学性质随相对湿度的变化情况，有助于追踪从初始水合阶段到MOF孔完全填充的吸附过程。初始水合作用始于使框架开放的Cr位点饱和的水分子，随后形成沿连接框架六边形孔的通道延伸的水链。在通道完全水合后，这些通道中存在的水逐渐合并并依次填充六边形孔。利用实验和计算红外光谱在分子水平上表征了MOF孔内氢键网络随相对湿度的发展情况。对OH伸缩振动带的详细分析表明，低频尾部源于强极化的氢键水分子，这表明实验样品中存在一些结构无序现象。还基于NU-1500-Cr中的水吸附机制提出了设计高效取水MOF材料的策略。
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