Adsorption selectivity studies of flexible metal-organic-frameworks by in situ and solid-state NMR spectroscopy

本项目旨在进一步加深对在外部刺激(QP1)下观察到的特殊、灵活的金属-有机骨架(MOF)的显著结构变化(切换)的基本理解。计划对灵活的MOF进行的调查是DFG研究单位内QP2的一部分。该项目在第二个资助期的主要重点将是调查柔性MOF对气体和液体混合物的吸附选择性(QP4)。这些研究的结果将有助于探索灵活的MOF模型系统的具体功能，例如，用于分离过程(QF1)。我们的研究将对子项目S1和S2中合成的财政部样本进行。此外，我们的目标是表征基于具有整合催化中心(QF2)的柔性MOF的催化剂。结合原位X射线衍射(S2)和原位EPR光谱(P2)的结果，计划中的研究可能会对开关过程及其对吸附选择性的影响有更深入和更全面的基础了解。该项目将从与理论小组(T1、T2)的合作中获益良多。研究将包括原位和固态核磁共振波谱。将使用13CO2/13CH4、13CO2/13CO、C3H8/C3H6(13C和/或2H标记)和129Xe/83Kr等混合气体进行原位核磁共振光谱吸附/解吸研究。液态吸附--包括液体混合物--将用固体核磁共振波谱研究柔性MOF上的吸附。将应用先进的固体核磁共振技术来表征MOF晶格及其结构变化，例如通过13C MAS核磁共振、1H-13C HETCOR(异核相关)和其他实验。吸附分子的状态以及它们与MOF晶格的相互作用将通过2H核磁共振光谱、交叉极化实验以及距离敏感技术如REDOR(旋转回波双共振)等实验来研究。因此，本研究项目旨在解决以下问题：1)骨架柔性总体上是否影响吸附选择性？回答这个问题将有助于设计具有高选择性的MOF，在分离过程、选择性催化等方面具有潜在的应用。2)柔性MOF与被吸附分子之间的哪些分子相互作用正在驱动结构转变，骨架柔性是否影响被吸附分子的状态？这些研究旨在了解混合物中气体或液体的类型对结构转变的影响，并有助于表征这些分子与MOF晶格的特定相互作用。

The present project aims to further develop the fundamental understanding of the pronounced structural changes (switching) observed for special, flexible metal-organic frameworks (MOFs) under external stimuli (QP1). The planned investigations on flexible MOFs are part of QP2 within this DFG Research Unit. The main focus of the project in the 2nd funding period will be the investigation of flexible MOFs with respect to their adsorption selectivity from mixtures of gases as well as liquids (QP4). The results of these investigations will help exploring specific functions of flexible MOF model systems, e.g., for separation processes (QF1). Our studies will be performed on MOF samples synthesized within sub-projects S1 and S2. Furthermore, we aim to characterize catalysts based on flexible MOFs with integrated catalytic sites (QF2). Together with the results of in situ X-ray diffraction (S2) and in situ EPR spectroscopy (P2), the planned studies are likely to deliver a deeper and more comprehensive fundamental understanding of the switching process and its influence upon the adsorption selectivity. The project will substantially profit from the collaboration with theoretical groups (T1,T2). Investigations will include on in situ and solid-state NMR spectroscopy. In situ NMR spectroscopic adsorption/desorption studies will be performed using gas mixtures like 13CO2/13CH4, 13CO2/13CO, C3H8/C3H6 (13C and/or 2H labeled) and 129Xe/83Kr. Liquid-state adsorption – including liquids mixtures - on flexible MOFs will be investigated by solid-state NMR spectroscopy. Advanced solid-state NMR techniques will be applied in order to characterize the MOF lattice and its structural changes, e.g., by 13C MAS NMR, 1H-13C HETCOR (heteronuclear correlation) and other experiments. The state of the adsorbed molecules as well as their interactions with the MOF lattice will be studied by experiments like 2H NMR spectroscopy, cross-polarization experiments as well as distance-sensitive techniques such as REDOR (rotational echo double resonance) etc. The present research project thus aims to address the following questions:1) Does framework flexibility in general influence the adsorption selectivity? Answering this question will help designing highly selective MOFs with potential applications for separation processes, selective catalysis, and others.2) Which molecular interactions between flexible MOFs and adsorbed molecules are driving the structural transitions and does framework flexibility influence the state of the adsorbed molecules? These studies aim to understand the influence of the type of gases or liquids in mixtures upon the structural transitions and will also contribute to characterize specific interactions of these molecules with the MOF lattice.