Kolbe Felicitas, Krause Simon, Bon Volodymyr, Senkovska Irena, Kaskel Stefan, Brunner Eike
Faculty of Chemistry and Food Chemistry, Chair of Bioanalytical Chemistry and Faculty of Chemistry and Food Chemistry, Chair of Inorganic Chemistry I, TU Dresden, D-01062 Dresden, Germany.
Chem Mater. 2019 Aug 27;31(16):6193-6201. doi: 10.1021/acs.chemmater.9b02003. Epub 2019 Jul 24.
Flexible metal-organic frameworks (MOFs) are capable of changing their crystal structure as a function of external stimuli such as pressure, temperature, and type of adsorbed guest species. DUT-49 is the first MOF exhibiting structural transitions accompanied by the counterintuitive phenomenon of negative gas adsorption. Here, we present high-pressure in situ Xe NMR spectroscopic studies of a novel isoreticular MOF family based on DUT-49. These porous materials differ only in the length of their organic linkers causing changes in pore size and elasticity. The series encompasses both, purely microporous materials as well as materials with both micropores and small mesopores. The chemical shift of the adsorbed xenon depends on xenon-wall interactions and thus on the pore size of the material. The xenon adsorption behavior of different MOFs can be observed over the whole range of relative pressure. Chemical shift adsorption/desorption isotherms closely resembling the conventional, uptake-measurement-based isotherms were obtained at 237 K where all materials are rigid. The comparable chemical environment of the adsorbed xenon in these isoreticular MOFs allows to establish a correlation between the chemical shift at a relative pressure of / = 1.0 and the mean pore diameter. Furthermore, the xenon adsorption behavior of MOFs is studied also at 200 K. Here, structural flexibility is found for DUT-50, a material with an even longer linker than that of the previously known DUT-49. Its structural transitions are monitored by Xe NMR spectroscopy. This compound is the second known MOF showing the phenomenon of negative gas adsorption. Further increase in the linker length results in DUT-151, a material with an interpenetrated network topology. In situ Xe NMR spectroscopy proves that this material exhibits another type of flexibility compared to DUT-49 and DUT-50. Further surprising observations are made for DUT-46. Volumetric xenon adsorption measurements show that this nonflexible microporous material does not exhibit any hysteresis. In contrast, the in situ Xe NMR spectroscopically detected xenon chemical shift isotherms exhibit a hysteresis even after longer equilibration times than in the volumetric experiments. This indicates kinetically hindered redistribution processes and long-lived metastable states of adsorbed xenon within the MOF persisting at the time scale of hours or longer.
柔性金属有机框架材料（MOFs）能够根据外部刺激（如压力、温度和吸附客体物种的类型）改变其晶体结构。DUT - 49是首个表现出结构转变并伴有负气体吸附这一违反直觉现象的MOF。在此，我们展示了基于DUT - 49的新型等规MOF家族的高压原位Xe NMR光谱研究。这些多孔材料仅在有机连接体的长度上有所不同，这导致了孔径和弹性的变化。该系列既包括纯微孔材料，也包括同时具有微孔和小介孔的材料。吸附氙的化学位移取决于氙 - 壁相互作用，进而取决于材料的孔径。不同MOF的氙吸附行为可在整个相对压力范围内观察到。在237 K时，所有材料均为刚性，获得了与基于传统摄取测量的等温线极为相似的化学位移吸附/解吸等温线。这些等规MOF中吸附氙的可比化学环境使得能够在相对压力(p/p_0 = 1.0)时的化学位移与平均孔径之间建立关联。此外，还在200 K下研究了MOF的氙吸附行为。在此发现，DUT - 50具有结构柔性，其连接体比先前已知的DUT - 49更长。通过Xe NMR光谱监测其结构转变。该化合物是第二个已知表现出负气体吸附现象的MOF。连接体长度的进一步增加产生了DUT - 151，一种具有互穿网络拓扑结构的材料。原位Xe NMR光谱证明，与DUT - 49和DUT - 50相比，这种材料表现出另一种类型的柔性。对于DUT - 46有进一步令人惊讶的发现。体积氙吸附测量表明，这种非柔性微孔材料不表现出任何滞后现象。相反，原位Xe NMR光谱检测到的氙化学位移等温线即使在比体积实验更长的平衡时间后仍表现出滞后现象。这表明吸附在MOF内的氙存在动力学受阻的再分布过程和持续数小时或更长时间的长寿命亚稳态。
