Crystallizing Atomic Xenon in a Flexible MOF to Probe and Understand Its Temperature-Dependent Breathing Behavior and Unusual Gas Adsorption Phenomenon

Flexible metal–organic frameworks
(MOFs) hold great promise
as smart materials for specific applications such as gas separation.
These materials undergo interesting structural changes in response
to guest molecules, which is often associated with unique adsorption
behavior not possible for rigid MOFs. Understanding the dynamic behavior
of flexible MOFs is crucial yet challenging as it involves weak host–guest
interactions and subtle structural transformation not only at the
atomic/molecular level but also in a nonsteady state. We report here
an in-depth study on the adsorbate- and temperature-dependent adsorption
in a flexible MOF by crystallizing atomic gases into its pores. Mn­(ina)2 shows an interesting temperature-dependent response toward
noble gases. Its nonmonotonic, temperature-dependent adsorption profile
results in an uptake maximum at a temperature threshold, a phenomenon
that is unusual. Full characterization of Xe-loaded MOF structures
is performed by in situ single-crystal and synchrotron
X-ray diffraction, IR spectroscopy, and molecular modeling. The X-ray
diffraction analysis offers a detailed explanation into the dynamic
structural transformation and provides a convincing rationalization
of the unique adsorption behavior at the molecular scale. The guest
and temperature dependence of the structural breathing gives rise
to an intriguing reverse of Xe/Kr adsorption selectivity as a function
of temperature. The presented work may provide further understanding
of the adsorption behavior of noble gases in flexible MOF structures.

柔性的金属-有机框架（MOFs）作为特定应用中的智能材料，如气体分离，具有巨大的应用潜力。这些材料在响应客体分子时会发生有趣的构型变化，通常与刚性MOFs所不具备的独特吸附行为相关联。深入理解柔性MOFs的动态行为至关重要却极具挑战性，因为它不仅涉及原子/分子层面的微弱主-客体相互作用，还包括非稳态中的微妙结构转变。本研究通过将原子气体结晶进其孔隙，对柔性MOFs的吸附剂和温度依赖性吸附进行了深入研究。Mn­(ina)2对稀有气体表现出有趣的温度依赖性响应。其非单调的温度依赖性吸附曲线导致在温度阈值处出现吸附量最大值，这一现象颇为罕见。通过原位单晶和同步辐射X射线衍射、红外光谱和分子建模，对Xe加载的MOF结构进行了全面表征。X射线衍射分析对动态结构转变提供了详细的解释，并对分子尺度上的独特吸附行为提供了令人信服的合理性论证。结构呼吸的客体和温度依赖性导致Xe/Kr吸附选择性随温度变化的逆向。本研究可能为进一步理解稀有气体在柔性MOF结构中的吸附行为提供新的见解。