The functionalisation of organic linkers in metal–organic frameworks (MOFs) to improve gas uptake is well-documented. Although the positive role of free carboxylic acid sites in MOFs for binding gas molecules has been proposed in computational studies, relatively little experimental evidence has been reported in support of this. Primarily this is because of the inherent synthetic difficulty to prepare MOF materials bearing free, accessible –COOH moieties which would normally bind to metal ions within the framework structure. Here, we describe the direct binding of CO₂ and C₂H₂ molecules to the free –COOH sites within the pores of MFM-303(Al). MFM-303(Al) exhibits highly selective adsorption of CO₂ and C₂H₂ with a high selectivity for C₂H₂ over C₂H₄. In situ synchrotron X-ray diffraction and inelastic neutron scattering, coupled with modelling, highlight the cooperative interactions of adsorbed CO₂ and C₂H₂ molecules with free –COOH and –OH sites within MFM-303(Al), thus rationalising the observed high selectivity for gas separation.

中文翻译：

---

二氧化碳和乙炔与金属有机框架中游离羧酸位点的结合

金属有机框架（MOF）中有机连接体的功能化可改善气体吸收已有充分记录。尽管在计算研究中已经提出了 MOF 中游离羧酸位点对于结合气体分子的积极作用，但支持这一点的实验证据相对较少。这主要是因为制备带有游离、可接近的-COOH部分的MOF材料存在固有的合成困难，这些部分通常会与框架结构内的金属离子结合。在这里，我们描述了CO ₂和C ₂ H ₂分子与MFM-303(Al)孔内的游离-COOH位点的直接结合。 MFM-303(Al)表现出对CO ₂和C ₂ H _{2的高选择性吸附，并且对C 2} H _{2 的}选择性高于C ₂ H ₄。原位同步加速器 X 射线衍射和非弹性中子散射与建模相结合，突出了 MFM-303(Al) 内吸附的 CO ₂和 C ₂ H ₂分子与自由 -COOH 和 -OH 位点的协同相互作用，从而合理化了观察到的结果气体分离选择性高。