Analysis and Refinement of Host-Guest Interactions in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a class of hybrid porous materials, characterised by their periodic assembly from metal ions and organic ligands through coordination bonds. Their high crystallinity, extensive surface area, and uniform, adjustable pore sizes make them promising candidates for a wide array of applications. These include gas adsorption and separation, substrate binding, and catalysis, of relevance to tackling pressing global issues such as climate change, energy challenges and pollution. In comparison to traditional porous materials such as zeolites and activated carbons, the design flexibility of the organic ligands in MOFs, coupled with their orderly arrangement with associated metal centres, allows for the precise engineering of uniform pore environments. This unique feature enables a rich variety of interactions between the MOF host and adsorbed gas molecules, which are fundamental to understanding their uptake capacity and selectivity to target gas molecules and thus to their overall performance. The designable property of MOFs promotes the development of advanced materials tailored for specific applications, thereby addressing emerging challenges across industry and society.
In this Account, a dataset for three-dimensional structured MOFs is constructed based upon the structural analysis of host-guest interactions using the largest experimental database, the Cambridge Structural Database (CSD). A full screening was performed on structures with guest molecules of H2, C2H2, CO2 and SO2. A relationship between the primary binding site, the isosteric heats of adsorption (Qst) and the adsorption uptake was extracted and established from this structural analysis. We review the available protocols that can refine host-guest interactions based on primarily our studies on the host-guest chemistry of MOFs. The methods include ligand functionalisation, variation of metal centres, formation of defects, addition of single atom sites, and pore control. In situ structural and dynamic investigations using diffraction and spectroscopic techniques are powerful tools to visualise the details of host-guest interactions upon the above modifications, affording key insights into their performance at a molecular level. Finally, we give an outlook of future research priorities in the study of host-guest chemistry in MOF materials. We hope this Account will cultivate rational development and improvement of future MOF-based sorbents for applications in challenging gas adsorption, separations and catalysis.