Structure-Property Relationships in MIL-101 MOFs: Influence of Organic Ligands on CO₂ Adsorption

Metal-organic frameworks (MOFs) of the MIL-101 type have garnered significant attention as promising materials for CO₂ capture due to their high surface areas, tunable porosity, and chemical versatility. Understanding how the choice of organic ligands and metal centers affects CO₂ adsorption capacity and selectivity is critical for the rational design of efficient adsorbents. This study investigates the impact of functionalizing MIL-101(Cr) frameworks with a diverse set of 21 carboxylate ligands varying in size, aromaticity, and geometry, alongside an exploration of isostructural variants incorporating different metal ions (Al, Sc, Mn, Fe, Ti, V, and Cr). Using Grand Canonical Monte Carlo simulations combined with Langmuir model fitting, we quantified adsorption capacities and affinities at 298 K, revealing that longer aromatic ligands generally enhance CO₂ uptake by increasing pore volume and promoting π-CO₂ interactions, while compact ligands favor stronger local affinity but lower capacity. Spatial density mapping demonstrated preferential CO₂ adsorption sites near tetrahedral cavities and metal nodes, influenced by ligand chemistry. Structural and textural descriptors such as accessible surface area and pore limiting diameter were correlated with adsorption performance, highlighting trade-offs between capacity and selectivity. Among metal variants, MIL-101(Al) and MIL-101(Sc) exhibited superior simulated CO₂ uptake, attributable to favorable local electronic environments as evidenced by atomic charge correlations. The findings underscore the critical role of both ligand design and metal center selection in optimizing MIL-101 frameworks for CO₂ capture applications, while also acknowledging potential stability challenges associated with highly porous, ligand-extended structures.