Hydroxy-substituted aromatic N-heterocycles as high-affinity CO ₂ adsorbents: a DFT study

Hydroxy-substituted aromatic N-heterocycles, including hydroxy pyridine (py), dihydroxy naphthyridines (nt), and trihydroxy pyridonaphthyridines (pn), have been investigated for their potential as CO₂ adsorbents using density functional theory (DFT) calculations. Building on the pioneering work of Luo <i>et al.</i>, who demonstrated exceptional CO₂ capture capacities in pyridine-containing anion-functionalized ionic liquids, this study extends the exploration to a broader range of N-heterocycles. These N-heterocycles exhibit exceptional CO₂ capture capabilities, driven by cooperative interactions between nitrogen and oxygen centres with CO₂. The adsorption capacity increases with the number of nitrogen centres and hydroxy groups, with py, nt, and pn systems binding one, two, and three CO₂ molecules, respectively. Notably, anionic N-heterocycles exhibit dramatically improved CO₂ adsorption compared to their neutral counterparts, forming covalent bonds with CO₂. The presence of counter cations, such as lithium or tetramethylphosphonium ions, further stabilizes CO₂ adsorption, resulting in shorter interaction distances and higher exergonic free energy values. Solvent effects modeled using monoethanolamine (MEA) indicate a modest reduction in interaction energies for neutral and anionic systems, while ion-paired systems exhibit enhanced CO₂ affinity in solution. Additionally, molecular electrostatic potential (MESP) analysis highlights the key adsorption sites and charge delocalization mechanisms that facilitate CO₂ capture. The study also finds that enol-keto transformations, which could lead to CO₂ conversion into carboxylates, are energetically unfavorable due to the loss of aromatic stability. These findings underscore the potential of hydroxy-substituted N-heterocycles, particularly in their anionic and cation-stabilized forms, as promising candidates for efficient CO₂ capture. The insights gained from this study provide valuable guidelines for the design of next-generation CO₂ sequestration materials and highlight new directions for experimental validation and real-world applications.