Hydroxy-Substituted Aromatic N-Heterocycles as High-Affinity CO2 Adsorbents: A DFT Study

Abstract

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 et al., 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.