Electronic structure dictates performance: why Mg-MOF-74 outperforms its peers in direct air capture of CO₂.

CONTEXT: The urgent need for scalable direct air capture (DAC) technologies is hindered by the lack of adsorbents that efficiently capture CO₂ from ambient air. While metal-organic frameworks (MOFs) such as the M-MOF-74 series show promise, a systematic electronic-level comparison of their DAC suitability is lacking. This study fills that gap by a systematic, electronic-level comparison of five M-MOF-74 variants (Mg, Ni, Co, Mn, Zn) using integrated DFT-GCMC analysis, establishing a clear structure-property relationship for DAC. Results identify Mg-MOF-74 as the optimal candidate, achieving the highest CO₂ uptake (5.03 mmol/g) and strongest binding energy (34.3 kJ/mol), which are attributed to its favorable electronic structure and open metal sites. We identify an optimal electronic configuration, moderate band gap, clear valence band maximum-conduction band minimum separation, and well-defined adsorption sites, which maximizes strong, selective physisorption at open metal sites. The work thus provides a blueprint for targeted material design, though the moisture sensitivity of Mg-MOF-74 underscores the need for hydrophobic functionalization in practical applications.

METHODS

Density functional theory (DFT) calculations were performed using the Dmol3 module in Materials Studio, employing the generalized gradient approximation with the Perdew-Burke-Ernzerhof (GGA-PBE) exchange-correlation functional. Grand Canonical Monte Carlo (GCMC) simulations were conducted using the sorption module in Materials Studio, with the COMPASS II force field applied for van der Waals interactions. The MOF-74 structures were obtained from the Cambridge Crystallographic Data Centre (CCDC) and optimized via the Forcite geometry optimization tool. Simulations were carried out under ambient conditions (298 K, 1 bar) to assess CO₂ adsorption isotherms, uptake capacities, and isosteric heats of adsorption.