Strategically designed azolyl-carboxylate MOFs for potential humid CO2 capture

Abstract

Development of solid sorbents with optimal CO₂ capture characteristics is key to improving the efficiency of PSA based CO₂ separation. Of the several potential sorbents, metal organic frameworks (MOFs) hold a key niche. This is owing to their modular tunable structures and manipulatable adsorption sites. Yet, developing a MOF that meets the multiple demands of a gas-separation sorbent remains a challenge. Particularly, tuning them to adsorb CO₂ over the more polar water is quite difficult. The presence of highly polarizing metal centers and oxygen-rich sites in MOFs makes it difficult to retain their CO₂ adsorption capacities under humid gas streams. However, tailoring the organic framework to incorporate humid CO₂ capture properties should be feasible. Along these lines, here we have developed a family of iso-structural MOFs built from bi-functional ligands that carry basic azolyl and chelating carboxylate groups together. Importantly, the framework is built by employing acetate moieties as ‘modulators’; they provide a hydrophobic lining to the pore-walls. This not only enables selective adsorption of CO₂, but also helps retain about 80% of the capture capacity even upon exposure to 75% RH. Along with its other advantageous features: high CO₂ uptake and selectivity (3 mmol g⁻¹ and s(CO₂–N₂) = ∼500 for 85N₂ : 15CO₂@303 K); surface area: ∼700 m² g⁻¹, working capacity: 2.6 mmol g⁻¹; optimal HOA (22–30 kJ mol⁻¹) and CO₂ kinetics (D_c = 1.02 × 10⁻⁸ m² s⁻¹), these MOFs can qualify as efficient candidates for humid CO₂ capture. Furthermore, we identify the most-favorable CO₂ adsorption sites via simulated annealing methods, from which the presence of polarized CO₂ molecules located adjacent to the π-electron rich pore walls can be seen. Importantly, these polarized molecules form T-shaped configurations among themselves via C(δ+ve)⋯O(δ−ve) interactions resembling those found in solid CO₂, a cooperative feature that is not observed in the other CO₂ molecules in the structure, which are not proximal to the polarizing walls.