Competitive Adsorption of Exhaust Gases in the Covalent Organic Framework DAAQ-TFP from Molecular Modeling

Covalent organic frameworks (COFs) offer a wide range of tunable porous structures with the potential for high stability against aggressive temperature and chemical conditions. These traits make COFs potential candidates for tuned adsorbent materials to target specific contaminants in aqueous waste streams and gaseous discharges. Herein, we investigate the adsorption phenomena of a 2D planar COF, DAAQ-TFP, with typical exhaust gases and calculate the adsorption isotherms, energetics, and binding motifs to inform choices in the design of COF features for targeted selectivity and properties. DAAQ-TFP demonstrates a preference for adsorbing reactive N_xO_y species, especially N₂O₅. N₂O₅ exhibits nearly double the interaction energy with DAAQ-TFP compared with other species. The internal pore vertices of DAAQ-TFP make a high-affinity binding regime from N_xO_y species and unburnt hydrocarbons such as heptane with peak adsorption energies of 18 and 11 kcal/mol, respectively. DAAQ-TFP flakes stack together in offset layers creating diagonalized pores and large surface areas. We found that in 89% of the cases DAAQ-TFP flakes would stack with a helical twist and in 11% of the cases it would stack in a parallel displaced fashion. This is driven by numerous π-stacking interactions summed over the interlayer interface. This study provides a model basis for COF adsorption simulation that may predict performance as well as the driving forces behind COF gas adsorption selectivity. DAAQ-TFP may serve as a meaningful adsorbent component to help immobilize and retain contaminants from combustion exhaust gaseous discharge.