New insight into impact of humidity on direct air capture performance by SIFSIX-3-Cu MOF

Removal of CO₂ from air is one of the key human challenges in battling global warming. SIFSIX-3-Cu is a promising metal-organic framework (MOF) suggested for carbon capture even at low CO₂ concentrations. However, the impact of humidity on its performance in direct air capture (DAC) is poorly understood. To evaluate the MOF performance for DAC application under humid conditions, we investigate the adsorption of H₂O, CO₂, and N₂ using density functional theory (DFT), grand canonical Monte Carlo (GCMC), and molecular dynamics (MD) simulations. The simulation results show a higher tendency of SIFSIX-3-Cu towards H₂O adsorption rather than CO₂ (and N₂). The results agree with the adsorption isotherms for the pure compounds from the Sips model. The extended Sips model shows 1.34 mmol g^-1 CO₂ adsorption at the atmospheric pressure and 298 K for the CO₂/N₂ mixture containing 400 ppm CO₂, and low CO₂ adsorption (less than 0.75 mmol g^-1) at a low relative humidity (RH) of 20%. This finding highlights the efficiency of SIFSIX-3-Cu for DAC in dry air and the negative impact of humidity on the CO₂ selective adsorption. Therefore, we suggest to consider the impairing of humidity effects when designing a SIFSIX-3-Cu-based CO₂ separation process and removal of any water vapor before introduction of the air to SIFSIX-3-Cu.