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

Removal of CO 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 CO 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 HO, CO, and N 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 HO adsorption rather than CO (and N). The results agree with the adsorption isotherms for the pure compounds from the Sips model. The extended Sips model shows 1.34 mmol g CO adsorption at the atmospheric pressure and 298 K for the CO/N mixture containing 400 ppm CO, and low CO adsorption (less than 0.75 mmol g) 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 CO selective adsorption. Therefore, we suggest to consider the impairing of humidity effects when designing a SIFSIX-3-Cu-based CO separation process and removal of any water vapor before introduction of the air to SIFSIX-3-Cu.