A molecular study of humid CO2 adsorption capacity by Mg-MOF-74 surfaces with ligand functionalization

The ligand on the surface is an important factor for the improvement of the CO2 adsorption capacity of a metal–organic framework (MOF). In this work, the hydroxyl and carboxyl functional groups that have different attractions to water molecules are connected separately to the metal–organic framework, Mg-MOF-74, and their effect on humid CO2 adsorption capacity is investigated using molecular dynamics simulations. The humid CO2 adsorption capacity of Mg-MOF-74, Mg-MOF-74-OH, and Mg-MOF-74-COOH present linear proportional relationships to the volumetric concentration of CO2. It is found Mg-MOF-74-COOH has the best CO2 adsorption capacity from the mixture gas with high humidity. Owing to the competitive adsorption of H2O and CO2 at the open sites, the more sensitive to H2O the MOFs are, the less they absorb CO2. Hence, Mg-MOF-74-COOH is mostly favorable for adsorbing CO2, followed by Mg-MOF-74 and Mg-MOF-74-OH. Meanwhile, the difference in adsorption heat between H2O and CO2 is over 3 times for Mg-MOF-74-OH; and it becomes about 1.5 times for Mg-MOF-74, while it is negligible for MOF-74-COOH. The results show that by synthesizing a MOF with high CO2 selective adsorption and low sensitivity to impurities, especially water molecules, it can be widely used in industrial applications, thus this work has a guiding significance for developing and synthesizing new materials by ligand functionalization of high surface area MOFs which are promising in carbon capture technology.