Mechanism of CO₂ adsorption on Mg/DOBDC with elevated CO₂ loading

Understanding the nature of CO₂ adsorption in porous materials is important to developing the next generation high performance adsorbents for CO₂ capture. The mechanism of CO₂ adsorption in a typical metal organic framework (MOF) material, named Mg/DOBDC (DOBDC = 1,4-dioxido-2,5-benzenedicarboxylate), was investigated by experiments and theoretical calculations. Needle-shaped Mg/DOBDC crystals were synthesized and characterized. CO₂ adsorption isotherm at ambient conditions on Mg/DOBDC was converted to obtain open metal site coverage by CO₂ with pressure. Binding energies of CO₂ on open metal site and organic linker site were calculated separately using density functional theory (DFT). Furthermore, step-by-step DFT calculations were performed by adding CO₂ molecules into Mg/DOBDC pores one after another to mimic the CO₂ adsorption process. The results show that CO₂ adsorption energy is as high as -55.5 kJ/mol on the open metal site (primary site) and about -27.1 kJ/mol on the organic linkers (secondary site). As CO₂ pressure increases, CO₂ molecules occupy the open metal sites first, then organic linker site due to the difference in adsorption energy between these two sites. Correspondingly, charges transfer from adsorbed CO₂ molecules to the framework continuously with increasing CO₂ loading, which results in the variation of electrostatic environment. When six CO₂ molecules are loaded in one single pore, each CO₂ molecule interacts with the framework with same distance and angle.