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.

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