Adsorption Sites and Binding Nature of CO2 in Prototypical Metal-Organic Frameworks: A Combined Neutron Diffraction and First-Principles Study

We report a detailed study of CO2 adsorption in two important metal organic framework (MOF) compounds (Mg-MOF-74 and HKUST-1). In both MOFs, the open metal ions were identified as the primary binding sites through neutron diffraction measurements. The relatively strong metal-CO2 binding was attributed to an enhanced electrostatic interaction, and vibrational mode analysis shows that the adsorbed CO2 molecule is strongly attached through one of its oxygen atoms while the rest of the molecule is relatively free. This high orientational disorder is the reason for the large apparent O-C-O bond bending angle derived from diffraction measurements. Our calculations give only a small degree of bond bending, suggesting that the CO2 adsorption on the open metal site is still largely physisorption. Interestingly, the overall metal-CO2 binding strength is right in the range which can facilitate both adsorption (CO2 capture) and desorption (MOP regeneration) under typical flue gas conditions.