Assessing the Role of Metal Identity on CO₂ Adsorption in MOFs Containing M–OH Functional Groups

Heterobimetallic analogues of CFA-1 [Zn1+zM4-zX4(bibta)3, bibta2– = 5,5′-bibenzotriazolate, M = Co (z = 0), Ni (z = 1), Cu (z = 2.3), X = Cl–, Br–, CH3CO2–] have been prepared via postsynthetic cation exchange. Subsequent postsynthetic X–/HCO3– ligand exchange followed by thermal activation generates nucleophilic M–OH groups at the Kuratowski-type metal nodes of the heterobimetallic metal-organic frameworks (MOFs). While the Cu-exchanged MOF suffered from degradation as a result of the postsynthetic modifications, the Co and Ni analogues (Co–OH and Ni–OH) proved to be stable to activation, and room-temperature isotherm measurements show steep CO2 uptake at pressures compatible with direct air capture and other trace CO2 removal applications. Ni–OH exhibits a greater low-pressure CO2 capacity and higher isosteric heat of adsorption than Co–OH and the all-Zn MOF, Zn–OH. In situ diffuse reflectance infrared (IR) spectroscopy experiments indicate that Co–OH and Ni–OH adsorb CO2 via a M–OH → M–O2COH chemisorption mechanism aided by intercluster hydrogen-bonding interactions. However, CO2 adsorption in Ni–OH gives rise to spectroscopic features that are not observed for Co–OH and Zn–OH and can be attributed to Ni-bicarbonate groups that do not engage in intercluster hydrogen bonding. Density functional theory (DFT) calculations performed on model clusters support the experimentally observed trend in CO2 affinity.