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), bibta(2-) = 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.