Comprehensive study of carbon dioxide adsorption in the metal–organic frameworks M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Analysis of the CO₂ adsorption properties of a well-known series of metal–organic frameworks M₂(dobdc) (dobdc⁴⁻ = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO₂ adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO₂ within many of these materials while systematically varying both the amount of CO₂ and the temperature. Unlike previous studies, we show that CO₂ adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO₂ adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO₂. For the weaker CO₂ adsorbents, significant elongation of the metal–O(CO₂) distances are observed and diffraction experiments additionally reveal that secondary CO₂ adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO₂ angles and trends in relative geometric properties and heats of adsorption in the M₂(dobdc)–CO₂ adducts.