Sizable dynamics in small pores: CO2 location and motion in the α-Mg formate metal-organic framework

Metal-organic frameworks (MOFs) are promising materials for carbon dioxide (CO2) adsorption and storage; however, many details regarding CO2 dynamics and specific adsorption site locations within MOFs remain unknown, restricting the practical uses of MOFs for CO2 capture. The intriguing a-magnesium formate (alpha-Mg-3(HCOO)(6)) MOF can adsorb CO2 and features a small pore size. Using an intertwined approach of C-13 solid-state NMR (SSNMR) spectroscopy, H-1-C-13 cross-polarization SSNMR, and computational molecular dynamics (MD) simulations, new physical insights and a rich variety of information have been uncovered regarding CO2 adsorption in this MOF, including the surprising suggestion that CO2 motion is restricted at elevated temperatures. Guest CO2 molecules undergo a combined localized rotational wobbling and non-localized twofold jumping between adsorption sites. MD simulations and SSNMR experiments accurately locate the CO2 adsorption sites; the mechanism behind CO2 adsorption is the distant interaction between the hydrogen atom of the MOF formate linker and a guest CO2 oxygen atom, which are ca. 3.2 angstrom apart.