(739b) Understanding CO2 in MOF-74:  Dynamics and Structure of Adsorbed CO2 with 13C NMR and DFT

Metal-organic frameworks are crystalline organic-inorganic hybrid materials that have been of interest for adsorption applications for their tunability, high crystallinity, and high surface areas. Mg-MOF-74 has long been of interest for its exceptional gravimetric uptake of CO₂ at flue-gas relevant conditions. CO₂ adsorption in Mg-MOF-74 is well studied, but most work has focused on understanding adsorption at the undercoordinated open-metal sites that line the pores. Despite this, loadings of CO₂ in Mg-MOF-74 at even modest partial pressures can exceed one CO₂ per open-metal site, requiring a more nuanced picture to understand the behavior and loading of CO₂. Recent neutron diffraction studies have shown that CO₂ occupies not only these primary adsorption sites, but also “secondary” and even “tertiary” sites that exist within the MOF pore. Here we present a joint experimental NMR and theory study that demonstrates that higher CO₂ loadings qualitatively alter the structure of adsorbed CO₂ within the MOF pore. Constructing special NMR hardware to deliver gases at variable temperature and pressure, we have found that the ¹³C NMR lineshape evolves in a predictable fashion as these different sites become occupied. We show that the time-averaged value of the second Legendre polynomial, P₂, which maps to the alignment of CO₂ with respect to the MOF pore axis, allows understanding of the so-called “sign” of the NMR lineshape—which shifts in appearance with loading in a highly sensitive and informative manner. The evolution of the NMR lineshape with CO₂ loading, which probes the aggregate dynamics and structure of adsorbed CO₂, agrees exceptionally well with the DFT-predicted loading-dependent structures of CO₂ within the Mg-MOF-74 pore through analysis in terms of P₂. NMR experiments also point to qualitative differences between the behavior of CO₂ in Mg-MOF-74 and Mg—Cd-MOF-74, which we rationalize in terms of theoretical calculations for mixed-metal MOF-74 systems and our calculations on Mg-MOF-74. Importantly, the NMR spectra are difficult to interpret without the detailed picture of adsorption sites provided by DFT, and the combined effect demonstrates how experiments can reinforce the DFT models of collective CO₂ behavior.