CO2 Dynamics in Pure and Mixed-Metal MOFs with Open Metal Sites

Metal–organic frameworks (MOFs), such as MOF-74, can have open metal sites to which adsorbates such as CO2 preferentially bind. 13C NMR of 13CO2 is highly informative about the binding sites present in Mg-MOF-74. We used this technique to investigate loadings between ∼0.88 and 1.15 molecules of CO2 per metal in Mg-MOF-74 at 295 K. 13C lineshapes recorded as a function of loading can be understood in terms of the dependence of the CO2 NMR frequency on the angle (θ) with respect to the CO2 axis and the channel of the MOF, reflected in the Legendre polynomial, P2. In the fast motion limit, the NMR spectra reveal the time-averaged value of P2, where θ is the angle between the instantaneous CO2 axis and the channel axis. DFT calculations were used to determine a weighted average of P2 in this regime and are in good agreement with experimental data. Static variable temperature 13C NMR from cryogenic temperatures to room temperature was used to investigate 13CO2 binding in Mg-MOF-74 loaded at two levels (∼0.88 a...