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

Abstract

Metal–organic frameworks (MOFs), such as MOF-74, can have open metal sites to which adsorbates such as CO<sub>2</sub> preferentially bind. <sup>13</sup>C NMR of <sup>13</sup>CO<sub>2</sub> 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 CO<sub>2</sub> per metal in Mg-MOF-74 at 295 K. <sup>13</sup>C lineshapes recorded as a function of loading can be understood in terms of the dependence of the CO<sub>2</sub> NMR frequency on the angle (θ) with respect to the CO<sub>2</sub> axis and the channel of the MOF, reflected in the Legendre polynomial, P<sub>2</sub>. In the fast motion limit, the NMR spectra reveal the time-averaged value of P<sub>2</sub>, where θ is the angle between the instantaneous CO<sub>2</sub> axis and the channel axis. DFT calculations were used to determine a weighted average of P<sub>2</sub> in this regime and are in good agreement with experimental data. Static variable temperature <sup>13</sup>C NMR from cryogenic temperatures to room temperature was used to investigate <sup>13</sup>CO<sub>2</sub> binding in Mg-MOF-74 loaded at two levels (~0.88 and 1.08 molecules of CO<sub>2</sub> per metal), revealing temperature-dependent lineshapes. We have investigated the effect of partial substitution of Cd for Mg in Mg-MOF-74 on the <sup>13</sup>CO<sub>2</sub> variable temperature NMR spectra. The chemical shift anisotropy (CSA) that leads to characteristic lineshapes of <sup>13</sup>C indicates that incorporation of Cd leads to weaker binding energies for adsorbed CO<sub>2</sub>.