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

Solid‐State NMR Spectroscopy: A Powerful Technique to Directly Study Small Gas Molecules Adsorbed in Metal–Organic Frameworks

“…[14,15] The CO 2 dynamics in Mg-MOF-74 was further investigated by Hayes and co-workers using various CO 2 loading levels. [18] The static 13 CS SNMR spectrum obtaineda tt he lowest loading agreedw ith that recorded by Reimer and co-workers. [14] However,a th igher loadings, the powder patterns were observed to change.…”

“…MOFs are promising CO 2 adsorbents, [13] and SSNMR spectroscopy has been employed to studyC O 2 chemisorptiona nd physisorption in severalM OFs. [12] This section focuses on physisorptiona nd demonstrates how guest dynamics, [14][15][16][17][18][19][20][21][22] MOF-CO 2 interaction strengths [16] and binding sites [16,17] can be investigated.…”

“…Long-and short-range CO 2 dynamics can both be studied using SSNMR spectroscopy.E ven thoughl ong-ranges tudies can provide important information on diffusivity, [19,22] local CO 2 dynamics has been the main focus for many of the recent SSNMR investigations.Hence, we restrict our currentd iscussion to local CO 2 dynamics studies. Various SSNMR experiments (e.g.,e xchange spectroscopy, [20] T 1 relaxation studies, [21] variable-temperature (VT) experiments [14][15][16]18] )h ave been used to probe CO 2 motions, and static VT 13 Ce xperiments are one of the most commonly employed techniques. Since the appearance of powder patterns is influenced by molecular orientations, and therefore changes in orientations (i.e.,d ynamics), motional modelsc an be directly extracted by simulating the VT spectra.…”

Welcoming Gallium- and Indium-Fumarate MOFs to the Family: Synthesis, Comprehensive Characterization, Observation of Porous Hydrophobicity, and CO2 Dynamics

“…NMR parameters relating to the CS and QI are described in more detail within the Supporting Information. 69 Ga and 71 Ga NMR spectra of MOFs such as MIL-53, MIL-61, MIL-96, MIL-120 and MIL-124 have been reported. [40][41] In this study, we have used NMR experiments at a magnetic field of 21.1 T to acquire 69 Ga and 71 Ga spectra of activated Ga-fumarate and Ga-fumarate-H 2 O.…”

“…69 Ga and 71 Ga NMR spectra of MOFs such as MIL-53, MIL-61, MIL-96, MIL-120 and MIL-124 have been reported. [40][41] In this study, we have used NMR experiments at a magnetic field of 21.1 T to acquire 69 Ga and 71 Ga spectra of activated Ga-fumarate and Ga-fumarate-H 2 O. Acquisition of both 69 Ga and 71 Ga spectra permits accurate determination of NMR parameters.…”