Characterization of Metal–Organic Frameworks: Unlocking the Potential of Solid-State NMR

Abstract

An exciting advance in materials science is the discovery of hybrid organic-inorganic solids known as metal-organic frameworks (MOFs). Although they have numerous important applications, the local structures, specific molecular-level features, and guest behaviors underpinning desirable properties and applications are often unknown. Solid-state nuclear magnetic resonance (SSNMR) is a powerful tool for MOF characterization as it provides information complementary to that from X-ray diffraction (XRD). We describe our novel pursuits in the three primary applications of SSNMR for MOF characterization: interrogating the metal center, targeting linker molecules, and probing guests. MOFs have relatively low densities, and the incorporated metals are often quadrupolar nuclei, making SSNMR detection difficult. Recently, we examined the local structures of metal centers (i.e., Mg,Ti, Cu,Zn, Ga,Zr, In,Ba, La,Al) in representative MOFs by SSNMR at a high magnetic field of 21.1 T, addressing several important issues: (1) resolving chemically and crystallographically nonequivalent metal sites; (2) exploring the origin of disorder around metals; (3) refining local metal geometry; (4) probing the effects of activation and adsorption on the metal local environment; and (5) monitoring in situ phase changes in MOFs. Organic linkers can be characterized by H,C, and O SSNMR. Although the framework structure can be determined by X-ray diffraction, hydrogen atoms cannot be accurately located, and thus the local structure about hydrogen is poorly characterized. Our work demonstrates that magic-angle spinning (MAS) experiments at very high magnetic field along with ultrafast spinning rates and isotope dilution enables one to obtain ultrahigh resolutionH MAS spectra of MOFs, yielding structural information truly complementary to that obtained from single-crystal XRD. Oxygen is a key constituent of many important MOFs but O SSNMR work on MOFs is scarce due to the low natural abundance ofO. O enriched MOFs can now be prepared in an efficient and economically feasible manner using solvothermal approaches involving labeled HO water; the resulting O SSNMR spectra provide distinct spectral signatures of various key oxygen species in representative MOFs. MOFs are suitable for the capture of the greenhouse gas CO and the storage of energy carrier gases such as H and CH. A better understanding of gas adsorption obtained using C,H, and O SSNMR will enable researchers to improve performance and realize practical applications for MOFs as gas adsorbents and carriers. The combination of SSNMR with XRD allows us to determine the number of adsorption sites in the framework, identify the location of binding sites, gain physical insight into the nature and strength of host-guest interactions, and understand guest dynamics.

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

“…Both the MOF framework and guest molecules can be studied through SSNMR spectroscopy (Figure ) . For guest adsorption and dynamics studies, different nuclei (e.g., 13 C, 17 O, 2 H) can be employed depending on the system under consideration.…”

“…Both the MOF framework and guest molecules can be studied through SSNMR spectroscopy (Figure1). [3][4][5][6][7][8] For guest ad-sorptiona nd dynamics studies, differentn uclei (e.g., 13 C, 17 O, 2 H) can be employed depending on the system under consideration. Due to the presence of anisotropic interactions, powderp atterns are often observed in static SSNMR spectra.…”

“…Here, we demonstrate the potential of SSNMR spectroscopy by showcasingr ecent SSNMR studies of small gas molecules (i.e.,c arbon dioxide, carbon monoxide, hydrogen gas and light hydrocarbons) adsorbed in MOFs. Many interesting works have been conducted over the years, such as those reviewed in references [3][4][5][6][7][8][10][11][12].H owever,d ue to the brief nature of this Concepta rticle, we limit our scope to the last six years and only selected works are highlighted.…”

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

“…13 C NMR experiments have been used to investigate CO 2 locations, dynamics, and relative binding strengths within MOFs. 35,38,[59][60][61][62][63][64][65][66][67][68][69][70][71][72][73] In this work, we acquired static VT 13 C NMR spectra of CO 2 -loaded Al-fumarate at temperatures ranging from 133 K to 373 K (Figure 11(a)), and those of CO 2 -loaded Ga-fumarate, In-fumarate-E, and In-fumarate-M were acquired at temperatures between 133 K to 293 K (Figure 11(b), (c), (d)); the extracted CS parameters are summarized in Table S7. At 293 K, the static 13 C NMR spectra of 13 CO 2 adsorbed in Al-fumarate features a powder pattern corresponding to a span () of 27(1) ppm and a skew () value equal to +1.…”

Solid-state NMR spectroscopy: an advancing tool to analyse the structure and properties of metal–organic frameworks

“…Examples are 27 Al or 51 V which exhibit high natural abundance and a moderately high magnetic moment. 27 Various commonly used metal ions in MOFs exhibit NMR-active isotopes with low natural abundance and small magnetic moments, for example 25 Mg and 67 Zn. [28][29][30] Special pulse sequences like CPMG (Carr-Purcell-Meiboom-Gill) can enhance the NMR signal thus enabling detection of such spectra.…”