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

Effect of Co-Adsorbed Guest Adsorbates on the Separation of Ethylene/Ethane Mixtures on Metal-Organic Frameworks with Open Metal Sites

Direct determination of the equilibrium adsorption and spectroscopic observation of adsorbent-adsorbate interaction is crucial to evaluate the olefin/paraffin separation performance of porous adsorbents. However, the experimental characterization of competitive adsorption of various adsorbates at atomic-molecular level in the purification of multicomponent gas mixtures is challenging and rarely conducted. Herein, solid-state NMR spectroscopy is employed to examine the effect of co-adsorbed guest adsorbates on the separation of ethylene/ethane mixtures on Mg-MOF-74, Zn-MOF-74 and UTSA-74. 1H MAS NMR facilitates the determination of equilibrium uptake and adsorption selectivity of ethylene/ethane in ternary mixtures. The co-adsorption of H2O and CO2 significantly leads to the degradation of ethylene uptake and ethylene/ethane selectivity. The detailed host-guest and guest-guest interactions are unraveled by 2D 1H-1H spin diffusion homo-nuclear correlation and static 25Mg NMR experiments. The experimental results verify H2O coordinated on open metal sites can supply a new adsorption site for ethylene and ethane. The effects of guest adsorbates on the adsorption capacity and adsorption selectivity of ethylene/ethane mixtures are in the following order: H2O>CO2>O2. This work provides a direct approach for exploring the equilibrium adsorption and detailed separation mechanism of multicomponent gas mixtures using MOFs adsorbents.

Chiral Metal-Organic Frameworks with Spectroscopic Methods: Towards Chemical Sensor Devices

Chiral Metal-Organic Frameworks (CMOFs) are a rapidly growing field reflecting their potential as selective and sensitive chemical sensors for chiral analytes. The highly tuneable nature of CMOFs enables the size, shape, and non-covalent interactions to be optimised towards specific analytes to engender strong intermolecular interactions and sensing responses. While CMOFs as chiral chemical sensor devices have been explored with electrochemical methods including differential pulse voltammetry (DPV), bipolar and chemiresistive sensing techniques, the CMOFs as chiral chemical sensors using spectroscopic methods has received significantly less attention. This review examines the synthesis of CMOFs for chemical sensors with spectroscopic methods such as photoluminescence, circular dichroism, and solid-state nuclear magnetic resonance with a view towards their incorporation into chemical sensor devices. Future directions of the field are highlighted for the generation of functional devices.

Cyclodextrin Metal-Organic Frameworks as a Drug Delivery System for Selected Active Pharmaceutical Ingredients

The cyclodextrin-based metal-organic frameworks (CD MOFs) are a suitable molecular platform for drug delivery systems of various active pharmaceutical ingredients (APIs). The low toxicity and cost-efficient synthesis make CD MOFs an attractive host for the encapsulation of APIs. In this study, we created a model system based on γCD-K MOFs with widely used drugs carmofur (HCFU), 5-fluorouracil (5-FU), and salicylic acid (HBA) to study host-guest encapsulation methods using different crystallization protocols. The host-guest complexes of API:CD MOF in an in-depth study were investigated by liquid chromatography-mass spectrometry (LC-MS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and 19F- and 13C-detected solid-state NMR spectroscopy (ssNMR). These techniques confirmed the structure and interaction sites within the encapsulation product in the host-guest complex. We also evaluated the toxicity and biocompatibility of the API:CD MOF complex using in vitro and in vivo methods. The cytotoxicity, hepatotoxicity, and neurotoxicity were established with cell lines of fibroblasts (BJ), human liver cell line (HepG2), and human oligodendrocytic cells (MO3.13). Then, Danio rerio was used as an in vivo experimental model of ecotoxicity. The results showed the choice of γCD-K-5 as the most protective and safe option for drug encapsulation to decrease its toxicity level against normal cells.

Surface Hydrophobicity and Guest Permeability in Polydimethylsiloxane-Coated MIL-53 as Studied by Solid-State Nuclear Magnetic Resonance Spectroscopy

Experimental characterization of the hydrophobic porous materials at the atomic and molecular levels is of great significance, but exploring their hydrophobicity characteristics and interactions with guest molecules with distinct polarity is still challenging. In this work, solid-state NMR is employed to characterize the surface hydrophobicity and explore the guest solvent permeability in polydimethylsiloxane (PDMS)-coated MIL-53. It was found that the PDMS-coated MIL-53 was hydrophobic to water and infiltrated to methanol, acetone, benzene, toluene, and ethylbenzene solvents. In addition, two types of guest solvents (methanol, acetone, benzene, toluene, and ethylbenzene), inside the pore and outside the pore of PDMS-coated MIL-53, were clearly identified using two-dimensional 1H-1H homo-nuclear correlation NMR experiments. Moreover, the membrane thickness of the PDMS-coated MIL-53 could be determined from the analysis of the 1H-1H spin diffusion buildup curves. Furthermore, the permeability of benzene, toluene, and ethylbenzene at different PDMS coating levels was extracted from 1H MAS NMR. The increase of the hydrophobic PDMS layer resulted in a decrease of the penetration of aromatic guests to the internal pore of MIL-53. This work provides deep insights into the understanding of guest solvent permeability of hydrophobic layer-coated MOFs in the application fields of catalysis and separation.

Nuclear magnetic resonance studies of carbon dioxide capture

Carbon dioxide capture is an important greenhouse gas mitigation technology that can help limit climate change. The design of improved capture materials requires a detailed understanding of the mechanisms by which carbon dioxide is bound. Nuclear magnetic resonance (NMR) spectroscopy methods have emerged as a powerful probe of CO2 sorption and diffusion in carbon capture materials. In this article, we first review the practical considerations for carrying out NMR measurements on capture materials dosed with CO2 and we then present three case studies that review our recent work on NMR studies of CO2 binding in metal-organic framework materials. We show that simple 13C NMR experiments are often inadequate to determine CO2 binding modes, but that more advanced experiments such as multidimensional NMR experiments and 17O NMR experiments can lead to more conclusive structural assignments. We further discuss how pulsed field gradient (PFG) NMR can be used to explore diffusion of adsorbed CO2 through the porous framework. Finally, we provide an outlook on the challenges and opportunities for the further development of NMR methodologies that can improve our understanding of carbon capture.

Defined metal atom aggregates precisely incorporated into metal-organic frameworks

Nanosized metal aggregates (MAs), including metal nanoparticles (NPs) and nanoclusters (NCs), are often the active species in numerous applications. In order to maintain the active form of MAs in "use", they need to be anchored and stabilised, preventing agglomeration. In this context, metal-organic frameworks (MOFs), which exhibit a unique combination of properties, are of particular interest as a tunable and porous matrix to host MAs. A high degree of control in the synthesis towards atom-efficient and application-oriented MA@MOF composites is required to derive specific structure-property relationships and in turn to enable design of functions on the molecular level. Due to the versatility of MA@MOF (derived) materials, their applications are not limited to the obvious field of catalysis, but increasingly include 'out of the box' applications, for example medical diagnostics and theranostics, as well as specialised (bio-)sensoring techniques. This review focuses on recent advances in the controlled synthesis of MA@MOF materials en route to atom-precise MAs. The main synthetic strategies, namely 'ship-in-bottle', 'bottle-around-ship', and approaches to achieve novel hierarchical MA@MOF structures are highlighted and discussed while identifying their potential as well as their limitations. Hereby, an overview of standard characterisation methods that enable a systematic analysis procedure and state-of-art techniques that localise MA within MOF cavities are provided. While the perspectives of MA@MOF materials in general have been reviewed various times in the recent past, few atom-precise MAs inside MOFs have been reported so far, opening opportunities for future investigation.

Slow CO2 Diffusion Governed by Steric Hindrance of Rotatory Ligands in Small Pores of a Metal-Organic Framework

Understanding the adsorption and diffusional dynamics of CO₂ in metal-organic frameworks (MOFs) is essential in the application of these materials to CO₂ capture and separation. We show that the dynamics of adsorbed CO₂ is related to the rotational motion of ligands located in the narrow pore windows of a MOF using solid-state nuclear magnetic resonance (NMR) spectroscopy. NMR analyses of local dynamics reveal that CO₂ adsorbed in the pore hinders the rotation of the ligands. The rate of diffusion of adsorbed CO₂ monitored by ¹³C NMR is much less than that in the larger pores of MOFs and decreases cooperatively with ligand mobility, which indicates that the rate of diffusion is influenced by the steric hindrance of the rotatory ligands. Adsorbed CH₄ also showed slow diffusion in the MOF, suggesting molecular size-selective effect of the mobile steric hindrance on the rate of adsorbate diffusion.

Multinuclear solid-state NMR: Unveiling the local structure of defective MOF MIL-120

Metal-organic frameworks (MOFs) are emerging materials with many current and potential applications due to their unique properties. One critical feature is that the physical and chemical properties of MOFs are tunable. One of the methods for tuning MOF properties is to introduce defects by design for desired applications. Characterization of MOF defects is important, but very challenging due to the local nature and short-range ordering. In this work, we have introduced the ordered vacancies (the defects) in the form of the coordinatively unsaturated sites (CUSs) into the framework of MOF MIL-120(Al). The creation of ordered vacancies is achieved by replacing one quarter of the BTEC (1,2,4,5-benzenetetracarboxylate) with BDC (benzene-1,4-dicarboxylate) linkers. Both parent and defective MOFs were characterized by multinuclear solid-state NMR spectroscopy. ¹H MAS NMR is used to characterize the hydrogen bonding in these MOFs, whereas ¹³C CP MAS NMR confirms unambiguously that the BDC is incorporated into the framework. One-dimensional ²⁷Al MAS NMR provides direct evidence of the coordinatively unsaturated Al sites (the defects). Furthermore, ²⁷Al 3QMAS experiments at 21.1 ​T allow direct identification of one penta-coordinated and three chemically inequivalent octahedral Al sites in the defective MIL-120(Al). Two of the above-mentioned octahedral Al sites are in the domain which appears defect-free. The third octahedral Al site is near the defective site. This work clearly demonstrates the power of solid-state NMR spectroscopy for characterization of defective MOFs.

Characterization of Chemisorbed Species and Active Adsorption Sites in Mg-Al Mixed Metal Oxides for High-Temperature CO2 Capture

Mg-Al mixed metal oxides (MMOs), derived from the decomposition of layered double hydroxides (LDHs), have been purposed as adsorbents for CO₂ capture of industrial plant emissions. To aid in the design and optimization of these materials for CO₂ capture at 200 °C, we have used a combination of solid-state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) to characterize the CO₂ gas sorption products and determine the various sorption sites in Mg-Al MMOs. A comparison of the DFT cluster calculations with the observed ¹³C chemical shifts of the chemisorbed products indicates that mono- and bidentate carbonates are formed at the Mg-O sites with adjacent Al substitution of an Mg atom, while the bicarbonates are formed at Mg-OH sites without adjacent Al substitution. Quantitative ¹³C NMR shows an increase in the relative amount of strongly basic sites, where the monodentate carbonate product is formed, with increasing Al/Mg molar ratios in the MMOs. This detailed understanding of the various basic Mg-O sites presented in MMOs and the formation of the carbonate, bidentate carbonate, and bicarbonate chemisorbed species yields new insights into the mechanism of CO₂ adsorption at 200 °C, which can further aid in the design and capture capacity optimization of the materials.

Solid-state NMR insights into alcohol adsorption by metal-organic frameworks: adsorption state, selectivity, and adsorption-induced phase transitions

Alcohol adsorption by metal-organic frameworks (ZIF-8 and ZIF-11) in aqueous solutions is investigated including alcohol mixtures. Solid-state ¹³C NMR spectroscopy is demonstrated to be well-suited for such liquid-phase adsorption studies at the molecular level. Adsorption-induced immobilization could be visualized. Finally, an unexpected phase transition of ZIF-11 was discovered.

A transition from solid effect to indirect cross effect with broadband microwave irradiation

Dynamic nuclear polarization (DNP) at high magnetic fields has become a prominent technique for signal enhancement in nuclear magnetic resonance (NMR). In static samples, the highest DNP enhancement is usually observed for high radical concentrations in the range of 15-40 mM. Under these conditions, the dominant DNP mechanism for broad-line radicals is the electron-electron spectral-diffusion-based indirect cross effect (iCE). To further increase the DNP performance, broadband microwave irradiation is often applied. Until now, the theory of iCE was not rigorously combined with broadband microwave irradiation. This paper fills this gap by extending the iCE theory to explicitly include broadband irradiation. We demonstrate that our theory allows for quantitative fitting of the DNP spectra lineshapes using four different datasets acquired at 3.4 T and 7 T. We find that the DNP mechanism changes with an increase in the excitation bandwidth. While with narrowband continuous-wave irradiation the DNP mechanism is a combination of the solid effect (SE) and iCE, it shifts toward iCE with increasing excitation bandwidth until, at high bandwidth, the iCE completely dominates the DNP spectrum - this effect was not accounted for previously.

Metal-organic frameworks (MOFs) based nanofiber architectures for the removal of heavy metal ions

Environmental heavy metal ions (HMIs) accumulate in living organisms and cause various diseases. Metal-organic frameworks (MOFs) have proven to be promising and effective materials for removing heavy metal ions from contaminated water because of their high porosity, remarkable physical and chemical properties, and high specific surface area. MOFs are self-assembling metal ions or clusters with organic linkers. Metals are used as dowel pins to build two-dimensional or three-dimensional frameworks, and organic linkers serve as carriers. Modern research has mainly focused on designing MOFs-based materials with improved adsorption and separation properties. In this review, for the first time, an in-depth look at the use of MOFs nanofiber materials for HMIs removal applications is provided. This review will focus on the synthesis, properties, and recent advances and provide an understanding of the opportunities and challenges that will arise in the synthesis of future MOFs-nanofiber composites in this area. MOFs decorated on nanofibers possess rapid adsorption kinetics, a high adsorption capacity, excellent selectivity, and good reusability. In addition, the substantial adsorption capacities are mainly due to interactions between the target ions and functional binding groups on the MOFs-nanofiber composites and the highly ordered porous structure.

Investigation of CO2 Orientational Dynamics through Simulated NMR Line Shapes*

The dynamics of carbon dioxide in third generation (i. e., flexible) Metal-Organic Frameworks (MOFs) can be experimentally observed by ¹³ C NMR spectroscopy. The obtained line shapes directly correlate with the motion of the adsorbed CO₂ , which in turn are readily available from classical molecular dynamics (MD) simulations. In this article, we present our publicly available implementation of an algorithm to calculate NMR line shapes from MD trajectories in a matter of minutes on any current personal computer. We apply the methodology to study an effect observed experimentally when adsorbing CO₂ in different samples of the pillared layer MOF Ni₂ (ndc)₂ (dabco) (ndc=2,6-naphthalene-dicarboxylate, dabco=1,4-diazabicyclo-[2.2.2]-octane), also known as DUT-8(Ni). In ¹³ C NMR experiments of adsorbed CO₂ in this MOF, small (rigid) crystals result in narrower NMR line shapes than larger (flexible) crystals. The reasons for the higher mobility of CO₂ inside the smaller crystals is unknown. Our ligand field molecular mechanics simulations provide atomistic insight into the effects visible in NMR experiments with limited computational effort.

Breaking the trade-off between selectivity and adsorption capacity for gas separation

The trade-off between selectivity and adsorption capacity with porous materials is a major roadblock to reducing the energy footprint of gas separation technologies. To address this matter, we report herein a systematic crystal engineering study of C₂H₂ removal from CO₂ in a family of hybrid ultramicroporous materials (HUMs). The HUMs are composed of the same organic linker ligand, 4-(3,5-dimethyl-1H-pyrazol-4-yl)pyridine, pypz, three inorganic pillar ligands, and two metal cations, thereby affording six isostructural pcu topology HUMs. All six HUMs exhibited strong binding sites for C₂H₂ and weaker affinity for CO₂. The tuning of pore size and chemistry enabled by crystal engineering resulted in benchmark C₂H₂/CO₂ separation performance. Fixed-bed dynamic column breakthrough experiments for an equimolar (v/v = 1:1) C₂H₂/CO₂ binary gas mixture revealed that one sorbent, SIFSIX-21-Ni, was the first C₂H₂ selective sorbent that combines exceptional separation selectivity (27.7) with high adsorption capacity (4 mmol·g⁻¹).

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Six isostructural hybrid ultramicroporous materials are prepared and characterized

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Crystal engineering approach enabled fine-tuning of pore size and chemistry

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Weak CO₂/strong C₂H₂ affinity resulted in high C₂H₂/CO₂ separation selectivities

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SIFSIX-21-Ni: benchmark selectivity/uptake capacity for C₂H₂/CO₂ separation

It is generally recognized that porous solids (sorbents) with high selectivity and high adsorption capacity offer potential for energy-efficient gas separations. Unfortunately, there is generally a trade-off between capacity and selectivity, which represents a roadblock to the utility of sorbents in key industrial processes. For example, acetylene (C₂H₂), an important fuel and chemical intermediate, is produced with CO₂ as an impurity, and the similar physicochemical properties of C₂H₂ and CO₂ mean that most sorbents are poorly selective. Hybrid ultramicroporous materials (HUMs) are candidates for gas separations as they exhibit benchmark selectivity for several key gas pairs. Unfortunately, existing HUMs are handicapped by low capacity. We report a new HUM, SIFSIX-21-Ni, that addresses the trade-off between selectivity and capacity that has plagued sorbents, as its high uptake and high selectivity renders it the new benchmark for C₂H₂/CO₂ separation performance.

Host-Guest Interaction in Ethylene and Ethane Separation on Zeolitic Imidazolate Frameworks as Revealed by Solid-State NMR Spectroscopy

The separation of ethane/ethylene mixture by using metal-organic frameworks (MOFs) as adsorbents is strongly associated with the pore size-sieving effect and the adsorbent-adsorbate interaction. Herein, solid-state NMR spectroscopy is utilized to explore the host-guest interaction and ethane/ethylene separation mechanism on zeolitic imidazolate frameworks (ZIFs). Preferential access to the ZIF-8 and ZIF-8-90 frameworks by ethane compared to ethylene is directly visualized from two-dimensional ¹ H-¹ H spin diffusion MAS NMR spectroscopy and further verified by computational density distributions. The ¹ H MAS NMR spectroscopy provides an alternative for straightforwardly extracting the adsorption selectivity of ethane/ethylene mixture at 1.1∼9.6 bar in ZIFs, which is consistent with the IAST predictions.

Solid-State NMR Spectroscopy: A Key Tool to Unravel the Supramolecular Structure of Drug Delivery Systems

In the past decades, nanosized drug delivery systems (DDS) have been extensively developed and studied as a promising way to improve the performance of a drug and reduce its undesirable side effects. DDSs are usually very complex supramolecular assemblies made of a core that contains the active substance(s) and ensures a controlled release, which is surrounded by a corona that stabilizes the particles and ensures the delivery to the targeted cells. To optimize the design of engineered DDSs, it is essential to gain a comprehensive understanding of these core-shell assemblies at the atomic level. In this review, we illustrate how solid-state nuclear magnetic resonance (ssNMR) spectroscopy has become an essential tool in DDS design.

Dynamics in Flexible Pillar[n]arenes Probed by Solid-State NMR

Pillar[n]arenes are supramolecular assemblies that can perform a range of technologically important molecular separations which are enabled by their molecular flexibility. Here, we probe dynamical behavior by performing a range of variable-temperature solid-state NMR experiments on microcrystalline perethylated pillar[n]arene (n = 5, 6) and the corresponding three pillar[6]arene xylene adducts in the 100-350 K range. This was achieved either by measuring site-selective motional averaged 13C 1H heteronuclear dipolar couplings and subsequently accessing order parameters or by determining 1H and 13C spin-lattice relaxation times and extracting correlation times based on dipolar and/or chemical shift anisotropy relaxation mechanisms. We demonstrate fast motional regimes at room temperature and highlight a significant difference in dynamics between the core of the pillar[n]arenes, the protruding flexible ethoxy groups, and the adsorbed xylene guest. Additionally, unexpected and sizable 13C 1H heteronuclear dipolar couplings for a quaternary carbon were observed for p-xylene adsorbed in pillar[6]arene only, indicating a strong host-guest interaction and establishing the p-xylene location inside the host, confirming structural refinements.

Combining In Situ Techniques (XRD, IR, and 13C NMR) and Gas Adsorption Measurements Reveals CO2-Induced Structural Transitions and High CO2/CH4 Selectivity for a Flexible Metal-Organic Framework JUK-8

Flexible metal-organic frameworks (MOFs) are promising materials in gas-related technologies. Adjusting the material to processes requires understanding of the flexibility mechanism and its influence on the adsorption properties. Herein, we present the mechanistic understanding of CO₂-induced pore-opening transitions of the water-stable MOF JUK-8 ([Zn(oba)(pip)]_n, oba^2- = 4,4'-oxybis(benzenedicarboxylate), pip = 4-pyridyl-functionalized benzene-1,3-dicarbohydrazide) as well as its potential applicability in gas purification. Detailed insights into the global structural transformation and subtle local MOF-adsorbate interactions are obtained by three in situ techniques (XRD, IR, and ¹³CO₂-NMR). These results are further supported by single-crystal X-ray diffraction (SC-XRD) analysis of the solvated and guest-free phases. High selectivity toward carbon dioxide derived from the single-gas adsorption experiments of CO₂ (195 and 298 K), Ar (84 K), O₂ (90 K)_, N₂ (77 K), and CH₄ (298 K) is confirmed by high-pressure coadsorption experiments of the CO₂/CH₄ (75:25 v/v) mixture at different temperatures (288, 293, and 298 K) and in situ NMR studies of the coadsorption of ¹³CO₂/¹³CH₄ (50:50 v/v; 195 K).

Solid-state NMR studies of the acidity of functionalized metal-organic framework UiO-66 materials

The acid strength of metal-organic frameworks plays a key role in their catalytic performance such as activity and selectivity during catalytic reactions. Solid-state nuclear magnetic resonance in combination with probe molecules including 2-¹³ C-acetone and pyridine-d₅ was employed to characterize the acid strength of UiO-66-X (X = -H, -2COOH, -SO₃ H). It was found that after introduction of the functional groups, the acid strength of UiO-66-2COOH and UiO-66-SO₃ H is considerably enhanced compared with that of parent UiO-66, with that of the former being similar to that of zeolite H-ZSM-5, and with that of the latter being slightly stronger than that of the former. Even though the acid density can efficiently be modified through changing the relative ratio in multivariate functionalized UiO-66-X, no significant alternation for the acid strength could be discerned in the MTV-UiO-66-X compared with acidic same-link counterpart. Theoretical calculations were employed to further confirm the acid strength of UiO-66-SO₃ H and UiO-66-2COOH.

Recent Advances of Solid-State NMR Spectroscopy for Microporous Materials

Microporous materials have attracted a rapid growth of research interest in materials science and the multidisciplinary area because of their wide applications in catalysis, separation, ion exchange, gas storage, drug release, and sensing. A fundamental understanding of their diverse structures and properties is crucial for rational design of high-performance materials and technological applications in industry. Solid-state NMR (SSNMR), capable of providing atomic-level information on both structure and dynamics, is a powerful tool in the scientific exploration of solid materials. Here, advanced SSNMR instruments and methods for characterization of microporous materials are briefly described. The recent progress of the application of SSNMR for the investigation of microporous materials including zeolites, metal-organic frameworks, covalent organic frameworks, porous aromatic frameworks, and layered materials is discussed with representative work. The versatile SSNMR techniques provide detailed information on the local structure, dynamics, and chemical processes in the confined space of porous materials. The challenges and prospects in SSNMR study of microporous and related materials are discussed.

Higher Magnetic Fields, Finer MOF Structural Information: 17O Solid-State NMR at 35.2 T

The spectroscopic study of oxygen, a vital element in materials, physical, and life sciences, is of tremendous fundamental and practical importance. ¹⁷O solid-state NMR (SSNMR) spectroscopy has evolved into an ideal site-specific characterization tool, furnishing valuable information on the local geometric and bonding environments about chemically distinct and, in some favorable cases, crystallographically inequivalent oxygen sites. However, ¹⁷O is a challenging nucleus to study via SSNMR, as it suffers from low sensitivity and resolution, owing to the quadrupolar interaction and low ¹⁷O natural abundance. Herein, we report a significant advance in ¹⁷O SSNMR spectroscopy. ¹⁷O isotopic enrichment and the use of an ultrahigh 35.2 T magnetic field have unlocked the identification of many inequivalent carboxylate oxygen sites in the as-made and activated phases of the metal-organic framework (MOF) α-Mg₃(HCOO)₆. The subtle ¹⁷O spectral differences between the as-made and activated phases yield detailed information about host-guest interactions, including insight into nonconventional O···H-C hydrogen bonding. Such weak interactions often play key roles in the applications of MOFs, such as gas adsorption and biomedicine, and are usually difficult to study via other characterization routes. The power of performing ¹⁷O SSNMR experiments at an ultrahigh magnetic field of 35.2 T for MOF characterization is further demonstrated by examining activation of the MIL-53(Al) MOF. The sensitivity and resolution enhanced at 35.2 T allows partially and fully activated MIL-53(Al) to be unambiguously distinguished and also permits several oxygen environments in the partially activated phase to be tentatively identified. This demonstration of the very high resolution of ¹⁷O SSNMR recorded at the highest magnetic field accessible to chemists to date illustrates how a broad variety of scientists can now study oxygen-containing materials and obtain previously inaccessible fine structural information.

Probing dynamics of carbon dioxide in a metal-organic framework under high pressure by high-resolution solid-state NMR

The application of high-resolution NMR analysis for CO2 adsorbed in a MOF under high pressure is reported for the first time. The results showed that CO2 adsorbed in MOF-74 had an unusually slow mobility (τ ∼ 10-8 s). CO2-CO2 interactions suppressed the mobility of CO2 under high pressure, which, in turn, would have contributed to the stability of CO2 at the adsorption sites.

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

Metal-organic frameworks (MOFs) gain increasing interest due to their outstanding properties like extremely high porosity, structural variability, and various possibilities for functionalization. Their overall structure is usually determined by diffraction techniques. However, diffraction is often not sensitive for subtle local structural changes and ordering effects as well as dynamics and flexibility effects. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is sensitive for short range interactions and thus complementary to diffraction techniques. Novel methodical advances make ssNMR experiments increasingly suitable to tackle the above mentioned problems and challenges. NMR spectroscopy also allows study of host-guest interactions between the MOF lattice and adsorbed guest species. Understanding the underlying mechanisms and interactions is particularly important with respect to applications such as gas and liquid separation processes, gas storage, and others. Special in situ NMR experiments allow investigation of properties and functions of MOFs under controlled and application-relevant conditions. The present minireview explains the potential of various solid-state and in situ NMR techniques and illustrates their application to MOFs by highlighting selected examples from recent literature.

High-Pressure in Situ 129Xe NMR Spectroscopy: Insights into Switching Mechanisms of Flexible Metal-Organic Frameworks Isoreticular to DUT-49

Flexible metal-organic frameworks (MOFs) are capable of changing their crystal structure as a function of external stimuli such as pressure, temperature, and type of adsorbed guest species. DUT-49 is the first MOF exhibiting structural transitions accompanied by the counterintuitive phenomenon of negative gas adsorption. Here, we present high-pressure in situ ¹²⁹Xe NMR spectroscopic studies of a novel isoreticular MOF family based on DUT-49. These porous materials differ only in the length of their organic linkers causing changes in pore size and elasticity. The series encompasses both, purely microporous materials as well as materials with both micropores and small mesopores. The chemical shift of the adsorbed xenon depends on xenon-wall interactions and thus on the pore size of the material. The xenon adsorption behavior of different MOFs can be observed over the whole range of relative pressure. Chemical shift adsorption/desorption isotherms closely resembling the conventional, uptake-measurement-based isotherms were obtained at 237 K where all materials are rigid. The comparable chemical environment of the adsorbed xenon in these isoreticular MOFs allows to establish a correlation between the chemical shift at a relative pressure of p/p ₀ = 1.0 and the mean pore diameter. Furthermore, the xenon adsorption behavior of MOFs is studied also at 200 K. Here, structural flexibility is found for DUT-50, a material with an even longer linker than that of the previously known DUT-49. Its structural transitions are monitored by ¹²⁹Xe NMR spectroscopy. This compound is the second known MOF showing the phenomenon of negative gas adsorption. Further increase in the linker length results in DUT-151, a material with an interpenetrated network topology. In situ ¹²⁹Xe NMR spectroscopy proves that this material exhibits another type of flexibility compared to DUT-49 and DUT-50. Further surprising observations are made for DUT-46. Volumetric xenon adsorption measurements show that this nonflexible microporous material does not exhibit any hysteresis. In contrast, the in situ ¹²⁹Xe NMR spectroscopically detected xenon chemical shift isotherms exhibit a hysteresis even after longer equilibration times than in the volumetric experiments. This indicates kinetically hindered redistribution processes and long-lived metastable states of adsorbed xenon within the MOF persisting at the time scale of hours or longer.

In Situ 13C NMR Spectroscopy Study of CO2/CH4 Mixture Adsorption by Metal-Organic Frameworks: Does Flexibility Influence Selectivity?

Metal-organic frameworks are promising candidates for selective separation processes such as CO₂ removal from methane (natural gas sweetening). Framework flexibility, that is, the ability of a MOF lattice to change its structure as a function of parameters like pressure, temperature, and type of adsorbed molecules, is only observed for some special compounds. The main question of our present work is: does framework flexibility influence the adsorption selectivity? As a direct quantitative method to monitor the adsorption of both, carbon dioxide and methane, we make use of high-pressure in situ ¹³C NMR spectroscopy of ¹³CO₂/¹³CH₄ gas mixtures. This method allows to distinguish between the two gases as well as between adsorbed molecules and the interparticle gas phase. Gas mixture adsorption is studied under isothermal conditions. The selectivity factor for CO₂ adsorption from CO₂/CH₄ mixtures is measured as a function of total gas pressure. The flexible material SNU-9 as well as the flexible and the nonflexible variant of DUT-8(Ni) are compared. Maximum selectivity factors for CO₂ are observed for the flexible variant of DUT-8(Ni) in its open, large-pore state. In contrast, the rigid variant of DUT-8(Ni) and SNU-9 especially in its intermediate state exhibits lower adsorption selectivity factors. This observation indicates significant influence of the framework elasticity on the adsorption selectivity.

New insights into solvent-induced structural changes of 13C labelled metal–organic frameworks by solid state NMR

The proposed ¹³C isotope-labelling scheme enables the in-depth analysis of site-specific host–guest interactions and adsorption complexes formed in MOFs.