Dynamics of Linkers in Metal-Organic Framework Glasses.

Metal-organic framework (MOF) glasses have emerged as a new class of organic-inorganic hybrid glass materials. Considerable efforts have been devoted to unraveling the macroscopic dynamics of MOF glasses by studying their rheological behavior; however, their microscopic dynamics remain unclear. In this work, we studied the effect of vitrification on linker dynamics in ZIF-62 by solid-state 2H nuclear magnetic resonance (NMR) spectroscopy. 2H NMR relaxation analysis provided a detailed picture of the mobility of the ZIF-62 linkers, including local restricted librations and a large-amplitude twist; these details were verified by molecular dynamics. A comparison of ZIF-62 crystals and glasses revealed that vitrification does not drastically affect the fast individual flipping motions with large-amplitude twists, whereas it facilitates slow cooperative large-amplitude twist motions with a decrease in the activation barrier. These observations support the findings of previous studies, indicating that glassy ZIF-62 retains permanent porosity and that short-range disorder exists in the alignment of ligands because of distortion of the coordination angle.

Understanding Alkene Interaction with Metal-Modified Zeolites: Thermodynamics and Mechanism of Bonding in the π-Complex.

Zeolites modified with metal cations are perspective catalysts for converting light alkenes to valuable chemicals. A crucial step of the transformation is an alkene interaction with zeolite to afford π-complex with metal cations. The mechanism of alkene bonding with cations is still unclear. To address this problem, propene adsorption on H+ (BroÌ·nsted acid site), Na+, Ca2+, Zn2+, Co2+, Cu2+, Cu+, and Ag+ cationic sites in ZSM-5 zeolite has been studied by quantum chemical calculations in terms of adsorption enthalpy, νC═C frequency, and natural bond orbital (NBO) analysis together with natural energy decomposition analysis (NEDA). It is revealed that the conventional concept of σ- and π-bonding is only partially applicable to alkene interaction with metal cations in zeolites. The orbital interaction between an alkene molecule and a metal site is more complex. Several different bonding mechanisms have been identified depending on the nature and electron configuration of the metal cation. This finding explains the complex correlations observed for propene π-complex stability and νC═C frequency shift or charge transfer from the alkene molecule. The results provide the basis for further understanding the interactions between alkenes and inorganic solid BroÌ·nsted and Lewis acids.

Ion Mobility in Hydroxy-Functionalized Ionic Liquids Depends on Cationic Clustering: Tracking the Alkyl Chain Length Behavior with Deuteron NMR Relaxation.

Observing and quantifying the like-charge attraction in liquids and solutions is still challenging. However, we showed that elusive cation-cation hydrogen bonding may govern the structure and interaction in hydroxyl-functionalized ionic liquids. Therefore, cationic cluster formation depends on the shape, charge distribution, and functionality of the ions. We demonstrated by means of solid-state 2H NMR spectroscopy that cationic clusters change the structure and dynamics of ionic liquids. With increasing alkyl chain length, we observed two deuteron quadrupole coupling constants for the OD groups, differing by about 30 kHz. The lower value was assigned to the cation-cation interaction, indicating that the average (c-c) hydrogen bonds are stronger than the (c-a) hydrogen bonds between the cation and the anion despite the repulsive and attractive Coulomb interaction in the first and latter cases. Ion mobility could be studied by 2H NMR spectroscopy, although the deuterons in the hydrogen-bonded clusters underwent fast exchange. Our results also showed that simple relaxation models are not applicable anymore and that anisotropic motion must be considered.

Mobility and separation of linear and branched C5 alkanes in UiO-66 (Zr) probed by 2H NMR and MD simulations.

The UiO-66 (Zr) metal-organic framework (MOF) is of notable interest due to its facile synthesis, robustness under a wide range of chemical and physical conditions and its capability to separate industrially relevant hydrocarbons mixtures. However, the knowledge of the molecular mechanisms behind these process remains limited. Here, we present a combined experimental (2H NMR) and computational study of the molecular mobility, transport and adsorption of C5 alkanes isomers in a dehydroxylated UiO-66 (Zr) MOF. We show that the tetrahedral cages of the MOF are the preferred adsorption location for both n-pentane and isopentane. In a binary mixture of the isomers, isopentane interacts more strongly with the material leading it to occupy more of the tetrahedral cages than n-pentane, resulting in an isopentane/n-pentane adsorption selectivity of αads = 2 (at 373 K). At the same time, the microscopic diffusivity for n-pentane, Dn (En = 18 kJ mol-1), is significantly lower than for isopentane, Diso (Eiso = 28 kJ mol-1), which results in a high separation selectivity for a n-pentane/isopentane mixture of α ≈ 13 (at 300 K). This shows that the UiO-66 MOF is indeed a promising active material for use in light hydrocarbon separation processes.

Salt Confined in MIL-101(Cr)-Tailoring the Composite Sorbents for Efficient Atmospheric Water Harvesting.

The adsorption method for atmospheric water harvesting (AWH) is considered as a promising heat-driven technology for potable water supply in arid regions. This research is focused on novel composite sorbents based on hygroscopic salts loaded in the pores of MIL-101(Cr) developed for AWH. The composites based on LiCl, LiBr, CaCl2 , and Ca(NO3 )2 were synthesized and comprehensively studied by SEM, XRD, N2 adsorption, and thermogravimetric methods. We evidence that the CaCl2 /MIL-101(Cr) composite demonstrates a high net water uptake of 0.52-0.59 g_(H2 O)/g_(composite) per cycle under conditions of Saudi Arabia and the Sahara desert as the reference regions with extra-dry climate, which exceeds the appropriate values for other adsorbents. It is shown that water adsorption on the composite cannot be presented as a combination of the adsorption on the components, thus indicating a synergistic effect. A detailed characterization of water coordination, mobility, and hydrogen bonding within the confined CaCl2 hydrates and salt solution using solid-state 2 H NMR spectroscopy has been performed. It is established that pore confinement promotes a prolonged transition to a dynamically melted state of the hydrated salt and a notable decrease of the melting temperature, which facilitates the molecular transport of water and causes the alteration of sorption properties of CaCl2 inside MIL-101 pores. Finally, the performance of AWH employing CaCl2 /MIL-101(Cr) was evaluated in terms of the fractions of water extracted and collected, and the specific energy consumption, demonstrating its high potential for AWH.

How Like-Charge Attraction Influences the Mobility of Cations in Hydroxyl-Functionalized Ionic Liquids.

Attractive interactions between ions of like charge remain an elusive concept. Observing and quantifying this type of interaction in liquids and solutions is still a major challenge. Recently, we have shown that cation-cation interactions are present in hydroxyl-functionalized ionic liquids and that they can be controlled by the shape, charge distribution and functionality of the ions. In the present study, we demonstrate that cationic cluster formation does not only change the local structures of the ionic liquids but also influences the dynamics of the cations in a characteristic way. We show that solid-state 2H NMR spectroscopy is well suited for the study of molecular motion, even if the hydrogen bonded species of interest are indistinguishable due to fast deuteron exchange. We also provide valuable information about the applicability of well-accepted relaxation models.

Zn-BTC MOF as Self-Template to Hierarchical ZnS/NiS2 Heterostructure with Improved Electrochemical Performance for Hybrid Supercapacitor.

Zn-BTC (H3BTC refers to 1, 3, 5-benzoic acid) MOF was used as a self-template and a zinc source to prepare ZnS/NiS2 with a layered heterogeneous structure as a promising electrode material using cation exchange and solid-phase vulcanization processes. The synergistic effect of the two metal sulfides enhances the application of ZnS/NiS2. And the high specific surface area and abundant active sites further promote the mass/charge transfer and redox reaction kinetics. In the three-electrode system, the specific capacitance was as high as 1547 F/g at a current density of 1 A/g, along with satisfactory rate capability (1214 F/g at 6 A/g) and cycling performance. Coupled with activated carbon (AC), the prepared hybrid device (ZnS/NiS2 as the positive electrode and AC as the negative electrode) (ZnS/NiS2/AC) can be operated under a potential window of 1.6 V and provides a high energy density of 26.3 Wh/kg at a power density of 794 W/kg. Notably, the assembled ZnS/NiS2//AC showed little capacity degradation after 5000 charge/discharge cycles.

The Influence of Deuterium Isotope Effects on Structural Rearrangements, Ensemble Equilibria, and Hydrogen Bonding in Protic Ionic Liquids.

We report strong isotope effects for the protic ionic liquid triethylammonium methanesulfonate [TEA][OMs] by means of deuterium solid-state NMR spectroscopy covering broad temperature ranges from 65 K to 313 K. Both isotopically labelled PILs differ in non-deuterated and fully deuterated ethyl groups of the triethyl ammonium cations. The N-D bond of both cations is used as sensitive probe for hydrogen bonding and structural ordering. The 2 H NMR line shape analysis provides the deuteron quadrupole coupling constants and the characteristics of a broad heterogeneous phase with simultaneously present static and mobile states indicating plastic crystal behavior. The temperatures where both states are equally populated differ by about 80 K for the two PILs, showing that deuteration of the ethyl groups in the trialkylammonium cations tremendously shifts the equilibrium towards the static state. In addition, it leads to a significant less cooperative transition, associated with a significantly reduced standard molar transition entropy.

Butane isomers mobility and framework dynamics in UiO-66 (Zr) MOF: Impact of the hydroxyl groups in zirconia cluster.

UiO-66 (Zr) is a metal-organic framework (MOF) known for its thermal and chemical stability and wide range of adsorption-based applications. This MOF exhibits high separation selectivity for butane isomers. It has been earlier inferred that the separation performance of the material depends on the hydroxylation state of the zirconia cluster. In this contribution, we apply 2H solid-state NMR to characterize the dynamics of both the MOF organic framework itself and butane isomers in hydroxylated and dehydroxylated forms of UiO-66. It is established that the rate of π-flipping and the amplitude of the phenylene ring plane librations in the framework are higher for the dehydroxylated form. Self-diffusion coefficients of butane isomers have been estimated for both forms of UiO-66. The diffusivity is higher for n-butane in the dehydroxylated form, whereas the diffusion of isobutane is not affected by the presence of OH groups in the zirconia cluster of the MOF. Higher diffusivity of n-butane in dehydroxylated form is accounted for by the larger effective diameter of the window between the adjacent cages in this form, which arises from faster rotation and larger amplitude of framework linker libration. This rationalizes the higher efficiency of the dehydroxylated form of UiO-66(Zr) material for butane isomers separation.

Structure, hydrogen bond dynamics and phase transition in a model ionic liquid electrolyte.

We show that solid-state NMR spectroscopy is a suitable method for characterizing the structure, hydrogen bond dynamics and phase transition behavior in protic ionic liquids (PILs). Deuteron line shape and spin relaxation time analysis provide a description of the structural and dynamical heterogeneity in the solid state of the model PIL triethyl ammonium bis(trifluoromethanesulfonyl)amide [TEA][NTf2]. Therein, we observed two deuteron quadrupole coupling constant for the ND bond of the TEA cation, indicating differently strong hydrogen bonds to the nitrogen and oxygen atoms of the NTf2 anion, as we could confirm by DFT calculations. The transition processes in the dynamically heterogeneous phase are characterized by two standard molar enthalpies and thus different stages of melting. We provide geometry, rates and energetics of the cation in the solid and liquid states of the PIL. Comparison with PILs having stronger interacting anions shows higher enthalpy change between the solid and liquid states, lower activation barriers of tumbling motion and higher amplitude of librational motion for the TEA cation in the presence of the weakly interacting anion NTf2. We provide reasonable relations between microscopic and macroscopic properties, as is relevant for any kind of application.

High-Temperature Quantum Tunneling and Hydrogen Bonding Rearrangements Characterize the Solid-Solid Phase Transitions in a Phosphonium-Based Protic Ionic Liquid.

We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosphonium bis(trifluoromethylsulfonyl)imide [C8 H17 PD3 ][NTf2 ] by means of solid-state NMR spectroscopy. Combined line shape and spin relaxation studies of the deuterons in the PD3 group of the octylphosphonium cation allow to map and correlate the phase behavior for a broad temperature range from 71 K to 343 K. In the solid PIL at 71 K, we observed a static state, characterized by the first deuteron quadrupole coupling constant reported for PD3 deuterons. A transition enthalpy of about 12 kJ mol-1 from the static to the mobile state with increasing temperature suggests the breaking of a weak, charge-enhanced hydrogen bond between cation and anion. The highly mobile phase above 100 K exhibits an almost disappearing activation barrier, strongly indicating quantum tunneling. Thus, we provide first evidence of tunneling driven mobility of the hydrogen bonded P-D moieties in the glassy state of PILs, already at surprisingly high temperatures up to 200 K. Above 250 K, the mobile phase turns from anisotropic to isotropic motion, and indicates strong internal rotation of the PD3 group. The analyzed line shapes and spin relaxation times allow us to link the structural and dynamical behavior at molecular level with the phase behavior beyond the DSC traces.

Dissecting the effects of water guest adsorption and framework breathing on the AlO4(OH)2 centres of metal-organic framework MIL-53 (Al) by solid state NMR and structural analysis.

The relationship between the adsorption of water on MIL-53 (Al) MOF, the structural phase of MIL-53 (Al), and the quadrupole coupling constant of 27Al framework aluminium atom (QCC) of the MOF AlO4(OH)2 centres (Al-sites) has been investigated by combining solid-state 27Al MAS NMR spectroscopy with XRD analysis and DFT calculations. It is established that 27Al QCC is primarily sensitive to water adsorption to the Al-sites and by a minor extent to the framework contraction/expansion interconversions. We thus conclude that the 27Al MAS NMR method is sensitive enough to differentiate the effects of pore contractions and water adsorption to Al-sites basing on the changes of the QCC value.

Molecular Insight into the Slow Dynamics of C4 Hydrocarbons in the Zeolitic-Imidazole Framework (ZIF-8).

The family of zeolitic-imidazole framework (ZIF) materials is currently considered for the challenging separation of C4 hydrocarbons. However, yet, the microscopic diffusion mechanism for these hydrocarbons in these narrow gate porous materials remains elusive by conventional methods due to its very slow nature. Experimental (solid-state 2H nuclear magnetic resonance-NMR) and computational (molecular dynamics-MD) approaches were applied together to derive slow diffusional dynamics of n-butane and 1-butene in ZIF-8. By means of the 2H NMR technique, we evidenced the presence of two adsorption sites for the guests localized inside the cages of ZIF-8 and in the vicinity of the gates. We characterized the molecular mobility at each site and revealed that the translational intercage diffusion is realized by a slow directional motion associated with the gate-crossing. MD simulations provide an in-depth analysis of the diffusion and fully support the proposed dynamics picture for both n-butane and 1-butene. These calculations enable the derivation of the diffusivity and barriers for the long-range diffusion of both hydrocarbons in ZIF-8 and unraveled the microscopic diffusion mechanism implying intracage and intercage motions. We show that this NMR approach combined with modeling is a valuable tool to probe the molecular mobility for slow diffusing species in ordered cagelike porous frameworks.

Selective Gas Uptake and Rotational Dynamics in a (3,24)-Connected Metal-Organic Framework Material.

The desolvated (3,24)-connected metal-organic framework (MOF) material, MFM-160a, [Cu3(L)(H2O)3] [H6L = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid)], exhibits excellent high-pressure uptake of CO2 (110 wt% at 20 bar, 298 K) and highly selective separation of C2 hydrocarbons from CH4 at 1 bar pressure. Henry's law selectivities of 79:1 for C2H2:CH4 and 70:1 for C2H4:CH4 at 298 K are observed, consistent with ideal adsorption solution theory (IAST) predictions. Significantly, MFM-160a shows a selectivity of 16:1 for C2H2:CO2. Solid-state 2H NMR spectroscopic studies on partially deuterated MFM-160-d12 confirm an ultra-low barrier (∼2 kJ mol-1) to rotation of the phenyl group in the activated MOF and a rotation rate 5 orders of magnitude slower than usually observed for solid-state materials (1.4 × 106 Hz cf. 1011-1013 Hz). Upon introduction of CO2 or C2H2 into desolvated MFM-160a, this rate of rotation was found to increase with increasing gas pressure, a phenomenon attributed to the weakening of an intramolecular hydrogen bond in the triazine-containing linker upon gas binding. DFT calculations of binding energies and interactions of CO2 and C2H2 around the triazine core are entirely consistent with the 2H NMR spectroscopic observations.

Characterization of Proton Dynamics for the Understanding of Conduction Mechanism in Proton Conductive Metal-Organic Frameworks.

Proton conductivity has been traditionally investigated with various materials such as organic polymers, metal oxides, and other inorganic and organic compounds because of their potential application in the electrochemical devices. In particular, during the last decade, crystalline porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) have received considerable attention in recent years, as solid-state proton conductors (SSPCs). To date, proton-conductive MOFs have achieved high performance in proton conductivity (>10-2  S cm-1 ) with rational design strategies. In addition, there are dedicated efforts to define the conduction pathway and mechanism using various experimental tools. In this review, we focus on the characterization of proton conductivity and molecular dynamics in hydrated MOFs, with selected examples to provide an understanding of the overall conduction mechanism.

Molecular Mobility of Tert-butyl Alcohol Confined in a Breathing MIL-53 (Al) Metal-Organic Framework.

We present a detailed solid-state NMR characterization of the molecular dynamics of tert-butyl alcohol (TBA) confined inside breathing metal-organic framework (MOF) MIL-53(Al). 27 Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL-53 material with partially shrunk channels. 2 H solid-state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two C 3 axes and librations of the molecule as a whole about the axis passing through the TBA C-O bond. These librations are realized by two distinct ways: fast molecule orientation change during the translational jump diffusion along the channel with characteristic time τD of about 10-9  s at 300 K; slow local librations at a single coordination site, representing framework hydroxyl groups, with τl ≈10-6  s at 300 K. Self-diffusion coefficient of the alcohol in the MOF has been estimated: D=3.4×10-10  m2 s-1 at 300 K. It has been inferred that both the framework flexibility and the interaction with framework hydroxyl groups define the dynamics of TBA confined in the channels of MIL-53 (Al).

Freezing the Motion in Hydroxy-Functionalized Ionic Liquids-Temperature Dependent NMR Deuteron Quadrupole Coupling Constants for Two Types of Hydrogen Bonds Far below the Glass Transition.

We measured the deuteron quadrupole coupling constants (DQCCs) for hydroxy-functionalized ionic liquids (ILs) with varying alkyl chain length over the temperature range between 60 and 200 K by means of solid-state NMR spectroscopy. For all temperatures, the 2H spectra show two DQCCs representing different types of hydrogen bonds. Higher values, ranging from 220 to 250 kHz, indicate weaker hydrogen bonds between cation and anion (c-a), and lower values varying from 165 to 210 kHz result from stronger hydrogen bonds between the OD groups of cations (c-c), in agreement with recent observations in infrared, neutron diffraction, and NMR studies. We observed different temperature dependencies for (c-a) and (c-c) hydrogen bonding. From the static pattern of the 2H spectra at the lowest temperatures, we derived the true DQCCs being up to 20 kHz larger than recently reported values measured at the glass transition temperature. We were able to freeze the librational motions of the hydrogen bonds in the ILs. The temperature dependence of the (c-a) and (c-c) cluster populations in the glassy state is opposite to that observed in the liquid state, partly anticipating the behavior of ILs tending to crystallize.

Transformation of a proton insulator to a conductor via reversible amorphous to crystalline structure transformation of MOFs.

In this study, a successful proton conduction modulation of MOFs, from an ionic insulator to an ionic conductor, is demonstrated through their structural transformation. It is shown that the reversible structural change from amorphous to crystalline phases allows for the reversible proton conduction modulation of MOFs. Moreover, the proton conduction mechanism of the ionic conductor phase is elucidated by 2H NMR analysis.

Counting cations involved in cationic clusters of hydroxy-functionalized ionic liquids by means of infrared and solid-state NMR spectroscopy.

In hydroxy-functionalized ionic liquids, two types of hydrogen bonding coexist: the conventional H-bonds between cation and anion (c-a) and those between cation and cation (c-c), although the interaction between like-charged ions is supposed to be much weaker due to the repulsive Coulomb forces. Counting the cations involved in either (c-a) or (c-c) clusters is a challenge. For that purpose, we recently performed neutron diffraction (ND) measurements and molecular dynamics (MD) simulations at and above room temperature accompanied by NMR solid-state experiments in the glassy state of the ILs. In principle, these methods are suitable for determining the populations of (c-a) and (c-c) cluster species. For different reasons we could only address single temperatures and/or small temperature intervals above 300 K. The by far largest temperature range with reasonable efforts is accessible by simple infrared (IR) spectroscopy. However, counting (c-a) or (c-c) hydrogen bonds is a difficult task due to the different transition dipole moments resulting in varying intensities and broad vibrational bands. Here we present a method for deriving the number of cations involved in (c-a) ion pairs from IR spectra in the OH stretch region. This procedure provides access to the equilibria of (c-a) and (c-c) hydrogen bonds as a function of temperature allowing derivation of the transition enthalpy.

Superprotonic Conductivity in Metal-Organic Framework via Solvent-Free Coordinative Urea Insertion.

Highly stable superprotonic conductivity (>10-2 S cm-1) has been achieved through the unprecedented solvent-free-coordinative urea insertion in MOF-74 [M2(dobdc), M = Ni2+, Mg2+; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] without an acidic moiety. The urea is bound to open metal sites and alters the void volume and surface functionality, which triggers a significant change in proton conductivity and diffusion mechanism. Solid-state 2H NMR revealed that the high conductivity was attributed to the strengthening of the hydrogen bonds between guest H2O induced by hydrogen bonds in the interface between H2O and the polarized coordinated urea.