Efficient Selective CO Composite Sorbent from Amino Acid Ionic Liquids and Silica Gel: H NMR Spectroscopy Provides Insight on the CO-Binding Mechanism and in-Pore Microscopic Viscosity.

A promising CO sorbent based on the [Emim][Gly] (1-ethyl-3-methylimidazolium glycinate)/silica gel composite has been studied. CO sorption experiments have shown that the optimum loading of [Emim][Gly] ionic liquid in silica gel is 40 wt.%.

The Nature of Structural Defects in ZIF-8 Revealed with H and P MAS NMR and X-Ray Absorption Spectroscopy.

The metal-organic frameworks (MOFs) attract interest as potential catalysts whose catalytic properties are driven by defects. Several methods have been proposed for the defects-inducing synthesis of MOFs. However, the active species formed on the defective sites remain elusive and uncharacterized, as the spectroscopic fingerprints of these species are hidden by the regular structure signals.

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 H nuclear magnetic resonance (NMR) spectroscopy.

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.

Mobility and separation of linear and branched C alkanes in UiO-66 (Zr) probed by H 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 (H NMR) and computational study of the molecular mobility, transport and adsorption of C alkanes isomers in a dehydroxylated UiO-66 (Zr) MOF.

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.

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.

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.

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][NTf]. 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 NTf anion, as we could confirm by DFT calculations.

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 [C H PD ][NTf ] by means of solid-state NMR spectroscopy. Combined line shape and spin relaxation studies of the deuterons in the PD 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 PD deuterons.

Dissecting the effects of water guest adsorption and framework breathing on the AlO(OH) 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 Al framework aluminium atom (QCC) of the MOF AlO(OH) centres (Al-sites) has been investigated by combining solid-state Al MAS NMR spectroscopy with XRD analysis and DFT calculations. It is established that Al 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 Al 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 C 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 H nuclear magnetic resonance-NMR) and computational (molecular dynamics-MD) approaches were applied together to derive slow diffusional dynamics of -butane and 1-butene in ZIF-8.

Dynamics of isobutane is a sensitive probe for framework breathing in MIL-53 (Al) MOF.

MIL-53 (Al) is an example of a MOF with a flexible framework featuring a broad range of applications such as hydrocarbon adsorption, separation and catalysis. Such processes are strongly influenced by the flexibility of the framework and thus require monitoring of the interrelation between the guest dynamics and framework breathing events. Here, we demonstrate that breathing of the framework can be monitored by probing the isobutane guest dynamics with H solid-state nuclear magnetic resonance (H NMR).

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). Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL-53 material with partially shrunk channels. H solid-state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two axes and librations of the molecule as a whole about the axis passing through the TBA C-O bond.

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 H 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.

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.

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 H NMR analysis.

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

Highly stable superprotonic conductivity (>10 S cm) has been achieved through the unprecedented coordinative urea insertion in MOF-74 [M(dobdc), M = Ni, Mg; 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 H NMR revealed that the high conductivity was attributed to the strengthening of the hydrogen bonds between guest HO induced by hydrogen bonds in the interface between HO and the polarized coordinated urea.

Dynamics of propene and propane in ZIF-8 probed by solid-state H NMR.

We present a detailed H NMR characterization of molecular mobility of propene and propane propagating though the microporous ZIF-8, a zeolitic imidazolate framework renowned for its outstandingly high separation selectivity for industrially relevant propene/propane mixtures. Experimental characterization of both propene and propane diffusivity in ZIF-8 has been provided. Using H NMR spin relaxation analysis, the motional mechanisms for propene and propane guests trapped within the ZIF-8 framework have been elucidated.

Simultaneous determination of deuteron quadrupole coupling constants and rotational correlation times: the model case of hydrogen bonded ionic liquids.

We show that deuteron quadrupole coupling constants (DQCCs), and reorientational correlation times of molecular bonds N-D that are involved in hydrogen bonding, can be determined from NMR T1 relaxation time experiments simultaneously. For this purpose, we used trialkylammonium-based protic ionic liquids (PILs) as model compounds. They exhibit high viscosities and wide liquid ranges that allow measurements far beyond the extreme narrowing region (ω0τc ≪ 1).

Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants-Comparison with Salt Bridges and Molecular Systems.

We present deuteron quadrupole coupling constants (DQCC) for hydroxyl-functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb-enhanced hydrogen bonds between cation and anion (c-a), and the unusual hydrogen bonds between cation and cation (c-c), which are present despite repulsive Coulomb forces. We measure these sensitive probes of hydrogen bonding by means of solid-state NMR spectroscopy. The DQCCs of (c-a) ion pairs and (c-c) H-bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids.

H Solid-State NMR Spectroscopy Reveals the Dynamics of a Pyridine Probe Interacting with Coordinatively Unsaturated Metal Sites of MIL-100(Al) Metal-Organic Frameworks.

Coordinatively unsaturated metal sites (CUS) play an important role in catalysis by metal-organic frameworks (MOF). Being an intrinsic part of the framework the CUS take the role of acidic sites active in industrially relevant processes such as condensation or oxidation reactions. The key step of such reactions represents the coordination of the reagents to CUS.

Guests Like Gear Levers: Donor Binding to Coordinatively Unsaturated Metal Sites in MIL-101 Controls the Linker's Rotation.

We present investigation of the effect of electron-donor guests on framework mobility in the metal-organic framework (MOF) MIL-101(Cr) monitored by solid state H NMR spectroscopy. In a guest-free material, the mobile phenylene fragments of the terephthalate (TP) linkers populate two fractions with notably different kinetic parameters for torsional motion. Two fractions of rotational motion are indicative of non-equivalence of TP linker binding to the Cr O trimer, the primary building unit of the MIL-101 framework.

Dynamical heterogeneities in ionic liquids as revealed from deuteron NMR.

The heterogeneity in dynamics has important consequences for understanding the viscosity, diffusion, ionic mobility, and the rates of chemical reactions in technology relevant systems such as polymers, metallic glasses, aqueous solutions, and inorganic materials. Herein, we study the spatial and dynamic heterogeneities in ionic liquids by means of solid state NMR spectroscopy. In the H spectra of the protic ionic liquid [TEA][OTf] we observe anisotropic and isotropic signals at the same time.