Daniil I. Kolokolov

Probing the Dynamics of CO2 and CH4 within the Porous Zirconium Terephthalate UiO-66(Zr): A Synergic Combination of Neutron Scattering Measurements and Molecular Simulations

Quasi-elastic neutron scattering (QENS) measurements combined with molecular dynamics (MD) simulations were conducted to deeply understand the concentration dependence of the self- and transport diffusivities of CH(4) and CO(2), respectively, in the humidity-resistant metal-organic framework UiO-66(Zr). The QENS measurements show that the self-diffusivity profile for CH(4) exhibits a maximum, while the transport diffusivity for CO(2) increases continuously at the loadings explored in this study. Our MD simulations can reproduce fairly well both the magnitude and the concentration dependence of each measured diffusivity. The flexibility of the framework implemented by deriving a new forcefield for UiO-66(Zr) has a significant impact on the diffusivity of the two species. Methane diffuses faster than CO(2) over a broad range of loading, and this is in contrast to zeolites with narrow windows, for which opposite trends were observed. Further analysis of the MD trajectories indicates that the global microscopic diffusion mechanism involves a combination of intracage motions and jump sequences between tetrahedral and octahedral cages.

Dynamics of Benzene Rings in MIL‐53(Cr) and MIL‐47(V) Frameworks Studied by 2H NMR Spectroscopy

Metal–organic frameworks (MOFs) combine metal oxide clusters and organic linkers in almost infinite manners. Because the variability in pore dimensions and chemical composition is larger than in zeolites, this class of hybrid porous solids has major potential applications in the fields of adsorption or separation of gases and liquids, catalysis, drug delivery, and others. 5] A remarkable feature of some MOFs is their flexibility. The MIL-53 type (MIL: Materials of Institut Lavoisier) is one of the best representatives of the “breathing” MOFs. This series of metal(III) terephthalates of formula (M(OH)·O2CC6H4CO2) (M = Al, Cr, Fe, Ga), is built up from chains of metal-centered octahedra sharing OH vertices, which are linked in the two other directions by terephthalate groups to create one-dimensional (1D) lozenge-shaped tunnels. Depending on the guest entrapped in the pores, MIL-53(Cr) has been shown to exhibit different crystalline states, corresponding to different pore openings, while the framework topology remains unchanged. The assynthesized form contains disordered terephthalic acid molecules in the pores and has a cell volume of 1440 . Upon calcination, the free acid is removed and the cell volume increases to 1486 , while it decreases to 1012 3 on hydration. This transition between large-pore (LP) and narrow-pore (NP) forms corresponding to anhydrous and hydrated states, respectively, is reversible. On the other hand, MIL-47(V), which is isostructural to MIL-53-LP but without the OH groups, has a rigid framework. 8] As a consequence, MIL-47(V) exhibits only type I adsorption isotherms, as expected for gas adsorption in a rigid nanoporous material. In contrast, steps in the adsorption isotherms of CO2 and various hydrocarbons occur in MIL-53(Cr) at room temperature, and are associated with two consecutive structural transitions. The transition from the LP to the NP form is observed at low concentration, and the NP-to-LP transition at higher loadings. The structural switching were evidenced by X-ray powder diffraction, adsorption microcalorimetry, and simulations. 10] This phenomenon is guest-dependent; for example, MIL-53(Cr) behaves as a rigid framework (LP form) on adsorption of certain small species (H2 and CH4), but is flexible for others (Xe). The magnitude of breathing can be related to the van der Waals volume of the guest molecule: the smaller the molecule the more MIL-53(Cr) is able to breathe. 11] The largest amplitude of breathing (ca. 40%) is obtained for the empty material. Surprisingly, the LP–NP transition can occur without any guest, simply by changing the temperature. This conclusion was drawn from elastic and inelastic neutron scattering measurements, which also showed the existence of a large temperature hysteresis in MIL-53(Al). It was suggested that the low-energy librational modes of the aromatic ring are coupled to the structural transition. In MOF-5, the softest twisting or torsional modes of benzene were calculated at similar energies. Although the energy of these modes is very low (20–80 cm ), no rotation of benzene was observed by quasi-elastic neutron scattering (QENS), the timescale of which ranges typically between 10 13 and 10 8 s. The energy barrier for 1808 (p) flips was indeed found to be relatively large, with estimates varying between 51.8 and 63 kJ mol . On the longer timescale of H NMR spectroscopy (> 10 7 s), the benzene rings in MOF-5 were found to be stationary at temperatures below 298 K, but p flips were observed at higher temperatures, and all benzene rings execute this motion at 373 K. More recently, an activation energy of 47.3 kJ mol 1 was obtained for the p-flip rate constant in MOF-5. In MIL-47 and MIL-53 frameworks, which have 1D pore systems, the dynamics of the benzene rings could more strongly influence the adsorption and transport properties compared to a MOF with 3D pore connectivity. For small molecules, 1D diffusion has been evidenced in MIL-47(V) and MIL-53(Cr) by QENS. Moreover, in molecular simulations of these two MILs, the framework was taken to be rigid, and no switching of molecules from one channel to another was observed. On a much longer timescale, which is relevant for macroscopic measurements, the rotational motion of benzene could play a role. We used solidstate H NMR to determine the flipping rate of benzene rings in MIL-47(V) and MIL-53(Cr) frameworks. An additional [*] D. I. Kolokolov, Dr. H. Jobic Universit Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de LYON 2. Av. A. Einstein, 69626 Villeurbanne (France) E-mail: herve.jobic@ircelyon.univ-lyon1.fr

Porous Metal–Organic Polyhedral Frameworks with Optimal Molecular Dynamics and Pore Geometry for Methane Storage

Natural gas (methane, CH4) is widely considered as a promising energy carrier for mobile applications. Maximizing the storage capacity is the primary goal for the design of future storage media. Here we report the CH4 storage properties in a family of isostructural (3,24)-connected porous materials, MFM-112a, MFM-115a, and MFM-132a, with different linker backbone functionalization. Both MFM-112a and MFM-115a show excellent CH4 uptakes of 236 and 256 cm3 (STP) cm–3 (v/v) at 80 bar and room temperature, respectively. Significantly, MFM-115a displays an exceptionally high deliverable CH4 capacity of 208 v/v between 5 and 80 bar at room temperature, making it among the best performing metal–organic frameworks for CH4 storage. We also synthesized the partially deuterated versions of the above materials and applied solid-state 2H NMR spectroscopy to show that these three frameworks contain molecular rotors that exhibit motion in fast, medium, and slow regimes, respectively. In situ neutron powder diffraction studies on the binding sites for CD4 within MFM-132a and MFM-115a reveal that the primary binding site is located within the small pocket enclosed by the [(Cu2)3(isophthalate)3] window and three anthracene/phenyl panels. The open Cu(II) sites are the secondary/tertiary adsorption sites in these structures. Thus, we obtained direct experimental evidence showing that a tight cavity can generate a stronger binding affinity to gas molecules than open metal sites. Solid-state 2H NMR spectroscopy and neutron diffraction studies reveal that it is the combination of optimal molecular dynamics, pore geometry and size, and favorable binding sites that leads to the exceptional and different methane uptakes in these materials.

Tailoring porosity and rotational dynamics in a series of octacarboxylate metal-organic frameworks

Significance A family of stable porous materials incorporating organic linkers and Cu(II) cations is reported. Their pores can be altered systematically by elongation of the ligands allowing a strategy of selective pore extension along one dimension. These materials show remarkable gas adsorption properties with high working capacities for CH4 (0.24 g g−1, 163 cm3 cm−3 at 298 K, 5–65 bar) for the most porous system. The mechanism of rotation of the organic groups in the solid state has been analyzed by NMR spectroscopy and rotational rates and transition temperatures analyzed. Significantly, we show that framework dynamics can be controlled by ligand design only, and this paves the way to understanding the role of molecular rotors within these materials. Modulation and precise control of porosity of metal-organic frameworks (MOFs) is of critical importance to their materials function. Here we report modulation of porosity for a series of isoreticular octacarboxylate MOFs, denoted MFM-180 to MFM-185, via a strategy of selective elongation of metal-organic cages. Owing to the high ligand connectivity, these MOFs do not show interpenetration, and are robust structures that have permanent porosity. Interestingly, activated MFM-185a shows a high Brunauer–Emmett–Teller (BET) surface area of 4,734 m2 g−1 for an octacarboxylate MOF. These MOFs show remarkable CH4 and CO2 adsorption properties, notably with simultaneously high gravimetric and volumetric deliverable CH4 capacities of 0.24 g g−1 and 163 vol/vol (298 K, 5–65 bar) recorded for MFM-185a due to selective elongation of tubular cages. The dynamics of molecular rotors in deuterated MFM-180a-d16 and MFM-181a-d16 were investigated by variable-temperature 2H solid-state NMR spectroscopy to reveal the reorientation mechanisms within these materials. Analysis of the flipping modes of the mobile phenyl groups, their rotational rates, and transition temperatures paves the way to controlling and understanding the role of molecular rotors through design of organic linkers within porous MOF materials.

Mobility of solid tert-butyl alcohol studied by deuterium NMR.

The molecular mobility of solid deuterated tert-butyl alcohol (TBA) has been studied over a broad temperature range (103–283 K) by means of solid-state 2H NMR spectroscopy, including both line shape and anisotropy of spin–lattice relaxation analyses. It has been found that, while the hydroxyl group of the TBA molecule is immobile on the 2H NMR time scale (τC > 10(–5) s), its butyl group is highly mobile. The mobility is represented by the rotation of the methyl [CD3] groups about their 3-fold axes (C3 rotational axis) and the rotation of the entire butyl [(CD3)3-C] fragment about its 3-fold axis (C3′ rotational axis). Numerical simulations of spectra line shapes reveal that the methyl groups and the butyl fragment exhibit three-site jump rotations about their symmetry axes C3 and C3′ in the temperature range of 103–133 K, with the activation energies and preexponential factors E1 = 21 ± 2 kJ/mol, k(01) = (2.6 ± 0.5) × 10(12) s(–1) and E2 = 16 ± 2 kJ/mol, k(02) = (1 ± 0.2) × 10(12) s(–1), respectively. Analysis of the anisotropy of spin–lattice relaxation has demonstrated that the reorientation mechanism of the butyl fragment changes to a free diffusion rotational mechanism above 173 K, while the rotational mechanism of the methyl groups remains the same. The values of the activation barriers for both rotations at T > 173 K have the values, which are similar to those at 103–133 K. This indicates that the interaction potential defining these motions remains unchanged. The obtained data demonstrate that the detailed analysis of both line shape and anisotropy of spin–lattice relaxation represents a powerful tool to follow the evolution of the molecular reorientation mechanisms in organic solids.

Characterization of Doubly Ionic Hydrogen Bonds in Protic Ionic Liquids by NMR Deuteron Quadrupole Coupling Constants: Differences to H-bonds in Amides, Peptides, and Proteins

We present the first deuteron quadrupole coupling constants (DQCCs) for selected protic ionic liquids (PILs) measured by solid-state NMR spectroscopy. The experimental data are supported by dispersion-corrected density functional theory (DFT-D3) calculations and molecular dynamics (MD) simulations. The DQCCs of the N-D bond in the triethylammonium cations are the lowest reported for deuterons in PILs, indicating strong hydrogen bonds between ions. The NMR coupling parameters are compared to those in amides, peptides, and proteins. The DQCCs show characteristic behavior with increasing interaction strength of the counterion and variation of the H-bond motifs. We report the similar presence of the quadrupolar splitting pattern and the narrow liquid line in the NMR spectra over large temperature ranges, indicating the heterogeneous nature of PILs.

CCDC 1499824: Experimental Crystal Structure Determination

Related Article: Yong Yan, Daniil I. Kolokolov, Ivan da Silva, Alexander G. Stepanov, Alexander J. Blake, Anne Dailly, Pascal Manuel, Chiu C. Tang, Sihai Yang, Martin Schroder|2017|J.Am.Chem.Soc.|139|13349|doi:10.1021/jacs.7b05453