Die Rotationsdynamiken von Benzolringen im flexiblen MIL‐53‐ und im starren MIL‐47‐Gerüst wurden durch 2H‐NMR‐Spektroskopie untersucht. In beiden Fällen werden 180°‐Flips beobachtet, wobei der Flip‐Prozess in MIL‐53 schneller ist. Zur Simulation der experimentellen Spektren (links) reicht die Berücksichtigung der quadrupolaren Wechselwirkung nicht aus (Mitte): Auch der paramagnetische Effekt muss einbezogen werden (rechts).

Owing to their inherent pore structure, porous metal−organic frameworks (MOFs) can undergo postsynthetic modification, such as loading extra-framework proton carriers. However, strategies for improving the proton conductivity for nonporous MOFs are largely lacking, although increasing numbers of nonporous MOFs exhibit promising proton conductivities. Often, high humidity is required for nonporous MOFs to achieve high conductivities, but to date no clear mechanisms have been experimentally identified. Here we describe the new materials MFM-550(M), [M(HL 1 )], (H 4 L 1 = biphenyl-4,4′-diphosphonic acid; M = La, Ce, Nd, Sm, Gd, Ho), MFM-550(Ba), [Ba(H 2 L 1 )], and MFM-555(M), [M(HL 2 )], (H 4 L 2 = benzene-1,4-diphosphonic acid; M = La, Ce, Nd, Sm, Gd, Ho), and report enhanced proton conductivities in these nonporous materials by (i) replacing the metal ion to one with a lower oxidation state, (ii) reducing the length of the organic ligand, and (iii) introducing additional acidic protons on the MOF surface. Increased framework proton density in these materials can lead to an enhancement in proton conductivity of up to 4 orders of magnitude. Additionally, we report a comprehensive investigation using in situ 2 H NMR and neutron spectroscopy, coupled with molecular dynamic modeling, to elucidate the role of humidity in assembling interconnected networks for proton hopping. This study constructs a relationship between framework proton density and the corresponding proton conductivity in nonporous MOFs, and directly explains the role of both surface protons and external water in assembling the proton conduction pathways.

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 À 2 S cm À 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.

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 2 H 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 × 10 6 Hz cf. 10 11 −10 13 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

is an important example of metal−organic frameworks (MOFs) with a flexible framework capable to efficiently separate ortho and para isomers of xylene at moderate temperatures. The MIL-53 MOF contains mobile terephthalate phenylene fragments that can be used as a dynamic probe to investigate the guest−host interactions and the origin of the separation selectivity. Here 2 H NMR spin alignment echo technique for the first time was applied to probe ultraslow structural mobility (0.1−1 kHz) in MOFs materials, with particular application to MIL-53(Al) saturated with ortho or para isomers of xylene. A specific influence of different isomers of xylene adsorbed in the MOF pores on the rotation of the phenylenes in MIL-53 for the temperature range with proved separation selectivity (T < 393 K) is shown. It has been established that the rotation of phenylene fragments is sensitive to the type of xylene isomer. The phenylenes' rotation performs 1 order of magnitude slower in the presence of o-xylene (k ortho = 70 s −1 ) compared to the same rotation in the presence of the other isomer, p-xylene (k para = 800 s −1 ) at T ∼ 373 K. This is rationalized by a stronger interaction of the ortho isomer with the linker than the para isomer. This finding offers an understanding of the molecular mechanism of p-and o-xylene separation by MIL-53: stronger interaction of o-xylene with organic linker compared to p-xylene provides separation of these isomers on MIL-53.

UiO-66 (Zr) is a thermally and chemically stable Zr-based metal–organic framework (MOF) that has proved to be promising for different hydrocarbon separation applications including membrane-based processes. Here, we present a combined experimental (2H NMR) and computational (molecular dynamics simulation) study of the molecular mobility and transport of n-butane and isobutane in the dehydroxylated UiO-66 (Zr) MOF. We show that the microscopic diffusivity D n for n-butane is significantly higher than D iso for isobutane, D n/D iso = 1000 at 300 K with a large difference in diffusion barriers, E n = 25 kJ mol–1 for n-butane and E iso = 50 kJ mol–1 for isobutane. High selectivity for kinetic-driven separation of the n-butane/isobutane mixture in UiO-66 (Zr)-based materials or membranes is expected based on this result.

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.

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