Hervé Jobic

Why hybrid porous solids capture greenhouse gases

Hybrid porous solids, with their tunable structures, their multifunctional properties and their numerous applications, are currently topical, particularly in the domain of adsorption and storage of greenhouse gases. Most of the data reported so far concern the performances of these solids in this domain, particularly in terms of adsorbed amounts of gas but do not explain at the atomic level why and how adsorption and storage occur. From a combination of structural, spectroscopic, thermodynamic experiments and of molecular simulations, this tutorial review proposes answers to these open questions with a special emphasis on CO(2) and CH(4) storage by some rigid and flexible hybrid porous materials.

Vibrational Spectroscopy with Neutrons

Several neutron techniques are being used to study catalytic systems [I]. We will limit ourselves here to inelastic neutron scattering (INS) which is a method of great potential interest to study vibrational modes of catalysts and of adsorbed molecules. INS is one of the numerous vibrational techniques available for a better understanding of surface phenomena. Each technique has its particular advantages for a given system in terms of spectral domain, resolution, sensitivity and experimental conditions. For example infrared spectroscopy is a very efficient method to detect adsorbed CO (see the contribution by Mauge et al.) but it is much less sensitive to adsorbed hydrogen.

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

Self and Transport Diffusivity of CO2 in the Metal-Organic Framework MIL-47(V) Explored by Quasi-elastic Neutron Scattering Experiments and Molecular Dynamics Simulations

Quasi-elastic neutron scattering measurements are combined with molecular dynamics simulations to determine the self-diffusivity, corrected diffusivity, and transport diffusivity of CO(2) in the metal-organic framework MIL-47(V) (MIL = Materials Institut Lavoisier) over a wide range of loading. The force field used for describing the host/guest interactions is first validated on the thermodynamics of the MIL-47(V)/CO(2) system, prior to being transferred to the investigations of the dynamics. A decreasing profile is then deduced for D(s) and D(o) whereas D(t) presents a non monotonous evolution with a slight decrease at low loading followed by a sharp increase at higher loading. Such decrease of D(t) which has never been evidenced in any microporous systems comes from the atypical evolution of the thermodynamic correction factor that reaches values below 1 at low loading. This implies that, due to intermolecular interactions, the CO(2) molecules in MIL-47(V) do not behave like an ideal gas. Further, molecular simulations enabled us to elucidate unambiguously a 3D diffusion mechanism within the pores of MIL-47(V).

Diffusion of linear and branched alkanes in ZSM-5. A quasi-elastic neutron scattering study

Abstract The diffusivities of long n-alkanes in ZSM-5 have been measured by quasi-elastic neutron scattering (QENS). Self-diffusion coefficients and activation energies for chains up to C14 have been obtained. Reasonable agreement is observed with single-crystal membrane experiments, except for C6, but large discrepancies are found with the zero-length column (ZLC) method. The diffusivities predicted by a hierarchical simulation diverge from those measured by QENS after C6, although the activation energies for diffusion are comparable. The discrepancy with MD simulations is larger. QENS results show that diffusion of branched alkanes is much more restricted than the one of linear alkanes in this structure.

Comparative QENS and PFG NMR diffusion studies of water in zeolite NaCaA

Quasi-elastic neutron scattering and pulsed field gradient NMR, in combination with temperature-programmed desorption, were applied to study water diffusion in NaCaA samples obtained by exchange from a batch of NaA zeolite of commercial origin. Both techniques exhibit similar trends, viz. increasing water diffusivities with increasing loading (at least up to medium pore filling factors) and with decreasing calcium content. There are, however, notable differences in both the absolute values of the diffusivities and their temperature dependencies. These differences are explained by the real structure of the zeolite samples and the different sensitivity of the measuring techniques to the deviations from ideal crystallinity.

Dynamics of ethane and propane in zeolite ZSM-5 studied by quasi-elastic neutron scattering

Abstract Quasi-elastic neutron scattering has been used to study the dynamics of ethane and propane in zeolite ZSM-5. The experiments were performed at different loadings for the two alkanes and at different temperatures for propane. The long-range translational motion of the molecules has been observed and has been interpreted with a jump diffusion model, the mean jump lengths being of the order of 10 A, slightly decreasing with increasing loading. The self-diffusion coefficients of ethane are about 2 × 10 −9 m 2 s −1 at 300 K, those of propane at the same temperature being smaller by a factor of two. An activation energy of 5 kJ mol −1 is obtained for the self-diffusion of propane. These results are in good agreement with the pulsed-field gradient n.m.r. measurements. In addition to the translational motion, a rotational motion of the two molecules is also observed; it is described by an uniaxial rotational diffusion model and rotational diffusion constants are derived.

Intracrystalline Transport Resistances in Nanoporous Zeolite X

By applying pulsed-field gradient nuclear magnetic resonance (PFG NMR) in comparison to quasi-elastic neutron scattering (QENS), we sense by measurement of the diffusion of n-octane on different length scales, transport resistances in faujasite type X (which is isostructural with type Y and differs by the lower Si/Al ratio only) with mutual distances of less than 300 nm. Direct observation of the real structure of zeolite X by transmission electron microscopy identifies them as stacking faults of mirror-twin type on (111)-type planes of the cubic framework. Thus, direct experimental proof is given that, in general, nanoporous host systems such as zeolite crystals cannot be considered as a mere arrangement of cavities. It is rather the presence of structural defects that dominates their properties as soon as transport phenomena with practical relevance, including chemical conversion by heterogeneous catalysis and chemical separation by molecular sieving and selective adsorption, become relevant.

Translational and rotational dynamics of methane in ZSM-5 zeolite: A quasi-elastic neutron scattering study

Abstract The translational and rotational motions of methane adsorbed at different loadings in NaZSM-5 have been studied by quasi-elastic neutron scattering at two temperatures: 200 and 250 K. The translational motion does not simply follow Ficks law, but a jump diffusion model with a Gaussian distribution of jump lengths satisfactorily simulates the experimental results. The diffusion coefficient that is obtained for long-range translational motion does not vary much on the loading in the range that was studied: it is of ⋍ 2.7 × 10 −5 cm 2 s −1 at 200 K and ⋍ 5.5 × 10 −5 cm 2 s −1 at 250 K. Good agreement is found between the neutron and n.m.r. results for this motion, but large discrepancies are observed with the macroscopic measurements. The rotational motion is well described by an isotropic rotational diffusion model, and this motion is found to be much slower in the zeolite than in physisorbed layers or in bulk solid methane.