Flexible Metal Organic Framework MIL-53 Research Articles

Facile shaping of flexible MIL-53(Al) for effective separation of propylene over propane

A facile granulation approach using polyvinyl butyral (PVB) as a polymer binder was developed to control the textural properties of the flexible metal–organic framework MIL-53(Al). The large pore (LP) structure of MIL-53(Al) contracted after granulation due to a strong interaction between MOF and polymer binder. This contraction enhances propylene's selectivity over propane at saturation pressure (region II), as confirmed by single-component adsorption isotherms. The adsorption kinetics showed faster adsorption of C3H6 compared to C3H8 over time in the case of the granules, while no difference was observed for the powder sample. Breakthrough experiments successfully separated C3H6 over C3H8 using MIL-53(Al)-B10% granules, whereas the pristine MIL-53(Al) powder did not separate the C3H8-C3H6 gas mixture. The breakthrough finding reaffirms that the observed gate-opening effect in MIL-53(Al) within pressure region I is ineffective for separating a mixed gas system containing propane and propylene. The constrained pores of the shaped granules could facilitate strong π-π stacking interactions with C3H6 and decrease the diffusion rate for C3H8 molecules. These findings indicate that the simple granulation approach can modulate the textural properties of flexible MIL-53(Al) and improve its selectivity for C3H6/C3H8 separation resulting from increased MOF-propylene affinity and kinetic factors.

Manipulated adsorption of C8 aromatics in MIL-53(Cr) through pre-adsorbing water molecules

The flexible metal-organic framework MIL-53(Cr) has been demonstrated to be an efficient adsorbent for liquid phase adsorption and separation, while the effect of this attractive “breathing effect” on its performance remains uncertain. In this research, by increasing the amount of introduced water, MIL-53(Cr) first shrank into a narrow pore form through bound water and then reopened due to the introduction of free water. Using MIL-53(Cr) with diverse amounts of water as an adsorbent, the liquid-phase adsorption and separation of C8 aromatics, including o-xylene (OX), p-xylene (PX), m-xylene (MX) and ethylbenzene (EB), was manipulated by the pre-adsorbed water in MIL-53(Cr). Pre-adsorbing water in MIL-53(Cr) induced a preference reversal between MX and PX, where dry samples preferentially adsorbed MX, while high-water-addition samples preferred PX instead. In addition, the reopened MIL-53(Cr) with high water addition hardly adsorbed EB, thereby increasing the selectivity of xylene isomers to EB. A longer equilibrium time was observed on MIL-53(Cr) with high water addition, during which the adsorption preference shifted from OX to MX to PX. Further molecular simulation provided a microscopic explanation that water molecules in the pores tended to package around MX and EB, hindering their interaction with frameworks or other adsorbates.

Elucidating the Vibrational Fingerprint of the Flexible Metal-Organic Framework MIL-53(Al) Using a Combined Experimental/Computational Approach.

In this work, mid-infrared (mid-IR), far-IR, and Raman spectra are presented for the distinct (meta)stable phases of the flexible metal–organic framework MIL-53(Al). Static density functional theory (DFT) simulations are performed, allowing for the identification of all IR-active modes, which is unprecedented in the low-frequency region. A unique vibrational fingerprint is revealed, resulting from aluminum-oxide backbone stretching modes, which can be used to clearly distinguish the IR spectra of the closed- and large-pore phases. Furthermore, molecular dynamics simulations based on a DFT description of the potential energy surface enable determination of the theoretical Raman spectrum of the closed- and large-pore phases for the first time. An excellent correspondence between theory and experiment is observed. Both the low-frequency IR and Raman spectra show major differences in vibrational modes between the closed- and large-pore phases, indicating changes in lattice dynamics between the two structures. In addition, several collective modes related to the breathing mechanism in MIL-53(Al) are identified. In particular, we rationalize the importance of the trampoline-like motion of the linker for the phase transition.

In Search of the Reason for the Breathing Effect of MIL53 Metal-Organic Framework: An ab Initio Multiconfigurational Study.

Multiconfigurational methods are applied to study electronic properties and structural changes in the highly flexible metal-organic framework MIL53(Cr). Via calculated bending potentials of angles, that change the most during phase transition, it is verified that the high flexibility of this material is not a question about special electronic properties in the coordination chemistry, but about overall linking of the framework. The complex posseses a demanding electronic structure with delocalized spin density, antifferomagnetic coupling and high multi-state character requiring multiconfigurational methods. Calculated properties are in good agreement with known experimental values confirming our chosen methods.

Diffusion of Light Hydrocarbons in the Flexible MIL-53(Cr) Metal–Organic Framework: A Combination of Quasi-Elastic Neutron Scattering Experiments and Molecular Dynamics Simulations

The dynamics of light hydrocarbons, including ethane, propane, and n-butane, is explored in the highly flexible metal–organic framework MIL-53(Cr) by combining quasi-elastic neutron scattering measurements and molecular dynamics simulations. The loading dependence of the self-diffusivity shows peculiarities, including (i) a relatively rapid decrease of Ds at low loading for ethane and propane and (ii) an unusual increase of Ds for n-butane at high loading, following a decreasing profile up to intermediate loading. These diffusion behaviors are analyzed in light of the structural flexibility of the solids upon alkane adsorption characterized by the neutron measurements. A 1D-type diffusion is evidenced for all alkanes with a jump sequence mainly ruled by the hydroxyl groups present at the surface of the MOF pore wall. This global translational motion is associated with a rotational dynamics that differs according to the nature of the alkane: whereas n-butane follows uniaxial displacements, ethane shows ran...

Interaction of methanol with the flexible metal-organic framework MIL-53(Fe) observed by inelastic neutron scattering

In situ powder X-ray diffraction and thermogravimetry show that the iron(III) carboxylate material MIL-53(Fe) expands in two steps upon exposure to methanol vapour to give a ‘half open’ phase and then a fully expanded structure; these correspond to the uptake of one and two molar equivalents of methanol, respectively. This knowledge allows interpretation of inelastic neutron scattering (INS) spectra, measured using TOSCA at ISIS, of the solid loaded with various amounts of water, methanol or d3-methanol (CD3OH), aided also by DFT simulation of INS spectra of the analogous MIL-53(Al) structure. The key signature of the expansion and contraction of the structure is the shift of bands in the 400cm−1 region: these are assigned as due to the librations of the terephthalate rings, involving motions that result in distortion of the aromatic ring and a rocking motion of the aromatic ring about the bonds to the carboxylate groups.

Adsorption of N/S heterocycles in the flexible metal–organic framework MIL-53(FeIII) studied by in situ energy dispersive X-ray diffraction

The adsorption of N/S-containing heterocyclic organic molecules in the flexible iron(III) terephthalate MIL-53, Fe(III)(OH)(0.6)F(0.4)(O2C-C6H4-CO2)·(H2O), from the liquid phase was studied with in situ energy dispersive X-ray diffraction (EDXRD), in order to follow the adsorption-induced expansion of the structure. For comparison with the diffraction data, liquid phase adsorption isotherms were recorded for uptake of benzothiophene, benzothiazole and indole in isopropanol and in heptane. The solvent not only influences pore opening but is also a competing guest. The in situ EDXRD experiments allow the kinetics of guest uptake and the competition with solvent to be monitored directly. Indole uptake is limited; this adsorbate is barely capable of opening the closed, either hydrated or dehydrated, MIL-53(Fe) structure, or of penetrating the isopropanol-containing material in the concentration range under study. When isopropanol is used as a solvent, the guest molecules benzothiophene and benzothiazole must be present at a certain threshold concentration before substantial adsorption into the metal-organic framework takes place, eventually resulting in full opening of the structure. The fully expanded structures of benzothiophene or benzothiazole loaded MIL-53(Fe) materials have Imcm symmetry and a unit cell volume of ca. 1600 Å(3), and upon uptake of the guest molecules by the closed form (unit cell volume ~1000 Å(3)) no intermediate crystalline phases are seen. Successful uptake by MIL-53(Fe) requires that the adsorbate is primarily a good hydrogen bond acceptor; additionally, based on UV-visible spectroscopy, a charge-transfer interaction between the S atoms of benzothiophene and the aromatic rings in the MOF pore wall is proposed.

Ab Initio Parametrized Force Field for the Flexible Metal-Organic Framework MIL-53(Al).

A force field is proposed for the flexible metal-organic framework MIL-53(Al), which is calibrated using density functional theory calculations on nonperiodic clusters. The force field has three main contributions: an electrostatic term based on atomic charges derived with a modified Hirshfeld-I method, a van der Waals (vdW) term with parameters taken from the MM3 model, and a valence force field whose parameters were estimated with a new methodology that uses the gradients and Hessian matrix elements retrieved from nonperiodic cluster calculations. The new force field predicts geometries and cell parameters that compare well with the experimental values both for the large and narrow pore phases. The energy profile along the breathing mode of the empty material reveals the existence of two minima, which confirms the intrinsic bistable behavior of the MIL-53. Even without the stimulus of external guest molecules, the material may transform from the large pore (lp) to the narrow pore (np) phase [Liu et al. J. Am. Chem. Soc.2008, 120, 11813]. The relative stability of the two phases critically depends on the vdW parameters, and the MM3 dispersion interaction has the tendency to overstabilize the np phase.

Comparative Guest, Thermal, and Mechanical Breathing of the Porous Metal Organic Framework MIL-53(Cr): A Computational Exploration Supported by Experiments

The breathing of the flexible metal organic framework MIL-53(Cr) has been widely explored by both experimental and modeling approaches upon the inclusion of different guest molecules within its porosity. This spectacular phenomenon has been only partially tackled by force field based simulations mainly due to the complexity of deriving a set of accurate potential parameters able to capture the associated structural transition implying a unit cell volume change up to 40%. Here, a new parametrization of a flexible force field for the MIL-53(Cr) framework is realized from an iterative procedure starting with the experimental structural data collected in the presence of CO2 as the guest molecule. Hybrid osmotic Monte Carlo simulations based on this refined force field are then successfully conducted to reproduce for the first time the complex shape of the CO2 adsorption isotherm in the whole range of pressures. The structural behavior of the MIL-53(Cr) under a wide range of applied temperature and pressure is...

Rapid and reversible formation of a crystalline hydrate of a metal–organic framework containing a tube of hydrogen-bonded water

The flexible metal-organic framework MIL-53(Cr) undergoes a dramatic volume expansion upon immersion in water at room temperature to form a crystalline hydrate in which water is held as a hydrogen-bonded tube: the hydration is readily reversible under ambient conditions as shown by time-resolved powder X-ray diffraction.

Explanation of the Adsorption of Polar Vapors in the Highly Flexible Metal Organic Framework MIL-53(Cr)

A comparison of the adsorption of water, methanol, and ethanol polar vapors by the flexible porous chromium(III) terephthalate MIL-53(Cr) was investigated by complementary techniques including adsorption gravimetry, ex situ X-ray powder diffraction, microcalorimetry, thermal analysis, IR spectroscopy, and molecular modeling. The breathing steps observed during adsorption strongly depend on the nature of the vapor. With water, a significant contraction of the framework is observed. For the alcohols, the initial contraction is followed by an expansion of the framework. A combination of IR analysis, X-ray diffraction, and computer modeling leads to the molecular localization of the guest molecules and to the identification of the specific guest-guest and host-guest interactions. The enthalpies of adsorption, measured by microcalorimetry, show that the strength of the interactions decreases from ethanol to water. Differential scanning calorimetry experiments on an EtOH/H(2)O mixture suggest a selective adsorption of ethanol over water.

Prediction of the Conditions for Breathing of Metal Organic Framework Materials Using a Combination of X-ray Powder Diffraction, Microcalorimetry, and Molecular Simulation

The adsorption of C1 to C4 linear hydrocarbons in the flexible metal organic framework MIL-53(Cr) has been followed by adsorption manometry coupled with microcalorimetry and Synchrotron X-ray powder diffraction. This experimental investigation was completed by molecular modeling. In the case of methane, the solid remains rigid whatever the adsorbate amount. However for the C2-C4 series, an increasing flexibility of the structure is observed, which is ascribed first to a breathing of the material from a large pore to a narrow pore form followed by a further expansion at high pressure. The collected thermodynamic and structural information suggests that a minimum adsorption enthalpy of ca. 20 kJ mol (-1) in the initial large pore structure of MIL-53(Cr) is required to induce the structural transition "large to narrow pore". Further, the enthalpy of adsorption can be used to predict the pressure at which the structure reopens. Finally, the magnitude of the breathing can be related to the size of the probe molecule via the van der Waals volume. The above trends have been successfully verified in the case of water and carbon dioxide. This combined experimental and theoretical approach gives the first elements for the prediction of whether or not the MIL53 and similar flexible structures will respond to gas loading and what would be the pressure required and further the amplitude of the induced breathing.