Tina Düren

Using molecular simulation to characterise metal–organic frameworks for adsorption applications

Molecular simulation is a powerful tool to predict adsorption and to gain insight into the corresponding molecular level phenomena. In this tutorial review, we provide an overview of how molecular simulation can be used to characterise metal-organic frameworks for adsorption applications. Particular attention is drawn to how these insights can be combined to develop design principles for specific applications.

Opening the gate: framework flexibility in ZIF-8 explored by experiments and simulations.

ZIF-8 is a zeolitic imidazole-based metal-organic framework with large cavities interconnected by narrow windows. Because the small size of the windows, it allows in principle for molecular sieving of gases such as H(2) and CH(4). However, the unexpected adsorption of large molecules on ZIF-8 suggests the existence of structural flexibility. ZIF-8 flexibility is explored in this work combining different experimental techniques with molecular simulation. We show that the ZIF-8 structure is modified by gas adsorption uptake in the same way as it is at a very high pressure (i.e., 14,700 bar) due to a swing effect in the imidazolate linkers, giving access to the porosity. Tuning the flexibility, and so the opening of the small windows, has a further impact on the design of advanced molecular sieving membrane materials for gas separation, adjusting the access of fluids to the porous network.

Flexibility and swing effect on the adsorption of energy-related gases on ZIF-8: combined experimental and simulation study

ZIF-8, a prototypical zeolitic porous coordination polymer, prepared via the self-assembly of tetrahedral atoms (e.g. Zn and Co) and organic imidazolate linkers, presents large cavities which are interconnected by narrow windows that allow, in principle, molecular sieving. However, ZIF-8 shows flexibility due to the swing of the imidazolate linkers, which results in the adsorption of molecules which are too large to fit through the narrow window. In this work, we assess the impact of this flexibility, previously only observed for nitrogen, and the level of agreement between the experimental and simulated isotherms of different energy-related gases on ZIF-8 (CO(2), CH(4) and alkanes). We combine experimental gas adsorption with GCMC simulations, using generic and adjusted force fields and DFT calculations with the Grimme dispersion correction. By solely adapting the UFF force field to reduce the Lennard-Jones parameter ε, we achieve excellent agreement between the simulated and experimental results not only for ZIF-8 but also for ZIF-20, where the transferability of the adapted force field is successfully tested. Regarding ZIF-8, we show that two different structural configurations are needed to properly describe the adsorption performance of this material, demonstrating that ZIF-8 is undergoing a structural change during gas adsorption. DFT calculations with the Grimme dispersion correction are consistent with the GCMC and experimental observations, illustrating the thermodynamics of the CH(4) adsorption sites and confirming the existence of a new adsorption site with a high binding energy within the 4-ring window of ZIF-8.

Elucidating the Breathing of the Metal–Organic Framework MIL-53(Sc) with ab Initio Molecular Dynamics Simulations and in Situ X-ray Powder Diffraction Experiments

Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal-organic framework (MOF) MIL-53(Sc) in response to changes in temperature over the range 100-623 K and adsorption of CO2 at 0-0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory accurately simulated the experimentally observed closure of MIL-53(Sc) upon solvent removal and the transition of the empty MOF from the closed-pore phase to the very-narrow-pore phase (symmetry change from P2(1)/c to C2/c) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations to mimic the CO2 adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc) framework to evolve freely in response to CO2 loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO2-containing intermediate and large-pore phases observed by experimental synchrotron X-ray diffraction studies with increasing CO2 pressure; this would not have been possible for the intermediate structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the intermediate structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53(Sc) upon temperature variation and CO2 adsorption. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids.

p-Xylene-Selective Metal-Organic Frameworks: A Case of Topology-Directed Selectivity

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(6)]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.

Molecular modelling of adsorption in novel nanoporous metal–organic materials

Grand canonical Monte Carlo and molecular dynamics simulations have been performed for methane, n-alkanes, cyclohexane and benzene in two novel nanoporous metal–organic materials. The first material, bipyridine molecular squares, consists of discrete square molecules with corners formed by rhenium complexes and edges formed by bipyridine links, giving a small cavity within each square. The material is considered in both its crystalline form and an amorphous packing of squares. The second material is IRMOF-1, a periodic, crystalline structure also with metal corners and organic bridging units. Adsorption isotherms and self-diffusion coefficients are reported and provide insight into molecular interactions in these materials.

Structural Chemistry, Monoclinic-to-Orthorhombic Phase Transition, and CO2 Adsorption Behavior of the Small Pore Scandium Terephthalate, Sc2(O2CC6H4CO2)3, and Its Nitro- And Amino-Functionalized Derivatives

The crystal structure of the small pore scandium terephthalate Sc(2)(O(2)CC(6)H(4)CO(2))(3) (hereafter Sc(2)BDC(3), BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO(2) has been examined. The structure of Sc(2)BDC(3) has been followed in vacuo over the temperature range 140 to 523 K by high resolution synchrotron X-ray powder diffraction, revealing a phase change at 225 K from monoclinic C2/c (low temperature) to Fddd (high temperature). The orthorhombic form shows negative thermal expansivity of 2.4 × 10(-5) K(-1): Rietveld analysis shows that this results largely from a decrease in the c axis, which is caused by carboxylate group rotation. (2)H wide-line and MAS NMR of deuterated Sc(2)BDC(3) indicates reorientation of phenyl groups via π flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH(2) and -NO(2) groups. The structure of Sc(2)(NH(2)-BDC)(3) has been determined by Rietveld analysis of synchrotron powder diffraction at 100 and 298 K and found to be orthorhombic at both temperatures, whereas the structure of Sc(2)(NO(2)-BDC)(3) has been determined by single crystal diffraction at 298 K and Rietveld analysis of synchrotron powder diffraction at 100, 298, 373, and 473 K and is found to be monoclinic at all temperatures. Partial ordering of functional groups is observed in each structure. CO(2) adsorption at 196 and 273 K indicates that whereas Sc(2)BDC(3) has the largest capacity, Sc(2)(NH(2)-BDC)(3) shows the highest uptake at low partial pressure because of strong -NH(2)···CO(2) interactions. Remarkably, Sc(2)(NO(2)-BDC)(3) adsorbs 2.6 mmol CO(2) g(-1) at 196 K (P/P(0) = 0.5), suggesting that the -NO(2) groups are able to rotate to allow CO(2) molecules to diffuse along the narrow channels.

Unusual adsorption behavior on metal-organic frameworks.

Metal-organic frameworks (MOFs) have shown adsorption behavior that is not observed in other microporous materials such as zeolites or activated carbons. This study used grand canonical Monte Carlo simulation to evaluate a particular form of behavior, which corresponds to the presence of unusual type V adsorption isotherms. Study of a series of MOFs in the IRMOF family, containing chemically similar linkers of different length, showed that the presence of type V adsorption depends on a fine balance between the strength of the fluid-fluid and fluid-solid interactions, which in turn is a strong function of the length of the linker and therefore the pore size. A transition from type V behavior to the more common type I behavior is observed as the temperature increases. The temperature at which this transition occurs increases, and the transition becomes more diffuse, as the length of the linker increases. This type V behavior leads to an interesting possibility in the design of MOF adsorbents for use in gas separation and gas storage applications.

A Computational and Experimental Approach Linking Disorder, High-Pressure Behavior, and Mechanical Properties in UiO Frameworks

Abstract Whilst many metal–organic frameworks possess the chemical stability needed to be used as functional materials, they often lack the physical strength required for industrial applications. Herein, we have investigated the mechanical properties of two UiO‐topology Zr‐MOFs, the planar UiO‐67 ([Zr6O4(OH)4(bpdc)6], bpdc: 4,4′‐biphenyl dicarboxylate) and UiO‐abdc ([Zr6O4(OH)4(abdc)6], abdc: 4,4′‐azobenzene dicarboxylate) by single‐crystal nanoindentation, high‐pressure X‐ray diffraction, density functional theory calculations, and first‐principles molecular dynamics. On increasing pressure, both UiO‐67 and UiO‐abdc were found to be incompressible when filled with methanol molecules within a diamond anvil cell. Stabilization in both cases is attributed to dynamical linker disorder. The diazo‐linker of UiO‐abdc possesses local site disorder, which, in conjunction with its longer nature, also decreases the capacity of the framework to compress and stabilizes it against direct compression, compared to UiO‐67, characterized by a large elastic modulus. The use of non‐linear linkers in the synthesis of UiO‐MOFs therefore creates MOFs that have more rigid mechanical properties over a larger pressure range.

Stabilization of Scandium Terephthalate MOFs against Reversible Amorphization and Structural Phase Transition by Guest Uptake at Extreme Pressure

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally nonporous materials. Here we report a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC = 1,4-benzenedicarboxylate) and its nitro-functionalized derivative (Sc2(NO2-BDC)3) and compare it to direct compression behavior in a nonpenetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations, and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against nonpenetrative hydrostatic fluids at high pressure, this study demonstrates a novel high-pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favorable adsorption sites.