Tuning the High-Pressure Phase Behaviour of Highly Compressible Zeolitic Imidazolate Frameworks: From Discontinuous to Continuous Pore Closure by Linker Substitution.

The high-pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF-62 family with the chemical composition M(im)2-x (bim)x is presented (M2+ =Zn2+ , Co2+ ; im- =imidazolate; bim- =benzimidazolate, 0.02≤x≤0.37). High-pressure powder X-ray diffraction shows that the materials contract reversibly from an open pore (op) to a closed pore (cp) phase under a hydrostatic pressure of up to 4000 bar. Sequentially increasing the bim- fraction (x) reinforces the framework, leading to an increased threshold pressure for the op-to-cp phase transition, while the total volume contraction across the transition decreases. Most importantly, the typical discontinuous op-to-cp transition (first order) changes to an unusual continuous transition (second order) for x≥0.35. This allows finetuning of the void volume and the pore size of the material continuously by adjusting the pressure, thus opening new possibilities for MOFs in pressure-switchable devices, membranes, and actuators.

Configurational Entropy Driven High-Pressure Behaviour of a Flexible Metal-Organic Framework (MOF).

Flexible metal-organic frameworks (MOFs) show large structural flexibility as a function of temperature or (gas)pressure variation, a fascinating property of high technological and scientific relevance. The targeted design of flexible MOFs demands control over the macroscopic thermodynamics as determined by microscopic chemical interactions and remains an open challenge. Herein we apply high-pressure powder X-ray diffraction and molecular dynamics simulations to gain insight into the microscopic chemical factors that determine the high-pressure macroscopic thermodynamics of two flexible pillared-layer MOFs. For the first time we identify configurational entropy that originates from side-chain modifications of the linker as the key factor determining the thermodynamics in a flexible MOF. The study shows that configurational entropy is an important yet largely overlooked parameter, providing an intriguing perspective of how to chemically access the underlying free energy landscape in MOFs.

Coordinated Water as New Binding Sites for the Separation of Light Hydrocarbons in Metal-Organic Frameworks with Open Metal Sites.

Metal-organic frameworks with open metal sites are promising materials for gas separations. Particularly, the M2(dobdc) (dobdc4- = 2,5-dioxidobenzenedicarboxylate, M2+ = Co2+, Mn2+, Fe2+, ...) framework has been the Drosophila of this research field and has delivered groundbreaking results in terms of sorption selectivity. However, many studies focus on perfect two-component mixtures and use theoretical models, e.g., the ideal adsorbed solution theory, to calculate selectivities. Within this work, we shed light on the comparability of these selectivities with values obtained from propane/propene multicomponent measurements on the prototypical Co2(dobdc) framework, and we study the impact of impurities like water on the selectivity. Despite the expected capacity loss, the presence of water does not necessarily lead to a decreased selectivity. Density functional theory calculations of the binding energies prove that the water molecules adsorbed to the metal centers introduce new binding sites for the adsorbates.

Increasing Alkyl Chain Length in a Series of Layered Metal-Organic Frameworks Aids Ultrasonic Exfoliation to Form Nanosheets.

Metal-organic framework nanosheets (MONs) are attracting increasing attention as a diverse class of two-dimensional materials derived from metal-organic frameworks (MOFs). The principles behind the design of layered MOFs that can readily be exfoliated to form nanosheets, however, remain poorly understood. Here we systematically investigate an isoreticular series of layered MOFs functionalized with alkoxy substituents in order to understand the effect of substituent alkyl chain length on the structure and properties of the resulting nanosheets. A series of 2,5-alkoxybenzene-1,4-dicarboxylate ligands (O2CC6H2(OR)2CO2, R = methyl-pentyl, 1-5, respectively) was used to synthesize copper paddle-wheel MOFs. Rietveld and Pawley fitting of powder diffraction patterns for compounds Cu(3-5)(DMF) showed they adopt an isoreticular series with two-dimensional connectivity in which the interlayer distance increases from 8.68 Å (R = propyl) to 10.03 Å (R = pentyl). Adsorption of CO2 by the MOFs was found to increase from 27.2 to 40.2 cm3 g-1 with increasing chain length, which we attribute to the increasing accessible volume associated with increasing unit-cell volume. Ultrasound was used to exfoliate the layered MOFs to form MONs, with shorter alkyl chains resulting in higher concentrations of exfoliated material in suspension. The average height of MONs was investigated by AFM and found to decrease from 35 ± 26 to 20 ± 12 nm with increasing chain length, with the thinnest MONs observed being only 5 nm, corresponding to five framework layers. These results indicate that careful choice of ligand functionalities can be used to tune nanosheet structure and properties, enabling optimization for a variety of applications.

Flexibility control in alkyl ether-functionalized pillared-layered MOFs by a Cu/Zn mixed metal approach.

Flexible metal-organic frameworks (MOFs) exhibit large potential as next-generation materials in areas such as gas sensing, gas separation and mechanical damping. By using a mixed metal approach, we report how the stimuli reponsive phase transition of flexible pillared-layered MOFs can be tuned over a wide range. Different Cu2+ to Zn2+ metal ratios are incorporated into the materials by using a simple solvothermal approach. The properties of the obtained materials are probed by differential scanning calorimetry and CO2 sorption measurements, revealing stimuli responsive behaviour as a function of metal ratio. Pair distribution functions derived from X-ray total scattering experiments suggest a distortion of the M2 paddlewheel as a function of the Cu content. We rationalize these phenomena by the different distortion energies of Cu2+ and Zn2+ ions to deviate from the square pyramidal structure of the relaxed paddlewheel node. Our work follows on from the large interest in tuning and understanding the materials properties of flexible MOFs, highlighting the large number of parameters that can be used for the targeted manipulation and design of properties of these fascinating materials.

Tuning the Mechanical Response of Metal-Organic Frameworks by Defect Engineering.

The incorporation of defects into crystalline materials provides an important tool to fine-tune properties throughout various fields of materials science. We performed high-pressure powder X-ray diffraction experiments, varying pressures from ambient to 0.4 GPa in 0.025 GPa increments to probe the response of defective UiO-66 to hydrostatic pressure for the first time. We observe an onset of amorphization in defective UiO-66 samples around 0.2 GPa and decreasing bulk modulus as a function of defects. Intriguingly, the observed bulk moduli of defective UiO-66(Zr) samples do not correlate with defect concentration, highlighting the complexity of how defects are spatially incorporated into the framework. Our results demonstrate the large impact of point defects on the structural stability of metal-organic frameworks (MOFs) and pave the way for experiment-guided computational studies on defect engineered MOFs.