About 700 scientists from 45 countries gathered in Dresden for the first time since the start of the COVID-19 pandemic to share their latest findings on metal-organic frameworks and open frameworks compounds.
Mercury porosimetry and in situ high pressure single crystal X-ray diffraction revealed the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction. The near-absence of hysteresis upon cycling exhibited by this robust MOF, akin to an ideal molecular spring, is associated with a constant work energy storage capacity of 40 J/gr. Molecular simulations were further deployed to uncover the free-energy landscape behind this unprecedented pressure-responsive phenomenon in the area of compliant hybrid porous materials. This discovery is of utmost importance from the perspective of instant energy storage and delivery.
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In theory, bimetallic UiO-66(Zr:Ce) and UiO-66(Zr:Hf) metal-organic frameworks (MOFs) are extremely versatile and attractive nanoporous materials as they combine the high catalytic activity of UiO-66(Ce) or UiO-66(Hf) with the outstanding stability of UiO-66(Zr). Using in situ high-pressure powder X-ray diffraction, however, we observe that this expected mechanical stability is not achieved when incorporating cerium or hafnium in UiO-66(Zr). This observation is akin to the earlier observed reduced thermal stability of UiO-66(Zr:Ce) compounds. To elucidate the atomic origin of this phenomenon, we chart the loss-of-crystallinity pressures of 22 monometallic and bimetallic UiO-66 materials and systematically isolate their intrinsic mechanical stability from their defect-induced weakening. This complementary experimental/computational approach reveals that the intrinsic mechanical stability of these bimetallic MOFs decreases nonlinearly upon cerium incorporation but remains unaffected by the zirconium:hafnium ratio. Additionally, all experimental samples suffer from defect-induced weakening, a synthesis-controlled effect that is observed to be independent of their intrinsic stability.
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The coadsorption of CO2/CH4 in the breathing MIL-53(Cr) under the application of an additional mechanical pressure is investigated through the use of an extended thermodynamic mean-field model. The focus is on the breathing behavior, negative gas adsorption (NGA), and selective adsorption of CO2 as well as to what degree the application of mechanical pressure influences this behavior. To this end, phase diagrams, coadsorption isotherms are constructed and the CO 2 /CH 4 selectivity is computed in terms of the vapor pressure of methane and carbon dioxide as well as the mechanical pressure. As a result, it was found that NGA can be induced by coadsorption of CO2 /CH4 gas mixtures with certain molar compositions. Finally, a specific adsorption/desorption cycle, which includes the application of an additional mechanical pressure, is proposed to allow for an increased CO2/CH4 selectivity as well as for an expected less energy demanding CO2 desorption step.
Herein we explore the unique potential of pillared-layered metal–organic frameworks of the DMOF-1 family for mechanical energy storage applications. In this work, we theoretically predict for the guest-free DMOF-1 a new contracted phase by exerting an external mechanical pressure of more than 200 MPa with respect to the stable phase at atmospheric pressure. The breathing transition is accompanied by a very large volume contraction of about 40%. The high transition pressures and associated volume changes make these materials highly promising with an outstanding mechanical energy work. Furthermore, we show that changing the nature of the metal allows to tune the behavior under mechanical pressure. The various phases were revealed by a combination of periodic density-functional theory calculations, force field molecular dynamics simulations and mercury intrusion experiments for DMOF-1(Zn) and DMOF-1(Cu). The combined experimental and theoretical approach allowed to discover the potential of these materials for new technological developments.
Knowledge of the thermodynamic potential in terms of the independent variables allows to characterize the macroscopic state of the system. However, in practice, it is difficult to access this potential experimentally due to irreversible transitions that occur between equilibrium states. A showcase example of sudden transitions between (meta)stable equilibrium states is observed for soft porous crystals possessing a network with long-range structural order, which can transform between various states upon external stimuli such as pressure, temperature and guest adsorption. Such phase transformations are typically characterized by large volume changes and may be followed experimentally by monitoring the volume change in terms of certain external triggers. Herein, we present a generalized thermodynamic approach to construct the underlying Helmholtz free energy as a function of the state variables that governs the observed behaviour based on microscopic simulations. This concept allows a unique identification of the conditions under which a material becomes flexible.
Open Access version available at UGent repositoryA gallium analogue of the commercially available Al-fumarate MOF A520 - recently identified as isotypic to MIL-53(Al)-BDC - has been synthesized and further characterized in its hydrated and dehydrated forms. The structural response under applied mechanical pressure of this MIL-53(Ga)-FA solid was investigated using advanced experimental techniques coupled with computational tools. Hg porosimetry and high-pressure X-Ray Powder Diffraction (XRPD) experiments evidenced that the pristine dehydrated large pore form undergoes an irreversible structure contraction upon an applied pressure of 85 MPa with an associated volume change of ca. 14% which makes this material promising for mechanical energy storage applications, in particular as a shock absorber. The breathing behavior was further rationalized performing a series of periodic Density Functional Theory (DFT) calculations with the construction of an energy profile as a function of volume for both MIL-53(Ga)-FA and its Aluminum analogue. As such we could fully unravel the microscopic origin of the difference in pressure-induced behavior for the aluminum and gallium fumarate based materials.
In this Article, we present a molecular-level understanding of the experimentally observed loss of crystallinity in UiO-66-type metal–organic frameworks, including the pristine UiO-66 to -68 as well as defect-containing UiO-66 materials, under the influence of external pressure. This goal is achieved by constructing pressure-versus-volume profiles at finite temperatures using a thermodynamic approach relying on ab initio derived force fields. On the atomic level, the phenomenon is reflected in a sudden drop in the number of symmetry operators for the crystallographic unit cell because of the disordered displacement of the organic linkers with respect to the inorganic bricks. For the defect-containing samples, a reduced mechanical stability is observed, however, critically depending on the distribution of these defects throughout the material, hence demonstrating the importance of judiciously characterizing defects in these materials.
This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
http://pubs.acs.org/doi/abs/10.1021/acs.chemmater.6b01956
Open Access version available at UGent repositoryThe aluminum fumarate MOF A520 or MIL-53-FA is revealed to be a promising material for mechanical energy-related applications with performances in terms of work and heat energies which surpass those of any porous solids reported so far. Complementary experimental and computational tools are deployed to finely characterize and understand the pressure-induced structural transition at the origin of these unprecedented levels of performance.
Open Access version available at UGent repository