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•CO2/CH4 selective adsorption in Metal Organic Frameworks (MOFs) was analyzed.•Computational screening of 3000 MOFs structures from CoREMOF database.•Selection of 13 MOFs with highest ...selectivity (>80 in dry and humid conditions)•Structures performing best in dry or in humid environment identified.•Competition between water and CO2 adsorption observed and discussed.
Competitive adsorption of water is an important issue in the adsorption-based industrial processes of bio- and flue gases separation. The dehumidification of gases prior to separation would increase process complexity and lower its economic interest. In this work, large-scale computational screening was applied to identify Metal-Organic Frameworks (MOFs) structures which exhibit high CO2/CH4 selectivity and total loading higher than 0.5 mol/kg (in the presence of water). High-throughput Grand Canonical Monte Carlo (GCMC) screening of nearly 3000 existing MOF materials was carried out. Initial selection assumed fixed values of pore limiting diameter (PLD) and Henry's constant for water and allowed one to preselect 764 structures. After GCMC simulations carried for 50/50 CO2/CH4 mixture, at ambient conditions (p = 1 bar, T = 298 K), and variable gas humidity (0%, 5%, 30% and 40%) the final selection revealed 13 most promising MOFs structures. We focused on analysis of the correlations between the properties of the selected MOFs and the separation selectivity. We show that the selectivity is a complex function of the porous materials characteristics and finding selective sorbent, performing well in dry and wet conditions requires careful analysis of available MOFs.
An effective separation of propylene/propane mixtures is one of the most important processes in the petrochemical industry. Incidentally, this separation is challenging due to the extensive ...similarities between both gases in terms of physicochemical properties such as, but not only limited to, boiling point, kinetic diameter, and molecular weight. A drive to switch to energy efficient processes, like adsorption or membrane separation, has highlighted several microporous metal organic frameworks as promising materials. In this work, we present a combined numerical and experimental investigation on propane and propylene adsorption in Zr-fumarate-MOF (also known as MOF-801), a small pore isoreticular analogue of UiO-66. Here, we demonstrate how the presence of structural defects can completely change the sorptive properties and separation performance of the Zr-fumarate-MOF, with the loss of sieving effects and a reversal of selectivity toward propane, as well as enhanced capacity and diffusion rates for C3-sized hydrocarbons. Extensive GCMC simulations performed on mixed defective supercells show that a ratio of missing-cluster defects of around 1/8 can best account for the experimental results. Furthermore, analysis of low-frequency phonon spectra is used to explain gaseous diffusion in the original pristine material. Finally, the thermodynamic preference for propane over propylene is confirmed through column breakthrough experiments, suggesting the potential applicability of the Zr-fumarate-MOF in this challenging separation.
Three types of phenomena occurring on both sides of the event horizon of spherically symmetric black holes are analyzed and discussed here. These phenomena are: a light ray orbiting a photon sphere ...and its analogue, the motion of a uniformly accelerated massive particle and a generalized Doppler effect. The results illustrate how the anisotropic dynamics of the interior of black holes, distinct in the cases both with and without an additional internal horizon, affect non-quantum behaviour.
A black hole in a Schwarzschild spacetime is considered. A transformation is proposed that describes the relationship between the coordinate systems exterior and interior to an event horizon. The ...application of this transformation permits considerations of the (a)symmetry of a range of phenomena taking place on both sides of the event horizon. The paper investigates two distinct problems of a uniformly accelerated particle. In one of these, although the equations of motion are the same in the regions on both sides, the solutions turn out to be very different. This manifests the differences of the properties of these two ranges.
Functional materials that respond to chemical or physical stimuli through reversible structural transformations are highly desirable for the integration into devices. Now, a new stable and flexible ...eightfold interpenetrated three‐dimensional (3D) metal–organic framework (MOF) is reported, Zn(oba)(pip)n (JUK‐8) based on 4,4′‐oxybis(benzenedicarboxylate) (oba) and 4‐pyridyl functionalized benzene‐1,3‐dicarbohydrazide (pip) linkers, featuring distinct switchability in response to guest molecules (H2O and CO2) or temperature. Single‐crystal X‐ray diffraction (SC‐XRD), combined with density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations, reveal a unique breathing mechanism involving collective motions of eight mixed‐linker diamondoid subnetworks with only minor displacements between them. The pronounced stepwise volume change of JUK‐8 during water adsorption is used to construct an electron conducting composite film for resistive humidity sensing.
Breathe and sense: A flexible interpenetrated metal–organic framework exhibits a unique breathing effect in response to guest molecules and temperature, involving collective motions of diamondoid subnetworks. The high stability is demonstrated to be ideal for system integration into threshold sensing architectures.
We report a microscopic model of the phonon-adsorption correlations in flexible metal–organic framework materials. We analyze the mechanism of the gate opening deformation using the notion of coupled ...phonon- and adsorption-induced structural transformation. Using the ZIF-8 structure as an example, we perform an analysis of transformation-related, low-frequency phonon modes of the framework. On the basis of structure-related quantities such as pore limiting diameter, void fraction, and adsorption uptake, we determine the conditions which lead to the gate opening transformation in ZIF-8. Energetic landscape of the deformation process is analyzed using grand thermodynamic potential of adsorption. We generalize our conclusions to other flexible ZIF structures with the same topology.
The structural transformations of periodic structures are very often initiated by the dynamical fluctuation of the equilibrium structure. The natural mechanical excitations in crystals are called ...phonons. If the energy of these fluctuations is low, they can easily be transformed into static deformations which define new structural properties of the materials. This is the case in so called gate opening transformations which modify the structure and the adsorptive properties of porous solids. Using the example of three SOD-type zeolitic imidazolate frameworks (ZIFs) containing linker molecules with different substituents, we show that analysis of low-frequency phonons obtained from density-functional theory (DFT) calculations allows one to model the observed gate opening and to understand the microscopic mechanism of this structural transformation.
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•Phonon approach to describe structural deformations in MOF is proposed.•Soft modes in ZIF-8 framework are calculated and related to structure deformation.•Mechanism of deformation in 3 chemically modified SOD cages is compared.•Correlation between phonon symmetry and ZIF-type structures' deformation is discussed.•Gate opening mechanism is explained.
Abstract
This review spotlights the role of atomic‐level modeling in research on metal‐organic frameworks (MOFs), especially the key methodologies of density functional theory (DFT), Monte Carlo (MC) ...simulations, and molecular dynamics (MD) simulations. The discussion focuses on how periodic and cluster‐based DFT calculations can provide novel insights into MOF properties, with a focus on predicting structural transformations, understanding thermodynamic properties and catalysis, and providing information or properties that are fed into classical simulations such as force field parameters or partial charges. Classical simulation methods, highlighting force field selection, databases of MOFs for high‐throughput screening, and the synergistic nature of MC and MD simulations, are described. By predicting equilibrium thermodynamic and dynamic properties, these methods offer a wide perspective on MOF behavior and mechanisms. Additionally, the incorporation of machine learning (ML) techniques into quantum and classical simulations is discussed. These methods can enhance accuracy, expedite simulation setup, reduce computational costs, as well as predict key parameters, optimize geometries, and estimate MOF stability. By charting the growth and promise of computational research in the MOF field, the aim is to provide insights and recommendations to facilitate the incorporation of computational modeling more broadly into MOF research.
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Soft porous crystals undergo large structural transformations under a variety of physical stimuli. Breathing-like transformations, occurring with a large volume change, have been ...associated with an existence of bi-stable or multi-stable crystal structures. Understanding of the mechanism of these transformations is essential for their potential applications in gas adsorption, separation and storage. However, the generic description is still missing. Here, we provide a detailed, multiscale qualitative and quantitative analysis of the adsorption-induced “breathing” transformations in two metal organic frameworks (MOFs): MIL-53(Al) which is a reference case of our approach, and recently synthesized JUK-8, which does not show any bistability without adsorbate. The proposed approach is based on atomistic simulations and does not require any empirical or adjustable parameters. It allows for a prediction of potential structural transformations in MOFs including the adsorption induced deformations derived from adsorption stress model. We also show that the quantitative agreement between calculated and experimental results critically depends on the quality of the dispersion energy correction. Our methodology represents a new, powerful tool for designing and screening of flexible materials, alternative and complimentary to experimental approaches.
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