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.
In this paper, three barostat coupling schemes for pressure control, which are commonly used in molecular dynamics simulations, are critically compared to characterize the rigid MOF-5 and flexible ...MIL-53(Al) metal–organic frameworks. We investigate the performance of the three barostats, the Berendsen, the Martyna–Tuckerman–Tobias–Klein (MTTK), and the Langevin coupling methods, in reproducing the cell parameters and the pressure versus volume behavior in isothermal–isobaric simulations. A thermodynamic integration method is used to construct the free energy profiles as a function of volume at finite temperature. It is observed that the aforementioned static properties are well-reproduced with the three barostats. However, for static properties depending nonlinearly on the pressure, the Berendsen barostat might give deviating results as it suppresses pressure fluctuations more drastically. Finally, dynamic properties, which are directly related to the fluctuations of the cell, such as the time to transition from the large-pore to the closed-pore phase, cannot be well-reproduced by any of the coupling schemes.
The flexibility of three MIL-47(V)-type materials (MIL-47, COMOC-2, and COMOC-3) has been explored by constructing the pressure versus volume and free energy versus volume profiles at various ...temperatures ranging from 100 to 400 K. This is done with first-principles-based force fields using the recently proposed QuickFF parametrization protocol. Specific terms were added for the materials at hand to describe the asymmetry of the one-dimensional vanadium-oxide chain and to account for the flexibility of the organic linkers. The force fields are used in a series of molecular dynamics simulations at fixed volumes but varying unit cell shapes. The three materials show a distinct pressure–volume behavior, which underlines the ability to tune the mechanical properties by varying the linkers toward different applications such as nanosprings, dampers, and shock absorbers.
Three different types of NH3 species can be simultaneously present on Cu(2+)-exchanged CHA-type zeolites, commonly used in Ammonia Selective Catalytic Reduction (NH3-SCR) systems. These include ...ammonium ions (NH4(+)), formed on the Brønsted acid sites, Cu(NH3)4(2+) complexes, resulting from NH3 coordination with the Cu(2+) Lewis sites, and NH3 adsorbed on extra-framework Al (EFAl) species, in contrast to the only two reacting NH3 species recently reported on Cu-SSZ-13 zeolite. The NH4(+) ions react very slowly in comparison to NH3 coordinated to Cu(2+) ions and are likely to contribute little to the standard NH3-SCR process, with the Brønsted groups acting primarily as NH3 storage sites. The availability/reactivity of NH4(+) ions can be however, notably improved by submitting the zeolite to repeated exchanges with Cu(2+), accompanied by a remarkable enhancement in the low temperature activity. Moreover, the presence of EFAl species could also have a positive influence on the reaction rate of the available NH4(+) ions. These results have important implications for NH3 storage and availability in Cu-Chabazite-based NH3-SCR systems.
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.
The UiO-66 metal organic framework is one of the most thermally and chemically stable hybrid materials reported to date. However, it is also accepted that the material contains structurally embedded ...defects, which may be engineered to enhance properties towards specific applications such as catalysis, sensing, etc. The synthesis conditions determine to a large extent the level of perfection of the material and additionally the catalytic activity may be enhanced by post-synthesis activation at high temperature under vacuum, in which defect coordinating species (H2O, HCl, monocarboxylic modulators, etc.) evaporate. The molecular level characterization of defects is extremely challenging from both theoretical and experimental points of view. Such experimental endeavor was recently proposed via experimental SXRD measurements, also unraveling the coordination of water on the Zr-O-Zr defect sites Angew. Chem., Int. Ed., 2015, 54, 11162-11167. The present work provides a theoretical understanding of defect structures in UiO-66(Zr) by means of periodic density functional theory calculations and ab initio molecular dynamics simulations. A range of defect structures are generated with different numbers of missing linkers. For each of the defects, the free energetic and mechanical stability is discussed and the coordination of water and charge balancing hydroxide ions is studied. For catalysis applications, the material is mostly pretreated to remove water by dehydration reactions. For each of the proposed defect structures, mechanistic pathways for dehydration reactions of the Zr-bricks are determined employing nudged elastic band (NEB) calculations. During the dehydroxylation trajectory, loose hydroxyl groups and terephthalate decoordinations are observed. Furthermore, dehydration reactions are lower activated if terephthalate linkers are missing in the immediate environment of the inorganic brick. The creation of defects and the dehydration processes have a large impact on the mechanical properties of the material, which is evidenced by lower bulk moduli and elastic constants for structures with more defects.
For the development of ab initio derived force fields, atomic charges must be computed from electronic structure computations, such that (i) they accurately describe the molecular electrostatic ...potential (ESP) and (ii) they are transferable to the force-field application of interest. The Iterative Hirshfeld (Hirshfeld-I or HI) scheme meets both requirements for organic molecules. For inorganic oxide clusters, however, Hirshfeld-I becomes ambiguous because electron densities of nonexistent isolated anions are needed as input. Herein, we propose a simple Extended Hirshfeld (Hirshfeld-E or HE) scheme to overcome this limitation. The performance of the new HE scheme is compared to four popular atoms-in-molecules schemes, using two tests involving a set of 248 silica clusters. These tests show that the new HE scheme provides an improved trade-off between the ESP accuracy and the transferability of the charges. The new scheme is a generalization of the Hirshfeld-I scheme, and it is expected that its improvements are to a large extent applicable to molecular systems containing elements from the entire periodic table.
We present the
in situ
synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66. The resulting Au@UiO-66 materials were characterized by means of N
2
sorption, XRPD, UV-Vis, ...XRF, XPS and TEM analysis. The Au nanoparticles (NP) are homogeneously distributed along the UiO-66 host matrix when using NaBH
4
or H
2
as reducing agents. The Au@UiO-66 materials were evaluated as catalysts in the oxidation of benzyl alcohol and benzyl amine employing O
2
as oxidant. The Au@MOF materials exhibit a very high selectivity towards the ketone (up to 100%). Regenerability and stability tests demonstrate that the Au@UiO-66 catalyst can be recycled with a negligible loss of Au species and no loss of crystallinity.
In situ
IR measurements of UiO-66 and Au@UiO-66-NaBH
4
, before and after treatment with alcohol, showed an increase in IR bands that can be assigned to a combination of physisorbed and chemisorbed alcohol species. This was confirmed by velocity power spectra obtained from the molecular dynamics simulations. Active peroxo and oxo species on Au could be visualized with Raman analysis.
We present the
in situ
synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66.
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