Atomistic molecular dynamics insights on water local structure and dynamics on different surfaces of zeolitic-imidazolate frameworks (original) (raw)
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Water nanodroplets confined in zeolite pores
Faraday Discussions, 2009
We provide a comprehensive depiction of the behaviour of a nanodroplet of x20 water molecules confined in the pores of a series of 3D-connected isostructural zeolites with varying acidity, by means of molecular simulations. Both grand canonical Monte Carlo simulations using classical interatomic forcefields and first-principles Car-Parrinello molecular dynamics were used in order to characterise the behaviour of confined water by computing a range of properties, from thermodynamic quantities to electronic properties such as dipole moment, including structural and dynamical information. From the thermodynamic point of view, we have identified the all-silica zeolite as hydrophobic, and the cationic zeolites as hydrophilic; the condensation transition in the first case was demonstrated to be of first order. Furthermore, in-depth analysis of the dynamical and electronic properties of water showed that water in the hydrophobic zeolite behaves as a nanodroplet trying to close its hydrogen-bond network onto itself, with a few short-lived dangling OH groups, while water in hydrophilic zeolites ''opens up'' to form weak hydrogen bonds with the zeolite oxygen atoms. Finally, the dipole moment of confined water is studied and the contributions of water self-polarisation and the zeolite electric field are discussed. † Electronic supplementary information (ESI) available: The two possible structures of [Al 9 Si 15 O 48 ] 9À satisfying Loewenstein's rule. For ESI see
Journal of Physical Chemistry C, 2018
Metal-organic frameworks are chemically versatile materials, and excellent candidates for many applications from carbon capture to drug delivery, through hydrogen storage. While most studies so far focus on the crystalline MOFs, there has been a recent shift to the study of their disordered states, such as defective structures, glasses, gels, and very recently liquid MOFs. Following the publication of the melting mechanism of zeolitic imidazolate framework ZIF-4, we use here molecular simulation in order to investigate the similarities and differences with two other zeolitic imidazolate frameworks, ZIF-8 and ZIF-zni. We perform first principles molecular dynamics simulations to study the melting phenomena and the nature of the liquids obtained, focusing on structural characterization at the molecular scale, dynamics of the species, and thermodynamics of the solid-liquid transition. We show how the retention of chemical configuration, the changes in the coordination network, and the variation of the porous volume in the liquid phase are influenced by the parent crystalline framework.
Hydrogen storage in zeolite imidazolate frameworks. A multiscale theoretical investigation
International Journal of Hydrogen Energy, 2011
ZIFs MP2 GCMC a b s t r a c t A multiscale approach is used to investigate the hydrogen adsorption in nanoporous Zeolite Imidazolate Frameworks (ZIFs) on varying geometries and organic linkers. Ab initio calculations are performed at the MP2 level to obtain correct interaction energies between hydrogen molecules and the ZIF structures. Subsequently, classical grand canonical Monte-Carlo (GCMC) simulations are carried out to obtain the hydrogen uptake of ZIFs at different thermodynamic conditions of pressure and temperature.
The Journal of Physical Chemistry C, 2017
Molecular dynamics simulations were carried out to explore the kinetic and structural properties of water diffusing through several types of zeolite with pore sizes that make them suitable for application as membranes for small ion filtering. Zeolites with pores in one and three dimensions were considered and insights into the effect of the structures of the zeolites were obtained by studying diffusion and ordering of the confined water. Interestingly, water molecules in the zeolites with pores in one-dimension showed up to 2.5-fold higher diffusivities than in those with pores in three-dimensions for a given water density and a similar pore diameter. The distribution of water molecules across pores and the number of water molecules in a specified region were also investigated to determine the effects of pore shape and size on the water assembly. The arrangement and the number of water molecules in a cluster were observed to depend heavily on the shape and size of pores. The present study provides a deeper understanding of how various structural features affect the dynamics and structure of the water within zeolites.
The Journal of Physical Chemistry B, 2010
Statistical-mechanics-based simulation studies at the atomistic level of argon (Ar), methane (CH 4), and hydrogen (H 2) sorbed in the zeolite imidazolate framework-8 (ZIF-8) are reported. ZIF-8 is a product of a special kind of chemical process, recently termed as reticular synthesis, which has generated a class of materials of critical importance as molecular binders. In this work, we explore the mechanisms that govern the sorption thermodynamics and kinetics of nonpolar sorbates possessing different sizes and strength of interactions with the metal-organic framework to understand the outstanding properties of this novel class of sorbents, as revealed by experiments published elsewhere. For this purpose, we have developed an in-house modeling procedure involving calculations of sorption isotherms, partial internal energies, various probability density functions, and molecular dynamics for the simulation of the sorbed phase over a wide range of occupancies and temperatures within a digitally reconstructed unit cell of ZIF-8. The results showed that sorbates perceive a marked energetic inhomogeneity within the atomic framework of the metal-organic material under study, resulting in free energy barriers that give rise to inflections in the sorption isotherms and guide the dynamics of guest molecules.
Molecular Dynamics Simulation of Water Diffusion in MFI-Type Zeolites
Journal of Physical Chemistry B, 2009
Molecular dynamics simulation using COMPASS force field has been employed to understand the dynamics of water diffusion and structuring in silicalite-1 and Na-ZSM-5 (Si/Al ) 95 and 191) samples at three different temperatures, 297, 354, and 393 K, at a water loading of 8 molecules per unit cell, in canonical ensemble. Diffusion coefficients were significantly reduced upon the introduction of aluminum atoms into the framework, together with charge balancing cations placed in their vicinity, since the ion-dipole interactions dominant in ZSM-5 samples are stronger than the H-bond interactions in silicalite-1. The activation energy of diffusion increased with decreasing Si/Al ratio. In the silicalite-1 and ZSM-5 samples, straight channels were observed to be preferred than the sinusoidal ones and the channel preference was not observed to be a strong function of either temperature or the Si/Al ratio. The ordered structures of the water molecules, forming clusters in the channels of silicalite-1 at low temperature was observed to be broken to some extent by increased temperatures, and decreased Si/Al ratio, resulting in less ordered structures. The positions of the water molecules in the straight and sinusoidal channels for the ZSM-5 samples were mainly determined by the location of the charge compensating cation(s) in the structure, as was shown by the concentration profiles.
Microporous and Mesoporous Materials, 2015
Theoretical methods are used for modelling the capture and storage of CO 2 in Zinc-Imidazolate-Frameworks (ZIFs). Density functional theory (DFT) is employed to establish the structure of three ZIFs with small, medium and large cages to predict the uptake behaviour of adsorbed water and carbon dioxide. Water hydration strength and configuration inside the material are investigated within and without the presence of CO 2. CO 2 is predominantly located around the inner surface of the cages in the ZIFs type ZIFn (n=1,4,6) through single and multipoint interactions. Water shows a stronger adsorption than carbon dioxide and can play an important role in solvating the CO 2 molecules inside the porous. In the final structures, H 2 O and CO 2 do not react with the surface to produce hydroxyl groups or carbonate-like species, respectively. Both species compete for the same adsorption sites. Their maximum adsorption occurs in systems with really accessible surfaces such us ZIF6. A detailed study of the surface-accessibility of a molecule on a "Connolly Surface" is reported, were ZIFs with extra-large pores plays a major role on the adsorption process. Simulated adsorption isotherms throughout MC simulations was obtained for a single CO 2 and H 2 O in which results give a ranking for the most appropriate process conditions.
Molecular simulations of water in hydrophobic microporous solids
Adsorption, 2008
This work reports Grand Canonical Monte-Carlo molecular simulation (GCMC) results of water adsorption in a priori hydrophobic microporous solids such as silicalite, a purely siliceous zeolite (Ø pore ∼ 5 Å) and C-Y, a pure carbon replica of zeolite Y (Ø pore ∼ 1 nm). At a first step, in both cases, the water-water interactions are described with the SPC model (calibrated for bulk liquid water) while watersubstrate interactions are calculated within the framework of the PN-TrAZ model. This adsorbate-zeolite potential decomposes into short range (repulsive, inductive and dispersive) interaction terms with transferable parameters plus, in the case of silicalite, an electrostatic interaction term based on SPC partial charges for water and ab initio charges for silicalite. With such a standard approach, we found that water fills the microporous volume in both materials at pressure value well below P 0 ; hence does not show a strong hydrophobic behaviour at variance with reference experiments (V. Eroshenko et al. in C. R. Phys. 3:111, 2002). This indicates that common models used to describe confined polar molecules are far from being operative. We show on the basis of periodic ab initio calculations that confined water molecules in silicate have a dipole value ∼10% smaller than that in the 3D liquid phase indicating that the environment felt by a confined water molecule in silicalite pores is not equivalent to that in the bulk liquid. This implies that classical simulations of polar molecules in ultra
Rsc Advances, 2012
A combined IR absorption and first principles modelling study of zeolite imidazolate frameworks (ZIFs) filled with hydrogen is presented. It is shown that hydrogen physisorbed in a ZIF results in a number of absorption lines at around 4131, 4121, 4480, 4700, 4880, 5100, and 5280 cm 21 , which are assigned to the Q(0ro-vibrational transitions of physisorbed H 2 , respectively. The latter three modes represent simultaneous excitation of molecular pairs, which implies that hydrogen physisorbed in ZIF occurs at a density close to that of the liquid and/or solid state. The adsorption onset temperature defined as the maximum temperature at which localized adsorption occurs was found to be around 80 K.
Journal of the American Chemical Society, 2016
Mixed-linker zeolitic imidazolate frameworks (ZIFs) are nanoporous materials that exhibit continuous and controllable tunability of properties like effective pore size, hydrophobicity, and organophilicity. The structure of mixed-linker ZIFs has been studied on macroscopic scales using gravimetric and spectroscopic techniques. However, it has so far not been possible to obtain information on unit-cell-level linker distribution, an understanding of which is key to predicting and controlling their adsorption and diffusion properties. We demonstrate the use of (1)H combined rotation and multiple pulse spectroscopy (CRAMPS) NMR spin exchange measurements in combination with computational modeling to elucidate potential structures of mixed-linker ZIFs, particularly the ZIF 8-90 series. All of the compositions studied have structures that have linkers mixed at a unit-cell-level as opposed to separated or highly clustered phases within the same crystal. Direct experimental observations of l...