An approach for computing Fick diffusivities directly from equilibrium molecular dynamics (MD) simulations is presented and demonstrated for a ternary chloroform–acetone–methanol liquid mixture. In ...our approach, Fick diffusivities are calculated from the Maxwell–Stefan (MS) diffusivities and the so-called matrix of thermodynamic factors. MS diffusivities describe the friction between different molecular species and can be directly computed from MD simulations. The thermodynamic factor describes the deviation from ideal mixing behavior and is difficult to extract from both experiments and simulations. Here, we show that the thermodynamic factor in ternary systems can be obtained from density fluctuations in small subsystems embedded in a larger simulation box. Since the computation uses the Kirkwood–Buff coefficients, the present approach provides a general route toward the thermodynamics of the mixture. In experiments, Fick diffusion coefficients are measured, while previously equilibrium molecular dynamics simulation only provided MS transport diffusivities. Our approach provides an efficient and accurate route to predict multicomponent diffusion coefficients in liquids based on a consistent molecular picture and therefore bridges the gap between theory and experiment.
Molecular valves are becoming popular for potential biomedical applications. However, little is known concerning their performance in energy and environmental areas. Zeolite RHO shows unique pore ...deformations upon changes in hydration, cation siting, cation type, or temperature–pressure conditions. By varying the level of distortion of double eight-rings, it is possible to control the adsorption properties, which confer a molecular valve behavior to this material. We have employed interatomic potentials-based simulations to obtain a detailed atomistic view of the structural distortion mechanisms of zeolite RHO, in contrast with the averaged and space group restricted information provided by diffraction studies. We have modeled four aluminosilicate structures, containing Li+, Na+, K+, Ca2+ cations. The distortions of the three different zeolite rings are coupled, and the six- and eight-membered rings are largely flexible. A large dependence on the polarizing power of the extra-framework cations and with the loading of water has been found for the minimum aperture of the eight-membered rings that control the nanovalve effect. The calculated energy barriers for moving the cations across the eight-membered rings are very high, which explains the experimentally observed slow kinetics of the phase transition as well as the appearance of metastable phases.
The stability and desirable crystal formation of organohalide perovskite semiconductors is of utmost relevance to ensure the success of perovskites in photovoltaic technology. Herein we have ...simulated the dynamics of ionic precursors toward the formation of embryonic organohalide perovskite CH3NH3PbI3 units in the presence of solvent molecules using Molecular Dynamics. The calculations involved, a variable amount of Pb(2+), I(-), and CH3NH3(+) ionic precursors in water, pentane and a mixture of these two solvents. Suitable force fields for solvents and precursors have been tested and used to carry out the simulations. Radial distribution functions and mean square displacements confirm the formation of basic perovskite crystalline units in pure pentane - taken as a simple and archetypal organic solvent. In contrast, simulations in water confirm the stability of the solvated ionic precursors, which prevents their aggregation to form the perovskite compound. We have found that in the case of a water/pentane binary solvent, a relatively small amount of water did not hinder the perovskite formation. Thus, our findings suggest that the cause of the poor stability of perovskite films in the presence of moisture is a chemical reaction, rather than the polar nature of the solvents. Based on the results, a set of force-field parameters to study from first principles perovskite formation and stability, also in the solid phase, is proposed.
We use molecular simulations to study the adsorption of ethane and ethylene in zeolitic imidazolate frameworks. The separation of these two compounds is a crucial step in many industrial processes, ...most of them related to production of ethylene. Separation methods such as fractional cryogenic distillation require large energy consumption that increases the costs of ethylene production. Here, we analyze the suitability of zeolitic imidazolate frameworks for the separation of these gases on the basis of structural and chemical features. We pay special attention to the effect exerted by other gases on the adsorption and diffusion of ethane and ethylene in the structures. We found that nitrogen has an important role in the separation process and depending on the structure, it can enhance or hinder the adsorption selectivity for ethane. The presence of gases other than nitrogen also causes an effect on the ethane/ethylene separation. A mixture containing hydrogen, oxygen, methane, carbon monoxide, carbon dioxide, ethane, and ethylene in zeolitic imidazolate frameworks is also investigated. Our results identify ZIFs with rho, crg, and lta topologies as good candidates for the separation of ethane and ethylene.
Adsorption of n-alkanes on high silica MFI and MEL zeolites was studied by means of experimental quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) and Monte Carlo ...simulations. An unusual, isobaric approach to adsorption measurements and simulations was applied. Good agreement between the experimental and calculated data observed for MFI indicates that the QE-TPDA is a reliable method for studying porosity-related adsorptive properties of molecular sieves. The calculated average occupation profiles confirmed limited mobility of hexane and heptane molecules adsorbed in the sinusoidal channels on the MFI, thus proving the concept of order–disorder phase transition postulated in explanation of the two-step desorption profiles of these alkanes observed for MFI zeolites. Partial agreement of the calculated isobars with the experimental data found for zeolite MEL indicated that adsorption of n-alkanes in this structure is more complex than assumed in the simulation model. However, two-step desorption profiles and immobilization of the molecules adsorbed in the straight channels of the MEL structure were also found for hexane.
We have used molecular simulations to analyze the effect of water on the adsorption of carbon monoxide in metal–organic frameworks. We have developed a model for carbon monoxide that not only ...reproduces the required experimental properties more accurately than previous models, but also takes into account the effect of the dipole moment on the interaction of the molecule with water and with the metal–organic framework. Our simulations were performed for Cu-BTC and IRMOF-1 with different contents of water, up to the highest water loading that each metal–organic framework can contain without losing stability. To carry out this study, we computed the entropies and energies of adsorption, the Henry coefficients, the adsorption isotherms in the dry and hydrated structures, and the molecular distributions of carbon monoxide based on the topology of the framework and on the interactions with the molecules of water. Our results show that the adsorption of carbon monoxide can be increased or decreased by controlling the amount of water in the structures.
The whole compositional range (Ge f = Ge/(Ge + Si) = 0 to 1) of zeolite STW has been synthesized and studied by a comprehensive combined experimental–theoretical approach. The yield of the zeolite ...goes through a maximum and then drops at the GeO 2 side of the series, following the inverse of the calculated free energy curve. The unit cell generally expands, roughly linearly, as the Ge f increases, but a notable resilience to expansion is observed at the high silica side. This can be attributed to the more rigid character of SiO 2 and the ability of Ge units to deform. Density functional theory calculations provide a new assignment of the previously controversial 19 F MAS NMR resonances for occluded fluoride, which is based not only on the number of Ge atoms in the double-4-ring units but also on the way they are associated (namely, no Ge, isolated Ge, Ge pairs or closed Ge clusters). While we found an overall good agreement between the experimental and theoretical trends in preferential occupation of different crystallographic sites by Ge, the theoretical models show more sharp and abrupt tendencies, likely due both to limitations of the approach and to kinetic factors that allow metastable configurations to actually exist.
Adsorption of hexane isomers in high silica MFI and MEL zeolites was studied by means of quasi-equilibrated temperature-programmed desorption and adsorption and Monte Carlo simulations. ...Configurational bias and continuous fractional component Monte Carlo were applied for these systems, and their efficiency and effectiveness were compared. All branched hexane isomers, i.e., 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane, were used. The agreement between experimental and calculated adsorption isobars confirmed that quasi-equilibrated temperature-programmed desorption and adsorption is an effective method for studying adsorption of branched alkanes in zeolites. Detailed literature review on adsorption of branched paraffins in MFI revealed diffusion limitations which make reaching adsorption equilibria of these molecules difficult with isothermal approach at low temperatures. Temperature-dependent measurements conducted in this work largely allowed avoiding such limitations. Adsorption equilibria of branched alkanes in MEL zeolite have been studied experimentally for the first time. Detailed analysis of the adsorption behavior of these systems revealed the presence of two adsorption sites located in the different intersections of the straight channels characteristic of the MEL framework. Molecular simulations for adsorption of mixtures of branched hexane isomers showed potential application of the MFI and MEL zeolites in separation of mono- and dibranched isomers.
This work aims to shed light on the performance of zeolitic imidazolate frameworks for hydrogen purification from coke oven gases (COG). Using molecular simulation, we model COG as a mixture of six ...gases and study the effect of ZIF topology on the separation performance. To do this, we compare similar structures, e.g., ZIF-8 and ZIF-11, and focus on obtaining information that explains why they behave differently while being so similar. Simulation results show that the structure with the smallest pore size best separates hydrogen from carbon monoxide and nonpolar molecules. The adsorption of carbon dioxide is also strongly affected by the polarizability of the structure. However, the adsorption of the other components (methane, carbon monoxide, nitrogen, and oxygen) is strongly dependent on their pore size. We also provide molecular information on the effect of phase transition on hydrogen purification using ZIF-7 as an example, which drastically changes the pore volume of the structure when it changes phase. These findings will help to select high-performance ZIFs for adsorption- or screening-based hydrogen purification.
Monte Carlo simulations were used to compute the solubility of the pure gases CO2, CH4, CO, H2, N2, and H2S in the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ...bmimTf2N. Simulations in the osmotic ensemble were performed to compute absorption isotherms at a temperature of 333.15 K using the versatile continuous fractional component Monte Carlo (CFCMC) method. The predicted gas solubilities and Henry constants are in good agreement with the experimental data. The Monte Carlo simulations correctly predict the observed solubility trend, which obeys the following order: H2S > CO2 > CH4 > CO > N2 > H2. Relevant separation selectivities for the precombustion process are calculated from the pure gas Henry constants and a comparison with experimental data is provided.
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