The effect of an external electric field (E) on the dielectric constant (ε) of several pure solvents [propylene carbonate (PC), ethylene carbonate (EC), acetonitrile (MeCN), and dimethyl carbonate (DMC)] and for several EC/DMC mixtures is explored using classical molecular dynamics simulation. Force fields were chosen that accurately predict the density and zero-field dielectric constant with respect to experiment. The simulation results for ε(E) for field strengths up to 0.4 V/Å are calculated and fit to the Booth model, which is a standard functional form for the dependence of the dielectric constant on electric field. For PC and DMC, the Booth model gives an excellent representation of the data at all electric fields studied. For EC and MeCN, the Booth model works well at lower field strengths (up to 0.15 and 0.2 V/Å, respectively), but at the higher electric fields, these systems are observed to crystallize, a phenomenon referred to as electrofreezing. This work will provide useful input data for the continuum modeling of devices, such as electric double-layer capacitors, that utilize organic electrolyte solvents.

Ethylene is an important industrial feedstock for the generation of commodity chemicals and is increasingly abundant as a by-product of natural gas production. In some cases, the reaction of ethylene can be enhanced by using it to create a gas-expanded liquid with the co-reactants dissolved in an organic solvent. Here, we present a molecular simulation study of the phase behavior, structure, and transport properties of mixtures of ethylene and methanol, in which the ethylene mole fraction is controlled by changing the pressure. For this ethylene-expanded methanol system, we report phase equilibria, volume expansion, liquid structure, translational diffusion constants, and rotational correlation times. The simulation results show excellent agreement with experimental values, where available, and the ethylene and methanol models show promise for use in further studies on related systems.

Gas-expanded liquids have received significant interest as catalytic reaction media. While most GXL studies involve CO2 as the expansion gas, there is growing interest in non-CO2 based GXLs, especially when the expansion gas is also a reactant. In this work, we focus on ethylene as an expansion gas, motivated by recent experimental studies on the catalytic epoxidation of ethylene using ethylene-expanded methanol/H2O2/water mixtures within metal doped silica mesopores. Reported simulation studies on GXLs, even for bulk properties, have been primarily limited to single-component or binary systems. Here we extend the use of simulation to the study of a bulk ternary GXL system - namely, ethylene-expanded mixtures of methanol and water mixtures. We investigate the phase behavior and transport properties in the liquid phase with respect to temperature, pressure and water content. The model force fields are validated by comparing compositions and transport properties to existing experiments. In addition, we study local liquid solvation structure as a function of composition.

Using grand canonical Monte Carlo (GCMC) and molecular dynamics simulation, we examine the phase equilibrium and transport of a gas-expanded liquid under confinement. The system chosen is ethylene-expanded methanol confined in model silica mesopores, but in equilibrium with the bulk mixture—a system that has received recent interest as a reaction medium, e.g., for epoxidation of ethylene. This system was studied at 20 °C and pressures ranging from 5 to 55 bar. In addition, two different pore surface chemistries were examined: a hydrophilic pore, in which the silica dangling bonds were terminated by −OH groups, and a model “hydrophobic” pore, in which the charges on the pore atoms (including the −OH groups) were turned off. The chemical potentials for the mixture necessary to perform the GCMC simulations were obtained using a novel Gibbs–Duhem integration method along a previously calculated binary vapor–liquid equilibrium curve. We find that the pressure significantly affects the ethylene mole fraction in the confined mixture. The pore surface chemistry has a significant effect on the composition and transport properties of the confined ethylene–methanol mixture, relative to the bulk. In addition, there are significant qualitative differences between the hydrophilic and hydrophobic pores with regard to the spatial distributions of the confined ethylene and methanol.

•Mechanism of Nb-doped silica-catalyzed C2H4 epoxidation by H2O2 has been examined.•Rate-limiting barrier (11.6 kcal/mol) is lower than reported for other catalysts.•This work will assist metal-doped mesoporous epoxidation catalyst development.A mechanistic study of ethylene epoxidation by hydrogen peroxide catalyzed by niobium doped in a silica mesopore is reported. Density functional theory calculations at the M06-L/aug-cc-pVDZ level were used to investigate the catalytic pathway. A five-step cycle is proposed. The initial steps are the adsorption of H2O2 to the Nb center followed by coordination of ethylene to the hydrogen peroxide. The rate-limiting step is the subsequent epoxidation of ethylene via transfer of an oxygen atom, which has a calculated enthalpic barrier of 11.6 kcal/mol relative to the preceding intermediate. This is followed by desorption of the ethylene oxide product and dehydration to regenerate the catalyst. The reaction barrier is lower than reported for other catalysts in the literature, consistent with recent experimental reports of the efficacy of Nb-doped mesoporous silica catalysts [Catal. Sci. Technol.,2014, 4, 4433–4439]. The mechanistic details elucidated in the present calculations may aid in the rational design of new epoxidation catalysts and thus the factors influencing the reaction, e.g., geometry and charge changes, are discussed.Schematic illustration of the catalytic cycle for ethylene epoxidation by Nb-doped silica

We examine the thermodynamics and intrinsic structure of the Al–Pb liquid–liquid interface using molecular dynamics simulation and embedded atom method potentials. The instantaneous interfacial positions, from which the intrinsic structure and the capillary fluctuation spectrum are determined, are calculated using a grid-based method. The interfacial free energy extracted from the capillary fluctuation spectrum is shown to be in excellent agreement with that calculated mechanically by integrating the stress profile. The intrinsic liquid–liquid interfacial density profile shows structural oscillations in the liquid phases in the interfacial region that are shown to be quantitatively similar to the radial distribution functions of the bulk liquid, consistent with theoretical predictions from classical density functional theory and with earlier simulations on liquid–liquid and liquid–vapor interfaces. In addition, we show the mean interfacial density profile for this system is well described as a convolution of the intrinsic density profile and the probability distribution of interfacial position.

We report CO2 adsorption data for four zeolitic imidazolate frameworks (ZIFs) to 55 bar, namely ZIF-7, ZIF-11, ZIF-93, and ZIF-94. Modification of synthetic conditions allows access to different topologies with the same metal ion and organic link: ZIF-7 (ZIF-94) having sod topology and ZIF-11 (ZIF-93) having the rho topology. The varying topology, with fixed metal ion and imidazolate functionality, makes these systems ideal for studying the effect of topology on gas adsorption in ZIFs. The experiments show that the topologies with the smaller pores (ZIF-7 and 94) have larger adsorptions than their counterparts (ZIF-11 and 93, respectively) at low pressures (<1 bar); however, the reverse is true at higher pressures where the larger-pore structures have significantly higher adsorption. Molecular modeling and heat of adsorption measurements indicate that while the binding potential wells for the smaller-pore structures are deeper than those of the larger-pore structures, they are relatively narrow and cannot accommodate multiple CO2 occupancy, in contrast to the much broader potential wells seen in the larger pore structures.