Rigorous Molecular Simulations Research Articles

Asymmetric membranes for gas separation: interfacial insights and manufacturing.

State-of-the-art gas separation membrane technologies combine the properties of polymers and other materials, such as metal-organic frameworks to yield mixed matrix membranes (MMM). Although, these membranes display an enhanced gas separation performance, when compared to pure polymer membranes; major challenges remain in their structure including, surface defects, uneven filler dispersion and incompatibility of constituting materials. Therefore, to avoid these structural issues posed by today's membrane manufacturing methodologies, we employed electrohydrodynamic emission and solution casting as a hybrid membrane manufacturing method, to produce ZIF-67/cellulose acetate asymmetric membranes with improved gas permeability and selectivity for CO2/N2, CO2/CH4, and O2/N2. Rigorous molecular simulations were used to reveal the key ZIF-67/cellulose acetate interfacial phenomena (e.g., higher density, chain rigidity, etc.) that must be considered when engineering optimum composite membranes. In particular, we demonstrated that the asymmetric configuration effectively leverages these interfacial features to generate membranes superior to MMM. These insights coupled with the proposed manufacturing technique can accelerate the deployment of membranes in sustainable processes such as carbon capture, hydrogen production, and natural gas upgrading.

High-throughput screening of metal–organic frameworks for hydrogen purification

In industry, the separation/purification of hydrogen from methane impurity is of significant importance, especially under the background of carbon dioxide mitigation towards carbon neutrality. Adsorption separation is an effective route for this application regarding energy cost and separation efficiency, for which the selection of adsorbents is the key. This work proposes a computer-aided high-throughput screening strategy for the selection of metal–organic frameworks (MOFs) for CH4/H2 separation, striving to accelerate the practice of discovering better adsorbents for this particular application. Firstly, a prescreening was conducted on 12,020 computation-ready three-dimensional MOFs based on intuitive structure and chemical analysis, shortlisting 5,096 candidates. Secondly, a molecular simulation-assisted structure–property relationship analysis was performed based on small sampled MOFs, from which two structure criteria were derived for high-performing MOFs. Based on the criteria, a screening procedure was then carried out, narrowing the candidate MOFs down to 2,408. The rigorous molecular simulation was again applied to assess the separation performance metrics of these MOFs, covering adsorption selectivity, working capacity, adsorbent performance score, regenerability and separation potential. By analysing the simulated performance indicators, top-performing MOFs were identified and a more detailed analysis of the structure-performance relationship was carried out. Additionally, the impact of several practical factors, covering humidity, operation temperature and inlet gas composition, on the performance of the selected adsorbents in practice was explored. It was observed that the robustness of MOFs at humidity depends on their hydrophilicity and preferential adsorption sites. Meanwhile, operation temperature can be manipulated to accommodate flexible production scenarios.

Surface properties of the ethanol/water mixture: Thickness and composition

Ethanol is a common amphiphilic solvent often used in conjunction with water. However, despite its widespread use, key questions regarding the thickness and composition of molecules at the ethanol/water/air surface remain unclear. Recent thermodynamic analyses, Bagheri and co-authors (2016) and Santos and Reis (2018), indicated that the interfacial thickness is not constant. However, the interfacial thickness from these two analyses follows opposite trends. This study aims to provide a detailed description of the thickness and composition of the interfacial layer by combining neutron reflectivity (NR) experiments with rigorous molecular simulation. The interfacial composition was determined from molecular simulations and, in conjunction with the Gibbs excess concentration, used to calculate the interfacial thickness. It was found that the thickness decreased exponentially and reached a plateau of ~ 8.2 Å. The results confirm the trends obtained thermodynamically from surface tension. The study also provides a new theoretical framework to describe the interfacial layer of water/alcohol mixtures.

Combining molecular dynamics and chemical process simulation: the SPEAD model

AbstractDiscontinuous molecular dynamics simulation and thermodynamic perturbation theory have been used to study the thermodynamic and transport properties of a large number of organic compounds. The fundamental basis of the approach relies on a stepwise characterization of the disperse interactions and blister potentials for hydrogen bonding. It has been demonstrated that these types of attractive interactions can be quantitatively treated by thermodynamic perturbation theory (TPT). The vapor pressure is predicted to roughly 10% average error with reduced temperatures generally extending to 0.45 while applying transferable characterizations of the molecular interaction potentials. For branched, aromatic, and naphthenic compounds, it is necessary to distinguish between primary, secondary, and tertiary bonding of the attached group as well as the branched site. One key to this accuracy is the distinction between isomers at the molecular level by the rigorous molecular dynamics simulation of the repulsive part of the potential. The molecular dynamics simulations also permit predictions of transport properties. The transport results account for the repulsive structure, branching, bond angles, etc. through rigorous molecular simulation but there is no theory for transport properties comparable to TPT. Therefore, predictions for transport properties are semiempirical in the sense that attractive forces have been correlated with experimental data. The accuracy for diffusivity is within 30% for non‐associative species. Disperse attractions appear to play a significant role in transport properties. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.