Dual-Force Zone Nonequilibrium Molecular Dynamics Simulations on Nanoporous Metal–Organic Framework Membranes for Separation of H₂/CH₄ Mixtures

Abstract

Recently, metal−organic frameworks (MOFs) have emerged as powerful nanoporous materials for selective separation of complex gas mixtures because of possible modifications of their structural characteristics. Herein, to investigate the effect of MOF functionalization on their gas separation performance at nanoscale, we have performed separation of H 2 /CH 4 mixtures through functionalized and unfunctionalized IRMOF-1 membranes. A molecular-level understanding is developed by employing a dual-force zone nonequilibrium molecular dynamics (DFZ-NEMD) simulation method. The DFZ-NEMD method was validated by performing H 2 /CH 4 separation through IRMOF-1 and comparing these results with experimental and other computational results reported in the literature. The linkers in different directions with respect to the gas flow were functionalized with acetamide, urea, and methyl groups to explore the effect of structural and chemical features of functional groups on gas separation performance. These results show that both the size and chemical nature of a functional group can play an important role in determining the size of the MOF nanopores and thus the selectivity and permeability of an MOF membrane. Moreover, the carbon atoms on the six-membered ring of linkers of IRMOF-1 were systematically functionalized with urea to understand the effect of the position of functional groups on the gas separation performance and mechanism. We show that by changing only this position, one could generate MOF pore entrances with a diverse density of functional groups, generating MOF membranes with different selectivities and permeabilities. Our gas separation results show that the change in this density of functional groups at the pore entrance can further result in different diffusion mechanisms, including Knudsen-type diffusion, adsorption−diffusion, and molecular sieving. Overall, molecular-level insights from the results of this research can pave a path for the synthesis of functionalized MOFs to control the separation of different gas molecules and hydrocarbons from their mixtures.

A review of recent advances and applications of machine learning in tribology

“…The perturbation of equilibrium may be necessitated by external environmental factors like force, temperature or pressure gradient, etc. 292 In the context of tribology, NEMD simulations, which involve applying a shear force to the simulated system, have also led to a better understanding of both dry friction, where the sliding surfaces are in close contact, and wet friction, where the surfaces are separated by a liquid lubricant. [293][294][295] A number of factors must be considered in order to perform NEMD simulations, including the inherent limitations of the time and length scales that can be modeled, as well as the accuracy of the representation of intramolecular and intermolecular interactions, similar to classical MD.…”

Metal–Organic Framework Membrane Constructor: A Tool for High-Throughput Construction of Metal–Organic Framework Membrane Models

“…18,49 The graphene layer is not always necessary in the NEMD systems, especially for those performed with specific methods, for instance, the concentration gradient−driven molecular dynamics 17 and dual-force zone NEMD methods. 19 The membrane model in the second system was built by using UiO-66. This material and its derivatives have been extensively used in studies reported 52−54 for fabricating MOF membranes in seawater purification applications.…”

“…This approach can reproduce surface effects on separation performance, such as the mass transfer resistance on membrane surfaces, and provide more realistic descriptions of membrane systems by examining the separation performance of membranes with different crystal orientations. However, only a few studies have been reported so far − that have used membrane models to investigate the gas separation of the MOF membranes. These studies are considerably limited when compared with the studies using the bulk models of MOFs, particularly in the field of high-throughput screening of MOF membranes .…”

Computational Simulation of CO2/CH4 Separation on a Three-Dimensional Cd-Based Metal–Organic Framework

“…MOFs are porous inorganic materials that have received much attention recently owing to their outstanding characteristics, such as significant surface area and porosity, thermal/chemical stability, and tunability. − MOFs are considered promising materials for a variety of potential applications across various fields, including gas storage and separation, chemical sensing, biomedical applications, adsorption, and heterogeneous catalysis. − They are considered as one of the most promising physical adsorbent materials in the process of separating CO 2 /CH 4 . The engineering design of MOFs for gas separation applications is currently a rapidly growing area of research. − Numerous experimental and simulation-based studies have been documented involving the separation of CO 2 from CH 4 through the use of MOFs. − Considering the quadrupole moment and polarizability of CO 2 , current research efforts to enhance CO 2 uptake and selectivity primarily involves strategies aimed at improving the interaction between CO 2 and the frameworks. These include, but are not limited to, the utilization of various open metal sites, the insertion of functional groups, , the development of smart adsorbents, and ligand shortening in MOFs. , An investigation showed that MOF-801­(Ce) displayed improved separation performance for CO 2 /N 2 and CO 2 /CH 4 compared to MOF-801­(Zr/Hf) .…”