Insights into adsorption and diffusion of CO2, CH4 and their mixture in MIL-101(Cr) via molecular simulation

Abstract

An in-depth comprehension of the adsorption and diffusion properties of CH4, CO2, and their mixture in MIL-101(Cr) is crucial for its application in the highly selective separation of CO2 and CH4 mixtures in biogas. Existing studies often overlook the influence of gas molecular radius and MIL-101 topological structure on adsorption and lack quantitative analysis of the primary and secondary adsorption sites of MIL-101. Additionally, there is a significant gap in research on the adsorption mechanism of gas mixtures. To address these gaps and to provide new knowledge, this study employs Monte Carlo and molecular dynamics simulations to evaluate various factors such as CH4 and CO2 adsorption isotherms, density distribution profiles, self-diffusion coefficients, radial distribution functions (RDF), and CO2/CH4 selectivity within MIL-101 metal-organic framework (MOF). The insightful and comprehensive atomic-level study reveals several unique and new findings, include 1) the RDF analysis shows that the C = C double bond of the benzene ring within MIL-101 primarily governs the adsorption of CH4, while the open metal center (Cr) exhibits significant adsorption effects on CO2; 2) the density distribution profile analysis indicates that CH4 molecules tend to localize in the larger and medium-sized cages, whereas CO2 molecules exhibit relatively uniform distribution. 3) evaluation of self-diffusivity coefficients and diffusion activation energies demonstrated that CH4 diffuses more easily than CO2, and in the CO2 and CH4 two-component gas, the existence of CH4 can further reduce the diffusion coefficient of CO2. This investigation provides a microscopic understanding of adsorption and diffusion phenomena within MIL-101, contributing to the overall comprehension of its behavior in gas separation applications.