Adsorption and molecular simulation of CO2 and CH4 in two-dimensional metal–organic frameworks with the same layered substrate

Abstract

Cu(1,3-BDC) (1,3-BDC = 1,3-benzenedicarboxylic acid) was found in the interpenetrated metal–organic frameworks [Cu(1,3-BDC)(H₂O)]·2H₂O and Cu(1,3-BDC)(PY)₂. We studied their CO₂, CH₄, and N₂ adsorption isotherms at high pressure (10 bar) and used density functional theory methods to locate their most stable configurations. We show that [Cu(1,3-BDC)(H₂O)]·2H₂O has a high adsorption selectivity of CO₂ over CH₄ because of the total lack of CH₄ adsorption. Although Cu(1,3-BDC)(PY)₂ has the same layered substrate as [Cu(1,3-BDC)(H₂O)]·2H₂O, except for the insertion of pyridine rings between layers, it adsorbs significantly more CH₄, leading to decreased CO₂/CH₄ adsorption selectivity and increased CH₄/N₂ adsorption selectivity. Our experimental and theoretical studies reveal that introducing suitable organic ligands provides more adsorption sites for CH₄, while the ligands' occupancy of the spaces between the layers prevents the spread of CO₂ molecules.