Separation of CO₂ from CH₄ Using Mixed-Ligand Metal−Organic Frameworks

Abstract

The adsorption of CO₂ and CH₄ in a mixed-ligand metal−organic framework (MOF) Zn₂(NDC)₂(DPNI) [NDC = 2,6-naphthalenedicarboxylate, DPNI = <i>N</i>,<i>N′</i>-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydiimide] was investigated using volumetric adsorption measurements and grand canonical Monte Carlo (GCMC) simulations. The MOF was synthesized by two routes: first at 80 °C for two days with conventional heating, and second at 120 °C for 1 h using microwave heating. The two as-synthesized samples exhibit very similar powder X-ray diffraction patterns, but the evacuated samples show differences in nitrogen uptake. From the single-component CO₂ and CH₄ isotherms, mixture adsorption was predicted using the ideal adsorbed solution theory (IAST). The microwave sample shows a selectivity of ∼30 for CO₂ over CH₄, which is among the highest selectivities reported for this separation. The applicability of IAST to this system was demonstrated by performing GCMC simulations for both single-component and mixture adsorption.