Enhanced Adsorption Selectivity of Carbon Dioxide and Ethane on Porous Metal–Organic Framework Functionalized by a Sulfur-Rich Heterocycle

Porous metal–organic framework [Zn₂(ttdc)₂(bpy)] (1) based on thieno [3,2-b]thiophenedicarboxylate (ttdc) was synthesized and characterized. The structure contains intersected zig-zag channels with an average aperture of 4 × 6 Å and a 49% (v/v) guest-accessible pore volume. Gas adsorption studies confirmed the microporous nature of 1 with a specific surface area (BET model) of 952 m²·g^–1 and a pore volume of 0.37 cm³·g^–1. Extensive CO₂, N₂, O₂, CO, CH₄, C₂H₂, C₂H₄ and C₂H₆ gas adsorption experiments at 273 K and 298 K were carried out, which revealed the great adsorption selectivity of C₂H₆ over CH₄ (IAST selectivity factor 14.8 at 298 K). The sulfur-rich ligands and double framework interpenetration in 1 result in a dense decoration of the inner surface by thiophene heterocyclic moieties, which are known to be effective secondary adsorption sites for carbon dioxide. As a result, remarkable CO₂ adsorption selectivities were obtained for CO₂/CH₄ (11.7) and CO₂/N₂ (27.2 for CO₂:N₂ = 1:1, 56.4 for CO₂:N₂ = 15:85 gas mixtures). The computational DFT calculations revealed the decisive role of the sulfur-containing heterocycle moieties in the adsorption of CO₂ and C₂H₆. High CO₂ adsorption selectivity values and a relatively low isosteric heat of CO₂ adsorption (31.4 kJ·mol^–1) make the porous material 1 a promising candidate for practical separation of biogas as well as for CO₂ sequestration from flue gas or natural gas.