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

Porous metal–organic framework [Zn(2)(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 m(2)·g(–1) and a pore volume of 0.37 cm(3)·g(–1). Extensive CO(2), N(2), O(2), CO, CH(4), C(2)H(2), C(2)H(4) and C(2)H(6) gas adsorption experiments at 273 K and 298 K were carried out, which revealed the great adsorption selectivity of C(2)H(6) over CH(4) (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 CO(2) adsorption selectivities were obtained for CO(2)/CH(4) (11.7) and CO(2)/N(2) (27.2 for CO(2):N(2) = 1:1, 56.4 for CO(2):N(2) = 15:85 gas mixtures). The computational DFT calculations revealed the decisive role of the sulfur-containing heterocycle moieties in the adsorption of CO(2) and C(2)H(6). High CO(2) adsorption selectivity values and a relatively low isosteric heat of CO(2) adsorption (31.4 kJ·mol(–1)) make the porous material 1 a promising candidate for practical separation of biogas as well as for CO(2) sequestration from flue gas or natural gas.