Enhancement of CO₂ Uptake and Selectivity in a Metal–Organic Framework by the Incorporation of Thiophene Functionality

High Resolution Image Download MS PowerPoint Slide The complex [Zn 2 (tdc) 2 dabco] (H 2 tdc = thiophene-2,5-dicarboxylic acid; dabco = 1,4-diazabicyclooctane) shows a remarkable increase in carbon dioxide (CO 2 ) uptake and CO 2 /dinitrogen (N 2 ) selectivity compared to the nonthiophene analogue [Zn 2 (bdc) 2 dabco] (H 2 bdc = benzene-1,4-dicarboxylic acid; terephthalic acid). CO 2 adsorption at 1 bar for [Zn 2 (tdc) 2 dabco] is 67.4 cm 3 ·g –1 (13.2 wt %) at 298 K and 153 cm 3 ·g –1 (30.0 wt %) at 273 K. For [Zn 2 (bdc) 2 dabco], the equivalent values are 46 cm 3 ·g –1 (9.0 wt %) and 122 cm 3 ·g –1 (23.9 wt %), respectively. The isosteric heat of adsorption for CO 2 in [Zn 2 (tdc) 2 dabco] at zero coverage is low (23.65 kJ·mol –1 ), ensuring facile regeneration of the porous material. Enhancement by the thiophene group on the separation of CO 2 /N 2 gas mixtures has been confirmed by both ideal adsorbate solution theory calculations and dynamic breakthrough experiments. The preferred binding sites of adsorbed CO 2 in [Zn 2 (tdc) 2 dabco] have been unambiguously determined by in situ single-crystal diffraction studies on CO 2 -loaded [Zn 2 (tdc) 2 dabco], coupled with quantum-chemical calculations. These studies unveil the role of the thiophene moieties in the specific CO 2 binding via an induced dipole interaction between CO 2 and the sulfur center, confirming that an enhanced CO 2 capacity in [Zn 2 (tdc) 2 dabco] is achieved without the presence of open metal sites. The experimental data and theoretical insight suggest a viable strategy for improvement of the adsorption properties of already known materials through the incorporation of sulfur-based heterocycles within their porous structures.