Lawrence Berkeley National Laboratory

The complex [Zn₂(tdc)₂dabco] (H₂tdc = thiophene-2,5-dicarboxylic acid; dabco = 1,4-diazabicyclooctane) shows a remarkable increase in carbon dioxide (CO₂) uptake and CO₂/dinitrogen (N₂) selectivity compared to the nonthiophene analogue [Zn₂(bdc)₂dabco] (H₂bdc = benzene-1,4-dicarboxylic acid; terephthalic acid). CO₂ adsorption at 1 bar for [Zn₂(tdc)₂dabco] is 67.4 cm³·g^-1 (13.2 wt %) at 298 K and 153 cm³·g^-1 (30.0 wt %) at 273 K. For [Zn₂(bdc)₂dabco], the equivalent values are 46 cm³·g^-1 (9.0 wt %) and 122 cm³·g^-1 (23.9 wt %), respectively. The isosteric heat of adsorption for CO₂ in [Zn₂(tdc)₂dabco] 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 CO₂/N₂ gas mixtures has been confirmed by both ideal adsorbate solution theory calculations and dynamic breakthrough experiments. The preferred binding sites of adsorbed CO₂ in [Zn₂(tdc)₂dabco] have been unambiguously determined by in situ single-crystal diffraction studies on CO₂-loaded [Zn₂(tdc)₂dabco], coupled with quantum-chemical calculations. These studies unveil the role of the thiophene moieties in the specific CO₂ binding via an induced dipole interaction between CO₂ and the sulfur center, confirming that an enhanced CO₂ capacity in [Zn₂(tdc)₂dabco] 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.