Enhancement of CO Uptake and Selectivity in a Metal-Organic Framework by the Incorporation of Thiophene Functionality.

The complex [Zn(tdc)dabco] (Htdc = thiophene-2,5-dicarboxylic acid; dabco = 1,4-diazabicyclooctane) shows a remarkable increase in carbon dioxide (CO) uptake and CO/dinitrogen (N) selectivity compared to the nonthiophene analogue [Zn(bdc)dabco] (Hbdc = benzene-1,4-dicarboxylic acid; terephthalic acid). CO adsorption at 1 bar for [Zn(tdc)dabco] is 67.4 cm·g (13.2 wt %) at 298 K and 153 cm·g (30.0 wt %) at 273 K. For [Zn(bdc)dabco], the equivalent values are 46 cm·g (9.0 wt %) and 122 cm·g (23.9 wt %), respectively. The isosteric heat of adsorption for CO in [Zn(tdc)dabco] at zero coverage is low (23.65 kJ·mol), ensuring facile regeneration of the porous material. Enhancement by the thiophene group on the separation of CO/N gas mixtures has been confirmed by both ideal adsorbate solution theory calculations and dynamic breakthrough experiments. The preferred binding sites of adsorbed CO in [Zn(tdc)dabco] have been unambiguously determined by in situ single-crystal diffraction studies on CO-loaded [Zn(tdc)dabco], coupled with quantum-chemical calculations. These studies unveil the role of the thiophene moieties in the specific CO binding via an induced dipole interaction between CO and the sulfur center, confirming that an enhanced CO capacity in [Zn(tdc)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.