Abstract
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.
Highlights
Carbon dioxide (CO2) release poses one of the biggest anthropogenic impacts to the environment
The main strategies for the incorporation of CO2 binding centers into metal−organic frameworks (MOFs) structures have been via the incorporation of basic centers such as amines functioning as Lewis bases[11−15] and adsorption at coordinatively unsaturated metal cations as Lewis acid sites.[16−21] The former strategy is a development of the traditional approach of CO2 capture by amines to form carbamates and shows high uptake and very good adsorption selectivity even under humid conditions and, significantly, increases the energy cost for regeneration of the adsorbate
We demonstrate that the incorporation of thiophene moieties with polarizable sulfur heteroatoms, capable of induced dipole−dipole interactions, results in a remarkable increase of the CO2 binding affinity of the microporous MOF [Zn2(tdc)2dabco] (H2tdc = thiophene-2,5-dicarboxylic acid; dabco = 1,4-diazabicyclooctane)
Summary
Carbon dioxide (CO2) release poses one of the biggest anthropogenic impacts to the environment. The main strategies for the incorporation of CO2 binding centers into MOF structures have been via the incorporation of basic centers such as amines functioning as Lewis bases[11−15] and adsorption at coordinatively unsaturated metal cations as Lewis acid sites.[16−21] The former strategy is a development of the traditional approach of CO2 capture by amines to form carbamates and shows high uptake and very good adsorption selectivity even under humid conditions and, significantly, increases the energy cost for regeneration of the adsorbate.
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