CO2 capture in a carbazole-based supramolecular polyhedron structure: the significance of Cu(ii) open metal sites

Abstract

A crystalline, porous Metal–Organic Polyhedron (MOP) 4 was readily assembled starting from 9H-carbazole-3,6-dicarboxylic acid, which was synthesised in only three steps. The single crystal X-ray diffraction analysis of the resulting supramolecule evidenced voids that were occupied by coordinated as well as uncoordinated solvent molecules from the solvothermal reaction. Careful analysis of its desolvation process, activating the sample at 150 °C and 10⁻⁵ bar (followed by FTIR), evidenced that all solvents were completely removed, leaving an empty but nevertheless crystalline MOP with coordinatively-unsaturated Cu(II) sites that was studied for gas adsorption using N₂ at 77 K, and CO₂ at 196 K. Notably, carbon dioxide was found to be strongly adsorbed in the activated 4 (12.1 mmol g⁻¹, 53.2 wt%) with a molar enthalpy of adsorption of ΔH = 65.12 kJ mol⁻¹, revealing a great selectivity over nitrogen, which in turn showed negligible uptake under the same activation conditions. In situ Raman experiments demonstrated the direct interaction of CO₂ molecules and coordinatively-unsaturated Cu(II) sites within 4.