Enhanced carbon dioxide capture upon incorporation of N,N'-dimethylethylenediamine in the metal-organic framework CuBTTri

Abstract

High capacity, high selectivity, and low-cost regeneration conditions are the most important criteria by which new adsorbents for post-combustion carbon dioxide capture will be judged. The incorporation of N,N'-dimethylethylenediamine (mmen) into H₃[(Cu₄Cl)₃ (BTTri)₈ (CuBTTri; H₃BTTri = 1,3,5-tri(1H-1,2,3-triazol-4-yl)benzene), a water-stable, triazolate-bridged framework, is shown to drastically enhance CO₂ adsorption, resulting in one of the best performing metal-organic frameworks for CO₂ separation reported to date. High porosity was maintained despite stoichiometric attachment of mmen to the open metal sites of the framework, resulting in a BET surface area of 870 m² g⁻¹. At 25 °C under a 0.15 bar CO₂/0.75 bar N₂ mixture, mmen-CuBTTri adsorbs 2.38 mmol CO₂ g⁻¹ (9.5 wt%) with a selectivity of 327, as determined using Ideal Adsorbed Solution Theory (IAST). The high capacity and selectivity are consequences of the exceptionally large isosteric heat of CO₂ adsorption, calculated to be −96 kJ mol⁻¹ at zero coverage. Infrared spectra support chemisorption between amines and CO₂ as one of the primary mechanisms of uptake. Despite the large initial heat of adsorption, the CO₂ uptake was fully reversible and the framework could be easily regenerated at 60 °C, enabling a cycling time of just 27 min with no loss of capacity over the course of 72 adsorption/desorption cycles.