An Ultramicroporous Metal–Organic Framework for CO₂ uptake and conversion

Capturing CO₂ from the atmosphere represents a key challenge, since CO₂ has been recognized as the primary anthropogenic greenhouse contributor to the increase in Earth’s average temperature. Their high porosity, tunable pore size, and large surface area make Metal–Organic Frameworks (MOFs) promising candidates to uptake and separate CO₂ from gaseous mixtures. In particular, the ultramicroporosity (pore size < 0,7 nm) and the presence of nitrogen atoms are crucial requirements in the design of MOFs for CO₂ separation. In 2021, some of us synthesized a new microporous MOF, formulated as [Co(trz₂An)]n·3H₂O (CAMOF1), by combining 3, 6-N-ditriazolyl-2,5-dihydroxy-1,4-benzoquinone (trz₂An), as an organic linker, with Co^II meta nodes in a 1:1 stoichiometric ratio. This MOF showed a high capability to separate CO₂ from natural gas. On this basis, since cobalt is classified as a critical raw material, by using Cu^II metal ions, a new isomorphous, robust, chemical, and thermally stable MOF, formulated as [Cu(trz₂An)]· 2.5H₂O (CAMOF2), was obtained. The synthesis performed by the hydrothermal approach was optimized in order to scale-up the reaction mixture by ten times. CAMOF2 is formed by cubic cavities with a void volume of 30 % due to the coordination to the N atom in the 4-position of the triazole ring, which induces an alternate orientation of Cu-anilate chains. Static and dynamic adsorption measurements revealed i) a remarkable carbon dioxide uptake, ii) a high selectivity in CO₂ separation in CO₂:N₂ gas mixtures, and iii) easy regeneration in mild conditions. Furthermore, preliminary CO₂ electroreduction studies show a good CAMOF2 capability to convert carbon dioxide to ethylene. In conclusion, by replacing the metal node in the MOF structure, a new microporous biocompatible multifunctional MOF has been obtained, with CO₂ adsorption and reduction capabilities.