Synthesis optimization, adsorption properties and spectroscopic investigation of an NH2-tagged Cu-based MOF with open metal sites

We report the rapid microwave-assisted solvothermal synthesis of a Cu-MOF (metal-organic framework) with open metal sites, focusing on understanding its CO2 capture properties in relation to phase purity and stability. A combined experimental and theoretical approach is used to identify the MOF's structural features involved in the adsorption process. Specifically, Cu(i) defects are found to play an important role in the CO2 adsorption process, with the Cu-1 sample, synthesized using an optimized ligand/Cu precursor ratio for the highest phase purity, exhibiting more abundant Cu(i) defects as well as the highest adsorption capacity. Grand canonical Monte Carlo simulations show that the Cu(i) sites exhibit a greater affinity for CO2 adsorption compared to the Cu(ii) sites. In situ soft and hard X-ray absorption fine structure spectroscopic techniques confirm the conversion of Cu(i) to Cu(ii) upon CO2 chemisorption, with this conversion being more pronounced in the core of the particles. The simulations are used to estimate the fraction of Cu(i) defects and Cu(ii) sites present within the Cu-1 and to validate the experimental isotherm. Overall, this study provides insights into the CO2 capture properties of this type of Cu-MOF and highlights the importance of phase purity and the role of defects in achieving high adsorption performance.