Enhancing the Adsorption Sites of NH2-MIL-125 via a Copper Modification Strategy for Improved CO2 Capture and CO2/N2 Separation

Abstract Developing efficient adsorbents for CO2 capture and CO2/N2 separation is of great significance for mitigating greenhouse gas emissions. In this study, NH2-MIL-125 was modified with Cu2+ using different copper precursors, including Cu(NO3)2, CuSO4, Cu(CH3COO)2, and CuCl2, to regulate the adsorption sites and improve the CO2 affinity of the framework. When synthesized using CuCl2 as the precursor, CuC-NH2-MIL-125 exhibited an enhanced CO2 adsorption capacity of 6.36 mmol g–1 at 273 K and 1 bar, representing a 58% improvement over pristine NH2-MIL-125 (4.01 mmol g–1). The CO2/N2 selectivity for this new material is 22.4 at 273 K, and it could also maintain stable adsorption performance over 20 consecutive cycles. In contrast, the use of Cu(NO3)2 and CuSO4 as the precursor significantly compromised the capture capacity of CO2, indicating that the copper precursor plays a critical role in determining the pore environment and accessible adsorption sites. Further analysis using different contents of CuCl2 was conducted, and theoretical calculations reveal that the Cu ion is anchored to the −NH2 groups while preserving the microporous framework and the specific surface area of CuC-NH2-MIL-125 was further increased. The Cu2+ coordination modifies the electronic environment of neighboring amino groups, which collectively enhance the material’s affinity for CO2. These findings highlight the decisive role of copper precursor selection in governing the CO2 capture performance of Cu-modified NH2-MIL-125 and identify CuCl2 derived CuC-NH2-MIL-125 as a promising adsorbent for practical CO2/N2 separation.