Synergistic Al/Fe bimetallic coordination in MOFs drives high-efficiency lithium adsorption and isotope separation: structural regulation and mechanistic insights

The rational design of high-performance adsorbents for lithium recovery from aqueous resources depends critically on understanding how metal doping modulates the structure and properties of metal-organic frameworks (MOFs). In this work, we systematically investigate the effects of Fe doping on the lithium adsorption and isotope separation performance of Al-MOFs through controlled synthesis of bimetallic Al/Fe-MOFs with varying metal ratios (1:4 to 4:1). The materials were characterized by XRD, XPS, TGA, and adsorption experiments, complemented by density functional theory calculations. Results show that optimal Fe incorporation (Al/Fe = 2:1) significantly improves Li + adsorption capacity (24.07 mg g −1 ) and isotopic selectivity ( α  = 1.033) compared to the undoped MOFs ( Q  = 12.97 mg g −1 , α  = 1.027). Structural analyses reveal Al/Fe-MOFs contraction (Δ d (002)  = 0.36 Å) and surface charge modification (pH ZPC shift from 10.26 to 8.73), while electronic structure studies demonstrate Fe 3+ /Fe 2+ redox activity promotes electron transfer. DFT calculations identify greatly increased adsorption energetics (−3.96 eV) through synergistic Al O/Fe O interactions, where the rigid Al O framework ensures structural stability and flexible Fe O sites support dynamic adsorption. The material exhibits excellent cycling stability (92 % capacity retention) due to dual stabilization mechanisms. These findings confirm that balanced bimetallic coordination in MOFs can enhance adsorption capacity and selectivity through geometric and electronic effects, providing valuable insights for designing advanced adsorbents for lithium recovery.