来源：ACS Publications
Separation of Rare Earth Elements (REEs) remains an open challenge for their closely related coordination chemistry and hydration. Neodymium (Nd) and yttrium (Y) form an exemplary pair for this open problem, since Nd is a priority target for recovery from end-of-life magnets while yttrium closely mimics the ionic radius and coordination behavior of heavy rare earth elements, which are common cocontaminants of Nd. Membrane-based separation has been recognized as an efficient and eco-friendly alternative to the conventional solvent extraction procedures. In this context, the development of novel membrane coatings with specific selectivity represents a fundamental challenge that encourages the search for nanostructures equipped with tailored binding sites. In this work, a Density Functional Theory study was conducted to elucidate the adsorption behavior of Y3+ and Nd3+ ions on citrate-functionalized carbon nanotube (CNT-Cit), with the aim of evaluating the potential of this functionalized nanostructure for selective membrane coatings. Adsorption, binding, and deformation energies, in addition to adsorption free energies, were systematically evaluated to quantify the adsorption and stability of metal complexes as a function of the binding topology. The study establishes a structure–energy relationship governing the REEs binding on the citrate-functionalized CNT. The work shows that the adsorption strength and coordination geometry are strongly dependent on the metal ion and functionalization topology. In particular, adsorption at the CNT sidewall site, involving the central carboxylate of citrate, is predicted to be exergonic for Nd and endergonic for Y, whereas adsorption at the CNT tip site was found to be too strong for ion transport. The exergonic and endergonic adsorption of Nd and Y on the citrate functional group at the CNT sidewall suggests that CNT-Cit will be selective for neodymium over yttrium. Although the present study does not directly simulate site-to-site hopping barriers or fluxes in assembled CNT networks, it provides molecular-level guidelines for the design of selective CNT-based networks to be used as membrane coatings for REE recovery.