Interaction-determined extraction capacity between rare earth ions and extractants: taking lanthanum and lutetium as models through theoretical calculations

Extractants play an important role in the separation and purification of rare earth elements (REEs), and extraction performance is the most effective tactic to evaluate whether an extractant shows complete involvement and fast action in the separation of REEs. In this paper, the complexation differences between lanthanum/lutetium (La/Lu) and four phosphoric acid extractants (C272, P227, P507 and P204) with very similar structures but different extraction abilities are analyzed by theoretical calculations for the first time. The results show that REEs are coordinated with two oxygen atoms in each dimer extractant to form three octatomic ring complexes. La is more likely to be complexed in the form of eight water molecules in aqueous solution, while Lu takes action with the extractant in the form of pure ions. Furthermore, La mainly hybridized with coordinated oxygen atoms via 4f, 5d and 6p orbitals, while Lu hybridized via 6s and 5d orbitals, and the difference between La and four extractants originates from 5d and 6p electrons while it mainly comes from the 6s orbital for Lu. It is really remarkable that the number of electrons transferred from La/Lu coordination oxygen atoms to REEs decreased in the order of P204 > P507 > P227 > C272, indicating that the extraction ability of P204 is the strongest, which is consistent with the experimental results. Based on this result, an extractant containing NH-substituted oxygen (HEHAEP) with stronger extraction capacity than P204 was designed. Our theoretical study laid a steadfast foundation for the design of extractants, and opened up a new way for the experimental synthesis of new extractants.