Yttrium separation by phosphorylcarboxylic acid and the underlying tetrad effect along lanthanide unveiled from different microscopic interactions

Due to similar physical and chemical properties, separating and purifying rare earth elements is challenging. Considering the environmental issue, the design and synthesis of efficient and green extractants are significant for this purpose. To this end, the underlying complexation between rare earth and the extractant should be clarified, and the long-historically existing tetrad effect along with the lanthanide family from La to Lu due to the different 4f orbital occupations should be figured out. Thus, within this study, taking our newly experimentally synthesized extractant, phosphorylcarboxylic acid, as an example and utilizing accurate quantum mechanical calculation, we comprehensively investigated the complexation behavior between rare earth and the extractant via the quantum theory of atoms in molecules (QTAIM), Mayer bond order (MBO), molecular orbital (MO) energy level, natural bond orbital (NBO), charge decomposition analysis (CDA), electron localization function (ELF) and extended transition state-natural orbitals for chemical valence (ETS-NOCV). It has been found that yttrium can be separated from the lanthanide series by the phosphorylcarboxylic acid, attributed to the poor ability of Y³⁺ to accept electrons and the weak orbital interactions compared to other rare earth cations. This paper also reveals the tetrad effect caused by the different occupations of 4f orbital electrons, providing a theoretical approach to comprehending the diverse extraction behaviors in the rare earth extraction process.