A Scalar Relativistic DFT Study on the Complexation Behavior and Selective Separation of Th(IV)/U(VI) With Four Amide Ligands Containing N-Heterocycles

Abstract

Achieving efficient separation of thorium and uranium is one of the main technical challenges for the nuclear fuel cycle of thorium-based molten salt reactors. In this work, the scalar relativistic density functional theory (DFT) calculations were adopted to deeply unravel the complexation behaviors and separation selectivity of Th⁴⁺/UO₂²⁺ with four amide ligands L_A, L_B, L_C, and L_D containing N-heterocycles. The analyses of dihedral angle, bond length, WBIs, IGMH, and ETS-NOCV indicated that the four ligands were more likely to coordinate with Th⁴⁺ in gas phase and that the interactions between metal ions and carbonyl oxygens in complexation were stronger than those between metal ions and pyridine nitrogen atoms. The QTAIM analysis showed that the metal-O/N bonds were typical coordination bonds, and the EDA revealed that the electrostatic and orbital interactions provided major contributions to attractive interaction. The thermodynamic study exhibited that L_C and L_D had stronger complexation ability than L_A and L_B and that the separation selectivity of the four ligands for Th⁴⁺/UO₂²⁺ was closely related to the solvent types. It was easier for the ligands to bind with Th⁴⁺ in solvents with hydroxyl (as water, 1-butanol,1-shexanol, 1-octanol). However, the ligands were preferential to bind with UO₂²⁺ in solvents without hydroxyl group (as nitrobenzene, acetone, cyclohexanone, cyclohexanone, and methyl benzoate). The selectivity coefficients of these ligands for Th⁴⁺/UO₂²⁺were generally over 98%. This work not only provides theoretical insights for Th (IV)/U(VI) separation based on PBIMO ligands but also offers an effective strategy for nuclear fuel cycle of thorium-based molten salt reactors.