Linear 2-Ethylhexyl Imidophosphoric Esters as Effective Rare-Earth Element Extractants

Abstract

Imidophosphoric organic esters containing phosphoryl groups are potential polydentate ligands and promising extractants of rare-earth elements. For their preparation, a monophosphazene salt [PCl3=N−PCl3]+[PCl6]− and short phosphazene oligomers of the general formula [Cl−(PCl2=N)n−PCl3]+[PCl6]−, where n = 4−7, were synthesized via living cationic polymerization of Cl3P=NSiMe3 and used as starting compounds. All phosphazenes were reacted with 2-ethylhexanol to obtain the corresponding esters of imidophosphoric acids (EIPAs). The formation of imidophosphoric acids occurs due to the phosphazene-phosphazane rearrangement of −P(OR)2=N− or −P(OH)(OR)=N− units, where R = 2-ethylhexyl. The prepared EIPAs were characterized by 1H, 31P NMR, and MALDI-TOF analyses and their extractive capacity towards lanthanide ions in aqueous solutions of nitric acid was examined. The EIPAs are mixtures of mono-, di-, and trifunctional compounds of the type HxA, where x = 1−3, which can form chelate complexes of lanthanide ions [Ln(A)z], where z = 3−6, depending on the chain length. The longer chain EIPAs are more suitable for collective rare-earth elements extraction. A comparison of the extraction properties of the EIPAs with the industrially used polyalkylphosphonitrilic acid (PAPNA) was drawn.

Keywords

• imidophosphoric acids
• phosphazenes
• extractants
• 2-ethylhexyl esters
• rare-earth elements
• living cationic polymerization