Smaller Amides for More Efficient Uranium Extraction

Amide extractants are promising CHON-based alternatives to tri-n-butyl phosphate for uranium recovery in nuclear fuel reprocessing due to their combustibility and improved radiolytic stability. We report a systematic structure-performance evaluation of 16 amide extractants to define correlations between structure, uranium loading, solvent viscosity, aggregation behavior, and degradation product solubility. Amide molecular weight (213–325 g/mol) and steric environment were tuned through variation of N,N-alkyl substitution and α-carbon branching. Uranium extraction was assessed using a direct dissolution approach in nitric acid media in the presence of representative fission product surrogates. Several amides formed neutral [(amide)2UO2(NO3)2] complexes with uranium loadings up to 0.60 M while maintaining low solution viscosity and avoiding third‑phase formation. Structure-property correlations indicate that optimal performance requires moderate molecular weight (213–283 g/mol), asymmetric N,N-substitution with a combined alkyl-chain length of 13 carbons or fewer, and together with limited steric crowding at the carbonyl oxygen atom to strike a balance between coordination strength and aggregation. Three candidates met these criteria and exhibited improved viscosity and phase stability relative to leading amide extractants, providing design principles and molecular-level guidance for the design of next-generation amide extractants for uranium reprocessing applications.