Guanidines are highly basic functional groups that are key to the function of various enzymes, medicines, toxins, disinfectants, and solvent extraction reagents among other uses. However, they are prone to both acid- and base-catalyzed hydrolysis to form ureas and amines. We hypothesized that steric hindrance close to the nitrogen atoms would raise the activation energy of nucleophilic attack at the central guanidine carbon atom, thereby increasing the guanidine longevity under stressing conditions. Experiments have been carried out under alkaline conditions to test this hypothesis, using NMR and GC analytical methods. Improved stability will specifically benefit the Next Generation Caustic-Side Solvent Extraction (NG-CSSX) process, which has remarkably high selectivity for the extraction of cesium from highly alkaline radioactive waste solutions. The solvent system requires the addition of an alkylguanidine, called a suppressor, to enable consistent and effective stripping of the cesium into a weakly acidic boric acid solution. Thus far, two alkylguanidines, N,N’-dicyclohexyl-N’’-isotridecylguanidine (DCiTG) and N,N’,N’’-tri(3,7-dimethyloctyl)guanidine (TiDG), have received the most attention for this role. However, DCiTG has undesirably low lipophilicity and washes out of the solvent, while TiDG is prone to degradation by alkaline hydrolysis. The loss of either alkylguanidine results in impaired stripping performance, eventually compromising the efficacy of the entire process. Here, new branched alkylguanidines with larger and sterically bulkier alkyl groups have been synthesized and found to be more stable potential replacements for DCiTG and TiDG. The ability of these alkylguanidines to maintain suppressor performance without adversely affecting phase-separation behavior and the stability of the alkylguanidines has been examined. The new alkylguanidines maintained the performance of DCiTG and TiDG, and as hypothesized, the bulkier alkyl groups were found to confer additional degradation resistance to the guanidine while also having greater lipophilicity. Therefore, these new alkylguanidines stand to improve the stability and reliability of the NG-CSSX process.

Development of more hydrolytically stable alkyl guanidines

ACS Fall 2021

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