Synthesis and Properties of Calix[4]arene‐bis[4‐(2‐ethylhexyl)benzo‐crown‐6], A Cesium Extractant with Improved Solubility

Abstract

The new cesium‐selective macrocycle calix[4]arene‐bis[4‐(2‐ethylhexyl) benzo‐crown‐6] (“BEHBCalixC6”) has been found to have improved solubility in a modified alkane diluent over calix[4]arene‐bis(4‐tert‐octylbenzo‐crown‐6) (“BOBCalixC6”). The synthesis of this new calixcrown extractant, together with its solubility and extraction properties, is described in this report. The solubility, cesium extraction, and potassium loading behavior are compared with those of BOBCalixC6, the macrocycle currently employed in the caustic‐side solvent extraction (CSSX) process for extracting cesium from alkaline nitrate solutions such as high‐level nuclear waste. Replacement of the tert‐octyl alkyl chains on the benzo‐crown portion of the calixcrown by 2‐ethylhexyl chains improves the equilibrium solubility of the free calixcrown in aliphatic diluents while not affecting the cesium extraction strength. Equilibrium concentrations of BEHBCalixC6 in Isopar L diluent modified with 0.5 M 1‐(2,2,3,3‐tetrafluoropropoxy)‐3‐(4‐sec‐butylphenoxy)‐2‐propanol (Cs‐7SB) over the course of 41 weeks of gentle agitation at 25°C maintained the initial concentration of 57 ± 2 mM, with no evidence of precipitation or third‐phase formation. In contrast, 57 ± 2 mM solutions of BOBCalixC6 prepared using sonication were not stable, with precipitation occurring to afford an equilibrium concentration of only 6.7 ± 0.2 mM after 41 weeks. Third‐phase tests showed that BEHBCalixC6 resists third‐phase formation as well as, or better than, BOBCalixC6. With increasing potassium loading from an alkaline sodium nitrate solution, both calixcrowns at 20 mM in modified Isopar L exhibited comparable onset of third‐phase formation, gauging by the decreasing mass balance in cesium extraction from the same solutions. However, the BEHBCalixC6 solvent exhibited no visible indications of third‐phase formation until the aqueous potassium concentration reached 0.75 M, whereas the BOBCalixC6 suffered visible third‐phase formation when the aqueous potassium nitrate concentration reached 0.25 M. Calixcrowns with higher solubility in alkane diluents may find application in cesium extraction from waste matrices containing moderate (≥0.2 M) quantities of potassium, such as that stored at the US Department of Energy's Hanford Site, where higher concentrations of the calixcrown extractant may be required to obtain practical cesium distribution coefficients for process flowsheets.