Synthesis of a cross phosphonic acid type calix[4]arene with two different spacers and its extractive separation of rare earth metals

Abstract

A phosphonic acid derivative of calix[4]arene in a cone configuration with two different alkylphosphonic acid substituents positioned at distal positions has been synthesized and used to investigate the extraction behavior towards rare earth metals. The new reagent exhibited both heavy rare earth metal extraction selectivity and very high extraction ability that is comparable to the separation efficiency of the commercial phosphonic acid extraction reagent, 2-ethylhexyl hydrogen 2-ethylhexylphosphonate, towards rare earth metals. The latter reagent is currently used as the most effective means for separating the rare earths. The high extraction and separation efficiency are largely attributed to the convergence of the functional groups in the above calix[4]arene derivative providing a preorganized metal binding site that reflects both the cyclic structure and the cone configuration of the calix[4]arene backbone. Any loss of “binding complementarity” due to the presence of different spacer groups connecting the two phosphonic acid groups to the calix[4]arene backbone is proposed to have been compensated by the operation of a chelate effect giving rise to high extraction ability being maintained. Extraction equilibrium constants and separation factors were estimated based on the extraction data. The stripping of the loaded rare earth metal ions with hydrochloric acid was also investigated.