Insights into the Coordination and Extraction of Yttrium(III) Ions with a Phenoxyacetic Acid Ionic-Liquid Extractant

Phenoxyacetic acid type extractants such as sec-octylphenoxyacetic acid (CA-12), sec-nonylphenoxyacetic acid (CA-100), and the resulting bifunctional ionic-liquid extractants were developed to separate yttrium from heavy rare-earth elements (REEs). To obtain information on the coordination chemistry of this type of bifunctional ionic-liquid extractant, the CA-12 analogue n-octylphenoxyacetic acid (HOCTPOA) was synthesized, and its structure was characterized by single-crystal X-ray diffraction analysis. Then, a bifunctional ionic-liquid extractant based on HOCTPOA, namely, [N-1888][OCTPOA] (N-1888 is methyltrioctylammonium) was synthesized. The study of the extraction of yttrium with [N-1888][OCTPOA] in chloride media was conducted by IR spectroscopy, logD versus logC slope analysis, ESI-MS, extended X-ray absorption fine structure (EXAFS), and DFT studies. The EXAFS data indicate that a Y-III extraction complex with phenoxyacetic acid formed. Three possible Y-III coordination cations were calculated by the DFT method, and the obtained Y-O bond lengths were comparable to the results from EXAFS fitting. A possible structure for the overall extracted complex was proposed, and this knowledge should contribute to a better understanding of the extraction process with phenoxyacetate-type bifunctional ionic liquids.