Extraction Properties of Novel Polyethylene Oxide Diphosphate-based Chelating Agents towards Lanthanides and their Solubilities in Supercritical Carbon Dioxide

These CO2-philic open-chain organophosphorous chelating ligands, i.e. diethylene glycol bis (2, 4, 4-trimethyl-1-pentyl) dimethyl diphosphate (EG2TP), diethylene glycol bis (3, 5, 5-trimethyl-1-hexyl) dimethyl diphosphate (EG2TH), and diethylene glycol dioctyl dimethyl diphosphate (EG2Oct), ethylene glycol bis(2-isopropoxyethyl) dimethyl diphosphate (EG2IPE), triethylene glycol bis(2-isopropoxyethyl) dimethyl diphosphate (EG3IPE), tetraethylene glycol bis(2-isopropoxyethyl) dimethyl diphosphate (EG4IPE) were synthesized and characterized. Solubilities of these ligands in scCO2 were determined at different combinations of temperature (313.15-333.15 K) and pressure (9-20 Mpa), which increase in the order of EG4IPE (MW= 554.50) < EG3IPE (MW= 510.45) < EG2Oct (MW= 518.56) < EG2IPE (MW= 466.40) < EG2TH (MW= 546.61) < EG2TP (MW= 518.56). Solubility parameter and connectivity index of these compounds were calculated and discussed, which indicated that branched side chains of the ligands play an important part in increasing solubility in scCO2. These experimental data are in agreement with computed data via a semi empirical model, in which the average absolute relative deviations lie in the range of 2.3% to 5.0%. Also the quantum chemical calculation and COSMO-RS method were introduced to estimate the structure effect on the interaction with CO2 by computing the charge density distribution on the surface of these compounds. As expected, better masking effect of the side chains results in higher solubility. The effect of these ligands on supercritical fluid extraction of selected rare earth metals (La, Ce, Pr, Nd, Sm, Gd, Er, and Yb) was investigated at 313.15 K and 20 Mpa. The extraction efficiency of the system was found to increase in the order EG4IPE < EG3IPE < EG2IPE < EG2Oct < EG2TP < EG2TH with a range from 55% to 92%. The rationale behind different selectivities towards these metals was also discussed in comparison to other traditional organophosphorous agents.