Synthesis Of Open-chain Crown Ether Like Diphosphate And Their Application In Supercritical Carbon Dioxide

As a green technology, supercritical CO2chelating extraction (SCCE) of metal ions has obvious superiority to the traditional extraction methods. With its fast speed, high extraction efficiency and non-toxic properties, SCCE has great application prospects in the field of environmental protection, nuclear industry, hydrometallurgy, materials science, food and medicine. Chelating agents affect a lot in the SCCE process. Higher solubility in SCCO2and the ability to form more stable metal complex are the essential demands. It is difficult to satisfy the requirements of the high extraction efficiency and also the metal extraction selectivity using these traditional chelating agents, thus we designed a serious of open chain crown ether like diphosphate chelating agents which have good solubility in scCO2. The ligands’ structure effect on their solubility and their selectivity to the metals were studied through the extraction experiments and also the quantum chemical calculations. The mechanism was also considered and good separation results were obtained in the extractions of lanthanides, heavy metals and alkaline earth metals.These open chain crown ether like diphosphate chelating agent were synthesized and purified by silica column. The ligands were characterized by elemental analysis, IR, ESI-MS and1H NMR spectra to identify the structures. Also the reaction mechanism was discussed.The solubilities (cloud point pressure) of the ligands in scCO2were determined at313.15～333.15K and9-20MPa. The solubilities of these ligands were found to reach10’3orders of magnitude, which were in the same order of fluorine-containing chelating agents. These data were well correlated with Bartle model and Chrastil model and the fitting results are satisfactory with AARD of3.4%-5.2%and1.23%～2.15%respectively. Also the solubility order of the ligands was in good agreement with the solubility parameter calculated by the group contribution method. The molecular connectivity index0χ was calculated and correlated with solubility of the ligands and the correlation coefficient is0.97in the case of EG2TP, EG2TH and EG2Oct. The interaction between the chelating agent and CO2were calculated by COSMO-RS method and the order of EG2Oct≈EG2IPE<EG2TH<EG2EH<EG2TP was similar as their solubility order, indicating that the branched side chains played an important part of solubility increasement. The molecular molar volume of the ligands was estimated and the molecular cluster size reduced as the temperature of scCO2increased.The mechanism of the coordination was discussed by characterization of the metal complex. The Mulliken charge of monophosphate, diphosphate with different designed alky side chain and the ligands prepared in this article were all calculated by Gauss software. Also the interaction energy ΔE between the ligands and the metal ions were estimated and selectivity to metals was predicted in the case of EG2IPE to the lanthanide metal ions.The operation conditions of the SCCE were optimized (P=20MPa, T=313.15K, stirring for1h,10μL of water and10μL chelating agent added). In the extraction of lanthanide ions, all the chelating ligands exhibited higher metal ion extraction capability (except EG2Ben), and the extraction efficiency could reach60%～90%without any modifier. The extraction efficiency of these ligands was consistent with their solubility order in scCO2. Better solublity of the ligands resulted in higher extraction efficiency of the metals. The side chain of ligand could influence the extraction process by the steric hindrance effect. Small steric hindrance favors for extraction of small radius ions and large side chain volume gives a better masking effect of the central metal ion resulting more stable metal complex. Heavy metal ions of Pb2+, Cd2+and Hg2+could be well removed under optimal condition. The extraction efficiency could be89%of Pb2+,78%of Cd2+and75%of Hg2+. Alkaline earth metal ions Ca2+, Sr2+and Ba2+could be extracted with49%-74%,50%-73%and16%-64%, respectively.