| The control of heavy metal pollution and the recycling of precious metals have increasingly become a hot issue in the world. Supercritical CO2(scCO?) is a promising alternative to the traditional organic solvents in the treatments of metals, due to its green features. However, the CO2-philic chelating ligands are necessary for scCO2extraction of metal ions. Up to now, it has been acknowledged that the fluorinated chelating ligands are quite superior to the non-fluorinated ones. Unfortunately, the high cost and environmental toxicity hampered their large-scale application. As a result, it is highly desirable to develop new efficient non-fluorinated chelating ligands for scCO2, but it is still a challenge job in this field. It could afford CO2-philic ligands by simply attaching CO2-philic non-fluorinated groups to the chelating heads, however, which generally exhibited low scCO2extraction efficiency of metal ions, because of much more emphasis on the CO2affinity of the free ligands in the designing process. Therefore, the new design methodology is quite necessary.A deeper insight into the impact of F-based groups on the interactions between the La((3-diketone)3chelates and CO2has been achieved by quantum chemistry and COSMO-RS calculations. It was clearly found that (1) the most negative F atoms can effectively disperse the trapped electrons in the highly polar chelation core, leading to the reduced polarity of the system;(2) the-CF3groups could enthapically enhance the CO2-philicity of metal chelates through the electrostatic interactions between the highly dipolar C-F bonds and the quadrupolar CO2. Subsequently, a new strategy for designing non-fluorinated ligands for scCO2was proposed by the combination of both concepts:(1) The positive charges on the central metal cation could be dispersed among the metal chelate system, avoiding highly-polar center;(2) The non-chelating donors could interact with CO2molecule, e.g. through the specific Lewis Acid (LA)-Lewis base (LB) interactions, energetically enhanced the CO2-philicity of the metal chelate. Accordingly, a series of new neutral organophosphorous open-chain crown ether analogues was designed,(MeO)(RO)P(O)-Xn-OP(O)(OR)(OMe), where the bridging chains X is-(OCH2CH2)n-or-(OCH(CH3)CH2)n-, n=2、3、4; R is the side chain of the phosphoryl group:n-Octyl,2-ethylhexyl and other long chains containing ether oxygen units. The COSMO-RS calculations suggested that the oxygenated bridging chains and the non-chelating O donors (P-O-C groups) in the new La(Ⅲ) chelate systems can provide the two favorable effects above, and subsequently the new ligands can enhance the CO2-philicity of metal chelates.The new series of chelating ligands were prepared by a modified method involving the catalyst,1H-tetrazole. POCl3, oligo(ethylene glcol)(or oligo(propylene glycol)), long-chained alcohol, methanol and triethylamine were employed as the reactants and toluene was as the reaction solvent. After three reactions steps, the crude products were separated and purified by silica gel column chromatography (acetone/dichloromethane). The final yields of the yellow oils were22-47%. The molecules were characterized by1H-NMR,13C-NMR, APCI-MS and element analysis.The cloud point pressures and solubilities of chelating ligands in SCCO2were determined by a static method at temperatures from (313.15to333.15) K and over a pressure range of (10to22) MPa. The solubilities of ligands were all high in the10-3orders of magnitude. The values calculated by the Bartle semi-empirical model exhibited good agreement with the experimental data. Additionally, the Henry’s constants (KH) of CO2in the new chelating ligands as molecule solvents (298.15K) were calculated by COSMO-RS method. The order of the1/KH was the same with that of the number (n) of oxygenated units in the bridging chains of oligo(ethylene glycol), namely that the smaller n was, the bigger1/KH was, the more CO2-philic the ligands were. Based on the results, COSMO-RS calculations were further performed for the linear molecules containing O, N or S atoms. In terms of the enhancement of the CO2-philicity, the S-based groups were more favorable than O-based groups, and the N-based group,-NO2, was more effective than the acetyl group. This was new important information for designing CO2-philic molecules.The extractability of the lanthanides (La3+, Gd3+and Yb3+) from the filter paper using scCO2and the new chelating ligands was preliminarily investigated at313K,20MPa by the static in-situ manner. These ligands exhibited high extraction efficiencies (close to80%) even without organic modifiers or fluorinated counter anions, which were much higher than that of tetraethylene glycol, tributyl phosphate and15-crown-5, encouragely, which reached the level of the fluorinated ones, e.g. the extraction efficiency of La3+by hexafluoroacetone (hfa) was70%under the condition:333K,15MPa,5%methanol by the static-dynamic coupling manner. In addition, these ligands showed different selectivities for lanthanide ions, e.g. X=-(OCH2CH2)n-, when n=2, the higher affinity for La3+; when n=3or4, the moderately higher selectivity for Gd3+. The results of quantum chemistry calculations and FT-IR spectroscopy analysis for the metal chelates demonstrated that if the perturbation effect for the pre-organized conformation of ligand caused by the metal ion was weaker, the ligand would present stronger affinity to the corresponding metal ion. |
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