EMSP Project Book Revision (DOE/OSTI--EMSP-1(Rev.1)-CD

Supercritical fluid CO 2 is an alternative solvent for extraction of metals. The solubility parameter of supercritical CO 2 varies with density resembling that of liquid hexane at moderate pressures in the supercritical region to those of chlorinated solvents at very high pressures. By changing density of supercritical CO 2 , the solvation environment of a metal chelate system can vary continuously and resembles over a wide range of solvents. Thus, supercritical CO 2 provides a unique system for studying solvation effects on metal chelation. This project is designed to investigate the solvation effects on cesium complexation with macrocyclic compounds including crown ethers and calixarene-crown ethers in CO 2 from liquid to supercritical region at high pressures. A powerful spectroscopic technique for studying cesium chelation is nuclear magnetic resonance (NMR). Cesium has only one isotope, Cs, with a nuclear spin I = 7/2. Popov et al. used NMR to study cesium complexation with crown ethers and cryptand. Variation of the Cs chemical shift as a function of the 18-crown-6/Cs mole ratio in different solvents indicates that the solvent plays an extremely important role in the complexation process. The magnitude of the chemical shift upon cesium complexation with cryptand 222 (which has a tridimensional cavity) is much larger than with the flexible 18-crown-6. The NMR chemical shift for Cs inside a cavity may be correlated to the configuration of the complex and the tightness of the fit. Our collaborator, Clem Yonker at Pacific Northwest National Lab (PNNL), recently developed simple techniques for NMR studies in supercritical fluids. In Yonker’s method, a hand pump is used to deliver the desired high pressure and to transport the dissolved solute in a fused-silica tubing placed in the NMR probe. Because capillary tubing can stand very high pressures, this technique is safe and capable of performing NMR experiments up to several kilo-bars. Yonker’s technique will be used to start our NMR experiments (proton, F and Cs) for studying cesium-crown ether complexation in supercritical fluids. The solubility of crown ethers such as dicyclohexano-18-crown-6 is reasonably high in supercritical CO 2 , >1x10 mol/L according to our preliminary data. The solubility of calixarene-crowns in supercritical CO 2 is much smaller, <1x10 mol/L. It is known that fluorination of ligand can significantly increase the solubility of metal chelates in supercritical CO 2 . To enhance the solubility of calixarene-crowns in supercritical CO 2 , synthesis of fluorinated calixarene-crowns are currently underway in our laboratory. For simplicity, in the beginning of this project we chose to compare the complexation of cesium with crown ethers in conventional solvent systems and in supercritical CO 2 systems.