Direct Extraction of Sodium Hydroxide by Calix[4]pyrrole-Based Ion-Pair Receptors.

Described in this work are calix[4]pyrrole-based ion-pair receptors, cis / trans - 1 and cis / trans - 2 , designed for the extraction of sodium hydroxide. An X-ray diffraction analysis of a single crystal of the cis - 1 ·NaOH isomer isolated from a mixture of cis / trans - 1 revealed a unique dimeric supramolecular structure. An average dimer in toluene- d 8 solution was inferred on the basis of diffusion-ordered spectroscopy (DOSY). Support for the proposed stoichiometry came from density functional theory (DFT) calculations. The structural stability of the dimeric cis - 1 ·NaOH complex in toluene solution was further confirmed by ab initio molecular dynamics (AIMD) simulation with explicit representation of solvent. Under conditions of liquid-liquid extraction (LLE), purified receptors cis - and trans - 2 were both found to remove NaOH from a pH 11.01 aqueous source phase into toluene with extraction efficiencies ( E %) of 50-60% when used equimolar to NaOH. However, in all cases, precipitation was observed. Complexities associated with precipitation could be avoided by immobilization of the receptors onto a chemically inert poly(styrene) resin by means of solvent impregnation. The use of solvent-impregnated resins (SIRs) eliminated precipitation in solution while retaining the extraction efficiency toward NaOH. This allowed both the pH and salinity of the alkaline source phase to be lowered.