Recovery of Propylene Glycol from Dilute Aqueous Solutions by Complexation with Organoboronates in Ion-Pair Extractants

Recovery of propylene glycol (1,2-PD) from aqueous solution was studied in batch experiments using extractants consisting of ion pairs of Aliquat 336 and phenylboronate in 2-ethylhexanol, toluene, o-xylene, or diisobutyl ketone. The heterogeneous complexation constant β11 calculated from the results at 25 °C was highest in 2-ethylhexanol (49−100 (mol/L)-1). The equilibrium water concentration in the extractants was 8−12 wt % and decreased with increasing uptake of 1,2-PD. Nearly all extractant/diluent systems exhibited stoichiometric overloading. Evidence for aggregation of the ion-pair extractant in the organic phase was found from water solubilization studies and 1H NMR spectroscopy studies. The complexation constant decreased with increasing temperature. Up to 80% of the extracted 1,2-PD was backextracted into water after acidification with CO2. The extractant could then be regenerated by stripping CO2 from solution at temperatures exceeding 110 °C. However, at these temperatures the color of the extractant changes, and the extraction capacity is reduced to about 60% of its original value. Regeneration by contacting with aqueous solutions of Na2CO3 did not cause extractant degradation; regeneration effectiveness increased with increasing pH.