CO₂ + ionic liquid biphasic system for reaction/product separation in the synthesis of cyclic carbonates

The possibility of using supercritical CO₂ (scCO₂) technology as a second step for extracting propylene carbonate produced from the coupling reaction between propylene oxide and CO₂, was investigated. The coupling reaction was performed under biphasic gas-liquid conditions, using a zinc(II) complex of arylhydrazones of β-diketones (Zn(II)-AHBD) as metal catalyst, combined with tetrabutylammonium bromide (TBABr). 1-Ethyl-3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]) and methyltrioctylammonium chloride ([ALIQUAT][Cl]) were explored as reaction solvents, in order to efficiently solubilize the catalyst and retain the catalyst inside the reactor during the product extraction step. Results obtained were compared with other common used solvents as methyl ethyl ketone (MEK), ethyl lactate (EL), and polyethylene glycol 400 (PEG 400). [ALIQUAT][Cl] was selected as the most promising solvent and scCO₂ extraction was effectively applied as a second step to isolate propylene carbonate from the catalyst system. Following this strategy, the catalyst system was reused three times, without loss of activity.