Bi-functional phosphonium poly(ionic liquid)s catalyzed CO₂-promoted hydration of ethylene oxide

The highly selective production of monoethylene glycol (MEG) from ethylene oxide (EO) is desirable at low H₂O/EO molar ratio, however, the development of an efficient heterogeneous catalyst for the reaction is a challenge. Herein, a series of bi-functional poly(ionic liquid)s (BF-PILs) were successfully synthesized by radical copolymerization of polymerizable phosphonium/ammonium ionic liquids, N-vinyl imidazole and cross-linker divinylbenzene (DVB). The structure and thermal stability of BF-PILs were characterized by NMR, FT-IR, elemental analysis and TGA. These BF-PILs could be swollen in the H₂O and MEG mixture and the swelling ratio (Q) increased with the increase of the MEG mole fraction. Under swelling in the H₂O and MEG mixture (1:1), BF-PILs presented an irregular channel structure with the diameter of 1–20 μm observed by the cryo-scanning electron microscope (cryo-SEM). BF-PILs were applied to catalyze the CO₂-promoted hydration of EO at a low H₂O/EO ratio of 1.5:1. The catalytic activities of phosphonium BF-PILs were superior to ammonium BF-PILs, and poly[(Ph)₃VBPBr-VIm] with the greatest Q in the H₂O and MEG mixture (1:1) gave highest yield (96.1%) and selectivity (97.3%) of MEG. The CO₂-promoted hydration of EO involved the cycloaddition of EO with CO₂ to produce ethylene carbonate (EC), and followed by the hydrolysis of EC to produce MEG. Kinetic experiments confirmed that phosphonium ionic liquid species mainly catalyzed CO₂ cycloaddition and imidazole catalyzed hydrolysis of EC. Furthermore, poly[(Ph)₃VBPBr-VIm] exhibited good generality and could be reused up to eight times without significant decrease of its initial activity.