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 copoly-merization 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 mu m 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.