Pathway modulation and substrate partitioning for CO2-assisted hydration of ethylene oxide catalyzed by swelling poly(ionic liquid)s

Abstract

Amine-functionalized swellable poly(ionic liquid)s (poly[BAVIm]Br-DVIm-n%) were synthesized by free radical copolymerization of amine-functional vinyl imidazolium ionic liquids and ionic cross-linker 1,8-triethylene glycoldiyl-3,3́-divinylimidazolium (DVIm). Poly[BAVIm]Br-DVIm-n% exhibited excellent swelling ability in the water, ethylene carbonate (EC), and ethylene glycol (MEG), and the swelling ratio (Q) decreased with the increase of the content of the crosslinker. The swollen state of poly[BAVIm]Br-DVIm-2.5% in water observed by cryogenic scanning electron microscopy (cryo-SEM) presented densely packed and interconnected pores (0.2–11 μm). Poly[BAVIm]Br-DVIm-n% were used to catalyze CO2-assisted hydration of ethylene oxide (EO) at a low H2O/EO ratio of 1:1, in which poly[BAVIm]Br-DVIm-2.5% with the highest Q in water gave the highest yield (87%) and selectivity (97%) of MEG. Moreover, EC was produced and subsequently disappeared during the process of CO2-assisted hydration of EO. A cascade reaction including the cycloaddition of CO2 with EO (EO + CO2 → EC) and the hydrolysis of the resulting EC (EC + H2O → MEG + CO2) in situ was elaborated. Poly[BAVIm]Br-DVIm-n% containing amine group and imidazolium ionic liquid synergistically catalyzed this cascade reaction, exhibiting a bifunctional effect. Poly[BAVIm]Br-DVIm-2.5% with the H2O, EC, and MEG enrichment ability in H2O/EO, EC/EO, and MEG/EO could spatially isolate EO and H2O, EC, MEG. The substrate partitioning of poly[BAVIm]Br-DVIm-2.5% could promote the two sequential reactions to be orthogonal and inhibit the side-reactions between MEG and EO, thus resulting in a higher reaction rate of poly[BAVIm]Br-DVIm-2.5% (0.0141 h−1) than that of homogeneous catalyst ([BAVIm]Br) (0.0074 h−1). Furthermore, poly[BAVIm]Br-DVIm-n% had good generality and could be reused effectively at least seven times without a significant decrease of its initial activity.