Aqueous Grafting Ionic Liquid on Graphene Oxide for CO₂ Cycloaddition

Abstract

In this study, a facile and environment-friendly approach was developed to immobilize water-soluble imidazolium- and pyridinium-based ionic liquids (ILs), i.e. [SmIm]X (X = Cl and I) and [SPy]I, on graphene oxide (GO) sheets in aqueous medium by controlling the grafting temperature. Several characterizations including OEA, FT-IR and XPS techniques have been applied to investigate the effects of the grafting temperature on the preparation for GO-ILs in aqueous medium. As-prepared composites were employed as heterogeneous catalysts for CO₂ cycloaddition. Results showed that the amount of [SmIm]I on GO sheets reached a high level (about 1.7 mmol/g GO) under the mild temperature (i.e. 40–60 °C), owing to the inhibition of the self-condensations among ILs molecules in water under the mild temperature. The resulting composites exhibited excellent catalytic activity to CO₂ cycloaddition, affording the maximum conversion of propylene oxide (PO) as about 93% in 4 h, much higher than that on [SmIm]I. It could be explained that the abundant hydrogen bonding donor (hydroxyl groups) on GO sheets assisted in the ring opening of PO, which promoted the reaction. These heterogeneous catalysts could be reused for at least five runs without significant loss in activity, implying the stable and reusable capability of as-prepared catalysts. The aqueous grafting mechanism was proposed here, which went through a two-step reaction. ILs with silane coupling agent are firstly hydrolyzed to silanol in water, and grafting reaction then happens by removing H₂O molecules under the mild temperature due to the low activation energy for dehydration. It differs greatly from the traditional grafting mechanism in organic solvents, which involves the direct dealcoholization under the high temperature because of the high activation energy for the reaction.

Graphical Abstract

Water-soluble [SmIm]I were efficiently grafted on GO sheets in water under the mild temperature (30–60 °C); while the predominant self-condesation happened among [SmIm]I molecules under the high temperature (beyond 60 °C) in comparison.