Multi-functionalization of GO with multi-cationic ILs as high efficient metal-free catalyst for CO2 cycloaddition under mild conditions

To optimize the synergistic effects among hydrogen bond donors, halogen anions and basic sites, we synthesized a series of graphene oxide (GO) materials multi-functionalized with silanol group, multi-cationic quaternary ammonium salt and tertiary amine in a one-pot approach. Hexamethylenetetramine which is cheap and rich with tertiary amine was used as precursor. As characterized by elemental analysis and X-ray photoelectron spectroscopy, the loading of quaternary ammonium salt and tertiary amine was up to 2.56 and 1.84 mmol/g, respectively. This is the first time that a multi-functionalized GO (MF-GO) enriched with multi-cationic quaternary ammonium salt was prepared and used for the cycloaddition of CO2 towards epoxides under mild conditions (90 °C, 2 MPa and 4 h) with high efficiency (TOF = 46.1 h−1) without the need of a solvent and a co-catalyst. The advantage of the combined use of the “multi-cationic approach” and “multi-synergetic strategy” was illustrated by comparing the performance of MF-GO with those of the GO samples that are endowed with different functional groups. A plausible mechanism is proposed for the cycloaddition reaction over MF-GO. Moreover, the MF-GO catalyst can be easily separated and reused for at least five times without significant loss of activity.