Carbon Dots as New Effective Catalysts for Knoevenagel Condensation Reaction and Synthesis of Cyclic Carbonates under Solvent-free Conditions

Carbon dots (CDs) as new efficient catalysts have been developed for the Knoevenagel condensation reaction and the synthesis of cyclic carbonates under solvent-free conditions. Six kinds of carbon dots with different numbers of amino groups were synthesized. The amine-terminated carbon dot (NCDs-1) was synthesized using arginine and ethylenediamine by a microwave technique, and its catalytic action was studied. NCDs-1 with the highest numbers of amino groups was used as new metal-free and effective heterogeneous catalysts to perform essential Knoevenagel condensation and cycloaddition reactions in solvent-free conditions. 1H NMR was used to demonstrate the Knoevenagel condensation mechanism, in situ Fourier infrared spectroscopy was used to investigate CO2 cycloaddition and a catalytic mechanism was postulated. NCDs-1 showed a yield of 96.3 +/- 1.5 % for Knoevenagel condensation of benzaldehyde and malononitrile. In addition, NCDs-1/TBAB showed a yield of (97.0 +/- 1.0 %) of 4-phenyl-1,3-dioxolan-2-one for styrene oxide at 100 degrees C and 0.1 MPa in 1.5 h. A simple and convenient method for catalytic reactions with amine-terminated carbon dots was developed. NCDs-1, which is the first carbon dot by the microwave method using ethylenediamine and L-arginine, can serve as an efficient heterogeneous catalyst for the synthesis of cyclic carbonates from epoxides with CO2 and Knoevenagel condensation under mild and solvent-free condition. A simple and convenient method for catalytic reactions with amine-terminated carbon dots was developed. image