Electrochemical bottom-up synthesis of biomass-derived carbon dots for promoting Knoevenagel condensation

Natural raw materials such as biomass and plant wastes are the most interesting feedstocks for the synthesis of carbonaceous nanomaterials due to their high availability, environmental compatibility, and affinity with Green Chemistry principles. Carbon dots (CDs), a new type of carbon nanomaterials, perform an important role in this matter by serving in the development of environmentally friendly catalysts. In this study, CDs were synthesized in good yields by using the electrochemical bottom-up synthesis starting from the liquid phase that is separated during the hydrothermal carbonization (HTC) process involving orange peel waste (OPW). The electrochemical setup was also optimized to synthesize CDs based on 5-hydroxymethylfurfural, which represents one of the main components of the liquid phase obtained by HTC. The comprehensive optical and chemo-physical characterization of CDs samples carried out by several techniques such as SEM, TEM, XRD, PL, TGA, and FTIR, highlighted their distinctive morphological and microstructural features. The obtained CDs were successfully employed to catalyse the Knoevenagel condensation, showing excellent results in terms of yield and reproducibility. In addition, the sustainable nature of the CDs was demonstrated by recycling the catalysts up to five cycles without significant activity loss.