A green photocatalytic carbon dots system based on plant resources for the conversion of CO2 with epoxides into cyclic carbonates

The combination of plant-derived carbon dots (CDs) with ammonium salts was demonstrated to provide an efficient, cost-effective and readily accessible metal-free photocatalytic system for the conversion of CO 2 with epoxides into cyclic carbonates, driven by visible light (300 W), under mild conditions (25 °C, 1 bar CO₂, 4 h). The catalytic system employs organic materials derived from plants such as cress, cardamom, fenugreek and barley, with tetrabutylammonium bromide (TBABr) as a co-catalyst. The CDs were synthesized through a hydrothermal method. Several analytical techniques, including ultraviolet–visible spectroscopy (UV–Vis), transmission electron microscopy (TEM), fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR), were employed to characterize the catalysts and products. Among the prepared carbon dots, CDs-Cress exhibited the highest photocatalytic activity, achieving a 99% yield of cyclic carbonates with ≥ 99% selectivity under mild conditions. The effect of reaction time was investigated, showing that a 63% yield was achieved after 1 h, whereas extending the reaction time to 4 h increased the yield to 99% while maintaining ≥ 99% selectivity, demonstrating the excellent performance of the developed photocatalytic system. The Cress-CDs catalyst could be easily recovered and reused over five consecutive runs without significant loss of activity, representing an environmentally benign and cost-effective route for cyclic carbonate synthesis from CO₂ and demonstrating strong potential for green chemistry applications.