Nitrogen‐Doped Carbon Dots: An Alternative Base Catalyst for Knoevenagel‐Doebner and Aldol Condensation Reactions

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Abstract
Nitrogen‐doped carbon dots (NCDs) synthesized via thermal pyrolysis of citric acid and urea have been explored as basic heterogeneous catalysts for Knoevenagel‐Doebner and Aldol condensation reactions. Two distinct NCDs were prepared at different carbonization temperatures: 140 °C (NCD1) and 300 °C (NCD2). Structural characterization revealed that NCD1 comprises hydrogen‐bonded clusters of small organic molecules with abundant surface functional groups (─OH, ─NH 2 , and ─CONH 2 ), while NCD2 features a crystalline graphitic carbon core with minimal surface‐passivated functional groups, similar to carbon quantum dots (CQDs). Comparative catalytic studies demonstrated superior performance of NCD1 over NCD2, achieving 75% conversion of benzaldehyde to cinnamic acid in Knoevenagel‐Doebner condensation and 71% conversion to benzalacetone in Aldol condensation. Based on XPS and CO 2 ‐TPD (Temperature programmed Desorption) studies, the enhanced catalytic activity of NCD1 could be attributed to its elevated surface Lewis basicity, arising from the presence of molecular domain rich in ─NH 2 functionalities.

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