Revolutionizing green catalysis: a novel amla seed derived biochar modified g-C3N4·SO3H catalyst for sustainable and versatile synthesis of bis-indoles†

Catalysis plays a vital role in green chemistry by improving process efficiency, reducing waste, and minimizing environmental impact. A biochar-modified g-C₃N₄·SO₃H (BCNSA) catalyst was developed using biochar derived from amla seed powder and CNSA. CNSA was synthesized via the reaction of g-C₃N₄ with chlorosulfonic acid. Both components were combined, pyrolyzed, purified, and comprehensively characterized using FTIR, XRD, FE-SEM, EDX, elemental mapping, TGA, and DTA studies to confirm the successful synthesis and structural integrity. The catalyst demonstrated exceptional efficiency in synthesizing bis-indole derivatives through reactions between substituted indoles (indole, 1-methyl indole, and 6-chloro indole) and carbonyl-containing compounds, including isatins (isatin, 7-(trifluoromethyl)isatin, 5-bromo isatin, and 5-fluoro isatin), aldehydes, cyclo-ketones, dimedone, and acetophenones. These reactions were carried out under simplified conditions using water as a green solvent, promoting sustainability and versatility. A total of 21 bis-indole products were synthesized within 5–45 minutes, achieving yields of 80–98% showcasing the catalyst's outstanding performance. Furthermore, the method was scaled up to gram-level synthesis, and green chemistry metrics were evaluated for all the products, highlighting the environmental and economic benefits of this approach.