Sustainable catalysis in the synthesis of bis(indolyl)methanes and 1,8-dioxo-octahydroxanthenes using cotton residue-derived sulfonated biochar.

Functionalization and modification of carbon-based materials has attracted considerable interest in organic synthesis because of their improved inherent properties. In this study, a series of novel -SO3H functionalized biowaste-derived biochars were prepared and thoroughly characterized using various physicochemical techniques such as Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area analysis. The acidic sites were determined using acid-base back-titration methods. The prepared materials were evaluated as heterogeneous catalysts for the synthesis of 1,8-dioxo-octahydroxanthenes and bis(indolyl) methane derivatives. The developed catalytic system operates efficiently under relatively mild conditions without the need for any co-catalyst, and provides a rapid and simple protocol for the synthesis of a wide variety of valuable heterocycles. Moreover, the present catalytic system can be efficiently recovered and reused for several consecutive cycles without any significant loss of activity. The excellent performance of the catalyst is further highlighted by outstanding results in green chemistry metrics such as E-factor, atom economy, reaction mass efficiency (RME), process mass intensity (PMI), and E-score in the synthesis of both 1,8-dioxo-octahydroxanthenes and bis(indolyl)methane derivatives, underscoring its sustainability and broad applicability.