Zeolitization of fly ash for synthesis of pharmaceutically important bis(indolyl)methane derivatives

Abstract

Waste valorization represents an essential strategy toward achieving sustainable development goal 12 (Responsible Consumption and Production). In this work, coal fly ash was successfully transformed into Zeolite A and employed as a heterogeneous catalyst for the synthesis of pharmaceutically relevant bis(indolyl)methane derivatives (BIMs). Structural and morphological studies, conducted using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), confirmed the formation of Zeolite A (ZA). Acid modification with 0.01 M HCl for 30 minutes at 60 °C resulted in ZA2/30, the most active catalyst among the five synthesized. The ZA2/30 exhibited enhanced surface area (56.1 to 66.2 m² g⁻¹) and pore size (5.4 to 6.3 nm) compared to ZA. Ammonia temperature programmed desorption (NH₃-TPD), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses revealed an increase in the concentration of Brønsted acid sites with decreasing Si/Al ratio, which directly correlated with improved catalytic activity. The optimized catalyst enabled the synthesis of BIMs in yields ranging from 44% to 94%, under mild and environmentally friendly conditions. The used catalyst was studied for reusability up to five cycles. Leaching studies confirmed the heterogeneous nature of the catalyst, demonstrated its structural stability and reusability during successive cycles. This study establishes fly ash-derived zeolites as eco-friendly catalysts for sustainable organic synthesis, reinforcing their role in advancing circular chemistry.