SO/SnO-Fly Ash as Bifunctional Catalyst for Microwave-Assisted Single-Step Condensation of 2-Naphthol and Aromatic Aldehydes

Coal fly ash was transformed into a bifunctional catalyst bearing strong Lewis–Bronsted acid sites to catalyze the condensation of 2-naphthol and aromatic aldehydes under microwave irradiation. Due to the high thermal stability of its component, fly ash was employed as support for the incorporation of sulfated tin oxide to enhance Lewis–Bronsted acid sites for organic reactions. Fly ash was first mechanically activated for 15 h to enhance its reactivity followed by loading of sulfated tin oxide as an active species in varying amounts (4.6–13.4 wt%) and analyzed by various surface characterization techniques. The Bronsted acid sites were developed from the surface hydroxyl groups associated with metal oxides that existed in the fly ash skeleton. In contrast, Lewis acid centers were generated due to the sulfated tin oxide bonded with the fly ash matrix. The prepared catalyst promoted the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes with a high yield of > 90%, which is attributed due to high crystallinity (> 70%) and evenly distributed Bronsted–Lewis acid sites. The catalyst was stable and successfully recycled five times without any significant leaching of Sn and other elements of fly ash. The present study is of great significance in utilization of fly ash for the development of heterogeneous catalyst with high stability, activity and low cost.