Synthesis of Coal Fly Ash-Based Solid Acids Mediated by Acidic Ternary Deep Eutectic Solvents

In this work, a ternary acidic deep eutectic solvent (DES) comprising zirconyl chloride, choline chloride, and malonic acid was employed to modify coal fly ash, followed by chlorosulfonic acid sulfonation to prepare a coal fly ash-derived solid acid catalyst (CFA–DES–A). The catalyst exhibited outstanding performance in the condensation of ethylene glycol with cyclohexanone, affording a 92.4% yield under mild conditions (95°C, 4 h), with negligible loss of activity after five consecutive cycles. Structural characterizations revealed that the synergistic interplay between the acidic medium and the high dissolution capacity of the DES was crucial for generating a structurally robust material with markedly enhanced surface area and a high density of strong acid sites. In contrast, modification with either an acidic aqueous solution alone or a neutral binary DES (ZrOCl2–ethylene glycol) failed to achieve comparable improvements.