Valorization of Coal Fly Ash into Fe-FAU Catalysts: Targeted Iron Recovery and Fly Ash Matrix Effects in Methane-to-Methanol Conversion

Abstract The valorization of industrial wastes into functional catalysts offers a route toward more sustainable chemical processes, particularly when the inherited complexity of the waste can be retained as a design variable. Here, coal fly ash was converted into Fe-FAU zeolite catalysts through an integrated waste-to-catalyst strategy in which the same industrial residue supplied both the framework-forming Si–Al fraction and the recoverable Fe precursor. During presynthesis acid treatment, Fe was selectively recovered as crystalline Fe(II) oxalate dihydrate, while the remaining aluminosilicate fraction was transformed into OSDA-free FAU zeolites and subsequently modified by recovered-Fe ion exchange and steaming. Using fly ashes with distinct inherited chemical environments, we show that source-dependent matrix effects govern framework evolution, acidity redistribution, Fe speciation, and catalytic behavior in N2O-assisted methane-to-methanol conversion. Isolated Fe3+-rich catalysts exhibited the highest initial methanol formation rate of 137 μmol gcat–1 h–1, but showed rapid activity loss associated with accessible acidity and coke formation. In contrast, the Ca-richer Fe-Z2 matrix moderated acid-site accessibility, favored small extra-framework Fe–oxo/hydroxo descriptors, and delivered stable methanol productivity of approximately 71.5 μmol gcat–1 h–1 with suppressed coke formation. Catalysts more susceptible to steaming-induced dealumination or Fe2O3-like aggregation showed lower sustained methanol productivity. Overall, this work links Fe recovery, OSDA-free FAU synthesis, recovered-Fe incorporation, and MTM catalysis in a single material-flow concept, demonstrating that fly ash acts not as a passive precursor but as a chemically active matrix that controls Fe-site distribution, acidity, and time-on-stream stability.