Valorization of Coal Fly Ash via a Novel SiFA/Chitosan/MnFe2O4 Composite Photocatalyst for Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran

The valorization of biomass-derived 5-hydroxymethylfurfural (HMF) and coal fly ash (CFA) offers opportunities for more sustainable chemical production. In this study, a silica-rich fraction recovered from CFA (SiFA) was combined with chitosan (CS) and manganese ferrite (MnFe2O4, MFO) to prepare a ternary SiFA/CS/MFO composite using co-precipitation and ionic gelation. The materials obtained at each stage of preparation were comprehensively characterized by X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS). Under photocatalytic conditions in acetonitrile, the composite achieved 92.6% HMF conversion with 90.5% selectivity toward 2,5-diformylfuran (DFF). In contrast, thermal catalytic oxidation at 100 °C using tert-butyl hydroperoxide (tBuOOH) resulted in 99.7% HMF conversion but only 5% DFF selectivity. These results highlight the potential of fly-ash derived silica-based composites for selective photocatalytic valorization of a biomass-derived platform.