Valorization of fly ash to Au/CaA zeolite catalyst for selective oxidation of HMF to HMFCA: A waste-to-wealth strategy

5-Hydroxymethyl-2-furan carboxylic acid (HMFCA) is an important raw material in the chemical and pharmaceutical industries. In this work, we developed a strategy to the utilization of fly ash resource to obtain CaA zeolite and employed as catalyst supports Au nanoparticles to obtain high catalytic performance for the oxidation of 5-hydroxymethylfurfural (HMF) to HMFCA using air as an oxidant. Herein, the catalyst was synthesized through cation-exchange and wet impregnation method. The composition, morphology, and structure of the as-prepared catalyst were characterized. The effects of surface chemistry, the amount of Au loading, catalyst dosage, reaction time and reaction temperature on catalytic performance for the selective oxidation of 5-HMF to HMFCA were systematically investigated. In particular, the optimal 1.0 wt% Au-CaA catalyst afforded a satisfactory HMFCA yield of 89.2% and selectively of 91.5% from HMF oxidation using air as the oxidant and KHCO 3 as base in water at 80 °C. This excellent catalytic performance is not only attributed to the high charge density and total acidity of CaA zeolite, but also ascribe to the strong acid sites of support and high dispersion of Au nanoparticles, which promoted the activation of reactants and further improved HMFCA selectivity. Moreover, the relationship between structure (Au particle size, basicity within zeolites and Au δ+ species) and the yield of HMFCA was concretely established. Reaction mechanism analysis revealed the pathway and the formation of Au δ+ species on the as-prepared catalyst was the crucial step. This work may be potential platforms for the effective catalytic synthesis of sustainable value-added chemicals from biomass, and has pioneered new and environmentally friendly applications for the resource utilization of fly ash.