Surfactant decorated hydrotalcite-supported polyoxometalates for aerobic oxidation of 5-hydroxymethylfurfural and monosaccharides
Trifunctional catalysts based on polyoxometalate (POM) and surfactant modified MgAl-layered double hydroxide (LDH) were synthesized and evaluated in aerobic oxidation of 5-hydroxymethylfurfural (5-HMF) and mono-saccharides. H5PMo10V2O40@MgnAl-Surf (wt%) (abbreviated as HPMoV@MgnAl-Surf (wt%), n represented the molar ratio between Mg to Al), presented tunable redox potential, Brønsted acidity, Lewis acidity, and basicity as changing the molar ratio of HPMoV to MgnAl-Surf and also n values. HPMoV@Mg4Al-Surf (23) was found to be most active to give 88.6 % selectivity to 2, 5-diformylfuran (DFF) at 94.6 % conversion of 5-HMF in dimethyl sulfoxide (DMSO), while presented 90.9 % selectivity to 2, 5-furandicarboxylic acid (FDCA) at 93.2 % conversion in water. The existence of surfactant being covalently bonded on Mg4Al-LDH provided a hydrophobic surrounding for concentrating reactants and repelling the product, which promoted the mass transfer among inter-layer gallery. Meanwhile, higher yields of DFF as 66.4 % and 50.6 % were achieved directly from fructose and glucose, which were contributed to the suitable balancing of acidity and basicity in multifunctional catalysts. The catalytic mechanism on glucose oxidation upon HPMoV@MgnAl-Surf was studied in detail to determine the triple-functional sites on pathway. Also, the oxidation of 5-HMF, fructose and glucose were achieved under atmospheric pressure of O2, showing the wide availability of HPMoV@Mg4Al-Surf (23). HPMoV@Mg4Al-Surf (23) showed good stability and duration for being reused in ten times without any leaching of HPMoV from Mg4Al-Surf due to HPMoV being embedded by surfactant.