POMs nanofibers for the oxidation of 5-HMF with O2

2,5-Diformylfuran (DFF) is a versatile compound that can be used as a precursor in the synthesis of various value-added products, which can be obtained from aerobic oxidation of 5-hydroxymethylfurfural (HMF). However, aerobic oxidation of 5-HMF gives rise to several pathways and products containing DFF, 2,5-furandicarboxylic acid (FDCA), or maleic anhydride (MA). To date, both homogeneous and heterogeneous metal catalysts have been explored for the oxidation of HMF to DFF with various oxidants. It was found that materials containing vanadium showed higher efficiency with conversions ranging from 84% to 100% and DFF yields of 82%-99.9% at 120-140 degrees C for 3-11 h in dimethyl sulfoxide (DMSO). Selective oxidation of 5-HMF depended on the nature of the catalysts and supports. Polyoxometalates (POMs) are widely applied in catalysis especially in catalytic transformation of biomass. Especially, POMs containing vanadium had been found to be potential for oxidation of HMF to produce a series of products including DFF and MA. The diversity in structure and components allows POMs to be most active in acidic and redox catalysis. Compared to their homogeneous catalysis, heterogeneous systems showed more favorable in easy separation and reuse, introduction of special active sites belonging to supports, enhancement in surface area. Under such circumstances, heterogeneous POM catalysts are developed using microporous, mesoporous and macroporous SiO2, TiO2 and ZrO2 as supports. Compared to powdered mesoporous materials, electrospun nanofibers are good candidates because of their high surface-to-volume ratios. Herein, trifunctional H5PMo10V2O40/meso-ZrO2(f) had been synthesized using combined electrospinning and surfactant pore-forming technology, which were characterized by IR, P-31 MAS NMR, XRD, SEM, TEM, and N-2 adsorption-desorption measurement. Combination of IR, 31P MAS NMR, XRD, BET surface areas, SEM and TEM could confirm that POM molecules were loaded on ZrO2 nanofibers (200-400 nm) with mesoporous structure. It was found that decorating ZrO2 nanofibers by H5PMo10V2O40 generated enhanced catalytic activity by emerging their unique individual properties of redox ability, Bronsted acidity, Lewis acidity, and nanofiber structure with higher surface area. H5PMo10V2O40/meso-ZrO2(f) materials were evaluated in aerobic oxidation of 5-HMF to DFF. The conversion of HMF followed the order: HPMoV/meso-ZrO2(5-f) (0.17 mmol g(-1), 60.1%) < HPMoV/meso-ZrO2(14-f) (0.22 mmol g(-1), 79.8%) < meso-ZrO2(f) (0.36 mmol g(-1), 92.3%) < HPMoV/ meso-ZrO2(23-f)(0.41 mmol g(-1), 94.2%) < HPMoV/ meso-ZrO2(28-f) (0.47 mmol g(-1), 95.6%) < HPMoV/meso-ZrO2(35-f) (0.51 mmol g(-1), 96.4%) < HPMoV (2.66 mmol g(-1), 99.9%), which followed the increase of HPMoV loading hence the Bronsted acidity. The DFF yields depended on their Lewis acidity as HPMoV/meso-ZrO2(35-f) (B/L=0.04:1, yield=74.6%) < HPMoV/meso-ZrO2(28-f) (B/L=16.8:100, yield=80.2%) < HPMoV/meso-ZrO2(23-f) (B/L=0.11:1, yield=89.9%) < HPMoV/meso-ZrO2(14-f) (B/L=0.71:1, yield =74.5%) < HPMoV/meso-ZrO2(5-f) (Lewis acidity/Bronsted basicity = 1.1:1, yield=56.8%). Therefore, balancing the L/B ratio could control the DFF generation. As results, H5PMo10V2O40/meso-ZrO2(23) nanofiber (23, represented the POM amount) was found to be most active in aerobic oxidation of 5-HMF to give 89.9% yield of DFF at 96.4% conversion in DMSO at 120 degrees C for 7 h. The mechanism study showed that the generated O-2 was the active site in this aerobic oxidation being catalyzed by H5PMo10V2O40. Moreover, H5PMo10V2O40/meso-ZrO2(f) showed good stability and duration for being reused at least ten times without leaching of POMs from ZrO2 nanofibers.