POMs nanofibers for the oxidation of 5-HMF with O<sub>2</sub>

Abstract

<p indent=0mm>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°C for <sc>3−11 h</sc> 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, ³¹P MAS NMR, XRD, SEM, TEM, and N₂ 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 <sc>(200−400 nm)</sc> 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, Brønsted 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) <sc>(0.17 mmol g⁻¹,</sc> 60.1%) < HPMoV/meso-ZrO₂(14-f) <sc>(0.22 mmol g⁻¹,</sc> 79.8%) < meso-ZrO₂(f)<sc>(0.36 mmol g⁻¹,</sc> 92.3%) < HPMoV/meso-ZrO₂(23-f)<sc>(0.41 mmol g⁻¹,</sc> 94.2%) < HPMoV/meso-ZrO₂(28-f) <sc>(0.47 mmol g⁻¹,</sc> 95.6%) < HPMoV/meso-ZrO₂(35-f) <sc>(0.51 mmol g⁻¹,</sc> 96.4%) < HPMoV <sc>(2.66 mmol g⁻¹,</sc> 99.9%), which followed the increase of HPMoV loading hence the Brønsted 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/Brønsted 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°C for <sc>7 h.</sc> The mechanism study showed that the generated O₂ 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.