Abstract:
The optimum condition of HMF oxidation to FDCA over 1%wt Palladium coated on activated carbon (Pd/AC) was achieved by variation of four parameters i.e. solvent media (water and acetonitrile), reaction temperature (80-150 o C), oxidizing agent (oxygen 5 bar and tertbutylhydroperoxide; TBHP to HMF = 9 :1) and concentration of sodium carbonate solution (0, 0.06 and 0.3 M). While the reaction time was fixed at 24 h, it was found that water and acetonitrile solvents gave different optimal conditions. In case of water media, total yield of HMF oxidation was optimized at total yield of products 53% under temperature 70 o C, Na2CO3 0.03 M and molar ratio of TBHP: HMF (9 :1), whereas total yield of HMF oxidation was optimal at 42% under temperature 110 o C, molar ratio of TBHP: HMF (9 :1) without weak base Na2CO3. The former condition was green and environmental friendly, so that Pd incorporated with second transition metal (i.e. Mn, Ni and Co) coated on activated carbon was tested. The catalysts were prepared by nanoparticle colloidal immobilization. The Pd catalysts incorporated with second transition metals were inferior while compared with Pd/AC. The Pd/AC gave selectivity of FFCA >HMFCA>FDCA whereas the other catalysts did not fully oxidize HMF to FDCA, but these catalysts were only selective to intermediate products i.e. HMFCA>FFCA. This is because the Pd catalysts incorporated with second transition metal coated on activated carbon contaminated with poly vinyl alcohol (PVA) that was a chelating chemical during catalyst preparation and it covered metal active sites leading to loss of activity. Besides that, average particle size of Pd metal coated on activated carbon was 6 nm smaller than others (14-24 nm), implying a good dispersion of active metal sites on Pd/AC. The second transition metal (Mn, Ni, Co) can assist agglomeration of Pd metal site during preparation.