Selective aqueous phase oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over Pt/C catalysts: influence of the base and effect of bismuth promotion

Abstract

5-Hydroxymethylfurfural (HMF) was quantitatively oxidized to 2,5-furandicarboxylic acid (FDCA) at 100 °C under 40 bar air in moderately basic aqueous solution in the presence of active carbon supported platinum and bismuth–platinum catalysts. The transformation of HMF into FDCA proceeded via the 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 2,5-diformylfuran (DFF) intermediates; both of these were very reactive and rapidly oxidized to 5-formylfurancarboxylic acid (FFCA), the subsequent oxidation of which was found to be the rate-limiting step. The preparation method of the platinum catalysts influenced the particle size of metallic platinum and modified the surface of the support, therefore determining the activity. The addition of a carbonate base (Na2CO3/HMF molar ratio = 2) led to faster overall conversion than bicarbonate (NaHCO3/HMF = 4) by maintaining an appropriate pH for the oxidation reaction. The ex situ or in situ addition of a bismuth promoter still accelerated the reaction; the highest activity was observed for a Bi/Pt molar ratio of ca. 0.2. Furthermore, the promoter helped to prevent some deactivation of the Pt catalyst upon recycling experiments. Quantitative conversion of HMF (0.1 M) and >99% yield of FDCA were achieved using a molar ratio of HMF to Pt of 100 and Na2CO3 as the homogeneous base in less than 2 h.