Functionalized Carbon Nanotubes for Biomass Conversion: The Base-Free Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid over Platinum Supported on a Carbon Nanotube Catalyst

The aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) is one of the most attractive reactions to establish biomass-based sustainable chemical processes. Supported Au and Pt catalysts have mainly been reported for this reaction, but excess amounts of base additives are generally required, which makes the process less green. Here, we demonstrate that Pt nanoparticles loaded on functionalized carbon nanotubes (CNTs) can catalyze the aerobic oxidation of HMF to FDCA in water without any base additives. Kinetic studies suggest a tandem reaction mechanism via 2,5-diformylfuran and 5-formylfurancarboxylic acid intermediates. It has been clarified that the oxygen-containing functional groups, in particular carbonyl/quinone and/or phenol groups, on CNT surfaces play crucial roles in FDCA formation. These functional groups could enhance the adsorption of HMF as well as the reaction intermediates from water and might facilitate hydrogen transfer. Biomass oxidation: Functionalized Pt nanoparticles supported on carbon nanotubes (Pt/CNT) catalyze the aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) in water in the absence of any base additives. The alcohol group in HMF is preferentially oxidized to form 2,5-diformylfuran (DFF) as a primary product. The carbonyl/quinone and/or phenol groups on the CNTs play crucial roles in the formation of FDCA via DFF and 5-formyl-2-furancarboxylic acid (FFCA).