Aerobic oxidation of HMF-cyclic acetal enables selective FDCA formation with CeO2-supported au catalyst

The utilization of 5‐(hydroxymethyl)furfural (HMF) for the large‐scale production of essential chemicals has been largely limited by the formation of solid humin as a by‐product, which prevents continuous operation of step‐wise batch‐type processes and continuous flow‐type processes. The reaction of HMF with 1,3‐propanediol produces an HMF‐acetal derivative that exhibits excellent thermal stability. Aerobic oxidation of the HMF‐acetal with a CeO2‐supported Au catalyst and Na2CO3 in water gives a 90‐95% yield toward furan 2,5‐dicarboxylic acid, an increasingly important commodity chemical for the biorenewables industry, from concentrated HMF‐acetal solutions (10‐20 wt%) without humin formation. The stability of the six‐membered acetal ring suppresses thermal decomposition and self‐polymerization of HMF in concentrated solutions. Kinetic studies supported by density functional theory calculations identify two crucial steps in the reaction mechanism, i.e., the partial hydrolysis of the acetal into 5‐formyl‐2‐furan carboxylic acid involving OH‐ and Lewis acid sites on CeO2, and subsequent oxidative dehydrogenation of the in situ generated hemiacetal involving Au nanoparticles. The present results represent a significant advance over the current state of the art, overcoming an inherent limitation of the oxidation of HMF to an important monomer for biopolymer production.