Reaction Mechanism and Kinetics of the Liquid-Phase Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

2,5-Furandicarboxylic acid (FDCA), which is currently prepared by catalyzing the oxidation of 5-hydroxymethylfurfural (HMF), is a biobased platform compound with broad market prospects. Herein, a detailed free-radical chain reaction mechanism consisting of both main and side reaction pathways was proposed to describe the liquid-phase air oxidation process of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over a Co/Mn/Br catalyst. The higher reactivity of the hydroxymethyl group than the aldehyde group for HMF oxidation was first explained according to the retardation effect of alcohols on free-radical reactions and the reactive activity of active hydrogen on the two substituents of HMF. Considering the low stability of HMF, the possible side reactions including condensation, decarboxylation, decarbonylation, and ring-opening for HMF oxidation were further investigated. Subsequently, to obtain more reliable model parameters, a lumped kinetic scheme was developed considering only the formation of important and detectable reactants, intermediates, and products. Since the total reaction rate is not sensitive to the reactant concentration, a fractional kinetic model was adopted to correlate the experimental results and evaluate the kinetic parameters.