Base-free synthesis of renewable furan-2,5-dicarboxylic acid (FDCA) over an earth-abundant magnetic catalyst

Being a biobased alternative to petroleum-based terephthalic acid (TPA), furan-2,5-dicarboxylic acid (FDCA) requires a low-cost and efficient catalytic system for its commercialization. This study describes FDCA production using whey permeate powder (WPP)-derived 5-(hydroxymethyl)furfural (HMF) in a base-free system over a magnetically recyclable catalyst composed of earth-abundant metals (Mn and Fe). The impact of MnCl2·4 H2O loading on the iron core was studied at different levels and found optimal at the ratio of 1:1. The in-situ oxidation of HMF was carried out using ozone and t-BuOOH as the oxidants in a γ-valerolactone (GVL) -water solvent system. It was observed that the ozone pre-treatment (3 h at room temperature) of HMF increased the FDCA yield from 39.7 % (without ozonation) to 60 % at a reaction condition of 130 °C for 2 h. The concentration profiles for the intermediates formed during HMF to FDCA oxidation i.e., 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), furan-2,5-dicarbaldehyde (DFF), and 5-formyl-2-furancarboxylic acid (FFCA) were analyzed at different reaction temperatures to evaluate the reaction kinetics. The kinetic study revealed that the present solvent-catalytic system favored HMFCA formation over DFF due to the requirement of higher activation energy for the latter. The HMF to FDCA catalytic solvent was tested for WPP, which afforded an overall FDCA yield of 14.6 % at 130 °C in 120 min. In addition, the Mn1Fe1 consisted of mixed phases of Mn3O4, Mn2O3, MnO2, Fe2O3, and Fe3O4 and demonstrated variation in oxidation states for Mn (Mn3+↔Mn2+) and Fe (Fe2+↔Fe3+) species.