Scheme 1

Possible pathways (A, B and C) to produce FDCA from 5-HMF obtained by dehydration of fructose. Pathway A involves oxidation of the hydroxyl group of 5-HMF to give DFF followed by stepwise oxidation of the aldehyde groups to form FFCA and then FDCA. In Pathway B, the aldehyde group on 5-HMF is first reduced to hydroxyl group to form BHMF, which is then oxidized via HMFCA to FFCA and finally to FDCA. Pathway C involves selective oxidation of the aldehyde group on 5-HMF to give HMFCA and the following oxidations are similar to that in Pathway B/C