Selective Oxidation of 5‑Hydroxymethylfurfural to 5‑Hydroxymethyl-2-furancarboxylic Acid Using Gluconobacter oxydans

5-Hydroxymethylfurfural (HMF), an important biobased platform chemical, can be potentially oxidized to several products that can serve as versatile building blocks for polymers. 5-Hydroxymethyl-2-furan carboxylic acid (HMFCA) is formed by incomplete oxidation of HMF but the reaction often suffers from substrate inhibition and overoxidation to other products. In this study, resting cells of Gluconobacteroxydans DSM 50049 were shown to oxidize HMF quantitatively to HMFCA with exquisite selectivity. Complete conversion of 31.5 g L^–1 crude HMF to HMFCA was achieved within 6 h under pH-controlled conditions. Initial productivity of 10 g L^–1 h^–1 was reduced to 2 g L^–1 h^–1 toward the end of the reaction. Thereafter, additional HMF added to the reaction mixture (12 g L^–1) was converted up to 94% within 17 h with 100% selectivity resulting in final HMFCA concentration of 44.6 g L^–1 and yield of 6.2 g g^–1 cell dry weight. Recovery of HMFCA from the reaction could be achieved by adsorption to anion exchange resins Amberlite IRA-400 (Cl^– form) and Ambersep 900 (OH^– form), the former showing higher binding (169 mg/g resin) and product recovery. Alternatively, liquid–liquid extraction with ethyl acetate provided a facile separation technique for the recovery of pure HMFCA.