A Review on Green and Efficient Synthesis of 5-Hydroxymethylfurfural (HMF) and 2,5-Furandicarboxylic Acid (FDCA) from Sustainable Biomass

The present work gives an overview of the approaches for green synthesis of 5-hydroxymethylfurfural (HMF) and 2,5 furan dicarboxylic acid (FDCA) focusing on reduction in the byproduct formation such as levulinic acid and humins. It was elucidated that the use of highly polar and aqueous solvents, high Brønsted acidity, high pH, reaction time, and temperature increases the levulinic acid formation in the reaction. In contrast, humin formation is increased by high basicity, temperature, and initial substrate concentration. Specific catalytic and solvent systems for minimizing the byproduct formation during HMF synthesis are also discussed along with options for use of microwave, ultrasound, and continuous processes. In terms of intensification, safety, and ease of scale-up, the use of ultrasound for HMF synthesis is promising; however, microreactors are currently a better choice for higher substrate concentration usage and higher yields. Effect of parameters like reaction time, temperature, oxidizing source, solvent, and base on FDCA synthesis is also analyzed. Depending on the reaction conditions and the reactor type, the optimum temperature typically should be between 80 and 160 °C, moderate bases, such as K2CO3 and KHCO3, are preferred to maintain a preferred pH of 7−11 and O2 is an ideal oxidizing agent. The reaction pathway for FDCA synthesis from HMF was also studied and the effect of the base and solvent elucidated. Finally, one-pot synthesis of FDCA from HMF and the strategies necessary to optimize the FDCA yield has been discussed. It was demonstrated that one-pot two-step FDCA synthesis from sugars provides enhanced FDCA yield, selectivity, and lower byproduct formation, compared to one-pot one-step synthesis.