Energy Densification of Biomass-Derived Furfurals to Furanic Biofuels by Catalytic Hydrogenation and Hydrodeoxygenation Reactions

The concomitant hydrolysis and dehydration of biomass-derived cellulose and hemicellulose to furfural (FUR) and 5-(hydroxymethyl)furfural (HMF) under acid catalysis allows a dramatic reduction in the oxygen content of the parent sugar molecules with a 100% carbon economy. However, most applications of FUR or HMF necessitate synthetic modifications. Catalytic hydrogenation and hydrogenolysis have been recognized as efficient strategies for the selective deoxygenation and energy densification of biomass-derived furfurals generating water as the sole byproduct. Efficient and eco-friendly catalysts have been developed for the selective hydrogenation of furfurals affording renewable furanic compounds such as 2-methylfuran, 2,5-dimethylfuran and 2-methyltetrahydrofuran with potential applications as biofuel, solvent and chemical feedstock. Hydrogen gas or hydrogen donor molecules, required for the above processes, can also be renewably obtained from biomass using catalytic processes, enabling a circular economy. In this review, the recent developments in the energy densification of furfurals to furanic compounds of commercial significance are elaborated, emphasizing the role of catalyst and the reaction parameters employed. Critical discussion on sourcing hydrogen gas required for the processes, using hydrogen donor solvents, catalyst design and the potential markets of furanic intermediates have been made. Critical evaluations of the accomplishments and challenges in this field are also provided.