Homogeneous Metal Salt Solutions for Biomass Upgrading and Other Select Organic Reactions

The growing interest in biomass as a renewable feedstock drives research toward innovative processes and performance-advantaged bioproducts. Homogeneous metal salt solutions are promising media owing to their diverse applications as Lewis and Brønsted acids, salting-in(out) agents, and solvents. Their multifaceted roles render them attractive but also complex to understand. Here, we aim at reviewing and analyzing the speciation of and the underlying mechanisms of metal salt catalysts by correlating the intrinsic properties of ions, obtained from spectroscopic and computational techniques, with experimental kinetics and the speciation–property relations from dilute to very concentrated (molten salt) solutions. We focus on (1) metal salt speciation in dilute aqueous solutions with emphasis on the glucose to 5-hydroxymethylfurfural (HMF) tandem reaction, (2) salt–solvent and salt–substrate interactions in the reactive extraction of HMF and furfural from aqueous media into an organic solvent, (3) cation–anion and salt–substrate interactions, and the structure of the hydration sphere of cations during the dissolution of lignocellulosic biomass in molten salt hydrates (MSHs), (4) the activity of ions in solution, acidity, and speciation in the hydrolysis of cellulose in MSHs, (5) metal salt speciation in organic solvents and ionic liquids for the conversion of sugars to HMF and for further HMF upgrade, and (6) applications of metal salts in the Prins and Diels–Alder reactions. Finally, we discuss remaining challenges in this field of homogeneous catalysis and occasionally draw parallels to heterogeneous catalysis.