Renewable Adipic Acid Production Via Metal-Free Cleavage of C-O Bonds in the Presence of Molecular H2 in Organic Acid Solvents

A central challenge in biomass valorization is the selective removal of oxygen-containing functional groups from platform chemicals, e.g., 5-hydroxymethylfurfural, to valuable chemicals without overhydrogenating the substrate to undesired products. Supported metal or metal oxide catalysts have shown promise in removing C-O bonds; however, many catalytically active metals/metal oxides are economically costly and can lead to major selectivity challenges, such as overhydrogenation or C-C cracking. Recently, we have developed a system for the selective removal of C-O single bonds, through employment of homogeneous or solid acids with molecular H₂ in organic acid solvents. In this system, cleavage of C-OH and ether bonds occurs readily without reducing higher order oxygenated functional groups such as carboxylic acids. This system is particularly relevant in forming adipic acid (AA), a key polymer precursor for producing Nylon 66, from biomass-derived tetrahydrofuran-2,5-dicarboxylic acid (THFDCA), where we have achieved an AA yield of 89% in the absence of any metal catalyst. In this work, we demonstrate key mechanistic insights in THFDCA hydrogenolysis to AA via computational input, kinetic measurements, and reactivity data, where we shed light onto the interplay between acids, iodides, molecular H₂, and the organic acid solvent.