Catalytic reduction systems of 2-furancarboxylic acid (FCA) and 2,5-furandicarboxylic acid (FDCA) with H2 without reduction of the carboxyl groups are reviewed. FCA and FDCA are produced from furfural and 5-hydroxymethylfurfural which are important platform chemicals in biomass conversions. Furan ring hydrogenation to tetrahydrofuran-2-carboxylic acid (THFCA) and tetrahydrofuran-2,5-dicarboxylic acid (THFDCA) easily proceeds over Pd catalysts. Hydrogenolysis of one C–O bond in the furan ring produces 5-hydroxyvaleric acid (5-HVA) and 2-hydroxyadipic acid. 2-Hydroxyvaleric acid is not produced in the reported systems. 5-HVA can be produced as the lactone form (δ-valerolactone; DVL) or as the esters depending on the solvent. These reactions proceed over Pt catalysts with good yields (~ 70%) at optimized conditions. Hydrogenolysis of two C–O bonds in the furan ring produces valeric acid and adipic acid, the latter of which is a very important chemical in industry and its production from biomass is of high importance. Adipic acid from FDCA can be produced directly over Pt-MoOx catalyst, indirectly via hydrogenation and hydrodeoxygenation as one-pot reaction using the combination of Pt and acid catalysts such as Pt/niobium oxide, or indirectly via two-step reaction composed of hydrogenation catalyzed by Pd and hydrodeoxygenation catalyzed by iodide ion in acidic conditions. Only the two-step method can give good yield of adipic acid at present.
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