Continuous Flow Hydrogenation of Lignin‐model Aromatic Compounds over Carbon‐supported Noble Metals

Abstract

An efficient continuous‐flow (CF) protocol was designed for the hydrogenation of lignin‐derived aromatics to the corresponding cycloalkanes derivatives. A parametric analysis of the reaction was carried out by tuning the temperature, the H2 pressure and the flow rate, and using diphenyl ether (DPE) as a model substrate, commercial Ru/C as a catalyst, and isopropanol as a solvent: at 25 °C, 50 bar H2, and a flow rate of 0.1 mL min−1, dicyclohexyl ether was achieved in an 86 % selectivity, at quantitative conversion. By‐products from the competitive C−O bond cleavage of DPE, cyclohexanol and cyclohexane, did not exceed 14 % in total. Remarkably, prolonged experiments demonstrated an excellent stability of the catalyst whose performance was unaltered for up to 420 min of time‐of‐stream. A substrate scope evaluation proved that under the same conditions used for DPE, a variety of substrates including alkoxy‐, allyl‐, and carbonyl‐functionalized phenols, biphenyl, aryl benzyl‐ and phenethyl ethers (10 examples) yielded the ring‐hydrogenated products with selectivity up to 99 % at complete conversion.