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

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.