Hydrothermally stable Ru/HZSM-5-catalyzed selective hydrogenolysis of lignin-derived substituted phenols to bio-arenes in water
In the context of arenes generated from bioresources, a hydrothermally stable catalyst Ru/HZSM-5 is reported for the quantitative conversion of lignin-derived phenol, anisole, guaiacols, and syringols into bioaromatic hydrocarbons in a one-pot aqueous-phase process in moderate conditions (240 °C, 2 bar H2), with a high carbon balance of 98.6%. The selective hydrodeoxygenation reaction pathway from guaiacol proceeded via an initial hydrogenolysis route to phenol with a rate of 33 mmol g−1 h−1, and subsequently phenol was directly hydrogenolyzed to benzene at a rate of 10 mmol g−1 h−1 over Ru/HZSM-5 in selected conditions. Moreover, routes via the hydrogenation of guaiacol, as well as the hydrogenolysis of catechol and anisole, have been rationally excluded via comparative kinetic studies of the individual steps of the hydrodeoxygenation of catechol, anisole, and phenol over Ru/HZSM-5 in identical conditions. The selective cleavage of guaiacol at the C–O position of the Csp3–OAr group is influenced by space hindrance and the environment of the reactant (gas phase, liquid phase, or acidic aqueous phase). In the condensed liquid phase and gas phase, catechol rather than phenol is formed as the major product from guaiacol over Ru/HZSM-5. However, in the highly diluted aqueous phase, phenol alone (without detected catechol) is formed as the dominant product. In addition, the pathway for guaiacol conversion is sensitive to the hydrogen pressure and temperature, which demonstrates that a relatively high temperature and a low hydrogen pressure are crucial for manipulating the coupled tandem hydrogenolysis routes. Among various oxide-loaded Ru catalysts, Ru/HZSM-5 has been confirmed to be the most efficient catalyst for the target selective hydrodeoxygenation of guaiacol, and the metal sites and support were found to be highly stable for four successive catalytic runs in the near-critical aqueous phase that was used.