Controlling C−OAryl Hydrogenolysis vs Aryl Hydrogenation in Lignin Model Depolymerization Using Ni‐, Rh‐ or Ni/Rh‐based Silica Catalysts

Abstract

AbstractThis paper focuses on the hydrogenolysis of 2‐phenoxy‐1‐phenylethanone, modelling a pre‐oxidized form of lignin using five catalysts of similar metal content, prepared by impregnation of Ni(II), Rh(III) and both metals onto Aerosil‐380 in the presence of NH3, followed by a reduction step. The materials were characterized in the dried as‐synthesized state and after consecutive reductive and oxidative treatments, showing the great dispersion of the metal (oxide nanoparticles with sizes<2 nm). Working with 1 mol % of the reduced metal (500 °C) per substrate at 180 °C with isopropanol as H‐donor, the most active catalyst, but also the least selective towards phenol, was initially found to be the Ni‐based one. Under similar conditions, the lower capacity of the Rh‐based catalyst to induce H transfer from isopropanol favoured phenol formation but resulted in much slower C−OAr cleavage. Despite a very high dispersion of the two metals in the bimetallic catalysts, no synergy was found, suggesting that Ni would be segregated at the surface. Finally, the best phenol productivity could be reached by reducing the Ni‐based catalyst at 650 °C, which led to a more efficient cleavage of C−OAr bonds. In this way, it was possible to produce 8 times more phenol per hour.