Support Effects in the Catalytic Transfer Hydrogenolysis of Diphenyl Ether over Ru Catalysts

The selective cleavage of aryl C–O bonds while avoiding hydrogenation of the aromatic ring is a crucial challenge in the reductive valorisation of lignin. Industrial Ru (5 wt%) catalysts supported on wood-derived activated carbon (RuCw) and δ/θ-Al 2 O 3 (RuAl), prepared by deposition–precipitation using Ru recovered from metal-containing waste streams, were compared in diphenyl ether (DPE) hydrogenolysis under catalytic transfer hydrogenolysis (CTH) with 2-propanol and under molecular H 2 . At 200 °C after 2 h, RuCw gave 92.0% DPE conversion and 63.6% aromatic yield under CTH conditions, compared with 42.8% and 33.4%, respectively, over RuAl. Normalizing apparent rate constants with respect to the Ru accessible by CO chemisorption reduced the activity di]erence to a factor of 1.2-1.4 between 160 and 200 °C, indicating that the accessible Ru population accounts for most of the support-dependent rate di]erence, although uncertainties in CO/Ru adsorption stoichiometry prevent assignment of the residual di]erence to intrinsic kinetics. The corresponding apparent activation energies were found to be 104 ± 7 and 85 ± 5 kJ mol-1. Under CTH conditions, aromatic selectivity followed nearly the same dependence on DPE conversion over both catalysts. At comparable conversion, 2-propanol gave higher aromatic selectivity than molecular H 2 . Under H 2 pressure, dicyclohexyl ether formation was consistent with a greater contribution from DPE ring hydrogenation before C–O cleavage. Under the reaction conditions examined, the support primarily determines the accessible Ru population and hence catalytic activity, whereas the mode of hydrogen supply governs the competition between C–O cleavage and aromatic-ring hydrogenation.