Catalytic Transfer Hydrogenolysis of Lignin-Derived Aromatic Ethers Promoted by Bimetallic Pd/Ni Systems

Catalytic transfer hydrogenolysis (CTH) of diphenyl ether (DPE), 2-phenethyl phenyl ether (PPE), and benzyl phenyl ether (BPE)—as model molecules of α-O-4 and β-O-4 as well as 4-O-5 lignin linkages—promoted by bimetallic Pd/Ni systems is reported. Pd/Ni (Pd loading of 3 wt %) catalysts were synthesized by using a simple and economic coprecipitation technique, and its detailed physicochemical characterization was performed by means of H2-TPR, XRD, TEM, and XPS analysis. In the presence of palladium as cometal, an almost complete conversion of DPE was reached after 90 min at a temperature of 240 °C while BPE and PPE C–O bond breaking could be achieved at milder reaction conditions. Pd/Ni bimetallic systems can be magnetically recovered and efficiently used up to eight consecutive recycling tests in the transfer hydrogenolysis of DPE. The investigated substrates were also tested using analogous Ni monometallic systems. Palladium as cometal present in the catalysts was proven to increase the C–O bond cleavage rates and decrease aromatic ring hydrogenation selectivity. The catalytic tests on all possible reaction intermediates clearly show that the hydrogenolysis cleavage in etheric C–O bond breaking was the rate-determining step under CTH conditions, while hydrogenations only take place in a successive step. Moreover, it has been demonstrated that the hydrogenation of phenol formed from CTH depends on the type of aryl groups that form the aromatic ether structure.