Unraveling the Metal–Acid Site Cooperation in Organic Acid-Modified NiPd/Al(PO3)3 for the Selective Cleavage of C–O Bridged Bonds in Lignin to Produce Aromatic Monomers

Selective cleavage of the C–O bridged bonds in lignin while minimizing the hydrogenation of aromatic rings remains a major challenge in the production of aromatic monomers. Tuning the metal and acid sites in bifunctional catalysts is a feasible strategy to improve the catalyst activity and selectivity for catalytic hydrogenolysis (CH) of lignin to aromatic monomers. Herein, a 4-(trifluoromethyl)salicylic acid (TFMSA)-modified NiPd/Al(PO₃)₃ catalyst was prepared via a facile impregnation method for the CH of lignin and its model compounds. The CH of the model compounds proceeded under kinetic control with negligible mass transfer resistance. Benzyl phenyl ether achieved 100% conversion on NiPd-TFMSA/Al(PO₃)₃ (versus 40% on unmodified NiPd/Al(PO₃)₃) with 100% selectivity of toluene and phenol at 180 °C and 0.5 MPa H₂. The catalyst also exhibited good applicability for the CH of other lignin model compounds, yielding high selectivity toward aromatic monomers. The enhanced CH activity and selectivity could be attributed to the synergistic interaction between Ni and Pd, as well as the formation of more oxygen vacancies and Lewis acid sites modified by TFMSA. Additionally, TFMSA modification significantly increased the turnover frequency (TOF) of Pd and acid sites of the catalyst. The active hydrogen transfer pathway was elucidated through active hydrogen trapping and isotope labeling experiments. The results revealed that the metal–acid sites cooperatively activated H₂ to generate active hydrogen radicals (H·) and protons (H⁺), which preferentially transferred to the oxygen atom, leading to selective cleavage of the C–O bond. The CH of alkali lignin over NiPd-TFMSA/Al(PO₃)₃ achieved a 52.9 wt % yield of lignin oil through the selective cleavage of C–O bridged bonds. Phenols, especially guaiacols, were the dominant aromatic monomers, accounting for 63.3% of the relative content in the lignin oil.