Unlocking Birch Lignin Hydrocracking through Tandem Catalysis: Unraveling the Role of Moderate Hydrogen Spillover

The trade-off between intensity of hydrogen spillover and efficient production of aromatics poses an insurmountable obstacle to the depolymerization of lignin into monomeric phenols (MPs), as effectively harnessing hydrogen spillover for degradating lignin into MPs while avoiding excessive hydrogenation of aromatic rings remains a formidable challenge. Herein, a Ni–Cu/Nb2O5@Fe3O4 composite with Ni–Cu alloy and oxygen vacancies (OVs) has been fabricated for birch lignin valorization. The interspecies electron transfer within the alloy significantly improves intrinsic catalytic inertness of Cu species while simultaneously reducing intrinsic catalytic activity of Ni species toward moderate hydrogen spillover. Moreover, the mutual alloying of Ni and Cu generates moderately strong Lewis acidic sites (LASs). This resulting moderate hydrogen spillover and LASs collaboratively promote the depolymerization of lignin into crude MPs with varying degrees of methoxy substituents. Subsequently, the coupling of O atom-specific OVs recognition with LASs profoundly facilitates Ph–OMe bond dissociation, ultimately leading to the one-step generation of MPs. The tandem catalysis of moderate hydrogen spillover and specific recognition results in a high yield of MPs (up to 58 wt %) in birch lignin depolymerization, which is approximately twice as high as the reported optimum heterogeneous catalysts. Both experimental and calculation results suggest that the presence of Cu species induces an unusual electron-donating ability of Ni species during the alloying process, which successively reconstitutes their intrinsic catalytic performance and Lewis acid property, and the specific recognition of OVs is mediated by an O atom electron-effect, giving rise to a rare tandem catalysis for harvesting lignin based-MPs.