Atomically dispersed nickel on nitrogen-doped activated carbon for reductive catalytic fractionation of birch toward high-yield propyl-substituted phenolic monomers.

Lignin-first fractionation is a promising strategy for the integrated valorization of lignocellulosic biomass, but its broader application is still hindered by the reliance on noble-metal catalysts. Herein, a low-loading Ni catalyst supported on N-doped activated carbon was developed for the reductive catalytic fractionation (RCF) of birch wood. Structural characterization confirmed that Ni species were highly dispersed and stabilized in Ni-NAC coordination environments. Under the optimized conditions of 240 °C, 3 MPa initial H2 pressure, 4 h, and methanol as solvent, the catalyst afforded a phenolic monomer yield of 48.2 wt%, in which 4-propylsyringol and 4-propylguaiacol were the dominant products, corresponding to 82.4% selectivity toward propyl-substituted monomers. Meanwhile, a high delignification degree of 94.4% was achieved, while 91.8% cellulose and 84.7% hemicellulose were retained in the solid residue. In comparison with Ni-AC, N-doped carbon, and the catalyst-free system, the Ni-N catalyst exhibited markedly enhanced performance for lignin depolymerization and stabilization, and showed activity comparable to that of commercial Ru/C under identical conditions. Model-compound studies further revealed that the Cα-OH and Cβ-O functionalities in β-O-4 linkages play decisive roles in promoting selective bond cleavage, whereas the Cγ-OH group mainly affects side-chain evolution. In addition, the catalyst showed good hydrothermal stability and partial recovery of activity after regeneration. This work demonstrates the potential of low-loading atomically dispersed Ni sites as an effective non-noble-metal platform for selective lignin depolymerization and carbohydrate-preserving biorefinery.