Highly Efficient Reductive Catalytic Fractionation of Lignocellulosic Biomass over Extremely Low-Loaded Pd Catalysts

The reductive catalytic fractionation (RCF) of lignocellulosic and herbaceous biomass over heterogeneous catalysts has been demonstrated to recover high-yield phenolic monomers and holocellulose-rich solids effectively, and these products could be further used to produce value-added chemicals and second-generation biofuel. Catalyst selection plays a critical role in the performance of the RCF process, and noble metal catalysts (e.g., Pt, Pd, and Ru) with a high loading of 5 wt % have been extensively used to obtain high-yield phenolic monomers and delignified holocellulose-rich solids. In this study, we demonstrated that the RCF of biomass over extremely low Pd loaded on N-doped carbon (CNx) support catalysts could produce phenolic monomers at approximately theoretical maximum yield and presented high holocellulose-rich solid recovery. When birch wood was converted over the catalyst with 0.25 wt % Pd loaded on CNx (Pd0.25/CNx) at 250 °C and an initial H2 pressure of 3.0 MPa for 3 h, a lignin-derived phenolic monomer carbon yield and highly delignified holocellulose recovery of 52.7 C % and 84.2 wt %, respectively, were achieved. The Pd0.25/CNx catalyst contained both ultrasmall Pd nanoclusters and single Pd atoms, which were stabilized on the N-functionalized carbon support. The highly activated hydrogenolysis and double-bond saturation that occurred over the Pd0.25/CNx catalyst dominantly produced 4-n-propyl guaiacol/syringol. In contrast, 4-n-propanol guaiacol/syringol with residual -OH groups was the major species obtained over the typical 5 wt % Pd/activated carbon catalyst. The plausible reaction pathways for the production of different types of phenolic monomers were discussed using density functional theory calculations. The excellent RCF performance of the Pd0.25/CNx catalyst was demonstrated using other types of biomass, such as oak, pine, and miscanthus. The successful use of extremely low-Pd-loaded catalysts is advantageous for implementing economically viable RCF techniques.