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 solid 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 the high loading of 5 wt% have been extensively used to obtain high-yield phenolic monomers and delignified holocellulose-rich solid. 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, 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 ultra-small 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. Contrarily, 4-n-propanol guaiacol/syringol with residual –OH groups were 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 the density functional theory calculation. 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.