Selective Hydrogenolysis of Lignin to Alkene-Functionalized Monomers over ZnO-Modified Carbon Nanotube Supported Pd Catalysts

Although current methods enable lignin extraction and depolymerization, the selective hydrogenolysis of lignin into alkene-functionalized monomers remains a significant scientific challenge. These monomers can be further functionalized into diverse natural products or pharmaceutical intermediates, thereby enabling higher-value utilization of biomass resources. Here, we report a catalyst with low Pd loading (0.4 wt%) supported on ZnO-modified carbon nanotubes (Pd/ZnNC) for the selective catalytic hydrogenolysis of the dimer β-O-4 lignin model compound (veratrylglycerol-β-guaiacyl ether, VG) via distinct reaction pathways. The Pd/ZnNC catalyst achieves a selectivity of 37.5% toward alkene-functionalized monomers (1,2-dimethoxy-4-allylbenzene and 1,2-dimethoxy-4-propenylbenzene), approaching 75% of the theoretical maximum selectivity (50%). In contrast, the Pd/CNT catalyst predominantly produces 3-(3,4-dimethoxyphenyl)-1-propanol, and excessive side-chain saturation of monomers are not conducive to subsequent utilization processes. Moreover, the Pd/ZnNC catalyst achieves a conversion rate seven-fold higher than that of commercial catalysts Pd/C (5 wt%). Experimental characterization results demonstrate that the ZnO sites on carbon nanotubes facilitate the dispersion of Pd nanoparticles, resulting in a reduction in particle size. Furthermore, the synergistic effect between ZnO and Pd active sites promotes the selective catalytic formation of monomers containing unsaturated C=C bonds. The DFT calculations reveal that the Cγ-OH elimination is facilitated by Pd nanoparticles supported on ZnO, which reduces the reaction energy barrier and promotes the generation of propenyl-substituted monomers. The pathway enabled by Pd/ZnNC catalyst offers a great potential for lignin depolymerization into high-value products.