Selective hydrogenolysis of lignin to alkene-functionalized monomers over ZnO-modified carbon nanotube supported Pd catalystsOA

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 dimerβ-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 is not conducive to subsequent utilization processes.Moreover,the Pd/ZnNC catalyst achieves a conversion rate sevenfold 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.