Self-hydrogen transfer hydrogenolysis of native lignin over Pd-PdO/TiO2

Lignin is a renewable aromatics resource that can be converted into high-value chemicals through the beta-O-4 linkage cleavage. Self-hydrogen transfer hydrogenolysis (STH) is a promising way to produce monomers from lignin through the beta-O-4 linkage cleavage with no additional hydrogen sources, but usually with low yield toward native lignin. Here, a Pd-PdO/TiO2 catalyst was prepared for the STH of native lignin. Compared with Pd/TiO2 and PdO/TiO2, Pd-PdO/TiO2 showed the highest activity in the STH of C-beta-O bond in beta-O-4 models. In the STH process, the C alpha H-OH in beta-O-4 linkage is dehydrogenated to form beta-O-4 ketone intermediates, and hydrogens are adsorbed on Pd sites to form a hydrogen pool which participates in the following hydrogenolysis of C-beta-O bond. DFT calculation shows that PdO more effectively activates the beta-O-4 ketone intermediate and shows lower activation energy toward C-beta-O bond cleavage compared with Pd. Pd acts as the dehydrogenation site and PdO activates the C-beta-O bond. The co-existence of Pd and PdO facilitates the C-beta-O bond cleavage of beta-O-4 linkage and about 40 wt % yields of lignin monomers can be obtained from poplar lignin at 180 degrees C.

Automated summary

A Pd-PdO/TiO2 catalyst was prepared for the self-hydrogen transfer hydrogenolysis (STH) of lignin, showing higher activity and yield compared with other catalysts. In the STH process, Pd and PdO play different roles, with Pd acting as the dehydrogenation site and PdO activating the C-beta-O bond.