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 β-O-4 linkage cleavage. Self-hydrogen transfer hydrogenolysis (STH) is a promising way to produce monomers from lignin through the β-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 Cβ−O bond in β-O-4 models. In the STH process, the CαH-OH in β-O-4 linkage is dehydrogenated to form β-O-4 ketone intermediates, and hydrogens are adsorbed on Pd sites to form a “hydrogen pool” which participates in the following hydrogenolysis of Cβ−O bond. DFT calculation shows that PdO more effectively activates the β-O-4 ketone intermediate and shows lower activation energy toward Cβ−O bond cleavage compared with Pd. Pd acts as the dehydrogenation site and PdO activates the Cβ−O bond. The co-existence of Pd and PdO facilitates the Cβ−O bond cleavage of β-O-4 linkage and about 40 wt % yields of lignin monomers can be obtained from poplar lignin at 180 ºC.