Conventional thermocatalytic methods for lignin depolymerization usually require severe conditions, which lead to low product selectivity. Photocatalysis, which utilizes solar energy to drive chemical reactions as nature does for the photosynthesis, is a promising method to challenge the reaction that is difficult by the conventional catalytic method. Here, we report an efficient photocatalytic system for the direct transformation of lignin into aromatic monomers under visible-light irradiation. We found that CdS was very efficient for the cleavage of β-O-4 bond under visible-light irradiation. Acetophenone, an oxidative product, and phenol, a reductive product, were formed almost in equal amounts, indicating that both photogenerated electrons and holes were used for the formation of aromatic monomers. This is further confirmed by the control experiments using electron and hole scavengers. Our mechanistic studies suggest that the α-C-H bond is first cleaved by a hole, forming benzyl radical intermediate, followed by photoreductive cleavage of the ether bond in PP-ol (Figure 1, Path B). The catalytic efficiency could be significantly enhanced by using CdS quantum dots (QDs). The CdS QDs catalyzed the conversion of a wide range of lignin model compounds to aromatic monomers. Furthermore, the CdS QDs demonstrated very unique activity in the conversion of real lignin at room temperature. The yield of monomers composed of high-value sinapic and coniferyl alcohols reached 16.2% in photocatalytic depolymerization of dioxasolv birch lignin. This yield is close to the theoretical maximum by considering the selective cleavage of β-O-4 bonds. In short, we discovered that simple CdS could catalyze the deploymerization of lignin into aromatic monomers at room temperature under visible light. The selectivity toward the cleavage of β-O-4 bond was very high and the activity could be enhanced by using CdS quantum dots. Both photogenerated electrons and holes were used for the formation of products and the reaction was highly atom efficient.

Catalytic transformation of lignin into aromatics under visible-light irradiation

255th National Meeting (2018)

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