Catalytic cleavage of the β-O-4 aryl ether bonds of lignin model compounds by Ru/C catalyst

Abstract

Lignin is a potential renewable feedstock for aromatic compounds. Lignin glues cellulose and hemicellulose together in a rigid structure that protects plants from weather, insects, and disease. This rigidity also poses a barrier to cleavage of lignin into aromatic compounds. Typically, lignin is depolymerized by metal-catalyzed hydrogenolysis of its β-O-4 aryl ether (Cβ-O) bonds; this process requires high H2 pressure. Here, we show that the abundant aliphatic hydroxyl groups (Cα-OH) in lignin structure, can serve as the hydrogen source in Ru-catalyzed hydrogenolysis of the Cβ-O bonds. We pretreated the Ru/C catalyst under reducing and oxidizing conditions to generate various Ru/RuO2 ratios. Then we investigated the effects of Ru and RuO2 on hydrogenolysis of the Cβ-O bonds of lignin model compounds. We used X-ray diffraction (XRD) and H2 Temperature-programmed reduction (TPR) to determine changes of grain size and Ru content of the Ru/C catalysts, respectively. Our results revealed that Ru/C catalyzed hydrogenolysis of β-O-4 aryl ether bonds with internal hydrogen (self-hydrogen) as a hydrogen source. The elimination of external H2 in lignin hydrogenolysis is an efficient approach for lignin conversion to valuable aromatic chemicals.