Evaluating Gold and Selenium Chemistry for Selective Transformations of Lignin Model Compounds

Abstract

Applications of gold and selenium chemistry are reported as novel approaches to promote lignin depolymerization into more valuable chemicals via selective oxidation reactions (alcohol oxidations and Baeyer‐Villiger reactions). In this study, we proposed two different oxidative methodologies using Au/SiO2 and phenylseleninic acid resin (PAR) as stable and reusable catalysts to promote selective transformations of the β‐O‐4 linkage of lignin model compounds. After evaluating the catalytic systems under batch conditions, they were both applied in a packed‐bed reactor for continuous flow operations. By using Au/SiO2 as a catalyst under flow conditions, ketones were efficiently obtained (up to 86% conversion) from the oxidation of alcohols with a residence time (tR) of 30 min. In the case of Baeyer‐Villiger oxidations catalyzed by phenylseleninic acid resin, the corresponding esters were obtained in up to 91% conversion (tR=30 min). Both systems efficiently catalyzed the conversion of the lignin model compounds.magnified image

Catalytic Strategies and Mechanism Analysis Orbiting the Center of Critical Intermediates in Lignin Depolymerization

“…Besides the hydrolysis of aryl esters and acetal esters generated from the lignin C α –C β or C Ar –C α bond cleavage by the BV oxidation, in 2017, Wang reported a K 2 CO 3 -promoted alcoholysis of the resulting ester products at 45 °C, affording various useful monomers from β-O-4 dimers in excellent yields (up to 99%). In addition, in 2018, Andrade achieved the BV oxidation of β-O-4 lignin models with a continuous flow reactor equipped with supported Au/SiO 2 to catalyze the oxidation of C α –OH to CO with H 2 O 2 and filled with the phenylseleninic acid resin to catalyze the BV oxidation of the generated C α ketones to esters with H 2 O 2 . In 2020, Hao developed a Mo–V-catalyzed system with H 2 O 2 to achieve the lignin C–C bond cleavage at room temperature via the above C α oxidation–BV oxidation pathway.…”