In-situ hydrogen-mediated catalytic transfer hydrogenolysis of Kraft lignin for highly selective production of monomeric phenols
Han-Bing Gao, Bing-Hu Xie, Yue-Lun Wang, Le‐Le Qiu, Yun‐Peng Zhao, Jian Xiao, Mei Zhong, Jing Liang, Jian Li, Fang‐Jing Liu, Jing‐Pei Cao
Abstract
Selective cleavage of aryl C O bonds through catalytic transfer hydrogenolysis (CTH) strategy using an in-situ hydrogen donor is a promising but challenging approach for the depolymerization and valorization of lignin. Herein, the depolymerization of lignin through the CTH approach has been effectively enhanced, selectively producing monomeric phenols. The CTH reactivity of various aryl ethers was evaluated, confirming the superior hydrogen-donating performance of isopropanol as the sole hydrogen source. The 100 % monomeric selectivity of aryl ethers indicated a preferential pathway for direct C O bond cleavage over aromatic ring hydrogenation. Increasing the reaction temperature and time can significantly enhance the CTH process, thereby promoting the stability of monomeric phenols due to intensified free radical reactions and a more robust supply of active hydrogen. Characterization results showed that macromolecular aromatic fragments within lignin were effectively converted by hydrocracking C O bonds and removing heteroatoms (O, N and S), yielding abundant monomeric phenols with 1–2 benzene rings. The CTH reaction follows a mechanism involving metal site-mediated isopropanol dehydrogenation and metal hydride formation. The heterolytic cleavage of in-situ hydrogen produces provides mobile H + and adsorbed H - at the active sites, with H + acting as the dominant active hydrogen species to attack the oxygen atoms within lignin.