Radical-induced β-H elimination through metal-support charge transfer in Ru(OH)x/C catalyst for enhanced lignin oxidative depolymerization in aqueous phase

Catalytic oxidative depolymerization of lignin is an important strategy for producing valuable oxygen-functionalized monoaromatic compounds. However, this process is less efficient over heterogeneous catalysts due to the slow rate-determining step of selective oxidation of secondary alcohol (C α -OH to C α =O) in β-O-4 lignin linkages. Herein, a facile and efficient Ru(OH) x /C catalyst was developed for the oxidative depolymerization of lignin model compounds and real lignin in aqueous phase using molecular oxygen as oxidant. Extensive characterizations revealed that molecular oxygen abstracts electrons from the conductive carbon matrix to generate superoxide radicals (•O 2 – ), which are subsequently converted to hydroperoxide radicals (HOO•) in the presence of water. The C α -OH in the β-O-4 linkages first reacts with Ru-OH to form a Ru-O-C(H)- metal alkoxide intermediate, which then undergoes β-H elimination with the help of HOO• to generate H 2 O 2 and surface Ru-O-C•- species. Hydrolysis of Ru-O-C•- leads to the generation of C α =O products and the regeneration of Ru-OH sites. Different from the traditional Ru(OH) x -catalyzed alcohol dehydration mechanism in which Ru abstracts β-H to form Ru-H, in this case, the β-H in the Ru-alkoxide intermediate is eliminated by the hydroperoxide radicals rather than Ru, avoiding the formation of inert Ru-H species, thereby accelerating the dehydrogenation of C α -OH to C α =O. Electrochemical measurements further confirmed the charge transfer from Ru(OH) x to carbon during alcohol oxidation, demonstrating that electron donation by alcohol occurs on Ru(OH) x , while electron abstraction by O 2 occurs on the carbon. The free-radical-involved β-H elimination reaction pathway endows Ru(OH) x /C with superior catalytic oxidation activity in the aqueous phase compared with traditional metal oxides supported Ru(OH) x (e.g. SiO 2 , Al 2 O 3 , TiO 2 ), and provides a fundamental basis for the rational design of aqueous phase oxidation catalysts.