Theoretical Mechanism of Rh-Containing Species Catalyzing the Hydrogenolysis of Lignin Model Compounds Using -CαO-H and -Cα-H Groups as Superior H-Sources.

Abstract

The hydrogenolysis of lignin model compounds (MCs) into high-value chemicals has received increasing attention, but their catalytic reaction mechanisms are not yet very clear. Here, we report the reaction mechanisms of the hydrogenolysis of MC into 4-acetylanisole (AAL) and guaiacol (GAL) catalyzed by LRuCl₃ (L = 4'-(4-methoxyphenyl)-2,2':6',2″-terpyridine) with MC, H₂, and 1-phenylethan-1-ol (PEO) as the H-sources in aqueous solution with the Bro̷nsted base (NaOH), at the M06/def2-TZVP, 6-311++G (d,p) theoretical level, namely, RS-Self, RS-H₂, and RS-PEO, respectively. After dissociation in NaOH aqueous solution, the LRuCl₃ compound can form a stable complex LRh (OH)₃ as the initial catalytically active species. Their rate-determining steps are related to the cleavage of the -C_αO-H bond in both RS-Self-and RS-PEO, whereas it is associated with the heterolysis of the H-H bond in RS-H₂. From NaOH aqueous solution, the OH^- ligand of LRh(OH)₃ promotes the cleavage of the -C_αO-H bond with MC and PEO as H-sources, whereas it advances the heterolysis of the H-H bond with H₂ as the H-source, owing to its strong Bro̷nsted basicity. Furthermore, the Rh center of LRh(OH)₃ mediates the cleavage of the -C_α-H bond, because of its Lewis acidity. The reaction activity lowers as RS-Self > RS-PEO ≫ RS-H₂, which is positively dependent on the protonation degree of the departing H atom. Compounds containing both -C_αO-H and -C_α-H groups can act as effective donors for H-sources. In the meantime, the stronger the electron-withdrawing groups they contain, the more pronounced the protonation of the -C_αO-H group and the higher the reaction activity in providing H-sources. The present results provide insights into utilizing lignin's own functional groups as a hydrogen source for high-value conversion.