Theoretical Mechanism\nof Rh-Containing Species Catalyzing\nthe Hydrogenolysis of Lignin Model Compounds Using −C<sub>α</sub>O–H and −C<sub>α</sub>–H Groups as Superior\nH‑Sources

The hydrogenolysis of lignin model compounds (MCs) into\nhigh-value\nchemicals has received increasing attention, but their catalytic reaction\nmechanisms are not yet very clear. Here, we report the reaction mechanisms\nof the hydrogenolysis of MC into 4-acetylanisole (AAL) and guaiacol\n(GAL) catalyzed by LRuCl<sub>3</sub> (L = 4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyridine)\nwith MC, H<sub>2</sub>, and 1-phenylethan-1-ol (PEO) as the H-sources\nin aqueous solution with the Bro̷nsted base (NaOH), at the M06/def2-TZVP,\n6-311++G (d,p) theoretical level, namely, RS-Self, RS-H<sub>2</sub>, and RS-PEO, respectively. After dissociation in NaOH aqueous solution,\nthe LRuCl<sub>3</sub> compound can form a stable complex LRh (OH)<sub>3</sub> as the initial catalytically active species. Their rate-determining\nsteps are related to the cleavage of the −C<sub>α</sub>O–H bond in both RS-Self-and RS-PEO, whereas it is associated\nwith the heterolysis of the H–H bond in RS-H<sub>2</sub>. From\nNaOH aqueous solution, the OH<sup>–</sup> ligand of LRh(OH)<sub>3</sub> promotes the cleavage of the −C<sub>α</sub>O–H\nbond with MC and PEO as H-sources, whereas it advances the heterolysis\nof the H–H bond with H<sub>2</sub> as the H-source, owing to\nits strong Bro̷nsted basicity. Furthermore, the Rh center of\nLRh(OH)<sub>3</sub> mediates the cleavage of the −C<sub>α</sub>–H bond, because of its Lewis acidity. The reaction activity\nlowers as RS-Self > RS-PEO ≫ RS-H<sub>2</sub>, which is\npositively\ndependent on the protonation degree of the departing H atom. Compounds\ncontaining both −C<sub>α</sub>O–H and −C<sub>α</sub>–H groups can act as effective donors for H-sources.\nIn the meantime, the stronger the electron-withdrawing groups they\ncontain, the more pronounced the protonation of the −C<sub>α</sub>O–H group and the higher the reaction activity\nin providing H-sources. The present results provide insights into\nutilizing lignin’s own functional groups as a hydrogen source\nfor high-value conversion.