Mechanism of CO in promoting the hydrogenation of levulinic acid to γ-valerolactone catalyzed by RuCl in aqueous solution.

A Ru-containing complex shows good catalytic performance toward the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) with the assistance of organic base ligands (OBLs) and CO. Herein, we report the competitive mechanisms for the hydrogenation of LA to GVL, 4-oxopentanal (OT), and 2-methyltetrahydro-2,5-furandiol (MFD) with HCOOH or H as the H source catalyzed by RuCl in aqueous solution at the M06/def2-TZVP, 6-311++G(d,p) theoretical level. Kinetically, the hydrodehydration of LA to GVL is predominant, with OT and MFD as side products. With HCOOH as the H source, initially, the OBL (triethylamine, pyridine, or triphenylphosphine) is responsible for capturing H from HCOOH, leading to HCOO and [HL]. Next, the Ru site is in charge of sieving H from HCOO, yielding [RuH] hydride and CO. Alternatively, with H as the H source, the OBL stimulates the heterolysis of H-H bond with the aid of Ru active species, producing [RuH] and [HL]. Toward the [RuH] formation, H as the H source exhibits higher activity than HCOOH as the H source in the presence of an OBL. Thereafter, H in [RuH] gets transferred to the unsaturated C site of ketone carbonyl in LA. Afterwards, the Ru active species is capable of cleaving the C-OH bond in 4-hydroxyvaleric acid, yielding [RuOH] hydroxide and GVL. Subsequently, CO promotes Ru-OH bond cleavage in [RuOH], forming HCO and regenerating the Ru-active species owing to its Lewis acidity. Lastly, between the resultant HCO and [HL], a neutralization reaction occurs, generating HO, CO, and OBLs. Thus, the present study provides insights into the promotive roles of additives such as CO and OBLs in Ru-catalyzed hydrogenation.