Mechanism of CO 2 in promoting the hydrogenation of levulinic acid to γ-valerolactone catalyzed by RuCl 3 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 2 . 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 2 as the H source catalyzed by RuCl 3 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 3+ site is in charge of sieving H - from HCOO - , yielding [RuH] 2+ hydride and CO 2 . Alternatively, with H 2 as the H source, the OBL stimulates the heterolysis of H-H bond with the aid of Ru 3+ active species, producing [RuH] 2+ and [HL] + . Toward the [RuH] 2+ formation, H 2 as the H source exhibits higher activity than HCOOH as the H source in the presence of an OBL. Thereafter, H - in [RuH] 2+ gets transferred to the unsaturated C site of ketone carbonyl in LA. Afterwards, the Ru 3+ active species is capable of cleaving the C-OH bond in 4-hydroxyvaleric acid, yielding [RuOH] 2+ hydroxide and GVL. Subsequently, CO 2 promotes Ru-OH bond cleavage in [RuOH] 2+ , forming HCO 3 - and regenerating the Ru 3+ -active species owing to its Lewis acidity. Lastly, between the resultant HCO 3 - and [HL] + , a neutralization reaction occurs, generating H 2 O, CO 2 , and OBLs. Thus, the present study provides insights into the promotive roles of additives such as CO 2 and OBLs in Ru-catalyzed hydrogenation.