Mechanism of CO2 in promoting the hydrogenation of levulinic acid to γ-valerolactone catalyzed by RuCl3 in aqueous solution

Abstract

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 CO₂. 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 H₂ as the H source catalyzed by RuCl₃ 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 Ru³⁺ site is in charge of sieving H^- from HCOO^-, yielding [RuH]²⁺ hydride and CO₂. Alternatively, with H₂ as the H source, the OBL stimulates the heterolysis of H-H bond with the aid of Ru³⁺ active species, producing [RuH]²⁺ and [HL]⁺. Toward the [RuH]²⁺ formation, H₂ 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 Ru³⁺ active species is capable of cleaving the C-OH bond in 4-hydroxyvaleric acid, yielding [RuOH]²⁺ hydroxide and GVL. Subsequently, CO₂ promotes Ru-OH bond cleavage in [RuOH]²⁺, forming HCO₃^- and regenerating the Ru³⁺-active species owing to its Lewis acidity. Lastly, between the resultant HCO₃^- and [HL]⁺, a neutralization reaction occurs, generating H₂O, CO₂, and OBLs. Thus, the present study provides insights into the promotive roles of additives such as CO₂ and OBLs in Ru-catalyzed hydrogenation.