Issue 20, 2024

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.

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

Supplementary files

Article information

Article type
Paper
Submitted
21 Feb 2024
Accepted
27 Apr 2024
First published
29 Apr 2024

Phys. Chem. Chem. Phys., 2024,26, 14613-14623

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

H. Min, J. Xiong, T. Liu, S. Fu, C. Hu and H. Yang, Phys. Chem. Chem. Phys., 2024, 26, 14613 DOI: 10.1039/D4CP00753K

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