Mechanistic Investigations into the Catalytic Levulinic Acid Hydrogenation, Insight in H/D Exchange Pathways, and a Synthetic Route to d₈-γ-Valerolactone

Abstract

γ-Valerolactone (GVL) is readily accessible by catalytic hydrogenation of carbohydrate-derived levulinic acid (LA) and is an attractive biobased chemical with a wide range of applications in both the chemical (e.g., as biomass-derived solvent) and the transportation fuel sector. In this study, we used isotopic labeling experiments to provide insights into the catalytic hydrogenation pathways involved in the conversion of LA to GVL under different reaction conditions using water as an environmentally benign solvent and Ru/C as a readily available catalyst. 2H NMR experiments combined with quantum chemical calculations revealed that deuterium atoms can be incorporated at different positions as well as the involvement of the different intermediates 4-hydroxypentanoic acid and α-angelica lactone (α-AL). The insight provided by these studies revealed an as of yet unexploited sequential deuteration route to synthesize fully deuterated LA and GVL. The route starts by the conversion of LA to α-AL followed by a selective deuteration of the acidic protons of α-AL by H/D exchange with D2O. Subsequent ring-opening in D2O (d2-AL to d3-LA) and exchange of the remaining protons of d3-LA via a keto-enol tautomerization by heating in D2O under acidic conditions gives d8-LA. Finally, the d8-LA is catalytically reduced at low temperature using Ru/C with D2 in D2O to d8-GVL.

Photo–Thermo-Dual Catalysis of Levulinic Acid and Levulinate Ester to γ-Valerolactone

“…To gain insights into the mechanistic pathways, control experiments were conducted in both photo- and thermocatalytic hydrogenation reactions of LA to GVL. The major proposed mechanistic pathway of the catalytic hydrogenation of LA to GVL proceeds via the hydrogenation of the carbonyl group of LA into 4-hydroxypentanoic acid (4-HPA) followed by the cyclization step of the corresponding alcohol to GVL. ,, Also, a handful of reports proposed that esterification of LA to ester was the side reaction pathway prior to the cyclization step when acidic heterogeneous catalysts and alcohols were employed. , First, ethyl levulinate (EL) was tested in the catalytic hydrogenation of EL to GVL with both systems. Fascinatingly, the photocatalytic conversion of EL gave GVL in excellent yields (>99%) (Scheme a).…”

Micro/mesoporous LTL derived materials for catalytic transfer hydrogenation and acid reactions of bio-based levulinic acid and furanics

“…The reaction of AnL to GVL involves acid and reduction chemistry (e.g., hydration, CTH and lactonization) (Li et al, 2018;Antunes et al, 2022aAntunes et al, , 2022b. According to the literature, the reversible reaction between AnL and LA may occur via hydration-dehydration (Dutta and Bhat, 2021;Liu et al, 2021;Yuan et al, 2021), but AnL was not detected in the experiments using LA as substrate, whereas LA was formed using AnL as substrate. This is in agreement with literature studies for Hfcontaining catalysts in alcohol media (Al-Shaal et al, 2015;Li et al, 2018;Antunes et al, 2022bAntunes et al, , 2022a.…”

Levulinic Acid Hydrogenation with Homogeneous Cu(I) Catalyst

“…This pathway is favorable at low temperatures (lower than 150 °C) and is the main route in homogeneous catalysis. [4,5,6] In a second pathway, LVA dehydration yields α-angelica lactone, followed by C=C bond hydrogenation. This route is typically favorable at high temperatures (above 150 °C) in acidic media.…”

“…GVL is obtained by the hydrogenation of the ketone group to produce 4‐hydroxypentanoic acid (4‐HPA), followed by intramolecular esterification. This pathway is favorable at low temperatures (lower than 150 °C) and is the main route in homogeneous catalysis [4,5,6] …”

“…In a second pathway, LVA dehydration yields α‐angelica lactone, followed by C=C bond hydrogenation. This route is typically favorable at high temperatures (above 150 °C) in acidic media [4 6,7] . In the last decade, γ‐valerolactone (GVL) has drawn considerable attention as a bio‐based solvent, fuel, and food additive [8,9] .…”