A Comparative Study of Electrochemical Reduction of Levulinic Acid on Various Electrodes in Organic Solvents

Studies on the electrochemical hydrogenation (ECH) of levulinic acid (LA) to valeric acid (VA) or γ-valerolactone (GVL) have mainly focused on the electrochemical reduction of LA in acidic aqueous solutions. However, the narrow range of applied potentials has hindered understanding of some mechanistic aspects of LA electrochemical conversion. Earlier, we discovered that employing proton-deficient non-aqueous reaction media provides more comprehensive insights into the mechanism of LA electrochemical reduction. Here, we conducted further investigations into the LA electroreduction process using cyclic voltammetry in various organic solvents on a Pt electrode and on various electrode materials in acetonitrile, both with and without the addition of proton donors. The products of the ECH processes were identified using HPLC. The solvent nature, the presence of proton donors, the electrode material, and the applied potential strongly influence the LA electroreduction process. This study reveals that LA, in the presence proton donors, can undergo electrochemical reduction through different pathways, depending on the difference (ΔE1/2) between the reduction half-wave potential of protons and LA on a certain electrode. When the difference is large, the LA reduction is incomplete and the formation of GVL is observed. Under the close reduction potentials of protons and LA, LA can be completely reduced to VA.

Highly efficient CuNi-ZrO2 nanocomposites for selective hydrogenation of levulinic acid to γ-valerolactone

CuNi-ZrO2 nanocomposites were prepared by a simple coprecipitation technique of copper, nickel and zirconium ions with potassium carbonate. The structures of the nanocomposites were characterized by N2 physical adsorption, XRD, H2-TPR and STEM-EDS. The Cu0.05Ni0.45-ZrO2 nanocomposite showed outstanding catalytic performance in hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL), especially NaOH solution (0.5 mol L-1) as a solvent. 100% LA conversion and > 99.9% GVL selectivity are achieved over Cu0.05Ni0.45-ZrO2 catalyst at 200 °C, 3 MPa for 1.5 h. Characterization results suggest that the excellent reactivity of the Cu0.05Ni0.45-ZrO2 may be due to a better reducibility of nickel oxide in the CuONiO-ZrO2, dispersion of Ni in the Cu0.05Ni0.45-ZrO2 compared to nickel oxide in the NiO-ZrO2 and Ni in the Ni0.5-ZrO2 and promotion of OH-. The results demonstrate that the Cu0.05Ni0.45-ZrO2 nanocomposite has potential to realize high efficiency and low-cost synthesis of liquid fuels from biomass.

Hydrogenation of Carboxylic Acids, Esters, and Related Compounds over Heterogeneous Catalysts: A Step toward Sustainable and Carbon-Neutral Processes

The catalytic hydrogenation of esters and carboxylic acids represents a fundamental and important class of organic transformations, which is widely applied in energy, environmental, agricultural, and pharmaceutical industries. Due to the low reactivity of the carbonyl group in carboxylic acids and esters, this type of reaction is, however, rather challenging. Hence, specifically active catalysts are required to achieve a satisfactory yield. Nevertheless, in recent years, remarkable progress has been made on the development of catalysts for this type of reaction, especially heterogeneous catalysts, which are generally dominating in industry. Here in this review, we discuss the recent breakthroughs as well as milestone achievements for the hydrogenation of industrially important carboxylic acids and esters utilizing heterogeneous catalysts. In addition, related catalytic hydrogenations that are considered of importance for the development of cleaner energy technologies and a circular chemical industry will be discussed in detail. Special attention is paid to the insights into the structure-activity relationship, which will help the readers to develop rational design strategies for the synthesis of more efficient heterogeneous catalysts.

Hierarchically porous single catalyst Ru/HPW/UiO-66 with synergistic acid/metal sites for one-pot catalytic synthesis of γ–Valerolactone

γ-valerolactone (GVL) is an important lignocellulosic platform molecule with huge potential for various industrial and chemical applications, which is usually produced by using Levulinic acid (LA) or its esters as...

Highly efficient and selective aqueous phase hydrogenation of aryl ketones, aldehydes, furfural and levulinic acid and its ethyl ester catalyzed by phosphine oxide-decorated polymer immobilized ionic liquid-stabilized ruthenium nanoparticles

Phosphine oxide-decorated polymer immobilized ionic liquid stabilized RuNPs catalyse the hydrogenation of aryl ketones with remarkable selectivity for the CO bond, complete hydrogenation to the cyclohexylalcohol and hydrogenation of levulinic acid to γ-valerolactone.

Heterogeneous Catalytic Hydrogenation of Levulinic Acid to γ-Valerolactone with Formic Acid as Internal Hydrogen Source

As one of the most promising biomass-based platform molecules, γ-valerolactone (GVL) can be synthesized from a variety of lignocellulosic feedstocks through different hydrogen supply pathways. Among these transformation routes, the hydrogenation of levulinic acid (LA) to GVL by using formic acid (FA) as the internal hydrogen source is regarded as a critical path for the sustainable development of renewable energy systems. Although a large number of studies on the synthesis of GVL have been reported, the FA/LA catalytic system has not been interpreted as thoroughly as it should be. In this Minireview, core concerns are focused on key issues and their effects in this FA/LA catalytic system. The catalytic mechanism, together with competitive adsorption behavior between FA and LA on heterogeneous catalysts, is presented. The effects of active metal species and catalyst supports on the overall catalytic performance are summarized, and the influences of key condition parameters, including the time, temperature, FA/LA molar ratios, and aqueous solvent, are discussed. In particular, impacts and improvements of coke deposition and metal leaching, which could greatly affect the catalyst stability, are analyzed in detail. Additionally, several feasible suggestions for the enhancement of the catalytic efficiency and stability are also proposed.

Transformation of Levulinic Acid to Valeric Biofuels: A Review on Heterogeneous Bifunctional Catalytic Systems

Valerate esters (VAEs) commonly derived from levulinic acid (LA), which is deemed as one of the most promising biomass platform molecules, have been hailed as "valeric biofuels" in recent years. The cascade transformation of LA to VAEs consists of a series of acid- and metal-catalyzed processes alternately, in which heterogeneous bifunctional catalysts are required for better catalytic performance. The transformation pathway from LA to VAEs is presented, and bifunctional catalytic systems for the cascade transformation of LA into valeric acid (VA) and its esters, as well as one-pot conversion processes, are reviewed. Additionally, effects of metal and acid sites on the catalytic performance are discussed in detail. Impacts of and improvements to coke deposition, which is determined to be the primary reason for the reduction in catalytic activity, are also analyzed. Finally, feasible suggestions are proposed for enhanced catalytic performance and a reduction in overall costs.

Enhanced Production of γ-Valerolactone with an Internal Source of Hydrogen on Ca-Modified TiO2 Supported Ru Catalysts

Calcium-modified titania supported Ru catalysts were synthesized and evaluated for the hydrogenation of levulinic acid with formic acid as an internal hydrogen source and water as a green solvent. A new elegant photoassisted method was developed for the synthesis of uniform-size and evenly distributed Ru particles on the titania surface. Compared with the counterpart catalysts prepared by classical wet impregnation, enhanced levulinic acid conversion and γ-valerolactone yield were obtained and further improved through modification of the support by introduction of calcium into the titania support. This synthesis approach resulted in a change of the surface and bulk properties of the support, namely a decrease in the anatase crystallite size and the formation of a new calcium titanate phase. As a consequence, the properties of the catalysts were modified, and smaller ruthenium particles that had stronger interactions with the support were obtained. This affected the strength of the CO adsorption on the catalyst surface and facilitated the reaction performance. The optimum size of Ru particles that allowed for most efficient levulinic acid conversion was established.

An Efficient and Reusable Embedded Ru Catalyst for the Hydrogenolysis of Levulinic Acid to γ-Valerolactone

To achieve a higher activity and reusability of a Ru-based catalyst, Ru nanoparticles were embedded in N-doped mesoporous carbon through a hard-template method. The catalyst showed excellent catalytic performance (314 h^-1 turnover frequency) and recyclability (reusable five times with 3 % activity loss) for the hydrogenolysis of levulinic acid to γ-valerolactone. Compared with the mesoporous carbon without N-doping and conventional activated carbon, the introduction of N-dopant effectively improved the dispersion of Ru nanoparticles, decreased the average size of Ru nanoparticles to as small as 1.32 nm, and improved the adsorption of levulinic acid, which contributed to the increase in the activity of the catalyst. Additionally, the embedding method increased the interaction between Ru nanoparticles and carbon support in contrast with the conventional impregnation method, thus preventing the Ru nanoparticles from migration, aggregation, and leaching from the carbon surface and therefore increasing the reusability of the catalyst.

Efficient hydrogenation of levulinic acid in water using a supported Ni–Sn alloy on aluminium hydroxide catalysts

Efficient hydrogenation of levulinic acid (LA) into γ-valerolactone (GVL) in water using supported Ni–Sn(1.4)/AlOH consisting of Ni₃Sn₂ alloy species was achieved with high selectivity towards GVL and the catalyst could be reused without any significant loss of activity and selectivity.

The conversion of levulinic acid into γ-valerolactone using Cu–ZrO2 catalysts

A series of Cu–ZrO₂ catalysts prepared by a co-precipitation method were studied for the hydrogenation of levulinic acid to give γ-valerolactone (GVL).

Acid-functionalized mesoporous carbon: an efficient support for ruthenium-catalyzed γ-valerolactone production

The hydrogenation of levulinic acid has been studied using Ru supported on ordered mesoporous carbons (OMCs) prepared by soft-templating. P- and S-containing acid groups were introduced by postsynthetic functionalization before the addition of 1 % Ru by incipient wetness impregnation. These functionalities and the reaction conditions mediate the activity and selectivity of the levulinic acid hydrogenation. The presence of S-containing groups (Ru/OMC-S and Ru/OMC-P/S) deactivates the Ru catalysts strongly, whereas the presence of P-containing groups (Ru/OMC-P) enhances the activity compared to that of pristine Ru/OMC. Under mild conditions (70 °C and 7 bar H2 ) the catalyst shows high selectivity to γ-valerolactone (GVL; >95 %) and high stability on recycling. However, under more severe conditions (200 °C and p H 2=40 bar) Ru/OMC-P is particularly able to promote GVL ring-opening and the consecutive hydrogenation to pentanoic acid.