Recent Advances in Ruthenium-Catalyzed Hydrogenation Reactions of Renewable Biomass-Derived Levulinic Acid in Aqueous Media

Abstract

Levulinic acid (LA) is classified as a key platform chemical for the development of future biorefineries, owing to its broad spectrum of potential applications and because it is simply available from lignocellulosic biomass through inexpensive and high-yield production routes. Catalytic hydrogenation reactions of LA into the pivotal intermediate compound γ-valerolactone (GVL), and beyond GVL to yield valeric acid (VA), 1,4-pentanediol (1,4-PDO), and 2-methyltetrahydrofuran (2-MTHF) have gained considerable attention in the last decade. Among the various transition metals used as catalysts in LA hydrogenation reactions, ruthenium-based catalytic systems have been the most extensively applied by far, due to the inherent ability of ruthenium under mild conditions to hydrogenate the keto functionality of LA selectively into an alcohol group to form 4-hydroxyvaleric acid intermediate, which yields GVL spontaneously after dehydration and cyclization. This review focuses on recent advances in the field of aqueous-phase ruthenium-catalyzed hydrogenation reactions of LA toward GVL, VA, 1,4-PDO, 2-MTHF, 2-pentanol, and 2-butanol. It employs heterogeneous catalysts on solid supports, and heterogeneous water-dispersible catalytic nanoparticles or homogeneous water-soluble catalytic complexes with biphasic catalyst separation, for the inter alia production of advanced biofuels such as valeric biofuels and other classes of liquid transportation biofuels, value-added fine chemicals, solvents, additives to gasoline, and to food as well. The significance of the aqueous solvent to carry out catalytic hydrogenations of LA has been highlighted because the presence of water combines several advantages: (i) it is highly polar and thus an ideal medium to convert polar and hydrophilic substrates such as LA; (ii) water is involved as a byproduct; (iii) the presence of the aqueous solvent has a beneficial effect and enormously boosts hydrogenation rates. In sharp contrast, the use of various organic solvents gives rise to a dramatic drop in catalytic activities. The promotional effect of water was proven by numerous experimental investigations and several theoretical studies employing various types of catalytic systems; (iv) the large heat capacity of water renders it an excellent medium to perform large scale exothermic hydrogenations more safely and selectively; and (v) water is a non-toxic, safe, non-inflammable, abundantly available, ubiquitous, inexpensive, and green/sustainable solvent.

Keywords: biofuels; biorefinery; hydrogenation; levulinic acid; platform chemical; renewable; water; γ-valerolactone.

Figure 1

Production routes of levulinic acid from cellulose and hemicellulose.

Figure 2

Routes based on the catalytic hydrogenation of LA to obtain advanced biofuels, chemicals and solvents.

Figure 3

Possible reaction mechanisms for the synthesis of GVL from LA.

Figure 4

Proposed mechanism for the dehydration of 4-hydroxyvaleric acid intermediate to yield by intramolecular esterification GVL (Ruppert et al., 2015).

Figure 5

Hydrogenation reaction of LA in D₂O.

Figure 6

Proposed mechanism for the hydrogenation of LA to GVL at 275°C on the surface of Ru/C catalyst in aqueous media.

Figure 7

Synthesis of the Ru/NH₂-γ-Al₂O₃ catalytic system.

Figure 8

Structures of the water-soluble ligands triphenylphosphinetrisulfonic acid trisodium salt (TPPTS), triphenylphosphinemonosulfonic acid monosodium salt (TPPMS), n-butyldiphenylphosphinedisulfonic acid disodium salt (BDPPDS), tris(2,4-dimethylphenyl)phosphinetrisulfonic acid trisodium salt (TDMPPTS), 1,3,5-triaza-7-phosphaadamantane (PTA), tris(2-carboxyethyl)phosphine (TCEP), bathophenanthrolinedisulfonic acid disodium salt (BPhDS), bathocuproinedisulfonic acid disodium salt (BCDS), 2-aminoethanesulfonic acid (Taurine), nitrilotriacetic acid trisodium salt (NTA·Na₃), ethylenediaminetetraacetic acid tetrasodium salt (EDTA·Na₄), 2,2′-biquinoline-4,4′dicarboxylic acid dipotassium salt (BQC), tris(2-pyridyl)phosphine (T₂PyP), N,N′-2,2′-bipyridine-4,4′-dicarboxylic acid (BPyDCA), diethylenetriaminepentakis(methylphosphonic acid) (DTPPA), diethylenetriaminepentaacetic acid pentasodium salt (DTPA·Na₅) and 3-pyridinesulfonic acid (3-PSA).

Figure 9

Hydrogenation of LA beyond GVL to yield VA for the production of valeric biofuels and 1,4-PDO followed by dehydration to form 2-MTHF.

Figure 10

Ring-opening pathways of GVL through breaking the C(1)-(O)2 bond which after hydrogenation eventually leads into 1,4-PDO and through C(4)-(O)2 bond breaking into VA.