Semi‐Batch Hydrotreatment of Lignin‐Derived Phenolic Compounds over Raney‐Ni with a Continuous Regeneration of the H‐Donor Solvent

Abstract

Lignin can be converted into useful precursors of fuels and fine chemicals by thermochemical conversion followed by catalytic hydrogenation using metal catalysts at severe reaction conditions. Thus, mild hydrogenation would significantly improve the sustainability of lignin valorization. Here, hydrogenation of phenols, alkylphenols, and methoxyphenols was achieved at mild reaction conditions (70 °C and atmospheric pressure) via H-transfer hydrogenation over Raney-Ni catalyst in 2-propanol and 2-butanol solvents. The transfer hydrogenation was feasible at the mild conditions, but the complexity of the reactant greatly decreased or even completely suppressed its reactivity. The position of the functional group (o-, m-, p-position) had a great effect on the reactivity of phenols. Moreover, 2-butanol enhanced the conversion of phenols in comparison with 2propanol. When comparing classic hydrogenation with H-transfer hydrogenation in presence of external H 2 , it was found that external H 2 not only regenerated H-donor solvent and ensured stable performance but also increased conversion of phenols and alkylphenols. On the other hand, the absence of external H 2 boosted the conversion of methoxy phenols. Finally, phenols extracted from a pyrolysis oil aqueous phase were hydrogenated. The conversion of phenols was greatly affected by competitive adsorption of different compounds present in the reaction mixture. External H 2 promoted hydrogenation of the complex reaction mixture and prevented condensation of the reactive species in contrast to the H-transfer hydrogenation.

Hydrogen‐transfer strategy in lignin refinery: Towards sustainable and versatile value‐added biochemicals

“…Isopropanol's efficacy in hydrogen transfer reactions has been the subject of extensive research. [119][120][121][122][123][124][125] Notably, during the cleavage of ether bonds via hydrogen transfer over a Ni/ Al 2 O 3 catalyst, isopropanol has been observed to enhance the formation of aromatic alkanes, phenols, and cyclohexanol in a partially reduced form. [90] The proposed mechanism, illustrated in Figure 3b, involves isopropanol's interaction with basic sites on the nickel catalyst surface, which facilitates the release of active hydrogen.…”

“…Isopropanol's efficacy in hydrogen transfer reactions has been the subject of extensive research [119–125] . Notably, during the cleavage of ether bonds via hydrogen transfer over a Ni/Al 2 O 3 catalyst, isopropanol has been observed to enhance the formation of aromatic alkanes, phenols, and cyclohexanol in a partially reduced form [90] .…”

Precisely Designed Nitrogen-Doped Mesoporous Carbon Sphere-Confined Electron-Deficient Pd Nanoclusters with Enhanced Catalytic Hydrogenation Performance

“…23 In phenol hydrogenation, phenol undergoes one-step hydrogenation to produce cyclohexanone using a green catalysis approach, which offers an optimal solution. 24,25 However, maintaining control over the selectivity of the desired product is a significant challenge during phenol hydrogenation because cyclohexanone readily undergoes hydrogenation to form cyclohexanol. 26 Another important reaction with practical significance is the selective hydrogenation of benzoic acid to cyclohexanecarboxylic acid, which is an important intermediate for the synthesis of caprolactam, quinolinone, etc.…”

“…The former approach often requires harsh reaction conditions such as high temperatures and high pressures (1–2 MPa, 140–180 °C), and many byproducts are produced, leading to a low yield (<10%) . In phenol hydrogenation, phenol undergoes one-step hydrogenation to produce cyclohexanone using a green catalysis approach, which offers an optimal solution. , However, maintaining control over the selectivity of the desired product is a significant challenge during phenol hydrogenation because cyclohexanone readily undergoes hydrogenation to form cyclohexanol . Another important reaction with practical significance is the selective hydrogenation of benzoic acid to cyclohexanecarboxylic acid, which is an important intermediate for the synthesis of caprolactam, quinolinone, etc. , However, hydrogenation of the benzoic acid aromatic ring often requires harsher reaction conditions (1–5 MPa H 2 , 150–250 °C) than the hydrogenation of other functional groups because the high resonance energy of the aromatic ring must be overcome.…”

Construction of Spirocyclic Pyrrolo[1,2-a]quinoxalines via Palladium-Catalyzed Hydrogenative Coupling of Phenols and Nitroarenes

“…In addition, phenols have strong p-π-conjugated structures, resulting in a strong dissociation energy of the phenolic hydroxyl group, which increases the breaking strength of the C–O bond. Hence, the inherent thermodynamic stability and kinetic inertness of phenolic compounds limit their application potential . Recently, the cross-coupling of phenols to construct nitrogen heterocycles via transfer hydrogenation has been elegantly demonstrated by Li et al .…”