Transfer Hydrogenation of Ketones with 2‐Propanol and Raney®Nickel

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

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 2-propanol. When comparing classic hydrogenation with H-transfer hydrogenation in presence of external H₂ , it was found that external H₂ 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 H₂ 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 H₂ promoted hydrogenation of the complex reaction mixture and prevented condensation of the reactive species in contrast to the H-transfer hydrogenation.

Hydrogenation of Bio-Oil Model Compounds over Raney-Ni at Ambient Pressure

Lignocellulosic biofuels are the most promising sustainable fuels that can be added to the crude oil pool to refill the dwindling fossil resources. In this work, we tested a Raney-Ni catalyst for the hydrogenation of four bio-oil model compounds and their binary mixtures to assess their reactivity under mild conditions suitable for bio-oil stabilization preceding green diesel production from lignocellulosic biomass. The hydrogenation experiments were performed at ambient hydrogen pressure at temperatures in the range 30–70 °C. Raney-Ni was found to hydrogenate all investigated model compounds efficiently; both carbonyl groups and double bonds were saturated. In addition, it was also active in the demethoxylation of guaiacol. When studying the binary mixtures, furfuryl alcohol was found to significantly inhibit the hydrogenation of the other model compounds (guaiacol and methyl isobutyl ketone) due to their very strong adsorption.

Highly dispersed Ni nanoparticles on mesoporous silica nanospheres by melt infiltration for transfer hydrogenation of aryl ketones

Nickel-based catalysts have been applied to the catalytic reactions for transfer hydrogenation of aryl ketones.

Elucidating the reactivity of methoxyphenol positional isomers towards hydrogen-transfer reactions by ATR-IR spectroscopy of the liquid–solid interface of RANEY® Ni

In the presence of Raney® Ni and 2-propanol, guaiacol is orientated parallel to the catalyst surface, whereas 3- and 4-methoxyphenol forms a titled adsorption surface complex.

Efficient synthetic protocols in glycerol under heterogeneous catalysis

The massive increase in glycerol production from the transesterification of vegetable oils has stimulated a large effort to find novel uses for this compound. Hence, the use of glycerol as a solvent for organic synthesis has drawn particular interest. Drawbacks of this green and renewable solvent are a low solubility of highly hydrophobic molecules and a high viscosity, which often requires the use of a fluidifying co-solvent. These limitations can be easily overcome by performing reactions under high-intensity ultrasound and microwaves in a stand-alone or combined manner. These non-conventional techniques facilitate and widen the use of glycerol as a solvent in organic synthesis. Glycerol allows excellent acoustic cavitation even at high temperatures (70-100 °C), which is otherwise negligible in water. Herein, we describe three different types of applications: 1) the catalytic transfer hydrogenation of benzaldehyde to benzyl alcohol in which glycerol plays the dual role of the solvent and hydrogen donor; 2) the palladium-catalyzed Suzuki cross-coupling; and (3) the Barbier reaction. In all cases glycerol proved to be a greener, less expensive, and safer alternative to the classic volatile organic solvents.

Nickel nanoparticles in hydrogen transfer reactions

The transfer hydrogenation of organic compounds is a much safer and more environmentally benign process than reduction reactions involving molecular hydrogen, metal hydrides, or dissolving metals. In transfer hydrogenation, 2-propanol is often preferred as the source of hydrogen because it is cheap, easy to remove, and environmentally friendly. This class of transformation has been mostly pursued through the use of expensive noble metals, such as Ru, Pd, and so forth; research involving cheaper catalytically active metals has been relatively neglected. On the other hand, alcohols have recently emerged as desirable alkylating agents, a useful alternative to organic halides, in reactions of hydrogen autotransfer, also known as the "borrowing of hydrogen" methodology. For instance, the α-alkylation of ketones with alcohols is an atom-efficient process that produces water as the only byproduct in the presence of a noble metal catalyst. Hydrogen autotransfer is also successful in the synthesis of amines through a reductive aza-Wittig reaction, which involves an iminophosphorane and primary alcohol under iridium catalysis. The in situ oxidation-Wittig olefination of primary alcohols with stabilized phosphorus ylides is a commonly practiced method in organic synthesis that precludes the necessity of handling aldehydes. These reactions are normally performed in one pot but sequentially; thus the course of the alcohol oxidation needs monitoring before the ylide addition. In this Account, we describe the development of our discovery that nickel(0), in the form of nanoparticles, can replace the more expensive noble metals in both transfer hydrogenation and hydrogen autotransfer reactions. These nanoparticles were found to catalyze the transfer hydrogenation of olefins and carbonyl compounds, as well as the reductive amination of aldehydes, with 2-propanol as the hydrogen donor. All reactions proceeded in the absence of base, and the catalyst could be easily and successfully reutilized in the case of the carbonyl compounds. The catalyst was fully characterized, and the reaction mechanism, kinetics, and heterogeneous nature of the process were established through a variety of experiments. Moreover, the nickel nanoparticles enabled the activation of primary alcohols for the α-alkylation of ketones and reductive aza-Wittig reaction, with the latter leading to secondary amines. For the first time, these two reactions were achieved with a catalyst that was not one of the noble metals. We also observed that nickel nanoparticles can activate alcohols in the presence of phosphorus ylides. In this case, although the autotransfer of hydrogen failed, the reaction could be used as a key tool to construct carbon-carbon double bonds. In this respect, we describe the one-pot synthesis of stilbenes from alcohols through a Wittig-type olefination reaction promoted by nickel nanoparticles. We report a wide range of polymethoxylated and polyhydroxylated stilbenes, including the naturally occurring polyphenol resveratrol. The utility of the nickel nanoparticles was exceptional in all of the aforementioned reactions when compared with other forms of nickel, including Raney nickel.