Synonyms

Ionic liquid in reduction of unsaturated bonds with molecular hydrogen; Molten salt in hydrogenation; Room temperature ionic liquid in hydrogenation

Definition

An ionic liquid (IL) is a salt with low melting point. Hydrogenation is a chemical reaction constituting the addition of pairs of hydrogen atoms to an unsaturated substrate, e.g., alkene or carbonyl compound, to reduce/saturate these organic compounds, usually in the presence of a transition-metal catalyst, e.g., palladium. Ionic liquid in hydrogenation refers to those ILs applied for the hydrogenation acting as solvent to solubilize a great number of organic and inorganic compounds or stabilizer to form ion pairs with charged intermediates and participate in polar transition states, resulting in impressive improvement of yields and selectivities.

Introduction

Ionic liquids can be applied for the hydrogenation acting as solvent to solubilize a great number of organic and inorganic compounds, or stabilizer to form ion pairs with charged intermediates and participate in polar transition states, resulting in impressive improvement of yields and selectivities of hydrogenation product.

Scientific Fundamentals

IL is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as 100 °C. Because of their distinctive properties such as high thermal stability, negligible vapor pressure, excellent dissolving capacity, easy recyclability, and diversiform structure/property modulation, ILs have been widely used as environmentally friendly catalysts/solvents in a wide range of chemical reactions.

On the other hand, hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound, usually in the presence of a transition-metal catalyst such as nickel, palladium, or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to an unsaturated substrate, e.g., alkene or carbonyl compound.

Among all of the options to improve the catalytic activity for hydrogenation, ILs have become remarkable medium or stabilizers. The positive effect of those fluids over hydrogenation is due to their inherent ionic nature, which favors in solubilizing a great number of organic and inorganic compounds, forming ion pairs with charged intermediates and participating in polar transition states. Indeed, ILs are capable of forming large supramolecular aggregates, therefore reducing activation barrier energies and facilitating the hydrogenation. This specific property gives ILs the title “entropic drivers,” resulting in very impressive improvements on yields and selectivities for hydrogenation. Therefore, transition-metal catalysts for hydrogenation in ILs often show improved catalytic activities over those conducted in traditional solvents. Furthermore, considering the estimation of 106 possible combinations of known cations and anions to form an IL, the tunable properties of ILs open unlimited possibilities for hydrogenation [1].

The recovery and reuse of catalysts have attracted a large interest in meeting the need for an environmentally benign hydrogenation process. Homogeneous catalysis has an advantage in its ability to display full catalytic activity, accompanied by the drawback of tedious separation and catalyst recovery. Immobilization of a homogeneous catalyst on a solid support realizes recycling catalyst readily. However, the resulting heterogeneous catalysts always show reduced catalytic activity. Combining efficient catalytic activity of homogeneous catalysts and recycling of heterogeneous ones have been a challenging project in hydrogenation reaction. The IL solvent clearly provides such a reaction system [1].

As aforementioned, direct immobilization of homogeneous catalyst on a support often results in gradual loss of activity, because the active species suffers a modification of chemical and physical properties. An alternative, benign, and efficient approach is the use of supported ionic liquid-phase (SILP) catalysts, where a homogeneous catalyst is dissolved in IL and impregnated on a porous support material [2]. The chemical nature of the homogeneous catalyst is preserved yet dissolved in a separate phase, which can easily be separated after the reaction, reused, or applied in a continuous flow process with high long-term stability. SILP catalysts are therefore considered as sustainable catalysts, because they combine the advantages of heterogeneous and homogeneous catalysis and make use of minimum amounts of IL/catalyst solutions in a highly efficient manner.

Key Applications

ILs Act as Medium/Stabilizer

As crucial medium or stabilizers, ILs could render the hydrogenation to proceed smoothly through preparation and stabilization of transition-metal nanoparticles, so as to realize the addition of pairs of hydrogen atoms to an unsaturated substrate, e.g., alkene or carbonyl compound. A representative hydrogenation of olefin with recyclable Ir nanoparticles in IL [BMI][PF6] (1-n-butyl-3-methylimidazoilum hexafluorophosphate) as reported by Dupont et al. (Fig. 1) [3], though several early examples of hydrogenation in ILs had been previously reported. The IL is a suitable medium for the preparation and stabilization of transition-metal nanoparticles. What is worth mentioning, the Ir nanoparticles in [BMI][PF6] maintained their hydrogenation efficiency for up to at least seven recycles, which demonstrated outstanding ability for the generation of recyclable hydrogenation systems.

Fig. 1
figure 1

Recyclable Ir nanoparticles in IL for hydrogenation of olefin. (Reprinted with permission from Ref. [3]. Copyright (2018) American Chemical Society)

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Besides olefins, arenes and various heteroarene derivatives, e.g., quinolines, pyridines, benzofurans, and furan, could also be hydrogenated efficiently with IL as medium/stabilizer to access the corresponding heterocycles, important molecules present in fine chemicals, agrochemicals, and pharmaceuticals [4].

Compared with extensive study on hydrogenation of carbon-carbon unsaturated bonds in ILs, hydrogenation of carbonyl compounds in IL seems rare. However, as one special kind of carbonyl compounds, carbon dioxide hydrogenation in IL has been widely investigated. As a major greenhouse gas and a cheap C1 resource, CO2 has become the focus of attention recently. The hydrogenation of CO2 to produce formic acid (CO2 + H2 → HCOOH) is promising for industrialization. However, recovery of the formic acid and recycling of the catalysts are still a problem. Combination of an IL and a supported ruthenium complex catalyzes the hydrogenation of CO2 with satisfactory activity and selectivity (Fig. 2) [5]. The resulting formic acid is easily collected, and the ionic liquid and catalyst can be reused directly after an easy separation step. In another example of CO2 hydrogenation to formic acid in IL, imidazolium-based IL associated with the acetate anion acts as a precursor for the formation of the catalytically active Ru-H species, as a catalyst stabilizer, and as an acid buffer, shifting the equilibrium toward free formic acid [6]. Moreover, the IL acts as an entropic driver, lowering the entropic contribution imposed by the IL surrounding the catalytically active sites. The favorable thermodynamic conditions enable the reaction to proceed efficiently at low pressures, and furthermore the immobilization of the IL onto a solid support facilitates the separation of formic acid.

Fig. 2
figure 2

Hydrogenation of carbon dioxide promoted by a task-specific IL. (Reprinted with permission from Ref. [5], Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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Biomass-derived substrates are receiving increasing interest as a part of a sustainable supply chain to chemical products and transportation fuels. IL-stabilized Ru nanoparticles could realize selective hydrogenation of biomass-based substrates [7]. The activity, selectivity, and stability of the nanoparticles can be influenced and controlled by variation of the ionic liquids. The catalytic performance is complementary to both classical homogeneous and heterogeneous catalysts. Recycling of the catalyst by supercritical CO2 extraction of the products is possible without significant loss in selectivity and reactivity. The depolymerization of cellulose could also be carried out in the IL through hydrogenation process [8].

The importance of enantioselective catalysis to pharmaceutical synthesis has led to many attempts to use ILs for hydrogenation. The combination of a racemic ligand and a chiral IL as an additive or reaction medium for asymmetric hydrogenation can lead to enantioselectivities that are identical to those obtained with the enantiopure ligand (Fig. 3) [9]. Moreover, the use of the chiral IL together with an enantiomerically pure ligand can result in an enhanced enantioselectivity and an inverted absolute configuration in the product compared to those obtained when organic solvents are used.

Fig. 3
figure 3

Enantioselective hydrogenation with racemic ligand in the presence of a chiral IL. (Reprinted with permission from Ref. [9], Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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Supported IL Phase (SILP) Catalysis

The immobilization of ligands with their transition-metal complexes in ionic liquid supports provided an efficient way for catalyst recovery and recycling. This strategy also leads to enhanced catalyst stability and avoids catalyst leaching. Because of their attractive features, the concept of SILP catalysis has received much attention in hydrogenation.

Most applications of SILP catalysis in this field concern the immobilization of Rh phosphine complexes for the hydrogenation of C=C double bonds. The Rh complex [Rh(norbornadiene)(PPh3)2][PF6] and [bmim][PF6] were impregnated on silica gel to obtain the resulting SILP catalyst (Fig. 4) [10]. The catalyst was used for the hydrogenation of alkenes. Notably, in the hydrogenation of 1-hexene, the SILPC showed an enhanced activity in comparison with homogeneous and biphasic IL/heptane reaction systems. In addition to the high activity, the supported ionic liquid catalysts also showed good long-term stability. The same catalyst was reused for 18 batch runs without any significant loss of activity.

Fig. 4
figure 4

Confined IL phase containing the rhodium complex on the surface of a silica gel support material. (Republished with permission of Royal Society of Chemistry)

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Besides alkenes, aldehydes could also be hydrogenated smoothly in supported SILP system containing a well-defined hydride Fe(II) PNP pincer complex [11]. The new SILP catalyst, with the optimum pore filling, was highly active exhibiting high turnover numbers and turnover frequencies under mild conditions without significant leaching of both the complex and the IL. In addition, the SILP catalyst allowed the hydrogenation of nitrobenzenes to arylamines to proceed under solvent-free or in neat water, room temperature, and normal pressure, giving nearly 100% yield and selectivity [12].

A Rh complex of 1,4-bisphosphine bearing two imidazolium salt moieties was successfully immobilized in an ionic liquid and reused several times for the hydrogenation of an enamide without significant loss of catalytic efficiency (Fig. 5) [10]. The experimental results showed that introduction of imidazolium salt moieties on the ligand backbone not only avoids the problem of catalyst leaching from the IL layer but also increases the catalyst stability.

Fig. 5
figure 5

Catalytic asymmetric hydrogenation in IL using chiral Rh complex of IL grafted 1,4-bisphosphine ligand. (Republished with permission of Royal Society of Chemistry)

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Cross-References