Ionic liquids: "normal" solvents or nanostructured fluids?

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Small-Angle X-ray Scattering Study of the Amphiphilic Bulk Nanostructure of Tetraalkylammonium Deep Eutectic Solvents

Deep eutectic solvents (DESs) are low-melting mixtures, often prepared from a salt and a molecular hydrogen bond donor. Like ionic liquids, DESs that contain at least one sufficiently amphiphilic component can form bicontinuous nanostructures consisting of polar and nonpolar domains, although this has not been widely explored for many DES combinations. Here, the bulk nanostructures of DESs comprising tetraalkylammonium bromide salts (tetrabutylammonium bromide, tetraoctylammonium bromide, and methyltrioctylammonium bromide) with alkanols and alkanoic acids of systematically varied chain lengths (C2, C6, C8, and C10) as hydrogen bond donors have been studied. Small-angle X-ray scattering techniques were used to identify the relationship between the alkyl chain length and functionality of the hydrogen bond donor on the nature of the amphiphilic nanostructures formed. These findings demonstrated that the amphiphilic nanostructures of the DESs were not affected by the functional group on the hydrogen bond donor, with these nanostructures influenced primarily by both the absolute and relative alkyl chain lengths of the salt and hydrogen bond donor.

From Conventional to Sustainable Catalytic Approaches for Heterocycles Synthesis

Synthesis of heterocyclic compounds is fundamental for all the research area in chemistry, from drug synthesis to material science. In this framework, catalysed synthetic methods are of great interest to effective reach such important building blocks. In this review, we will report on some selected examples from the last five years, of the major improvement in the field, focusing on the most important conventional catalytic systems, such as transition metals, organocatalysts, to more sustainable ones such as photocatalysts, iodine-catalysed reaction, electrochemical reactions and green innovative methods.

Insight into the glycerol extraction from biodiesel using deep eutectic solvents

CONTEXT

The main challenge of large-scale biofuel production is related to the extraction of its undesired impurities including glycerol, water, methanol, soap/catalyst, free fatty acids, glycerides, and others. There are many ways to remove glycerol, and herein, the one alternative is the extraction of glycerol from biodiesel by deep eutectic solvents. In this regard, the mixture of a choline chloride (ChCl) and urea, methyltriphenylphosphonium chloride (MTPPCl), and ethylene glycol (EGL), as a deep eutectic solvent (DES), is effective in removing glycerol from biofuel.

METHODS

In this work, we have investigated the formation mechanism of ChCl and urea, and then MTPPCl and EGL, as a DES, and then extraction of glycerol from biofuel via DES implementing density functional theory (DFT) by Gaussian09 software, B3LYP basis set, and classical all-atom molecular dynamics (MD) simulations by Gromacs software, GROMOS force field. DFT approximation demonstrates that Cl ion plays an important binding role in the formation of complexes ChCl/urea-based DES + biofuel and in MTPPCl/EGL-based DES + biofuel. We have also considered the formation and change of hydrogen bonds upon the formation of these systems using the DFT method. Large HOMO-LUMO gaps in ChCl/urea-based DES + biofuel and in MTPPCl/urea-based DES + biofuel demonstrate the stability of the complexes. The results of MD work have stated that the chloride ion formed bonding with the choline/ethylene glycol EGL, while still weakly intermolecular interacting with the urea/methyltriphenylphosphonium in ChCl/urea- and MTPPCl/EGL-based DESs. Further results of MD simulations stated that the DESs had a higher intermolecular interaction with glycerol in comparison with biofuel, thereby favoring the extraction process of glycerol from model biofuel.

HIGHLIGHTS

• Intermolecular interactions of choline chloride and urea, methyl triphenyl phosphonium chloride, and ethylene glycol-based DESs and their applications in the extraction of glycerol from biofuel studied by DFT calculations and classical all-atom molecular dynamics simulations. • Calculated outputs of DFT calculations and classical all-atom molecular dynamics simulations for DESs and their applications in the extraction of glycerol from biofuel were discussed in detail. • The molecular formation mechanism of choline and methyl triphenyl phosphonium-based DESs and their application in the extraction process of glycerol from biofuel were summarized.

A Deep Eutectic Solvent Thermomorphic Multiphasic System for Biocatalytic Applications

The applicability of a thermomorphic multiphasic system (TMS) composed of a hydrophobic deep eutectic solvent (DES) and an aqueous potassium phosphate buffer with a lower critical solution temperature (LCST) phase change for homogeneous biocatalysis was investigated. A lidocaine‐based DES with the fatty acid oleic acid as a hydrogen‐bond donor was studied. Phase diagrams were determined and presented within this study. We tested different additional components to the solvent system and observed a decrease in the cloud point of approximately 0.026 °C per concentration unit. Distribution studies revealed a clear distribution of the protein in the aqueous buffer phase (>95 %), whereas the hydrophobic substrate and educt accumulated (>95 %) in the DES‐enriched layer. Finally, a reduction catalyzed by horse liver alcohol dehydrogenase was performed in a larger‐scale experiment, and the biocatalyst could be recycled by simply removing the DES phase for three recycling runs.

Chemistry Dissolved in Ionic Liquids. A Theoretical Perspective

The theoretical treatment of ionic liquids must focus now on more realistic models while at the same time keeping an accurate methodology when following recent ionic liquids research trends or allowing predictability to come to the foreground. In this Perspective, we summarize in three cases of advanced ionic liquid research what methodological progress has been made and point out difficulties that need to be overcome. As particular examples to discuss we choose reactions, chirality, and radicals in ionic liquids. All these topics have in common that an explicit or accurate treatment of the electronic structure and/or intermolecular interactions is required (accurate methodology), while at the same time system size and complexity as well as simulation time (realistic model) play an important role and must be covered as well.

Density functional theory of alkali metals at the IL/graphene electrochemical interface

The mechanism of charge transfer between metal ions and graphene in the presence of an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate) is investigated by means of density functional theory calculations. For that purpose, two different comparisons are established: (i) the behavior of Li⁺ and K⁺ when adsorbed onto the basal plane of graphene and (ii) the differences between Li⁺ approaching the carbon surface from the basal plane and being intercalated through the edge plane of trilayer graphene. In the first case, it is found that the metal ions must overcome high energy barriers due to their interaction with the ionic liquid before reaching an equilibrium position close to the interface. In addition, no significant charge transfer between any of the metals and graphene takes place until very close energetically unfavorable distances. The second configuration shows that Li⁺ has no equilibrium position in the proximity of the interface but instead has an equilibrium position when it is inside the electrode for which it has to cross an energy barrier. In this case, the formation of a LiC₁₂ complex is observed since the charge transfer at the equilibrium distance is achieved to a considerable extent. Thus, the interfacial charge transfer resistance on the electrode in energy devices based on ionic liquids clearly depends not only on the binding of the ionic liquid to the metal cations and their ability to form a dense solvation shell around them but also on the surface topography and its effect on the ion packing on the surface.

Electrochemical Synthesis of Unique Nanomaterials in Ionic Liquids

The review considers the features of the processes of the electrochemical synthesis of nanostructures in ionic liquids (ILs), including the production of carbon nanomaterials, silicon and germanium nanoparticles, metallic nanoparticles, nanomaterials and surface nanostructures based on oxides. In addition, the analysis of works on the synthesis of nanoscale polymer films of conductive polymers prepared using ionic liquids by electrochemical methods is given. The purpose of the review is to dwell upon an aspect of the applicability of ILs that is usually not fully reflected in modern literature, the synthesis of nanostructures (including unique ones that cannot be obtained in other electrolytes). The current underestimation of ILs as an electrochemical medium for the synthesis of nanomaterials may limit our understanding and the scope of their potential application. Another purpose of our review is to expand their possible application and to show the relative simplicity of the experimental part of the work.

Effects of Ionic Liquids on the Nucleofugality of Chloride

The nucleofugality of chloride has been measured in solvent mixtures containing ionic liquids for the first time, allowing reactivity in these solvents to be put in context with molecular solvents. Using well-described electrofuges, solvolysis rate constants were determined in mixtures containing different proportions of ethanol and the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide; the different solvent effects observed as the mixture changed could be explained using interactions of the ionic liquid with species along the reaction coordinate, determined using temperature dependent kinetic studies. The solvolysis data allowed determination of the nucleofugality of chloride in these mixtures, which varied with the proportion of salt in the reaction mixture, demonstrating quantitatively the importance of the amount of ionic liquid in the reaction mixture in determining reaction outcome. Nucleofugality data for chloride were determined in seven further ionic liquids, with the reactivity shown to vary over more than an order of magnitude. This outcome illustrates that the components of the ionic liquid are critical in determining reaction outcome. Overall, this work quantitatively extends the understanding of solvent effects in ionic liquids and demonstrates the potential for such information to be used to rationally select an ionic liquid to control reaction outcome.

Advances Achieved by Ionic-Liquid-Based Materials as Alternative Supports and Purification Platforms for Proteins and Enzymes

Ionic liquids (ILs) have been applied in several fields in which enzymes and proteins play a noteworthy role, for instance in biorefinery, biotechnology, and pharmaceutical sciences, among others. Despite their use as solvents and co-solvents, their combination with materials for protein- and enzyme-based applications has raised significant attention in the past few years. Among them, significant advances were brought by supported ionic liquids (SILs), in which ILs are introduced to modify the surface and properties of materials, e.g., as ligands when covalently bond or when physiosorbed. SILs have been mainly investigated as alternative supports for enzymes in biocatalysis and as new supports in preparative liquid chromatography for the purification of high-value proteins and enzymes. In this manuscript, we provide an overview on the most relevant advances by using SILs as supports for enzymes and as purification platforms for a variety of proteins and enzymes. The interaction mechanisms occurring between proteins and SILs/ILs are highlighted, allowing the design of efficient processes involving SILs. The work developed is discussed in light of the respective development phase and innovation level of the applied technologies. Advantages and disadvantages are identified, as well as the missing links to pave their use in relevant applications.

Influence of Alkyl Chain Length on Thermal Properties, Structure, and Self-Diffusion Coefficients of Alkyltriethylammonium-Based Ionic Liquids

The application of ionic liquids (ILs) has grown enormously, from their use as simple solvents, catalysts, media in separation science, or electrolytes to that as task-specific, tunable molecular machines with appropriate properties. A thorough understanding of these properties and structure-property relationships is needed to fully exploit their potential, open new directions in IL-based research and, finally, properly implement the appropriate applications. In this work, we investigated the structure-properties relationships of a series of alkyltriethylammonium bis(trifluoromethanesulfonyl)imide [TEA-R][TFSI] ionic liquids in relation to their thermal behavior, structure organization, and self-diffusion coefficients in the bulk state using DSC, FT-IR, SAXS, and NMR diffusometry techniques. The phase transition temperatures were determined, indicating alkyl chain dependency. Fourier-transformed infrared spectroscopy studies revealed the structuration of the ionic liquids along with alkyl chain elongation. SAXS experiments clearly demonstrated the existence of polar/non-polar domains. The alkyl chain length influenced the expansion of the non-polar domains, leading to the expansion between cation heads in polar regions of the structured IL. 1H NMR self-diffusion coefficients indicated that alkyl chain elongation generally caused the lowering of the self-diffusion coefficients. Moreover, we show that the diffusion of anions and cations of ILs is similar, even though they vary in their size.