Hydroxyl-functionalized Ionic Liquids Research Articles

Ion Mobility in Hydroxy-Functionalized Ionic Liquids Depends on Cationic Clustering: Tracking the Alkyl Chain Length Behavior with Deuteron NMR Relaxation.

Observing and quantifying the like-charge attraction in liquids and solutions is still challenging. However, we showed that elusive cation-cation hydrogen bonding may govern the structure and interaction in hydroxyl-functionalized ionic liquids. Therefore, cationic cluster formation depends on the shape, charge distribution, and functionality of the ions. We demonstrated by means of solid-state 2H NMR spectroscopy that cationic clusters change the structure and dynamics of ionic liquids. With increasing alkyl chain length, we observed two deuteron quadrupole coupling constants for the OD groups, differing by about 30 kHz. The lower value was assigned to the cation-cation interaction, indicating that the average (c-c) hydrogen bonds are stronger than the (c-a) hydrogen bonds between the cation and the anion despite the repulsive and attractive Coulomb interaction in the first and latter cases. Ion mobility could be studied by 2H NMR spectroscopy, although the deuterons in the hydrogen-bonded clusters underwent fast exchange. Our results also showed that simple relaxation models are not applicable anymore and that anisotropic motion must be considered.

How Like-Charge Attraction Influences the Mobility of Cations in Hydroxyl-Functionalized Ionic Liquids

Attractive interactions between ions of like charge remain an elusive concept. Observing and quantifying this type of interaction in liquids and solutions is still a major challenge. Recently, we have shown that cation-cation interactions are present in hydroxyl-functionalized ionic liquids and that they can be controlled by the shape, charge distribution and functionality of the ions. In the present study, we demonstrate that cationic cluster formation does not only change the local structures of the ionic liquids but also influences the dynamics of the cations in a characteristic way. We show that solid-state 2H NMR spectroscopy is well suited for the study of molecular motion, even if the hydrogen bonded species of interest are indistinguishable due to fast deuteron exchange. We also provide valuable information about the applicability of well-accepted relaxation models.

Synthesis and characterization of physicochemical properties of two new hydroxyl-functionalized ionic liquids

Two hydroxyl functionalized ionic liquids, 1-(2-hydroxyethyl)-3-methylimidazolium acetate [HEMIM][AC], and 1-(2-hydroxyethyl)-3-methylimidazolium propionate [HEMIM][Pro] ionic liquids were synthesized. The ionic liquids were characterized by 1H NMR and 13C NMR respectively. The density ρ, surface tension γ, and viscosity η of the two ionic liquids were measured in the temperature range (288.15–333.15 K). The contributions of anionic structure to molecular volume Vm, standard molar entropy S0 and the crystal energy UPOT, as well as the variation rules of surface entropy Sa, surface energy Ea and sound velocity W were calculated and discussed. In addition, according to Eyring viscosity equation and thermodynamic equation, the activation Gibbs energy of viscous flow activation ΔG≠ is calculated at different temperatures. And the influence of different anions on the thermodynamic properties of ionic liquids was discussed.

Novel hydroxyl-functionalized ionic liquids as efficient catalysts for the conversion of CO2 into cyclic carbonates under metal/halogen/cocatalyst/solvent-free conditions

Environmentally friendly synthesis route to carbonates from CO2 and epoxides catalysed by novel hydroxyl-functionalized ionic liquids under metal/halogen/cocatalyst/solvent-free conditions.

Hydrogen Bonds between Ions of Opposite and Like Charge in Hydroxyl-Functionalized Ionic Liquids: an Exhaustive Examination of the Interplay between Global and Local Motions and Intermolecular Hydrogen Bond Lifetimes and Kinetics.

Hydroxyl-functionalized ionic liquids (ILs) represent a new interesting class of ILs where hydrogen bonds (HBs) play an important role: here, "typical" HBs between cations and anions (ca) are competing with "atypical" HBs connecting pairs of cations (cc). We study the equilibrium and kinetics of (cc) and (ca) HBs in 1-(n-hydroxyalkyl)-pyridinium bis(trifluoromethlysulfonyl)imide [HOCnPy][NTf2] ILs by means of molecular dynamics simulations. (cc) HBs are found to be between 0.96 and 3.76 kJ mol-1 stronger than their (ca) counterparts, depending on the alkyl chain length. HB lifetimes and kinetics are analyzed by means of HB population and reactive flux correlation functions. Essentially, four different HB lifetimes have to be considered, spanning about 3 orders of magnitude, each valid in its own right and each associated with different aspects of HB breaking and HB reformation. The long-time limiting behavior of the HB population correlation function is controlled by diffusion of the ions and can be quantitatively described by analytical expressions. The short-time HB behavior is tied to the localized dynamics of the hydroxyl group exploring its local solvation environment. A minimalist kinetic two-domain model is introduced to realistically describe the time evolution of the HB population correlation function for both (ca) and (cc) HBs over 5 orders of magnitude. By employing the reactive flux method, we determine the kinetics of HB breaking, unaffected by diffusion processes. We determine both, the ultrafast upper boundary and the average rate of HB breaking, allowing recrossing-events during the transient relaxation time period. For sufficiently long alkyl chains, all those computed HB lifetimes indicate a higher kinetic stability of (cc) HBs over (ca) HBs; for short chains, it is vice-versa.

Synthesis, vibrational and thermal properties of new functionalized 1- (2-hydroxyethyl) -3-methylimidazolium dihydrogenophosphate ionic liquid

Very recently, the hydroxyl-functionalized ionic liquids have gained immense interest and were developed for a number of interesting applications. In this work and for the first time new hydroxyl-functionalized namely; 1-(hydroxyethyl)-3-methylimidazolium dihydrogenophosphate ionic liquid [EtOHMIM+][H2PO4−] was synthesized in our laboratory. The synthesis is based on an alkylation reaction of 1-methylimidazole followed by anion exchange. The obtained IL is characterized by 1H-NMR, 13C-NMR spectroscopy. Their experimental vibrational spectroscopy have been investigated using Infrared and Raman spectroscopy, based on the infrared (IR) and Raman spectroscopies results, complete vibrational assignments have been performed. Besides, thermal properties of this hydroxyl-functionalized IL were investigated by using following techniques, (i) Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG) in the temperature range from 20 to 600 C°, (ii) differential scanning calorimetry (DSC) from-100 °C to 200 °C. The thermogravimetry analysis was coupled with mass spectrometry in order to assess the influence of anion on the measured property.

Kinetics of Hydrogen Bonding between Ions with Opposite and Like Charges in Hydroxyl-Functionalized Ionic Liquids.

Hydrogen-bonded structures and their lifetimes in ionic liquids (ILs) are governed by the subtle balance between Coulomb interactions, hydrogen bonding, and dispersion forces. Despite the dominant Coulomb interaction, local and directional hydrogen bonds (HBs) can play an important role in the behavior of ILs. Compared to water, the archetype of hydrogen-bonded liquids, ILs have larger constituents and higher viscosities but are typically lacking a three-dimensional HB network. Hydroxyl-functionalized ionic liquids are even more special: regular HBs between cations and anions (ca) are accompanied by HBs between pairs of cations (cc). Recently, infrared (IR) measurements have suggested that the (cc) HBs are even stronger than their (ca) counterparts and their strength can be controlled via the hydroxyalkyl chain length. In this paper, we show by means of molecular dynamics (MD) simulations that the presence of HBs has a profound effect on the molecular mobility of the ions. We investigate the kinetic mechanism of hydrogen bonding in ILs and show that the lifetimes and hence the stability of (cc) HBs increase with the chain length, making them more stable than the respective (ca) HBs. The observed HB equilibrium can explain the peculiar chain length dependence of the relative molecular mobilities of the ions by a direct comparison between hydroxyl-functionalized ILs with their nonfunctionalized counterparts.

Aqueous solutions of hydroxyl-functionalized ionic liquids: Molecular dynamics studies

A series of aqueous solutions of 1-(n-hydroxyalkyl)-3-(n-hydroxyalkyl) imidazolium bromide ([HOCnCmOHIm][Br], with n and m = 2, 6,10 and 14) were studied by atomistic molecular dynamics simulations. Structural properties were characterized by the radial distribution functions between different pairs, angular distributions and aggregation numbers. Dynamics of the system has been investigated by computing the diffusion of the ions and molecules. Structures of the aggregates formed depend upon the length of the hydroxyalkyl chains. The long-distance spatial correlations observed in solutions with cations having long chain substituent are arising due to the formation of intercalated structures. A thin film like structure is formed in solutions having longer hydroxyalkyl chains, with the structure stabilized by the dispersion interactions between the interdigitated alkyl chains and the hydrogen bond formation between the hydroxyl group of a cation with head group of a different cation. Anions are dispersed near the surface of the film.

Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants-Comparison with Salt Bridges and Molecular Systems.

We present deuteron quadrupole coupling constants (DQCC) for hydroxyl‐functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb‐enhanced hydrogen bonds between cation and anion (c–a), and the unusual hydrogen bonds between cation and cation (c–c), which are present despite repulsive Coulomb forces. We measure these sensitive probes of hydrogen bonding by means of solid‐state NMR spectroscopy. The DQCCs of (c–a) ion pairs and (c–c) H‐bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (c–c) species successfully compete with the (c–a) ion pairs and dominate the cluster populations. Equilibrium constants obtained from molecular‐dynamics (MD) simulations show van't Hoff behavior with small transition enthalpies between the differently H‐bonded species. We show that cationic‐cluster formation prevents these ILs from crystallizing. With cooling, the (c–c) hydrogen bonds persist, resulting in supercooling and glass formation.

Cluster Formation through Hydrogen Bond Bridges across Chloride Anions in a Hydroxyl-Functionalized Ionic Liquid.

Several recent studies of hydroxyl-functionalized ionic liquids (ILs) have shown that cation-cation interactions can be dominating these materials at the molecular level when the anion involved is weakly interacting. The hydrogen bonds between the like ions led to the formation of interesting chain-like, ring-like, or distinct dimeric (i. e. two ion pairs) supermolecular clusters. In the present work, vibrational spectroscopy (ATR-IR and Raman) and density functional theory (DFT) calculations of the hydroxyl-functionalized imidazolium ionic liquid C2 OHmimCl indicate that anion-cation hydrogen bonding interactions are dominating, leading to the formation of distinct dimeric ion pair clusters. In this arrangement, the Cl- anions function as a bridge between the cations by establishing bifurcated hydrogen bonds with the OH group of one cation and the C(2)-H of another cation. Cation-cation interactions, on the other hand, do not play a significant role in the observed clusters.

Alkyl chain length effects of hydroxyl-functionalized imidazolium ionic liquids in the ionothermal synthesis of LiFePO4

LiFePO4 is one of the most promising cathode materials for lithium ion batteries. To investigate the role of the cation in the ionothermal synthesis of LiFePO4, three imidazolium-based ionic liquids, containing hydroxyl-terminated alkyl chains of various lengths, were synthesized using microwave-assisted methods: 1-(3-hydroxypropyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide, 1-(6-hydroxyhexyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide, and 1-(9-hydroxynonyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide. An ionothermal synthesis was attempted with each ionic liquid, using various precursors. The resulting solid products were investigated using powder X-ray diffraction and solid state nuclear magnetic resonance spectroscopy. The yield of LiFePO4 increased with an increase in the hydroxyl-terminated alkyl chain length on the imidazolium.

Isolating the role of hydrogen bonding in hydroxyl-functionalized ionic liquids by means of vaporization enthalpies, infrared spectroscopy and molecular dynamics simulations.

The enthalpy of vaporization is mainly the amount of the energy needed for transferring quantities from the liquid into the gas phase. It simply describes the energy required to overcome the interaction energy between quantities if those evaporate as monomers as is the case for molecular liquids. The situation for ionic liquids (ILs) is more complex. We do not know the delicate composition of different types of interaction, neither for the liquid nor for the gas phase. Additionally, we have to consider that ILs evaporate as ion pairs which carry substantial interaction energy of all kind into the vapor phase. In this study, we measured the vaporization enthalpies of well-selected hydroxyl-functionalized and non-hydroxyfunctionalized ILs. In particular, we focussed on the case of hydroxyl-functionalized ILs providing possible hydrogen bonding between cation and anion in the liquid as well as in the gas phase. With infrared spectroscopy, we showed that all the hydroxyl groups are involved in hydrogen bonding in the liquid state of the ILs. However, molecular dynamics simulations showed that the evaporating ion pairs also include this hydrogen bond. A detailed analysis of the potential energies for all IL constituents showed that the hydrogen bond hinders favourable interaction between the polarizable ring of the cations and the anions leading to higher vaporization enthalpies for the hydroxyl-functionalized ILs.

Theoretical Study of Ionic Liquid Clusters Catalytic Effect on the Fixation of CO2

Conversion of carbon dioxide (CO2) to valuable materials is important to academia and the chemical industry. Fixation of carbon dioxide catalyzed by ionic liquids (ILs) is an attractive option. Much work has been done in the fixation of carbon dioxide catalyzed by ILs, but most of it focused on reactions catalyzed by one molecule. In this work, we studied the existence of IL clusters in a series of systems containing hydroxyl-functionalized ILs (HFILs) through a series of theoretical calculations. The intermolecular interactions and microstructures of IL clusters were studied by molecular dynamics (MD) simulations. The results indicated that three-ion clusters to six-ion clusters exist in systems, while the number of clusters decreased as the cluster size increased. Instead of studying the single ion or ionic pair catalytic processes, we gained more insight into the catalytic effects of ionic liquid clusters on the fixation of CO2 by using density functional theory (DFT) calculations. Results suggested that the activity of ILs could be enhanced to a certain extent in the presence of clusters.

Structural Motifs in Cold Ternary Ion Complexes of Hydroxyl-Functionalized Ionic Liquids: Isolating the Role of Cation-Cation Interactions.

We address the competition between intermolecular forces underlying the recent observation that ionic liquids (ILs) with a hydroxyl-functionalized cation can form domains with attractive interactions between the nominally repulsive positively charged constituents. Here we show that this behavior is present even in the isolated ternary (HEMIm+)2NTf2- complex (HEMIm+ = 1-(2-hydroxyethyl)-3-methylimidazolium) cooled to about 35 K in a photodissociation mass spectrometer. Of the three isomers isolated by double resonance techniques, one is identified to exhibit direct contact between the cations. This linkage involves a cooperative H-bond wherein the OH group on one cation binds to the OH group on the other, which then attaches to the basic N atom of the anion. Formation of this motif comes at the expense of the usually dominant interaction of the acidic C(2)H group on the Im ring with molecular anions, as evidenced by isomer-dependent shifts in the C(2)H vibrational fundamentals.

Fluorescence quenching of coumarin 153 by hydroxyl-functionalized room temperature ionic liquids

Steady-state absorption and fluorescence as well as time-resolved fluorescence of coumarin 151 (C151) and coumarin 153 (C153) were measured in hydroxyl-functionalized ionic liquids ([HOEmim][BF4] and [HOEmim][N(CN)2]) and in nonhydroxyl-functionalized ionic liquids ([Emim][BF4] and [Emim][N(CN)2]). Both the steady-state fluorescence and time-resolved fluorescence observations reveal that hydroxyl-functionalized ionic liquid quenches the fluorescence of C153 while the nonhydroxyl-functionalized ionic liquid does not. We also measured the time-resolved fluorescence anisotropy of C151 and C153 in both [HOEmim][BF4] and [Emim][BF4]. It is found that the ratio of the rotational relaxation lifetime of C153 in [HOEmim][BF4] with respect to that in [Emim][BF4] is about 15% larger than that of C151 in [HOEmim][BF4] with respect to that in [Emim][BF4], indicating extra interaction between C153 and [HOEmim][BF4] exists except the effect of the viscosity of ionic liquid.

Extraction of l-Tryptophan by Hydroxyl-Functionalized Ionic Liquids

In this work, several hydroxyl-functionalized ionic liquids (ILs) were synthesized and used to extract tryptophan from the aqueous phase. The experimental results indicated that the hydroxyl-functionalized ILs presented better extraction ability than the dialkylimidazolium-based ILs. The extraction efficiency was significantly influenced by the pH of the aqueous phase. Moreover, the cationic form of tryptophan preferred to transfer into the IL phase, thereby suggesting a cation-exchange extraction mechanism. Driving forces of the extraction processes included hydrogen bonding, hydrophobicity, π–π stacking, and the screening effect. Among these factors, hydrogen bonding was the main parameter, as suggested by the results of thermodynamic analysis. After extraction, [C1C6OHim]NTf2 could be reused by using the pH swing effect. These findings demonstrate that [C1C6OHim]NTf2 is a good alternative to conventional solvents for recovering tryptophan from the aqueous phase.

Hydroxyl-functionalized ionic liquid for activation and conversion of CO2 and methanol into dimethyl carbonate

A series of hydroxyl-functionalized ionic liquids such as dialkyl (or alkyl) di-(tri-) hydroxyethyl ammonium hydroxide, tetrahydroxyethyl ammonium hydroxide and alkylhydroxyethyl or dihydroxyethyl immidazolium hydroxide were synthesized and characterized by FT-IR and 1H NMR. These ionic liquids containing hydroxyls were used as catalysts for direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide. The results showed that benzylmethyldihydroxyethyl ammonium hydroxide (BzMDH) is an effect catalyst with raised catalytic activity and high selectivity to DMC. The effect of hydroxyl groups on the activation of methanol and CO2 were explored. Furthermore, the reaction mechanism of this synthetic reaction was discussed.

Self-Assembly of Cations in Aqueous Solutions of Hydroxyl-Functionalized Ionic Liquids: Molecular Dynamics Studies.

The effect of presence of a hydroxyl-functionalized alkyl chain of varying carbon number on the self-assembly of cations in aqueous solutions of 1-(n-hydroxyalkyl)-3-decylimidazolium bromide (where the alkyl groups are ethyl, butyl, heptyl, and decyl) has been studied using atomistic molecular dynamics simulations. Spontaneous self-assembly of cations to form aggregates with hydrophobic core and hydrophilic surface is observed. The shape of the aggregates changes from quasispherical in the case of cations with hydroxyheptyl or smaller substituent chain, to a thin film like intercalated aggregate in the case of cations with hydroxydecyl chain. Cations with hydroxydecyl substituent chain exhibit long-range spatial correlations, and the anions are associated with cations to a greater extent due to the higher surface charge density of the aggregate. The ordered film like aggregate is stabilized by the dispersion interactions between the intercalated substituent chains and the intermolecular hydrogen bonds formed between the alkoxy oxygen atoms and the hydrogen atoms of the imidazolium ring. The cations form less compact aggregates with lower aggregation number than their nonhydroxyl analogues in the corresponding aqueous solutions. The intracationic and aggregate structures are governed by the length of the hydroxyalkyl chain.

Mechanism of fixation of CO2 in the presence of hydroxyl-functionalized quaternary ammonium salts

Utilization of CO2 (carbon dioxide) to synthesize useful organic compounds has attracted much attentions from the academia and industry. Recently, the fixation of CO2 catalyzed by using four hydroxyl-functionalized ionic liquids (ILs) has been successfully achieved. However, the detailed mechanism is still ambiguous. We performed a theoretical study on the mechanism of the fixation of CO2 catalyzed by 2-hydroxyl-ethyl-triethylammonium bromide (HETEAB) using the DFT method. The structural and energetic information for each possible route were obtained. Furthermore, the similarities and differences of two different catalysts, HETEAB and 2-hydroxyl-ethyl-tributylammonium bromide (HETBAB), were discussed in detail. In addition, the result shows that the OH functional group is one essential factor to improve the catalyst ability. Except that, including active H atom in the IL is the other major factor to refine the catalytic performance.

Efficient absorption of ammonia with hydroxyl-functionalized ionic liquids

The efficient ammonia absorption capacities of hydroxyl-functionalized ILs are due to the strong hydrogen bonding between the N atom of NH3 and the H atom of the hydroxyl on cation.