The effect of pressure on cation-cellulose interactions in cellulose/ionic liquid mixtures

Low-Viscosity Ether-Functionalized Ionic Liquids as Solvents for the Enhancement of Lignocellulosic Biomass Dissolution

Due to the substantial usage of fossil fuels, the utilization of lignocellulosic biomass as renewable sources for fuels and chemical production has been widely explored. The dissolution of lignocellulosic biomass in proper solvents is vital prior to the extraction of its important constituents, and ionic liquids (ILs) have been found to be efficient solvents for biomass dissolution. However, the high viscosity of ILs limits the dissolution process. Therefore, with the aim to enhance the dissolution of lignocellulosic biomass, a series of new ether-functionalized ILs with low viscosity values were synthesized and characterized. Their properties, such as density, viscosity and thermal stability, were analyzed and discussed in comparison with a common commercial IL, namely 1-butyl-3-methylimidazolium chloride (BMIMCl). The presence of the ether group in the new ILs reduces the viscosity of the ILs to some appreciable extent in comparison to BMIMCl. 1-2(methoxyethyl)-3-methylimidazolium chloride (MOE-MImCl), which possesses the lowest viscosity value among the other ether-functionalized ILs, demonstrates an ability to be a powerful solvent in the application of biomass dissolution via the sonication method. In addition, an optimization study employing response surface methodology (RSM) was carried out in order to obtain the optimum conditions for maximum dissolution of biomass in the solvents. Results suggested that the maximum biomass dissolution can be achieved by using 3 weight% of initial biomass loading with 40% amplitude of sonication at 32.23 min of sonication period.

Pressure-Dependent Confinement Effect of Ionic Liquids in Porous Silica

The effect of confining ionic liquids (ILs) such as 1-ethyl-3-methylimidazolium tetrafluoroborate [C₂C₁Im][BF₄] or 1-butyl-3-methylimidazolium tetrafluoroborate [C₄C₁Im][BF₄] in silica matrices was investigated by high-pressure IR spectroscopy. The samples were prepared via the sol-gel method, and the pressure-dependent changes in the C–H absorption bands were investigated. No appreciable changes were observed in the spectral features when the ILs were confined in silica matrices under ambient pressure. That is, the infrared measurements obtained under ambient pressure were not sufficient to detect the interfacial interactions between the ILs and the porous silica. However, dramatic differences were observed in the spectral features of [C₂C₁Im][BF₄] and [C₄C₁Im][BF₄] in silica matrices under the conditions of high pressures. The surfaces of porous silica appeared to weaken the cation-anion interactions caused by pressure-enhanced interfacial IL-silica interactions. This confinement effect under high pressures was less obvious for [C₄C₁Im][BF₄]. The size of the cations appeared to play a prominent role in the IL-silica systems.

Utilizing Infrared Spectroscopy to Analyze the Interfacial Structures of Ionic Liquids/Al₂O₃ and Ionic Liquids/Mica Mixtures under High Pressures

The interfacial interactions between ionic liquids (1,3-dimethylimidazolium methyl sulfate and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate) and solid surfaces (mesoporous aluminum oxide and mica) have been studied by infrared spectroscopy at high pressures (up to 2.5 GPa). Under ambient pressure, the spectroscopic features of pure ionic liquids and mixtures of ionic liquids/solid particles (Al₂O₃ and mica) are similar. As the pressure is increased, the cooperative effect in the local structure of pure 1,3-dimethylimidazolium methyl sulfate becomes significantly enhanced as the imidazolium C–H absorptions of the ionic liquid are red-shifted. However, this pressure-enhanced effect is reduced by adding the solid particles (Al₂O₃ and mica) to 1,3-dimethylimidazolium methyl sulfate. Although high-pressure IR can detect the interactions between 1,3-dimethylimidazolium methyl sulfate and particle surfaces, the difference in the interfacial interactions in the mixtures of Al₂O₃ and mica is not clear. By changing the type of ionic liquid to 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, the interfacial interactions become more sensitive to the type of solid surfaces. The mica particles in the mixture perturb the local structure of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate under high pressures, forcing 1-ethyl-3-methylimidazolium trifluoromethanesulfonate to form into an isolated structure. For Al₂O₃, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate tends to form an associated structure under high pressures.

Interactions of ionic liquids and surfaces of graphene related nanoparticles under high pressures

High-pressure infrared spectroscopy was used to study the interactions between 1-methyl-3-propylimidazolium iodide [MPIM]I and graphene-based nanoparticles. The results obtained at ambient pressure indicate the imidazolium ring of the cation to be a more favorable moiety for adsorption than alkyl C-H groups at ambient pressure. Upon increasing the pressure, the dominant C²-H band of pure [MPIM]I yields a significant red frequency shift. As the mixtures, i.e., graphene oxide (GO)/[MPIM]I, reduced graphene oxide (RGO)/[MPIM]I, and graphene (G)/[MPIM]I, were compressed, mild shifts in the C²-H absorption frequency were observed. The absence of drastic red-shifts suggests that the local C²-H structures may be perturbed by the addition of GO, RGO, and G under high pressures. When pure [MPIM]I was compressed from ambient to 0.4 GPa, the alkyl C-H band at ca. 2964 cm^-1 was blue-shifted to 2984 cm^-1. This discontinuous jump occurring around 0.4 GPa becomes less obvious for the mixtures GO/[MPIM]I, RGO/[MPIM]I, and G/[MPIM]I. The results of this study suggest that the addition of GO, RGO, and G can disturb the local structures of alkyl C-H under high pressures, demonstrating that high pressures may have the potential to tune the strength of ionic liquid-surface interactions and the performance of energy storage devices (e.g. supercapacitors).

Cation/macromolecule interaction in alkaline cellulose solution characterized with pulsed field-gradient spin-echo NMR spectroscopy

As a breakthrough to the traditional ¹H diffusometry, the interaction of cations with cellulose is investigated via⁷Li and ²³Na PFG-SE NMR. The diffusion coefficient of Li⁺ decreases more than that of Na⁺ with the addition of cellulose, which indicates a stronger binding of LiOH with the macromolecule. Therefore, a new, facile, accurate and repeatable method to characterize ion/polymer interactions is established.

Solvation Structure of Uracil in Ionic Liquids

The local solvation environment of uracil dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate has been studied using neutron diffraction techniques. At solvent:solute (ionic liquid:uracil) ratios of 3:1 and 2:1, little perturbation of the ion-ion correlations compared to those of the neat ionic liquid are observed. We find that solvation of the uracil is driven predominantly by the acetate anion of the solvent. While short distance correlations exist between uracil and the imidazolium cation, the geometry of these contacts suggest that they cannot be considered as hydrogen bonds, in contrast to other studies by Araújo et al. (J. M. Araújo, A. B. Pereiro, J. N. Canongia-Lopes, L. P. Rebelo, I. M. Marrucho, J. Phys. Chem. B 2013, 117, 4109-4120). Nevertheless, this combination of interactions of the solute with both the cation and anion components of the solvents helps explain the high solubility of the nucleobase in this media. In addition, favourable uracil-uracil contacts are observed, of similar magnitude to those between cation and uracil, and are also likely to aid dissolution.