Structures and Electronic Properties of Lithium Chelate-Based Ionic Liquids

Does the active hydrogen atom in the hydantoin anion affect the physical properties, CO2 capture and conversion of ionic liquids?

Compared to the effect of the active hydrogen atom in the cation in protic ionic liquids (ILs) on their properties and applications, there are very few reports on the role of the active hydrogen atom in the anion. In order to better understand the role of the active hydrogen atom in the anion, the physical properties, CO2 capture and conversion of three hydantoin-based anion-functionalized ILs ([P4442][Hy], [P4442]2[Hy], and [HDBU][Hy]) have been investigated via experiments, spectroscopy, and DFT calculations in this work. The results show that the active hydrogen atom in the anion can form anionic hydrogen bonding networks, which significantly increase the melting point and viscosity and decrease the basicity of the IL, thereby weakening its ability to capture and convert CO2. Interestingly, [P4442][Hy] undergoes a solid/liquid two-phase transition during CO2 absorption/desorption due to the formation of quasi-intramolecular hydrogen bonding between the active hydrogen atom and the O- atom of the absorbed CO2, suggesting that the presence of the active hydrogen atom gives [P4442][Hy] the potential to be an excellent molecular switch. As there is no active hydrogen atom in the anion of [P4442]2[Hy], it shows excellent CO2 capture and conversion performance through the double-site interaction. [HDBU][Hy] shows the weakest catalytic CO2 conversion due to the presence of active hydrogen atoms on both its anion and cation. Therefore, the active hydrogen atom in the anion may play a more important role in the properties and potential applications of ILs than the active hydrogen atom in the cation.

The interactions of chelate-based ionic liquids: from the perspective of vaporization enthalpy

The vaporization enthalpies of [C_nTPP][Cu(F₆-acac)₃] (n = 14, 16) and [C_nmim][Cu(F₆-acac)₃] (n = 8, 10), a new type of chelate-based ionic liquids (ChILs), were measured by thermogravimetric analysis (ΔglHom(T_av) = 73.2-83.7 kJ mol^-1), which decreased compared to their non-chelate counterparts (difference up to 34.1 kJ mol^-1). This can be explained by the fact that the increase in the polar region size by chelate ions mainly leads to the decrease in Coulomb forces based on the micro-biphasic separation in ionic liquids. Vaporization enthalpies at experimental temperatures were adjusted to a reference temperature by the Verevkin's method, and compared with the data calculated by the Kabo's method, which illustrated clear differences on ChILs. In addition, some physicochemical properties of [C_nTPP][Cu(F₆-acac)₃] (n = 14, 16) were measured including density, viscosity, conductivity, and surface tension.

LiCl-promoted-dehydration of fructose-based carbohydrates into 5-hydroxymethylfurfural in isopropanol

The carbohydrate-derived 5-hydroxymethylfurfural (HMF) is one of the most versatile intermediate chemicals, and is promising to bridge the growing gap between the supply and demand of energy and chemicals. Developing a low-cost catalytic system will be helpful to the production of HMF in industry. Herein, the commercially available lithium chloride (LiCl) and isopropanol (i-PrOH) are used to construct a cost-effective and low-toxic system, viz., LiCl/i-PrOH, for the preparation of HMF from fructose-based carbohydrates, achieving ∼80% of HMF yield under the optimum conditions. The excellent promotion effect of LiCl on fructose conversion in i-PrOH could be attributed to the synergistic effect of LiCl with i-PrOH through the LiCl-promoted and i-PrOH-aided dehydration process, and the co-operation of LiCl and i-PrOH for stabilizing the as-formed HMF by hydrogen/coordination bonds, giving a low activation energy of 68.68 kJ mol-1 with a pre-exponential factor value of 1.2 × 104 min-1. The LiCl/i-PrOH system is a substrate-tolerant and scalable catalytic system, fructose (scaled up 10 times), sucrose, and inulin also give 73.6%, 30.3%, and 70.3% HMF yield, respectively. Moreover, this system could be reused 8 times without significant loss of activity. The readily available and low-toxic LiCl, the sustainable solvent (i-PrOH), the renewable starting materials, and the mild reaction conditions make this system promising and sustainable for the industrial production of HMF in future.

Structural and electronic properties of CuII, CoII, and NiII-containing chelate-based ionic liquids

Potential applications of chelate-based ionic liquids depend on the structural and electronic properties of this class of liquid materials. Due to the large size of chelated metal anions, the high number of potential interaction sites could lead to complex intermolecular interactions, but metal-based anions have many degrees of freedom, and they have great potential in the structural modification of the physical properties of ionic liquids. To explore the influence of varying the metal center of anions and the length of carbon chains of cationic species, four single crystal structures of chelate-based ionic liquids, ([C₁₀mim][M(F₆-acac)₃], M = Cu, Co, and Ni) and [C₆mim][Cu(F₆-acac)₃] were obtained. Taking these as the initial configurations, theoretical efforts were made to understand the structural and electronic properties. The hydrogen bonding energies of the primary hydrogen bonding interaction, in the range of 17.7-20.9 kJ mol^-1, follow the order of [C₁₀mim][Ni(F₆-acac)₃] < [C₁₀mim][Co(F₆-acac)₃] <[C₁₀mim][Cu(F₆-acac)₃], and [C_nmim][Cu(F₆-acac)₃] (n≠ 10) < [C₁₀mim][Cu(F₆-acac)₃], while the experimental viscosities exhibit an opposite trend. Furthermore, by NBO analysis, the more negative charge on the oxygen atoms of anions shows the stronger hydrogen bonding of imidazolium with C²H. The reliability of the theoretical method was supported by the comparison between the simulated and experimental infrared and UV/vis spectra. This work is useful in increasing the understanding of the structure-property relationship of chelate-based ionic liquids and furthering the rational design of novel ionic liquids.

Empirical study of physicochemical and spectral properties of CuII-containing chelate-based ionic liquids

The structure–property relations of chelate-based ionic liquids were systematically explored through the study of physicochemical and spectral properties.