Biogas upgrading—Computer-aided ionic liquid absorbent design and process evaluation

In the petrochemical industry, the separation of propylene/propane is usually paid much attention due to its high energy-consuming and environmental challenges. In this work, an energy-efficient extractive distillation (ED) separation process of propylene/propane is proposed by introducing a tailor-made ionic liquid (IL) solvent. The structure optimization of ILs is performed with a novel design objective in the computer-aided ionic liquid design (CAILD) method. By solving a formulated CAILD-based mixed-integer nonlinear programming (MINLP) problem, the ethylimidazolium ethyl sulfate ([C2IM][EtSO4]) is identified as the best IL solvent. The separation performance of [C2IM][EtSO4] in the ED process of propylene/propane is then evaluated through rigorous process simulation in Aspen Plus. For comparison, the conventional separation process of propane/propylene is also simulated and optimized. Based on the optimized operations, in-depth energy and economic assessment are conducted for an industrial-scale separation of propane/propylene in a Chinese refining industry. Remarkably, the [C2IM][EtSO4]-based ED process has 68.9% energy savings and 74.7% cost reduction when compared to the conventional process.

Ionic liquids (ILs), molten salts that are liquid at room temperature, are considered as a potential green replacement for toxic volatile organic compounds in many applications. These liquids exhibit unique properties such as negligible vapor pressure, nonflammability, wide liquid range, high thermal and chemical stabilities, and high solvating capacities for inorganic, organic, and polymeric compounds. In this paper, computer-aided molecular design of potential ILs for cellulose dissolution was performed. A quantitative structure–activity relationship (QSAR) model was first developed to predict cellulose solubility in ILs using the group contribution (GC) and artificial neural network (ANN) methods. A mixed integer nonlinear programming (MINLP) problem was then formulated with an objective function that maximizes the QSAR model. The solution to the MINLP problem given by genetic algorithm (GA) corresponded to the optimal ILs structure for cellulose dissolution. For example, the cellulose solubility in IL...

Although ionic liquids (ILs) have been widely explored as solvents for extractive desulfurization (EDS) of fuel oils, systematic studying of the optimal design of ILs for this process is still scarce. The UNIFAC‐IL model is extended first to describe the EDS system based on exhaustive experimental data. Then, based on the obtained UNIFAC‐IL model and group contribution models for predicting the melting point and viscosity of ILs, a mixed‐integer nonlinear programming (MINLP) problem is formulated for the purpose of computer‐aided ionic liquid design (CAILD). The MINLP problem is solved to optimize the liquid‐liquid extraction performance of ILs in a given multicomponent model EDS system, under consideration of constraints regarding the IL structure, thermodynamic and physical properties. The top five IL candidates preidentified from CAILD are further evaluated by means of process simulation using ASPEN Plus. Thereby, [C5MPy][C(CN)3] is identified as the most suitable solvent for EDS. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1013–1025, 2018

A computer-aided ionic liquid design (CAILD) method was employed to screen suitable ionic liquids (ILs) for use as additives in electrolytes. By formulating and solving a CAILD-based mixed-integer non-linear programming problem, 100 ILs with desired properties were systematically and rapidly identified from 1,089,450 candidates. Subsequently, a new type of IL, 3-(2-amino-2-oxoethyl)-1-vinylimidazolium bis(trifluoromethylsulfonyl)amide ([VAIM][TFSA]), was selected due to its unsaturated functional groups such as vinyl and amide as it may potentially inhibit the water and HF contents in the electrolyte. As a proof of concept, this IL was synthesized for the first time, and then, some electrolytes containing 0, 0.5, 1.0, and 3.0 wt % [VAIM][TFSA] were systemically studied in LTO/Li batteries. Our results reveal that the basic groups of [VAIM][TFSA] are capable of slowing down the release of water and HF in the electrolyte for a long time, while the unsaturated vinyl group could promote the formation of the film on the anode. Remarkably, cells with [VAIM][TFSA] (especially 0.5 wt %), after 300 cycles, present much higher specific capacity and better rate properties than their counterparts without the IL additive.

To demonstrate a new family of ionic liquids (ILs), i.e., "solvate" ionic liquids, the properties (thermal, transport, and electrochemical properties, Lewis basicity, and ionicity) of equimolar molten mixtures of glymes (triglyme (G3) and tetraglyme (G4)) and nine different lithium salts (LiX) were investigated. By exploring the anion-dependent properties and comparing them with the reported data on common aprotic ILs, two different classes of liquid regimes, i.e., ordinary concentrated solutions and "solvate" ILs, were found in the glyme-Li salt equimolar mixtures ([Li(glyme)]X) depending on the anionic structures. The class a given [Li(glyme)]X belonged to was governed by competitive interactions between the glymes and Li cations and between the counteranions (X) and Li cations. [Li(glyme)]X with weakly Lewis basic anions can form long-lived [Li(glyme)](+) complex cations. Thus, they behaved as typical ionic liquids. The lithium "solvate" ILs based on [Li(glyme)]X have many desirable properties for lithium-conducting electrolytes, including high ionicity, a high lithium transference number, high Li cation concentration, and high oxidative stability, in addition to the common properties of ionic liquids. The concept of "solvate" ionic liquids can be utilized in an unlimited number of combinations of other metal salts and ligands, and will thus open a new field of research on ionic liquids.

Isotopic H/D or 6/7Li substitution Raman spectroscopy was applied to new kinds of ionic liquids; N-methylimidazole (C1Im) and acetic acid (CH3COOH) as the pseudo-protic ionic liquid (pPIL), and both of the neat and the 2,2,3,3-tetrafluoropropyl ether (HFE) diluted Li-glyme solvate ionic liquids (SIL) [Li(Gn)][TFSA] (Gn, glyme n = 3 or 4); TFSA, bis(trifluoromethanesulfonyl)amide) to clarify the proton transfer or the Li+ solvation/ion pair formation. The isotopic substitution Raman (ISR) spectra were obtained as the difference between the samples containing the same composition except the substituted isotope. The calculated and theoretical ISR spectra were also evaluated for comparison. With the C1Im-CH3COOH(D) pPIL, the Raman bands attributable to the C1Im/C1HIm+ gave signals of differential shape, and they were well reproduced with the curve fitting by taking the small amount of C1HIm+ and CH3COO- generation into consideration. The ISR spectra for the SIL were well explained by the formation of the Li-TFSA contact ion pair (CIP) and the solvent shared ion pair (SSIP) in the [Li(G3)][TFSA] SIL. In addition, the ISR spectra for the HFE-diluted [Li(G4)][TFSA] SIL clearly proved that the HFE hardly coordinates to the Li+ in the HFE-diluted SIL. Here, the ISR spectroscopy is proposed as a new tool for studying the ion solvation and the ion pair formation in ionic liquids.

Redox flow batteries have attracted much attention as the large-scaled energy storage device for such renewable energy sources as solar cells and windmills. Typical redox flow batteries use aqueous electrolytes because of low cost, low toxicity and high safety. However, the electromotive force of the redox flow batteries using aqueous electrolytes is limited by the electrochemical potential window of the aqueous electrolytes. The enlargement of the electromotive force is expected by using non-aqueous aprotic electrolytes. Although aprotic organic electrolytes are typical of non-aqueous electrolytes, flammability of organic solvents is unfavorable for the large-scaled batteries from the point of view of safety. The equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4) has been known to form the solvate ionic liquid consisting of a stable complex cation, [Li(G4)]+ and an anion, TFSA– [1]. The solvate ionic liquid has several favorable properties, such as high thermal stability, high Li ion concentration and low vapor pressure. In the present study, the redox reaction of tris(2,2'-bipyridine)iron(III/II) complexes, [Fe(bpy)3]3+/2+, has been investigated in the solvate ionic liquid containing carbon fiber as an electronic conductor aiming at its application to non-aqueous redox flow batteries. The redox reaction of [Fe(bpy)3]3+/2+ was observed at 0.36 V vs. Ag|Ag(I), which was corresponding to about 4 V vs. Li|Li(I), in [Li(G4)]TFSA containing 10 mM [Fe(bpy)3](TFSA)2. The diffusion coefficients of [Fe(bpy)3]2+ and [Fe(bpy)3]3+ were estimated to be 4.4 × 10–8 and 3.1 × 10–8 cm2 s–1, respectively, at 25 °C. The slow diffusion of the electroactive species was ascribed to the high viscosity of [Li(G4)]TFSA (122 mPa s [2]) and disadvantageous to battery application because the faradaic current is limited by diffusivity of the species. In order to diminish the diffusion limitation of the faradaic current, carbon fiber was added to the solvate ionic liquid as an electronic conductor. The carbon fiber was dispersed homogeneously in the solvate ionic liquid. The electronic conduction through the carbon fiber was confirmed by electric conductivity measurement. The redox reaction of [Fe(bpy)3]3+/2+ was found to be possible on the carbon fiber dispersed in the solvate ionic liquid. The reversible charge-discharge of a cell composed of Li anode and the solvate ionic liquid containing [Fe(bpy)3]3+/2+ and the carbon fiber using a solid-state Li ion conductor was confirmed to be possible with the average cell voltage of 3.8 V and the coulombic efficiency of 99%.

Anion–cation and ion–solvent interaction of some typical ionic liquids in solvents with different dielectric constant

The bottom-up synthesis of titanate nanoflakes in ionic liquid solvents was carried out by heating mixtures of titanium isopropoxide (TIP), 40% tetrabutylammonium hydroxide (TBAOH) aqueous solution, and ionic liquids with a molar ratio of 1TIP : xTBAOH : (10 − x)ionic liquid : 22xH2O. Using tetrabutylammonium chloride (TBACl) as an ionic liquid, layered titanate crystals with TBA+ as an interlayer cation were synthesized by the acid–base reaction of TBAOH and titanic acid, which was formed by the hydrolysis of TIP, in the range of 0.5 ≤ x ≤ 2.5. The use of 1,2,3-trimethylimidazolium (mmmIm+) methylsulfate as an ionic liquid also led to the formation of layered titanate with mmmIm+ ions as an interlayer cation, although the amount of layered titanate was smaller than that in the case using TBACl. The solvent exchange from ionic liquids to aqueous solution by dialyzing the products caused the exfoliation of layered titanate, providing titanate nanoflakes with a lateral size of about 3 nm. Moreover, by comparing the bottom-up syntheses of layered titanate in ionic liquids and in water, the crystallization behavior of layered titanate was examined. The amount of TBA+ or mmmIm+, which acts as an interlayer cation of layered titanate, influenced the nucleation of layered titanate rather than the solubility of titanate. A significant amount of TBA+ or mmmIm+ in the ionic liquid solvents accelerated the nucleation of layered titanate crystals, providing small titanate nanoflakes. Furthermore, the formed titanate species existed as exfoliated nanosheets or nanoflakes in water solvent, but were present as layered titanate crystals in ionic liquids. However, this difference in the state of titanates had little influence on the growth rates of layered titanate crystals. Thus, the bottom-up synthesis of layered titanates in ionic liquids was useful for the formation of very small titanate nanoflakes.

This study investigated the prospect of using aqueous mixture of 1-butylpyridinium tetrafluoroborate ([Bpy][BF4]) ionic liquid (IL) and monoethanolamine (MEA) as solvent in post-combustion CO2 capture (PCC) process. This is done by analysis of the process through modelling and simulation. In literature, reported PCC models with a mixture of IL and MEA solvent were developed using equilibrium-based mass transfer approach. In contrast, the model in this study is developed using rate-based mass transfer approach in Aspen Plus®. From the results, the mixed aqueous solvent with 5–30 wt% IL and 30 wt% MEA showed 7%–9% and 12%–27% less specific regeneration energy and solvent circulation rate respectively compared to commonly used 30 wt% MEA solvent. It is concluded that the IL concentration (wt%) in the solvent blend have significant impact on specific regeneration energy and solvent circulation rate. This study is a starting point for further research on technical and economic analysis of PCC process with aqueous blend of IL and MEA as solvent.

Blended ionic liquid systems for macroalgae pretreatment

The equimolar mixtures of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and glyme (CH3-O-(CH2-CH2-O) n -CH3) are classified as solvate ionic liquids. They have been investigated for the electrolytes of electrochemical devices because of low vapor pressure, high electrochemical stability, and the acceptable ionic conductivity [1]. In particular, they are suitable electrolytes for sulfur cathode batteries because of the low solubility of polysulfides, Li2S x [2]. In order to apply the LiTFSA-glyme solvate ionic liquids to the electrolytes for practical rechargeable lithium batteries, the improvement of rate performance is one of the significant issues because the diffusion of Li species is generally slow due to the high viscosity of solvate ionic liquids. We have already reported that the local viscosity near the interface between an electrode and the solvate ionic liquids drastically changes during deposition and dissolution of Li on the electrode using the electrochemical quartz crystal microbalance (EQCM), which can monitor not only the change of mass of the electrode but also the change of the viscosity and density near the electrode [3]. It is expected that an increase in the local viscosity with increase in the local concentration of Li species particularly affect the rate performance of the batteries using the solvate ionic liquids. In the present study, the effects of temperature on the change of local viscosity near the electrode during dissolution of Li were investigated in LiTFSA-tetraglyme (G4) solvate ionic liquids. The change of product of local viscosity and density (ηρ) was monitored during dissolution of Li deposited on a Ni-coated quartz crystal electrode in the LiTFSA-G4 solvate ionic liquids in the temperature range from 293 to 308 K with different current densities. An increase in the ηρ was observed during dissolution of Li, reflecting a descending concentration distribution of Li species was produced by diffusion of Li species from the electrode surface to the bulk of the solvate ionic liquid. The variation of the ηρ before and after dissolution for –0.25 mA cm–2 was larger than that for –0.15 mA cm–2 at a constant temperature, indicating the variation is a function of the concentration gradient depending on the current density. When the current density is constant, the variation of the ηρ decreased with elevating temperature, probably due to a decrease in the local concentration of Li species at the electrode surface by a decline in the viscosity. The limiting current density for dissolution of Li also increased with elevating temperature in a coin-type cell. Moreover, the performance of a LiFePO4 cathode, such as polarization and capacity, was remarkably improved especially during extraction of Li+ from the LiFePO4cathode at elevated temperature. These results indicated operation at higher temperatures is effective in improving the rate performance of the batteries using solvate ionic liquids, while most of conventional rechargeable batteries have to be cooled in order to avoid deterioration of their performance. This work was partially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST). [1] T. Tamura et al., Chem. Lett., 39, 753 (2010). [2] K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013). [3] N. Serizawa et al., J. Electrochem. Soc., 160, A1529 (2013).

COSMO-descriptor based computer-aided ionic liquid design for separation processes. Part I: Modified group contribution methodology for predicting surface charge density profile of ionic liquids

Ionic liquids (ILs) are salts possessing many properties that make them attractive for use in a wide range of applications including batteries. One of the biggest drawbacks of ILs is that they tend to be more viscous than conventional solvents, which means low ionic conductivity when used as electrolytes in batteries. One effective way to lower the viscosities is to mix ILs with low viscosity additives such as molecular solvents. An obvious disadvantage of mixing molecular solvents with ILs is that the extremely low volatility of the IL is compromised by the finite vapor pressure of the diluent. One strategy to overcome this is to mix ILs with molecular solvents that are themselves of very low volatility. One such class of compounds are the glymes, which are oligoethers having a CH3O-(CH2CH2O)n-CH3 structure. In addition to having low volatility, glymes are effective at dissociating salts because of the electron donating nature of their ether oxygen groups. They also tend to have high thermal and electrochemical stability, which has led many groups to investigate mixing glymes with alkali metal salts (most commonly Li+ [NTf2]) for use as electrolytes in batteries. The alkali metal cation tends to coordinate with the oxygen atoms of the glyme, which can significantly lower the melting point of the salt. Such systems usually retain the attractive properties of pure ILs but the transport properties are improved. They are considered a new class of ILs and have been termed “solvate ILs”. Tetraethylene glycol dimethyl ether (tetraglyme or G4) is a particularly promising glyme. While there are many studies devoted to the properties of Li-based solvate ILs, much less work has been done investigating the properties of mixtures containing glymes and more conventional ionic liquids. In the current work, the density and viscosity of the IL [C6mim][NTf2]) and its mixtures with tetraglyme were measured at different temperatures and concentrations. Molecular dynamics (MD) simulations were also carried out on the mixtures in order to compute the density, self-diffusivity, viscosity, ideal ionic conductivity and liquid structure. We find that both the density and viscosity drop with increasing temperature and G4 concentration. Detailed analysis of the simulation results suggests that G4 preferentially solvates the cations, thereby weakening the interactions with the anions and resulting in faster dynamics, particularly for the anions. Such behavior was previously observed in solvate ionic liquids. Based on the simulation results of a model G4 system, it was further revealed that the localized negative charges on the oxygen atoms of G4 are primarily responsible for the preferential solvation of cations. If G4 (or a similar solvent) instead had large localized positive charges, it would tend to preferentially solvate the anions, resulting in increased cation mobility. This suggests a strategy for selecting molecular solvents that will selectively enhance cation dynamics.

Solvate ionic liquid-based ionogels for lithium metal batteries over a wide temperature range