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Special Issue "Ionic Liquids and Deep Eutectic Solvents for Synthesis, Materials and Energy"

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Synthesis".

Deadline for manuscript submissions: closed (10 January 2017)

Special Issue Editor

Guest Editor
Prof. Dr. Hua Zhao

Department of Chemistry and Forensic Science, Savannah State University, Savannah, GA, USA
Website | E-Mail
Interests: biocatalysis; ionic liquids and deep eutectic solvents; biofuel; desulfurization; DNA-based asymmetric synthesis; medicinal molecules

Special Issue Information

Dear Colleagues,

As many of you are aware of, ionic liquids (ILs) and deep eutectic solvents (DES), as nonvolatile alternatives to conventional organic solvents, have gained tremendous attention over the past two decades in various aspects of chemical applications. In particular, these novel solvents have recently been found to be very valuable for many catalytic and biocatalytic reactions/processes with targeted applications for new materials (such as nanomaterials and biopolymers) and energy (such as cellulosic ethanol, biodiesel, dye-sensitized solar cells (DSSCs) and batteries). This Special Issue will be a collection of high-calibre papers focusing on transition metal catalysis, organocatalysis and biotransformations involving ionic liquids and deep eutectic solvents, and preferably are associated with applications in materials and energy. We anticipate this Issue will be highly visible and attractive to the scientific community of synthesis, materials and energy. This Special Issue will entertain contributions covering all the aspects broadly indicated by the keywords. Reviews articles by experts of relevant subjects are highly welcomed.

Prof. Dr. Hua Zhao
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • ionic liquid
  • deep eutectic solvent
  • catalysis
  • biocatalysis
  • biofuel
  • electrochemical
  • dye-sensitized solar cells (DSSCs)
  • battery
  • materials
  • nanomaterials
  • polymer

Published Papers (12 papers)

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Research

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Open AccessArticle Ionic Liquid as an Efficient Medium for the Synthesis of Quinoline Derivatives via α-Chymotrypsin-Catalyzed Friedländer Condensation
Molecules 2017, 22(5), 762; doi:10.3390/molecules22050762
Received: 30 March 2017 / Revised: 26 April 2017 / Accepted: 4 May 2017 / Published: 8 May 2017
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Abstract
An efficient, convenient, and eco-friendly biocatalytic approach was developed for the synthesis of quinoline derivatives via the α-chymotrypsin-catalyzed Friedländer reaction. Interestingly, α-chymotrypsin exhibited higher catalytic activity in an ionic liquid (IL) aqueous solution as compared to that observed in our previous relevant study,
[...] Read more.
An efficient, convenient, and eco-friendly biocatalytic approach was developed for the synthesis of quinoline derivatives via the α-chymotrypsin-catalyzed Friedländer reaction. Interestingly, α-chymotrypsin exhibited higher catalytic activity in an ionic liquid (IL) aqueous solution as compared to that observed in our previous relevant study, which was conducted using an organic solvent, and a series of substrates gave similar excellent yields at lower reaction temperature and under reduced enzyme-loading conditions. Full article
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Open AccessArticle The Influence of Anion Shape on the Electrical Double Layer Microstructure and Capacitance of Ionic Liquids-Based Supercapacitors by Molecular Simulations
Molecules 2017, 22(2), 241; doi:10.3390/molecules22020241
Received: 4 January 2017 / Revised: 25 January 2017 / Accepted: 2 February 2017 / Published: 16 February 2017
Cited by 1 | PDF Full-text (3410 KB) | HTML Full-text | XML Full-text
Abstract
Room-temperature ionic liquids (RTILs) are an emerging class of electrolytes for supercapacitors. In this work, we investigate the effects of different supercapacitor models and anion shape on the electrical double layers (EDLs) of two different RTILs: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Emim][Tf2N]) and 1-ethyl-3-methylimidazolium
[...] Read more.
Room-temperature ionic liquids (RTILs) are an emerging class of electrolytes for supercapacitors. In this work, we investigate the effects of different supercapacitor models and anion shape on the electrical double layers (EDLs) of two different RTILs: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Emim][Tf2N]) and 1-ethyl-3-methylimidazolium 2-(cyano)pyrrolide ([Emim][CNPyr]) by molecular dynamics (MD) simulation. The EDL microstructure is represented by number densities of cations and anions, and the potential drop near neutral and charged electrodes reveal that the supercapacitor model with a single electrode has the same EDL structure as the model with two opposite electrodes. Nevertheless, the employment of the one-electrode model without tuning the bulk density of RTILs is more time-saving in contrast to the two-electrode one. With the one-electrode model, our simulation demonstrated that the shapes of anions significantly imposed effects on the microstructure of EDLs. The EDL differential capacitance vs. potential (C-V) curves of [Emim][CNPyr] electrolyte exhibit higher differential capacitance at positive potentials. The modeling study provides microscopic insight into the EDLs structure of RTILs with different anion shapes. Full article
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Open AccessArticle Highly Efficient Enzymatic Preparation of Daidzein in Deep Eutectic Solvents
Molecules 2017, 22(1), 186; doi:10.3390/molecules22010186
Received: 31 October 2016 / Revised: 18 January 2017 / Accepted: 19 January 2017 / Published: 22 January 2017
Cited by 1 | PDF Full-text (3158 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Daidzein, which is scarce in nature, has gained significant attention due to its superior biological activity and bioavailability compared with daidzin. So far, it has been widely used in the medicine and health care products industries. The enzymatic approach for the preparation of
[...] Read more.
Daidzein, which is scarce in nature, has gained significant attention due to its superior biological activity and bioavailability compared with daidzin. So far, it has been widely used in the medicine and health care products industries. The enzymatic approach for the preparation of daidzein has prevailed, benefitted by its high efficiency and eco-friendly nature. Our present research aimed at providing a preparation method of daidzein by enzymatic hydrolysis of daidzin in a new “green” reaction medium-deep eutectic solvents (DESs). Herein, the DESs were screened via evaluating enzyme activity, enzyme stability and the substrate solubility, and the DES (ChCl/EG 2:1, 30 vol %) was believed to be the most appropriate co-solvent to improve the bioconversion efficiency. Based on the yield of daidzein, response surface methodology (RSM) was employed to model and optimize the reaction parameters. Under these optimum process conditions, the maximum yield of 97.53% was achieved and the purity of daidzein crude product reached more than 70%, which is more efficient than conversions in DESs-free buffer. Importantly, it has been shown that DESs medium could be reused for six batches of the process with a final conversion of above 50%. The results indicated that this procedure could be considered a mild, environmentally friendly, highly efficient approach to the economical production of daidzein, with a simple operation process and without any harmful reagents being involved. Full article
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Open AccessArticle Nano-Structural Investigation on Cellulose Highly Dissolved in Ionic Liquid: A Small Angle X-ray Scattering Study
Molecules 2017, 22(1), 178; doi:10.3390/molecules22010178
Received: 21 December 2016 / Revised: 12 January 2017 / Accepted: 18 January 2017 / Published: 21 January 2017
Cited by 2 | PDF Full-text (3991 KB) | HTML Full-text | XML Full-text
Abstract
We investigated nano-structural changes of cellulose dissolved in 1-ethyl-3-methylimidazolium acetate—an ionic liquid (IL)—using a small angle X-ray scattering (SAXS) technique over the entire concentration range (0–100 mol %). Fibril structures of cellulose disappeared at 40 mol % of cellulose, which is a significantly
[...] Read more.
We investigated nano-structural changes of cellulose dissolved in 1-ethyl-3-methylimidazolium acetate—an ionic liquid (IL)—using a small angle X-ray scattering (SAXS) technique over the entire concentration range (0–100 mol %). Fibril structures of cellulose disappeared at 40 mol % of cellulose, which is a significantly higher concentration than the maximum concentration of dissolution (24–28 mol %) previously determined in this IL. This behavior is explained by the presence of the anion bridging, whereby an anion prefers to interact with multiple OH groups of different cellulose molecules at high concentrations, discovered in our recent work. Furthermore, we observed the emergence of two aggregated nano-structures in the concentration range of 30–80 mol %. The diameter of one structure was 12–20 nm, dependent on concentration, which is ascribed to cellulose chain entanglement. In contrast, the other with 4.1 nm diameter exhibited concentration independence and is reminiscent of a cellulose microfibril, reflecting the occurrence of nanofibrillation. These results contribute to an understanding of the dissolution mechanism of cellulose in ILs. Finally, we unexpectedly proposed a novel cellulose/IL composite: the cellulose/IL mixtures of 30–50 mol % that possess liquid crystallinity are sufficiently hard to be moldable. Full article
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Open AccessCommunication Deep Eutectic Solvents as Novel and Effective Extraction Media for Quantitative Determination of Ochratoxin A in Wheat and Derived Products
Molecules 2017, 22(1), 121; doi:10.3390/molecules22010121
Received: 11 November 2016 / Revised: 3 January 2017 / Accepted: 10 January 2017 / Published: 12 January 2017
Cited by 3 | PDF Full-text (717 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
An unprecedented, environmentally friendly, and faster method for the determination of Ochratoxin A (OTA) (a mycotoxin produced by several species of Aspergillus and Penicillium and largely widespread in nature, in wheat and derived products) has, for the first time, been set up and
[...] Read more.
An unprecedented, environmentally friendly, and faster method for the determination of Ochratoxin A (OTA) (a mycotoxin produced by several species of Aspergillus and Penicillium and largely widespread in nature, in wheat and derived products) has, for the first time, been set up and validated using choline chloride (ChCl)-based deep eutectic solvents (DESs) (e.g., ChCl/glycerol (1:2) and ChCl/ urea (1:2) up to 40% (w/w) water) as privileged, green, and biodegradable extraction solvents. This also reduces worker exposure to toxic chemicals. Results are comparable to those obtained using conventional, hazardous and volatile organic solvents (VOCs) typical of the standard and official methods. OTA recovery from spiked durum wheat samples, in particular, was to up to 89% versus 93% using the traditional acetonitrile-water mixture with a repeatability of the results (RSDr) of 7%. Compatibility of the DES mixture with the antibodies of the immunoaffinity column was excellent as it was able to retain up to 96% of the OTA. Recovery and repeatability for durum wheat, bread crumbs, and biscuits proved to be within the specifications required by the current European Commission (EC) regulation. Good results in terms of accuracy and precision were achieved with mean recoveries between 70% (durum wheat) and 88% (bread crumbs) and an RSDr between 2% (biscuits) and 7% (bread). Full article
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Open AccessArticle Characterization of the Micromorphology and Topochemistry of Poplar Wood during Mild Ionic Liquid Pretreatment for Improving Enzymatic Saccharification
Molecules 2017, 22(1), 115; doi:10.3390/molecules22010115
Received: 29 November 2016 / Revised: 20 December 2016 / Accepted: 9 January 2017 / Published: 11 January 2017
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Abstract
Ionic liquids (ILs) as designer solvents have been applied in biomass pretreatment to increase cellulose accessibility and therefore improve the enzymatic hydrolysis. We investigated the characterization of the micromorphology and the topochemistry of poplar wood during 1-ethyl-3-methylimidazolium acetate pretreatment with mild conditions (90
[...] Read more.
Ionic liquids (ILs) as designer solvents have been applied in biomass pretreatment to increase cellulose accessibility and therefore improve the enzymatic hydrolysis. We investigated the characterization of the micromorphology and the topochemistry of poplar wood during 1-ethyl-3-methylimidazolium acetate pretreatment with mild conditions (90 °C for 20 and 40 min) by multiple microscopic techniques (FE-SEM, CLSM, and CRM). Chemical composition analysis, XRD, cellulase adsorption isotherm, and enzymatic hydrolysis were also performed to monitor the variation of substrate properties. Our results indicated that the biomass conversion was greatly enhanced (from 20.57% to 73.64%) due to the cell wall deconstruction and lignin dissolution (29.83% lignin was removed after incubation for 40 min), rather than the decrystallization or crystallinity transformation of substrates. The mild ILs pretreatment, with less energy input, can not only enhance enzymatic hydrolysis, but also provide a potential approach as the first step in improving the sequential pretreatment effectiveness in integrated methods. This study provides new insights on understanding the ILs pretreatment with low temperature and short duration, which is critical for developing individual and/or combined pretreatment technologies with reduced energy consumption. Full article
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Open AccessArticle Cooperative Reinforcement of Ionic Liquid and Reactive Solvent on Enzymatic Synthesis of Caffeic Acid Phenethyl Ester as an In Vitro Inhibitor of Plant Pathogenic Bacteria
Molecules 2017, 22(1), 72; doi:10.3390/molecules22010072
Received: 6 November 2016 / Revised: 16 December 2016 / Accepted: 28 December 2016 / Published: 2 January 2017
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Abstract
It is widely believed that lipases in ionic liquids (ILs) possess higher enzyme activity, stability and selectivity; however, reaction equilibrium is always limited by product inhibition, and the product is difficult to separate from non-volatile ILs using distillation. To solve this problem, using
[...] Read more.
It is widely believed that lipases in ionic liquids (ILs) possess higher enzyme activity, stability and selectivity; however, reaction equilibrium is always limited by product inhibition, and the product is difficult to separate from non-volatile ILs using distillation. To solve this problem, using trialkylphosphine oxide (TOPO) as a complexing agent, a novel biphase of reactive solvent and IL was firstly reported for caffeic acid phenethyl ester (CAPE) production from methyl caffeate (MC) and 2-phenylethanol (PE) catalyzed by lipase via transesterification. The effects of the reaction parameters and their action mechanism were investigated, and the inhibition of CAPE against bacterial wilt pathogen Ralstonia solanacearum was firstly measured. The MC conversion of 98.83% ± 0.76% and CAPE yield of 96.29% ± 0.07% were obtained by response surface methodology in the 25 g/L TOPO-cyclohexane/[Bmim][Tf2N] (1:1, v/v); the complex stoichiometry calculation and FTIR spectrum confirmed that the reversible hydrogen-bond complexation between TOPO and caffeates significantly enhances the cooperative effect of two phases on the lipase-catalyzed reaction. The temperature was reduced by 14 °C; the MC concentration increased by 3.33-fold; the ratio of catalyst to donor decreased by 4.5-fold; and Km decreased 1.08-fold. The EC50 of CAPE against R. solanacearum was 0.17–0.75 mg/mL, suggesting that CAPE is a potential in vitro inhibitor of plant pathogenic bacteria. Full article
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Open AccessArticle Deep Eutectic Solvent-Based Microwave-Assisted Method for Extraction of Hydrophilic and Hydrophobic Components from Radix Salviae miltiorrhizae
Molecules 2016, 21(10), 1383; doi:10.3390/molecules21101383
Received: 21 August 2016 / Revised: 11 October 2016 / Accepted: 14 October 2016 / Published: 17 October 2016
Cited by 2 | PDF Full-text (7385 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Deep eutectic solvents (DESs) have attracted significant attention as a promising green media. In this work, twenty-five kinds of benign choline chloride-based DESs with microwave-assisted methods were applied to quickly extract active components from Radix Salviae miltiorrhizae. The extraction factors, including temperature,
[...] Read more.
Deep eutectic solvents (DESs) have attracted significant attention as a promising green media. In this work, twenty-five kinds of benign choline chloride-based DESs with microwave-assisted methods were applied to quickly extract active components from Radix Salviae miltiorrhizae. The extraction factors, including temperature, time, power of microwave, and solid/liquid ratio, were investigated systematically by response surface methodology. The hydrophilic and hydrophobic ingredients were extracted simultaneously under the optimized conditions: 20 vol% of water in choline chloride/1,2-propanediol (1:1, molar ratio) as solvent, microwave power of 800 W, temperature at 70 °C, time at 11.11 min, and solid/liquid ratio of 0.007 g·mL−1. The extraction yield was comparable to, or even better than, conventional methods with organic solvents. The microstructure alteration of samples before and after extraction was also investigated. The method validation was tested as the linearity of analytes (r2 > 0.9997 over two orders of magnitude), precision (intra-day relative standard deviation (RSD) < 2.49 and inter-day RSD < 2.96), and accuracy (recoveries ranging from 95.04% to 99.93%). The proposed DESs combined with the microwave-assisted method provided a prominent advantage for fast and efficient extraction of active components, and DESs could be extended as solvents to extract and analyze complex environmental and pharmaceutical samples. Full article
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Open AccessCommunication Exploiting 1,2,3-Triazolium Ionic Liquids for Synthesis of Tryptanthrin and Chemoselective Extraction of Copper(II) Ions and Histidine-Containing Peptides
Molecules 2016, 21(10), 1355; doi:10.3390/molecules21101355
Received: 29 August 2016 / Revised: 5 October 2016 / Accepted: 9 October 2016 / Published: 13 October 2016
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Abstract
Based on a common structural core of 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyridine, a number of bicyclic triazolium ionic liquids 13 were designed and successfully prepared. In our hands, this optimized synthesis of ionic liquids 1 and 2 requires no chromatographic separation. Also in
[...] Read more.
Based on a common structural core of 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyridine, a number of bicyclic triazolium ionic liquids 13 were designed and successfully prepared. In our hands, this optimized synthesis of ionic liquids 1 and 2 requires no chromatographic separation. Also in this work, ionic liquids 1, 2 were shown to be efficient ionic solvents for fast synthesis of tryptanthrin natural product. Furthermore, a new affinity ionic liquid 3 was tailor-synthesized and displayed its effectiveness in chemoselective extraction of both Cu(II) ions and, for the first time, histidine-containing peptides. Full article
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Open AccessArticle Enzymatic Synthesis of Glucose-Based Fatty Acid Esters in Bisolvent Systems Containing Ionic Liquids or Deep Eutectic Solvents
Molecules 2016, 21(10), 1294; doi:10.3390/molecules21101294
Received: 26 August 2016 / Revised: 20 September 2016 / Accepted: 20 September 2016 / Published: 27 September 2016
Cited by 3 | PDF Full-text (3128 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Sugar fatty acid esters (SFAEs) are biocompatible nonionic surfactants with broad applications in food, cosmetic, and pharmaceutical industries. They can be synthesized enzymatically with many advantages over their chemical synthesis. In this study, SFAE synthesis was investigated by using two reactions: (1) transesterification
[...] Read more.
Sugar fatty acid esters (SFAEs) are biocompatible nonionic surfactants with broad applications in food, cosmetic, and pharmaceutical industries. They can be synthesized enzymatically with many advantages over their chemical synthesis. In this study, SFAE synthesis was investigated by using two reactions: (1) transesterification of glucose with fatty acid vinyl esters and (2) esterification of methyl glucoside with fatty acids, catalyzed by Lipozyme TLIM and Novozym 435 respectively. Fourteen ionic liquids (ILs) and 14 deep eutectic solvents (DESs) were screened as solvents, and the bisolvent system composed of 1-hexyl-3-methylimidazolium trifluoromethylsulfonate ([HMIm][TfO]) and 2-methyl-2-butanol (2M2B) was the best for both reactions, yielding optimal productivities (769.6 and 397.5 µmol/h/g, respectively) which are superior to those reported in the literature. Impacts of different reaction conditions were studied for both reactions. Response surface methodology (RSM) was employed to optimize the transesterification reaction. Results also demonstrated that as co-substrate, methyl glucoside yielded higher conversions than glucose, and that conversions increased with an increase in the chain length of the fatty acid moieties. DESs were poor solvents for the above reactions presumably due to their high viscosity and high polarity. Full article
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Review

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Open AccessReview The Mechanism of Room-Temperature Ionic-Liquid-Based Electrochemical CO2 Reduction: A Review
Molecules 2017, 22(4), 536; doi:10.3390/molecules22040536
Received: 10 February 2017 / Revised: 18 March 2017 / Accepted: 25 March 2017 / Published: 28 March 2017
Cited by 3 | PDF Full-text (5657 KB) | HTML Full-text | XML Full-text
Abstract
Electrochemical CO2 conversion technology is becoming indispensable in the development of a sustainable carbon-based economy. While various types of electrocatalytic systems have been designed, those based on room-temperature ionic liquids (RTILs) have attracted considerable attention because of their high efficiencies and selectivities.
[...] Read more.
Electrochemical CO2 conversion technology is becoming indispensable in the development of a sustainable carbon-based economy. While various types of electrocatalytic systems have been designed, those based on room-temperature ionic liquids (RTILs) have attracted considerable attention because of their high efficiencies and selectivities. Furthermore, it should be possible to develop more advanced electrocatalytic systems for commercial use because target-specific characteristics can be fine-tuned using various combinations of RTIL ions. To achieve this goal, we require a systematic understanding of the role of the RTIL components in electrocatalytic systems, however, their role has not yet been clarified by experiment or theory. Thus, the purpose of this short review is to summarize recent experimental and theoretical mechanistic studies to provide insight into and to develop guidelines for the successful development of new CO2 conversion systems. The results discussed here can be summarized as follows. Complex physical and chemical interactions between the RTIL components and the reaction intermediates, in particular at the electrode surface, are critical for determining the activity and selectivity of the electrocatalytic system, although no single factor dominates. Therefore, more fundamental research is required to understand the physical, chemical, and thermodynamic characteristics of complex RTIL-based electrocatalytic systems. Full article
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Open AccessReview Pretreatment of Lignocellulosic Biomass with Ionic Liquids and Ionic Liquid-Based Solvent Systems
Molecules 2017, 22(3), 490; doi:10.3390/molecules22030490
Received: 10 January 2017 / Revised: 15 March 2017 / Accepted: 16 March 2017 / Published: 20 March 2017
Cited by 2 | PDF Full-text (1830 KB) | HTML Full-text | XML Full-text
Abstract
Pretreatment is very important for the efficient production of value-added products from lignocellulosic biomass. However, traditional pretreatment methods have several disadvantages, including low efficiency and high pollution. This article gives an overview on the applications of ionic liquids (ILs) and IL-based solvent systems
[...] Read more.
Pretreatment is very important for the efficient production of value-added products from lignocellulosic biomass. However, traditional pretreatment methods have several disadvantages, including low efficiency and high pollution. This article gives an overview on the applications of ionic liquids (ILs) and IL-based solvent systems in the pretreatment of lignocellulosic biomass. It is divided into three parts: the first deals with the dissolution of biomass in ILs and IL-based solvent systems; the second focuses on the fractionation of biomass using ILs and IL-based solvent systems as solvents; the third emphasizes the enzymatic saccharification of biomass after pretreatment with ILs and IL-based solvent systems. Full article
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