The Keggin type heteropolyacids (HPAs) have attracted increasing attention due to their strong Bronsted acidity and excellent redox properties, which could play an important role in accelerating the conversion of bio-derived molecules. In this work, heteropolyacid (HPA, H₄PMo₁₁VO₄₀) encapsulated by silica was synthesized by a sol-gel method and a sequential silylation technique (HPA@SiO₂-N₂-S). The as-synthesized material was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The FT-IR spectra show that the HPA anions preserved their Keggin structure when incorporated into the catalyst. The XRD patterns show that HPA molecules are uniformly dispersed within the silica network. The SEM and TEM images confirm that the catalyst was composed of spherical nanometer-sized particles. The porous properties of the catalysts measured by the N₂ adsorption-desorption isotherms indicate that the Brunauer, Emmett and Teller (BET) surface area of pure SiO₂ was 287 m²·g^-1, but upon encapsulation of HPA into the silica matrix, a lower surface area (245 m²·g^-1) was measured for the resulting material. In addition, the pore diameter was reduced after silylation. Furthermore, the hydrophobicity of the catalysts was investigated by the measurement of contact angle (CA) with water. The SiO₂ and SiO₂/HPA catalysts were completely hydrophilic and the contact angle was close to 0°. However, the contact angle of the silylated catalyst was determined to be 137°, indicating that the silylation procedure significantly increased the hydrophobicity of the catalyst. The as-prepared catalysts were also used as heterogeneous catalysts for the selective oxidation of glycerol. The prepared material exhibited excellent catalytic activity towards glycerol oxidation, in which the glycerol can be selectively transformed into formic acid (ca. 70% selectivity) and glycolic acid (ca. 27% selectivity) using H₂O₂ as an oxidant under mild reaction conditions. The effect of the silylation procedure on the recyclability of catalyst was also investigated in this work. The characterizations described above indicated that silylation procedure can significantly increase the hydrophobicity and limit the pore sizes, resulting in high leach-resistance towards HPA, thus improving the recyclability of the silica-encapsulated HPA catalyst, as compared to the SiO₂/HPA catalyst prepared with the conventional impregnation method. Furthermore, the conversion in the second catalytic run is even higher than that of the initial run, which is likely because more active sites are exposed after the first run. The catalyst can be reused for at least five cycles without any leaching of HPA. The spent catalyst did not undergo structural changes, as revealed by FT-IR, XRD, and SEM characterization. Moreover, it was found that the strong Bronsted acid additives played a crucial role in the catalytic oxidation of glycerol.

The anion-functionalization strategy has been proposed and applied for the synthesis of macro-porous resins [IRA-900][An], thus realizing anultra-high SO₂ adsorption capacity (>10 mmol·g^-1) at 101.3 kPa and 20 ℃. Compared with the normal azole-based anion-functionalized resins, the poly(imidazolyl)borate anion-functionalized resin [IRA-900][B(Im)₄] exhibited an outstanding adsorption capacity at low SO₂ partial pressures (10.62 mmol·g^-1 at 20 ℃ and 10.13 kPa). From the results of the IR spectrum investigation and DFT calculations, the multiple-site adsorption mechanism was verified. On account of the unique tetrahedral configuration of [B(im)₄], the conjugation and electronic communication between the electronegative nitrogen atoms were disrupted, making them behave as local reactive sites. Therefore, at least four electronegative nitrogen atoms could be provided by one [B(im)₄] to react with SO₂ without evident adsorption enthalpy deterioration (from -50.6 kJ·mol^-1 to -37.2 kJ·mol^-1) during the continuous SO₂ capture; this was responsible for the ultra-high SO₂ adsorption capacity achieved by [IRA-900][B(Im)₄] at low partial pressures. Moreover, the thermal stability and reversibility of [IRA-900][B(Im)₄] for SO₂ capture and desorption were investigated. Six cycles where the adsorption was carried out at 20 ℃ and 10.13 kPa and the regeneration was performed at 70 ℃ demonstrated the adequate reversibility of [IRA-900][B(Im)₄] for SO₂ capture, showing the resin's great potential for industrial desulfurization. Thus, the anion-functionalization strategy and multiple-site adsorption behavior provide new perspectives to realize effective SO₂ capture from flue gas.

Carbon dioxide (CO₂) is one of the main greenhouse gases that can be utilized as a useful C₁ source owing to its abundance, non-toxicity, and renewability. In fact, the transformation of carbon dioxide into valuable organic molecules has attracted considerable attention over the past decades. One-pot multicomponent reactions generally proceed with more than two different raw materials reacting in one pot, thus simplifying the reaction in operation and workup. In this regard, a three-component reaction of CO₂, propargylic alcohols, and nucleophiles such as amines, water, and alcohols, to prepare useful carbonyl compounds (e.g., carbamates, oxazolidinones, α-hydroxyl ketones, and organic carbonates) is particularly appealing because of the advantages of step and atom economy. From a mechanistic point of view, the three-component reaction of CO₂, a propargylic alcohol, and a nucleophile is a type of cascade reaction, involving the carboxylative cyclization of CO₂ and propargylic alcohol, and subsequent reaction of a nucleophile with the in situ formed α-alkylidene cyclic carbonate. On the other hand, reactions involving CO₂ are generally thermodynamically unfavorable because of the thermodynamic stability and kinetic inertness of CO₂. Cyclic carbonates are widely used in organic synthesis, and their preparation from vicinal diols and CO₂ represents a green synthetic method because biomass is utilized as the source of vicinal diols. However, the low yields of cyclic carbonates are obtained in most cases because of thermodynamic limitations and deactivation of the catalyst by water, which is the co-product of cyclic carbonates. The most commonly used method to improve the yields of cyclic carbonates involves the addition of dehydrating agents. However, decreased selectivity is often observed because of the side reaction of vicinal diols with the hydrolysis products of the dehydrating agent. In addition, the reaction of 2-aminoethanols and CO₂ to obtain the corresponding 2-oxazolidinones also encounters the analogous thermodynamic limitation. To solve this problem, an efficient three-component reaction of CO₂, propargylic alcohols, and nucleophiles was developed to offer thermodynamically favorable ways for converting CO₂ into cyclic carbonates and 2-oxazolidinones with vicinal diols or 2-aminoethanols as nucleophiles. In this strategy, water is not generated and the α-alkylidene cyclic carbonate formed from CO₂ and propargylic alcohol as the actual carbonyl source reacts with vicinal diol or 2-aminoethanol to give the corresponding cyclic carbonates or 2-oxazolidinones in high yields and selectivity with the simultaneous formation of hydroxyketones. This review aims to summarize and discuss the recent advances in three-component reactions of CO₂, propargylic alcohols, and nucleophiles to prepare various carbonyl compounds promoted by both metal catalysts and organocatalysts.

In recent years, deep eutectic solvents (DESs) have attracted considerable attention. They have been applied in many fields such as dissolution and separation, electrochemistry, materials preparation, reaction, and catalysis. The DESs are generally formed by the hydrogen bonding interactions between hydrogen-bond donors (HBDs) and acceptors (HBAs). Knowledge of the thermal stability of DESs is very important for their application at high temperatures. However, there have been relatively few studies on the thermal stability of DESs. Herein, a systematic investigation on the thermal stability of 40 DESs was carried out using thermal gravimetric analysis (TGA), and the onset decomposition temperatures (T_onset) of these solvents were obtained. The most important conclusion drawn from this work is that the thermal behavior of DESs is quite different from that of ionic liquids. The anions or cations of ionic liquids decompose first, followed by the decomposition of the opposite ion at elevated temperatures. On the other hand, the DESs generally first decompose to HBDs and HBAs at high temperatures through the weakening of the hydrogen bond interactions. Subsequently, the HBDs with relatively low boiling points or poor stabilities undergo volatilization or decomposition; the HBAs also undergo volatilization or decomposition but at a higher temperature. For example, the most commonly used HBA choline chloride (ChCl) begins to decompose at around 250 ℃. The hydrogen bond plays an important role in the thermal stability of DESs. It hinders the "escape" of molecules and requires greater energy to break than pure HBAs and HBDs, which causes the T_onset of DESs to shift to higher temperatures. Note that the thermal stability of HBDs has a crucial effect on the T_onset of DESs. The HBDs would decompose or volatilize first during TGA because of their relatively poor thermal stability or lower boiling points. The more stable the HBDs are, the greater would be the T_onset values of the corresponding DESs. Further, the effects of anions on HBAs, molar ratio of HBAs to HBDs, and heating rate in fast scan TGA have been discussed. As the heating rate increased, the TGA curves of DESs shifted to higher temperatures gradually, and the temperature hysteretic effect became prominent when the rate reached 10 ℃?min?1. From an industrial application point of view, there is an overestimation of the onset decomposition temperatures of DESs by T_onset, so the long-term stability of DESs was investigated at the end of the study. This study could help understand the thermal behavior of DESs (progressive decomposition) and provide guidance for designing DESs with appropriate thermal stability for practical applications.

Carbon dioxide is a green C₁ resource that can be efficiently recycled by catalytic transformation into value-added chemicals. Formamides are important intermediates and solvents that are used extensively in pharmaceutical, daily-chemical, and petrochemical industry. Therefore, it is worthwhile to synthesize formamides with CO₂ and amines. In this review, the main advancements in the synthesis of formamides by using CO₂ as the C₁ feedstock with noble metal catalysts (Ir, Pd, Pt, Ru, Rh, etc.), non-noble metal catalysts (Ni, Mo, Cu, Fe, Co, Zn, Al, etc.), organocatalysts, and catalyst-free systems have been summarized. In addition, the role of the reducing agents such as H₂, silanes, and boranes involved in these transformations has also been reviewed. In addition, the reaction mechanisms with the different catalyst systems have been discussed.

In this study, the influence of an external electric field (EEF) on the vibrational spectra of an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium hexfluorophosphate (EMIMPF₆), in the wavenumber range from 0 to 4000 cm⁻¹ was probed by molecular dynamics (MD) simulation at 350 K. The results showed that the experimentally obtained spectrum could be reproduced by the calculated vibrational bands (VBs) in the wavenumber range from 400 to 4000 cm⁻¹ using MD simulation without any EEF. When the EEF applied increased from 0 to 9 V·nm⁻¹, the VB intensities at 50.0 and 199.8 cm⁻¹ increased continuously and then tended to be saturated, while the VB intensities from 400 to 4000 cm⁻¹ decrease and eventually disappear. Moreover, the VB at 50.0 cm⁻¹ was red-shifted to ~16.7 cm⁻¹ and then increased to 33.3 cm⁻¹ as the EEF was increased from 0 to 2 and then to 3 V·nm⁻¹ and higher. The VB at 3396.6 cm⁻¹ was redshifted to ~16.7 cm⁻¹ and then increased to 33.3 cm⁻¹ as the EEF was increased from 0 to 3 and then to 4 V·nm⁻¹ and higher; however, the position of other VBs from 0 to 4000 cm⁻¹ remain almost unchanged. Based on further analysis of the simulation results and previously reported studies, for the VB at 50.0 cm⁻¹, the increasing EEF enhances the polarity between cations and anions; thus, the difference in dipole moment between the cations and the anions increases, which continually increases the VB intensity until saturation is reached. For the VB at 199.8 cm⁻¹, the increasing EEF intensifies the twisting of the ethyl chain, which enhances the VB intensity until saturation. For the other VBs from 400 to 4000 cm⁻¹, the increasing EEF makes the orientation of the cations and anions in EMIMPF₆ more consistent; thus, it can be conjectured that such consistent orientation may weaken the VB intensities and can even make them disappear. The redshift of VB at 50.0 cm⁻¹ may occur because the EEF breaks the distribution of the electrostatic field inside EMIMPF₆ and then weakens the interactions between cations and anions. The redshift of VB at 3396.6 cm⁻¹ may be attributed to the EEF weakening the stretching vibration of the hydrogen bonds formed between the N atoms and the acidic hydrogen atoms on the cationic imidazolium rings. The EEF does not change the positions of the other VBs because the inherent stretching, bending, and rocking vibration of functional groups are not affected by the EEF.

Small-sized zeolite ZSM-5 for a wide SiO₂/Al₂O₃ ratio range was prepared using a small amount of colloidal silicalite-1 as the active seeds. The thus-prepared small-sized ZSM-5 samples have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption analysis, temperature-programmed ammonium desorption (NH₃-TPD) analysis, and adsorbed pyridine infrared spectroscopy (Py-IR). The use of the active silicalite-1 seeds was effective in directing the reaction towards the formation of the MFI phase, avoiding the impure phases and reducing the crystal sizes. The prepared sample exhibited aggregated morphologies when a lower ratio of starting batch SiO₂/Al₂O₃ (SiO₂/Al₂O₃ ratio = 30) was used. The aggregates, with the size of ~500 nm, were formed with nano-sized primary crystals 50 nm in size, possessing large external surface area (84.9 m²·g⁻¹) and secondary pore volume (0.22 cm³·g⁻¹) and relatively regular mesopores. Different morphologies could be observed when the SiO₂/Al₂O₃ ratio was increased (SiO₂/Al₂O₃ ratio = 60–120). ZSM-5 with the size of 200 nm could be prepared, with the external surface area and the secondary pore volume being ~60 m²·g⁻¹ and 0.10 cm³·g⁻¹, respectively. It should be highlighted that all the prepared samples could be directly ion-exchanged to obtain the acidic H-form samples without complete blocking of the micropores due to the low dose of the organic structure-directing agent. The obtained acidic H-form samples exhibited acidic properties similar to the samples ion-exchanged after calcination and the conventional ZSM-5 possessing similar SiO₂/Al₂O₃ ratio, showing catalytic performance comparative to the catalytic conversion of methanol to olefins. Compared with conventional methods, this method largely reduced the use of organic templates and avoided the subsequent combustion before ion-exchange. The method is green and cost-effective, possessing wide potentials in the industrial processes.

Because of the unique acid-base behavior of amino acids, we combined glycine as the anion with an ether group as the cation and prepared a novel ionic liquid (IL), 1-methoxyethyl-3-methylimidazolium glycine [MOEMIM][Gly]. The formation of this IL was confirmed by ¹H-NMR, ¹³C-NMR, differential scanning calorimetry (DSC) and thermogravimetry (TG) analyses. The density, ρ, and surface tension, γ, of the ILs were measured in the temperature range from 298.15 K to 338.15 K at intervals of 5 K using the standard addition method, because the strong hydrogen bonds between the IL [MOEMIM][Gly] and trace amounts of water made it, extremely difficult to remove water by convention methods. The calculated molar volume, V_m, of [MOEMIM][Gly] increased with increasing temperature. The thermal expansion coefficient, α, was also obtained based on the density values. In terms of the molar surface Gibbs free energy, g_s for [MOEMIM][Gly], the traditional Eötvös equation is improved so that it has physical significance, that is, the intercept C₀ represents molar surface enthalpy, h, which is a temperature-independent constant and the slop C₁ = −($\partial $g_s/$\partial $T)_p is molar surface entropy, s, it shows that modified Eötvös equation is not only an empirical equation, but also a strict thermodynamic one. In addition, by a combination of g_s and the modified Eötvös equation, the surface tension values of [MOEMIM][Gly] were estimated and compared with the experimental values; both sets of data were in good accordance with each other.

Ionic liquids (ILs) have become one of the most rapidly growing new research areas of ionic liquid and have attracted considerable attention from industry and the academic community since they are derived from natural ions and are heralded a new "natural ILs" or "bio-ILs". ILs can be expected to find application in all of the biological, medical, and pharmaceutical sciences. However, the fundamental physical properties of an IL are extremely important in determining if a particular IL is appropriate for a given application. Recently, there is a developing trend in the literature towards estimation of thermodynamic properties for IL's, which is to be commended because it provides valuable insight into the origins of the behaviour of ILs. Acidic ionic liquids have been applied in desulfuration process widely. Phosphoric acid IL is one kind of acidic ILs, which are also stable and economical efficiency. ILs [C_nmim][H₂PO₄] (n= 3, 4, 5, 6; 1-alkyl-3-methylimidazolium phosphonate) were synthesized and characterized. An on-line solution-reaction isoperibol calorimeter was constructed. It consists of a water thermostat, a 200 mL pyrex-glass plated silver Dewar, a 4 mL glass sample cell, a calibration heater, a glass-sheathed thermistor probe, an amplifier, a circuit used as an A/D converter and a personal computer for data acquisition and processing. The performance and accuracy of the calorimetric system was tested by measuring the molar enthalpy of solution of KCl in water and THAM [tris-(hydroxymethyl) aminomethane] in 0.1 mol∙dm⁻³ HCl(aq) at 298.15 K. The molar enthalpies of solutions of the ILs [C_nmim][H₂PO₄] (n= 3, 4, 5, 6) at various molalities in water were measured by using a solution-reaction isoperibol calorimeter at 298.15 K respectively. According to the Pitzer's electrolyte solution theory, the molar standard enthalpy of the solution of [C_nmim][H₂PO₄] (n = 3, 4, 5, 6), Δ_solH_m⁰, and Pitzer's parameters, β_MX^(0)L, β_MX^(1)L, and C_MX^ϕ, were calculated and deduced. The standard molar enthalpies of solution of [C_nmim][H₂PO₄] (n = 3, 4, 5, 6) decreases with increasing alkyl chain length. When we make a plot with the standard molar enthalpy of solution vs carbon number in alkyl chains of the ionic liquid, a straight line was obtained with a correlation coefficient 0.99 and a slope of 1.54. The mean contribution per methylene (―CH₂―) group to the standard molar enthalpies of the solution of [C_nmim][H₂PO₄] (n = 3, 4, 5, 6) was then obtained, which will give reliable estimation of other IL properties.

The electrocatalytic reduction of CO₂ to C₂H₄ is a topic of great interest. It is known that the preparation of efficient catalysts for this transformation is the key factor that determines the yield of C₂H₄. In this study, we prepared 1-octyl-3-methylimidazole functionalized graphite sheets (ILGS) in a facile manner by the electro-exfoliation of pure graphite rod in an aqueous solution of 1-octyl-3-methylimidazolium chloride (OmimCl : H₂O = 1 : 5, V/V) at 10 V. They were then dispersed in an aqueous solution of copper chloride and sodium citrate. Subsequent reduction with sodium borohydride led to the formation of a composite comprised of cuprous oxide supported on Omim-functionalized graphite sheets (Cu₂O/ILGS). This composite was found to be an efficient catalyst for the electroreduction of carbon dioxide to ethylene. The as-made materials were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray diffraction (XRD). The TEM images showed that the ILGS were composed of multiple layers of graphene. The XRD pattern and Raman spectrum indicated that the surface of the ILGS possessed several defects. In the electro-exfoliation process, the defects in the ILGS were modified in situ by covalent bonding with Omim groups, which was also confirmed by XPS. The Cu₂O nanoparticles with an average diameter of 5 nm were uniformly distributed on the surface of the ILGS because the Omim groups grafted to the graphite sheets acted as anchors and prevented their aggregation by the steric effect. The electrocatalytic activities of Cu₂O/ILGS for CO₂ reduction were measured at different voltages in 0.1 mol L^–1 KHCO₃ aqueous solution under ambient temperature and pressure. These experiments showed that the catalytic performance of the Cu₂O/ILGS composite was determined by cuprous oxide, while the ILGS displayed nearly no catalytic activity in the electroreduction of carbon dioxide. The faradaic efficiency of hydrogen and carbon dioxide reduction products changed with the reaction time because of the reduction of Cu₂O to Cu under the electroreduction conditions. The faradaic efficiency of ethylene was ~14.8% at –1.3 V (versus reversible hydrogen electrode). The performance of Cu₂O/ILGS in the catalytic electroreduction of carbon dioxide was attributed to the stabilization of the Cu₂O nanoparticles by the nest-like microstructures in the Cu₂O/ILGS composite.

The self-assembly behavior of block copolymers and their assembled micellar morphologies have attracted considerable attention because of their potential applications in biomedicine, drug delivery, and catalysis. Herein we report that CO₂-expanded liquids (CXLs) facilitate the morphology control of the self-assembled aggregates (SAAs) of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) formed in CO₂-expanded toluene. It is found that the anti-solvent effect of CXLs can successfully regulate the self-assembly behavior of the copolymer PS-b-P4VP. The difference in amphiphilicity between PS and P4VP block is reduced with increasing pressure of CO₂-expanded toluene owing to the anti-solvent effect of CO₂. In addition, this diminished difference may influence the interfacial tension at the P4VP core-PS corona interface, which triggers a morphological change of the aggregate. The SAA structures are dependent on both CXL pressure and copolymer composition under the experimental conditions implemented in this work. The morphological evolution of the SAAs in CXLs exhibits remarkable pressure dependence. As the pressure increases, the SAA structure of PS₁₆₈-b-P4VP₄₂₀ transits from primarily spheres (0.1 MPa) to mostly interconnected rods (6.35 MPa), the SAA of PS₇₉₀-b-P4VP₂₆₃ evolves from small vesicles (0.1 MPa) to large compound vesicles (LCVs, 6.35 MPa), whereas the PS₁₅₃-b-P4VP₁₅₃₀ counterpart switches from large compound micelles (LCMs, 0.1 MPa) to mainly large compound vesicles (LCVs, 6.35 MPa). Moreover, transmission electron microscopy (TEM) data on constant copolymer composition implies that the packing parameter p of the SAAs increases with the CXLs pressure. Especially, under the experimental conditions employed in this work, we find that the major factor controlling the SAA shape in conventional toluene is the copolymer composition, while in CO₂-expanded toluene, the dominant factor controlling the SAA morphology might be the varying contact area between shell-forming segment PS and the CXLs with increasing pressure. This work proves that the CXL method facilitates the modulation of morphology of the SAAs, and opens a green route for the development of new nano-functional materials.

The increasing anthropogenic emission of CO₂ leads to global warming, to address which three strategies can be considered: (1) decrease fossil fuel consumption through increased utilization efficiency and lower per capita consumption; (2) replace fossil fuels with renewable energy sources like wind, tidal, solar, and biomass energies; (3) utilize CO₂ efficiently. Despite efforts to reduce energy use and increase the use of carbon-neutral biofuels, it seems that fossil fuels will continue to be a major energy source for the next few decades. Tremendous effort is therefore being focused on developing effective technologies for CO₂ capture and transformation. In particular, the transformation of CO₂ into fuels and chemicals via reduction with renewable hydrogen is a promising strategy for mitigating global warming and energy supply problems. The hydrogenation of CO₂, especially to C₂₊ hydrocarbons and oxygenates, has sparked growing interest. The C₂₊ species can be used as entry platform chemicals for existing value chains, thus providing more advantages than C₁ compounds. However, optimizing catalyst design by integrating multifunctionalities for both CO₂ activation and C-C coupling remains an ongoing challenge. Here, we provide a timely review on the recent progress that has been made in the hydrogenation of CO₂ to higher-order alkanes, olefins, and alcohols by various heterogeneous catalysts. The thermodynamics and kinetics, as well as possible reaction pathways for CO₂ hydrogenation, are discussed. The hydrogenation of CO₂ to hydrocarbons usually involves the initial generation of CO via a reverse water-gas shift (RWGS) reaction followed by hydrogenation of the CO intermediate. The RWGS reaction proceeds through a redox route and an associative pathway. "CH_x" insertion (carbide-type) and "CO" insertion are two proposed mechanisms for this Fischer-Tropsch-like synthesis. Fe-or Co-based catalysts have been widely used to catalyze the hydrogenation of CO₂ to C₂₊ hydrocarbons via the CO intermediate. C₂₊ hydrocarbons can also be obtained by combining CH₃OH synthesis with the methanol-to-hydrocarbon process (MTH). This reaction pathway has been realized over bifunctional systems comprising a CH₃OH synthesis catalyst and an MTH catalyst. Alternatively, CO₂ hydrogenation can occur via a RWGS reaction to the CO intermediate, and subsequent formation of higher alcohols from syngas. Higher alcohols (mostly CH₃CH₂OH) have been produced by using a hybrid tandem catalyst. Understanding of the activation mechanism, precise C-C coupling, and synergy control between the two active components requires further research. In the final part, we describe the future challenges and opportunities in heterogeneous catalysis of CO₂ hydrogenation. The combination of calculations (precise theoretical models) and experiments (in-situ spectroscopic techniques) will facilitate the design of advanced catalysts to achieve both high CO₂ conversion and C₂₊ product selectivity.

Deep eutectic solvents (DESs) are regarded as a new class of green solvents because of their unique properties such as easy synthesis, low cost, environmental friendliness, low volatility, high dissolution power, high biodegradability, and feasibility of structural design. DESs have been widely applied for the separation of mixtures as alternatives to conventional solvents. A DES usually consists of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). HBAs include amides, thiourea, amines, imidazole, azole, alcohols, acids and phenol. HBAs include quaternary ammonium salts, quaternary phosphonium salts, imidazolium-based salts, dication based salts, inner salts, and molecular imidazole and its analogues. Therefore, there are numerous DESs available for use in different applications. With an in-depth understanding of the common and novel properties of DESs, researchers have prepared and applied DESs to various types of separations. We first introduce the composition of DESs, including various HBDs and HBAs frequently used in the literature. Second, the properties of DESs, including phase diagrams, melting points, density, viscosity, and conductivity, are summarized. Third, recent applications of DESs in the separation of mixtures are reviewed, including the absorption of acidic gases (CO₂, SO₂ and H₂S), the extraction of bioactive compounds, extraction of sulfur-and nitrogen-containing compounds from fuel oils, extraction of phenolic compounds from oils, separation of mixtures of aromatic and aliphatic compounds, separation of alcohol and water mixtures, removal of glycerol from biodiesel, separation of alcohol and ester mixtures, removal of radioactive nuclear contaminants, and separation of isomer mixtures of benzene carboxylic acids. DESs are used in two ways for the separation of mixtures. (1) A DES prepared in advance is used as a solvent to separate a component from a mixture by selective dissolution or absorption of specific compound(s), such as the absorption of SO₂ using betaine+ethylene glycol DES. Here, DESs are used like traditional solvents. (2) A DES is formed in situ during the separation of mixtures by adding a HBA to a mixture containing one or more HBDs, such as the removal of phenol from an oil mixture using choline chloride, where a phenol+choline chloride DES is formed during the separation process and the formed DES does not dissolve in the oil phase. Although various DESs have been broadly developed for the separation of mixtures, research continues in the field of DESs, including analysis of the physicochemical properties of DES, especially during extraction or absorption, development of functional DESs for high-efficiency separations, development of efficient methods to regenerate DESs, and combined use of DESs with other techniques to improve separation processes. This article describes general trends in the development of DESs for separation and evaluates the problematic or challenging aspects of DESs in the separation of mixtures.

Thiazolidine-2-thiones have attracted much attention because of their unique bioactivity and have been widely used in the fields of medicine and synthetic heterocyclic chemistry. In this work, a simple and convenient route for the synthesis of 3-arylthiazolidine-2-thiones by direct condensation of 2-(arylamino) alcohols with carbon disulfide (CS₂) catalyzed by the ionic liquid 1-butyl-2, 3-dimethylimidazolium acetate (BmmimOAc) has been developed. A series of ionic liquids were used as catalysts in the model reaction of 2-(phenylamino) ethanol with CS₂. The results showed that only the acetate ionic liquids have catalytic activity, perhaps owing to the basicity of the acetate anion. Among these acetate ionic liquids, BmmimOAc showed the highest catalytic activity and was selected as the best catalyst. The effects of reaction time, reaction temperature, amount of BmmimOAc, and CS₂ to 2-(phenylamino) ethanol molar ratio were investigated in detail. The following were found to be the optimal reaction conditions for direct condensation of 2-(phenylamino) ethanol with CS₂: reaction time, 6 h; reaction temperature, 130 ℃; 10% molar fraction of BmmimOAc; and CS₂ to 2-(phenylamino) ethanol molar ratio of 5 : 1. Under these optimized reaction conditions, the product 3-phenylthiazolidine-2-thione was obtained in 97% yield. The reaction scope was explored by investigating the reactions of various 2-(arylamino) alcohols with CS₂. The results showed that the 2-(arylamino) alcohols with electron-donating substituents, electron-withdrawing substituents, or high steric hindrance could be smoothly converted to the corresponding products in excellent isolated yields of 83%–95%. NMR characterizations and mass spectrum indicated that the acetate anion of BmmimOAc could react spontaneously with CS₂ to form the thioacetate anion (CH₃COS⁻). Ionic liquid 1-butyl-2, 3-dimethylimidazolium thioacetate (BmmimCOS) might be the actual catalyst in the reaction of 2-(arylamino) alcohols with CS₂. ¹H and ¹³C NMR spectroscopies were used to study the interactions between BmmimCOS and substrates 2-(phenylamino) ethanol and CS₂. The NMR spectra showed the hydrogen bonding interactions between BmmimCOS and 2-(phenylamino) ethanol. The CH₃COS⁻ anion of BmmimCOS could activate the 2-(phenylamino) ethanol in the catalytic system. Based on the characterization results, a possible reaction mechanism was proposed. Firstly, BmmimOAc reacted spontaneously with CS₂ to form BmmimCOS. Then, CH₃COS⁻ of BmmimCOS activated 2-(phenylamino) ethanol via hydrogen bonding. Subsequently, CS₂ reacted with the activated 2-(phenylamino) ethanol to form the intermediate. Finally, the intermediate was subjected to intramolecular cyclization to form the final product 3-phenylthiazolidine-2-thione.