Shaojuan Zeng

Ionic-Liquid-Based CO2 Capture Systems: Structure, Interaction and Process

The inherent structure tunability, good affinity with CO2, and nonvolatility of ionic liquids (ILs) drive their exploration and exploitation in CO2 separation field, and has attracted remarkable interest from both industries and academia. The aim of this Review is to give a detailed overview on the recent advances on IL-based materials, including pure ILs, IL-based solvents, and IL-based membranes for CO2 capture and separation from the viewpoint of molecule to engineering. The effects of anions, cations and functional groups on CO2 solubility and selectivity of ILs, as well as the studies on degradability of ILs are reviewed, and the recent developments on functionalized ILs, IL-based solvents, and IL-based membranes are also discussed. CO2 separation mechanism with IL-based solvents and IL-based membranes are explained by combining molecular simulation and experimental characterization. Taking into consideration of the applications and industrialization, the recent achievements and developments on the transport properties of IL fluids and the process design of IL-based processes are highlighted. Finally, the future research challenges and perspectives of the commercialization of CO2 capture and separation with IL-based materials are posed.

Improving SO2 capture by tuning functional groups on the cation of pyridinium-based ionic liquids

In this work, three kinds of novel functionalized ionic liquids (ILs) [NEt2C2Py][SCN], [C4OPy][SCN] and [C4CNPy][SCN] were developed by introducing a tertiary amino group, ether group and nitrile group on the pyridinium cation to improve SO2 absorption performances. Among the investigated ILs, [NEt2C2Py][SCN] showed the highest absorption capacity of 1.06 gSO2 gIL−1 under ambient conditions due to a combination of chemical and physical absorption. By contrast, the enhancement in SO2 capacity by [C4CNPy][SCN] and [C4OPy][SCN] is mainly ascribed to the stronger physical interaction between ILs and SO2 than the conventional IL [C4Py][SCN]. Meanwhile, higher SO2/CO2 selectivity was also obtained using these functionalized ILs, which was increased up to 41% comparing with that of [C4Py][SCN]. Moreover, the effect of water on SO2 capacity and the absorption mechanism were studied. The results indicated that the presence of water caused a slight decrease in SO2 capacity of [C4CNPy][SCN] and [C4OPy][SCN] because of physical absorption, whereas a slight increase in SO2 capacity by [NEt2C2Py][SCN] due to the formation of hydrogen sulfite salts through chemical absorption. In addition, three kinds of cation-functionalized ILs could remain the stable absorption performance after five cycles of absorption and desorption, implying these ILs show great potentials for SO2 capture.

Extractive desulfurization of fuel using N-butylpyridinium-based ionic liquids

Sulfur compounds in fuels have become one of the sources of serious environmental problems. The extractive desulfurization using ionic liquids (ILs) has attracted great attention in recent years. In this work, the pyridinium-based ionic liquids (ILs) N-butylpyridinium thiocyanate ([C4Py][SCN]), N-butylpyridinium bis(trifluoromethylsulfonyl)imide ([C4Py][NTf2]), and N-butylpyridinium dicyanamide ([C4Py][N(CN)2]) were used as extractants for desulfurization of model fuels. The results demonstrate that the structure of the anion influences the extractive performance of ILs, following the order of [NTf2] benzothiophene (BT) > 4,6-dimethyldibenzothiophene (4,6-DMDBT). Moreover, the [C4Py][N(CN)2] can be recycled at least 4 times with little decrease in the desulfurization activity.

Imidazole tailored deep eutectic solvents for CO2 capture enhanced by hydrogen bonds

Deep eutectic solvents (DESs) have emerged as promising alternative candidates for CO2 capture in recent years. In this work, several novel DESs were firstly prepared to enhance CO2 absorption. Structural and physical properties of DESs were investigated, as well as their absorption performance of CO2. A distinct depression in the melting point up to 80 K of DESs was observed compared with that of BMIMCl. The observed red shifts of the C2H group in an imidazolium ring and its chemical shifts downfield in NMR spectra are indicative of a hydrogen bond interaction between BMIMCl and MEA. In particular, CO2 uptake in MEA : ILs (4 : 1) at room temperature and atmospheric pressure is up to 21.4 wt%, which is higher than that of 30 wt% MEA (13%). A hydrogen bond related mechanism was proposed in which ILs act as a medium to improve CO2 uptake through hydrogen bonds. Finally, the firstly reported overall heat of CO2 absorption is slightly higher than that of 30 wt% MEA, implying that the hydrogen bonds of DESs contribute to the overall heat of CO2 absorption. This study reveals that the heat of CO2 absorption can be tailored by the proper molar ratio of MEA and ILs.

Ether-functionalized ionic liquid based composite membranes for carbon dioxide separation

The efficient separation of CO2 from other light gases has received growing attentions due to its importance in reducing greenhouse gas emissions and applications in gas purification. In this work, we developed a series of composite membranes composed of ether-functionalized pyridinium-based ionic liquids ([EnPy][NTf2]) and cellulose acetate (CA) polymer matrices to improve CO2 separation performance. CA + [EnPy][NTf2] and CA + [CnPy][NTf2] composite membranes were fabricated by a casting method. The CO2, N2 and CH4 permeabilities of the CA + IL composite membranes were measured, and the CO2/N2 and CO2/CH4 permselectivities were further calculated. The results showed that the CA + 40 wt% [E1Py][NTf2] composite membrane exhibits approximately a seven-fold increase in CO2 permeability with CO2/N2 and CO2/CH4 permselectivities of 32 and 24, respectively. The characterization results showed that the mechanical properties and thermal stabilities of the CA + [E1Py][NTf2] composite membranes are affected by both plasticizing effect and affinity of the ILs for the gases, which also lead to the changes in the CO2/N2 and CO2/CH4 permselectivities. Compared with membranes containing the non-functionalized analogues [CnPy][NTf2], the addition of [EnPy][NTf2] improves the ideal permselectivities of CA + IL composite membranes, whereas it decreases slightly the gas permeabilities.

Protic ionic liquid [Bim][NTf2] with strong hydrogen bond donating ability for highly efficient ammonia absorption

Cost-efficient and environmentally benign treatment of NH3-containing exhaust gas has been a challenge. Ionic liquids (ILs) due to their unique structures and properties are recognized as potential solvents to absorb NH3. In this study, three types of ILs, including [Bmim][NTf2], [Bim][NTf2], [HOOC(CH2)3mim][NTf2], were designed and synthesized with various hydrogen bond donating abilities and characterized based on their thermodynamic dissociation constants (pKa). The results showed that protic ionic liquid (PIL) [Bim][NTf2] with a moderate pKa value had the highest NH3 absorption capacity (up to 2.69 mol NH3 per mol IL, 313 K, 100 kPa). Experimental characterization and theoretical calculations verified that such extremely high NH3 absorption capacity for [Bim][NTf2] resulted from the interactions between H-3 on the imidazole ring and the NH3 molecule and NH3 molecules sintering themselves. Furthermore, stable absorption performance was recorded for [Bim][NTf2] over four cycles, implying potential applications in the industry for NH3 recycling.

Deep Desulfurization of Gasoline Fuel using FeCl3-Containing Lewis-Acidic Ionic Liquids

The FeCl3-containing Lewis-acidic ionic liquids (ILs) [C6mim]Cl/FeCl3(1:1.5), [C6mim]Cl/FeCl3(1:2), [C8mim]Cl/FeCl3(1:1.5), and [C8mim]Cl/FeCl3(1:2) were used as extractants for desulfurization of model fuel and gasoline fuel, respectively. The results demonstrate that these ILs are effective for the removal of sulfur compounds from model fuel under different mass ratio of IL to model fuel (1:1, 1:3, 1:5, 1:10) at 25°C. The extractive performance of ILs increased as the molar ratio of FeCl3 to [Cnmim]Cl(n = 6, 8) varied from 1:1 to 1:2. The selectivity of sulfur compounds by extraction process followed the order of dibenzothiophene (DBT)>benzothiophene (BT) > 4,6-dimethyldibenzothiophene (4,6-DMDBT). The sulfur removal of gasoline fuel containing sulfur content of 440.3 ppmw could be up to 85.8%; that is to say that the sulfur content of gasoline fuel varied from 440.3 ppmw to 62.4 ppmw after one extraction stage. Moreover, the [C6mim]Cl/FeCl3(1:2) can be recycled for at least 4 times with a little decrease in the desulfurization activity.

Multi-scale simulation of the 1,3-butadiene extraction separation process with an ionic liquid additive

A multi-scale simulation method is proposed to enable screening of ionic liquids (ILs) as entrainers in extractive distillation. The 1,3-butadiene production process with acetonitrile (ACN) was chosen as a research case to validate the feasibility of the methodology. Ab initio calculations were first carried out to further understand the influence of ionic liquids on the selectivity of ACN and the solubility of C4 fractions in [CnMIM][PF6](n = 2–8), [C2MIM][X] (X = BF4−, Cl−, PF6−, Br−), by investigating the microstructure and intermolecular interaction in the mixture of C4 fractions and several selected ionic liquids. It was found that the selectivity of the ionic liquid is determined by both its polarity and hydrogen-bonding ability. Based on the analysis, a suitable ionic liquid was chosen. With the ab initio calculation, a priori prediction of thermophysical data of the IL-containing system was performed with COSMO-RS. The calculation revealed that the selectivity of the extractive solvent was increased by an average of 3.64% after adding [C2MIM][PF6]. With above calculations, an improved ACN extraction distillation process using ILs as an entrainer was proposed, and a configuration for the new process was constructed. Based on the established thermodynamic models which have considered the properties from the molecular structure of ILs, process simulation was performed to obtain the process parameters which are important for the new process design. The simulation results indicated that the temperatures at the bottom of the extractive distillation column with the ionic liquid as an additive are lowered by an average of 3.1 °C, which is significant for inhibition of polymerization. We show that the ACN consumption using this process can be lowered by 24%, and the energy consumption can likewise be lowered by 6.62%.

Pebax-based composite membranes with high gas transport properties enhanced by ionic liquids for CO2 separation

Membrane-based separation technology has been reported as one of the possible methods to efficiently and economically separate carbon dioxide (CO2). To provide synergistic enhancements in the gas separation performance, organic polymer (Pebax 1657), zeolite imidazolate framework-8 (ZIF-8) nanoparticles, and ionic liquid (IL) have been integrated to develop three-component composite membranes. To achieve high separation performance of three-component membranes, the effects of IL anions and ZIF-8 content on gas permeability and selectivity were investigated first. The ILs were 1-butyl-3-methyl imidazolium ([Bmim]) cation based on different anions of bis(trifluoromethylsulfonyl)imide ([NTf2]), dicyanamide ([DCA]), and tetrafluoroborate ([BF4]). Gas transport properties of all the prepared membranes were investigated at 23 °C and 1 bar. The results showed that the anion of IL is a key factor to determine the CO2 permeability of the membranes, which is similar to the principle of CO2 solubility in pure ILs. In addition, ZIF-8 could increase both CO2 diffusivity and solubility coefficients of the Pebax/ZIF-8 membranes, resulting in a two-fold increase in the CO2 permeability. For the Pebax/ZIF-8(15%)/[Bmim][NTf2] membranes, it has been revealed that [Bmim][NTf2] acts as a low molecular weight additive, leading to a more amorphous structure and a higher FFV (fractional free volume) of the membranes, which are beneficial for gas diffusion. The addition of IL can improve the compatibility between the inorganic particles and the polymer matrix; thus, the non-selective voids decrease, which leads to a higher CO2/N2 selectivity. The CO2 permeability of the Pebax/ZIF-8(15%)/IL(80%) membrane was 4.3 times that of the pure Pebax membrane without sacrificing the CO2/N2 selectivity. Therefore, the high gas transport properties of the Pebax/ZIF-8/IL membranes make them promising candidates for CO2-effective separation materials.

Ionic Liquids: Advanced Solvents for CO 2 Capture

As one of promising advanced solvents, ionic liquids (ILs) have become more attractive for CO2 capture due to their unique properties, special structures and potential energy saving efficiency. This chapter mainly reviews the research progress on CO2 capture with ILs, focusing on the CO2 absorption capacity of conventional ILs, task-specific ILs and ILs based mixtures as well as the comparison and analysis. The influence of cations, anions and functional groups of ILs on the CO2 absorption was analyzed and the mechanisms of physisorption and chemisorption were revealed using experimental test and molecular simulation results. Especially considering the real applications of the new ILs-based capture technologies, the research on process simulation and assessment of CO2 capture processes was also reviewed. Finally, we discussed the challenges and opportunities of transferring the lab-scale research results to practical industries processes, and also present some perspectives of ILs based novel technologies.