Ionic Liquids for CO2 Capture: Comparison
Please note this is a comparison between Version 2 by Jessie Wu and Version 1 by SYED AWAIS ALI.

Ionic Liquids (ILs) are the molten salts with a melting point lower than 100 °C and behave as a liquid at room temperature. ILs properties can be altered by the careful selection of the anion and cation. ILs have bulky organic cations with low molecular symmetry and small organic/inorganic anions that are in their molten form in their pure states under ambient conditions. Global warming is one of the major problems in the developing world, and one of the major causes of global warming is the generation of carbon dioxide (CO2) because of the burning of fossil fuels. Burning fossil fuels to meet the energy demand of households and industries is unavoidable. The current commercial and experimental techniques used for capturing and storing CO2 have serious operational and environmental constraints. The amine-based absorption technique for CO2 capture has a low absorption and desorption ratio, and the volatile and corrosive nature of the solvent further complicates the situation. To overcome all of these problems, researchers have used ionic liquids (ILs) and deep eutectic solvents (DESs) as a replacement for commercial amine-based solvents. ILs and DESs are tunable solvents that have a very low vapor pressure, thus making them an ideal medium for CO2 capture. Moreover, most ILs and DESs have low toxicity and can be recycled without a significant loss in their CO2 capture capability.

  • ionic liquids
  • ,CO2 capture
  • ,products
  • ,Catalytic conversion of CO2
  • ,Economics of ionic liquids for CO2 capture

1. Introduction

Long before the industrial revolution, the temperature of the Earth was controlled by greenhouse gases that naturally occurred, such as water vapor and clouds [1]. Greenhouse gases (GHGs) are transparent to sunlight, but they do not allow the heat radiated from the Earth to escape from the atmosphere and absorb a portion of it, thus keeping the temperature of the Earth uniform [2][3]. However, in the last few decades, the industrialization and burning of fossil fuels have generated anthropogenic GHGs, resulting in an imbalance in the temperature of the Earth, and researchers have coined the term climate change for this phenomenon [4]. The effect of climate change is different for different countries, and it is independent of the amounts of GHGs emitted by a particular country. The countries that are worst hit by climate change have qite small amounts of GHG emissions [5]. The major GHGs include carbon dioxide (75%), fluorinated gases (2%), methane (18%), and nitrous oxide (4%) [6]. Carbon dioxide (CO2) is the major contributor to greenhouse gases because of fossil fuel burning in the transportation, heating, manufacturing, and electricity sectors [7][8]. The Intergovernmental Panel on Climate Change (IPCC) has shown that 79% of CO2 produced from fossil fuels is used for power generation, and the main contributors are coal power plants, with a share of 60% [9][10]. Hence, it is highly necessary to devise an efficient strategy for CO2 removal from waste gases and its utilization for the production of different chemicals [11].

2. Conversion of CO2 into Valuable Products Using Ionic Liquids

The conversion of CO2 into valuable products involves several major steps, such as CO2 diffusion, adsorption, catalytic conversion, product distribution from the catalyst, and finally, the diffusion of the product to the bulk phase for solution separation [12][13]. Both the capture and conversion of CO2 are essential features to achieve high-value products from CO2 [14]. In photochemical, biochemical, and electrochemical reduction methods, ILs facilitate CO2 conversion into quality products. This is the most reliable method for the electrochemical reduction of chemicals and CO2. [15]. Both chemicals and epoxides were used for the conversion of CO2 into linear and cyclic carbonates through cycloaddition reactions. Due to its remarkable ability for electrical energy storage from natural sources such as the sun, product conversion efficiency, and product selectivity, electrochemical CO2 reduction is prevalent [16].
The cycloaddition of CO2 with epoxides for its conversion via ILs is regarded as the most prominent approach toward the model conversion reaction. Peng et al. utilized ILs to facilitate the catalytic cycloaddition of CO2 into epoxides without any involvement of organic solvents, even at 2 MPa and 110 °C [17]. Lkushima’s and Wang’s groups refined the rate of conversion and yield using supercritical CO2 at pressures and temperatures of 8–14 MPa and 100–160 °C, respectively. The advantages of supercritical CO2 comprise the rapid equilibrium and non-usage of organic solvents for the separation of reactants and products [18][19]. However, a lot of efforts have been made for the improvement and design of advanced IL synthesis via polymerization and functionalization [20] or through the use of support methods, such as MOF introduction [21], polymer support, and silica support [22]. Introducing metal elements into ILs by metal doping or immobilization also enhances their selectivity and efficiency in the cycloaddition of CO2 [14][23]. ILs can act as catalysts and as solvents for the cycloaddition of CO2 without involving the supercritical state, immobilization, or metals. Han et al. reported on the selectivity behavior and efficiency of task-specific ILs as solvents and catalysts at a pressure and temperature of 1 bar and 30–60 °C [24]. Many efforts have been made to reduce the viscosity of ILs to make them economical by improving their reaction time [25]. Wang et al. reported on the use of epoxide in propargylic alcohol for the cycloaddition of CO2 to prepare α-alkylidene cyclic carbonate. The reaction was performed at a pressure and temperature of 1 bar and 60 °C, consuming 200 mol% of the IL without involving another solvent [26].
Zhang et al. reported that the superbase-derived PIL 1,8-Diazabicyclo[5.4.0]undec-7-ene acetate [DBU] [Ac] could catalyze carbamate formation from a solution of silica ester, amine, and CO2 in acetonitrile, even at a pressure and temperature of 5 MPa CO2 and 150 °C [27]. Amines based on aromatics showed lower activity than aliphatic amines because of the low PKa values of aromatic amines [27]. This is mainly attributed to the hydrogen-bonding interactions between aniline and the acetate anion of ILs. They require a protonated cation and basic anion to facilitate the efficient and smooth preparation of carbamate. The significant restrictions on using the results of this report were the presence of co-solvents (i.e., acetonitrile) and harsh conditions (such as 5 MPa and 150 °C). The synthesis of carbamate from CO2 catalyzed by ILs could be improved even at low CO2 pressure and mild temperature by designing new tasks for metal-free ILs [14].
In order to obtain C1-C21 hydrocarbons from CO2 hydrogenation, Qadir et al. increased the conditions by raising the temperature to 150 °C and pressure to 6.8 MPa for H2 and 1.7 MPa for CO2 without any involvement of supporting solvents (such as DMSO and H2O) using Ruthenium Iron (RuFe) as a catalyst and 1-butyl-3-methylimidazolium bis[trifluoromethyl)sulfonyl] imide [BMIM] [Tf2N] as a solvent [28]. They used a hydrophobic IL, [BMIM] [Tf2N], involving the same mechanism as that of the IL in Figure 1. Hydrocarbon production via media of non-basic ILs involves two steps: CO2 conversion to CO by the reverse water–gas shift reaction on the surface of RuFe and subsequent chain circulation via Fischer–Tropsch synthesis [28].
Figure 1. IL cage on RuFe nanocatalysts for CO2 to HCOOH (basic IL) selective hydrogenation using hydrocarbons (via non-basic IL).With Permission from Ref. [28]. Copyright 2018, American Chemical Society.
In light of Qadir’s report, the involvement of the RuFe catalyst at a high temperature and pressure was a necessary condition to produce hydrocarbons. Melo et al. raised the systematic temperature above 150 °C to obtain CH4 in a yield of 69%, maintaining a comparable pressure (4–6 MPa H2 with a total H2/CO2 pressure of 8 MPa) through the involvement of Ru nanoparticles as the catalyst in 1-octyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) imide [OMIM] [Tf2N]. The enhancement in the concentration of the catalyst and the temperature would also increase the CH4 yield and TON [29].
Deng et al. successfully studied the fascinating combination of the reaction and separation using a system of cesium hydroxide (CsOH)-IL catalysts for the carbonylation of both aromatic and aliphatic amines with CO2 at pressure and temperature of 6 MPa and 170 °C, respectively, to give rise to the formation of urea derivatives [30]. The product could also be recovered again using drying and filtration via the addition of water to the reaction mixture. The high pressure/temperature and the presence of CsOH would necessitate more energy [30]. Jing et al. used 1-butyl-3-methylimidazolium hydroxide [BMIM] [OH] by combining the benefits of ILs with the simplicity of CsOH to avoid the utilization of the relatively expensive and risky CsOH technique. Although the conditions for CO2 conversion were still very tight (170 °C and 5.5 MPa), they expanded the range of amines to include benzylamine, cyclohexylamine, and aliphatic amines [31].

3. Economics of Ionic Liquid-Based CO2 Capture

The cost of CO2 capture using ILs is a challenging aspect of utilizing ILs at an industrial scale to attain the economic commercialization of this technology. However, significant research efforts in the past decades have been made to develop ILs for efficient CO2 capture [32][33]. It was successfully achieved by improving the hydrothermal stability and the CO2 capacity of ILs [34]. However, the overall cost includes the costs of the synthesis of ILs [35], CO2 adsorption and desorption, and the regeneration of ILs [36]. The main problem is the high material costs and solvent requirements for synthesizing ILs. However, these costs are still significantly greater than those for conventional solvents and amine-based solvents [37][38]. Numerous experiments have been carried out to make ILs cost-effective for CO2 capture and conversion [39]. The main benefit of ILs over conventional amine-based solvents during regeneration is their ability to absorb CO2 [40] and other acidic gases, with the advantage of requiring minimum energy compared with conventional solvents based on amines [41][42]. Nevertheless, these systems may not be economically comparable to conventional solvents due to the current high demand for and price of solvents [33][43].
In terms of the techno-economic intentions of ILs in CO2 absorption, Riva et al. proposed an alternative operative cost of 83 USD/t CO2 using 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]-imide ([Emim] [NTf2]) for post-combustion CO2 capture [44], whereas the cheapest possible cost previously attained by Martinez et al. was 90 USD/t CO2 using 1-ethyl-3-methylimidazolium dicyanamide ([Emim] [DCN]) [45]. García et al. conducted a techno-economic evaluation for ILs such as 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)-sulfonyl]imide [Hmim] [NTf2]), 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide [Emim] [NTf2], and trihexyl(tetradecyl)phosphoniumbis[(trifluoromethyl)-sulfonyl]imide ([P66614] [NTf2]) for upgrading biogas, employing [Emim] [NTf2] as a cost-effective alternative with a total cost of USD 271 per metric ton of CO2 captured [46]. T. E. Akinola et al. investigated CO2 removal using a solution of MEA and H2O (30/30/40 wt%) with 1-butylpyridinium tetrafluoroborate, [Bpy] [BF4], and found an expected cost (25 USD/t CO2) to obtain a cost-efficient and energy-efficient gas separation technique [47]. This is due to the reduced operating cost of the [Bpy] [BF4]-MEA-based process as a result of the low utility cost [48]. Y. Huang et al. performed a cost analysis for the feasibility of using [Bpy] [BF4]-MEA-based solutions compared to the conventional MEA-based process and achieved a lower cost ranging from USD 70 to USD 60–62.5 per metric ton of CO2 [49]. Shiflett et al. also performed an equilibrium-based simulation for 1-butyl-3-methylimidazolium acetate, [Bmim] [acetate], which showed an affinity toward CO2 with a total cost of about 140 USD/t CO2 [50].
One of the core issues that stands in the way of commercialisation is the high cost of ILs. However, in one of the recent modelling and simulation studies on the ILs production process shows that ILs can be produced at lower cost ($1.24 kg−1), which is in comparison with most of the organic solvents such as acetone or ethyl acetate with a cost of $1.30–$1.40 kg−1 [51]. Similarly, in another study, the extraction of aromatic hydrocarbon from aliphatic hydrocarbon with 4-methyl-N-butylpyridinium tetrafluoroborate was modelled using ASPEN resulting in a positive margin of about €20 million per year [52]. These results indicate that ILs are not necessarily expensive, and therefore large-scale ILs-based processes can become a commercial reality provided that some industry is ready to take up the project.

4. Difficulties and Drawbacks of Using Ionic Liquids to Capture CO2

ILs have been developed as potential sorbents for carbon capture and conversion operations. Despite the many advancements in ILs for carbon capture, there is still a need for significant advancements in ILs for the carbon capture and utilization (CCU) process [11]. To begin with, the majority of ILs used in the mature binding process are pyridine- or imidazole-based, which exhibits biological toxicity [53][54]. ILs disintegrate near the normal boiling point, and therefore, it is difficult to determine their critical properties. As a result, effective techniques for obtaining the essential properties of ILs must be developed [55].
One of the primary problems in CO2 capture using ILs is their high viscosity, which leads to a decrease in CO2 solubility in ILs. There are several approaches to this problem that need to be thoroughly explored and investigated. One approach is to use a combination of water and amines with ILs, for which the best composition and process conditions must be found [56]. The cost of ILs used for CO2 capture is another drawback. In comparison to amines, ILs used in CO2 capture processes are more expensive than conventional solvents. Although the cost of ILs on a large scale (less than USD 40/kg) can be substantially lower than the present lab-scale pricing (about USD 1000/kg), they are still 10–20 times more expensive than conventional solvents [55].
Researchers must develop easy and cost-effective synthesis methods for CO2 capture using ILs as target solvents. To employ ILs as an absorption method on a wide scale, the right system design and operating conditions must be chosen [57]. Large-scale applications are one of the main problems with IL-based membranes. Lab-scale experiments are carried out by changing a single parameter (such as time) under ideal conditions. However, under actual conditions, the parameters vary at the same time, making the procedure extremely complicated [58]. For example, when technology is used to extract CO2 from industrial exhaust gases, the conditions are vastly different from those seen in lab-scale research. This includes the flue gas composition, which contains SO2, H2O, N2, CO2, and O2 and may contain ash, NOx, CO, and other tiny particles. Another difficulty with flue gas streams is pressure loss. The loss of ILs from the surface of the membrane owing to dispersion, evaporation, and displacement causes the membrane to perform poorly [59].
Supported ionic liquid membranes (SILMs) have been developed to effectively separate CO2 from various gas mixtures, particularly N2 and CH4. However, the stabilization of ILs on the membrane support, the degradation of membranes, and variation in membrane thickness are still the main challenges to obtaining better gas permeability and selectivity in real operating conditions [12]. However, research on process evaluations of CO2 capture mediated by ILs is still limited to date, and more experimentally based and theoretical simulation techniques are encouraged to drive progress in IL-based technologies. The major difficulties in developing a proper CO2 absorption system using ILs on an industrial scale are the high viscosity, availability, cost, compatibility, and purity of ILs [60].


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