Rotten Eggs Revaluated: Ionic Liquids and Deep Eutectic Solvents for Removal and Utilization of Hydrogen Sulfide

Hydrogen sulfide (H2S) is highly toxic and one of the problematic impurities in industrial gas streams, calling for H2S removal down to single-digit ppm levels to protect health and environment, and not to harm to the downstream processes. Here, we discuss the recent developments and challenges of current H2S removal technologies. Furthermore, we present a comprehensive review of H2S removal in ionic liquids (ILs), IL-based solvents/adsorbents/membranes, and deep eutectic solvents (DESs) due to their unique advantages. We analyze theoretical studies to better understand the microscopic details behind H2S removal. We discuss new research on IL/DES-based H2S removal processes from an industrial perspective. Finally, we summarize the utilization of H2S in IL/DES-based systems for the recovery of sulfur and hydrogen, and synthesis of value-added chemicals. This review will provide both general and in-depth knowledge of the achievements, difficulties, and research priorities in developing novel ILs/DESs for efficient and sustainable H2S removal and utilization.


INTRODUCTION
Hydrogen sulfide (H 2 S) is regularly present in biogas, natural gas, refined gas, liquefied petroleum gas, and other industrial gas streams, which is one of the major problems for many industries due to its high toxicity, corrosivity, etc. The toxicity verge of H 2 S is about 10 ppm, and the damage to health increases rapidly with the increase of H 2 S concentration. 1 Besides, the weak acidity of H 2 S causes serious corrosion to piping and production facilities. 2 A trace amount of H 2 S is poisonous for many metal catalysts. 3 H 2 S, on the other hand, is an important feedstock of the productions of sulfur, 4 hydrogen, 5 and some value-added metal sulfides and thio-organic chemicals, such as bis (2-phenylethyl) sulfide. 6 Therefore, both removal and further utilization of H 2 S from gas streams are all of great importance for human health, environmental protection, downstream operation, and resource reutilization.
The concentration of H 2 S can be quite different depending on the sources, and thus different technologies have been developed for H 2 S removal both in academic research laboratories and industries, to fulfill the international environmental regulations and to achieve a desirable removal of H 2 S. In general, these technologies can be classified as absorption, adsorption, membrane separation, and biological removal. 7 Membrane separation is an energy-efficient technology, but the high cost of the membrane modules, their poor resistance to corrosion, and short lifetime all limit their industrial applications. Biological methods have the advantages of both mild operating conditions and low cost, but they are currently not able to treat H 2 S at moderate to high concentrations, and the technology is still underdeveloped. 8 Chemical absorption and adsorption are the most widely used methods due to their simple operation, high maturity, strong adaptability, and high efficiency. For the chemical absorption, alkaline solutions, such as amine and sodium hydroxide (NaOH), are usually used as absorbents via acid−base reactions. 9,10 In the industrial adsorption, metal oxides are important chemical adsorbents, and they react with H 2 S to form stable sulfides. 11 The current status associated with different H 2 S removal technologies is summarized in Figure 1.
Related to the utilization of H 2 S, nonthermal plasma (NTP), electrochemistry, and the Claus process are general methods for sulfur and hydrogen recovery. NTP includes dielectric barrier, 12 rotating glow, 13 DC corona, 14 gliding arc, 15 and microwave plasmas. 16 In the studies conducted on the first three plasmas, the energy costs are high. Gliding arc and microwave plasmas are more effective with high energy efficiency. However, all plasma technologies are currently not suitable for industrial application as H 2 S needs to be diluted in advance. Electrochemical conversion of H 2 S is usually carried out at room temperature, which is an energy-efficient technology. However, direct conversion of H 2 S suffers from the drawback of anode passivation due to the aggregation of sulfur. 17 Indirect electrochemical conversion of H 2 S by introducing redox couples (such as Fe 3+ /Fe 2+ ) is a feasible way to avoid the passivation of sulfur on the anode, but it is difficult to separate sulfur from the electrolytes. 18 The Claus process is the most common method for the recovery of sulfur from H 2 S, in which H 2 S is catalyzed to elemental sulfur and water. 19 The Claus process is always operated above the dew point of sulfur (180°C) to avoid the deactivation of catalysts due to the precipitation of sulfur on their surface. Such a high temperature is not beneficial to the conversion of H 2 S as the reaction is highly exothermic. The development of a liquid-phase Claus reaction realizes the oxidation of H 2 S under relatively mild conditions. In the wetoxidation process, aqueous iron(III) chelate is an attractive catalytic solution for the low sulfur recovery processes below 6 ton-S/day, but it still suffers from challenges, such as low concentrations of ferric complexes in solution, degradation of iron(III) chelate, and side reactions during H 2 S oxidation. Therefore, there is a pressing demand to find a novel reaction medium with a favorable conversion efficiency, low volatility and toxicity, and high stability and regeneration performance for efficient utilization of H 2 S.
Ionic liquids (ILs) are molten salts below 100°C, which consist of organic cations, including imidazolium, pyridinium, phosphonium, and ammonium, as well as organic/inorganic anions. Nowadays, ILs have been considered as "green solvent" for a variety of applications attributed to their many unique advantages, such as negligible vapor pressure, high thermal stability, and high liquid range. Moreover, their physicochemical properties can be easily tuned to fulfill a specific application demand by selecting appropriate cations and anions, or by introducing functional groups into ILs. Deep eutectic solvents (DESs), composed of hydrogen-bond acceptors (HBAs) and hydrogen-bond donors (HBDs), have received more attention because of their additional advantages compared to ILs, such as low cost, low environmental impact, easy preparation, no purification, etc. In the field of gas purification, ILs and DESs have been hot research topics in recent years and some ILs/ DESs showed attractive application potential for the capture of CO 2 , 20 H 2 S, 21 SO 2 ,22 and other gases.
In H 2 S capture, the inherent polarity of ILs increases their affinity to polar H 2 S molecules giving a convincing reason to develop ILs for H 2 S removal. Jou et al. 23 were the first to investigate H 2 S absorption in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF 6 ]). However, in a physisorption, the conventional ILs can only be applied for gas streams with high partial pressure of H 2 S. Later, functionalized ILs were reported to show an increased H 2 S absorption capacity even at low partial pressures. For instance, the H 2 S solubility in triethylbutylammonium N,N-dimethyl-glycinate ([N 2224 ]- [DMG]) increased up to 106.1 mg-H 2 S/g-IL at 60°C and 1 bar. 24 Even though some ILs show good performance, their high viscosities limit their large-scale applications. To overcome this drawback, several strategies have been proposed: typically, mixing high viscosity ILs with less viscous organic solvents and/ or water, 25−27 also, using ILs as key components in adsorbents and membranes, such as IL/metal organic framework (IL/ MOF) composites, 28 and supported IL membranes. 29 In recent years, deep eutectic solvents (DESs), analogous to ILs, have received more attention, because they have many additional advantages, such as low cost, low environmental impact, easy preparation, and no purification required, etc. More and more DESs have been developed for H 2 S removal. 30 Owing to the unique advantages of both ILs and DESs, intensive work is also ongoing to develop ILs and DESs for the utilization of H 2 S. 31,32 Several review articles about H 2 S removal using different technologies have already been published, witnessing the importance to suppress the emissions of H 2 S. Shah et al. 33 presented recent progress on H 2 S capture by using polar liquids, oxides, zeolites, MOFs, and membranes. Chiappe et al. 34 discussed the specific influence of anions, cations, and functional groups of ILs on H 2 S capture and oxidation. Wang et al. 35 reviewed the selective absorption of H 2 S/CO 2 and CO 2 /CH 4 in pure ILs. Haider et al. 36 reviewed the simultaneous capture of acid gases using ILs. Kumar et al. 37 discussed the potential application of IL−amine blends and binding organic liquids for natural gas sweetening. Liu et al. 38 summarized the recovery of sulfur from H 2 S in metal-based ILs. However, to the best of our knowledge, no review is specifically dedicated to cover IL-based solvents, IL-reinforced adsorbents and membranes, to discuss both the theoretical studies and industrial perspectives of the IL/ DES-based sorbents for H 2 S removal, as well as to analyze the potential of developing ILs/DESs for H 2 S conversion to produce sulfur, hydrogen, and value-added chemicals.
In this review, the research work on H 2 S removal with chemical absorption and adsorption methods from 2018 is surveyed and discussed, considering their promising prospects. H 2 S removal by using pure ILs, IL-based solvents, IL-reinforced adsorbents/membranes, and DESs is summarized. The theoretical studies, including quantum chemical calculations, Molecular Dynamics simulations, thermodynamic models, and Machine learning approaches for predicting H 2 S solubility are reviewed to investigate molecular interactions and mechanisms between IL/DES-systems and H 2 S. On the large scale applications, process design and industrial perspectives of using ILs and DESs for H 2 S removal are discussed. Finally, developing ILs and DESs for recycling and utilization of H 2 S is summarized.   40,41 N-Methyldiethanolamine (MDEA) has gradually appeared in industrial processes due to its low energy cost, low corrosive impact on equipment, and high absorption capacity. MDEA has also been used together with other alkanolamines to provide a synergetic effect and improved performance. For example, Tian et al. 42 studied H 2 S absorption in a tray column with MEA-activated MDEA aqueous solutions as solvents. Their results showed that the addition of MEA could increase both H 2 S absorption capacity and removal efficiency.
It is common that CO 2 is involved as a cocomponent in the gas streams together with H 2 S. A considerable amount of research work has been conducted for such streams. For example, Zhan et al. 43 prepared a mixed solution of MDEA and piperazine (PZ) for simultaneous absorption of H 2 S and CO 2 in a rotating packed bed. In some industrial applications, there is a need to separate H 2 S from CO 2 , and thus a novel alkanolamine absorbent with high H 2 S selectivity needs to be developed. For this purpose, hindered amines, such as 2-(tert-butylamino)ethanol (TBE) and 2-amino-2-methyl-1-propanol (AMP), were found as potential absorbents for selective H 2 S removal. 44 However, for the chemical absorption with alkanolamines, the main issues, associated with the commercial MEA and DEA, are their high energy requirement during regeneration, high solvent loss, and degradation. 8 MDEA is a promising solvent with a high H 2 S absorption capacity, low energy usage, low corrosion rate, and good degradation resistance, but it shows a relatively low kinetics. 43 MDEA has also been used together with other alkanolamines to improve absorption performance. However, application of the mixed solvents complicates the H 2 S absorption mechanisms, and makes it difficult to regenerate and recycle. Overall, the technology based on chemical absorption needs to be further optimized to become more efficient, and more efforts should be directed to developing novel solvents with a high degradation resistance, high absorption capacity and kinetics, and desirable regeneration capacity.
2.2. Adsorption. The strong affinity between the used adsorbent and H 2 S molecules is the main mechanism to achieve a high H 2 S selectivity and adsorption capacity. Besides, the morphology of the adsorbents, such as their size and shape, can also influence the H 2 S selectivity over other components in gas streams. The most common adsorbents include metal oxides, carbon materials, zeolites, and metal−organic frameworks (MOFs). 11 Of them, metal oxides have been widely studied and applied in the industry due to their low cost, high adsorption capacity, and abundance.
Iron oxide (Fe 2 O 3 ) is a potential adsorbent for lowtemperature H 2 S removal with the removal rate less than 200 kg-S/day. 45 The adsorption of H 2 S using iron oxides is usually carried out at temperatures below 500°C. This is because high temperatures are undesirable for the exothermic sulfurization reaction, even if kinetically beneficial. 46 The commercialization of Fe 2 O 3 for H 2 S removal is limited due to their low specific surface area and small pore-volume, as well as the pores getting blocked during the adsorption. As a result, Fe 2 O 3 is commonly combined with other materials, such as mesostructured silica supports (M41S), 47 semicoke, 46 oxygenated porous carbon (OPC), 48 red clay, 49 and so on, to increase the stability of Fe 2 O 3 and the desulfurization performance. Zinc oxide (ZnO), as a commonly used desulfurizer, has good thermal stability and strong desulfurization ability. In recent years, H 2 S removal at not so high, or even room temperature, has received wide attention in order to reduce energy usage. However, the adsorption capacity of ZnO is quite low at room temperature due to slow kinetics. Supporting ZnO on porous materials (e.g., mesoporous silica, 50 activated carbon, 51,52 ceramic, 53 molecular sieve, 54 semicoke, 55 etc.), mixing with other metal oxides, 56−58 and doping with impurity atoms 59 are simple and effective methods to increase the specific area of the adsorbents, and thus improve H 2 S adsorption capacity. Copper oxide (CuO) has been increasingly developed as an adsorbent for H 2 S removal due to its higher reaction equilibrium constant with H 2 S compared with Fe 2 O 3 and ZnO. Nevertheless, aggregation and low regeneration are the main drawbacks of CuO, which is similar Industrial & Engineering Chemistry Research pubs.acs.org/IECR Review to most metal oxides. To solve these problems and enhance desulfurization performance, different porous materials, including activated carbon nanofibers 60 and mesoporous silica, 61 were introduced into CuO nanoparticles. Metal oxides are promising adsorbents for H 2 S removal from different gas streams due to their high reactivity. However, issues such as low porosity, low surface area, aggregation during regeneration, and metal evaporation remain as major drawbacks, leading to a reduced H 2 S adsorption capacity. 62 The preparation of metal oxide-based adsorbents by combining them with porous materials, supporting them on other metal oxides, and doping them with impurity atoms to improve the specific adsorption area and further enhance the structural stability has been proposed to tackle the issues. The adsorption performance of several metal oxide-based adsorbents has been reported, while their regeneration is still difficult. Therefore, future research should focus on developing metal oxide-based adsorbents to not only increase H 2 S adsorption capacity but also enhance the regeneration capacity.

ILS AND DESS FOR H 2 S REMOVAL
As already pointed out, chemical absorption technology using alkanolamines has disadvantages due to high solvent loss, a high energy requirement, and low thermal stability; while adsorption using metal oxides shows a low adsorption capacity and poor regeneration capacity. ILs have become potential candidates for H 2 S removal to overcome the above-listed drawbacks of current conventional sorbents. This is highly due to their extremely low vapor pressure, low heat capacity, high thermal stability, chemically tunable nature, and a strong affinity to the polar H 2 S molecule in particular, as illustrated in Figure 2. As summarized in Table 1, ILs are used in different forms, including pure IL absorbents, IL-based solvents, IL-reinforced adsorbents, and membranes. Further, the research work on developing DESs for H 2 S removal is also included in this section. The structure of ILs and DESs are summarized as shown in Figure 3.
3.1. ILs as Pure Absorbents. 3.1.1. "Physical" ILs. According to the structure of ILs and the interaction with H 2 S, ILs can be divided into "'physical'" and "functionalized" ILs. The reported physical ILs for H 2 S removal mainly include imidazolium-based and pyridinium-based ILs. Jou et al. 23 Figure 4a. 69 The possible reason is that the cation with a longer alkyl chain has a higher free volume, which can weaken the interaction between the cation and anion of IL, and thus increase H 2 S solubility. Interestingly, Zhao et al. 70 found that increasing the alkyl chain of the anion could also promote the absorption of H 2 S in carboxylate ILs due to the increased alkalinity of the anion (Figure 4b).
In some cases, selective absorption of H 2 S from CO 2 is favored to recycle these two gases as feedstocks for downstream industries, and thus developing ILs with large H 2 S/CO 2 selectivity is quite attractive.  72 was also found to be preferential for the separation of H 2 S over CO 2 , indicating the great potential of "'physical'" ILs for selective separation of H 2 S over CO 2 . The interaction between the physical ILs and H 2 S is usually weak (mainly van der Waals forces), resulting in low regeneration energy, but the limited H 2 S absorption capacity makes these ILs only suitable for the removal of H 2 S when its partial pressure is high.
3  24 and high H 2 S solubilities of 48.1−106.1 mg-H 2 S/g-IL with H 2 S/CO 2 selectivities of 13−26 were obtained at 60°C and 1 bar. The selectivity can be even higher than 100 at low pressures and high temperatures due to the strong interaction of carboxyl···H 2 S···amine and the large difference in binding energy between anion-H 2 S and anion-CO 2 . This can also explain the reason that the solubilities of H 2 S in 1-butyl-3methylimidazolium acetate ([Bmim][Ac]) are much higher than those for CO 2 reported by Haghtalab et al. 74 Protic ILs are considered as better absorbents for the separation of H 2 S from CO 2 . H 2 S/CO 2 selectivities could be up to 8 76 The same research group also designed a novel type of hydrophobic protic IL containing a free tertiary amine group and used it for H 2 S removal. 77 79 The particularly strong interaction between the N atom in anion and H 2 S resulted in a higher H 2 S absorption capacity compared with those reported in the literature, which further confirmed the exceptionally high potential of protic ILs for selective H 2 S removal.
3.2. IL-Based Solvents. Although many functionalized ILs show a high H 2 S solubility and H 2 S/CO 2 selectivity, they are still limited by the low absorption kinetics resulting from the high viscosities, which hinders their practical applications. This limitation did encourage several researchers to explore new ILbased solvents by adding low viscous organic solvents and/or water to ILs. For example, when tetraethylammonium L-alanate ([N 2222 ][L-Ala]) mixed with low viscous EG with the mass ratio of 1:1 at 40°C and the H 2 S partial pressure of 0.05 bar, the absorption reached equilibrium in about 480 min, much shorter than that using pure IL (1400 min). 25 83 This observation indicated that the amino group in the anion of [N 1111 ][G1y] not only reacted with H 2 S, but also promoted the absorption of H 2 S in aqueous MDEA by increasing the alkalinity of the solution. On the contrary, both H 2 S absorption capacity and absorption rate decreased with increasing the concentration of MDEA. This is because, at a low partial pressure of H 2 S, adding more MDEA could not promote the absorption of H 2 S, but rather increased the viscosity of solutions, and thus limited the absorption and diffusion of H 2 S.
Sometimes, the temperature of H 2 S in the industrial gas streams with H 2 S can reach hundreds of degrees Celsius, while the conventional absorption technologies for H 2 S removal can only be operated at relatively low temperatures (<80°C) in order to reduce the solvent loss and increase H 2 S absorption capacity. As a result, cooling processes are always needed before absorption, which leads to a high energy demand. To achieve H 2 S absorption at high temperatures, Liu et al. 84

IL-Reinforced Adsorbents and Membranes.
Combining ILs with adsorbents or membranes is another potential strategy to overcome the high viscosity of ILs and enhance H 2 S adsorption efficiency. Metal−organic frameworks (MOFs) can be ideal supporters owing to their large pore volume, high surface area, and designable structure. When [Bmim][Cl] was immobilized on Cu-TDPAT, a high selectivity of H 2 S/CH 4 (= 611) was obtained compared with the pure Cu-TDPAT (= 141) under the same conditions. This phenomenon can be explained by the "like dissolves like" principle, which means polar H 2 S is easier to dissolve into polar IL compared with CH 4 . 28 Ishak et al. 88  Ma et al. 90 prepared a novel metal-based IL, triethylamine hydrochloride copper chloride (Et 3 NHCl·CuCl 2 ), and compared the adsorption performance of H 2 S in pure zeolite, cyclodextrin-grafted zeolite (CDGZ), zeolite-supported IL (ILzeolite), and cyclodextrin-modified zeolite-supported IL (IL-CDGZ). The order of H 2 S removal capacity is CDGZ < zeolite < IL-zeolite < IL-CDGZ. Introducing cyclodextrin (CD) to zeolite decreased H 2 S removal capacity due to its inappropriate cavity size. The chemically active species of IL, including amines and Cu 2+ , could associate with H 2 S, and thus increase H 2 S capacity of IL-zeolite. In the system of IL-CDGZ, some IL-cations filled the cavity of CD and enlarged the pore size. The combined effect of CD, zeolite, and IL achieves a good H 2 S removal performance of CDGZ. Later, the H 2 S adsorption performance of metalbased IL immobilized sol−gel derived silica (IL/silica gel) was also investigated by the same group. 91 It was found that both the components (metal and halogen) and the loading amount of IL could greatly influence the H 2 S removal performance. The much better performance compared with pure silica gel and IL is attributed to the formation of nanometer-sized and highconcentrated IL because of the confinement of IL in the silica gel.
The use of conventional polymeric membranes is limited by their competitive relationship between permeability and selectivity. Combining ILs with polymeric membranes gives many opportunities for designing novel membranes with both high H 2 S permeability and selectivity. Zhang et al. 29  3.4. DESs. Recently, DESs have been widely studied owing to their benign characteristics, such as low cost, low environmental impact, easy preparation, without purification process, and no byproduct generation. 94 Considering the unique properties of DESs and the observed promising absorption performance for weak acidic CO 2 , the application of DESs for acidic H 2 S removal should provide great potential, although the number of current studies is still very low.
Choline chloride (ChCl)/urea is one of the most common DESs for gases separation. The measurement of H 2 S, CO 2 , and CH 4 solubilities in ChCl/urea with the molar ratios of 1:1.5, 1:2.0, and 1:2.5 showed that H 2 S solubility decreased with the decrease of ChCl/urea ratio, which is different for CO 2 and CH 4 showing the highest value in ChCl/urea (1:2). 95 This is because the Cl of ChCl could form a strong hydrogen bond with the H atom of H 2 S during the absorption of H 2 S in the ChCl/urea systems, and the decrease of the molar ratio weakened the strength of interaction, which caused the decrease of H 2 S solubility. However, the solubilities of CO 2 and CH 4 were affected by the free volume of solvents. ChCl/urea (1:2.0) has the lowest melting point, exhibiting the biggest free volume, and in turn, showed the highest absorption performance for CO 2 and CH 4 among the studied ChCl/urea. Recently, [Bmim][Cl]/ imidazole (2:1) was proven to be a promising candidate for the selective separation of H 2 S from CO 2 in various processes as it has ultrahigh selectivities (up to 30.9 at 25°C) and high stability of removal performance in moisture environment with low water content. 96 Wu et al. 97 98 The combination of PEI and ChCl/EG (1:2) (PEI/FDES@EG) was identified as the most promising desulfurizer owing to its optimal H 2 S removal efficiency. For PEI/FDES@EG with 25% PEI, H 2 S removal efficiency could maintain above 95% at least 80 min at 30°C due to the strong interaction between the N of amine group in PEI and the H of H 2 S, while it dropped sharply at the beginning for ChCl/EG (1:2). Very recently, a novel kind of task-specific DESs using quaternary ammonium salts as HBAs and azoles as HBDs were prepared by Shi et al., 99 which showed the highest H 2 S absorption capacity and H 2 S/CO 2 selectivity compared with the reported DESs. Moreover, both alkalinity and free volume could influence the solubility of H 2 S, as the larger is the alkalinity and free volume the higher is the H 2 S absorption capacity, which is in agreement with the results obtained from the ILs.
Liu et al. 100 prepared an ETA/DES solution by dissolving ethanolamine (ETA) into ChCl/EG (1:2, molar ratio) with a weight concentration of 20%. Then the DES-based nanofluid (NF) systems (NF@Cu-x%) were further prepared by suspending Cu nanoparticles into the ETA/DES solution with different weight concentrations (x) of Cu. The DES-based NF systems showed quite low H 2 S removal efficiency in the absence of ETA, indicating that H 2 S removal performance of the NF systems is attributed to the effect of ETA. Meanwhile, Cu nanoparticles were served as the promoter for the interaction of H 2 S and ETA, and thus the H 2 S removal efficiency increased with the increasing concentration of Cu.
Considering the great potential and advantages of supported ILs, the study on the H 2 S removal performance of supported DESs (SDESs) should be meaningful for a wider application of DESs. Mao et al. 101 prepared a novel kind of SDESs using fumed silica (FS) as the supporting material, and TAE x CuCl 2+x with various molar ratios of triethylamine hydrochloride (TEACl) and cupric chloride (CuCl 2 ) as the loading substance. SDES with 10 wt % loading of TAECuCl 3 (TAECuCl 3 @FS/10 wt %) has the optimum desulfurization performance at 30°C with the highest adsorption efficiency (molar ratio of Cu to absorbed H 2 S, n H2S /n Cu is 0.87), which is about 3.56 times higher than the pure TAECuCl 3 under the same condition. This is because the interaction between FS and DES could reduce the size of surface TAECuCl 3 microclusters and thus promote the transformation of metal active sites. After adsorption, a small amount of H 2 S was oxidized to sulfur and sulfate ions, and most of the H 2 S was transferred to Cu 2 S. This study suggests that SDESs are potential sorbents for an efficient removal of H 2 S.
3.5. Summary and Outlook of ILs and DESs for H 2 S Removal. Some conclusions regarding H 2 S removal in ILs and DESs are summarized in Figure 6. H 2 S absorption in physical ILs has been widely studied due to the favorable properties of ILs. On the basis of the analysis presented above, both cation and anion of ILs have a great influence on H 2 S absorption performance. The cation with a longer alkyl chain has higher molar volume, which could weaken the interaction between the cation and anion of ILs, and thus increase H 2 S solubility. Physical ILs are only suitable for the removal of H 2 S when its partial pressure is high. Developing functionalized ILs is a promising way to improve H 2 S absorption capacity and H 2 S/ CO 2 selectivity at low pressures. Protic ILs have been proven to be favorable absorbents for the separation of H 2 S from CO 2 . The strong interactions between H 2 S and the anion of carboxylate, azole-based, and dual Lewis base functionalized ILs are the main causes of high H 2 S absorption capacity in these ILs. Nevertheless, the high viscosities are the main bottleneck of most functionalized ILs, which limit their large-scale applications. Mixing ILs with low viscous aqueous and organic solvents reduces the viscosity of ILs and promotes the absorption kinetics of H 2 S. Water and EG were often selected to increase the absorption rate of H 2 S in ILs due to their low viscosities. ILs can be also mixed with alkanolamines, where ILs play the roles to stabilize absorbents, and/or to promote the absorption of H 2 S in alkanolamines. Besides, the development of HPCs/ILs and PHPCs/ILs realized efficient H 2 S absorption at high temperatures. The combination of ILs with adsorbents and membranes is another potential strategy to overcome the high viscosity of ILs and improve H 2 S sorption performance. MOFs, modified Industrial & Engineering Chemistry Research pubs.acs.org/IECR Review zeolites, and sol−gel derived silica are all promising supporters of ILs due to their porous structure and high surface area. DESs have been considered as one of the most desirable solvents for the selective removal of H 2 S because of their benign properties. However, most reported DESs to date, especially functionalized DESs, have high viscosity due to the strong hydrogen-bond network in DESs, which are unadaptable for industrial H 2 S removal. Developing aqueous and supported DESs is an alternative way to minimize unfavorable effects of the high viscosity of DESs, but related research is still limited, thus further investigation is required to gain an in-depth understanding of the roles of DESs, and to screen more appropriate DESs for efficient H 2 S removal. It should be noted that even supported ILs/DESs exhibit attractive performance, they are only suitable for the gas streams with low H 2 S concentrations, while IL/DES-based liquid sorbents can be used for bulk H 2 S removal.
On the basis of the discussions presented above, the development of ILs/DESs-based sorbents is still one of the most potential directions for H 2 S removal. Therefore, the following aspects deserve further investigations: (1) Computational chemistry should be carried out to understand the mechanism of action between H 2 S and different ILs/DESs, and provide guidelines for the optimization of the structural design of task-specific ILs and DESs with high H 2 S absorption capacity and selectivity. (2) Carry out relevant research on the evaluation of technology and economy of IL/DES-based H 2 S removal processes, such as detailed analysis on energy requirement and total cost of a process, for commercial-scale adaptation of IL/ DES-based technologies. (3) Explore the developing potentials of ILs and DESs for the utilization of H 2 S to widen the application of ILs and DESs, and to build a more sustainable H 2 S removal and utilization process.

THEORETICAL STUDIES
A large part of studies have been experiments to investigate H 2 S removal performance with ILs and DESs as sorbents, such as absorption capacity and selectivity, as described in the above sections. Meanwhile, theoretical studies, including quantum chemical calculations, Molecular Dynamics simulations, thermodynamic models, and Machine learning approaches for the prediction of H 2 S solubility, are very important to clarify mechanisms and provide valuable information for designing and screening novel IL-based and DES-based sorbents, even if such research is still limited.

Quantum Chemical Calculations.
Quantum chemical calculations are highly useful tools to calculate the geometry structures and analyze the interaction mechanisms of H 2 S in ILs and DESs, and in general, they are often combined with experimental studies for a verification of the theory but also to better understand the measurements. Quantum chemical calculations are mainly used to analyze the nature of H 2 S capture in ILs/DESs and provide guidelines for designing and screening suitable ILs/DESs. The density functional theory (DFT) method combined with hybrid Becke 3-Lee−Yang−Parr (B3LYP) exchange-correlation function has been widely applied in quantum chemical calculations of ILs, in which electron correlation is well considered. 105 For example, a computational study at the density functional theory DFT/B3LYP level with the basis sets of 6-311+G(d) and 6-311++G(2d,2p) was performed by Jalili et al. 67 to understand the nature of H 2 S and CO 2 in [C n mim][Tf 2 N]. The binding energy of anion− cation complexes calculated in the basis set of 6-311+G(d) decreases from the cations of [C 2 mim] + to [C 6 mim] + , and then increases for [C 8 mim] + . However, a decreasing trend was observed from [C 2 mim] + to [C 8 mim] + by using 6-311+ +G(2d,2p). Moreover, an excellent linear correlation between the calculated absolute value of the energies of [C n mim][Tf 2 N] at the B3LYP/6-311++G(2d,2p) level and Henry's constant of H 2 S and CO 2 were observed, which confirmed that a more polarized and diffused basis set 6-311++G(2d,2p) provides more accurate results, and the increase of alkyl chain decreased the interaction energy of anion and cation, promoting the interaction of IL and H 2 S. This observation could also explain the gradually increased H 2 S solubility with increasing alkyl chain of [PF 6 ]-based ILs as reported by Safavi et al. 68 It was also found that the molecules of H 2 S and CO 2 are directed toward the less electronegative N atom, rather than the most electronegative F atom of the [Tf 2 N] − anion, as shown in Figure 7. Quantum chemical calculations on the H 2 S-anion by Pomelli et al. 106 and those on the CO 2 -anion by Bhargava et al. 107 76 Similar results were obtained by Zhang et al. 78 in their calculations of the interaction between the azole-based protic IL and H 2 S. They stated that the extremely high H 2 S absorption capacity is attributed to the chemical combination of the H atom in the H 2 S molecule and the N atom in the anion of ILs.
Quantum chemical calculations were also used to understand the different behavior of H 2 S, CO 2 , and CH 4 in DESs. 95 In the system of H 2 S + ChCl/urea (1:2.0), Cl and H of ChCl formed strong hydrogen bonds with H and S of H 2 S, respectively. For CO 2 -ChCl/urea (1:2.0), hydrogen bonds were also formed between the Cl of ChCl with C of CO 2 , and H of urea with O of CO 2 . While only van der Waals interactions were observed in the system containing CH 4 and ChCl/urea (1:2.0). As mentioned in section 3.4, the combination of PEI and ChCl/EG could greatly increase the H 2 S removal efficiency. Quantum chemical calculations on the complexes of H 2 S-PEI, H 2 S-ChCl, and H 2 S-EG confirmed that the interaction between PEI and H 2 S was the dominant driving force of high H 2 S removal performance, while DES (ChCl/EG) only played a role of solvent. 98 4.2. Molecular Dynamics Simulations. Molecular Dynamics (MD) simulations have been used as an important method to understand the structural and energetic phenomena, as well as the thermodynamic and transport properties of ILs at the molecular level. 110−113 The solubility of H 2 S can be obtained through MD simulations by calculating the solvation free energy using free energy perturbation (FEP), Bennett acceptance ratio methods (BAR), and so on. Sanchez-Badillo et al. 114  Moreover, the contribution of solvation enthalpy to free energy is twice of entropy, and, therefore, the H 2 S absorption in ILs is an enthalpy-driven process.
Salehin et al. 115 calculated the solvation free energy and thermodynamics properties of H 2 S in five cholinium ([Chl])based amino acid ILs using the BAR method with Optimized Potentials for Liquids Simulations (OPLS) force field. It showed that the free energy of H 2 S in IL is more negative compared with that in the aqueous system, indicating H 2 S can be more easily solvated in ILs. The radial and spatial distribution functions displayed that the H atoms of H 2 S are more aligned toward the anion, while the S atom of H 2 S is surrounded by cations, which agreed with the results from the literature. 114 Moreover, the IL with the lowest Henry's constant has the weakest interaction energy between anion and cation, which would lead to an increase of free volume for H 2 S to occupy and thus improve H 2 S solubility. Amhamed et al. 116 performed all-atom MD simulations to study the absorption of H 2 S, CO 2 , and CH 4

in choline benzolate ([Chl][BE]) and choline lactate ([Chl]-[Lac]
). The selectivities of H 2 S/CH 4 and CO 2 /CH 4 were estimated to be more than 105 and 104, respectively, which indicated that [Chl]-based ILs are potential candidates for acid gas removal.
RDFs between H 2 S with CH 4 and the components of caprolactam (CPL)-based DES (CPL/TBABr with a molar ratio of 1:1) were computed by Karibayev et al., 117 to explore the interactions within the DES, and to examine the interactions between the DES with H 2 S and CH 4 in natural gas. As shown in Figure 8, the highest peak at a distance of 3.9 Å indicated the strong interaction between the H 2 S molecule and the Br of the DES. Moreover, the peaks of H 2 S-TBA and H 2 S-CPL were slightly higher than that of H 2 S-CH 4 , thus favoring the selective removal of H 2 S from CH 4 . Besides, the strength of the interactions (peak height) within the DES was marginally

. It was found that the interactions formed between H(H 2 S)/C(CO 2 ) and Cl([Emim][Cl]
) dominate the capture of H 2 S and CO 2 , but the former is much stronger than the latter, resulting in remarkable H 2 S/CO 2 selectivities. These results enhanced the understanding of the molecular interactions within the DES and their interactions with H 2 S. As mentioned above, MD simulations have been used to understand the interaction of anion−cation, and H 2 S-ILs/DESs. MD simulations can also predict H 2 S solubility in ILs and DESs, but the precision strongly depends on the methods used for the calculation of solvation free energy, the force fields, and the structure of ILs/DESs. For example, Sanchez-Badillo et al. 114 evaluated united-atom and different all-atom force field types for [Bmim]-based ILs with the anions of [PF 6 ] − , [BF 4 ] − , and [Cl] − , and found that some all-atom force fields are unsuitable for these ILs. But Amhamed et al. 116 found that the GROMOS 53A6 allatom force field is suitable to predict selectivity in [Chl]-based ILs. Overall, MD simulations are mainly used to calculate the solubility trend of H 2 S in ILs and DESs, while thermodynamic models will be used to predict H 2 S solubility with high precision.
4.3. Thermodynamic Modeling of H 2 S Solubility. Experimental measurements can be time-consuming and dangerous because of the high toxicity of H 2 S, and even expensive. Therefore, it is desirable to obtain H 2 S solubility in ILs and DESs theoretically. A variety of theoretical tools, such as equation of state (EoS) and conductor-like screening model for real solvents (COSMO-RS) have been established for calculating the H 2 S solubility in ILs, which are discussed in this section and summarized in Figure 9.
4.3.1. Cubic EoS. EoS is a powerful tool in representing thermodynamic properties of pure ILs and the H 2 S + IL mixtures, and thus has been widely used to predict H 2 S solubility in ILs. The first prediction for H 2 S + IL mixtures was done by using relatively simple cubic EoS, including Peng-Robinson (PR) and EoS Redlich-Kwong (RK). In these cases, the IL was modeled as a whole molecule without specification about its structure and consideration of the association effect.
Fauńdez et al. 118 used a modified PR EoS to correlate the solubilities of H 2 S and SO 2 in ILs, and found that the model results are acceptable in all cases with the absolute average deviations (AAD) below 7.6% for H 2 S/IL mixtures and 2.6% for SO 2 /IL mixtures. Shojaeian et al. 119 applied the Peng-Robinson two-state (PR-TS) EoS model to predict the solubility of H 2 S in the imidazolium-based ILs with the assumption that crossassociation or self-association interaction is presented in binary mixtures. It was found that the predictions of VLE are reliable with an AAD of 3.40%.
Shiflett et al. 120   Statistical associating fluid theory (SAFT) EoS is a method for investigating the phase behavior of both nonassociating and associating chain fluids, in which the Helmholtz free energy is given as the sum of different contributions. Several versions of SAFT have been proposed to represent the H 2 S solubility in ILs, such as SAFT-VR (variable range), 129 perturbed-chain SAFT (PC-SAFT), 129,130 electrolyte perturbed-chain SAFT (ePC-SAFT), 131,132 and soft-SAFT. 133,134 Rahmati-Rostami et al. 129 calculated H 2 S solubilities in six imidazolium-based ILs using the SAFT-VR and PC-SAFT models, and both showed good agreement with the experimental data. Moreover, by studying the effect of selfassociation of the H 2 S molecules, they made a conclusion that self-association plays an important role in modeling the systems. Baramaki et al. 130 modeled H 2 S solubility in various ILs using PC-SAFT, and acceptable results were obtained in most of the cases with the AAD lower than 10%. ePC-SAFT has been developed to predict the properties of ILs and gas solubilities (e.g., H 2 S, CO 2 , and CH 4 ) in ILs. 131 In their work, two strategies that (1) ILs were treated as neutral molecules, and (2) ILs were modeled as two charged ions, were examined. For the second strategy, four selfassociation schemes, including nonassociating, 2-sites (hydrogen bond formed from one donor of cation and one acceptor of anion of ILs as shown in Figure 10), 3-sites (hydrogen bonds formed from two donors and one acceptor), and 4-sites (hydrogen bonds formed from two donors and two acceptors) schemes were investigated. It was found that the inclusion of the electrolyte term in the second strategy improved the predictive capability of the model. Among different schemes, the 4-site association scheme provided the best results with AARD values of 2.76%−6.62% for H 2 S-IL mixtures, and 1.54%−4.98% for CO 2 −IL mixtures. soft-SAFT EoS is suited for describing the phase behavior of ILs and their mixtures. 133,140 For example, good agreement was found for all the calculations of solubilities of H 2 S, SO 2 , and NH 3 in three imidazolium-based ILs using soft-SAFT with experimental data. 133 Later, the same group presented solubility data for several gases (H 2 S, CO 2 , CO, CH 4 , H 2 , and SO 2 ) in [Bmim][MeSO 4 ] in wide temperature and pressure ranges using the same model. The predicted Henry's constants and selectivities showed that SO 2 and H 2 S were preferentially absorbed in IL. 134 Baramaki et al. 141 6 ] by applying different EoS models. For the prediction of equilibrium pressure, the percent deviations were 6.0−10.8 for Soave−Redlich−Kwong (SRK), 6.0−10.6 for PR, 2.6−9.6 for CPA, and 3.4−6.1 for PC-SAFT. For the prediction of vapor phase composition, the percent deviations were 3.1− 4.9 for SRK, 3.3−4.9 for PR, 3.1−7.0 for CPA, and 3.8−4.9 for PC-SAFT. The results indicated that the EoS can describe the phase behavior of H 2 S/CO 2 /IL systems reliably, and the performance of PC-SAFT is better. However, most of the EoS models are limited to specific systems, and in general, the studied ILs are those that physically absorb H 2 S.

COSMO-RS.
The COSMO-RS model, which is based on quantum chemistry calculations, can predict thermodynamic properties of solutions independent of experimental data. 142,143 COSMO-RS was used by Mechergui et al. 144 to predict H 2 S and CO 2 solubility in four bis(2-ethylhexyl) sulfosuccinate-based ILs, and it was found that the H-bonding interactions in the ILs and those between the gases and ILs have a crucial influence on the solubility. COSMO-RS was also combined with EoS to perform prediction. In the work by Mortazavi-Manesh et al., 145 COSMO-RS was combined with PR to estimate the solubilities and selectivities of H 2 S, CO 2 , CH 4 , and C 2 H 6 in ILs, in which COSMO-RS was used to calculate activity coefficients of H 2 S, CO 2 , CH 4 , and C 2 H 6 in ILs, and PR was used to correct the nonideality in the vapor phase. The results suggested that the ILs with higher molecular surface area showed higher absorption solubility of H 2 S and C 2 H 6 , while the ILs with higher molecular weight are beneficial for dissolving more CH 4  N] − , by using the universal quasi-chemical activity coefficient (UNIQUAC), nonrandom two-liquid (NRTL) and COSMO-RS models. The interaction parameters in UNIQUAC and NRTL were optimized in each temperature, and thus the accuracy in prediction was improved. COSMO-RS shows a larger absolute relative deviation (ARD). In conclusion, COSMO-RS can estimate H 2 S solubility in ILs without experimental data, but the accuracy is often lower than that of the other models. Therefore, this model is mainly used for evaluating the structure−property relation and screening the candidate ILs for a specific task. 4.3.5. GC-EoS. Since COSMO-RS is not so accurate and the number of different ILs is huge, it is necessary to develop   154 was also reported and obtained satisfactory results. Even the research related to H 2 S + IL systems is still scarce; these successful cases for predicting other gases suggest the great potential of GC-EoS to predict H 2 S solubility in ILs.  Table 2.
ANN is a nonlinear mathematical model with the advantages of high uniformity of analysis, nonlinearity, parallelism, and capability to challenge fuzzy and imprecise data. 167 Shafiei et al. 155 employed the acentric factor (ω), critical temperature (T c ), and critical pressure (P c ) of ILs as input parameters to build a multilayer feed-forward neural network (MLFFNN) model to predict the H 2 S solubility in various imidazoliumbased ILs. A low ADD of 4.58% for 465 experimental data points was obtained. Sedghamiz et al. 156 predicted the solubilities of CO 2 and H 2 S in ILs using the feed-forward neural network (FFNN), and the modeling results were compared with empirical correlations and PR. The accuracy of FFNN is higher than both empirical correlations and PR. Moreover, multi-layer perceptron ANN (MLP-ANN) was proposed by the research groups of Amedi, 157 Fauńdez,158 and Hosseini,159 to investigate H 2 S solubility in ternary mixtures containing CO 2 . Desirable predictions by using the MLP-ANN method indicate that MLP-ANN is a good alternative method for the estimation of H 2 S solubility in ILs.
GEP, as a full-fledged genotype-phenotype system, is inspired by genetic algorithms and genetic programming. 168 Ahmadi et al. 160 evaluated the performance of the GEP model in estimating the solubility of H 2 S in ILs with the input variables of ω, T c , and P c , accompanied by temperature and pressure. A quite high coefficient of determination (R 2 = 0.9902) and extremely low mean absolute relative error (MARE = 0.0438%) were obtained. Industrial & Engineering Chemistry Research pubs.acs.org/IECR Review Support vector machine (SVM) is an intelligent method, which converts the nonlinear input space to a high-dimensional feature space and discovers a hyperplane by the means of nonlinear mapping. 169 LSSVM is the variant of SVM. It has the ability to handle large-scale data sets with a higher convergence rate and less complexity. 170 H 2 S solubility in 11 imidazoliumbased ILs has been estimated by Ahmadi et al. 161 through the LSSVM model with a genetic algorithm as an optimization scheme. The obtained results agree well with the experimental data, and the corresponding R 2 and mean squared error (MSE) are 0.9976 and 0.00006651, respectively. Baghban et al. 162 also employed LSSVM to predict H 2 S solubility in 27 different ILs. In this proposed model, only 15 different chemical structures of ILs were used as input parameters, while previously reported models required many input parameters, such as ω, T c , and P c for each IL.
In the SGB method, randomness is introduced into its sequential fitting using subsamples of the training data for each iteration to improve the performance of gradient boosting. 171 The application of SGB for the prediction of H 2 S solubility in ILs was first reported by Soleimani et al., 163 with the input variables of ω, T c , and P c of ILs, accompanied by operating temperature and pressure. It was found that SGB has desirable performance with a quite high correlation coefficient (R) of 0.9995, and low MARE of 0.02220. The comparative studies with EoS, GEP, and LSSVM indicated that SGB is more efficient and reliable than other models due to its higher accuracy and robustness. Figure 11 illustrates an example for the prediction of H 2 S solubility in [Hmim][Tf 2 N] with different models.
The above mentioned models are able to predict H 2 S solubility with a high precision. However, they still meet some obstacles as experimental data need to be employed as the input. To overcome this problem, Zhao et al. 164 reported an ELM algorithm for predicting H 2 S solubility in ILs with the number of fragments as the input parameters, in which 1282 pieces of data for 27 ILs were included. The R 2 , average absolute relative deviation (AARD) and root-mean-square error (RMSE) for the whole set were 0.997, 4.12%, and 0.0168, respectively, indicating that ELM can be used to estimate H 2 S solubility in ILs over wide ranges of temperatures and pressures. Later, a new type of molecular descriptors, electrostatic potential surface (S EP ), was proposed by the same group to obtain information at the electron level for the prediction of H 2 S solubility in ILs using ELM. 166 In this work, two new quantitative models were established based on the S EP parameters of isolated cation and anion (ELM 1 ), or ion pair structure of each IL (EML 2 ). The ELM 2 model showed better prediction ability since the effect of the corresponding opposite charge of ILs was included in the calculation. All the aforementioned results proved that ML methods have great potential to estimate the H 2 S solubility in ILs. It should be noted that a high number of experimental data is usually needed to get high prediction accuracy when using ML methods, and a small number of experimental data will result in erroneous predictions.

PROCESS SIMULATION OF H 2 S REMOVAL IN ILS
AND DESS Evaluation of the technologies are always required for a largescale adaptation of any process. A combination of experimental and computational methods is the most common and powerful way to carry out process analysis. For H 2 S removal, physical absorption using methanol and the mixture of dimethyl ether and polyethylene glycol is applied in industrial processes, which are called Rectisol 172 and Selexol. 173 However, Rectisol shows high volatility and low H 2 S selectivity in the presence of CO 2 ; the high viscosity of Selexol results in low absorption kinetics. 174 Chemical amine-based processes show the drawbacks of high solvent loss and energy usage. As mentioned above, recent developments have demonstrated that IL/DES-based sorbents are promising and offer desirable performance for desulfurization from different gas streams owing to their unique properties compared with conventional physical and chemical absorbents. Therefore, there is a pressing demand for evaluating the feasibility and implementation in industrial desulfurization.
Santiago et al. 175 combined predictive model and process simulations to select promising ILs for H 2 S capture. Specifically,   Figure  12. In this case, the removal ratios of CO 2 and H 2 S could reach 97.6% and 95.3%, respectively, whereas in the Rectisol process as reported by Liu et al., 177 the removal ratios of CO 2 and H 2 S were 89.7% and 99.89%, respectively. Besides, the recovery ratio of IL was high, up to 99.91%. Later, Taheri et al. 178 compared the removal efficiency, energy usage, and total direct capital cost of [Bmim][Tf 2 N]-based purification process with the Rectisol process as a reference. The electricity used to capture 1 kg of CO 2 was estimated to be 1.150 and 1.176 MJ using the [Bmim][Tf 2 N]-based and Rectisol processes, respectively, meaning that the IL-based process is more energy-efficient. However, the total direct capital cost of the IL-based process is higher than that of Rectisol process due to the high selling price and still expensive recovery cost of IL.
Kazmi et al. 179 studied the removal of acid gases from natural gas through an amine-based process with an imidazolium-based cationic IL, such as [Bmim][PF 6 ], as absorbent. They found that the removal efficiency can go up to 99% in the IL-based process, and 99.77% IL could be recovered. The estimated energy demand and total annualized cost were reduced by 85.6% and 50.7% compared to the amine-based process. This indicated that the IL-based process has a great potential for industrial applications.
The similar characteristics as ILs and the lower price make DESs more attractive for large-scale H 2 S removal. Haider et al. 180 did use Aspen Plus to evaluate the performance of aqueous ChCl/urea for biogas upgrading at 36 bar. The upgraded biogas was further liquefied for economical and safe transportation. During this process, the biogas is upgraded by selective absorption of H 2 S (≤10 ppm) and CO 2 (99% removal), with biomethane recovery (≥97 wt %) with a purity of 99 wt %. For the process with the DES of 70% concentration, the savings in the overall capital cost, overall operating cost, and total annualized costs were 2.80%, 25.82%, and 14.26%, respectively, compared with MEA-based process, and 1.41%, 16.85%, and 8.71% were saved compared with the [Bmim][PF 6 ]-based process. This study concludes that DESs have the potential to replace conventional solvents and provide feasible solutions for cost-related issues of ILs.

EVALUATION OF ILS AND DESS FOR H 2 S REMOVAL
6.1. Viscosity and H 2 S Absorption Capacity. Viscosity and H 2 S absorption capacity provide important references for the screening of candidate ILs and DESs since they are the main influential factors for H 2 S removal efficiency. Besides, H 2 S removal conditions (high pressure or low pressure) and removal types (selective or simultaneous removal of H 2 S and CO 2 ) will also affect the selection of ILs and DESs. In this part, different  Functionalized ILs can be used as attractive absorbents for low pressure H 2 S removal. As can be seen in Figure 14a,  The viscosity data of most DESs used for H 2 S removal are lacking, and thus we cannot make an evaluation of DESs based on their viscosity directly. In fact, most reported DESs to date have very high viscosity at room temperature, which is attributed to the strong hydrogen-bond network between HBAs and HBDs   75 and these two ILs can be used in industry when the operation temperature is lower than 120°C.
6.3. Cost. The high price of ILs compared with conventional solvents results in a high investment cost of ILs. But the use of ILs may also increase the removal performance of H 2 S and reduce the energy usage of the processes. Therefore, economic evaluation is always needed before using these ILs in pilot-scale application. Taheri et al. 178 [For] can be synthesized using a one-step method, and the price of raw material is low. Therefore, these two functionalized ILs are promising for industrial H 2 S removal. Economic evaluation of a DESs-based process is also performed, and 14.26% total annualized cost saving was obtained for aqueous ChCl/urea (1:2) of 70% concentration compared with the amine-based technology, indicating the high economic feasibility of ChCl/ urea. 180 7. ILS AND DESS FOR UTILIZATION OF H 2 S 7.1. Recovery of Sulfur and Hydrogen. Sulfur is one of the important chemicals for sulfuric acid (H 2 SO 4 ) production in the industry. Therefore, the recovery of sulfur from H 2 Scontained gas streams is an attractive way to reuse resources and achieve sustainable development. The Claus process is the most mature technology for sulfur recovery through a two-step reaction (eqs 1 and 2). In the thermal step, H 2 S is oxidized to SO 2 in the furnace at 1000−1200°C; and then elemental sulfur is produced from the reaction of H 2 S and SO 2 in the presence of catalysts at 200−350°C. 200 For this process, reasonable efficient conversion of H 2 S is difficult because of the thermodynamic limitations, and the high-temperature results in large energy requirements. The liquid-phase Claus reaction has shown great progress for the recovery of sulfur under relatively mild conditions. However, the main obstacles include the volatile and toxic nature of organic solutions and other challenges, such as the operating temperature should be above the melting point of sulfur (112.8°C), which is still high and needs to be further optimized. Sulfur oxo-acid salts would also be formed as byproducts during the Claus reaction, even in the IL-based reaction medium. To prevent the side-reaction, He et al. 202 synthesized a type of hydrophobic iron-based IL (Fe(III)-IL) for the oxidation of H 2 S to sulfur. It was found that the removal efficiency of H 2 S was up to 99% without a side reaction. The reaction mechanism is as follows: To overcome the high cost of ILs, the relatively low cost metal-based DESs have also been synthesized for H 2 S conversion. Liu et al. 206 synthesized a Fe-based DES 4Fe/ ChCl using FeCl 3 ·6H 2 O and ChCl with the molar ratio of 4:1 for H 2 S removal and sulfur recovery. Results showed that low temperature was beneficial to the oxidation of H 2 S, and above 98% H 2 S could be removed within 2 h at 25°C. Besides, the existence of a moderate amount of water could promote the absorption reaction of H 2 S in DES, which is consistent with that in the Fe-based ILs. Later, the same group developed a new system Fe-5MEA-DES by dissolving 1 wt % FeCl 3 into ChCl/ EG, and then MEA was added with a molar ratio to FeCl 3 of 5:1. 207 The desirable performance with H 2 S removal efficiency of 100% within 80 min at 30°C was observed, due to the synergistic effect of Fe 3+ and MEA by forming a complex compound. As mentioned in section 3.4, Cu 2 S was obtained during the adsorption of H 2 S in supported DESs composed of TAECuCl 3 and fumed silica. Actually, Cu 2 S could be further transferred to sulfur and CuSO 4 during the regeneration process, and thus can recover elemental sulfur. 101 The above studies verify that Fe(III)-based ILs and metalbased DESs have great potential for sulfur recovery. However, their regeneration rate by air or O 2 is very slow due to the low solubility of O 2 and the low reaction kinetics in ILs and DESs. Besides, hydrogen, as a clean energy source with high energy density, was not recovered during the oxidation process of H 2 S.
The thermal decomposition of H 2 S at high temperature (>1000°C) is the most direct way for H 2 S splitting to obtain hydrogen and sulfur. 208 However, the highly endothermic nature of this reaction results in unsatisfactory low values of conversion even at high temperatures. It was reported that the conversion of H 2 S is only 20% at 1010°C. 209 Such a high temperature makes the thermal decomposition method not a good choice for industrial hydrogen recovery from H 2 S since it is very energy-intensive. Reducing reaction temperature and using electrical energy originated from renewable sources are two main strategies to reduce energy usage and enhance the economic feasibility of the H 2 S conversion process. At present, electrolysis has been used for hydrogen and sulfur recovery from H 2 S by direct and indirect routes. Direct transformation of H 2 S is preferred, but the aggregation of sulfur formed on the anode causes the serious passivation of the anode. Indirect electrolysis could avoid the problem of anode passivation, but the separation of flocculent sulfur is usually difficult. Therefore, no commercialscale research has been reported for H 2 S conversion to produce hydrogen and sulfur based on electrolysis. Inspired by the successful application of ILs in enhancing the performance of sulfur recovery from H 2 S and the great potential of electrolysis, Guo et al., 210 established a new nonaqueous desulfurization system (Fe(III/II)−IL/DMF), in which the controlledpotential electrolysis method was used for the regeneration of the desulfurizer. It was found that a quite high desulfurization efficiency of 99.96% can be obtained in Fe(III/II)−IL/DMF, but the method to separate sulfur from the electrolytes is not mentioned. More recently, Ma et al. 211 established an effective method for direct H 2 S electrolysis at 50°C in the IL-TGDE-MEA electrolyte using hydroxy-functionalized IL [C 3 OHmim]- [BF 4 ] as the supporting electrolyte, tetraethylene glycol dimethyl ether (TGDE) as the solvent, and MEA as the absorbent of H 2 S. No sulfur was attached on the surface of the anode after 7 h due to its quite high solubility in electrolyte at 50°C , indicating that direct electrolysis of H 2 S in this IL-based solvent did not result in problems linked to anodic passivation. Sulfur was precipitated when the temperature was reduced to 20°C , which makes it easy to recover sulfur and reuse the electrolyte. Besides, the mild reaction condition could reduce the volatilization and degradation of MEA. Therefore, this process is also environmentally friendly. This study provides a promising method for the efficient recovery of hydrogen and sulfur from the direct electrolysis of H 2 S, even if only lab-scale tests are performed.
7.2. Synthesis of Value-Added Chemicals. Conversion of H 2 S to value-added chemicals is another feasible way to use H 2 S. Mishra et al. 6 used the IL of trihexyltetradecylphosphonium chloride (THTDPC) as the catalyst for the utilization of H 2 S to synthesize bis(2-phenylethyl) sulfide (PES). More specifically, H 2 S was first absorbed into aqueous MDEA, which was followed by the reaction of H 2 S-enriched MDEA with 2-bromoethylbenzene (2-BEB) to produce PES in the presence of THTDPC. The initial reaction rate in the system with the IL concentration of 0.03 kmol/m 3 is three times higher than that in the system without IL. The final selectively of PBS could reach 100%. Zhang et al. 212 developed a catalytic conversion of H 2 S into mercaptan acids by using a series of hydrophobic ILs as both absorbents and catalysts. Among them, bis(2-dimethylaminoethyl) ether bis(trifluoromethylsulfonyl)imide [BDMAEEH][Tf 2 N] showed the highest catalytic performance for the reaction of H 2 S and α-methylacrylic acid to produce 2-methyl-3-sulfanylpropanoic acid with a high αmethylacrylic acid conversion ratio of 98% at 90°C. After the reaction, the product can be separated by water extraction without the addition of any organic solvent. The favorable characteristics, including no organic solvent, high conversion ratio, and good IL recyclability, make this new method more

CONCLUSIONS AND FUTURE PROSPECTS
In this work, recent developments of conventional chemical absorption and adsorption, including their challenges, are reviewed. Fundamental research on H 2 S removal using pure ILs, IL-based solvents, IL-reinforced adsorbents/membranes, and DESs are summarized and discussed. ILs and DESs presented advantages in the desulfurization process compared with conventional absorption and adsorption technologies. Theoretical studies, including quantum chemical calculations, MD simulations, and thermodynamic models, and machine learning approaches for predicting H 2 S solubility are summarized to analyze the interaction mechanism between H 2 S and IL/ DES, and to screen suitable ILs with high H 2 S solubility. From process simulation, the lower energy usage and total annualized cost evidenced the feasibility of IL-based and DES-based processes to replace conventional Rectisol and amine-based processes for industrial desulfurization. Moreover, several ILs and DES are selected from the choices based on their viscosity, H 2 S removal capacity, thermal stability, and cost. Finally, the application of ILs and DESs in the conversion of H 2 S provides an effective way for resource reutilization. However, a number of issues still exist and need to be addressed in the future as illustrated in Figure 15. First, from the viewpoint of IL selection, the complicated synthetic routes, large viscosity, and still high price of most ILs potentially increasing the total running cost are still unfavorable factors for the scale-up of an IL-based process for H 2 S removal. To simplify the synthetic step, protic ILs are getting more attention in research. The development of IL-based solvents and IL-reinforced adsorbents/membranes could overcome the negative effect of viscosity and thus lower capital cost. DESs have shown specific advantages of the simple synthetic process, biodegradable capability, and cheap raw material cost, which may bring about a breakthrough in the near future.
Second, with respect to experimental research related to H 2 S removal, current work with ILs/DESs is still relatively new, and lots of effects have not been deeply studied and understood. Studies on H 2 S absorption using ILs/DESs in mixed gases are scarce and need to be complemented, to provide fundamental knowledge for conducting the practical processes, since H 2 S solubility may be more or less influenced by the presence of other gases.
Third, combining quantum chemical calculations and MD simulations with experimental studies are important methods to understand the relationship of ILs/DESs-properties−H 2 S removal performance, and thus to design task-specific ILs/ DESs. However, related work is not yet rewarding enough. Various thermodynamic models have been used for estimating H 2 S solubility in ILs. Nevertheless, most of the EoS models are limited to the specific system and need to be advanced to reduce the amount of experimental work in predicting H 2 S solubility due to numerous combinations of anions and cations of ILs and extreme operating conditions. COSMO-RS could predict the thermodynamic properties independent of experimental data, but it is often not sufficient to accurately predict H 2 S solubility, which needs to be further modified. Machine learning approaches are promising models due to their simplicity, flexibility, and high precision, but they are not adequately rich in molecular information. However, they can be used to find hidden correlations in large data sets and also to provide more Industrial & Engineering Chemistry Research pubs.acs.org/IECR Review accurate force field models for molecular simulations from quantum chemistry calculations. This has not been done yet in the context of H 2 S removal but will most likely be seen in the near future. Fourthly, although a large amount of experimental studies on the absorption of H 2 S using ILs and DESs have been reported, pilot scale research related to H 2 S removal in pure ILs, IL-based solvents, and DESs is still lacking. Therefore, future studies should focus more on the process design to analyze the technical, economic, and environmental aspects, and to evaluate the feasibility of IL/DES-based desulfurization processes in industrial applications. Moreover, the potential issues during the use of ILs/DESs, for example, corrosion, toxicity, environmental impact, and long-term stability should be addressed and studied.
Finally, developing H 2 S utilization technologies in ILs/DESssystems is an efficient way to reuse H 2 S. However, only a limited number of ILs and DESs have been used for the conversion of H 2 S. In the future, more efforts should be put into the investigation of functionalized ILs and DESs. Quantum chemistry calculations and MD simulations can also be used to further in-depth understanding of the roles of ILs and DESs in H 2 S conversion. Additionally, the evaluation of technoeconomic-environmental impacts for H 2 S utilization processes should also be conducted to judge their industrial feasibility in ILs/DESs-based systems.