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

  • Fangfang Li
    Fangfang Li
    Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, Sweden
    More by Fangfang Li
  • Aatto Laaksonen
    Aatto Laaksonen
    Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, Sweden
    Division of Physical Chemistry, Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm 10691, Sweden
    Center of Advanced Research in Bionanoconjugates and Biopolymers, “Petru Poni”Institute of Macromolecular Chemistry, Iasi 700469, Romania
    State Key Laboratory of Materials-Oriented and Chemical Engineering, Nanjing Tech University, Nanjing 211816, China
  • Xiangping Zhang*
    Xiangping Zhang
    CAS Key Laboratory of Green Process and Engineering, Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
    School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
    *Email: [email protected]
  • , and 
  • Xiaoyan Ji*
    Xiaoyan Ji
    Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, Sweden
    *Email: [email protected]
    More by Xiaoyan Ji
Cite this: Ind. Eng. Chem. Res. 2022, 61, 7, 2643–2671
Publication Date (Web):February 11, 2022
https://doi.org/10.1021/acs.iecr.1c04142

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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.

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1. Introduction

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Hydrogen sulfide (H2S) 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 H2S is about 10 ppm, and the damage to health increases rapidly with the increase of H2S concentration. (1) Besides, the weak acidity of H2S causes serious corrosion to piping and production facilities. (2) A trace amount of H2S is poisonous for many metal catalysts. (3) H2S, 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 H2S from gas streams are all of great importance for human health, environmental protection, downstream operation, and resource reutilization.
The concentration of H2S can be quite different depending on the sources, and thus different technologies have been developed for H2S removal both in academic research laboratories and industries, to fulfill the international environmental regulations and to achieve a desirable removal of H2S. 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 H2S 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 H2S to form stable sulfides. (11) The current status associated with different H2S removal technologies is summarized in Figure 1.

Figure 1

Figure 1. Current status of H2S removal technologies.

Related to the utilization of H2S, 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 H2S needs to be diluted in advance. Electrochemical conversion of H2S is usually carried out at room temperature, which is an energy-efficient technology. However, direct conversion of H2S suffers from the drawback of anode passivation due to the aggregation of sulfur. (17) Indirect electrochemical conversion of H2S by introducing redox couples (such as Fe3+/Fe2+) 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 H2S, in which H2S 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 H2S as the reaction is highly exothermic. The development of a liquid-phase Claus reaction realizes the oxidation of H2S under relatively mild conditions. In the wet-oxidation 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 H2S 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 H2S.
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 CO2, (20) H2S, (21) SO2, (22) and other gases.
In H2S capture, the inherent polarity of ILs increases their affinity to polar H2S molecules giving a convincing reason to develop ILs for H2S removal. Jou et al. (23) were the first to investigate H2S absorption in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]). However, in a physisorption, the conventional ILs can only be applied for gas streams with high partial pressure of H2S. Later, functionalized ILs were reported to show an increased H2S absorption capacity even at low partial pressures. For instance, the H2S solubility in triethylbutylammonium N,N-dimethyl-glycinate ([N2224][DMG]) increased up to 106.1 mg-H2S/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 H2S 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 H2S. (31,32)
Several review articles about H2S removal using different technologies have already been published, witnessing the importance to suppress the emissions of H2S. Shah et al. (33) presented recent progress on H2S 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 H2S capture and oxidation. Wang et al. (35) reviewed the selective absorption of H2S/CO2 and CO2/CH4 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 H2S 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 H2S removal, as well as to analyze the potential of developing ILs/DESs for H2S conversion to produce sulfur, hydrogen, and value-added chemicals.
In this review, the research work on H2S removal with chemical absorption and adsorption methods from 2018 is surveyed and discussed, considering their promising prospects. H2S 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 H2S solubility are reviewed to investigate molecular interactions and mechanisms between IL/DES-systems and H2S. On the large scale applications, process design and industrial perspectives of using ILs and DESs for H2S removal are discussed. Finally, developing ILs and DESs for recycling and utilization of H2S is summarized.

2. Recent Status of Chemical Absorption and Adsorption

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2.1. Chemical Absorption

Chemical absorption using alkanolamines is currently one of the most mature technologies for industrial H2S removal from gases. Conventional processes with amines are mainly applied in the packed/tray columns under conditions of 30–50 °C and 5–205 bar. (1) The current scale for H2S removal in amines with subsequent sulfur recovery in Claus plants is up to 50 tons-S/day. (39) Monoethanolamine (MEA) and diethanolamine (DEA) are the most frequently used absorbents. (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 H2S absorption in a tray column with MEA-activated MDEA aqueous solutions as solvents. Their results showed that the addition of MEA could increase both H2S absorption capacity and removal efficiency.
It is common that CO2 is involved as a cocomponent in the gas streams together with H2S. 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 H2S and CO2 in a rotating packed bed. In some industrial applications, there is a need to separate H2S from CO2, and thus a novel alkanolamine absorbent with high H2S 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 H2S 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 H2S 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 H2S 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 H2S molecules is the main mechanism to achieve a high H2S selectivity and adsorption capacity. Besides, the morphology of the adsorbents, such as their size and shape, can also influence the H2S 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 (Fe2O3) is a potential adsorbent for low-temperature H2S removal with the removal rate less than 200 kg-S/day. (45) The adsorption of H2S 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 Fe2O3 for H2S 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, Fe2O3 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 Fe2O3 and the desulfurization performance. Zinc oxide (ZnO), as a commonly used desulfurizer, has good thermal stability and strong desulfurization ability. In recent years, H2S 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 H2S adsorption capacity. Copper oxide (CuO) has been increasingly developed as an adsorbent for H2S removal due to its higher reaction equilibrium constant with H2S compared with Fe2O3 and ZnO. Nevertheless, aggregation and low regeneration are the main drawbacks of CuO, which is similar 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 H2S 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 H2S 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 H2S adsorption capacity but also enhance the regeneration capacity.

3. ILs and DESs for H2S Removal

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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 H2S 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 H2S 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 H2S removal is also included in this section. The structure of ILs and DESs are summarized as shown in Figure 3.

Figure 2

Figure 2. Challenges of chemical absorption and adsorption technologies, and the beneficial features of ILs for H2S removal.

Figure 3

Figure 3. Structures of (a) ILs and (b) DESs.

Table 1. Summary of the Absorption Performance of H2S in ILs and IL-Based Solvents
absorbentT (°C)P (bar)H2S solubility (mg-H2S/g-sorbent)remarksref
IL Used as Pure Absorbent
[Bmim][PF6]25–130up to 96 [Bmim][PF6] would be useful only for the absorption of H2S with high partial pressure (23)
[Bmim][PF6]30–70up to 10 The trend in affinity of H2S: [Bmim][Tf2N] > [Bmim][BF4] > [Bmim][PF6]. (63)
[Bmim][BF4]
[Bmim][Tf2N]
[Hmim][PF6]30–70up to 11 The trend in affinity of H2S: [Hmim][BF4] > [Hmim][PF6] ≈ [Hmim][Tf2N]. (64)
[Hmim][BF4]
[Hmim][Tf2N]
[Hemim][OTf]30–80up to 18 The trend in affinity of H2S: [HOemim][Tf2N] > [HOemim][OTf] > [HOemim][PF6] > [HOemim][BF4]. (65)
[Hemim][PF6]
[Hemim][Tf2N]
[Emim][PF6]30–80up to 20 The trend in affinity of H2S: [Emim][Tf2N] > [Emim][PF6]. (66)
[Emim][Tf2N]
[Emim][EtSO4]30–80up to 16 The diffusion coefficient of H2S in [Emim][EtSO4] is about 2 orders of magnitude as that of CO2. (102)
[Hemim][BF4]30–80up to 11 The solubility of H2S is higher than CO2 in [Hemim][BF4]. (103)
[C8mim][Tf2N]30–80up to 20 The solubility of H2S is about twice as high as that of CO2 in [C8mim][Tf2N]. (67)
[Emim][eFAP]30–80up to 20 Solubility of H2S is more than twice that of CO2 at fixed condition; (104)
Acid gas solubility increased with the increase of – CF3 group in anion.
[C8mim][PF6]30–80up to 20 Both H2S and CO2 solubility increased by increasing the alkyl chain length of cation. (68)
[Emim][OTf]30–80up to 30 Solubility of H2S is more than four times that of CO2. (71)
[Bmim][OTf]30–80up to 32 Solubility of H2S is more than four times that of CO2. (72)
[C4Py][Tf2N]30–60up to 6 H2S solubility: [C8Py][SCN] > [C6Py][SCN] > [C4Py][SCN] > [C4Py][Tf2N] > [C4Py][NO3] > [C4Py][BF4]; (69)
[C4Py][SCN]H2S/CO2 selectivity is about 8.99 in [C4Py][SCN] at 30 °C.
[C4Py][NO3]
[C4Py][BF4]
[C6Py][SCN]
[C8Py][SCN]
[NEMH][Ac]25–45up to 1.096 H2S absorption in these ILs is a physical process; (70)
[NEMH][Pro]H2S solubility increased with the increase of alkyl chain on anion.
[NEMH][Bu]
[NEMH][MOAc]
[NHEMH][Ac]
[NHEMH][MOAc]
[TEAH][Ac]
[TEAH][Pro]
[TEAH][Bu]
[Emim][Ac]20–60up to 3.5 Solubility of H2S increased with the increase of alkalinity of anions; (73)
[Emim][Pro]Both physical and chemical absorptions were thought to occur in the systems.
[Emim][Lac]
[Bmim][Ac]
[Hmim][Ac]
[N2224][DMG]601106.1CO2 solubility in these ILs was quite limited due to the coupling effect of the two Lewis base group. (24)
[N2224][IMA]601∼48.1
[N2224][NIA]601∼48.6
[MDEAH][Ac]30–60up to 1.2 Protic ILs showed larger solubility for H2S than for CO2. (75)
[MDEAH][For]
[DMEAH][Ac]
[DMEAH][For]
[Bmim][Ac]30–701–22 Chemical absorption was taken place between [Bmim][Ac] and H2S. (74)
[TMPDA][Tf2N]251∼12.4H2S solubility followed the sequence of: [TMHDA][Tf2N] > [BDMAEE][Tf2N] ≫ [TMPDA][Tf2N]. (77)
[BDMAEE][Tf2N]25142.5
[TMHDA][Tf2N]40150.4
[TMGH][PhO]401138.4The strong interaction between [PhO] and H2S is beneficial for increasing H2S solubility. (76)
[DBUH][PhO]401110.8
[Hmim][PhO]401110.1
[P4444][PhO]40185.2
[DBNH][1,2,4-triaz]401211.4The strong interaction between H2S and protic ILs (N•••H–S) is responsible for the high H2S solubility. (78)
[DBNH][1,2,3-triaz]401191.3
[DBUH][1,2,4-triaz]401180.0
[DBUH][1,2,3-triaz]401160.6
[DBNH][Im]401223.0The strong interaction between H2S and protic ILs (N•••H–S) is responsible for the high H2S solubility. (79)
[DBUH][Im]401194.5
[DBNH][Pyr]401231.5
[DBUH][Pyr]401180.5
IL-Based Solutions
[N2222][l-Ala]/EG (1:1)a400.0572.3Addition a certain amount of EG could increase both H2S solubility and absorption rate. (25)
[N2222][β-Ala]/EG (1:1)a70.2
[N2222][Gly]/EG (1:1)a80.5
[N2222][l-Pro]/EG (1:1)a54.2
MEA/[MEA][Ac] (1:1)a251-The high H2S removal efficiency was attributed to hydramine; (80)
DEA/[DEA][Ac] (1:1)aILs increased the stability of blended absorbents.
TEA/[TEA][Ac] (1:1)a
MDEA/[MDEA][Ac] (1:1)a
40 wt % [BDMAEE][Ac]25161.0H2S and CO2 solubility decreased with the increase of [BDMAEE][Ac] concentration. (26)
DIPA + [Bmim][Ac] in water with concentration of (50 wt % + 5 wt %), (50 wt % + 10 wt %), (30 wt % + 5 wt %), (30 wt % + 10 wt %), and (50 wt % + 50 wt %)40–752–25 H2S and CO2 solubility increased with the concentration of DIPA. (27)
50 wt % [TDMAPAH][Ac]40up to 1.298.4The cation with multiple Lewis base group can promote the absorption of H2S. (82)
50 wt % [PMDPTAH][Ac]84.0
50 wt % [TMPDA][Ac]58.2
7.5 wt % [N1111][Gly] + 40 wt % MDEA300.00410.12Addition of a small amount of IL could greatly increase H2S loading. (83)
[C8mim][BF4] in PC with mass fraction of 20%, 50% and 80%30–60up to 10 The selectivity of H2S/CO2 increased with increasing concentration of [C8mim][BF4]. (81)
[Bmim]3PW12-xMoxO40/[Bmim][Cl]2001 Desulfurization mechanism: H2S + O2 → H2S + S. (84)
[Bmim]3+xPMo12-xVxO40/[Bmim][Cl]
16-PHPC/[Bmim][HCO3]951 The absorption capacity of H2S is reduced after regeneration due to the loss of peroxo species. (87)
a

Mass ratio.

3.1. ILs as Pure Absorbents

3.1.1. “Physical” ILs

According to the structure of ILs and the interaction with H2S, ILs can be divided into “‘physical’” and “functionalized” ILs. The reported physical ILs for H2S removal mainly include imidazolium-based and pyridinium-based ILs. Jou et al. (23) were the first to study the absorption of H2S in [Bmim][PF6] at the temperatures of 25–130 °C and pressures up to 96 bar. The pressure-dependent H2S solubility indicated that [Bmim][PF6] is a physical solvent. H2S absorption in physical ILs is affected by both the cations and anions. For the imidazolium-based ILs with different cations and anions, the solubilities of H2S in mole fraction basis follow the order of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][Tf2N]) > 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) > [Bmim][PF6], (63) 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF4]) > 1-hexyl-3-methylimidazolium hexafluorophosphate ([Hmim][PF6]) ≈ 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Hmim][Tf2N]), (64) 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Hemim][Tf2N]) ≥ 1-(2-hydroxyethyl)-3-methylimidazolium trifluoromethanesulfonate ([Hemim][OTf]) > 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate ([Hemim][PF6]) > 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ([Hemim][BF4]), (65) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf2N]) > 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]). (66) Also, the H2S solubilities in [Cnmim]-based ILs with [Tf2N] and [PF6] anions did increase by increasing the alkyl chain length of cation. (67,68) The same conclusion was obtained by investigating H2S absorption in the pyridinium ([CnPy])-based ILs with the anion of thiocyanate ([SCN]) as shown in 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 H2S solubility. Interestingly, Zhao et al. (70) found that increasing the alkyl chain of the anion could also promote the absorption of H2S in carboxylate ILs due to the increased alkalinity of the anion (Figure 4b).

Figure 4

Figure 4. (a) Effect of cations of [CnPy]-based ILs on H2S absorption at 40 °C. Figure reprinted from ref (69). Copyright 2018 American Chemical Society. (b) Effect of anions of carboxylate ILs on H2S absorption at 25 °C. Figure reprinted with the permission from ref (70). Copyright 2018 Elsevier.

In some cases, selective absorption of H2S from CO2 is favored to recycle these two gases as feedstocks for downstream industries, and thus developing ILs with large H2S/CO2 selectivity is quite attractive. To this end, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][OTf]) was selected for selective H2S removal. It was found that the solubility of H2S was more than four times that of CO2 under fixed temperature and pressure. (71) Moreover, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([Bmim][OTf]) (72) was also found to be preferential for the separation of H2S over CO2, indicating the great potential of “‘physical’” ILs for selective separation of H2S over CO2. The interaction between the physical ILs and H2S is usually weak (mainly van der Waals forces), resulting in low regeneration energy, but the limited H2S absorption capacity makes these ILs only suitable for the removal of H2S when its partial pressure is high.

3.1.2. Functionalized ILs

Functionalized ILs have been developed to improve the H2S absorption capacity at low pressures. As reported, [Emim]-based ILs with different carboxylate anions, including acetate ([Ac]), propionate ([Pro]), and lactate ([Lac]), showed much higher H2S solubility compared with the commonly used ILs at low pressures. (73) More specifically, the solubility of H2S in [Emim][Ac] at 60 °C and 1 bar is 18 times higher than that in [Emim][Tf2N] (66) under the same conditions. Moreover, the H2S solubility increased dramatically in these ILs at low pressures (0–0.5 bar) and almost linearly at high pressures (>0.5 bar), which means both chemical and physical absorptions can occur during the absorption of H2S. Later, H2S solubility and the H2S/CO2 selectivity in three functionalized ILs, triethylbutylammonium N,N-dimethyl-glycinate ([N2224][DMG]), triethylbutylammonium 1-imidazole acetate ([N2224][IMA]), and triethylbutylammonium nicotinate ([N2224][NIA]) were studied, (24) and high H2S solubilities of 48.1–106.1 mg-H2S/g-IL with H2S/CO2 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···H2S···amine and the large difference in binding energy between anion-H2S and anion-CO2. This can also explain the reason that the solubilities of H2S in 1-butyl-3-methylimidazolium acetate ([Bmim][Ac]) are much higher than those for CO2 reported by Haghtalab et al. (74)
Protic ILs are considered as better absorbents for the separation of H2S from CO2. H2S/CO2 selectivities could be up to 8.9 and 15.1 at 30 °C in two protic ILs, that is, methyldiethanolammonium acetate ([MDEAH][Ac]) and dimethylethanolammonium acetate ([DMEAH][Ac]), respectively, which are much higher than that in commonly used ILs (2–4). (75) This is because the Brønsted acid–base interaction between the protonated nitrogen in the cation and [Ac] could reduce the affinity of CO2 with [Ac], and thus decrease the solubility of CO2. However, unlike the CO2 molecule, H2S still interacts with the electron donor of protic ILs due to its active protons. The same conclusion was drawn when using the protic IL tetramethylguanidinium phenolate ([TMGH][PhO]) for H2S removal by Huang et al. (76) The same research group also designed a novel type of hydrophobic protic IL containing a free tertiary amine group and used it for H2S removal. (77) H2S solubilities were 42.5 and 50.4 mg-H2S/g-IL in bis(2-dimethylaminoe-thyl)ether bis(trifluoromethylsulfonyl)imide ([BDMAEE][Tf2N]) and N,N,N′,N′-tetramethyl-1,6-hexanediamine bis(trifluoromethylsulfonyl)imide ([TMHDA][Tf2N]) with high selectivities of 37.2 (25 °C) and 29.5 (40 °C) at 1 bar, respectively, which are attributed to the strong interactions between H2S and the tertiary amine group in the cation.
Using 1,2,4–1H-imidazolide ([1,2,4-triaz]) and 1,2,3–1H-imidazolide ([1,2,3-triaz]) as the anions of protic ILs could further increase the absorption capacity of H2S. For example, [DBNH][1,2,4-triaz] showed high H2S absorption capacity (211.4 mg-H2S/g-IL at 1 bar), with a H2S/CO2 selectivity of 4.6 at 40 °C. (78) When the anion of [DBNH][1,2,4-triaz] changed to [Pyr], the capacity of H2S could even reach 231.5 mg-H2S/g-IL at 1 bar and 40 °C with H2S/CO2 selectivity of 2.0, indicating that [DBNH][Pry] is a potential absorbent for simultaneous capture of H2S and CO2. (79) The particularly strong interaction between the N atom in anion and H2S resulted in a higher H2S absorption capacity compared with those reported in the literature, which further confirmed the exceptionally high potential of protic ILs for selective H2S removal.

3.2. IL-Based Solvents

Although many functionalized ILs show a high H2S solubility and H2S/CO2 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 IL-based solvents by adding low viscous organic solvents and/or water to ILs. For example, when tetraethylammonium l-alanate ([N2222][l-Ala]) mixed with low viscous EG with the mass ratio of 1:1 at 40 °C and the H2S partial pressure of 0.05 bar, the absorption reached equilibrium in about 480 min, much shorter than that using pure IL (1400 min). (25) IL can also be used to stabilize the absorbent. When equal amounts of MEA and ethanolammonium acetate ([MEA][Ac]) were mixed, the removal efficiency of H2S was nearly 100%, much higher compared with the pure [MEA][Ac] (1.2%). (80) In this system, MEA is the main component, responsible for H2S absorption chemically, while IL plays a key role in improving the stability of blended absorbents. This interplay gives the MEA/[MEA][Ac] system a good desulfurization performance and recyclability. Research work has also been carried out for selective absorption of H2S from CO2. The mixture of 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]) and propylene carbonate (PC) showed higher H2S solubility and H2S/CO2 selectively compared with pure PC. (81)
Aqueous IL-based solutions have been widely studied for H2S removal. The molar solubilities of H2S and CO2 increased with increasing H2O concentration in bis(2-dimethylaminoethyl)ether acetate ([BDMAEE][Ac]) due to their dissolution in H2O. (26) Even though the equilibrium H2S/CO2 selectivity was limited, the kinetic selectivity of H2S/CO2 in aqueous [BDMAEE][Ac] was high. Therefore, the aqueous IL solution is a promising candidate for separating H2S from CO2. Aqueous solutions of three different Lewis base functionalized protic ILs were prepared by Zheng et al., (82) for H2S absorption. A high H2S solubility of 98.4 mg-H2S/g-IL was obtained in 50 wt % N,N,N-tris(3-(dimethylamino)propyl)ammonium acetate ([TDMAPAH][Ac]) solution at 40 °C and 1 bar, indicating the advantage of multiple Lewis base group of cation on desulfurization.
Moreover, ILs have also been combined with aqueous alkanolamine solutions. As shown in Figure 5, increasing diisopropanolamine (DIPA) content and [Bmim][Ac] concentration can intensify the selective absorption of H2S in the presence of CO2. (27) It was also observed that adding a small amount of tetramethylammonium glycinate ([N1111][Gly]) into aqueous MDEA solution can significantly increase the H2S solubility. (83) This observation indicated that the amino group in the anion of [N1111][G1y] not only reacted with H2S, but also promoted the absorption of H2S in aqueous MDEA by increasing the alkalinity of the solution. On the contrary, both H2S absorption capacity and absorption rate decreased with increasing the concentration of MDEA. This is because, at a low partial pressure of H2S, adding more MDEA could not promote the absorption of H2S, but rather increased the viscosity of solutions, and thus limited the absorption and diffusion of H2S.

Figure 5

Figure 5. Solubility of H2S in the presence of CO2 in DIPA + [Bmim][Ac] at 50 °C. Figure reprinted with permission from ref (27). Copyright 2017 Elsevier.

Sometimes, the temperature of H2S in the industrial gas streams with H2S can reach hundreds of degrees Celsius, while the conventional absorption technologies for H2S removal can only be operated at relatively low temperatures (<80 °C) in order to reduce the solvent loss and increase H2S absorption capacity. As a result, cooling processes are always needed before absorption, which leads to a high energy demand. To achieve H2S absorption at high temperatures, Liu et al. (84) dissolved a certain amount of heteropoly compounds (HPCs), such as [Bmim]3PW12-xMoxO40 and [Bmim]3+xPMo12-xVxO40, into 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) for H2S removal at 200 °C. It was found that the desulfurization capacity enhanced with the increase in the number of Mo atoms in [Bmim]3PW12-xMoxO40/[Bmim][Cl], confirming that Mo atoms work better for H2S removal than W atoms do in the anion of HPCs. Meanwhile, HPC was the dominant deriving force for H2S removal, while IL only served as solvent and reaction medium. Among the studied solutions with different amounts of Mo atoms, [Bmim]5PMo10V2O40/[Bmim][Cl] showed the optimal H2S removal performance and good regeneration ability. Later, the same group studied the macroscopic kinetics of the desulfurization in [Bmim]3PMo12O40/[Bmim][Cl] and proved that the absorption was driven by diffusion. (85) Moreover, the desulfurization efficiency can still be kept 100% after six times recycle of [Bmim]3PMo12O40/[Bmim][Cl]. (86) The desulfurization performance was also investigated in several peroxo-heteropoly compound (PHPC)/[Bmim]-based ILs with different anions, including [Cl], [Tf2N], [PF6], [BF4], and [HCO3], (87) and cetyltrimethylammonium peroxophosphomolybdate (16-PHPC)/[Bmim][HCO3] showed the highest H2S removal efficiency (nearly 100%) for 2 h at 95 °C. All the above-mentioned studies provided a novel method for efficient H2S removal at high temperatures by using HPCs/ILs and PHPCs/ILs solutions.

3.3. IL-Reinforced Adsorbents and Membranes

Combining ILs with adsorbents or membranes is another potential strategy to overcome the high viscosity of ILs and enhance H2S 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 H2S/CH4 (= 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 H2S is easier to dissolve into polar IL compared with CH4. (28) Ishak et al. (88) compared the stability of choline (Chl)-based ILs/isoreticular MOF (IRMOF-1) composites with different anions of ILs, such as [SCN], [OTf], and methyl sulfate ([MeSO4]). They found that a smaller-sized anion of IL contributed to a better stability since it could easily diffuse inside the pores of IRMOF-1. Solubilities of H2S and CH4 in [Chl][SCN], calculated from the solvation free energy by using the Bennet acceptance ratio (BAR) method, showed that both H2S solubility and H2S/CH4 selectivity were significantly increased compared with pure IL and IRMOF-1, owing to the low affinity of IRMOF-1 toward H2S, as well as the strong interaction between the cation and anion in [Chl][SCN] and also its high viscosity. Later, the same research group studied [Chl][Ala]/IRMOF-1 for selective H2S/CO2 capture. (89) One H2S molecule could combine with two molecules through carboxylate groups (−COO) in [Ala] anion via hydrogen bonds, while one molecule of CO2 was only hydrogen bonded with one molecule of amino group (−NH2) in the [Ala] anion. The stronger interaction between H2S and the [Ala] anion resulted in a higher H2S adsorption capacity. All these research studies indicate that IL/MOF composites are promising adsorbents for H2S separation from CH4 and CO2.
Ma et al. (90) prepared a novel metal-based IL, triethylamine hydrochloride copper chloride (Et3NHCl·CuCl2), and compared the adsorption performance of H2S in pure zeolite, cyclodextrin-grafted zeolite (CDGZ), zeolite-supported IL (IL-zeolite), and cyclodextrin-modified zeolite-supported IL (IL-CDGZ). The order of H2S removal capacity is CDGZ < zeolite < IL-zeolite < IL-CDGZ. Introducing cyclodextrin (CD) to zeolite decreased H2S removal capacity due to its inappropriate cavity size. The chemically active species of IL, including amines and Cu2+, could associate with H2S, and thus increase H2S 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 H2S removal performance of CDGZ. Later, the H2S adsorption performance of metal-based 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 H2S removal performance. The much better performance compared with pure silica gel and IL is attributed to the formation of nanometer-sized and high-concentrated 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 H2S permeability and selectivity. Zhang et al. (29) prepared several supported ionic liquid membranes (SILMs) composed of polyvinylidene difluoride (PVDF) with neutral [Bmim]-based ILs containing the anions of [PF6], [BF4], [Tf2N], [OTf], and a basic [Bmim][Ac]. The H2S permeability in these SILMs with neutral ILs increased with an increase of the basicity of the anion. H2S permeability in [Bmim][OTf]/PVDF membrane could reach 4303 barrers. The selectivity of CO2/CH4 was 50.7, while that for H2S/CO2 was only 4.0, at 0.1 bar and 40 °C. For [Bmim][Ac]/PVDF, the H2S permeability was 7304 barrers, whereas the selectivities of H2S/CH4 and H2S/CO2 were 136 and 11.7, respectively, under the same condition. This means that [Bmim][OTf]/PVDF can be used for simultaneous removal of CO2 and H2S, while [Bmim][Ac]/PVDF is beneficial to the selective separation of H2S from CO2 and CH4. Akhmetshina et al. (92) prepared another kind of SILMs using [Bmim][BF4] and a microfiltration tetrafluoroethylene-vinylidene fluoride composite membrane (MFFK-1). H2S achieved the highest permeability (∼380 barrers), followed by CO2 (∼80 barrers) and CH4 (∼12 barrers). Bhattacharya et al. (93) studied the permeabilities of H2S, CO2, air, and CH4, respectively, in a mixed matrix membrane synthesized with [Emim][EtSO4] and poly(ether-block-amide) (PEBA). The observed trend for different gases follows H2S > CO2 > air > CH4. These studies further confirmed that SILMs are promising candidates for selective H2S removal.

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 CO2, the application of DESs for acidic H2S 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 H2S, CO2, and CH4 solubilities in ChCl/urea with the molar ratios of 1:1.5, 1:2.0, and 1:2.5 showed that H2S solubility decreased with the decrease of ChCl/urea ratio, which is different for CO2 and CH4 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 H2S during the absorption of H2S in the ChCl/urea systems, and the decrease of the molar ratio weakened the strength of interaction, which caused the decrease of H2S solubility. However, the solubilities of CO2 and CH4 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 CO2 and CH4 among the studied ChCl/urea. Recently, [Bmim][Cl]/imidazole (2:1) was proven to be a promising candidate for the selective separation of H2S from CO2 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) synthesized two kinds of DESs by mixing tetrabutylammonium bromide (TBABr) and ChCl with carboxylic acid (including Pro, Ac, and For). It was demonstrated that the carboxylic acid with a higher acidity would cause a lower H2S solubility both in TBABr/carboxylic acid and ChCl/carboxylic acid systems. Besides, TBABr-based DESs exhibited higher H2S solubility than ChCl-based DESs under similar conditions due to the higher electronegativity of Cl and hydroxyl group in ChCl, which results in more complex hydrogen-bonding networks in ChCl-based DESs and decreases the overall space to absorb H2S. Moreover, H2S solubility increases with the decrease of temperature and increase of pressure, behaving like a typical physical absorption.
Inspired by functionalized ILs, novel functionalized DESs (FDESs) were prepared by adding polyethylenimine (PEI) to the ChCl-based DESs, including ChCl/Gly (1:2), ChCl/EG (1:2), ChCl/propylene glycol (PG) (1:2), and ChCl/urea (1:2), to enhance H2S removal efficiency. (98) The combination of PEI and ChCl/EG (1:2) (PEI/FDES@EG) was identified as the most promising desulfurizer owing to its optimal H2S removal efficiency. For PEI/FDES@EG with 25% PEI, H2S 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 H2S, 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 H2S absorption capacity and H2S/CO2 selectivity compared with the reported DESs. Moreover, both alkalinity and free volume could influence the solubility of H2S, as the larger is the alkalinity and free volume the higher is the H2S 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 H2S removal efficiency in the absence of ETA, indicating that H2S 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 H2S and ETA, and thus the H2S removal efficiency increased with the increasing concentration of Cu.
Considering the great potential and advantages of supported ILs, the study on the H2S 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 TAExCuCl2+x with various molar ratios of triethylamine hydrochloride (TEACl) and cupric chloride (CuCl2) as the loading substance. SDES with 10 wt % loading of TAECuCl3 (TAECuCl3@FS/10 wt %) has the optimum desulfurization performance at 30 °C with the highest adsorption efficiency (molar ratio of Cu to absorbed H2S, nH2S/nCu is 0.87), which is about 3.56 times higher than the pure TAECuCl3 under the same condition. This is because the interaction between FS and DES could reduce the size of surface TAECuCl3 microclusters and thus promote the transformation of metal active sites. After adsorption, a small amount of H2S was oxidized to sulfur and sulfate ions, and most of the H2S was transferred to Cu2S. This study suggests that SDESs are potential sorbents for an efficient removal of H2S.

3.5. Summary and Outlook of ILs and DESs for H2S Removal

Some conclusions regarding H2S removal in ILs and DESs are summarized in Figure 6. H2S 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 H2S 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 H2S solubility. Physical ILs are only suitable for the removal of H2S when its partial pressure is high. Developing functionalized ILs is a promising way to improve H2S absorption capacity and H2S/CO2 selectivity at low pressures. Protic ILs have been proven to be favorable absorbents for the separation of H2S from CO2. The strong interactions between H2S and the anion of carboxylate, azole-based, and dual Lewis base functionalized ILs are the main causes of high H2S 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 H2S. Water and EG were often selected to increase the absorption rate of H2S 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 H2S in alkanolamines. Besides, the development of HPCs/ILs and PHPCs/ILs realized efficient H2S 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 H2S sorption performance. MOFs, modified zeolites, and sol–gel derived silica are all promising supporters of ILs due to their porous structure and high surface area.

Figure 6

Figure 6. Schematic representing key characteristics of pure ILs, IL-based solvents, IL-reinforced adsorbents and membranes, and DESs for H2S removal.

DESs have been considered as one of the most desirable solvents for the selective removal of H2S 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 H2S 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 H2S removal. It should be noted that even supported ILs/DESs exhibit attractive performance, they are only suitable for the gas streams with low H2S concentrations, while IL/DES-based liquid sorbents can be used for bulk H2S 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 H2S removal. Therefore, the following aspects deserve further investigations: (1) Computational chemistry should be carried out to understand the mechanism of action between H2S and different ILs/DESs, and provide guidelines for the optimization of the structural design of task-specific ILs and DESs with high H2S absorption capacity and selectivity. (2) Carry out relevant research on the evaluation of technology and economy of IL/DES-based H2S 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 H2S to widen the application of ILs and DESs, and to build a more sustainable H2S removal and utilization process.

4. Theoretical Studies

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A large part of studies have been experiments to investigate H2S 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 H2S 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.

4.1. Quantum Chemical Calculations

Quantum chemical calculations are highly useful tools to calculate the geometry structures and analyze the interaction mechanisms of H2S 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 H2S 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 H2S and CO2 in [Cnmim][Tf2N]. The binding energy of anion–cation complexes calculated in the basis set of 6-311+G(d) decreases from the cations of [C2mim]+ to [C6mim]+, and then increases for [C8mim]+. However, a decreasing trend was observed from [C2mim]+ to [C8mim]+ by using 6-311++G(2d,2p). Moreover, an excellent linear correlation between the calculated absolute value of the energies of [Cnmim][Tf2N] at the B3LYP/6-311++G(2d,2p) level and Henry’s constant of H2S and CO2 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 H2S. This observation could also explain the gradually increased H2S solubility with increasing alkyl chain of [PF6]-based ILs as reported by Safavi et al. (68) It was also found that the molecules of H2S and CO2 are directed toward the less electronegative N atom, rather than the most electronegative F atom of the [Tf2N] anion, as shown in Figure 7. Quantum chemical calculations on the H2S-anion by Pomelli et al. (106) and those on the CO2-anion by Bhargava et al. (107) confirmed the conclusion.

Figure 7

Figure 7. Minimum energy structures of (A) H2S-[Tf2N], and (B) CO2-[Tf2N]. Figure reprinted from ref (67). Copyright 2012 American Chemical Society.

Handy et al. (108) reported that the higher H2S solubility compared with that of CO2 in [Bmim][Br] is attributed to the much shorter H-bond of the H2S-cation (2.38 Å) than that of the CO2-cation (3.18 Å) by using the B3LYP/6-311G(2d,p) level. To reveal how the structures of cations and anions of ILs affect H2S solubility, Zhou et al. (109) calculated the interaction of H2S with different ILs, including 4-bis(2-hydroxypropyl)-1,1,3,3-tetramethyl guanidinium tetrafluoroborate ([TMGHPO2][BF4]), tetramethyl guanidinelactate (TMGL), and [Bmim]-based ILs with the anions of [Cl], [BF4], [PF6], [OTf], and [Tf2N] by using B3LYP/6-311++G(d,p) level. The orders of the hydrogen bond strength and interaction energy of H2S-IL were consistent with that for the H2S solubility determined experimentally, revealing that hydrogen bonds are the main driving force of H2S absorption in these ILs, which is also in agreement with other experimental and calculation investigations. (67,108)
Different from the absorption mechanism of H2S in the ILs with common anions, the active protons of H2S are ionizable in the anion-functionalized ILs. For instance, DFT calculations for the phenolic ([PhO]) IL-H2S complexes verified that the active proton of H2S was transferred to the oxygen atom of [PhO] and formed a solid O–H bond during H2S absorption. (76) Similar results were obtained by Zhang et al. (78) in their calculations of the interaction between the azole-based protic IL and H2S. They stated that the extremely high H2S absorption capacity is attributed to the chemical combination of the H atom in the H2S molecule and the N atom in the anion of ILs.
Quantum chemical calculations were also used to understand the different behavior of H2S, CO2, and CH4 in DESs. (95) In the system of H2S + ChCl/urea (1:2.0), Cl and H of ChCl formed strong hydrogen bonds with H and S of H2S, respectively. For CO2- ChCl/urea (1:2.0), hydrogen bonds were also formed between the Cl of ChCl with C of CO2, and H of urea with O of CO2. While only van der Waals interactions were observed in the system containing CH4 and ChCl/urea (1:2.0). As mentioned in section 3.4, the combination of PEI and ChCl/EG could greatly increase the H2S removal efficiency. Quantum chemical calculations on the complexes of H2S-PEI, H2S-ChCl, and H2S-EG confirmed that the interaction between PEI and H2S was the dominant driving force of high H2S 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 H2S 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) performed MD simulations to obtain the Henry’s constant and solvation thermodynamic properties of H2S in [Bmim][Cl], [Bmim][BF4], and [Bmim][PF6] using the FEP method with united-atom force fields. The order of the predicted Henry’s constants in these three ILs is [Bmim][Cl] < [Bmim][BF4] < [Bmim][PF6], which is in agreement with the experimental results reported by Pomelli et al. (106) The radial distribution functions (RDFs) indicated that the H atoms of H2S are oriented toward anions, while the S atom of H2S is surrounded by cations. The shortest distance between the H atoms in H2S and the [Cl] anion resulted in the highest H2S solubility of [Bmim][Cl]. Moreover, the contribution of solvation enthalpy to free energy is twice of entropy, and, therefore, the H2S absorption in ILs is an enthalpy-driven process.
Salehin et al. (115) calculated the solvation free energy and thermodynamics properties of H2S 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 H2S in IL is more negative compared with that in the aqueous system, indicating H2S can be more easily solvated in ILs. The radial and spatial distribution functions displayed that the H atoms of H2S are more aligned toward the anion, while the S atom of H2S 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 H2S to occupy and thus improve H2S solubility. Amhamed et al. (116) performed all-atom MD simulations to study the absorption of H2S, CO2, and CH4 in choline benzolate ([Chl][BE]) and choline lactate ([Chl][Lac]). The selectivities of H2S/CH4 and CO2/CH4 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 H2S with CH4 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 H2S and CH4 in natural gas. As shown in Figure 8, the highest peak at a distance of 3.9 Å indicated the strong interaction between the H2S molecule and the Br of the DES. Moreover, the peaks of H2S-TBA and H2S-CPL were slightly higher than that of H2S-CH4, thus favoring the selective removal of H2S from CH4. Besides, the strength of the interactions (peak height) within the DES was marginally decreased after it was mixed with the natural gas, but the distance at which the peaks appeared remained the same, indicating that the molecular structure of the DES did not change during the absorption of H2S. To explain why [Emim][Cl]/imidazole DESs showed high H2S solubilities and low CO2 solubilities, MD simulations were performed by Wu et al., (96) to determine the active sites of DESs for interaction with H2S and CO2. It was found that the interactions formed between H(H2S)/C(CO2) and Cl([Emim][Cl]) dominate the capture of H2S and CO2, but the former is much stronger than the latter, resulting in remarkable H2S/CO2 selectivities. These results enhanced the understanding of the molecular interactions within the DES and their interactions with H2S.

Figure 8

Figure 8. (A) RDFs between H2S with CH4 and the components of DES, and (B) RDFs between the components of DES before and after mixing with natural gas. Figure reprinted from ref (117). Copyright 2020 American Chemical Society.

As mentioned above, MD simulations have been used to understand the interaction of anion–cation, and H2S-ILs/DESs. MD simulations can also predict H2S 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 [PF6], [BF4], 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 all-atom force field is suitable to predict selectivity in [Chl]-based ILs. Overall, MD simulations are mainly used to calculate the solubility trend of H2S in ILs and DESs, while thermodynamic models will be used to predict H2S solubility with high precision.

4.3. Thermodynamic Modeling of H2S Solubility

Experimental measurements can be time-consuming and dangerous because of the high toxicity of H2S, and even expensive. Therefore, it is desirable to obtain H2S 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 H2S solubility in ILs, which are discussed in this section and summarized in Figure 9.

Figure 9

Figure 9. Classification of thermodynamic models for the prediction of H2S solubility in ILs.

4.3.1. Cubic EoS

EoS is a powerful tool in representing thermodynamic properties of pure ILs and the H2S + IL mixtures, and thus has been widely used to predict H2S solubility in ILs. The first prediction for H2S + 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.
Faúndez et al. (118) used a modified PR EoS to correlate the solubilities of H2S and SO2 in ILs, and found that the model results are acceptable in all cases with the absolute average deviations (AAD) below 7.6% for H2S/IL mixtures and 2.6% for SO2/IL mixtures. Shojaeian et al. (119) applied the Peng-Robinson two-state (PR-TS) EoS model to predict the solubility of H2S in the imidazolium-based ILs with the assumption that cross-association 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. (120,121) developed a generic RK EoS to evaluate the vapor–liquid–liquid equilibrium (VLLE) of H2S/[Bmim][PF6], CO2/[Bmim][PF6], H2S/[Bmim][MeSO4] and CO2/[Bmim][MeSO4]. The calculated results showed that [Bmim][MeSO4] has higher H2S/CO2 selectivity compared with [Bmim][PF6], which is inconsistent with the experimental results. Jallili et al. (67) used this model to determine the solubilities of H2S and CO2 in [Omim][Tf2N] at 30–80 °C and the pressures up to 20 bar. The Krichevsky-Kasarnovsky (KK) equation, the model comprising the extended Henry’s law and the Pitzer’s virial expansion, and the generic RK EoS were used to correlate the experimental values, and the RK model showed the best correlation with the experimental data for IL/CO2/H2S ternary mixtures. Same results were also obtained by Safavi et al. (68) when correlating the measured data for [Omim][PF6]/CO2/H2S ternary mixtures.

4.3.2. CPA EoS

The cubic plus association (CPA) EoS, which combines both cubic and association terms, has also been applied successfully to predict H2S solubility in different imidizolium-based ILs. Haghtalab et al. (74) modeled the simultaneous solubilities of H2S and CO2 in [Bmim][Ac] by applying CPA and the reaction equilibrium thermodynamic model. The calculated AADs for the ternary H2S/CO2/IL system were estimated to be 18.8% for H2S and 13.7% for CO2, respectively. Panah et al. (122) used CPA to calculate H2S solubilities in 14 imidazolium-based ILs, and the values of AAD were lower than 10% for all the cases. Moreover, modeling H2S solubility in [Omim][Tf2N], (123) [Bmim][MeSO4], (124) [Emim][PF6], (125) [Omim][PF6], (126) [Emim][Ac], and [Hmim][Ac] (127) verified that CPA is able to provide acceptable results for the binary H2S/imidazolium-based IL mixtures. Sousa et al. (128) used CPA to model the solubilities of H2S in 2-hydroxyethylammonium acetate ([2-HEA][Ac]), bis(2-hydroxyethyl)ammonium acetate ([B-2-HEA][Ac]), and 2-hydroxyethyldiethylammonium hydrogen diacetate ([2-HEDEA][H(Ac)2]) at temperatures of 25–45 °C and atmospheric pressure, by considering the 4-sites association scheme for the H2S and 2-sites association scheme for the ILs. The AADs for the H2S/IL mixtures in the three ammonium-based ILs were 6.5%, 6.9%, and 8.9%, respectively.

4.3.3. SAFT EoS

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 H2S 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 H2S 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 self-association of the H2S molecules, they made a conclusion that self-association plays an important role in modeling the systems. Baramaki et al. (130) modeled H2S 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., H2S, CO2, and CH4) in ILs. (131,135−139) Ji et al. (131) studied the solubilities of H2S, O2, CO, and H2 in [Cnmim]-based ILs (2 ≤ n ≤ 8) with the anion of [Tf2N], [PF6], and [BF4] using ePC-SAFT. The results indicated that ePC-SAFT could predict quantitatively the solubility of H2S under temperatures of 30–80 °C and pressures up to 20 bar. Later, the work was extended to other commonly used ILs (composed of the IL-cations of [Cnmim]+, [Cnpy]+, [Cnmpy]+, [Cnmpyr]+, and [THTDP]+, and the IL-anions of [Tf2N], [PF6], [BF4], [OTf], [DCA], [SCN], [C1SO4], [C2SO4], [eFAP], Cl, [Ac], and Br), making it possible to predict gas solubility in a wide variety of ILs. Al-Fnaish et al. (132) used PC-SAFT to investigate the solubilities of H2S and CO2 in [Cnmim][Tf2N] (n = 2, 4, 6, and 8). 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 self-association 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 H2S-IL mixtures, and 1.54%–4.98% for CO2–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 H2S, SO2, and NH3 in three imidazolium-based ILs using soft-SAFT with experimental data. (133) Later, the same group presented solubility data for several gases (H2S, CO2, CO, CH4, H2, and SO2) in [Bmim][MeSO4] in wide temperature and pressure ranges using the same model. The predicted Henry’s constants and selectivities showed that SO2 and H2S were preferentially absorbed in IL. (134)

Figure 10

Figure 10. Illustrative example of hydrogen bonding between cation and anion of [Hmim][Tf2N]. Figure reprinted with permission from ref (132). Copyright 2017 Elsevier.

Baramaki et al. (141) investigated the ternary systems of H2S/CO2/[Omim][Tf2N], H2S/CO2/[Omim][PF6], and H2S/CO2/[Bmim][PF6] 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 H2S/CO2/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 H2S.

4.3.4. 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 H2S and CO2 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 H2S, CO2, CH4, and C2H6 in ILs, in which COSMO-RS was used to calculate activity coefficients of H2S, CO2, CH4, and C2H6 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 H2S and C2H6, while the ILs with higher molecular weight are beneficial for dissolving more CH4. Besides, the ILs containing the cations of [N4111]+, [pmg]+, and [tmg]+, and the anions of [BF4], [NO3], and [MeSO4] showed high selectivities for H2S/CO2, H2S/CH4, and H2S/C2H6. Kamgar et al. (146) predicted VLE of H2S/[Hmim]-based ILs with the anions of [PF6], [BF4], and [Tf2N], 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 H2S 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 predictive EoS models to reduce the amount of experimental work in predicting H2S solubility. The group contribution equation of state (GC-EoS) derived by the combination of four well-known theories (van der Waals EoS, Carnahan–Starling expression for hard spheres, NRTL, and group contribution principle), (147) has been successfully applied to model gas + IL systems. This model takes into account the interactions between different functional groups of the molecules instead of different molecules themselves. In this case, several ILs containing similar functional groups do not need to estimate new interaction parameters, and less amount of experimental data will be required, which makes this model easily applicable to various ILs. Breure et al. (148) were the first to model binary systems of CO2 and imidazolium-based ILs with the anions of [PF6] and [BF4] using GC-EoS. Good agreement between experimental and calculated solubilities was obtained for pressures up to 100 MPa. Subsequently, application of GC-EoS to model the binary systems, including ILs with the anions of [Tf2N], (149) [NO3], (150) and tetracyanoborate [TCB] (151) with gases such as SO2, (152) alkanes, (153) H2, (149) and CO, (154) was also reported and obtained satisfactory results. Even the research related to H2S + IL systems is still scarce; these successful cases for predicting other gases suggest the great potential of GC-EoS to predict H2S solubility in ILs.

4.4. Machine Learning Approaches for Predicting H2S Solubility

Machine learning (ML) approaches also offer predictions for H2S-IL systems. Examples of these models include the artificial neural network (ANN), (155−159) gene expression programming (GEP), (160) least square support vector machine (LSSVM), (161,162) stochastic gradient boosting (SGB) tree, (163) and extreme learning machine (ELM). (164−166) Predictions of H2S solubility in different ILs with ML approaches are discussed below and summarized in Table 2.
Table 2. Summary of the Research Work on the Prediction of H2S Solubility in ILs Using ML Approaches
ML approachno. of data pointsno. of ILsinput parametersmodel performanceref
MLFFNN-ANNTotal: 465; Training: 372; Test: 9311Acentric factor, critical temperature and pressure of ILs, pressure, and temperatureR2: 0.9922 (155)
MSE: 0.00025
FFNN-ANNTotal: 664; Training: 70%; Validation: 15%; Test: 15%14Acentric factor, critical temperature and pressure of ILs, pressure, and temperatureR2: 0.9987 (156)
AAD: 2.07%
MLP-ANNTotal: 664; Training: 554; Test: 11013Critical temperature, pressure and molecular weight of ILs, pressure, and temperatureR2: 0.9951 (for H2S/IL) (157)
MSE: 0.000117 (for H2S/IL)
R2: 0.9955 (for H2S/CO2/IL)
MSE: 0.000082 (for H2S/CO2/IL)
MLP-ANNTotal: 496; Training: 392; Test: 10412Critical temperature and pressure of ILs, pressure, and temperatureAAD < 4% (158)
MLP-ANNTotal: 1243; Training: 80%; Test: 20%33Acentric factor, critical temperature and pressure of ILs, pressure, and temperatureR2: 0.9976 (159)
AARD: 2.57%
RMSE: 0.0087
GEPTotal: 465; Training: 80%; Test: 20%11Acentric factor, critical temperature and pressure of ILs, pressure, and temperatureR2: 0.9902 (160)
MARE: 0.0438%
LSSVMTotal: 465; Training: 80%; Test: 20%11Acentric factor, critical temperature and pressure of ILs, pressure, and temperatureR2: 0.9976 (161)
MES: 0.00006651
LSSVMTotal: 1282; Training: 962; Test: 3202715 different chemical structures of ILs, temperature, and pressureR2: 0.9941 (162)
RMSE: 0.0122
SGBTotal: 465; Training: 80%; Test: 20%11Acentric factor, critical temperature and pressure of ILs, pressure, and temperatureR: 0.9995 (163)
MARE: 0.02220
ELMTotal: 1282; Training: 1026; Test: 25627Pressure, temperature, and numbers of fragmentsR2: 0.990 (164)
RMSE: 0.0301
ELM1Total: 1318; Training: 1055; Test: 26328Pressure, temperature, and SEP parameters of isolated cation and anionR2: 0.991 (166)
AARD: 5.87%
ELM2Total: 1318; Training: 1055; Test: 26328Pressure, temperature, and SEP parameters of ion-pair of each ILR2: 0.994 (166)
AARD: 3.84%
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 (Tc), and critical pressure (Pc) of ILs as input parameters to build a multilayer feed-forward neural network (MLFFNN) model to predict the H2S solubility in various imidazolium-based ILs. A low ADD of 4.58% for 465 experimental data points was obtained. Sedghamiz et al. (156) predicted the solubilities of CO2 and H2S 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) Faúndez, (158) and Hosseini, (159) to investigate H2S solubility in ternary mixtures containing CO2. Desirable predictions by using the MLP-ANN method indicate that MLP-ANN is a good alternative method for the estimation of H2S 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 H2S in ILs with the input variables of ω, Tc, and Pc, accompanied by temperature and pressure. A quite high coefficient of determination (R2 = 0.9902) and extremely low mean absolute relative error (MARE = 0.0438%) were obtained.
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) H2S solubility in 11 imidazolium-based 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 R2 and mean squared error (MSE) are 0.9976 and 0.00006651, respectively. Baghban et al. (162) also employed LSSVM to predict H2S 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, Tc, and Pc 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 H2S solubility in ILs was first reported by Soleimani et al., (163) with the input variables of ω, Tc, and Pc 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 H2S solubility in [Hmim][Tf2N] with different models.

Figure 11

Figure 11. Estimated H2S molar fraction in [Hmim][Tf2N] at 60 °C by various models. Figure reprinted with permission from ref (163). Copyright 2017 Elsevier.

The above mentioned models are able to predict H2S 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 H2S solubility in ILs with the number of fragments as the input parameters, in which 1282 pieces of data for 27 ILs were included. The R2, 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 H2S solubility in ILs over wide ranges of temperatures and pressures. Later, a new type of molecular descriptors, electrostatic potential surface (SEP), was proposed by the same group to obtain information at the electron level for the prediction of H2S solubility in ILs using ELM. (166) In this work, two new quantitative models were established based on the SEP parameters of isolated cation and anion (ELM1), or ion pair structure of each IL (EML2). The ELM2 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 H2S 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.

5. Process Simulation of H2S Removal in ILs and DESs

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Evaluation of the technologies are always required for a large-scale adaptation of any process. A combination of experimental and computational methods is the most common and powerful way to carry out process analysis. For H2S 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 H2S selectivity in the presence of CO2; 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 H2S capture. Specifically, COSMO-RS was used to select promising ILs according to thermodynamic criteria. Then Aspen Plus was applied to test the absorption performance of H2S in the industrial absorption column. It showed that the ILs that exhibited desirable H2S absorption performance in the thermodynamic aspect may present a weak operating performance because of their limited transport properties. Wang et al. (176) developed a multilevel screening method for simultaneous removal of H2S and CO2 from natural gas. First, potential ILs were prescreened based on the Henry’s law constant-based absorption-selectivity-desorption index (ASDI) at infinite dilution conditions. Then, their simultaneous H2S and CO2 removal performance was further evaluated based on vapor–liquid equilibrium (VLE) of ILs + gas mixture (H2S, CO2 and CH4). Following this, several key physical properties of selected ILs, including melting point, viscosity, and thermal and chemical stability, were assessed by the group contribution methods to find out suitable ILs for industrial applications. Finally, IL-based acid gases removal processes were simulated with Aspen Plus to evaluate the feasibility. After evaluation, phosphate [BeMPYO][H2PO4] and [EMIM][H2PO4] were identified as the two best absorbents. This work provides a comprehensive method for evaluating the potential of ILs in industrial H2S removal processes.
Wang et al. (21) designed a novel IL-based process for simultaneous removal of H2S and CO2 from syngas at room temperature using Aspen Plus. By comparing the physical properties of different ILs, [Bmim][Tf2N] was selected for further investigation because of its high solubility of acid gases and low viscosity. The process flow diagram based on [Bmim][Tf2N]-based syngas purification was shown in Figure 12. In this case, the removal ratios of CO2 and H2S could reach 97.6% and 95.3%, respectively, whereas in the Rectisol process as reported by Liu et al., (177) the removal ratios of CO2 and H2S 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][Tf2N]-based purification process with the Rectisol process as a reference. The electricity used to capture 1 kg of CO2 was estimated to be 1.150 and 1.176 MJ using the [Bmim][Tf2N]-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.

Figure 12

Figure 12. Flow diagram of [Bmim][Tf2N]-based syngas purification process. Figure reprinted with permission from ref (21). Copyright 2019 Elsevier.

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][PF6], 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 H2S 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 H2S (≤10 ppm) and CO2 (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][PF6]-based process. This study concludes that DESs have the potential to replace conventional solvents and provide feasible solutions for cost-related issues of ILs.

6. Evaluation of ILs and DESs for H2S Removal

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6.1. Viscosity and H2S Absorption Capacity

Viscosity and H2S absorption capacity provide important references for the screening of candidate ILs and DESs since they are the main influential factors for H2S removal efficiency. Besides, H2S removal conditions (high pressure or low pressure) and removal types (selective or simultaneous removal of H2S and CO2) will also affect the selection of ILs and DESs. In this part, different ILs and DESs will be evaluated based on their viscosity and H2S absorption capacity, as well as the H2S removal conditions and removal types.
For H2S removal at high pressures, physical ILs are suitable absorbents considering their low regeneration energy due to the weak interaction between ILs and H2S. Figure 13 summarized the viscosities and Henry’s constant of different physical ILs. Since high viscosity could result in low mass transfer, only ILs with low viscosity (less than 70 mPa·S at 30 °C) were selected for further evaluation, which include [Emim][Tf2N], [Emim][OTf], [Bmim][Tf2N], [Emim][eFAP], [Hmim][Tf2N], [Bmim][OTf], [Hemim][Tf2N], [C4Py][Tf2N], and [C4Py][SCN]. As shown in Figure 13b, ILs with a longer alkyl chain of cation, and with the anion of [Tf2N], [OTf], and [SCN] have a lower Henry’s constant. Ideal ILs should have low viscosity and a low Henry’s constant of H2S. To meet this requirement, ILs with the viscosity lower than 70 mPa·S and the Henry’s constant below 15 bar at 30 °C were chosen. These ILs were then divided into two groups for selective (H2S/CO2 selectivity > 4) and simultaneous (H2S/CO2 selectivity < 3) removal of H2S and CO2. Finally, we made a conclusion that [Emim][OTf], [Bmim][OTf], and [C4Py][SCN] are attractive absorbents for selective removal of H2S, while [Emim][Tf2N], [Bmim][Tf2N], and [Hmim][Tf2N] can be used for simultaneous removal of both H2S and CO2.

Figure 13

Figure 13. (a) Viscosities and (b) Henry’s constant and H2S/CO2 selectivities of physical ILs at 30 °C (H2S/CO2 selectivity is equal to HCO2/HH2S, where HCO2 and HH2S are Henry’s constants of CO2 and H2S. Viscosity and Henry’s constant data taken from refs (63−69,71,72,102−104,and181−193)).

Functionalized ILs can be used as attractive absorbents for low pressure H2S removal. As can be seen in Figure 14a, most reported functionalized ILs for H2S removal are protic ILs due to their advantages of simple synthetic routes and good affinity with H2S. Similar to the evaluation criteria of physical ILs, functionalized ILs with viscosities less than 60 mPa·S at 35 °C or 50 mPa·S at 40 °C were first selected for further discussion. [NEMH]-based ILs with the anion of [Pro], [Bu], and [Ac] exhibited high H2S removal capacities at 1 bar, and their viscosities were lower than 6 mPa·S at 35 °C, indicating a great application potential for industrial H2S removal processes. However, experimental data about CO2 absorption capacity in these ILs have not been reported yet, and thus it is still unclear what kind of H2S removal (selective or simultaneous removal of H2S and CO2) that [NEMH]-based ILs applies. [DBNH][Pry], [DBNH][Im], [DBUH][Pry], [DBUH][Im], [DBNH][1,2,4-Triaz], [DMEAH][Ac], and [DMEAH][For] also showed low viscosities. Since all these functionalized ILs showed satisfied H2S absorption capacity, only H2S/CO2 selectivity was compared as shown in Figure 14b. The last three ILs are potential candidates for selective H2S absorption because of their high H2S/CO2 selectivities (>4). Especially, the H2S/CO2 selectivity of [DMEAH][For] is higher than 10. [DBNH][Pry], [DBNH][Im], [DBUH][Pry], [DBUH][Im] are promising absorbents for simultaneous removal of H2S and CO2 considering their low H2S/CO2 selectivities.

Figure 14

Figure 14. (a) Viscosities of functionalized ILs at 35 or 40 °C and (b) H2S/CO2 selectivities of functionalized ILs at 40 °C and 1 bar (H2S/CO2 selectivity is equal to nH2S/nCO2, where nH2S and nCO2 are the molar absorption capacity of H2S and CO2 in ILs. Viscosity and selectivity constant data taken from refs (24,70,73−79,and194)).

The viscosity data of most DESs used for H2S 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 of DESs. Therefore, finding a way to use high viscous DESs is also important since some of them have excellent H2S removal performance. Since ChCl/urea shows good biodegradability, and mixing DESs with low viscous solvents is a simple and feasible method to reduce viscosity, aqueous ChCl/urea should be a promising candidate for H2S removal at high pressure. It is reported that the viscosity of pure ChCl/urea (1:2) is 1571 mPa·S at 25 °C, which reduces to 45.06 mPa·S when 36.47% (molar fraction) water exists in ChCl/urea (1:2). (195)

6.2. Thermal Stability

Thermal stability of ILs/DESs is another important factor for deciding their suitability for gas separation processes. The decomposition temperatures of [Emim][Tf2N], [Bmim][Tf2N], [Hmim][Tf2N], [Emim][OTf], [Bmim][OTf] are up to 400 °C, (196−198) and [C4Py][SCN] is 220 °C. (69) The decomposition temperature of ChCl/urea (1:2) is 172 °C. (199) The high stability of these physical ILs and ChCl/urea (1:2) verify that the selected ILs and DES can be used for H2S removal without decomposition during the regeneration. For functionalized ILs, [DBNH][Pry], [DBNH][Im], [DBUH][Pry], [DBUH][Im], and [DBNH][1,2,4-Triaz] are unsuitable for industrial H2S removal due to their low decomposition temperatures (<80 °C). (78,79) The decomposition temperatures of [DMEAH][Ac] and [DMEAH][For] are about 120 °C, (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 H2S 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) compared [Emim][Tf2N] with the reference Rectisol process for H2S removal, and it was found that the direct capital cost of [Emim][Tf2N] is four times higher than the Rectisol process due to the high price of IL. However, ILs as green solvent can be easily recycled, for example, the recovery ratio of [Emim][Tf2N] can be as high as 99.91%, (21) so its advantages are obvious for long-term applications. In fact, the complicated synthetic route is one of the main reasons for the high price of ILs, and simplifying the synthetic routes could reduce cost. [DMEAH][Ac] and [DMEAH][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 H2S 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 H2S

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7.1. Recovery of Sulfur and Hydrogen

Sulfur is one of the important chemicals for sulfuric acid (H2SO4) production in the industry. Therefore, the recovery of sulfur from H2S-contained 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, H2S is oxidized to SO2 in the furnace at 1000–1200 °C; and then elemental sulfur is produced from the reaction of H2S and SO2 in the presence of catalysts at 200–350 °C. (200) For this process, reasonable efficient conversion of H2S 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.
(1)
(2)
It has been found that ILs can be highly appropriate solvents for the absorption of H2S. This fact promotes the investigation of ILs as alternative media for the capture and oxidation of H2S. Huang et al. (201) reported an IL-mediated Claus reaction under mild conditions without the addition of any catalysts. Among the ILs with different cations and anions, [Hmim][Cl] was found to be the most effective reaction media for the oxidation of H2S, in which the oxidation of H2S to S8 was almost complete within 3 min with a high conversion ratio of 96.4% at 40 °C, which is quite higher than that in the organic solvent–diglycol monomethyl ether (81.0%) under the same condition. The desirable performance of ILs as reaction media was attributed to the strong affinity of ILs with H2S and SO2.
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 H2S to sulfur. It was found that the removal efficiency of H2S was up to 99% without a side reaction. The reaction mechanism is as follows:
(3)
(4)
Later, Wang et al. (203) demonstrated that 1-butyl-3-methylimidazolium tetrachloroferrate ([Bmim]Fe(III)Cl4) can be recycled without obvious loss of H2S oxidative-absorption capability. The same group demonstrated that the acidity and Fe(III) concentration of Fe(III)-ILs have significant influence on the oxidation of H2S to S8 and the reduction of [Bmim]Fe(III)Cl4 to [Bmim]Fe(II)Cl4H by varying molar ratios of [Bmim][Cl] and FeCl3·6H2O. (32) Meanwhile, in the presence of water, the oxidative-absorption of H2S increased, and the emission of volatile HCl decreased with the increase of water content in the range of 5.88–80%. (204) However, high content of water could lead to the decomposition of the organic cations of ILs due to the reaction of water with Fe2+ and O2, (205) and thus only low water content in Fe-based ILs is acceptable. Li et al. (31) explored the absorption and oxidation of H2S in the ILs of triethylamine hydrochloride·ferric chloride (Et3NHCl·FeCl3) with different molar ratios of Et3NHCl and FeCl3. The IL of 1.5 Et3NHCl·FeCl3 showed the highest sulfur capacity of 21.78 mg-H2S/g-IL at 40 °C and 1 bar, with a relatively high oxidation efficiency of 87.9%. Moreover, this IL could be reused five times without any obvious loss of sulfur capacity. All the above studies have shown that the oxidation of H2S in Fe(III)-ILs has high removal and oxidation efficiencies, high regeneration capacity, and no side reaction.
To overcome the high cost of ILs, the relatively low cost metal-based DESs have also been synthesized for H2S conversion. Liu et al. (206) synthesized a Fe-based DES 4Fe/ChCl using FeCl3·6H2O and ChCl with the molar ratio of 4:1 for H2S removal and sulfur recovery. Results showed that low temperature was beneficial to the oxidation of H2S, and above 98% H2S could be removed within 2 h at 25 °C. Besides, the existence of a moderate amount of water could promote the absorption reaction of H2S 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 % FeCl3 into ChCl/EG, and then MEA was added with a molar ratio to FeCl3 of 5:1. (207) The desirable performance with H2S removal efficiency of 100% within 80 min at 30 °C was observed, due to the synergistic effect of Fe3+ and MEA by forming a complex compound. As mentioned in section 3.4, Cu2S was obtained during the adsorption of H2S in supported DESs composed of TAECuCl3 and fumed silica. Actually, Cu2S could be further transferred to sulfur and CuSO4 during the regeneration process, and thus can recover elemental sulfur. (101)
The above studies verify that Fe(III)-based ILs and metal-based DESs have great potential for sulfur recovery. However, their regeneration rate by air or O2 is very slow due to the low solubility of O2 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 H2S.
The thermal decomposition of H2S at high temperature (>1000 °C) is the most direct way for H2S 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 H2S 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 H2S 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 H2S conversion process. At present, electrolysis has been used for hydrogen and sulfur recovery from H2S by direct and indirect routes. Direct transformation of H2S 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 commercial-scale research has been reported for H2S conversion to produce hydrogen and sulfur based on electrolysis. Inspired by the successful application of ILs in enhancing the performance of sulfur recovery from H2S and the great potential of electrolysis, Guo et al., (210) established a new nonaqueous desulfurization system (Fe(III/II)–IL/DMF), in which the controlled-potential 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 H2S electrolysis at 50 °C in the IL-TGDE-MEA electrolyte using hydroxy-functionalized IL [C3OHmim][BF4] as the supporting electrolyte, tetraethylene glycol dimethyl ether (TGDE) as the solvent, and MEA as the absorbent of H2S. 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 H2S 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 H2S, even if only lab-scale tests are performed.

7.2. Synthesis of Value-Added Chemicals

Conversion of H2S to value-added chemicals is another feasible way to use H2S. Mishra et al. (6) used the IL of trihexyltetradecylphosphonium chloride (THTDPC) as the catalyst for the utilization of H2S to synthesize bis(2-phenylethyl) sulfide (PES). More specifically, H2S was first absorbed into aqueous MDEA, which was followed by the reaction of H2S-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/m3 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 H2S 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][Tf2N] showed the highest catalytic performance for the reaction of H2S and α-methylacrylic acid to produce 2-methyl-3-sulfanylpropanoic acid with a high α-methylacrylic 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 attractive for H2S conversion to highly valuable products. Xiong et al. (213) developed a novel mild method for H2S conversion to produce mercaptan alcohol with tertiary amine-functionalized protic ILs serving as both solvents and catalysts. [BDMAEEH][MeOAc] showed the highest conversion of 99% with the highest selectivity of 74%. These methods will open new opportunities for the utilization of H2S.

8. Conclusions and Future Prospects

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In this work, recent developments of conventional chemical absorption and adsorption, including their challenges, are reviewed. Fundamental research on H2S 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 H2S solubility are summarized to analyze the interaction mechanism between H2S and IL/DES, and to screen suitable ILs with high H2S 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, H2S removal capacity, thermal stability, and cost. Finally, the application of ILs and DESs in the conversion of H2S 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.

Figure 15

Figure 15. Challenges and future prospects for H2S removal and utilization in ILs/DESs.

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 H2S 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 H2S removal, current work with ILs/DESs is still relatively new, and lots of effects have not been deeply studied and understood. Studies on H2S absorption using ILs/DESs in mixed gases are scarce and need to be complemented, to provide fundamental knowledge for conducting the practical processes, since H2S 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–H2S 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 H2S 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 H2S 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 H2S 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 accurate force field models for molecular simulations from quantum chemistry calculations. This has not been done yet in the context of H2S removal but will most likely be seen in the near future.
Fourthly, although a large amount of experimental studies on the absorption of H2S using ILs and DESs have been reported, pilot scale research related to H2S 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 H2S utilization technologies in ILs/DESs-systems is an efficient way to reuse H2S. However, only a limited number of ILs and DESs have been used for the conversion of H2S. 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 H2S conversion. Additionally, the evaluation of techno-economic-environmental impacts for H2S utilization processes should also be conducted to judge their industrial feasibility in ILs/DESs-based systems.

Author Information

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  • Corresponding Authors
    • Xiangping Zhang - CAS Key Laboratory of Green Process and Engineering, Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, ChinaSchool of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, ChinaOrcidhttps://orcid.org/0000-0002-1431-0873 Email: [email protected]
    • Xiaoyan Ji - Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, Sweden Email: [email protected]
  • Authors
    • Fangfang Li - Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, SwedenOrcidhttps://orcid.org/0000-0001-9841-8285
    • Aatto Laaksonen - Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, SwedenDivision of Physical Chemistry, Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm 10691, SwedenCenter of Advanced Research in Bionanoconjugates and Biopolymers, “Petru Poni”Institute of Macromolecular Chemistry, Iasi 700469, RomaniaState Key Laboratory of Materials-Oriented and Chemical Engineering, Nanjing Tech University, Nanjing 211816, ChinaOrcidhttps://orcid.org/0000-0001-9783-4535
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Swedish Energy Agency (P47500-1), the Swedish Research Council, and partial support from a grant from Ministry of Research and Innovation of Romania (CNCS-UEFISCDI, Project No. PN-III-P4-ID-PCCF-2016-0050, within PNCDI III).

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