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J. Phys. Chem. B 2007, 111, 13014-13019
Influence of the Interaction between Hydrogen Sulfide and Ionic Liquids on Solubility: Experimental and Theoretical Investigation Christian Silvio Pomelli,*,† Cinzia Chiappe,‡ Ana Vidis,§ Ga´ bor Laurenczy,§ and Paul J. Dyson*,§ Institut des Sciences et Inge´ nierie Chimiques, Ecole Polytechnique Fe´ de´ rale de Lausanne (EPFL), CH-1015 Lausanne, Swizerland, and Dipartimento di Chimica Bioorganica e Biofarmacia and Dipartimento di Chimica e Chimica Industriale, UniVersita` di Pisa, 56126 Pisa, Italy ReceiVed: August 1, 2007; In Final Form: September 7, 2007
The solubility of H2S in a series of 1-butyl-3-methylimidazolium ([bmim]+) based ionic liquids (ILs) with different anions, chloride, tetrafluoroborate ([BF4]-), hexafluorophosphate ([PF6]-), triflate ([TfO]-), and bis(trifluoromethyl)sulfonylimide ([Tf2N]-), and in a series of [Tf2N] ILs with different cations, i.e., N-alkylN′-methylimidazolium, 2-methyl-N-methyl-N′-alkyimidazolium, N-alkylpyridinium, N-butyl-N-methylpyrrolidinium, and N-alkyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium has been determined using medium-pressure NMR spectroscopy. The observed solubilities are significantly higher than those reported for many other gases in ILs, suggesting the occurrence of specific interactions between H2S and the examined ILs. Quantum chemical calculations have been used to investigate at a molecular level the interaction between H2S and the [bmim]+-based ILs.
Introduction
CHART 1
Hydrogen sulfide (H2S) is produced along with methane in many natural gas fields, as well as from hydrodesulfurization of crude oils containing sulfur compounds (e.g., thiophene, benzothiophene, and dibenzothiophene).1 A variety of discrete sources for H2S in petroleum have been identified including bacterial reduction of sulfate to H2S (BSR), thermal decomposition of sulfides in kerogen and/or oil, and thermochemical reduction of sulfate to H2S (TSR).2 Separation of H2S from petroleum refinery and coal gasification process streams represents a continuing challenge for the petroleum refining industry, which generally uses traditional chemical engineering separation technologies including distillation, adsorption, membrane processes, absorption and stripping, and extraction. Recently, three patents have been published suggesting the possibility of using ionic liquids, directly or after being loaded onto a porous solid, for the removal of H2S from natural gas feedstreams or for fluid storage.3 Ionic liquids (ILs), salts having melting points near or below room temperature, have attracted considerable attention as new reaction media in the past decade or so.4 In particular, their ionic nature induces a very low volatility, which makes them suitable not only for use as solvents, but also for gas separation.5 The nonvolatile nature of ionic liquids plays two important roles in this latter application. First, cross-contamination of the gas stream by the solvent during operation may be avoided; consequently, solvent is not lost and there is no ensuing air pollution. Second, regeneration of the solvent is easy. A simple flash or mild distillation step is all that is required to remove the gas from the solvent, again without cross-contamination. Furthermore, most ILs are chemically and physically stable, in * To whom correspondence should be addressed. E-mail: paul.dyson@ epfl.ch (P.J.D.);
[email protected] (C.S.P.). † Dipartimento di Chimica e Chimica Industriale, Universita ` di Pisa. ‡ Dipartimento di Chimica Bioorganica e Biofarmacia, Universita ` di Pisa. § EPFL.
general over a wider range than volatile organic solvents and under a wide range of operating conditions. The solubility of organic and inorganic compounds (solid, liquid, or gas) strongly depends on the chemical nature of the solute and on the IL structure. Simple aliphatic compounds are generally only sparingly soluble in the common ILs, whereas more polar compounds show somewhat greater solubility. Carbon dioxide shows exceptional solubility in several ILs,6 carbon monoxide is less soluble in many ILs than in many common organic solvents,7 and hydrogen is only slightly soluble.8,9 Knowledge of the solubilities and the diffusivities of gases in ILs is needed to design ionic liquid based separation processes. Even though there are many recent papers concerning gas solubilities in ionic liquids,10 to the best of our knowledge, no data for the solubility of H2S in ILs have been reported. Furthermore, as gas solubilities in ionic liquids are governed by the interactions between the gas molecules and the solvent molecules (hydrogen-bonding, dipole-dipole, and dipoleinduced dipole interactions and dispersive forces), experimental and/or theoretical data on the nature of the interaction of H2S with IL components may be useful to determine the best IL for H2S extraction.
10.1021/jp076129d CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007
Interaction between H2S and Ionic Liquids
J. Phys. Chem. B, Vol. 111, No. 45, 2007 13015
Figure 1. Representative 1H NMR spectra of emim[Tf2N] before (black) and after (red) saturation with H2S at 1400 kPa. The signal corresponding to H2S is indicated. Inset: measured (blue) and calculated (red) 1H NMR spectra of emim[Tf2N] fitted using the Matlab program. The H2S signal was integrated versus the CH3 signal of the alkyl chain of ionic liquid cation.
TABLE 1: Viscosity and Density of the ILs Used in the Investigation Determined at 25 °C ionic liquid bmim[PF6] bmim[BF4] bmim[BF4] + 5% H2O bmimCl bmim[Tf2N] bmim[TfO] emim[Tf2N] emmim[Tf2N] a
viscosity (cP) 450 219 69 90 35
density (g cm-3) 1.36 1.26 1.10 (1.04)a 1.43 1.29 1.54 1.51
ionic liquid 4mbnpy[Tf2N] bnpy[Tf2N] bmim[Tf2N] bpy[Tf2N] mbpy[Tf2N] bmmim[Tf2N] eetamine[Tf2N] betamine[Tf2N]
viscosity (cP)
69 9.9 48
density (g cm-3) 1.51 1.52 1.43 1.44 1.41 1.42 1.44 1.45
At 85 °C.
We have investigated H2S solubilities in different 1-butyl3-methylimidazolium ([bmim]+) based ILs and in a series of bis(trifluoromethyl)sulfonylimide ([Tf2N]-) based ILs (see Chart 1). In addition, quantum chemical calculations (QM) have been performed to investigate the interaction between H2S and the employed ILs based on the imidazolium cation with the anions Cl-, tetrafluoroborate ([BF4]-), hexafluorophosphate ([PF6]-), triflate ([TfO]-), and [Tf2N]-. Along with the energetic aspects of the interactions, structural features have been computed. Experimental Details Materials. High-purity H2S (purity 99.7%) was obtained from Carbagas. The ionic liquids N-ethyl-N′-methylimidazolium bis(trifluoromethylsulfonyl)imide (emim[Tf2N]), 2-methyl-Nmethyl-N′-ethylimidazolium bis(trifluoromethylsulfonyl)imide (emmim[Tf2N]), 2-methyl-N-methyl-N′-butylimidazolium bis(trifluoromethylsulfonyl)imide (bmmim[Tf2N]), N-butyl-N′methylimidazolium bis(trifluoromethylsulfonyl)imide (bmim[Tf2N]), N-butylpyridinium bis(trifluoromethylsulfonyl)imide (bpy[Tf2N]), N-benzylpyridinium bis(trifluoromethylsulfonyl)imide (bz[Tf2N]), N-butyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide (4mbzpy[Tf2N]), N-benzyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (mbpy[Tf2N]), N-ethyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (eetamine[Tf2N]), N-butyl-N,N-
dimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (betamine[Tf2N]), N-butyl-N′-methylimidazolium tetrafluoroborate (bmim[BF4]), N-butyl-N′-methylimidazolium hexafluorophosphate (bmim[PF6]), and N-butyl-N′-methylimidazolium chloride (bmimCl) were synthesized using established procedures.11 Table 1 lists the ILs used in this study, along with two relevant properties: viscosity and density. Generally, before each experiment ILs were dried and degassed under vacuum for at least 12 h at 80 °C to remove water. NMR spectra were measured on a Bruker DRX 400 MHz spectrometer. NMR data were analyzed using WinNMR 6.1 (Bruker). IR spectra were recorded on a Perkin-Elmer FT-IR 2000 system. Electrospray ionization mass spectra were recorded on a ThermoFinnigan LCQTM Deca XP Plus quadrupole ion trap instrument on samples diluted in acetonitrile as described previously.12 Measurement Details. H2S solubility was determined by NMR spectroscopy under a pressure of 1400 kPa of H2S using a sapphire NMR tube. A sample of IL was added to a sapphire NMR tube and pressurized with H2S (1400 kPa) several times; between each pressurization the samples were shaken for 24 h, until no change in the H2S signal was observed. NMR spectra (1H, 13C, 31P, 19F, 11B) were recorded before and after addition of H2S. A shift due to the H2S addition was observed for the 1H NMR spectra, but no significant changes were observed for the 13C, 31P, 19F, and 11B NMR spectra (∆ < 1 ppm); typical
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TABLE 2: Solubility of H2S in [bmim]+-Based ILs at 25 °C and 1400 kPa solubility solvent
(M)
bmimCl 34.2 (37.8)a bmim[BF4] 17.9 bmim[BF4] + 5% H2O 19.2 bmim[TfO] 15.6 bmim[PF6] 12.7 bmim[Tf2N] 11.8 MeOH 101 thf 92.1 a
(mole fraction)
V increase (%)
0.86 (0.87)a ( 0.03 0.79 ( 0.02 0.79 ( 0.02 0.78 ( 0.02 0.72 ( 0.02 0.77 ( 0.03 0.80 ( 0.02 0.96 ( 0.04
59 58 62 65 65 250 300
Measured at 85 °C.
TABLE 3: Solubility of H2S in [Tf2N]--Based ILs at 25 °C and 1400 kPa solubility cation
(M)
(mole fraction)
V increase (%)
emim emmim 4mbnpy bnpy bmim bpy mbpy bmmim eetamine betamine
18.2 17.8 17.5 17.4 11.8 11.6 11.3 11.2 10.1 9.6
0.81 ( 0.02 0.82 ( 0.03 0.85 ( 0.03 0.84 ( 0.03 0.77 ( 0.03 0.89 ( 0.03 0.90 ( 0.03 0.79 ( 0.02 0.74 ( 0.02 0.73 ( 0.02
71 65 68 65 65 60 60 62 45 40
1H
NMR spectra are shown in Figure 1. The H2S solubility was determined from the 1H NMR spectra which were fitted with the Matlab program (Figure 1, inset). In all the examined ionic liquids the solubility measurements were carried out at 25 °C; bmim[Cl] was also measured at 85 °C (although due to the high solubility of H2S bmim[Cl] is a liquid at 25 °C). In addition, the solubility of H2S in bmim[BF4] containing 5% water was determined. Electrospray ionization mass spectrometry (ESI-MS) was used to characterize the ILs before and after addition of H2S. In all cases strong peaks indicative of the parent ions were observed (indicating that the ILs do not react with the H2S). Aggregates composed of the anions, cations, and H2S were also observed in some cases, as noted elsewhere. IR spectra for each ionic liquid before and after addition (and subsequent release) of H2S were also recorded. Computational Details. Quantum chemical calculations were performed using version 03 of the Gaussian package.13 The geometries were optimized at both the B3LYP and the MP2 levels of theory using the CEP-121G(d,p) basis set. For three systems calculations using the 6-311++G** basis set were performed. The results obtained from these systems confirmed the trend found using the time less expensive CEP-121G(d,p) basis set. Results and Discussion Tables 2 and 3 list the H2S solubilities at 1400 kPa for the ILs tested in this study and two molecular solvents for comparison. For the [bmim]+-based ILs (Table 2) the solubility (expressed as a molar fraction) decreases in the order Cl- > [BF4]- > [TfO]- > [Tf2N]- . [PF6]-. It is moderately affected by the temperature and practically unaffected by the presence of water (the addition of 5% water to [bmim][BF4] increases the molar concentration of H2S, whereas the molar fraction remains practically unchanged). The nature of the cation has a
Figure 2. Minimum energy structures found for the H2S-Acomplexes: (a) H2S-Cl-; (b) H2S-[BF4]-; (c) H2S-[PF6]-; (d) H2S[Tf2N]-; (e) H2S-[TfO]-. Color code: green, chloride; yellow, sulfur; hell gray, hydrogen; light blue, fluoride; pink, boron; orange, phosphorus; red, oxygen; gray, carbon.
moderate effect on solubility, as evidenced by the values reported in Table 3 for the [Tf2N]--based ILs. The solubilities (expressed as molar fractions) range from 0.72 to 0.9, with the lower values for the two N-alkyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium salts. In all cases, the solubilities are significantly higher than those reported for most of the previously examined gases and even higher than that of CO2.6 Comparable solubilities have been recently reported for ammonia.14 After the submission of this paper, a paper reporting the solubility of H2S in [bmim][PF6] was published.15 It is noteworthy that the solubility value at the same temperature and comparable pressure was very similar to that found in this investigation, further validating our experimental approach and the reproducibility of the data. The high solubility of H2S in these ionic media suggests the possibility of specific interactions between this gas and the ILs. In addition, the occurrence of irreversible reactions between H2S and ILs can be excluded on the basis of the ESIMS, IR, and NMR spectroscopic data, which showed that the ionic liquids remain unchanged following release of the H2S. Attempts to correlate the solubility of H2S in these media with some solvent parameters, such as the Kamlett-Taft parameters,16 were not possible. A very poor correlation was obtained by plotting the solubility (expressed as a molarity or molar fraction) with respect to the H-bond basicity (β) or H-bond basicity and H-bond acidity (R). To obtain an understanding of the interactions between H2S and the ILs at a molecular level, quantum chemical calculations have been performed. The geometries were optimized at both the B3LYP and the MP2 levels of theory using the CEP-121G(d,p) basis set. Several kinds of complexes could be hypothesized. However, the acidic nature of the gas suggested that its solubility in ILs might be determined primarily by the interaction with the anions. This hypothesis was further supported by previous theoretical studies17 investigating the interaction
Interaction between H2S and Ionic Liquids
J. Phys. Chem. B, Vol. 111, No. 45, 2007 13017
TABLE 4: Energetic and Geometric Data on H2S-Anion- Complexesa anion ClBF4PF6Tf2NTfOH2S a
method/ CEP-121g(d,p) B3LYP MP2 B3LYP MP2 B3LYP MP2 B3LYP MP2 B3LYP MP2 B3LYP MP2
H2S-A- complex E0 + ZPE
anion E0 + ZPE
-26.349 035 -26.132 430 -111.217 152 -110.496 144 -163.272 573 -162.222 980 -261.822 215 -260.131 263 -52.373 736 -52.004 454
-14.987 285 -14.877 122 -99.869 045 -99.249 927 -151.927 751 -150.979 177 -250.479 062 -248.886 719 -41.024 320 -40.758 063 -11.335 027 -11.232 421
∆E0 + ZPE
R1
R2
-16.776 -14.386 -8.211 -8.661 -6.149 -7.145 -5.112 -7.610 -9.033 -8.770
1.956 2.019 1.978 2.058 2.103 2.165 2.217 2.336 1.901 1.969
2.563 2.439 2.634 2.922 2.868 2.396
Absolute energies in hartress, relative energies in kilocalories per mole, and distances in angstroms.
between water and ionic liquids. Therefore, the molecular systems initially taken into account were the 1:1 complexes between H2S and Cl-, BF4-, PF6-, Tf2N-, and TfO- anions, defined here as H2S-A-. Subsequently, calculations on the complexes between H2S and the [bmim][A] ion pairs ([A]- ) Cl-, BF4-, PF6-) have also been performed. These latter calculations were performed at the B3LYP level, exclusively. The 1:2 complexes H2S-2A- were discarded due to the high solubility of H2S in ionic liquids. Moreover, the instability of these complexes was evidenced also in the case of H2O.17 Different 1:1 complexes between H2S and the above-mentioned anions were constructed and preoptimized at the B3LYP level. Minimum structures were fully optimized at the MP2 levels, giving the final structures reported in Figure 2. Energetic and geometrical data about the H2S-A- complexes are summarized in Table 4. The R1 value (Å) refers to the minimum distance between a H2S hydrogen and the closest atoms of the neighboring anion, whereas the R2 value refers to the comparable distance with the other H2S hydrogen. R2 is not applicable in the case of the Cl- anion since the second H2S hydrogen is not involved in any apparent interaction with the anion. Fully optimized geometries and normal modes are provided in the Supporting Information. The optimized structures of the H2S-A- complexes revealed three types of spatial arrangements of the constituent molecules. In H2S-Cl- the three atoms S-H-Cl are nearly aligned (the angle is greater than 173°). For the spherical fluorinated anions BF4- and PF6- both protons of H2S interact simultaneously with two fluorine atoms of the same anion. However, neither of these complexes shows the expected C2V symmetry previously found for the analogous complex between water and BF4-.17 In the complexes of H2S, the two H-F distances differ by about 0.5 Å (actually 0.8 Å for PF6- at the MP2 level). The H2S-BF4- complex has Cs symmetry, whereas the complex with PF6- is completely asymmetric. The calculated wavenumbers (cm-1) of the vibrational modes of H2S in theH2S-BF4- and H2S-PF6- complexes at the B3LYP/CEP-121G(d,p) level are reported in Table 5. From the data listed in Table 5 it can be seen that both the complexes present a very low frequency scissoring-like normal mode (F-B-S) that is probably related to the permutation of the fluorine atom positions. Finally, the complexes with TfOand Tf2N- anions are characterized by a chelated structure.18 It is noteworthy that in the case of H2S-TF2N- there is a slight difference between the B3LYP and MP2 structures. However, both complexes exhibit the scissoring-like normal mode, which is related to the permutation of the oxygen atom positions (see the Supporting Information). In all the complexes examined both H2S and the anions show very small distorsions arising from complexation. The largest
TABLE 5: Distances (R1, R2, Å) and Frequencies (ω, cm-1) of the H2S-Ion Pair Complexesa anion ClBF4PF6-
R1 ω R1 R2 ω R1 R2 ω
H2S-ion pair complex
H2S-Acomplex
2.185 163.31 2.109 3.104 102.88 2.166 3.440 90.79
1.956 181.49 1.978 2.563 114.83 2.103 2.634 110.12
a Calculations were performed at the B3LYP/CEP-121G(d,p) level with full geometry optimization.
difference in the observed H-S-H angle in the H2S complex with respect to the free molecule is 0.12°, found in the H2SCl- complex at both levels of theory. With the BF4- and PF6anions, the variation in the F-B-F angles is less than 1°, whereas F-P-F changes by less than 0.5°. Finally, the lengths of the H-S, B-F, and P-F bonds directly involved in interactions within the complex are elongated by less than 0.1 Å. Since it is known that the [bmim]+ cation is able to interact with its counteranion via C2-H, C4-H, and C5-H, and that these interactions might affect the ability of the anions to interact with H2S, a series of stable ion pairs of [bmim]Cl, [bmim][BF4], and [bmim][PF6] were optimized. The optimized ion pairs were subsequently used to investigate the interaction of the ionic liquid anion with H2S. The structures and normal modes of the ion pairs (Table 5 and Figure 3) show that the H2S interaction with the anion is virtually unaffected by the presence of the cation: the bond distances are similar, the frequency variation being due to the differences in reduced mass of the ion pair with respect to the free anion. Moreover, the relative position of the anion and cation in the H2S-ion pair complexes is very similar to that observed in the related ion pairs in the absence of H2S. The interaction energies listed in Table 4 are in the range 7-14 kcal/mol at the B3LYP and the MP2 levels, indicating that these interactions are as strong as the traditional hydrogen bond. The strength of the interaction decreases on going from Cl- to PF6-, with all the other anions representing an intermediate situation. The order based on the energies estimated at the MP2/121g(d,p) level (Cl- > [TfO]- > [BF4]- > [Tf2N]- > [PF6]-) is qualitatively in agreement with that obtained from solubility data. It is noteworthy that, in contrast to water, which has been reported15 to be unable to give a stable complex with PF6- at both the B3LYP/6-31G* and MP2/6-31G* levels, the
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Figure 3. Energy-minimized structures of H2S-ion pair complexes. Color code: green, chloride, yellow, sulfur, light gray, hydrogen; light blue, fluoride; pink, boron; orange, phosphorus; red, oxygen; dark gray, carbon.
TABLE 6: Stabilization Energies and Some Vibrational Frequencies for (H2S)N-Anion Complexesa anion
N
∆(E0 + ZPE)/N
ω(H-S)
ω(H-S) scaled
none Cl-
0 1 2 8 exptl 1 4 exptl
0 -14.386 -12.570 -8.816
2778, 2805 1990, 2781 2288, 2365, 2785, 2787 2666, 2671, 2681, 2684, 2696c
2615, 2626b 1868, 2611 2148, 2220, 2614, 2616 2512, 2526, 2528, 2540 1918, 2324, 2454, 2590 2563, 2628 2596, 2597, 2626 2588
BF4-
-8.661 -6.734
2730, 2799 2756, 2757, 2787c
a Computational vibrational frequencies were scaled according to an empirical factor and compared with some experimental IR bands. Frequencies in inverse centimeters. Energies in kilocalories per mole. b From CRC. c Some low-intensity modes were omitted.
present calculations indicate a greater ability of the more acidic H2S molecule to interact with this anion. The high values of the maximum concentration of H2S in ILs lead to the hypothesis that (H2S)NX complexes, where X is one of the IL anions, exist in the solution. Computational evidence to support this hypothesis includes the small frequency normal modes discussed above, which indicate the presence of empty coordination positions in which the ligand can move. In this way the H2S molecule can change its position and orientation with respect to the anion, while retaining a strong interaction with the anion. Thus, it is necessary to consider motions such as “switching” modes, and the structures of these complexes are given in the Supporting Information. The optimized geometries and normal modes for N ) 2 and 8 with Cl- and N ) 1 and 4 with BF4- complexes were calculated. A tentative optimization of an N ) 12 complex with Cl- led to the expulsion of four H2S molecules to the second coordination shell. For these calculations, we have considered exclusively the two smaller anions because they have a larger bond strength. All the geometries were optimized at the MP2/ CEP-121G(d,p) level. The stabilization energies, corrected for the ZPE, and the stretching frequencies of the H-S bond, are reported in Table 6 together with the IR spectra that appear after solvation of H2S. The scissoring frequencies overlap too much with the solvent absorptions to be considered. Since scaling of frequencies is a common practice in theoreticalexperimental comparisons of IR spectra, Table 6 also reports the calculated frequencies scaled by an empirical factor of 0.9387. This factor was obtained by confrontation of the computational and experimental frequencies for free H2S [0.9387 ) (2615 + 2626)/(2778 + 2805)]. We used this approximate method because in the literature a scaling factor for MP2/CEP121G(d,p) is not available; however, the scaling factor for MP2/ 6-311G(d,p) is 0.9496.19 The comparison between experimental and computational data has to be considered with some caution. From an experimental point of view, these switching modes give rise to small bands overlapped with a considerable amount of solvent noise. From
the computational point of view, these clusters present a lot of local minima, and a molecular dynamics based approach would be methodologically more correct, although beyond the scope of this paper. However, it is worth noting that the scaled computed frequencies for (H2S)8Cl- and (H2S)4BF4- resemble the experimental values, giving further support to the presence of complexes with high coordination numbers in solution. Conclusions The behavior of H2S in several ILs (solubility, ability to give specific interactions) has been investigated for the first time. Extremely high solubilities in ILs have been observed, and H2S is stable with no sign of any reaction with the ILs under the investigated experimental conditions. Ab initio calculations provide understanding at a molecular level of the observed high solubility of H2S in ionic liquids. The H2S molecules strongly interact with Cl-, [TfO]-, [BF4]-, and [Tf2N]- anions. At variance with water, calculations indicate that H2S is able to interact also with the PF6- anion, although the stability of the H2S-PF6- complex is lower than those of the other investigated H2S-A- complexes. The interaction energies of these complexes are in the range of 7-14 kcal/mol at the B3LYP and the MP2 levels, indicating that these interactions are comparable in strength to traditional hydrogen bonds. The relationship between H2S and the IL anions is related to an incomplete solvation shell. The 1:1 complexes can be considered as fragments of the complete solvation shell that are probably present in liquid H2S. Although the gas-phase calculations may be different from those that occur in the liquid and solid states, the ab initio results reported here (or eventually calculations at a higher level of theory) may be valuable to refine the parameters for the H2Sanion interaction and to investigate the interaction of H2S with bulk ionic liquids by classical molecular dynamics. These studies are in progress and will be reported in the near future. Finally, from an applicative point of view, the high solubility of H2S in ILs provides a significant opportunity for removal of
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