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Theoretical and experimental investigation on the capture of H2S in a series of ionic liquids
Journal of Molecular Graphics and Modelling 68 (2016) 87–94
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Theoretical and experimental investigation on the capture of H2 S in a series of ionic liquids Xinming Zhou a , Bobo Cao b , Shuangyue Liu b , Xuejun Sun b,∗ , Xiao Zhu b,∗ , Hui Fu a,∗ a b
College of Science, China University of Petroleum, Shandong, Qingdao 266580, China School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China
a r t i c l e
i n f o
Article history: Received 25 October 2015 Received in revised form 20 June 2016 Accepted 20 June 2016 Available online 21 June 2016 Keywords: Absorption H2 S DFT Ionic liquid Hydrogen bond
a b s t r a c t H2 S absorptions in ionic liquids (ILs), including tetramethyl guanidinelactate (TMGL), 4-bis(2hydroxypropyl)-1,1,3,3-tetramethyl guanidinium tetrafluoroborate ([TMGHPO2 ][BF4 ]) and 1-butyl-3methylimidazolium cation ([BMIM]+ ) with the anions chloride ([Cl]− ), tetrafluoroborate ([BF4 ]− ), hexafluorophosphate ([PF6 ]− ), triflate ([TfO]− ), bis-(trifluoromethyl) sulfonylimide ([Tf2 N]− ), were studied in experiment and computational methods. [TMGHPO2 ][BF4 ] showed the best H2 S absorption capacity among the seven ILs. Density functional theory (DFT) calculations were applied to reveal the absorption mechanisms. Interaction energy results were consistent with absorptivities (molar ratio of H2 S in IL) measured in experiment. As the best candidate absorbent, [TMGHPO2 ][BF4 ] was chosen as an example to characterize the hydrogen bonds and orbital interactions between H2 S and [TMGHPO2 ][BF4 ] in atoms in molecules (AIM) and natural bond orbital (NBO) methods, respectively. IR spectrums obtained in both experimental and computational method were used to characterize the features of absorption process. The results indicated that H2 S was physically absorbed by ILs, in which hydrogen bond was the driving force. © 2016 Elsevier Inc. All rights reserved.
1. Introduction Hydrogen sulfide (H2 S) is usually produced with methane, other synthetic gas and hydrodesulfurization process of crude oil, which is a highly toxic and corrosive gases [1]. Aqueous alkanolamine solutions is a kind of widely used industrial solvent, which can remove H2 S from industrial natural gas, refinery tail gas and synthesis gas. However, in the process, the loss of alkanolamine volatilization of and the water transfer to the vapor will form the corrosive degradation and byproducts which has many disadvantages and makes the process economically expensive [2]. In recent years, ionic liquids (ILs) have been widely used to study CO2 capture [2,3], due to the unique properties, such as the minimal volatile, design properties, thermal stability and liquid form in a wide temperature range [4]. IL is a promising candidate absorbent for the gas separation [5–10]. Besides, ILs were also employed to study harmful gas capture [11–17].
∗ Corresponding authors. E-mail addresses: [email protected] (X. Zhou), [email protected] (B. Cao), [email protected] (S. Liu), [email protected] (X. Sun), qfnu [email protected] (X. Zhu), [email protected] (H. Fu). http://dx.doi.org/10.1016/j.jmgm.2016.06.013 1093-3263/© 2016 Elsevier Inc. All rights reserved.
A few years ago, people began to focus on the absorption of H2 S in ILs [18–22]. Jou and Mather [18] first determined H2 S solubility in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6 ]) under the conditions of temperatures range of 298.15-403.15 K and pressures up to 9.6 MPa. Pomelli et al. [19] reported the solubility of H2 S in [BMIM]-based ILs with different anions. Morsi et al. [20] published the solubility and transfer coefficient of H2 S in ammonium-based ILs. Jaliliet al. [21–26] measured and provided H2 S solubility at various temperature and pressure. Huang [27] given that dual Lewis base fictionalizations of ILs had highly efficient and selective capture of H2 S. Wang [28] reported oxidative absorption of H2 S by iron-containing ILs. In this paper, the absorptions of H2 S in the chosen ILs are studied in both experimental and computational method, including tetramethyl guanidinelactate (TMGL), 4-bis(2-hydroxypropyl)-1,1,3,3tetramethyl guanidinium tetrafluoroborate ([TMGHPO2 ][BF4 ]) and [BMIM] based ILs with different anions, including chloride ([Cl]− ), tetrafluoroborate ([BF4 ]− ), hexafluorophosphate ([PF6 ]− ), triflate ([TfO]− ), bis-(trifluoromethyl) sulfonylimide ([Tf2 N]− ). The hydrogen bonds between H2 S and ILs are investigated by the vibrational frequency analyses (VFA), natural bond orbital (NBO) and atom in molecule (AIM) methods.
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2. Experiments 2.1. Materials High-purity H2 S (purity 99.7%) was purchased from Zibo Anze Standard Gas Co. Ltd. (Zibo, China). The [BMIM][Cl], [BMIM][PF6 ], [BMIM][BF4 ], [BMIM][TfO] and [BMIM][Tf2 N] were purchased from Lanzhou (Lanzhou, China) Center for Green Chemistry with the purity of 99% and water mass fractions less than 10−3 (g/g), respectively. 1,1,3,3-tetramethylguanidine (TMG, 99%), propylene oxide (99%), ethanol (99%), toluene (99.8%), fluoroboric acid (50 wt.% solution in water) were purchased from Air Liquide. The TMGL [12] and [TMGHPO2 ][BF4 ] [13] were synthesized and purified before absorption process. To remove traces of volatile compounds, the ILs were kept in a vacuum (about 10−6 bar) at 343 K for 24 h prior. 2.2. Synthesis of TMGL and [TMGHPO2 ][BF4 ] To synthesize TMGL, TMG dissolved in ethanol was added into a flask in a water bath of 25 ◦ C under mechanical stirring. Then, the solution consisting of lactic acid and ethanol was added into the flask. The mole ratio of TMG and lactic acid was 1:1. The reaction process was allowed to proceed for 2.5 h. After the evaporation of solvent under vacuum, the residue was obtained, which was further treated with active carbon and washed by ethanol. The colorless product was obtained after ethanol was evaporated under vacuum. The solution of TMG in toluene was added into a flask in a water bath of 25 ◦ C under mechanical stirring. Then, propylene oxide was slowly dropped into the flask, in which the mole ratio of TMG and propylene oxide was 1:1. The reaction process was allowed to proceed for 36 h, where after fluoroboric acid was slowly added to the solution in an ice bath under mechanical stirring. The mixture was heated (70 ◦ C) to remove solvent under reduced pressure, and the residue ([TMGHPO][BF4 ]) was dry under high vacuum (0.1 kPa). Then, an additional equivalent propylene oxide was used to react with [TMGHPO][BF4 ] and the future treatments were similar with the steps mentioned above. An colorless product ([TMGHPO2 ][BF4 ]) was obtained. The 1 H-NMR spectrum of TMGL and [TMGHPO2 ][BF4 ] are given Figs. S1 and S2 (Electronic Supporting information) to ensure lack of critical impurities. 2.3. Apparatus and measurements H2 S absorptivity was measured by applying isochoric saturation technique [29]. It is worth mentioning that absorptivity in this work refers to molar ratio of H2 S in IL. The apparatus used for H2 S absorption measurements is schematically represented in Fig. 1. The apparatus was composed of a H2 S cylinder, pressure regulators, magnetic stirrers with heating, oil bath, and glass container filled with IL and off-gas absorption by NaOH solution. The lowpressure gauge had an uncertainty of approximately (0.001 bar) in the experimental pressure range. For a typical experiment, the desired amount of IL was loaded in the glass container (the weight was m1 , about 5 g) using an electronic balance (Sartorius BS224S, uncertainty of 0.001 g), and the air in the system was eliminated by the vacuum pump, Then H2 S was charged into the glass container from the cylinder, and the liquid phase was stirred. The system was considered to have reached equilibrium if the pressure of the system had been unchanged over time for 2 h. Then the pressure of the system was recorded, and the glass container filled with IL was weighed (the weight was m2 ). Thus, the absorption of H2 S in a given IL was determined by the shifted quality of the glass container (m2 -m1 ). To verify the validation of the apparatus, we determined the absorption of CO2 in a chloride and urea mixture with this apparatus, and the results were consistent with ref. [30].
To support the computational results, an attenuated total reflection fourier transform infrared (FT-IR ATR) spectra of 2BP8HQ were registered using Varian FTS1000 FT-IR spectrometer with Diamond/ZnSe prism (4000–525 cm−1 ; number of scans: 250; resolution: 1 cm−1 ) to study the variations of IR spectrum of [TMGHPO2 ][BF4 ] before and after H2 S absorption. 3. Computational methods In recent years, density functional theory (DFT) method has been widely used since it is a cost-effective and reliable method for calculation chemistry. The hybrid Becke 3-Lee-Yang-Parr (B3LYP) exchange-correlation function combined with the 6-311 + +G(d,p) basis set are widely applied in theoretical studies of ionic liquids [16,31]. B3LYP method has been confirmed that it can supply reliable interaction energies, in which electron correlation is well considered. Polarizable functions and diffuse functions are used for all atoms in the 6-311 + +G(d,p) basis set, which is important for the description of hydrogen bonding interactions. The interaction energy (E) considering the basis set superposition errors (BSSE) correction using the counterpoise (CP) method is estimated [32]. The vibrational frequency studies of H2 S, ILs and the complexes have also been performed at the same level. To account for errors due to the neglecting electron correlation and the basis set incompleteness, considering B3LYP systematic errors with a scaling factor of 0.983 up to 1700 cm−1 and 0.958 for greater than 1700 cm−1 [33,34]. What is more, to better understand the nature of the intermolecular H-bonding interactions in the H2 S, different ILs and complexes, natural bond orbital (NBO) and atoms in molecule (AIM) have also been carried out at the same level. All computations are carried out by using Gaussian 09 program package [35]. 4. Results and discussion 4.1. Experimental absorptivity of H2 S Absorptivity of H2 S in the seven ILs is determined by using isochoric saturation technique (Fig. 1), which was depicted in Fig. 2a. Table S1 (Electronic Supporting information) shows the molar ratio of H2 S in seven ILs at 30 ◦ C. It can be seen that [TMGHPO2 ][BF4 ] is the best candidate for H2 S absorption, due to the high molar ratio of 0.502. However, the absorptivity of H2 S in the other six ILs does not exceed 0.38. The absorptivity order of H2 S in seven ILs is: [TMGHPO2 ][BF4 ] > [BMIM][Tf2 N] > [BMIM][TfO] > [BMIM][Cl] > [BMIM][BF4 ] > [BMIM][PF6 ] > TMGL. 4.2. Analysis of geometries and interaction energies In order to gain the possible interaction modes between H2 S and seven kinds of ILs, the electrostatic potential surfaces of them have been calculated at B3LYP/6-311 + +G(d,p) level. As shown in Fig. S3, the favorable sites (more negative charges and red area) for proton attack in anion are concentrated on the regions around chloride, oxygen atoms in carbonyl and fluorine atoms in tetrafluoroborate or hexafluorophosphate. The most possible interaction sites can be observed, such as the regions near the chloride, oxygen atom and fluorine atoms (more positive charges and blue area). It can be seen that these atoms are easy to form hydrogen bonds. For cation, the highly positive regions (deep blue) are mainly concentrated on around C H in [BMIM]+ , hydrogen atom of amine group of TMGL and the hydrogen atoms of hydroxyl group of [TMGHPO2 ]+ . Commonly, the regions with larger positive charge densities are easier to form hydrogen bonds. With the above analysis, the large area of positive charge density in cation and the area of negative charge density are easy to form hydrogen bonds. Thus the reason-
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Fig. 1. Apparatus of absorption of H2 S in ILs: (1) H2 S cylinder; (2) gas cylinder pressure regulator; (3) and (4) valves; (5) volume rotameter; (6) magnetic stirrers with heating; (7) temperature sensor; (8) oil bath; (9) glass container filled with IL; (10) off-gas absorption by NaOH.
able configurations of ILs and H2 S-IL complexes are constructed and optimized to locate the most stable configurations. Fig. 3 shows the optimized configuration of seven kinds of ILs, in which the main hydrogen bonds between cation and anion are depicted. Take [TMGHPO2 ][BF4 ] as an example, the hydrogen bonds are formed by hydrogen atoms in hydroxyl group with fluorine atoms in [BF4 ]− , which can be contributed to the large positive charge density of C H on the imidazole ring. This is consistent with the electrostatic potential energy surface analysis. For details, three hydrogen bonds in [TMGHPO2 ][BF4 ] are O42-H43· · · F44, N31H32· · ·F46 and C4-H6· · ·F48 with the corresponding bond length of 1.74 Å, 2.09 Å and 2.07 Å, respectively. Besides, strong hydrogen bonds are also formed by atoms of hydrogen and oxygen in TMGL. Structures of the complexes formed by H2 S with [BMIM]Cl, [BMIM][BF4 ], [BMIM][PF6 ], [BMIM][TfO], [BMIM][Tf2 N], TMGL, and [TMGHPO2 ][BF4 ] are optimized at B3LYP/6–311++G(d,p) level, which are shown in Fig. 4, respectively. The interaction energy of H2 S with the seven ILs are calculated at B3LYP/6–311++G(d,p) level corrected by using the counterpoise (CP) method, which is shown in Fig. 2b. It can be found that the interaction energy decreases with the order of [TMGHPO2 ][BF4 ] (14.90 kJ/mol) > [BMIM][Tf2 N] (10.24 kJ/mol) > [BMIM][TfO] (9.30 kJ/mol) > [BMIM](8.85 kJ/mol) > [BMIM][BF4 ] (7.95 kJ/mol) > [BMIM][PF6 ] [Cl] (7.06 kJ/mol)> TMGL (5.71 kJ/mol). As we expected, the “trend” is consistent with the experimental observation. Due to the excellent absorptivity and larger interaction energies of [TMGHPO2 ][BF4 ], the complex formed by [TMGHPO2 ][BF4 ] and H2 S is taken as an example to discuss the intermolecular interaction. Four different conformers of the H2 S/[TMGHPO2 ][BF4 ] complex, marked A, B, C and D, respectively, are optimized and shown in Fig. S4, in which
H2 S is located in four different positions of [TMGHPO2 ][BF4 ]. The electronic energies of four configurations are obtained and compared to select the most stable conformer. The values of A–D are −14.86, −14.95, −15.17 and −15.32 kJ/mol, respectively. It can be easily found that conformer D is the most stable conformer. The same way is used to select the most stable conformer of complexes formed by H2 S with the other ILs. As shown in Fig. S4, the hydrogen bond lengths of S49-H50· · ·F48 (2.27 Å) and S49-H51· · ·F44 (2.33 Å) are much short than the sum of their van der Waals radius, which suggests the existence strong hydrogen bonds between H2 S and [TMGHPO2 ][BF4 ]. As we expected, hydrogen bond strength and interaction energy between H2 S and ILs are consistent with the absorptivity of H2 S in the chosen ILs. It is reasonable to believe that hydrogen bonds are the driving force of H2 S absorption in ILs. This result is further demonstrated by the following analysis. 4.3. Vibrational frequency analysis [TMGHPO2 ][BF4 ] and the H2 S/[TMGHPO2 ][BF4 ] complex have 44 and 47 atoms, as well as have 138 and 147 normal vibrational modes, respectively. The harmonic vibrational frequencies are calculated using the B3LYP method at 6-311++G(d,p) level. The infrared spectra data and the corresponding vibrational frequencies and modes of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] are listed in Tables S2 and S5 respectively. The diagrams of the experimental and the theoretical infrared spectra of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] are shown in Fig. 5 and Fig. S3, respectively. Differences between the frequencies obtained using the scaled B3LYP method and that of the experiment are found
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Fig. 2. Absorptivity (molar ratio of H2 S in IL) of H2 S in the seven ILs at different temperatures (a) and interaction energies of between H2 S and the seven ILs calculated at B3LYP/6-311 + +G(d,p) level with the basis set superposition errors (BSSE) correction (b).
Table 1 Changes of the vibrational frequency for the interaction between H2 S, [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ]. Species
Cald Freq
Infrared
Exp Freq
Vibrational mode assignment
H2 S
1207.14 2681.31 2696.81
4.5488 2.4205 2.7739
1214.5a 2721.9a 2733.4a
ı(S49-H2 ) s (S49-H2 ) as (S49-H2 )
[TMGHPO2 ][BF4 ]
3151.93 3477.31 3546.97
6.9535 147.7190 630.4850
3139.56 3458.99 3543.63
as (H6-C4) (N31-H32) (O42-H43)
H2 S/[TMGHPO2 ][BF4 ]
1200.69 2672.08 2684.41 3156.82 3496.65 3577.68
4.1687 14.6414 1.3224 6.3248 114.0510 542.6947
1196.26 2216.82 2326.17 3146.52 3478.96 3575.70
ı(S49-H2 ) s (S49-H51) as (S49-H50) as (H6-C4) as (N31-H32) as (O42-H43)
a
The experiment results of H2 S come from Ref. [38].
significantly decrement, so the scale factor correction is necessary. The change of vibrational frequency in relation to our
discussion are shown in Table 1, it can be found that the some changes of vibrational frequency are happened, compared H2 S
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Fig. 3. Structures of [BMIM][BF4 ], [BMIM][Cl], [BMIM][PF6 ], [BMIM][Tf2 N], [BMIM][TfO], TMGL and [TMGHPO2 ][BF4 ] optimized at B3LYP/6-311 + +G(d,p) level.
with H2 S/[TMGHPO2 ][BF4 ]: the asymmetric stretching vibration of as (S49-H50) is red shift from 2696.81 to 2684.41 cm−1 , and symmetric stretching vibrational frequency of s (S49-H51) decreased from 2681.31 to 2672.08 cm−1 in H2 S/[TMGHPO2 ][BF4 ]. Meanwhile the scissoring vibration of ı(S49-H2 ) is red shift from 1207.14 to 1200.69 cm−1 , which is mainly because the hydrogen bonds of S49H50· · ·F48 and S49-H51· · ·F44 weaken S H bonds. Compared with [TMGHPO2 ][BF4 ], stretching vibrational frequency of O42-H43, N31-H32 and C4-H6 in [TMGHPO2 ]+ are blue shift from 3546.97, 3477.31 and 3151.93 cm−1 to 3577.68, 3496.65 and 3496.65 cm−1 , respectively, and the bond distances of H43· · ·F44, H6· · ·F48 and H32· · ·F46 are increased from 1.72, 2.07 and 2.01 Å to 1.74, 2.09 and 2.07 Å, respectively. It illustrates that the O42-H43, N31-H32 and C4-H6 are strengthen and the hydrogen bonds of H43· · ·F44, H32· · ·F46 and H6· · ·F48 are weakened. 4.4. NBO and AIM analysis NBO analysis can provide atomic population in the molecule and the intermolecular hyperconjugative interactions[36]. The data of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] complex are calculated
at B3LYP/6-311++G(d,p) level. Second order perturbation stabilization energies, E(2)s, of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] complex in relation to our discussion are shown in Table 2. As we all know, the larger E(2) values correspond to the stronger orbital interactions. For [TMGHPO2 ][BF4 ], it can be easily find that the E(2) of LP3(F44) → *(O42-H43) is as high as 27.11 kJ/mol, which indicates the charges may transfer from LP3(F44) to *(O42-H43) due to its strong orbital interactions. At the same time, E(2) of LP3(F48) → *(C4-H6) and LP3 (F46) → *(N31-H32) are 8.745 and 8.284 kJ/mol, respectively, indicating that three kinds of hydrogen bonds: O42-H43· · ·F44, C4-H6· · ·F48 and N31-H32· · ·F46 are formed. The results are consistent with the above analysis. In the H2 S/[TMGHPO2 ][BF4 ] complexes, E(2) of (F44) → *(O42-H43), (F48) → *(C4-H6) and (F46) → *(N31-H32) are 26.60, 6.234 and 6.067 kJ/mol, respectively. Compared with that of [TMGHPO2 ][BF4 ], it can be seen that E(2) of H2 S/[TMGHPO2 ][BF4 ] complexes are obviously smaller than those of [TMGHPO2 ][BF4 ], indicating that the absorption of H2 S weaken the strength of three kind of hydrogen bonds. In particular, E(2) of LP(2)F48 → *(S49-H50) and LP(2)F44 → *(S49-H51) are 2.008 and 0.7113 kJ/mol, respectively, which suggests that there is an stronger interaction between
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Fig. 4. Structures of the complexes formed by H2 S with ILs, including H2 S/[BMIM][BF4 ], [Tf2 N], H2 S/[BMIM][TfO], H2 S/TMGL and H2 S/[TMGHPO2 ][BF4 ], optimized at B3LYP/6-311++G(d,p) level.
H2 S/[BMIM][Cl],
H2 S/[BMIM][PF6 ],
H2 S/[BMIM]-
Table 2 The main donor-acceptor interactions and their second order perturbation stabilization energies, E(2), in [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] calculated at B3LYP/6311 + +G(d,p) levels. Species
Donor(i)
Acceptor(j)
E(2)/(kJ/mol)
E(j)-E(i)/(a.u.)
[TMGHPO2 ][BF4 ]
LP(3)F44 LP(3)F48 LP(3)F46 LP(1)F44 LP(1)F46 LP(2)F46
BD*(1)O42-H43 BD*(1)C4-H6 BD*(1)N31-H32 BD*(1)O42-H43 BD*(1)N31-H32 BD*(1)C34-H37
27.11 8.745 8.284 5.104 2.929 2.761
0.86 0.77 0.78 1.34 1.34 0.80
H2 S/[TMGHPO2 ][BF4 ]
LP(3)F44 LP(3)F48 LP(3)F46 LP(1)F44 LP(2)F46 LP(2)F48 LP(2)F44 LP(2)S49 LP(1)F48
BD*(1)O42-H43 BD*(1)C4-H6 BD*(1)N31-H32 BD*(1)O42-H43 BD*(1)C34-H37 BD*(1)S49-H50 BD*(1)S49-H51 BD*(1)C4-H5 BD*(1)S49-H50
26.60 6.234 6.067 4.686 2.427 2.008 0.7113 0.7113 0.4184
0.86 0.77 0.78 1.36 0.80 0.57 0.58 0.62 1.17
H2 S and [BF4 ]− i.e. existence of S49-H50· · ·F48 and S49-H51· · ·F44 hydrogen bonds, which are in good agreement with the above analysis. The electron density ((r)) and the Laplacian of the electron density (∇ 2 r(r)) as well as the eigenvalue (i) of Hessian matrix of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] complex calculated by B3LYP/6-311++G(d,p) level are shown in Table 3. Molecular graph of H2 S/[TMGHPO2 ][BF4 ] complex are shown in Fig. S6, in which the red points and yellow points represent the bond critical point (BCP)
and ring critical point (RCP), respectively. An atom ring can be easily seen, which is formed the atoms of S49, H50, F48, B45, F44 and H51. Commonly, (r)s and ∇ 2 r(r)s at BCPs are used to identify the bond strengths and types. Herein, these parameters are used to characterize the intermolecular bonds between H2 S and [TMGHPO2 ][BF4 ]. The results show that the parameter are all in the range of the criteria of hydrogen bonds proposed by Popelier [37], which is well consistent with the analysis of geometries and interaction energies. The r(r) values of F44-H43, F48-H6 and F46-H32 are in the region
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Table 3 Electron density (BCP), Laplacian of the electron density (∇ 2 BCP) and matrix eigenvalues (1 , 2 , 3 ) calculated at B3LYP/6-311+ +G(d,p) levels. A B Bond
r(r)
∇ 2 r(r)
1
2
3
[TMGHPO2 ][BF4 ]
F44-H43 F48-H6 F46-H43 F48-H28 F48-B45 F44-B45 F46-B45 F47-B45
0.03602 0.01909 0.01399 0.009704 0.1557 0.1476 0.1477 0.1778
−0.03079 −0.01650 −0.01348 −0.01004 −0.1941 −0.1786 −0.1782 −0.2408
−0.05899 −0.02451 −0.01623 −0.01029 −0.3812 −0.3464 −0.3463 −0.4772
−0.05786 −0.02355 −0.01467 −0.009414 −0.3766 −0.3430 −0.3428 −0.4758
0.2400 0.1140 0.08482 0.05986 1.534 1.404 1.402 1.916
H2 S/[TMGHPO2 ][BF4 ]
F44-H43 F48-H6 F48-H28 F48-B45 F44-B45 F46-B45 F47-B45 S49-H5 F44-H51 F48-H50
0.03429 0.01614 0.007844 0.1544 0.1452 0.1503 0.1786 0.004459 0.007087 0.01005
−0.02933 −0.01449 −0.008481 −0.1901 −0.1731 −0.1838 −0.2424 −0.003211 −0.007505 −0.0099
−0.05542 −0.01991 −0.007865 −0.3757 −0.3361 −0.3579 −0.4805 −0.002976 −0.006950 −0.01098
−0.05399 −0.01913 −0.006677 −0.3709 −0.3325 −0.3540 −0.4791 −0.002402 −0.006351 −0.01020
0.2267 0.09699 0.04847 10510 1.361 1.447 1.929 0.01822 0.04332 0.06081
are S49-H50· · ·F48 and S49-H51· · ·F44, respectively, which is the driving force of H2 S absorption in [TMGHPO2 ][BF4 ]. The effect is significantly higher than that of other six ILs. The computational results show that the absorption of H2 S in guanidinium based IL are a physical process. By analyzing of the vibration frequency changes in the absorption process, it can be seen that the formation of hydrogen bond between H2 S and [BF4 ]− weaken the hydrogen bonding between [BF4 ]− and [TMGHPO2 ]+ . The nature of the hydrogen bond is studied through the NBO and AIM method. The results confirmed the existence of strong hydrogen bonding interactions between H2 S and [BF4 ]− , and this interaction strength is stronger than that between [TMGHPO2 ]+ and water. Our results show that theoretical calculation data can reveal the mechanism of H2 S absorption of guanidine ILs, which is consistent with the experimental results and provides the theoretical guidance for further study. Acknowledgements
Fig. 5. Experimental IR spectrum of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ].
of 0.01399-0.03602 au., it may be due to three kinds of hydrogen bonds: O42-H43· · ·F44, C4-H6· · ·F48 and N31-H32· · ·F46 are formed, respectively. Compared with H2 S/[TMGHPO2 ][BF4 ] complex, it can be found that r(r) values of F48-B45 (0.1544 a.u.), F46-B45 (0.1503 a.u.) and F44-B45(0.1452 a.u.) are less than those of [TMGHPO2 ][BF4 ], respectively, which illustrates that the hydrogen bonds of F48· · ·H50-S49 and F44· · ·H51-S49 are formed in the absorption process of H2 S, respectively. At the same time, the corresponding r(r) values of F44-H43, F48-H6 and F46-H32 are decreased. 5. Conclusions In this paper, we study the absorption of H2 S in [BMIM][Tf2 N], [BMIM][TfO], [BMIM]Cl, [TMGHPO2 ][BF4 ], [BMIM][BF4 ], [BMIM][PF6 ] and TMGL which are optimized at B3LYP/6-311 + +G(d,p) level. There are three kinds of hydrogen bonds in [TMGHPO2 ][BF4 ]: O42-H43· · ·F44, C4-H6· · ·F48 and N31-H32· · ·F46. At the same time, we calculate the structure of H2 S/[TMGHPO2 ][BF4 ] complex. According to the interaction energy analysis, [TMGHPO2 ][BF4 ] shows the highest efficiency in the process of H2 S absorption, which is consistent with the experimental results. Hydrogen bonds formed between H2 S and [BF4 ]−
This work was supported by the National Natural Science Foundation of China (No. 21203250 and 21206085) and the Natural Science Foundation of Shandong Province (ZR2010BM024). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jmgm.2016.06. 013. References [1] K. Huang, D.N. Cai, Y.L. Chen, Y.T. Wu, X.B. Hu, Z.B. Zhang, Thermodynamic validation of 1-alkyl-3-methylimidazolium carboxylates as task-specific ionic liquids for H2 S absorption, AIChE J. 59 (2013) 2227–2235. [2] B. Cao, J. Du, S. Liu, X. Zhu, X. Sun, H. Sun, Carbon dioxide capture by amino-functionalized ionic liquids: DFT based theoretical analysis substantiated by FT-IR investigation, RSC Adv. 6 (2016) 10462–10470. [3] X. Sun, C. Huang, Z. Xue, T. Mu, An environmentally benign cycle to regenerate chitosan and capture carbon dioxide by ionic liquids, Energy Fuels 29 (2015) 1923–1930. [4] T. Mu, B. Han, Structures and thermodynamic properties of ionic liquids In: Structures and Interactions of Ionic Liquids, vol. 151, Springer, 2014, pp. 107–139. [5] B.E. Gurkan, J.C. de la Fuente, E.M. Mindrup, L.E. Ficke, B.F. Goodrich, E.A. Price, et al., Equimolar CO2 absorption by anion-functionalized ionic liquids, J. Am. Chem. Soc. 132 (2010) 2116–2117. [6] X. Cheng, T. Mu, X. Wang, X. Guo, L. Zou, Low pressure solubilities of vinyl chloride in ionic liquids, J. Chem. Eng. Data 53 (2008) 2807–2809. [7] X. Cheng, G. Yang, T. Mu, X. Guo, X. Wang, Absorption of vinyl chloride by room temperature ionic liquids, Clean–Soil Air Water 37 (2009) 245–248.
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Journal of Molecular Graphics and Modelling journal homepage: www.elsevier.com/locate/JMGM
Theoretical and experimental investigation on the capture of H2 S in a series of ionic liquids Xinming Zhou a , Bobo Cao b , Shuangyue Liu b , Xuejun Sun b,∗ , Xiao Zhu b,∗ , Hui Fu a,∗ a b
College of Science, China University of Petroleum, Shandong, Qingdao 266580, China School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China
a r t i c l e
i n f o
Article history: Received 25 October 2015 Received in revised form 20 June 2016 Accepted 20 June 2016 Available online 21 June 2016 Keywords: Absorption H2 S DFT Ionic liquid Hydrogen bond
a b s t r a c t H2 S absorptions in ionic liquids (ILs), including tetramethyl guanidinelactate (TMGL), 4-bis(2hydroxypropyl)-1,1,3,3-tetramethyl guanidinium tetrafluoroborate ([TMGHPO2 ][BF4 ]) and 1-butyl-3methylimidazolium cation ([BMIM]+ ) with the anions chloride ([Cl]− ), tetrafluoroborate ([BF4 ]− ), hexafluorophosphate ([PF6 ]− ), triflate ([TfO]− ), bis-(trifluoromethyl) sulfonylimide ([Tf2 N]− ), were studied in experiment and computational methods. [TMGHPO2 ][BF4 ] showed the best H2 S absorption capacity among the seven ILs. Density functional theory (DFT) calculations were applied to reveal the absorption mechanisms. Interaction energy results were consistent with absorptivities (molar ratio of H2 S in IL) measured in experiment. As the best candidate absorbent, [TMGHPO2 ][BF4 ] was chosen as an example to characterize the hydrogen bonds and orbital interactions between H2 S and [TMGHPO2 ][BF4 ] in atoms in molecules (AIM) and natural bond orbital (NBO) methods, respectively. IR spectrums obtained in both experimental and computational method were used to characterize the features of absorption process. The results indicated that H2 S was physically absorbed by ILs, in which hydrogen bond was the driving force. © 2016 Elsevier Inc. All rights reserved.
1. Introduction Hydrogen sulfide (H2 S) is usually produced with methane, other synthetic gas and hydrodesulfurization process of crude oil, which is a highly toxic and corrosive gases [1]. Aqueous alkanolamine solutions is a kind of widely used industrial solvent, which can remove H2 S from industrial natural gas, refinery tail gas and synthesis gas. However, in the process, the loss of alkanolamine volatilization of and the water transfer to the vapor will form the corrosive degradation and byproducts which has many disadvantages and makes the process economically expensive [2]. In recent years, ionic liquids (ILs) have been widely used to study CO2 capture [2,3], due to the unique properties, such as the minimal volatile, design properties, thermal stability and liquid form in a wide temperature range [4]. IL is a promising candidate absorbent for the gas separation [5–10]. Besides, ILs were also employed to study harmful gas capture [11–17].
∗ Corresponding authors. E-mail addresses: [email protected] (X. Zhou), [email protected] (B. Cao), [email protected] (S. Liu), [email protected] (X. Sun), qfnu [email protected] (X. Zhu), [email protected] (H. Fu). http://dx.doi.org/10.1016/j.jmgm.2016.06.013 1093-3263/© 2016 Elsevier Inc. All rights reserved.
A few years ago, people began to focus on the absorption of H2 S in ILs [18–22]. Jou and Mather [18] first determined H2 S solubility in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6 ]) under the conditions of temperatures range of 298.15-403.15 K and pressures up to 9.6 MPa. Pomelli et al. [19] reported the solubility of H2 S in [BMIM]-based ILs with different anions. Morsi et al. [20] published the solubility and transfer coefficient of H2 S in ammonium-based ILs. Jaliliet al. [21–26] measured and provided H2 S solubility at various temperature and pressure. Huang [27] given that dual Lewis base fictionalizations of ILs had highly efficient and selective capture of H2 S. Wang [28] reported oxidative absorption of H2 S by iron-containing ILs. In this paper, the absorptions of H2 S in the chosen ILs are studied in both experimental and computational method, including tetramethyl guanidinelactate (TMGL), 4-bis(2-hydroxypropyl)-1,1,3,3tetramethyl guanidinium tetrafluoroborate ([TMGHPO2 ][BF4 ]) and [BMIM] based ILs with different anions, including chloride ([Cl]− ), tetrafluoroborate ([BF4 ]− ), hexafluorophosphate ([PF6 ]− ), triflate ([TfO]− ), bis-(trifluoromethyl) sulfonylimide ([Tf2 N]− ). The hydrogen bonds between H2 S and ILs are investigated by the vibrational frequency analyses (VFA), natural bond orbital (NBO) and atom in molecule (AIM) methods.
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2. Experiments 2.1. Materials High-purity H2 S (purity 99.7%) was purchased from Zibo Anze Standard Gas Co. Ltd. (Zibo, China). The [BMIM][Cl], [BMIM][PF6 ], [BMIM][BF4 ], [BMIM][TfO] and [BMIM][Tf2 N] were purchased from Lanzhou (Lanzhou, China) Center for Green Chemistry with the purity of 99% and water mass fractions less than 10−3 (g/g), respectively. 1,1,3,3-tetramethylguanidine (TMG, 99%), propylene oxide (99%), ethanol (99%), toluene (99.8%), fluoroboric acid (50 wt.% solution in water) were purchased from Air Liquide. The TMGL [12] and [TMGHPO2 ][BF4 ] [13] were synthesized and purified before absorption process. To remove traces of volatile compounds, the ILs were kept in a vacuum (about 10−6 bar) at 343 K for 24 h prior. 2.2. Synthesis of TMGL and [TMGHPO2 ][BF4 ] To synthesize TMGL, TMG dissolved in ethanol was added into a flask in a water bath of 25 ◦ C under mechanical stirring. Then, the solution consisting of lactic acid and ethanol was added into the flask. The mole ratio of TMG and lactic acid was 1:1. The reaction process was allowed to proceed for 2.5 h. After the evaporation of solvent under vacuum, the residue was obtained, which was further treated with active carbon and washed by ethanol. The colorless product was obtained after ethanol was evaporated under vacuum. The solution of TMG in toluene was added into a flask in a water bath of 25 ◦ C under mechanical stirring. Then, propylene oxide was slowly dropped into the flask, in which the mole ratio of TMG and propylene oxide was 1:1. The reaction process was allowed to proceed for 36 h, where after fluoroboric acid was slowly added to the solution in an ice bath under mechanical stirring. The mixture was heated (70 ◦ C) to remove solvent under reduced pressure, and the residue ([TMGHPO][BF4 ]) was dry under high vacuum (0.1 kPa). Then, an additional equivalent propylene oxide was used to react with [TMGHPO][BF4 ] and the future treatments were similar with the steps mentioned above. An colorless product ([TMGHPO2 ][BF4 ]) was obtained. The 1 H-NMR spectrum of TMGL and [TMGHPO2 ][BF4 ] are given Figs. S1 and S2 (Electronic Supporting information) to ensure lack of critical impurities. 2.3. Apparatus and measurements H2 S absorptivity was measured by applying isochoric saturation technique [29]. It is worth mentioning that absorptivity in this work refers to molar ratio of H2 S in IL. The apparatus used for H2 S absorption measurements is schematically represented in Fig. 1. The apparatus was composed of a H2 S cylinder, pressure regulators, magnetic stirrers with heating, oil bath, and glass container filled with IL and off-gas absorption by NaOH solution. The lowpressure gauge had an uncertainty of approximately (0.001 bar) in the experimental pressure range. For a typical experiment, the desired amount of IL was loaded in the glass container (the weight was m1 , about 5 g) using an electronic balance (Sartorius BS224S, uncertainty of 0.001 g), and the air in the system was eliminated by the vacuum pump, Then H2 S was charged into the glass container from the cylinder, and the liquid phase was stirred. The system was considered to have reached equilibrium if the pressure of the system had been unchanged over time for 2 h. Then the pressure of the system was recorded, and the glass container filled with IL was weighed (the weight was m2 ). Thus, the absorption of H2 S in a given IL was determined by the shifted quality of the glass container (m2 -m1 ). To verify the validation of the apparatus, we determined the absorption of CO2 in a chloride and urea mixture with this apparatus, and the results were consistent with ref. [30].
To support the computational results, an attenuated total reflection fourier transform infrared (FT-IR ATR) spectra of 2BP8HQ were registered using Varian FTS1000 FT-IR spectrometer with Diamond/ZnSe prism (4000–525 cm−1 ; number of scans: 250; resolution: 1 cm−1 ) to study the variations of IR spectrum of [TMGHPO2 ][BF4 ] before and after H2 S absorption. 3. Computational methods In recent years, density functional theory (DFT) method has been widely used since it is a cost-effective and reliable method for calculation chemistry. The hybrid Becke 3-Lee-Yang-Parr (B3LYP) exchange-correlation function combined with the 6-311 + +G(d,p) basis set are widely applied in theoretical studies of ionic liquids [16,31]. B3LYP method has been confirmed that it can supply reliable interaction energies, in which electron correlation is well considered. Polarizable functions and diffuse functions are used for all atoms in the 6-311 + +G(d,p) basis set, which is important for the description of hydrogen bonding interactions. The interaction energy (E) considering the basis set superposition errors (BSSE) correction using the counterpoise (CP) method is estimated [32]. The vibrational frequency studies of H2 S, ILs and the complexes have also been performed at the same level. To account for errors due to the neglecting electron correlation and the basis set incompleteness, considering B3LYP systematic errors with a scaling factor of 0.983 up to 1700 cm−1 and 0.958 for greater than 1700 cm−1 [33,34]. What is more, to better understand the nature of the intermolecular H-bonding interactions in the H2 S, different ILs and complexes, natural bond orbital (NBO) and atoms in molecule (AIM) have also been carried out at the same level. All computations are carried out by using Gaussian 09 program package [35]. 4. Results and discussion 4.1. Experimental absorptivity of H2 S Absorptivity of H2 S in the seven ILs is determined by using isochoric saturation technique (Fig. 1), which was depicted in Fig. 2a. Table S1 (Electronic Supporting information) shows the molar ratio of H2 S in seven ILs at 30 ◦ C. It can be seen that [TMGHPO2 ][BF4 ] is the best candidate for H2 S absorption, due to the high molar ratio of 0.502. However, the absorptivity of H2 S in the other six ILs does not exceed 0.38. The absorptivity order of H2 S in seven ILs is: [TMGHPO2 ][BF4 ] > [BMIM][Tf2 N] > [BMIM][TfO] > [BMIM][Cl] > [BMIM][BF4 ] > [BMIM][PF6 ] > TMGL. 4.2. Analysis of geometries and interaction energies In order to gain the possible interaction modes between H2 S and seven kinds of ILs, the electrostatic potential surfaces of them have been calculated at B3LYP/6-311 + +G(d,p) level. As shown in Fig. S3, the favorable sites (more negative charges and red area) for proton attack in anion are concentrated on the regions around chloride, oxygen atoms in carbonyl and fluorine atoms in tetrafluoroborate or hexafluorophosphate. The most possible interaction sites can be observed, such as the regions near the chloride, oxygen atom and fluorine atoms (more positive charges and blue area). It can be seen that these atoms are easy to form hydrogen bonds. For cation, the highly positive regions (deep blue) are mainly concentrated on around C H in [BMIM]+ , hydrogen atom of amine group of TMGL and the hydrogen atoms of hydroxyl group of [TMGHPO2 ]+ . Commonly, the regions with larger positive charge densities are easier to form hydrogen bonds. With the above analysis, the large area of positive charge density in cation and the area of negative charge density are easy to form hydrogen bonds. Thus the reason-
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Fig. 1. Apparatus of absorption of H2 S in ILs: (1) H2 S cylinder; (2) gas cylinder pressure regulator; (3) and (4) valves; (5) volume rotameter; (6) magnetic stirrers with heating; (7) temperature sensor; (8) oil bath; (9) glass container filled with IL; (10) off-gas absorption by NaOH.
able configurations of ILs and H2 S-IL complexes are constructed and optimized to locate the most stable configurations. Fig. 3 shows the optimized configuration of seven kinds of ILs, in which the main hydrogen bonds between cation and anion are depicted. Take [TMGHPO2 ][BF4 ] as an example, the hydrogen bonds are formed by hydrogen atoms in hydroxyl group with fluorine atoms in [BF4 ]− , which can be contributed to the large positive charge density of C H on the imidazole ring. This is consistent with the electrostatic potential energy surface analysis. For details, three hydrogen bonds in [TMGHPO2 ][BF4 ] are O42-H43· · · F44, N31H32· · ·F46 and C4-H6· · ·F48 with the corresponding bond length of 1.74 Å, 2.09 Å and 2.07 Å, respectively. Besides, strong hydrogen bonds are also formed by atoms of hydrogen and oxygen in TMGL. Structures of the complexes formed by H2 S with [BMIM]Cl, [BMIM][BF4 ], [BMIM][PF6 ], [BMIM][TfO], [BMIM][Tf2 N], TMGL, and [TMGHPO2 ][BF4 ] are optimized at B3LYP/6–311++G(d,p) level, which are shown in Fig. 4, respectively. The interaction energy of H2 S with the seven ILs are calculated at B3LYP/6–311++G(d,p) level corrected by using the counterpoise (CP) method, which is shown in Fig. 2b. It can be found that the interaction energy decreases with the order of [TMGHPO2 ][BF4 ] (14.90 kJ/mol) > [BMIM][Tf2 N] (10.24 kJ/mol) > [BMIM][TfO] (9.30 kJ/mol) > [BMIM](8.85 kJ/mol) > [BMIM][BF4 ] (7.95 kJ/mol) > [BMIM][PF6 ] [Cl] (7.06 kJ/mol)> TMGL (5.71 kJ/mol). As we expected, the “trend” is consistent with the experimental observation. Due to the excellent absorptivity and larger interaction energies of [TMGHPO2 ][BF4 ], the complex formed by [TMGHPO2 ][BF4 ] and H2 S is taken as an example to discuss the intermolecular interaction. Four different conformers of the H2 S/[TMGHPO2 ][BF4 ] complex, marked A, B, C and D, respectively, are optimized and shown in Fig. S4, in which
H2 S is located in four different positions of [TMGHPO2 ][BF4 ]. The electronic energies of four configurations are obtained and compared to select the most stable conformer. The values of A–D are −14.86, −14.95, −15.17 and −15.32 kJ/mol, respectively. It can be easily found that conformer D is the most stable conformer. The same way is used to select the most stable conformer of complexes formed by H2 S with the other ILs. As shown in Fig. S4, the hydrogen bond lengths of S49-H50· · ·F48 (2.27 Å) and S49-H51· · ·F44 (2.33 Å) are much short than the sum of their van der Waals radius, which suggests the existence strong hydrogen bonds between H2 S and [TMGHPO2 ][BF4 ]. As we expected, hydrogen bond strength and interaction energy between H2 S and ILs are consistent with the absorptivity of H2 S in the chosen ILs. It is reasonable to believe that hydrogen bonds are the driving force of H2 S absorption in ILs. This result is further demonstrated by the following analysis. 4.3. Vibrational frequency analysis [TMGHPO2 ][BF4 ] and the H2 S/[TMGHPO2 ][BF4 ] complex have 44 and 47 atoms, as well as have 138 and 147 normal vibrational modes, respectively. The harmonic vibrational frequencies are calculated using the B3LYP method at 6-311++G(d,p) level. The infrared spectra data and the corresponding vibrational frequencies and modes of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] are listed in Tables S2 and S5 respectively. The diagrams of the experimental and the theoretical infrared spectra of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] are shown in Fig. 5 and Fig. S3, respectively. Differences between the frequencies obtained using the scaled B3LYP method and that of the experiment are found
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Fig. 2. Absorptivity (molar ratio of H2 S in IL) of H2 S in the seven ILs at different temperatures (a) and interaction energies of between H2 S and the seven ILs calculated at B3LYP/6-311 + +G(d,p) level with the basis set superposition errors (BSSE) correction (b).
Table 1 Changes of the vibrational frequency for the interaction between H2 S, [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ]. Species
Cald Freq
Infrared
Exp Freq
Vibrational mode assignment
H2 S
1207.14 2681.31 2696.81
4.5488 2.4205 2.7739
1214.5a 2721.9a 2733.4a
ı(S49-H2 ) s (S49-H2 ) as (S49-H2 )
[TMGHPO2 ][BF4 ]
3151.93 3477.31 3546.97
6.9535 147.7190 630.4850
3139.56 3458.99 3543.63
as (H6-C4) (N31-H32) (O42-H43)
H2 S/[TMGHPO2 ][BF4 ]
1200.69 2672.08 2684.41 3156.82 3496.65 3577.68
4.1687 14.6414 1.3224 6.3248 114.0510 542.6947
1196.26 2216.82 2326.17 3146.52 3478.96 3575.70
ı(S49-H2 ) s (S49-H51) as (S49-H50) as (H6-C4) as (N31-H32) as (O42-H43)
a
The experiment results of H2 S come from Ref. [38].
significantly decrement, so the scale factor correction is necessary. The change of vibrational frequency in relation to our
discussion are shown in Table 1, it can be found that the some changes of vibrational frequency are happened, compared H2 S
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Fig. 3. Structures of [BMIM][BF4 ], [BMIM][Cl], [BMIM][PF6 ], [BMIM][Tf2 N], [BMIM][TfO], TMGL and [TMGHPO2 ][BF4 ] optimized at B3LYP/6-311 + +G(d,p) level.
with H2 S/[TMGHPO2 ][BF4 ]: the asymmetric stretching vibration of as (S49-H50) is red shift from 2696.81 to 2684.41 cm−1 , and symmetric stretching vibrational frequency of s (S49-H51) decreased from 2681.31 to 2672.08 cm−1 in H2 S/[TMGHPO2 ][BF4 ]. Meanwhile the scissoring vibration of ı(S49-H2 ) is red shift from 1207.14 to 1200.69 cm−1 , which is mainly because the hydrogen bonds of S49H50· · ·F48 and S49-H51· · ·F44 weaken S H bonds. Compared with [TMGHPO2 ][BF4 ], stretching vibrational frequency of O42-H43, N31-H32 and C4-H6 in [TMGHPO2 ]+ are blue shift from 3546.97, 3477.31 and 3151.93 cm−1 to 3577.68, 3496.65 and 3496.65 cm−1 , respectively, and the bond distances of H43· · ·F44, H6· · ·F48 and H32· · ·F46 are increased from 1.72, 2.07 and 2.01 Å to 1.74, 2.09 and 2.07 Å, respectively. It illustrates that the O42-H43, N31-H32 and C4-H6 are strengthen and the hydrogen bonds of H43· · ·F44, H32· · ·F46 and H6· · ·F48 are weakened. 4.4. NBO and AIM analysis NBO analysis can provide atomic population in the molecule and the intermolecular hyperconjugative interactions[36]. The data of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] complex are calculated
at B3LYP/6-311++G(d,p) level. Second order perturbation stabilization energies, E(2)s, of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] complex in relation to our discussion are shown in Table 2. As we all know, the larger E(2) values correspond to the stronger orbital interactions. For [TMGHPO2 ][BF4 ], it can be easily find that the E(2) of LP3(F44) → *(O42-H43) is as high as 27.11 kJ/mol, which indicates the charges may transfer from LP3(F44) to *(O42-H43) due to its strong orbital interactions. At the same time, E(2) of LP3(F48) → *(C4-H6) and LP3 (F46) → *(N31-H32) are 8.745 and 8.284 kJ/mol, respectively, indicating that three kinds of hydrogen bonds: O42-H43· · ·F44, C4-H6· · ·F48 and N31-H32· · ·F46 are formed. The results are consistent with the above analysis. In the H2 S/[TMGHPO2 ][BF4 ] complexes, E(2) of (F44) → *(O42-H43), (F48) → *(C4-H6) and (F46) → *(N31-H32) are 26.60, 6.234 and 6.067 kJ/mol, respectively. Compared with that of [TMGHPO2 ][BF4 ], it can be seen that E(2) of H2 S/[TMGHPO2 ][BF4 ] complexes are obviously smaller than those of [TMGHPO2 ][BF4 ], indicating that the absorption of H2 S weaken the strength of three kind of hydrogen bonds. In particular, E(2) of LP(2)F48 → *(S49-H50) and LP(2)F44 → *(S49-H51) are 2.008 and 0.7113 kJ/mol, respectively, which suggests that there is an stronger interaction between
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Fig. 4. Structures of the complexes formed by H2 S with ILs, including H2 S/[BMIM][BF4 ], [Tf2 N], H2 S/[BMIM][TfO], H2 S/TMGL and H2 S/[TMGHPO2 ][BF4 ], optimized at B3LYP/6-311++G(d,p) level.
H2 S/[BMIM][Cl],
H2 S/[BMIM][PF6 ],
H2 S/[BMIM]-
Table 2 The main donor-acceptor interactions and their second order perturbation stabilization energies, E(2), in [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] calculated at B3LYP/6311 + +G(d,p) levels. Species
Donor(i)
Acceptor(j)
E(2)/(kJ/mol)
E(j)-E(i)/(a.u.)
[TMGHPO2 ][BF4 ]
LP(3)F44 LP(3)F48 LP(3)F46 LP(1)F44 LP(1)F46 LP(2)F46
BD*(1)O42-H43 BD*(1)C4-H6 BD*(1)N31-H32 BD*(1)O42-H43 BD*(1)N31-H32 BD*(1)C34-H37
27.11 8.745 8.284 5.104 2.929 2.761
0.86 0.77 0.78 1.34 1.34 0.80
H2 S/[TMGHPO2 ][BF4 ]
LP(3)F44 LP(3)F48 LP(3)F46 LP(1)F44 LP(2)F46 LP(2)F48 LP(2)F44 LP(2)S49 LP(1)F48
BD*(1)O42-H43 BD*(1)C4-H6 BD*(1)N31-H32 BD*(1)O42-H43 BD*(1)C34-H37 BD*(1)S49-H50 BD*(1)S49-H51 BD*(1)C4-H5 BD*(1)S49-H50
26.60 6.234 6.067 4.686 2.427 2.008 0.7113 0.7113 0.4184
0.86 0.77 0.78 1.36 0.80 0.57 0.58 0.62 1.17
H2 S and [BF4 ]− i.e. existence of S49-H50· · ·F48 and S49-H51· · ·F44 hydrogen bonds, which are in good agreement with the above analysis. The electron density ((r)) and the Laplacian of the electron density (∇ 2 r(r)) as well as the eigenvalue (i) of Hessian matrix of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ] complex calculated by B3LYP/6-311++G(d,p) level are shown in Table 3. Molecular graph of H2 S/[TMGHPO2 ][BF4 ] complex are shown in Fig. S6, in which the red points and yellow points represent the bond critical point (BCP)
and ring critical point (RCP), respectively. An atom ring can be easily seen, which is formed the atoms of S49, H50, F48, B45, F44 and H51. Commonly, (r)s and ∇ 2 r(r)s at BCPs are used to identify the bond strengths and types. Herein, these parameters are used to characterize the intermolecular bonds between H2 S and [TMGHPO2 ][BF4 ]. The results show that the parameter are all in the range of the criteria of hydrogen bonds proposed by Popelier [37], which is well consistent with the analysis of geometries and interaction energies. The r(r) values of F44-H43, F48-H6 and F46-H32 are in the region
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Table 3 Electron density (BCP), Laplacian of the electron density (∇ 2 BCP) and matrix eigenvalues (1 , 2 , 3 ) calculated at B3LYP/6-311+ +G(d,p) levels. A B Bond
r(r)
∇ 2 r(r)
1
2
3
[TMGHPO2 ][BF4 ]
F44-H43 F48-H6 F46-H43 F48-H28 F48-B45 F44-B45 F46-B45 F47-B45
0.03602 0.01909 0.01399 0.009704 0.1557 0.1476 0.1477 0.1778
−0.03079 −0.01650 −0.01348 −0.01004 −0.1941 −0.1786 −0.1782 −0.2408
−0.05899 −0.02451 −0.01623 −0.01029 −0.3812 −0.3464 −0.3463 −0.4772
−0.05786 −0.02355 −0.01467 −0.009414 −0.3766 −0.3430 −0.3428 −0.4758
0.2400 0.1140 0.08482 0.05986 1.534 1.404 1.402 1.916
H2 S/[TMGHPO2 ][BF4 ]
F44-H43 F48-H6 F48-H28 F48-B45 F44-B45 F46-B45 F47-B45 S49-H5 F44-H51 F48-H50
0.03429 0.01614 0.007844 0.1544 0.1452 0.1503 0.1786 0.004459 0.007087 0.01005
−0.02933 −0.01449 −0.008481 −0.1901 −0.1731 −0.1838 −0.2424 −0.003211 −0.007505 −0.0099
−0.05542 −0.01991 −0.007865 −0.3757 −0.3361 −0.3579 −0.4805 −0.002976 −0.006950 −0.01098
−0.05399 −0.01913 −0.006677 −0.3709 −0.3325 −0.3540 −0.4791 −0.002402 −0.006351 −0.01020
0.2267 0.09699 0.04847 10510 1.361 1.447 1.929 0.01822 0.04332 0.06081
are S49-H50· · ·F48 and S49-H51· · ·F44, respectively, which is the driving force of H2 S absorption in [TMGHPO2 ][BF4 ]. The effect is significantly higher than that of other six ILs. The computational results show that the absorption of H2 S in guanidinium based IL are a physical process. By analyzing of the vibration frequency changes in the absorption process, it can be seen that the formation of hydrogen bond between H2 S and [BF4 ]− weaken the hydrogen bonding between [BF4 ]− and [TMGHPO2 ]+ . The nature of the hydrogen bond is studied through the NBO and AIM method. The results confirmed the existence of strong hydrogen bonding interactions between H2 S and [BF4 ]− , and this interaction strength is stronger than that between [TMGHPO2 ]+ and water. Our results show that theoretical calculation data can reveal the mechanism of H2 S absorption of guanidine ILs, which is consistent with the experimental results and provides the theoretical guidance for further study. Acknowledgements
Fig. 5. Experimental IR spectrum of [TMGHPO2 ][BF4 ] and H2 S/[TMGHPO2 ][BF4 ].
of 0.01399-0.03602 au., it may be due to three kinds of hydrogen bonds: O42-H43· · ·F44, C4-H6· · ·F48 and N31-H32· · ·F46 are formed, respectively. Compared with H2 S/[TMGHPO2 ][BF4 ] complex, it can be found that r(r) values of F48-B45 (0.1544 a.u.), F46-B45 (0.1503 a.u.) and F44-B45(0.1452 a.u.) are less than those of [TMGHPO2 ][BF4 ], respectively, which illustrates that the hydrogen bonds of F48· · ·H50-S49 and F44· · ·H51-S49 are formed in the absorption process of H2 S, respectively. At the same time, the corresponding r(r) values of F44-H43, F48-H6 and F46-H32 are decreased. 5. Conclusions In this paper, we study the absorption of H2 S in [BMIM][Tf2 N], [BMIM][TfO], [BMIM]Cl, [TMGHPO2 ][BF4 ], [BMIM][BF4 ], [BMIM][PF6 ] and TMGL which are optimized at B3LYP/6-311 + +G(d,p) level. There are three kinds of hydrogen bonds in [TMGHPO2 ][BF4 ]: O42-H43· · ·F44, C4-H6· · ·F48 and N31-H32· · ·F46. At the same time, we calculate the structure of H2 S/[TMGHPO2 ][BF4 ] complex. According to the interaction energy analysis, [TMGHPO2 ][BF4 ] shows the highest efficiency in the process of H2 S absorption, which is consistent with the experimental results. Hydrogen bonds formed between H2 S and [BF4 ]−
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