Low viscous Protic ionic liquids functionalized with multiple Lewis Base for highly efficient capture of H2S

Journal of Molecular Liquids 263 (2018) 209–217

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Low viscous Protic ionic liquids functionalized with multiple Lewis Base for highly efficient capture of H2S Wentao Zheng, Dongsheng Wu, Xi Feng, Jinling Hu, Feng Zhang, You-Ting Wu ⁎, Xing-Bang Hu ⁎ School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210000, PR China

a r t i c l e

i n f o

Article history: Received 22 March 2018 Received in revised form 25 April 2018 Accepted 26 April 2018 Available online 1 May 2018 Keywords: Multiple Lewis base Protic ionic liquid H2S removal Low viscosity Large absorption capacity Thermodynamics

a b s t r a c t Three kinds of aqueous multiple Lewis base functionalized protic ionic liquids (MLB-PILs) solutions are designed for highly efficient absorption of H2S. These MLB-PILs are tethered with tertiary amine groups on their cations and acetate as anions. 1H NMR, 13C NMR spectra, their physical properties (e.g. densities and viscosities) and thermal properties are carefully characterized. Impressively, the viscosities of MLB-PILs are b25 cP at ambient temperature, which is obviously lower than other ILs reported in literature. The H2S solubility of MLB-PILs are superior compared with other reported H2S absorbents, which can reach 0.65–1.92 mol∙mol−1 at 1 bar and 313.2 K. With the assumption of complex formation between H2S and MLB-PILs, the reaction equilibrium thermodynamic model (RETM) is adopted to correlate the experimental solubility. Basing on the fitting results, the thermodynamic parameters such as Henry's law constants H and reaction equilibrium constants K are obtained. Furthermore, the enthalpy change ΔHSOL and the entropy change ΔSSOL are also calculated to analyze the absorption process of H2S in these MLB-PILs. According to the results, the MLB-PILs are believed to have potential use in gas sweetening. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen sulfide (H2S) is highly toxic and corrosive, but widely exists in natural gas and other syngas streams [1,2]. It is required to remove H2S from fuel gases in consideration of safe transportation and high fuel-source utilization. Aqueous alkanolamines solutions (e.g. monoethanolamine (MEA), diethanolamine (DEA), N methyldiethanolamine (MDEA), and diisopropanolamine (DIPA)) are the most widely used chemical solvents for removing H2S from gas streams with low H2S content (2%–3%) [3–5]. Organic solvents such as methanol, propylene carbonate, polyethylene glycol dimethyl ether, N methylpyrrolidone are used as physical absorbents for the bulk removal of H2S in some commercial processes, e.g. the Rectisol process [6]. However, the above-mentioned solvents have many disadvantages, such as the volatile loss of alkanolamine, high energy for regenerating the absorbents, and degradation of the alkanolamine to form corrosive byproducts [1]. All these disadvantages go against the general requirements for the processes to be environmentally benign, energy saving, and highly efficient [7]. In order to find promising candidates to replace traditional organic solvents in the purification of H2S, scientists start to focus on ionic liquids (ILs) [7]. ILs, a kind of liquid electrolytes composed entirely of ions at room temperature or below 100 °C, have drawn worldwide ⁎ Corresponding authors. E-mail addresses: [email protected], (Y.-T. Wu), [email protected] (X.-B. Hu).

https://doi.org/10.1016/j.molliq.2018.04.129 0167-7322/© 2018 Elsevier B.V. All rights reserved.

attention since they were discovered [8–14]. ILs have many unique properties including extremely low vapor pressure, high thermal stability, designable structure, and excellent solvency for many compounds [15–17]. In recent years, ILs have been widely used in the field of gas separation [18–21]. A number of papers have been published on absorption of CO2 and SO2 by various ILs [22–34]. However, the solubility of H2S is available in only a few common ILs based on imidazolium [35–37], in which the uptake of H2S remains at low level. For instance, the absorption capacities of [emim][EtSO4], [bmim][PF6] are only 0.018 mol/mol and 0.075 mol/mol at 1 bar and 313.2 K [38,39]. Therefore, these common ILs are economically unfavorable for the industrial application for H2S removal because the large volume circulation and regeneration of ILs require intensive energy input. The exploration of task-specific ILs for H2S absorption has been done in our group [1,2,40,41]. It has been revealed in our laboratory that carboxylatebased ILs could be considered as task-specific ILs for H2S absorption owing to the weak Lewis base nature of carboxylate, which has good affinity with the hydrogen protons in H2S [2]. In particular, the solubility of H2S in [emim][Ac] is 0.48 mol/mol under ambient pressure at 313.2 K, with the capacity being dozens of times larger than the normal ILs [1]. Huang et al. designed a series of phenolic ILs for highly efficient and selective absorption of H2S, which is resulting from anionic strong basicity and cationic hydrogen-bond donation [42]. However, those functionalized ILs are usually synthesized by nucleophilic substitution and anion exchange, in which expensive reactants and tedious reaction/separation steps are usually required [40,42]. In addition, the

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viscosities of those ILs are pretty large, some of which are even up to 1000 cP at room temperature, leading to the slow mass transfer and intensive energy consumption for material transportation [2]. It is widely accepted that high cost and viscosity of ILs are two main disadvantages that restrict their prospects in industrial application. Therefore, the major purpose of our research is to find potential ILs with high absorption capacity of H2S as well as low cost and viscosity. Protic ionic liquids (PILs) are an important subgroup of ILs, which can be easily prepared by the direct neutralization of a Brønsted acid and base [43,44]. In comparison with aprotic ILs, PILs have a number of unique properties, including simple one-step synthesis process, low cost and viscosity [45]. The leading feature that distinguishes PILs from other ILs is the proton transfer from the acid to the base, leading to the presence of proton-donor and proton-acceptor sites [46]. These sites may form hydrogen bond with the hydrogen protons in H2S, which makes it possible to enhance the solubility of H2S in the PILs. It has been found that carboxylate-based ILs are a class of H2S-philic ILs due to their Lewis base properties [2]. Inspiringly, the tertiary amine group belongs to Lewis base as well, whose lone pair electrons are expected to attract the hydrogen protons in H2S. In this work, multiple Lewis base functionalized PILs (MLB-PILs) tethered with tertiary amine on their cations and acetate as anions are designed for highly efficient absorption of H2S. Generally speaking, the thermal stability of PILs is lower than that of aprotic ILs [40,43]. It is proposed to use the PILs together with water so that the disadvantages mentioned above can be overcome greatly [43]. In addition, the addition of water can reduce the viscosities of the MLB-PILs, which can facilitate the mass transfer and transportation in industrial application. The thermodynamic solubility of H2S in aqueous MLB-PILs (50 wt%) was determined at 298.2–323.2 K and 0–1.2 bar. Comparison of the designed aqueous MLB-PILs with other ILs as well as organic solvents was performed to justify the advantages of MLB-PILs. The interaction mechanisms between PILs and H2S were illustrated on the basis of RETM analysis. Thermodynamic parameters such as Henry's law constants H, reaction equilibrium constants K, the enthalpy change ΔHSOL and the entropy change ΔSSOL were also calculated to evaluate the behavior of H2S absorption in MLB-PILs. 2. Experiments

Fig. 1. Chemical structure of the MLB-PILs prepared in this work.

400 MHz spectrometer using CDCl3 as the solvent with TMS as the internal standard. Densities were determined using an Anton Paar DMA 5000 type automatic density meter with a precision of 0.000001, which was calibrated using distilled water. Viscosities were measured on a HAAKE Rheostress 600 viscometer with an uncertainty of ±1% in relation to the full scale. The thermal gravimetric (TG) analysis traces of these three MLB-PILs were determined on a Netzsch STA 449C instrument from room temperature to 673.2 K at a heating rate of 10 K/min under N2 atmosphere.

2.1. Materials 2.3. H2S-absorption experiments H2S (99.9%) was purchased from Nanjing Guanghua Gas Co. Ltd. The analytical reagents of Tetramethyl 1,3 diaminopropane (TMDAP), N,N,N′,N′,N′ Pentameth yldipropylenetriamine (PMDPTA), N,N, N tris (3 Dimethylaminopropyl)amine (TDMAPA), acetic acid (HAc) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received without further purification. 2.2. Preparation and characterization of PILs The three MLB-PILs prepared in this work are [TMDAPH][Ac], [PMDPTAH][Ac] and [TDMAPAH][Ac] (Fig. 1). All of them were prepared using the established method according to the literature [43]. In brief, a round-bottom flask coupled with a magnetic stirrer is charged with TMDAP (0.1 mol) and an appropriate amount of water, the acetic acid (0.1 mol) is added slowly at 273.2 K over a period of 30 min. The mixture is stirred for 24 h at this temperature to finish the reaction. The other two aqueous PILs ([PMDPTAH][Ac] and [TDMAPA][Ac]) are performed using the same method. In order to characterize the pure MLB-PIL, a majority of water is removed by rotary evaporation to generate a residual solution that contained the required ILs [40]. Then, the MLB-PIL samples are kept in vacuum at 328.2 K for 24 h to remove traces of water, generating yellow or pale yellow liquids (target MLBPILs). The molecular structures of the target PILs were verified by using 1H and 13C NMR spectra, which were recorded on a Bruker

The apparatus for the determination of H2S solubility in the aqueous MLB-PILs solutions is similar to that reported in our group's previous work [40,41]. The whole device consists of two stainless-steel chambers with volumes of 119.376 (V1) and 47.600 cm3 (V2), respectively. The bigger chamber, named the gas reservoir, isolates H2S before it contacts with the aqueous MLB-PIL samples in the smaller chamber. The smaller chamber, named as the equilibrium cell, is equipped with a magnetic stirrer. The temperatures (T) of both chambers are controlled by a water bath with an uncertainty of ±0.1 K. The pressures in the two chambers are monitored by using two pressure transducers (Wide Plus Precision Instruments CO. Ltd.; Accuracy: ±0.2%; Scale: 0–0.6 MPa). The pressure transducers are connected to a Numeric Instrument (WP-D821–200-1212-N-2P) to record the pressure changes online. Typically, a known mass (w) of aqueous MLB-PIL sample is placed into the equilibrium cell and the air in the two chambers is evacuated. The residual pressure in the equilibrium cell, resulting from the vapor pressures of water, is recorded to be P0. H2S from its gas cylinder is then fed into the gas reservoir to a pressure recorded as P1. The needle valve between the two chambers is then turned on to let a part of H2S be introduced to the equilibrium cell to react with the aqueous MLB-PIL sample and then turned off. The absorption equilibrium is considered to be reached when the pressures of the two chambers remain constant for at least 2 h. The equilibrium pressures are denoted as P2 for the

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equilibrium cell and P3 for the gas reservoir, respectively. The partial pressure of H2S in the equilibrium cell is Pe = P2 − P0. The H2S uptake, n(Pe), can thus be calculated using the following equation: [1]. nðP e Þ ¼ ρg ðP 1 ÞV 1 −ρg ðP 3 ÞV 1 −ρg ðP e ÞðV 2 −w=ρPIL Þ

ð1Þ

where ρg(Pi) represents the density of H2S in mol∙cm−3 at Pi (i = 1, 3, e) and T. ρPIL is the density of the absorbent in g∙cm−3 at T. Continual determinations of solubility data at elevated pressures are performed by introducing more H2S into the equilibrium cell to reach new equilibrium. At the end of the experiments, the residue H2S in the equilibrium cell is introduced to an off-gas absorber which contains the NaOH and CuSO4 aqueous solution, preventing the poisonous H2S from leaking into the atmosphere. Duplicate experiments are run for each PIL system to obtain the averaged values of gas solubility. The relative deviation of the absorption data in this study is well within ±1%. 3. Result and discussion 3.1. Characterization of MLB-PILs 1

H and 13C NMR spectra data of all the prepared MLB-PILs are given as follows: [TMDAPH][Ac]: 1H NMR (400 MHz, CDCl3, 298.2 K, TMS), δ (ppm): 11.42 (s, 1H), 2.54–2.47 (m, 4H), 2.35 (s, 12H), 1.98 (s, 3H), 1.83–1.74 (m, 2H); 13C NMR (400 MHz, CDCl3, 298.2 K, TMS), δ (ppm): 176.63, 56.46, 44.14, 23.39, 22.74. [PMDPTAH][Ac]: 1H NMR (400 MHz, CDCl3, 298.2 K, TMS), δ (ppm): 11.06 (s, 1H), 2.46–2.40 (m, 8H), 2.32 (s, 12H), 2.25 (s, 3H), 1.98 (s, 3H), 1.77–1.66 (m, 4H); 13C NMR (400 MHz, CDCl3, 298.2 K, TMS), δ (ppm): 176.77, 56.91, 55.02, 44.41, 41.56, 23.97, 22.98. [TDMAPAH][Ac]: 1H NMR (400 MHz, CDCl3) δ 10.10 (s, 1H), 2.49–2.37 (m, 12H), 2.32 (t, 18H), 1.96 (s, 3H), 1.70–1.59 (m, 6H); 13C NMR (400 MHz, CDCl3) δ 176.79, 57.59, 51.84, 45.05, 24.76, 23.01. The 1H, 13C NMR spectra of the MLB-PILs are found to be in good agreement with their corresponding chemical structures and no impurities are found according to the characterization results. 3.2. Physical and thermal properties of aqueous MLB-PILs Densities and viscosities of liquid absorbents are basic data for designing the gas separation process, which have an influence on the absorption of gas in the absorbents. The density and viscosity of the aqueous MLBPILs solutions in the temperature range of 298.2–328.2 K were measured (Fig. 2). It is found that the densities decrease almost linearly with the temperature increasing, whereas the viscosities decrease in an exponential manner. In addition, with the increase of the length and/or the number of the cation's alkyl chain, the density shows a slight decrease correspondingly, which is a common phenomenon resulted from the increasing of the intermolecular distance caused by the steric hindrance of the alkyl chain [47]. Viscosity is one of the most important factors in determining the absorption performance of a liquid absorbent. A highly viscous absorbent not only has a slow diffusion rate of gas into liquid but also requires intensive energy input in materials transport [40]. The viscosities of all these three aqueous MLB-PILs are lower than 25 cP at ambient temperature (Fig. 2), which are well below those traditional ILs and functionalized ILs whose viscosities are usually up to hundreds of centipoise [2,41]. For example, viscosities of the triethylbutylammonium N,Ndimethylglycinate ([N2224][DMG]), triethylbutylammonium 1-imidazole acetate ([N2224][IMA]), and triethylbutylammonium nicotinate ([N2224] [NIA]) at room temperature are 2747 cP, 2876 cP and 3307 cP, respectively [2]. Similarly, the hydrophobic protic IL, such as N,N,N′,N ′ tetramethyl 1,6 hexanediamine bis(trifluoromethylsulfonyl)imide ([TMHDA][Tf2N]), also suffers from high viscosity (N1600 cP at 298.2 K)

Fig. 2. Densities (a) and viscosities (b) of aqueous MLB-PILs as a function of temperature (■: [TMDAPH][Ac]; ●: [PMDPTAH][Ac]; ▲: [TDMAPAH][Ac]).

[41]. In comparison with these functionalized ILs, the aqueous MLB-PILs are more applicable in industry in consideration of the viscosity. The thermal gravimetric (TG) analysis curves of these three MLBPILs were shown in Fig. 3. The decomposition temperatures (Td) were taken as the onset of mass loss, defined as the intersection of the baseline before decomposition and the tangent to the mass loss afterward [48]. It was found that the initial decomposition temperature of [TMDAPH][Ac], [PMDPTAH][Ac] and [TDMAPAH][Ac] was 421.2 K, 431.2 K and 466.2 K, respectively. These values were comparable to other reported functionalized ILs [42,48], indicating the good stability of these three MLB-PILs. In addition, the MLB-PIL with more Lewis base groups was found to decompose at a higher temperature, resulting from the strong ionic linkage between the cation and the anion [48]. 3.3. H2S absorption in aqueous MLB-PILs The solubility of H2S in the three aqueous MLB-PILs solutions at 313.2 K was measured in the pressure range of 0–1.2 bar (Fig. 4) (The corresponding solubility data are available in Supporting Information). It is found that the absorption isotherms of H2S in these three samples show typically non-ideal profiles. The solubility of H2S rises steeply

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W. Zheng et al. / Journal of Molecular Liquids 263 (2018) 209–217 Table 1 A summary of the solubility of H2S in MLB-PILs and other absorbents. Entry

Absorbents

T (K)

Solubility (mol∙mol−1)

Ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

[TDMAPAH][Ac] (50 wt%) [PMDPTAH][Ac] (50 wt%) [TMDAPH][Ac] (50 wt%) MDEA (50 wt%) MDEA (35 wt%) [emim][Tf2N] [bmim][PF6] [N2224][IMA] [N2224][IMA] + 7%H2O [N2224][NIA] [emim][Ac] [MDEAH][For] [MDEAH][Ac] [DMEAH][For] [DMEAH][Ac] [TMPDA][Tf2N] [BDAMEE][Tf2N] [TMHDA][Tf2N]

313.2 313.2 313.2 313.2 343.2 313.2 313.2 313.2 313.2 333.2 313.2 313.2 313.2 313.2 313.2 313.2 298.2 298.2

1.92 1.15 0.65 0.84 0.43 0.07 0.08 0.50 0.60 0.32 0.44 0.06 0.10 0.09 0.13 0.16 0.52 0.67

this work this work this work [50] [50] [37] [39] [2] [2] [2] [1] [40] [40] [40] [40] [41] [41] [41]

Fig. 3. TG curves for the MLB-PILs.

with pressure in the low-pressure region and then levels off at higher pressures. It is supposed that H2S can be strongly trapped by the MLBPILs through strong interactions at low-pressure region and physical absorption dominates the process under high H2S partial pressure. The solubility of H2S in the three aqueous MLB-PILs follows the order of [TDMAPAH][Ac] N [PMDPTAH][Ac] N [TMDAPH][Ac] under the same pressure, which is consistent with the sequence of the number of Lewis base groups (or the tertiary amine groups) on the MLB-PILs. The result confirms our expectation that the tertiary amine has strong affinity for H2S due to its lone pair electrons. In addition, lengthening the alkyl chain and/or increasing the number of the side chain of the cation can also lead to a slight increase in the absorption capacity. This is because ILs with longer or more alkyl chains possess larger free volumes to accommodate more H2S molecules in physical manner, which has been confirmed via modeling by Bara and co-workers recently [49]. To compare the aqueous MLB-PILs with other absorbents, the solubility of H2S in aqueous MDEA [50], common ILs [37,39], carboxylatesbased functionalized ILs [1,2,40] and hydrophobic PILs [41] at ordinary pressure was presented in Table 1. It is clear that the absorption capacities of MLB-PILs in this work are incredibly large. For instance, 1 mol [TDMAPAH][Ac] could absorb 1.92 mol H2S at 1 bar, which is almost 28 and 24 times of the normal ILs 1 ethyl 3 methylimidazolium bis (trifle oromethanesulfonyl)amide, [emim][Tf2N] (Entry 6) and

Fig. 4. Solubility of H2S in aqueous MLB-PILs (■: [TMDAPH][Ac]; ●: [PMDPTAH][Ac]; ▲: [TDMAPAH][Ac]).

1 butyl 3 methylimidazolium hexafluorophosphate, [bmim][PF6] (Entry 7), respectively. In particular, the absorption capacity of [TDMAPAH][Ac] is even up to 1.45 mol/mol at a pressure as low as 0.2 bar (Fig. 4), which is still much larger than other absorbents at 1 bar (Entries 4–18). This is the largest solubility of H2S in liquid absorbents obtained so far. In addition, the absorption capacities of the aqueous [TDMAPAH][Ac] (Entry 1) and aqueous [PMDPTAH][Ac] (Entry 2) are superior to the aqueous MDEA solutions (Entries 4 and 5), which is the commonly used chemical solvent for the removal of H2S in industry. Comparing the H2S solubility in the MLB-PILs and other functionalized ILs with Ac− as the anion (Entries 11, 13 and 15) at 1 bar, it is found that the absorption capacities of MLB-PILs are several or even multiple times larger, justifying the advantage of the cation with multiple Lewis base groups on the absorption of H2S. As for the hydrophobic PILs (Entries 16–18), only [TMHDA][Tf2N] exhibits a comparable H2S solubility with the aqueous [TMDAPH][Ac], but much lower than that of the aqueous [TDMAPAH][Ac] and the aqueous [PMDPTAH] [Ac], not to mention the fact that the viscosity of [TMHDA][Tf2N] is up to 1600 cP at 298.2 K [41]. Considering the low cost, low viscosity and high absorption capacity, the aqueous MLB-PILs are believed to be an attractive alternative to other absorbents for the separation of H2S. 3.4. Thermodynamic modeling The temperature dependence of H2S absorption isotherms in these three aqueous MLB-PILs solutions were measured in the temperature ranges of 298.2–328.2 K (Fig. 5) (The corresponding solubility data are available in Supporting Information). It is found that the solubility of H2S decreases significantly with the increasing temperature, which is a typical phenomenon in most gas absorption process [1,51]. In order to simulate the H2S absorption in ILs, the reaction equilibrium thermodynamic model (RETM) was used to correlate the experimental solubility data. Comparing with other thermodynamic models such as the equation of state (RK, PR, or SAFT-EoS) and activity coefficients model based on group contribution method (UNIFAC), the advantage of RETM on the simulation of H2S solubility data is that only Henry's constants and reaction equilibrium constants are included as the parameters while other models require a number of parameters [41]. It has been mentioned that both physical absorption and chemical reaction occurred in the process of H2S absorption in MLB-PILs. Therefore, the RETM is especially suitable for modeling the H2S absorption behavior as both Henry's law constant (H) and the reaction equilibrium constant (K) are included [1,41]. The reaction mechanisms between H2S and the MLB-PILs need to be known to implement the RETM. It is noted that the reaction mechanism of 1:2 (one H2S molecule react with two IL molecules) is more likely to

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occur, such as the absorption of H2S in [emim][Ace], [bmim][Ace] and [BDMAEE][Tf2N] [1,41]. Generally speaking, these ILs contain only one functional group (reaction site) that can bind with the hydrogen protons in H2S. However, the MLB-PILs used in this work have multiple Lewis bases to react with H2S. It is inferred that a different reaction mechanism may be observed and it may vary with the number of Lewis base in the MLB-PILs. As for the [TMDAPH][Ac], both the free tertiary amine and the acetate anion can bind with H2S. As there are two active hydrogen protons in H2S, the possible maximum stoichiometric ratio is 1:1 (one H2S molecule reacts with one [TMDAPH][Ac] molecule). The 1:2 reaction could also occur during the absorption process. Therefore, the possible reactions taking place during the absorption of H2S in aqueous [TMDAPH] [Ac] can be written as: Aðg Þ þ BðlÞ→ABðlÞ

ð2Þ

Aðg Þ þ 2BðlÞ→AB2 ðlÞ

ð3Þ

In Eqs. 2 and 3, species A and B represent H2S and [TMDAPH][Ac], respectively; g and l stand for gas phase and liquid phase, respectively. The reaction equilibrium constants K1° and K2° of Eqs. 2 and 3 can be expressed as:

K1

°

K2

°

mAB m° ¼ P mB  γB ° m P°

ð4Þ

m γ AB2 AB2 m° ¼
 P mB 2  γ B m° P°

ð5Þ

γAB

The physical absorption can be expressed by the Henry's law: P ¼ Hγ A

mA m°

ð6Þ

The mass balance for H2S and [TMDAPH][Ac] can be written as: mA0 ¼ mA þ mAB þ mAB2

ð7Þ

mB0 ¼ mB þ mAB þ 2mAB2

ð8Þ

In Eqs. 4–8, K1° and K2° are the reaction equilibrium constants of reactions 2 and 3, respectively; P is the H2S pressure in bar; H is the Henry's law constant in bar; m° is the standard molality (1 mol∙kg−1); P° is the standard pressure (1 bar); mA, mB, mAB and mAB2 are the molalities of free H2S, free [TMDAPH][Ac], complex AB, and complex AB2 in mol∙kg−1 in the liquid phase; γA, γB, γAB and γAB2 are the corresponding activity coefficients of the species mentioned above in the liquid phase; mA0 is the total solubility of H2S in mol∙kg−1 in [TMDAPH][Ac]; mB0 is the initial molality of [TMDAPH][Ac] in mol∙kg−1 in the liquid phase. It is reasonable to assume that the value of γA is 1 as most of the absorbed H2S in the [TMDAPH][Ac] exists in complexed state so that the concentration of free H2S (mA) is very low. As for the values of γB and γAB2, it is noted that only the product of activity coefficients is needed in the calculation of K° as shown in Eqs. 4 and 5. In order to avoid the calculation of γB and γAB2, the product of activity coefficients in Eqs. 4 and 5 is assumed to be constant. Combing Eqs. 4–8, the

Fig. 5. Solubilities of H2S in three aqueous MLB-PILs at different temperatures (□: 298.2 K; ○: 313.2 K; △: 328.2 K; Lines: fitting results). (a) [TMDAPH][Ac]; (b) [PMDPTAH][Ac]; (c) [TDMAPAH][Ac].

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relationship between the total solubility of H2S in [TMDAPH][Ac] and H2S pressure can be expressed as:

mA0 ¼

" sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi#   8mB0 K 2 P 2 2 4K 2 PmB0 þ 1−K 1 P þ1 1− ð1 þ K 1 P Þ2 8K 2 P

þ

P H

ð9Þ

Similarly, for the [PMDPTAH][Ac] system, one more free tertiary amine is added in comparison with [TMDAPH][Ac]. Therefore, the possible maximum stoichiometric ratio is 3:2 (three H2S molecules react with two [PMDPTAH][Ac] molecules). The possible reactions in the absorption of H2S are shown as follows: Aðg Þ þ BðlÞ→ABðlÞ

ð2Þ

Aðg Þ þ 2BðlÞ→AB2 ðlÞ

ð3Þ

3Aðg Þ þ 2BðlÞ→A3 B2 ðlÞ

ð10Þ

The reaction equilibrium constant K°3 for Reaction (10) can be written as: mA3 B2 γA3 B2 ° K 3 ° ¼  3  m 2 mB P γB m ° °

ð11Þ

P

The final RETM reaction obtained to correlate the total solubility of H2S in ILs to the partial pressure of H2S is: mA0 ¼

  P þ K 1 PmB þ K 2 P þ 3K 3 P 3 mB 2 H

ð12Þ

in which the equation of mB is: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   ð1 þ K 1 P Þ2 þ 8 K 2 P þ K 3 P 3 mB0 −ðK 1 P þ 1Þ   mB ¼ 4 K 2 P þ K 3 P3

ð13Þ

As for [TDMAPAH][Ac], there are four reaction sites that can bind with the hydrogen protons in H2S. The possible maximum stoichiometric ratio is 2:1 (two H2S molecules react with one [TDMAPAH][Ac] molecule), which can be expressed as: 2Aðg Þ þ BðlÞ→A2 BðlÞ

ð14Þ

The corresponding equilibrium constant K°4 for Reaction (14) is written as:

K4 °

mA B γ A2 B 2° ¼  2 m mB P γB ° P° m

   P  þ K 1 P þ 2K 4 P 2 mB þ K 2 P þ 3K 3 P 3 mB 2 H

Table 2 Thermodynamic parameters of H2S–MLB-PIL systems calculated from the RETM Model. T (K) 298.2

313.2

328.2

4.64 ± 0.05 1.43 ± 0.07 5.56 ± 0.42 0.999

0 2.55 ± 0.18 1.41 ± 0.05 0.996

0 0.61 ± 0.06 1.45 ± 0.08 0.991

[PMDPTAH][Ac] 21.68 ± 2.84 K1 0 K2 15.40 ± 121.5 K3 H (bar) 1.14 ± 1.54 2 0.989 R

8.28 ± 0.53 0 3.73 ± 7.62 1.53 ± 0.83 0.996

1.75 ± 0.75 0 0.99 ± 3.69 1.19 ± 1.39 0.991

[TDMAPAH][Ac] 46.28 ± 14.68 K1 6.31 ± 18.38 K2 7.86E-8 ± 1.84 K3 926.75 ± 314.99 K4 H (bar) 2.58 ± 0.36 0.999 R2

17.30 ± 2.72 0.06 ± 2.90 1.10E-7 ± 0.79 143.64 ± 28.80 4.93 ± 1.76 1

4.91 ± 0.06 0 5.79E-6 ± 0.00 19.02 ± 0.14 6.48 ± 0.32 1

ð15Þ

The RETM equation derived from Eqs. 2, 3, 10 and 14 is written as: mA0 ¼

were adopted to correlate the solubility of H2S in [TMDAPH][Ac], [PMDPTAH][Ac] and [TDMAPAH][Ac], respectively, and the values of H, Ki (i = 1, 2, 3 and 4) at different temperatures were obtained and shown in Table 2. It is found that the correlation coefficients R2 at different temperatures are larger than 0.99 for most of the conditions, indicating the accuracy of the RETM model. As for the aqueous [TMDAPH] [Ac] solution, it is found that the equilibrium constant K1 is 4.64 at 298.2 K but 0 at high temperatures. It implies that the 1:1 reaction can occur at room temperature, i.e. one [TMDAPH][Ac] molecule can bind with one H2S molecule. With the temperature increasing, it needs two [TMDAPH][Ac] molecules to trap one H2S molecule, implying that the availability of [TMDAPH][Ac] drops as the temperature increases. This is consistent with the phenomenon that H2S solubility declines rapidly with the increasing temperature. For the system of [PMDPTAH][Ac], the equilibrium constants of Reaction (3) (K2) is 0 at all the measured temperatures, implying that the 1:2 reaction does not occur for the [PMDPTAH][Ac] system. It is inferred from the values of K1 and K3 that both 1:1 and 3:2 reactions are easily to take place so that the absorbed H2S is more likely to exist in the form of AB and A3B2. For the system of [TDMAPAH][Ac], it is shown that K1 and K4 are much larger in comparison with K2 and K3, indicating that the 1:1 reaction and the 2:1 reaction are more likely to happen during the absorption of H2S in the aqueous [TDMAPAH][Ac], whereas the other two kinds of reactions (1:2 and 3:2) almost do not occur. The values of K4 are of a magnitude larger than that of K1, implying that the model correlation favors a reaction mechanism that one IL molecule reacts chemically with two H2S molecules to form the complex of A2B type. By comparing data from different systems, the overall equilibrium constant K are found to increase with sequence of [TMDAPH][Ac] b [PMDPTAH][Ac] b [TDMAPAH][Ac], which is consistent with the case of solubility variation. As for the Henry's law constant H, its values are in the range of 1.14–6.48 bar, being a magnitude smaller than those of H2S in both normal ILs (20–40 bar) [1] and other functionalized ILs such as [DMEAH][Ac], [MDEAH][Ac] [40] and [emim][Ace] [41], implying the good affinity of H2S to the MLB-PILs in physical manner. In addition, H is observed to follow the sequence [PMDPTAH][Ac] b [TMDAPH] [Ac] b [TDMAPAH][Ac] in general, which is not in consistence with the experimental observations of H2S solubility in these PILs. Therefore, it is inferred that the solubility of H2S in these MLB-PILs is dominated by the chemical absorption rather than the physical absorption. The reaction equilibrium constants Ki vary negatively and dramatically with the increase of temperature, implying that the increasing temperature can weaken the interaction between H2S and the MLB-

ð16Þ

where mB is expressed as: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi   2    1 þ K 1 P þ K 4 P 2 þ 8 K 2 P þ K 3 P 3 mB0 − K 4 P 2 þ K 1 P þ 1   mB ¼ 4 K 2 P þ K 3 P3 ð17Þ where K1, K2 K3 and K4 are the reaction equilibrium constants based on the concentrations of species but not on the activities. Eqs. 9, 12 and 16

[TMDAPH][Ac] K1 K2 H (bar) R2

W. Zheng et al. / Journal of Molecular Liquids 263 (2018) 209–217

PILs so that the absorbed H2S can be easily removed by heating. Subsequently, the enthalpy change ΔHSOL and the entropy change ΔSSOL of H2S absorption in the MLB-PILs were calculated according to their solubility data at different temperatures to investigate the strength of interactions between the absorbents and H2S. ΔHSOL and ΔSSOL can be calculated from the following well-known Eqs. (20) and (21) [51], where x is the molar ratio of H2S to MLB-PIL. ΔHSOL ¼ R

∂LnP   ∂ 1 =T


ΔSSOL ∂LnP ¼− R ∂LnT

! ð18Þ x

 ð19Þ x

ΔHSOL and ΔSSOL can thus be obtained from the slopes of the curve LnP vs. 1/T and the curve LnP vs. LnT, respectively. Fig. 6 showed the dependence of LnP on 1/T and LnT for [TDMAPAH][Ac] at x = 1 mol∙mol−1. The values of ΔHSOL and ΔSSOL for the three aqueous MLB-PILs at different concentrations were listed in Table 3. It is shown that the absolute

215

Table 3 ΔHSOL and ΔSSOL of the absorption of H2S in MLB-PIL systems. x (mol∙mol−1)

ΔHSOL (kJ∙mol−1)

ΔSSOL (J∙mol−1∙K−1)

[TMDAPH][Ac] 0.3 0.4 0.5

−43.5 ± 1.3 −41.9 ± 3.5 −40.2 ± 5.1

−139 ± 6 −134 ± 13 −128 ± 18

[PMDPTAH][Ac] 0.5 0.6 0.7

−59.8 ± 5.3 −55.9 ± 5.6 −48.7 ± 6.4

−191 ± 14 −179 ± 15 −156 ± 18

[TDMAPAH][Ac] 0.4 0.7 1.0 1.5

−56.7 ± 2.4 −52.7 ± 2.0 −49.2 ± 1.4 −42.2 ± 0.6

−181 ± 5 −169 ± 4 −157 ± 2 −135 ± 0

value of ΔHSOL decreases slightly with the increase of H2S solubility. This is because there are excessive molecules of MLB-PIL at low uptake of CO2 to interact strongly with H2S [43]. In addition, it is found that the values of ΔHSOL (−40 to −60 kJ∙mol−1) for these three MLB-PIL systems are all lower than that of H2S in aqueous organic amines [50]. In particular, for the system of [TMDAPH][Ac], ΔHSOL is found to range from −40 to −44 kJ∙mol−1, being only about twice of the hydrogen bonding energy (−20 kJ∙mol−1) [1]. All these data indicate that the strength of the H2S–IL bond is weak to moderate, so that the absorption can be easily reversed by heating. In contrast to the small absolute values of ΔHSOL, the entropy change ΔSSOL is found to be much larger, with values ranging from −128 to −191 J∙mol−1∙K−1. It is known that the Gibbs free energy is defined as: ΔGSOL ¼ ΔH SOL −T∙ΔSSOL

ð20Þ

It is inferred that the effects of the smaller ΔHSOL and the larger ΔSSOL can be superimposed to result in the less negative ΔGSOL (lower absolute value). Therefore, the aqueous H2S-saturated MLB-PILs can be easily to undergo desorption at a relatively low temperature, and the energy input required in the regeneration process can be reduced, making these aqueous MLB-PILs be a kind of energy-efficient absorbents. 3.5. Recycling of MLB-PILs To examine the recyclability of the MLB-PILs for H2S absorption, the aqueous H2S-saturated [TDMAPAH][Ac] in an airtight vessel was heated to 353.2 K for 50 mins, as described in our previous work [1,2,40,43]. The released H2S was then removed from the vessel by multiple evacuations to the NaOH solution. In order to prevent the evaporation of water, the time of each evacuation lasted for only about 5 s. The regenerated solution was then reused for the measurement of gas solubility for the next cycle of H2S absorption–desorption experiment. The absorption–desorption experiments were performed for five cycles, and Fig. 7 exhibits the solubility of H2S in the aqueous [TDMAPAH][Ac] at 313.2 K and 1 bar during the five cycles. It is revealed that the absorption capacity of the absorbent in each cycle is close to each other with a maximum loss in the capacity of about 6%. In fact, most of the capacity loss appears in the first to the second cycle, since high vacuum is not realized (the pressure of the system at the end of regeneration is usually larger than 15 kPa). The high regeneration efficiency demonstrates that the MLB-PILs can trap H2S reversibly. 4. Conclusion

Fig. 6. Calculation of the absorption enthalpy change ΔHSOL (a) and entropy change ΔSSOL (b) of H2S absorption in [TDMAPAH][Ac].

In summary, to find highly effective ILs for the removal of H2S from fuel gases, a multiple Lewis base functionalization strategy was proposed and three novel MLB-PILs tethered with tertiary amine groups have been designed as the highly efficient absorbents. All of them

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W. Zheng et al. / Journal of Molecular Liquids 263 (2018) 209–217

Fig. 7. Recycling of [TDMAPAH][Ac] for the absorption of H2S (absorption condition: 313.2 K and 1 bar; desorption condition: 353.2 K and multiple evacuations).

display low viscosities (b25 cP at 298.2 K), which is beneficial for their industrial applications. In addition, the H2S solubility in these aqueous MLB-PILs is significantly larger (0.65–1.92 mol∙mol−1 at 313.2 K and 1 bar) comparing with other H2S absorbents, validating their great potential application in natural gas sweetening and opening up a new path to designing task-specific ILs for the capture of H2S. The RETM equation is utilized to correlate successfully the solubility data. The fitting results favor reaction mechanisms of AB2, AB/A3B2, and A2B complex formation for the [TMDAPH][Ac], the [PMDPTAH][Ac], and the [TDMAPAH][Ac] system, respectively. Furthermore, thermodynamic parameters such as the Henry's constant H, the equilibrium constant K, the enthalpy change ΔHSOL and the entropy change ΔSSOL are also obtained to evaluate the absorption of H2S into the three aqueous MLB-PILs. All the results demonstrate that the aqueous MLB-PILs may have potential use in the desulfurization of natural gas or syngas streams. Acknowledgement The authors appreciate the financial support of the National Natural Science Foundation of China (Nos. 21376115, 21576129 and 21676134). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2018.04.129. 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 H2S absorption, AICHE J. 59 (2013) 2227–2235. [2] K. Huang, D.N. Cai, Y.L. Chen, Y.T. Wu, X.B. Hu, Z.B. Zhang, Dual Lewis base functionalization of ionic liquids for highly efficient and selective capture of H2S, ChemPlusChem 79 (2014) 241–249. [3] J. David Lawson, A.W. Garst, Gas sweetening data: equilibrium solubility of hydrogen sulfide and carbon dioxide in aqueous monoethanolamine and aqueous diet hanolamine solutions, J. Chem. Eng. Data 21 (1976) 20–30. [4] C.X. Li, W. Fürst, Representation of CO2 and H2S solubility in aqueous MDEA solutions using an electrolyte equation of state, Chem. Eng. Sci. 55 (2000) 2975–2988. [5] J.G. Lu, Y.F. Zheng, D.L. He, Selective absorption of H2S from gas mixtures into aqueous solutions of blended amines of methyldiethanolamine and 2 tertiarybutylamino 2 ethoxyethanol in a packed column, Sep. Purif. Technol. 52 (2006) 209–217. [6] O.R. Rlvas, J.M. Prausnitz, Sweetening of sour natural gases by mixed-solvent absorption: solubilities of ethane, carbon dioxide, and hydrogen sulfide in mixtures of physical and chemical solvents, AICHE J. 25 (1979) 975–984.

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