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Absorption of H2S and CO2 in Aqueous Solutions of Tertiary-Amine Functionalized Protic Ionic Liquids Kuan Huang, Jia-Yin Zhang, Xingbang Hu, and Youting Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03049 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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Graphic Abstract

ACS Paragon Plus Environment

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Absorption of H2S and CO2 in Aqueous Solutions of Tertiary-Amine Functionalized Protic Ionic Liquids Kuan Huanga,b*, Jia-Yin Zhanga, Xing-Bang Hub, You-Ting Wub* a

Poyang Lake Key Laboratory of Environment and Resource Utilization (Nanchang University),

Ministry of Education; School of Resources Environmental and Chemical Engineering, Nanchang University, Nanchang, Jiangxi 330031, China. b

School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210023,

China. ABSTRACT The development of ionic liquid-based absorbents for selective separation of H2S from CO2 is of interest to natural gas industry. In this work, a series of tertiary-amine functionalized protic ionic liquids (TA-PILs) were synthesized by neutralizing one of the tertiary amine groups in diamine compounds with acetic acid, leaving the other tertiary amine group as the functional site for selective absorption of H2S. To overcome the high viscosity of pure ionic liquids, TA-PILs were dissolved in water to give mixed absorbents. The effects of TA-PIL species, TA-PIL concentration and experimental temperature on the solubilities of H2S and CO2, and absorption kinetics of H2S and CO2 in aqueous TA-PILs were investigated systematically. The reaction equilibrium constants and absorption enthalpy of H2S and CO2 in aqueous TA-PILs were also calculated by correlating the solubility data with reaction equilibrium thermodynamic model (RETM). Results demonstrated that aqueous TA-PILs have high absorption capacities for H2S. Although the equilibrium selectivities of H2S/CO2 in aqueous TA-PILs are minimized, they still display significantly kinetic selectivities for the two gases. More importantly, the TA-PILs


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designed are featured with low cost and facile synthesis, making them more attractive than other functionalized ionic liquids reported in the literature for application in selective separation of H2S from CO2. KEYWORDS Hydrogen Sulfide; Carbon Dioxide; Protic Ionic Liquids; Tertiary Amine; Selective Absorption


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INTRODUCTION Natural gas has been regarded as one of the most promising alternative energy sources for industrial production and human activity, because of its cleanliness and convenience.1 Its share of the world’s energy consumption structure has reached 24.1 % in 2016, and may surpass coal and petroleum to become the world’s largest main energy source in the near future.1 The active component in natural gas is methane (CH4), with some other unwanted impurities such as hydrogen sulfide (H2S) and carbon dioxide (CO2). H2S is highly toxic and corrosive, and one of its burning product—sulfur dioxide (SO2)—is the major pollutant causing acid rain. Therefore, it is necessary to remove H2S from natural gas before its utilization, to ensure the safety of process and avoid the production of other gas pollutant. Aqueous organic amines are the most widely used absorbents for H2S removal from natural gas in the industry, owing to their reversibly chemical reactivity to H2S and low cost.2-3 The concentrated H2S is then fed to sulfur recovery unit (SRU) and converted to elemental sulfur (S8) through the well-known Claus process for ease of storage and transportation.4 To increase the efficiency of Claus process, CO2 should be excluded from the feed gas for SRU. Therefore, the selective separation of H2S from CO2 is also required in addition to the selective separation of H2S from CH4. To this end, the organic amines used for H2S removal should be tertiary amines, for example methyldiethanolamine (MDEA).2 Unlike primary amines or secondary amines, tertiary amines can not react with CO2 directly due to the absence of active protons. In aqueous solutions, CO2 should be hydrated to form carbonic acid first, and then carbonic acid reacts with tertiary amines through proton transfer. In contrast, H2S can react with tertiary amines directly as itself contains active protons. As a result, the absorption of CO2 in aqueous tertiary amines is much slower than that of H2S, and the selective separation of H2S from CO2 can be realized through the difference in absorption kinetics.


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However, the high volatility of organic amines is a significant problem associated with the amine scrubbing process, because it may produce volatile organic compounds (VOCs) to influence the safety of process and cause secondary pollution. Ionic liquids (ILs) have been proposed as a class of state-of-the-art materials in place of traditional organic solvents for gas separation process, because they have many unique properties such as wide liquid range, negligible volatility and high thermal stability.5-6 Especially, the unlimited combinations of cations and anions make it possible to design the structure of ILs according to specific tasks.5-6 As for the use of ILs in H2S removal from natural gas, some pioneers have determined the solubilities of H2S in a wide range of normal ILs composed of 1-alkyl-3-methylimidazolium cations

([Rmim])

and

anions

hexafluorophosphate

including

([PF6]),

chloride

([Cl]),

tetrafluoroborate

trifluoromethanesulfonate

([BF4]), ([TfO]),

bis(trifluoromethanesulfonyl)imide ([Tf2N]), methylsulfate ([MeSO4]), ethylsulfate ([EtSO4]), and tris(pentafluoroethyl)trifluorophosphate ([eFAP]).7-20 The absorption of H2S in these normal ILs display physical behavior due to the weak interaction of H2S with them, and the Henry’s constants fall in the range of 20~40 bar at 298.2 K. Furthermore, the solubilities of H2S in normal ILs are only 2~4 times as those of CO2, implying their poor ability for the selective separation of H2S from CO2. As the concentration of H2S in natural gas is normally very low, varying from ppm level to percent level, absorbents with chemical affinity to H2S are more preferred in most cases.21 To this end, our group designed several kinds of functionalized ILs that display chemical absorption for H2S. The first example is carboxylate-based ILs with weak alkalinity, which can absorb 0.2~0.4 mol/mol of H2S at 293.2 K and 0.1 bar, and 0.5~0.8 mol/mol of H2S at 293.2 K and 1 bar.22 The Lewis acid-base interaction between H2S and carboxylate anions was disclosed by


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thermodynamic modeling. However, carboxylate-based ILs can not differentiate H2S from CO2 in equilibrium solubilities, with the ideal selectivities of 1~2, as they can also react with CO2 through the formation of carbene-CO2 adduct23. Inspired by the chemistry of tertiary amines, we further designed tertiary-amine functionalized ILs, which can absorb comparable amount of H2S in relative to carboxylate-based ILs.24-25 Depending on the alkalinity of tertiary amine groups, the interaction between H2S and tertiary-amine functionalized ILs is based on either Lewis acid-base interaction or proton transfer reaction. In the absence of water, the ideal selectivities of H2S/CO2 in tertiary-amine functionalized ILs can reach 10~40 at 298.2 K and 1 bar. Although the difference in equilibrium solubilities of H2S and CO2 in tertiary-amine functionalized ILs diminishes significantly in the presence of water, the two gases can still be separated kinetically. The carboxylate-based ILs and tertiary-amine functionalized ILs mentioned above suffer from complicated synthesis and/or require expensive reactants, which limit their practical application in the industry. Protic ILs (PILs) are a subcategory of ILs, which are of low cost and can be facilely prepared from the one-step neutralization of corresponding acid and base.26 Our group have demonstrated that alkanolammonium carboxylate-based PILs exhibit much higher solubilities of H2S than other normal ILs, with the Henry’s constants falling in the range of 3~10 bar at 303.2 K.27 However, the absorption of H2S in those PILs is still physical, making the solubilities of H2S in them rather inferior to that in tertiary-amine functionalized ILs. In this work, we innovatively designed a series of tertiary-amine functionalized PILs (TA-PILs), as shown in Scheme 1. The idea is to neutralize one of the tertiary amine groups in diamine compounds with acetic acid to form PILs, and leave the other tertiary amine group as the functional site for selective absorption of H2S. The TA-PILs combine the advantages of PILs (i.e., low cost and facile synthesis) and tertiary-amine functionalized ILs (i.e., high H2S


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solubilities and H2S/CO2 selectivities). Another issue associated with ILs for gas separation application is their high viscosity, which requires extra energy to transport them in pipelines and disfavors the diffusion of gases. A common referred solution is to integrate ILs and other solvents to form hybrid materials.28 Herein, mixed absorbents were constructed by dissolving TA-PILs in water, and the absorption behavior of H2S and CO2 in prepared aqueous solutions of TA-PILs were investigated systematically to demonstrate their potential application in the selective separation of H2S from CO2. EXPERIMENTAL Materials H2S (99.99 mol%) and CO2 (99.99 mol%) were supplied from Nanjing Messer Gas Co. Ltd., China. N,N,N’,N’-tetramethyl-1,2-ethylenediamine (TMEDA, 99 wt.%), N,N,N’,N’-tetramethyl1,3-propanediamine (TMPDA, 99 wt.%), bis(2-dimethylaminoethyl)ether (BDMAEE, 99 wt.%), bis(2,2-morpholinoethyl)ether (BMEE, 99 wt.%) and acetic acid (AcOH, 99 wt.%) were purchased from Aladdin Chemical Reagent Co. Ltd., China. All the chemicals were used directly without further purification. Synthesis and Characterizations The TA-PILs were synthesized by one-step neutralization of diamine compounds with equimolar AcOH. Taking the synthesis of [TMEDA][AcO] as an example: an aqueous solution of AcOH was added dropwise to an aqueous solution of TMEDA under vigorous stirring; the reaction was kept in an ice bath for 12 h to ensure the half protonation of TMEDA; most of the water was then removed by rotary evaporation; the final product was dried under vacuum at 353.2 K for 24 h. The synthesized TA-PILs were characterized by 1H NMR spectra,

13

C NMR

spectra, elemental analysis and mass spectra. 1H NMR and 13C NMR spectra were collected on a


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Bruker DPX 400 MHz spectrometer, using D2O as the solvent with TMS as the internal standard. Elemental analysis was performed on an Elementar Vario EL II system. Mass spectra were taken on a Shimadazu LC-MS 2020 spectrometer. Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449 C instrument from room temperature to 873 K at a heating rate of 10 K/min under N2. Characterization results are presented in the Supporting Information. There may be neutral base and acid in protic ILs due to the insufficient proton transfer between the base and acid. However, it is believed that the percentage of neutral base and acid in the four TA-PILs prepared in this work is negligible, because the signals of neutral base and acid can not be observed in the NMR spectra. The aqueous solutions of TA-PILs were prepared by dissolving specific amount of TA-PILs in water. Alternatively, the aqueous solutions of TA-PILs could also be prepared directly from raw materials by adjusting the amount of water used during the synthesis of TA-PILs, implying the viability of producing aqueous TA-PILs as H2S absorbents in large scale. The densities of aqueous TA-PILs were determined using an Anton Paar DMA 5000 automatic densiometer with a precision of 0.00001 g/cm3. The viscosities of aqueous TA-PILs were measured on a Brookfield LVDV-II+Pro viscometer with an uncertainty of ±1 % in relation to the full scale. Measurement of gas solubilities The apparatus for measuring gas solubilities in aqueous TA-PILs is the same as that reported in our previous work.25 The whole apparatus consists of two 316 L stainless steel chambers, the volumes of which are 120.802 cm3 (V1) and 47.368 cm3 (V2) respectively. The bigger chamber is used as gas reservoir to isolate the gas before it contacts with the absorbent in the smaller chamber. The smaller chamber is used as equilibrium cell, and equipped with a magnetic stirrer. The temperatures (T) of both chambers are controlled using a water bath with an


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uncertainty of ±0.1 K. The pressures in two chambers are monitored using two Wideplus-8 pressure transducers with an uncertainty of ±0.2 % in relation to the full scale. The pressure transducers are connected to a Wideplus-80 digital displayer to record the pressure change online. In a typical run, a known mass (w) of absorbent was loaded into the equilibrium cell, and the air in two chambers was evacuated. The residual pressure in equilibrium cell was recorded to be P0 ( [TMPDA][AcO] >> [TMEDA][AcO] >


10

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[BMEE][AcO]; therefore, [BDMAEE][AcO] was selected as the representative for following investigations. Physical properties Since the physical properties of absorbents are fundamental data for the process design of gas separation, we measured the densities and viscosities of aqueous solutions of [BDMAEE][AcO] at different concentrations and temperatures, and results are presented in Figure 2. Generally, the density decreases almost linearly, while the viscosity decreases nonlinearly, with the increase of temperature. The density and viscosity data can be fitted with Equations (2) and (3) respectively:

ρ = a + bT η =η0exp(

(2)

D ) T − T0

(3)

where ρ is the density in cm3/g, η is the viscosity in cP, T is the temperature in K, a, b, η0, D and T0 are empirical parameters. Fitting results are summarized in Table 1. A widely concerned issue associated with ILs is their high viscosity. However, the viscosities of aqueous [BDMAEE][AcO] prepared in this work are quite low due to the dilution effect of water, with the values falling in the range of 8.02~48.0 cP at 293.2 K, being comparable to those of aqueous MDEA29. Therefore, the aqueous [BDMAEE][AcO] have promising application in the industrial gas separation process. Effect of concentration Figure 3 shows the solubilities of H2S and CO2 in aqueous solutions of [BDMAEE][AcO] with different concentrations at 298.2 K. In general, the molar ratio solubilities of H2S and CO2 decrease with the increase of [BDMAEE][AcO] concentration, i.e., the decrease of water weight percentage, which can be explained by the fact that water itself can dissolve the two gases30,31.


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Since the solubilities of H2S and CO2 are defined as the molar ratio of gas/TA-PIL, the amounts of H2S and CO2 absorbed by water are counted on [BDMAEE][AcO]. As a consequence, decreasing the amount of water in aqueous [BDMAEE][AcO] (i.e., increasing the concentration of [BDMAEE][AcO]) would cause the decrease of relative solubilities of H2S and CO2 (i.e., molar ratio of gas/TA-PIL). It is also possible that the extent of gas absorption can not compensate the additional moles of [BDMAEE][AcO] introduced, thus making the relative solubilities of H2S and CO2 decreased. However, the dependence of CO2 solubilities on [BDMAEE][AcO] concentration is more significant than that of H2S solubilities. For example, the solubilities of CO2 at 1 bar decrease by 54 % (from 0.926 to 0.430 mol/mol), while the solubilities of H2S at 1 bar decrease by only 9 % (from 1.044 to 0.950 mol/mol), with the [BDMAEE][AcO] concentration increasing from 40 wt.% to 70 wt.%. According to the chemistry of tertiary amines, the reaction mechanism of TA-PILs with H2S and CO2 can be depicted in Scheme 2. Obviously, water is an essential component for the reaction of CO2 with [BDMAEE][AcO], while not for the reaction of H2S with [BDMAEE][AcO]. As a result, the increase of [BDMAEE][AcO] concentration, i.e., the decrease of water weight percentage, has more pronounced impact on the reactivity of CO2 to [BDMAEE][AcO]. Thus, the equilibrium solubilities of H2S and CO2 in aqueous [BDMAEE][AcO] can be differentiated by adjusting the concentration of [BDMAEE][AcO], and aqueous [BDMAEE][AcO] with higher concentration have larger selectivities of H2S/CO2. It should be noted herein that the preparation of aqueous [BDMAEE][AcO] with concentrations higher than 70 wt.% failed probably due to the limited solubility of [BDMAEE][AcO] in water. It was observed that [BDMAEE][AcO] can not be completely dissolved in water at room temperature if the weight percentages of [BDMAEE][AcO] are higher than 70 % in [BDMAEE][AcO]/H2O mixtures. They can not form


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homogeneous solutions. Therefore, the highest concentration of [BDMAEE][AcO] we investigated for H2S and CO2 absorption is 70 wt.%. Table 2 summarizes the solubilities of H2S and ideal selectivities of H2S/CO2 in aqueous [BDMAEE][AcO], as well as those in other absorbents reported in the literature. The ideal selectivities of H2S/CO2 were calculated by Equation (4):

Ideal selectivity of H2S/CO2 =

Solubility of H2S Solubility of CO2

(4)

The solubilities of H2S in aqueous [BDMAEE][AcO] are much higher than those in other ILs, including normal ILs (e.g., [emim][Tf2N])12, carboxylate-based ILs (e.g., [emim][Ac])22, tertiaryamine

functionalized

ILs

(e.g.,

[N2224][IMA],

[N2224][NIA],

[TMPDA][Tf2N]

and

[BDMAEE][Tf2N])24-25 and protic ILs (e.g., [DMEAH][Ac] and [DMEA][For])27, and comparable to that in aqueous MDEA32. The equilibrium selectivities of H2S/CO2 in aqueous [BDMAEE][AcO] are comparable to those in normal ILs, carboxylate-based ILs and aqueous MDEA, but much inferior to those in tertiary-amine functionalized ILs and protic ILs. It should be noted herein that the selectivity data of tertiary-amine functionalized ILs in Table 2 were reported for pure ILs, and their H2S/CO2 selectivities vanished in the presence of water according to our previous work.25 Actually, the selective separation of H2S from CO2 by aqueous tertiary-amine based absorbents is realized mainly through kinetic selectivities2, which should also be the case for aqueous [BDMAEE][AcO]. The kinetically selective separation of H2S from CO2 by aqueous [BDMAEE][AcO] will be demonstrated in the following section. Another point should be pointed out is that aqueous [BDMAEE][AcO] can be facilely synthesized from cheap raw materials, making them more attractive than other ILs reported in the literature for application in the selective separation of H2S from CO2.


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Since the absolute solubilities of H2S in terms of wt.% are more concerned for industrial applications than the relative solubilities of H2S in terms of mol/mol, the absolute solubilities of H2S in terms of wt.% were also calculated, and results are presented in Table 2. It is found that, if the solubility data are defined as the weight percentage of gas in absorbents, the absolute solubilities of H2S in aqueous [BDMAEE][AcO] increase with the increase of [BDMAEE][AcO] concentration. Therefore, [BDMAEE][AcO] is the key component for gas absorption. The absolute solubilities of H2S in aqueous [BDMAEE][AcO] are impressive as well, being much higher than those in normal ILs (e.g., [emim][Tf2N])12, tertiary-amine functionalized ILs (e.g., [TMPDA][Tf2N] and [BDMAEE][Tf2N])25 and protic ILs (e.g., [DMEAH][Ac] and [DMEA][For])27, and comparable to carboxylate-based ILs (e.g., [emim][Ac])22, tertiary-amine functionalized ILs (e.g., [N2224][IMA] and [N2224][NIA])24 and aqueous MDEA32. Absorption kinetics To investigate the possibility of using aqueous [BDMAEE][AcO] for kinetically selective separation of H2S from CO2, the absorbed amount of H2S and CO2 in 70 wt.% [BDMAEE][AcO] as a function of absorption time were determined. To compare on the same basis, ~3.2 g of absorbent was loaded into the equilibrium cell, and H2S or CO2 was introduced to initial pressure of ~1.3 bar. The pressure decay in the equilibrium cell was recorded online until it remained constant, and the absorbed amount of gases at different time were calculated according to Equation (1); results are shown in Figure 4. It can be seen that the absorption of H2S in 70 wt.% [BDMAEE][AcO] reaches equilibrium within 5 min, while the absorption of CO2 still does not reach equilibrium after 30 min. Therefore, the kinetic selectivities of H2S/CO2 in aqueous [BDMAEE][AcO] are still very high, although the equilibrium selectivities are minimized. Effect of temperature


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Figure 5 shows the solubilities of H2S and CO2 in 70 wt.% [BDMAEE][AcO] at different temperatures. As can be seen, the solubilities of H2S and CO2 decrease with the increase of temperature, which is within expectation as the gas absorption process is normally exothermic. However, the dependence of CO2 solubilities on temperature is more significant than that of H2S solubilities. For example, the solubilities of CO2 at 1 bar decrease by 89 % (from 0.430 to 0.047 mol/mol), while the solubilities of H2S at 1 bar decrease by only 67 % (from 0.950 to 0.312 mol/mol), with the temperature increasing from 298.2 to 333.2 K; thus, the equilibrium selectivities of H2S/CO2 in 70 wt.% [BDMAEE][AcO] are improved by 3 times (from 2.2 to 6.6, see Table 3). Therefore, higher temperature is beneficial for the selective separation of H2S from CO2 in aqueous [BDMAEE][AcO]. In our previous work,27 we reported that the equilibrium selectivities of H2S/CO2 in protic ILs (i.e., [MDEAH][Ac], [MDEAH][For], [DMEAH][Ac] and [DMEAH][For]) decrease slightly (no more than 20%) with the increase of temperature. However, in this work, we found that the equilibrium selectivities of H2S/CO2 in TA-PILs (i.e., [BDMAEE][AcO]) increase significantly (by 3 times) with the increase of temperature. This phenomenon can be explained by the fact that the protic ILs reported in our previous work enable only physical absorption for H2S, while the TA-PILs reported in this work enable chemical absorption for H2S. For the physical absorption of H2S in protic ILs, the negative effect of temperature on the solubilities of H2S is very significant because of the weak interaction of protic ILs with H2S. Nonetheless, for the chemical absorption of H2S in TA-PILs, the negative effect of temperature on the solubilities of H2S is not that significant because of the strong interaction of TA-PILs with H2S. That is to say, the increase of temperature would result in much more decreased H2S solubilities in protic ILs than in TA-PILs. Thermodynamic modeling


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In order to better understand the absorption behavior of H2S and CO2 in aqueous [BDMAEE][AcO], we used the reaction equilibrium thermodynamic model (RETM), which was proposed in our previous work22 and widely used in the thermodynamic modeling of chemical absorption of acidic gases25,

33-39

, to correlate the solubilities of H2S and CO2 in 70 wt.%

[BDMAEE][AcO]. For the absorption of H2S in aqueous [BDMAEE][AcO], the overall reaction can be expressed by Equation (5): (5)

A(g) + C(l) → AC(l)

where A, C and AC stand for H2S, [BDMAEE][AcO] and ammonium hydrosulfide respectively; g and l represent gas phase and liquid phase respectively. The absorption of H2S in aqueous [BDMAEE][AcO] can be divided into two processes: physical absorption and chemical reaction. That is to say, the absorbed H2S exists in two forms in the liquid phase: free H2S and complexed H2S. The physical absorption, describing the equilibrium between gas-phase H2S and liquidphase free H2S, can be described by Henry’s law Equation (6). The chemical reaction, describing the equilibrium between gas-phase H2S and liquid-phase comlexed H2S, can be described by reaction equilibrium Equation (7). mA m°

(6)

mAC K A° = m° PA mC ⋅ P° m°

(7)

PA = H m,A

The mass balance of H2S and [BDMAEE][AcO] can be expressed by Equations (8) and (9) respectively. (8)

mA0 = mA + mAC


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Energy & Fuels

mC0 = mC + mAC

(9)

In Equations (6)~(9), PA is the pressure of H2S in bar; Hm,A is the Henry’s law constant of H2S in bar; K A° is the reaction equilibrium constant; m˚ is the standard molality (1 mol/kg); P˚ is the standard pressure (1 bar); mA, mC and mAC are the molality of free H2S, free [BDMAEE][AcO] and ammonium hydrosulfide in mol/kg in the liquid phase; mA0 is the total solubility of H2S in mol/kg in aqueous [BDMAEE][AcO]; mC0 is the initial molality of [BDMAEE][AcO] in mol/kg in aqueous [BDMAEE][AcO]. Combining Equations (6)~(9) and the following RETM Equation (10) can be obtained after simple derivation: mA0 =

K A° PA mC0 P + A ° K A PA + 1 H m,A

(10) For the absorption of CO2 in aqueous [BDMAEE][AcO], the overall reaction can be expressed by Equation (11): (11)

B(g) + C(l) + D(l) → BCD(l)

where B, C, D and BCD stand for CO2, [BDMAEE][AcO], water and ammonium bicarbonate respectively; g and l represent gas phase and liquid phase respectively. The absorption of CO2 in aqueous [BDMAEE][AcO] can also be divided into two processes: physical absorption and chemical reaction. That is to say, the absorbed CO2 exists in two forms in the liquid phase: free CO2 and complexed CO2. The physical absorption, describing the equilibrium between gas-phase CO2 and liquid-phase free CO2, can be described by Henry’s law Equation (12). The chemical reaction, describing the equilibrium between gas-phase CO2 and liquid-phase comlexed CO2, can be described by reaction equilibrium Equation (13).


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PB = H m,B

mB m°

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(12)

mBCD m° K B° = PB mC mD ⋅ ⋅ P° m° m° (13) The mass balance of CO2, [BDMAEE][AcO] and water can be expressed by Equations (14)~(16) respectively. mB0 = mB + mBCD

(14)

mC0 = mC + mBCD

(15)

mD0 = mD + mBCD

(16)

In Equations (12)~(16), PB is the pressure of CO2 in bar; Hm,B is the Henry’s law constant of CO2 in bar; K B° is the reaction equilibrium constant; m˚ is the standard molality (1 mol/kg); P˚ is the standard pressure (1 bar); mB, mC, mD and mBCD are the molality of free CO2, free [BDMAEE][AcO], free water and ammonium bicarbonate in mol/kg in the liquid phase; mB0 is the total solubility of CO2 in mol/kg in aqueous [BDMAEE][AcO]; mC0 is the initial molality of [BDMAEE][AcO] in mol/kg in aqueous [BDMAEE][AcO]; mD0 is the initial molality of water in mol/kg in aqueous [BDMAEE][AcO]. Considering that the absorbed amount of CO2 in aqueous [BDMAEE][AcO] is much lower than the initial molality of water in the experimental pressure range, for example the initial molality of water in 70 wt.% [BDMAEE][AcO] is 16.667 mol/kg, while the solubility of CO2 in 70 wt.% [BDMAEE][AcO] at 298.2 K and 1 bar is only 1.368 mol/kg, it is reasonable to assume


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that mD equals to mD0. Thus, combining Equations (12)~(16) and the following RETM Equation (17) can be obtained after simple derivation: K B° PB mC0 mD0 P mB0 = ° + B K B PB mD0 + 1 H m,B

(17)

The RETM Equations (10) and (17) were used to correlate the solubilities of H2S and CO2 in 70 wt.% [BDMAEE][AcO] respectively, to give the reaction equilibrium constants at different temperatures; results are shown in Figure 5 and Table 4. As can be seen, the RETM Equations can fit the experimental solubility data very well, with the values of R2 better than 0.99. The value of K A° is 2.85 at 298.2 K, indicating the moderately chemical reaction of H2S with [BDMAEE][AcO]. In comparison, the value of K B° is only 0.0219 at the same temperature, indicating the weakly chemical reaction of CO2 with [BDMAEE][AcO]. Therefore, the reactivity of H2S to [BDMAEE][AcO] is two magnitudes higher than that of CO2 in 70 wt.% aqueous solution. The absorption enthalpy of H2S and CO2 (∆H) in 70 wt.% [BDMAEE][AcO] were then calculated according to the van’t Hoff Equation (18) by drawing linear fit between lnK˚ and 1/T: ∂ ln K ° ∆H =− R ∂ (1/ T )

(18)

where R is the gas constant (8.314 J/mol·K); results are shown in Figure 6 and Table 4. The values of ∆HA and ∆HB were calculated to be -42.7 and -44.7 kJ/mol respectively, being comparable to the absorption enthalpy of H2S and CO2 in 50 wt.% MDEA32.

Recyclability To evaluate the recyclability of aqueous [BDMAEE][AcO] for H2S absorption, H2Ssaturated 70 wt.% [BDMAEE][AcO] was heated to 353.2 K under a vacuum of 0.01 bar for 2 h, and then reused for H2S absorption. Since water should evaporate from the mixture under the


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desorption conditions, water was supplemented to the equilibrium cell to keep the concentration of [BDMAEE][AcO] in the absorbent unchanged after each desorption experiment. However, [BDMAEE][AcO] is stable enough at 353.2 K. As shown in Figure 7, the weight loss of [BDMAEE][AcO] is negligible after heated at 353.2 K for 10 h. The absorption-desorption experiments were performed for five times, and Figure 8 shows the solubilities of H2S at 298.2 K and 1 bar throughout the five cycles. It can be seen that the absorption of H2S in 70 wt.% [BDMAEE][AcO] is completely reversible, and the solubilities of H2S remain almost unchanged after five times of recycling. In practical application, the regenerated H2S needs to be further dried because the water is inevitably evaporated out during the desorption process. Drying is also a necessary process for the industrial capture of H2S with aqueous organic amines. However, the organic amines are also easily evaporated out during the desorption process. In contrast, the evaporation of TA-PILs is negligible in the capture of H2S with aqueous TA-PILs. Therefore, aqueous TA-PILs still show advantages in saving energy over aqueous organic amines for H2S capture. Considering the protic nature of TA-PILs, the possible hydrolysis of them in aqueous solutions under high temperature may have influence on the selective separation of H2S from CO2. In order to examine the possible hydrolysis of TA-PILs in aqueous solutions and its effect on the selective separation of H2S from CO2, the 1H NMR and

13

C NMR spectra of 70 wt.%

aqueous [BDMAEE][AcO] after heated at 353.2 K for 48 h were collected, and compared with those of fresh sample (see Figure 9). It is found that no obvious change in chemical shifts can be observed, implying that the hydrolysis of TA-PILs is negligible under the desorption temperature. Furthermore, the solubilities of H2S and CO2 at 298.2 K in 70 wt.% aqueous [BDMAEE][AcO] after heated at 353.2 K for 48 h were also determined, and compared with


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those in fresh sample (see Figure 10). It is found that no obvious change in H2S solubilities and CO2 solubilities can be observed, validating the negligible effect of the hydrolysis of TA-PILs on the selective separation of H2S from CO2.

CONCLUSIONS In summary, a series of tertiary-amine functionalized protic ionic liquids (TA-PILs) were designed for the selective separation of H2S from CO2 in aqueous solutions. [BDMAEE][AcO] displayed the highest H2S and CO2 solubilities among the four TA-PILs synthesized owing to the strong alkalinity of free tertiary amine group in [BDMAEE][AcO]. The molar ratio solubilities of H2S and CO2 in aqueous [BDMAEE][AcO] decrease with the increase of [BDMAEE][AcO] concentration or experimental temperature; however, the dependence of CO2 solubilities on [BDMAEE][AcO] concentration or experimental temperature is more significant than H2S solubilities. Therefore, the equilibrium solubilities of H2S and CO2 in aqueous [BDMAEE][AcO] can be differentiated by adjusting the [BDMAEE][AcO] concentration or experimental temperature. Even though the equilibrium H2S/CO2 selectivities of [BDMAEE][AcO] are minimized in aqueous solutions, the selective separation of H2S from CO2 can still be realized in aqueous [BDMAEE][AcO] through kinetic difference. Thermodynamic modeling with reaction equilibrium thermodynamic model (RETM) disclosed that the reactivity of H2S to [BDMAEE][AcO] is two magnitudes higher than that of CO2. The results obtained in this work, together with the low cost and facile synthesis of TA-PILs, making the aqueous solutions of [BDMAEE][AcO] promising candidates for selective separation of H2S from CO2 in natural gas industry.

AUTHOR INFORMATION Corresponding Authors


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Page 22 of 42

*E-mails: [email protected]; [email protected]

Notes The authors declare no conflicts of interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21376115 and 21576129) and the Natural Science Foundation of Jiangxi Province (No. 20171BAB203019). K. H. thanks the sponsorship from Nanchang University.

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Table 1. Empirical parameters in Eqs. (2) and (3) fitted from experimental densities and viscosities 40 wt.% 50 wt.% 60 wt.% 70 wt.% Parameters [BDMAEE][AcO] [BDMAEE][AcO] [BDMAEE][AcO] [BDMAEE][AcO] a 1.213 1.238 1.256 1.287 b×104 -6.227 -6.791 -7.399 -8.566 η0×10-5 1808 4857 1.227 21896 D 1033 741.5 3832 304.4 T0 162.1 177.0 11.84 208.6


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Table 2. Summary of H2S solubilities and ideal H2S/CO2 selectivities in aqueous [BDMAEE][AcO] and other absorbents reported in the literature Absorbents Temperature H2S solubilitiesa H2S/CO2 selectivitiesa H2S solubilitiesa (K) (mol/mol) (wt.%) 40 wt.% [BDMAEE][AcO] 298.2 1.044 1.1 0.061 50 wt.% [BDMAEE][AcO] 298.2 1.004 1.1 0.072 60 wt.% [BDMAEE][AcO] 298.2 1.073 1.3 0.090 70 wt.% [BDMAEE][AcO] 298.2 0.950 2.2 0.093 [emim][Tf2N] 303.2 0.072 2.6 0.006 [emim][Ac] 293.2 0.585 1.4 0.105 [N2224][IMA] 298.2 0.846 9.3 0.096 [N2224][NIA] 298.2 0.867 12.3 0.095 [TMPDA][Tf2N] 298.2 0.158 6.0 0.013 [BDMAEE][Tf2N] 298.2 0.546 37.2 0.040 [DMEAH][Ac] 303.2 0.195 9.8 0.043 [DMEAH][For] 303.2 0.112 11.2 0.027 50 wt.% MDEA 298.2 0.944 1.0 0.118 a Data at 1 bar.


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Table 3. H2S solubilities and ideal H2S/CO2 selectivities in 70 wt.% aqueous [BDMAEE][AcO] at different temperatures Temperature (K) H2S solubilitiesa (mol/mol) H2S/CO2 selectivitiesa 298.2 0.950 2.2 313.2 0.722 3.6 333.2 0.312 6.6 a Data at 1 bar.


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Table 4. Thermodynamic parameters of H2S and CO2 absorption in 70 wt.% [BDMAEE][AcO] at different temperatures Temperature (K) H2S absorption CO2 absorption ∆HA ∆HB K A° K B° R2 R2 (kJ/mol) (kJ/mol) 298.2 2.85 0.996 -42.7 0.0219 0.999 -44.7 313.2 0.938 0.999 0.00744 0.999 333.2 0.459 0.998 0.00325 0.990


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Scheme 1. Chemical structures of TA-PILs.


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Scheme 2. Reaction mechanism of TA-PILs with H2S and CO2.


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Figure 1. Solubilities of H2S (A) and CO2 (B) in 60 wt.% aqueous solutions of different TAPILs at 298.2 K.


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Figure 2. Densities (A) and viscosities (B) of aqueous solutions of [BDMAEE][AcO] with different concentrations.


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Figure 3. Solubilities of H2S (A) and CO2 (B) in aqueous solutions of [BDMAEE][AcO] with different concentrations at 298.2 K.


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Figure 4. Pressure decay (A) and instant absorption amount (B) of gases in 70 wt.% aqueous solution of [BDMAEE][AcO] (temperature: 298.2 K; initial pressure: ~1.3 bar; absorbent mass: ~3.0 g).


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Figure 5. Solubilities of H2S (A) and CO2 (B) in 70 wt.% aqueous solution of [BDMAEE][AcO] at different temperatures.


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Figure 6. Linear fit between lnK˚ and 1/T for H2S and CO2 absorption in 70 wt.% aqueous solution of [BDMAEE][AcO].


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Figure 7. Isothermal TGA profile for [BDMAEE][AcO] at 353.2 K under flowing N2


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Figure 8. Recycling of H2S absorption in 70 wt.% aqueous solution of [BDMAEE][AcO] (absorption: 298.2 K, 1 bar; desorption: 353.2 K, 0.01 bar).


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Figure 9. 1H (A and C) and 13C NMR (B and D) spectra of 70 wt.% aqueous [BDMAEE][AcO] before (A and B) and after (C and D) heated at 353.2 K for 48 h.


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Figure 10. H2S and CO2 solubilities at 298.2 K in 70 wt.% aqueous [BDMAEE][AcO] before and after heated at 353.2 K for 48 h.


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Graphic Abstract

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