Selective Separation of Hydrogen Sulfide with Pyridinium-Based Ionic

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Selective Separation of Hydrogen Sulfide with Pyridinium-based Ionic Liquids Xue Wang, Shaojuan Zeng, Junli Wang, Dawei Shang, Xiangping Zhang, Jindun Liu, and Yatao Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04477 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Industrial & Engineering Chemistry Research

Selective Separation of Hydrogen Sulfide with Pyridinium-based Ionic Liquids Xue Wang,

a, b

Shaojuan Zeng,

a,

* Junli Wang, c Dawei Shang, a Xiangping Zhang,

a,

d,

* Jindun Liu, b and Yatao Zhang b

a

Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of

Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China. b

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou

450001, China. c

State Key Laboratory of Chemical Resource Engineering, Beijing University of

Chemical Technology, Beijing, 100029, China. d

College of Chemical and Engineering, University of Chinese Academy of Sciences,

Beijing 100049, China

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT In this work, pyridinium-based ionic liquids (ILs) were prepared and showed excellent performance in selective separation of H2S owing to existing active proton in H2S molecule. A series of pyridinium-based ILs were synthesized and characterized, and their physical properties including density, viscosity and thermal decomposition temperatures as well as H2S separation performances were systematically studied. The results indicated that the solubilities of H2S and CO2 in ILs increase significantly with increasing pressure and decrease with increasing temperature, while slightly increase with the growing length of the alkyl chains on the cations. Based on the analysis of experimental results, the molar solubility of H2S in these ILs declines following the order: [C8Py][SCN] > [C6Py][SCN] > [C4Py][SCN] > [C4Py][NTf2] > [C4Py][NO3] > [C4Py][BF4]. Further, the results revealed that the Henry's law constants of H2S in the corresponding ILs are much lower than those of CO2, indicating that the ILs can effectively and selectively separate H2S. Among the studied ILs with the same cation, [C4Py][SCN] has highest H2S/CO2 selectivity up to 8.99 at 303.15 K, which is about 1.5 to 4 times larger than that of the conventional imidazolium-based ILs. Moreover, the [C4Py][SCN] can also keep the stable absorption performance after five consecutive absorption and desorption cycles, implying that ILs can serve as good absorbents for H2S separation application. KEYWORDS: Pyridinium-based ionic liquids; H2S Solubility; Selectivity; Thermodynamic properties

2


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1. INTRODUCTION

Due to energetic and environmental problems, natural gas has been standing out as a clean alternative energy source to coal and oil. Considering the requirements of natural gas transportation and processing, the removal of acid gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2) is required.1 Besides, as a kind of toxic and corrosive gas, H2S is harmful to the environment and the human being, and CO2 results in the decrease of caloric value of natural gas. Therefore, the removing of H2S and CO2 from natural gases becomes very important. Currently, among the existing gases removal or treatment techniques, organic amines, such as monoethanolamine (MEA)、diethanolamine (DEA) and N-methyldiethanolamine (MDEA),2, 3 are widely used as the absorbents to remove H2S and CO2. Although organic amines have high gas absorption capacity, this method has its inherent drawbacks, such as volatility, corrosion, oxidative degradation and high energy consumption for regeneration. To address the challenges and meet the general requirement of economy and environment,4 the development of new alternative absorbents for H2S and CO2 capture is of great significance. Ionic liquids (ILs) have been reported exhibiting good affinity to acid gases. They are composed of cations and anions, which can be used as potential alternatives to replace organic substances used in natural gas treatment, owing to their unique properties, such as low vapor pressures, high thermal stability, and tunable structures.5-9 Up to date, a growing number of articles have been published on H2S and CO2 absorption in ILs.10-15 Jou et al.16 firstly reported H2S solubility in 3



1-butyl-3-methylimidazolium

hexafluorophosphate

([Bmim][PF6])

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under

the

conditions of temperatures range of 298.15-403.15 K and pressures up to 96.0 bar, which shows high H2S solubility of 0.84 mol H2S/mol IL, and the Henry’s law constants of H2S are much lower than those of CO2. Subsequently, Jalili et al.17 took different imidazolium-based ILs ([Bmim][PF6], [Bmim][BF4], and [Bmim][NTf2]) as examples to absorb H2S and made a comparation. It was found that H2S solubility in three ILs follows this order: [Bmim][NTf2] > [Bmim][BF4] > [Bmim][PF6], and H2S solubility in [Bmim][NTf2] is 0.79 molar fraction at 298.15 K and 14.0 bar. Furthermore, taking the effect of anions during the absorption process into consideration, researchers proposed a series of the imidazolium-based ILs for H2S and CO2 capture. Huang et al.18 determined a plenty of H2S solubility data in 1-alkyl-3-methylimidazolium carboxylates ILs at the temperatures from 293.15 to 333.15 K and pressures up to 3.5 bar. For instance, the solubility of H2S in 1-ethyl-3-methylimidazolium acetate ([Emim][Ace]), 1-ethyl-3-methylimidazolium propionate ([Emim][Pro]) and 1-ethyl-3-methylimidazolium lactate ([Emim][Lac]) are 0.53, 0.63 and 0.45 mol H2S/mol IL under ambient pressure at 303.15 K, respectively, indicating that the solubility of H2S increases dramatically with the increasing alkalinity of the anions. Besides H2S solubility, H2S/CO2 selectivity is also an important parameter to address natural gas purification.19-21 Although the solubility of CO2 in ILs has been widely investigated, the solubility of H2S and H2S/CO2 separation selectivity were relatively scarce, and most studies of the H2S focused mainly on the 4


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imidazolium-based ILs. Huang et al. designed and synthesized functionalized imidazolium-based ILs for highly efficient separate H2S from CO2, such as hydrophobic

protic

ILs

tethered

with

tertiary

amine

group,22

1-alkyl-3-methylimidazolium carboxylates,18 and dual Lewis-base functionalized ILs.23 These functionalized ILs can improve H2S solubility and H2S/CO2 selectivity. However, complicated synthesis process, higher viscosity and cost of these ILs disfavor their applications in industries. Therefore, ILs as absorbents for industrial applications still need further design and development. As we know, understanding the effect of ILs on H2S absorption performances and the mechanism between ILs and H2S is the key to the design of ILs, while the structure-property relationship has not been systematically studied up to now. Considering the low cost and toxicity, high thermal stability and biodegradability of pyridinium-based ILs, the major purpose of this work is to provide a guide for the design of new functionalized ILs by the structure-property relationship of pyridinium-based ILs. In this study, a series of pyridinium-based ILs with different cations and anions, including

N-butylpyridinium

bis(trifluoromethylsulfonyl)imide

([C4Py][NTf2]),

N-butylpyridinium tetrafluoroborate ([C4Py][BF4]), N-butylpyridinium thiocyanate ([C4Py][SCN]),

N-butylpyridinium

nitrate

([C4Py][NO3]),

N-hexylpyridinium

thiocyanate ([C6Py][SCN]) and N-octylpyridinium thiocyanate ([C8Py][SCN]) were synthesized, characterized and used for selective separation of H2S. H2S and CO2 solubilities at the temperatures range from 303.15 to 333.15 K and pressures from 0 to 20.0 bar, as well as the regeneration and recyclability of ILs were studied. 5



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Furthermore， Henry’s law constants, H2S/CO2 selectivity, and the thermodynamic properties including enthalpies, Gibbs free energies, entropies of H2S and CO2 in the pyridinium-based ILs were also calculated to explain their dissolution mechanisms. 2. EXPERIMENTAL SECTION 2.1. Materials H2S (purity > 99.99%) and CO2 (purity > 99.99%) were supplied by Beijing Beiwen Gas Factory. Chemicals of analytical grade were used for the synthesis of the ILs.

Sodium

tetrafluoroborate

(NaBF4,

98.0%),

lithium

bis(trifluoromethylsulfonyl)imide (LiNTf2 , 98.5%) and sodium thiocyanate (NaSCN, 98.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium nitrate (NaNO3 , 99.0%) were purchased from Beijing Chemical Works Co., Ltd. Ethyl acetate, dichloromethane and acetone were analytical grade and were purchased from Beijing Chemical Company. N-butylpyridinium bromide ([C4Py][Br], 97.0%)), N-hexylpyridinium bromide ([C6Py][Br], 97.0%)), and N-octylpyridinium bromide ([C8Py][Br], 97.0%)) were obtained from Linzhou Keneng Materials Technology Co., Ltd. All the ILs above were used after further purification. The purification process of ILs was according to the reported method.24 2.2. Synthesis, Characterization and Physical Properties of the Pyridinium-based ILs A series of pyridinium-based ILs including N-butylpyridinium tetrafluoroborate ([C4Py][BF4]), N-butylpyridinium bis(trifluoromethylsulfonyl)imide ([C4Py][NTf2]), N-butylpyridinium

thiocyanate

([C4Py][SCN]),

N-butylpyridinium

6


nitrate

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([C4Py][NO3]),

N-hexylpyridinium

thiocyanate

([C6Py][SCN])

and

N-octylpyridinium thiocyanate ([C8Py][SCN]) were synthesized according to our previous methods.25 All the ILs were dried under vacuum for 48 h at 323.15 K before use. The structures of all ILs in the study were shown in Figure 1. The structures of these ILs were confirmed by 1H NMR and 13C NMR spectroscopy with a Bruker 600 spectrometer in dimethyl sulfoxide (d6-DMSO). FTIR spectra were recorded in the range of 400-4000 cm-1 on a Thermo Nicolet 380 spectrometer. The results of FTIR spectrum and NMR analysis data of prepared pyridinium-based ILs were shown in Figure S1 (see the Supporting Information). The water content of ILs was measured with volumetric Karl Fischer titration (Metrohm, 787 KF Titrino,) and found to be less than 0.20 wt %. The bromide content of the ILs was determined by a PXSJ-226 Series ion meter (INESA Scientific Instrument Co. Ltd.) and found to be less than 0.40 wt %. The water contents and the residual halide contents of pyridinium-based ILs detail data were in Table S1 in supporting information. The density and viscosity of ILs were measured using a density meter (Anton Paar DMA 5000) and an automated micro viscometer (Anton Paar Lovis 2000 M/ME), respectively. The thermal decomposition temperatures were obtained on a TGA (Q5000 V3.15 Build 263) by heating samples from room temperature to 873.15 K at a heating rate of 10 K min-1 under N2 atmosphere at a flow rate of 25 mL/min. The thermal decomposition temperature was defined as the temperature corresponding to the loss 5% mass fraction of ILs. Table 1 showed the estimated uncertainties for the experimental measurement of each physical property. 7



2.3. Apparatus and Measurements H2S and CO2 solubilities in ILs under different conditions were measured by the gas-liquid equilibrium apparatus, which was similar to our previous work.26-29 The whole device consists of two stainless steel chambers whose volumes were 486.8 ± 0.1 ml and 31.6 ± 0.1 ml, respectively. The bigger chamber, as storage tank, isolated the gas before it contacted with the ILs in the smaller chamber, and the smaller chamber as the absorption vessel with a magnetic stirrer was used to speed up the reaction. In a typical experiment, about 6 g ILs were placed into the absorption vessel and then absorption vessel was placed in a state of vacuum. Subsequently, H2S in storage tank was injected into the absorption vessel slowly through the valve. At the same time, magnetic stirrer was opened to enhance the absorption. The pressure was measured by pressure transmitter with uncertainties of ± 0.5 kPa, and the pressure transducer were connected to a Numeric Instrument to record the pressure changes online. The temperature of the absorption vessel was controlled by a thermostatic water bath with an accuracy of ± 0.1 K. The absorption equilibrium was assumed when two pressures transducers kept constant for more than 30 min. Then more H2S could be injected into the absorption vessel to reach another equilibrium. The equilibrium pressure data has been determined more than three times with standard relative deviation being less than 2.0 %. The standard relative deviation is calculated by the equation: %RSD =

∑ ∑

∑

× 100.

Finally, according to the variation of the pressure in the absorption vessel and the storage tank, gas solubility was calculated with the Peng Robinson (PR) equation.30 8


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The PR equation was employed H2S solubility and shown as follows: p=

− 1 − + + −

' (' α = 0.45724 2 )( b = 0.0778

( 3 )(

α = -1 + . 1 − /0.1 2' 4 k = 0.3746 + 1.5422ω − 0.269227' 5

Where P is the pressure, T is the temperature, R is the gas constant and a, b, α are correlation coefficients. Tc, Pc, Tr are the critical temperature, critical pressure, and relative temperature, respectively, and ω is the eccentric factor. The molar volume under different temperature and pressure can be calculated by the formula, and the solubility of gas in different ILs can be evaluated by the following calculation: ; @ − AB 89:; = ∆ = ?= 6 ,; ,@

C9:; =

89:; 7 89:; + 8AB

Where Vs is the volume of the storage vessel, VA is the volume of the absorption vessel, VIL is the volume of ILs, Vm, S and Vm, A are the molar volume of storage tank and absorption vessel, ngas is the amount of gas dissolved in ILs and nIL is the amount of the ILs. To evaluate the feasibility of the apparatus, CO2 solubility in [Bmim][BF4] was measured at 298.15 K and compared with the available values reported from the

9



literatures. Figure 2 showed that the solubility is similar to that in literatures.31, 32 The relative deviations (%RD) between our data and the literature data are less than 4.0 %, which proves that our experiment apparatus and data are credible. %RD is calculated DEF G HEIJ D

by the equation: %RD=

EF G

× 100 where KLM and KNOP are experimental data

and literature data, respectively. In this work, gas solubility is calculated by the measured equilibrium pressure. The standard uncertainties of solubility data are mainly produced by error propagation from the pressure uncertainties, but from the uncertainties of temperature, volume, mass, and density are very small and can be neglected. The standard relative deviations (%RSD) for H2S and CO2 in ILs are less than 3.0 %, and the detail data have been provided in the supporting information. 2.4. Thermodynamic Properties The gas solubility can be expressed as Henry’s law constant. The Henry’s law constant H is defined as the following equation, in which f1 (T, p) represent the fugacity of gas, Peq is the equilibrium partial pressure and ϕ1 (T, P) is the fugacity coefficient can be calculated from equation. WX , ) )LY ∅X , ) H = lim = ?= 8 U→0 CX CX The ideal selectivity of H2S/CO2 ([\ ]⁄^_ ) in these ILs is calculated according to the following equation. [\ ]⁄^_ =

a^_ 9 a\ ] 10


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The enthalpy (∆solH), the entropy (∆solS), and the Gibbs free energy (∆solG) of solvation are calculated by the following equations, respectively. ∆;bNc d = e8a ⁄) 0 10

∆;bNc a = −

∆;bNc [ =

f-e8a ⁄)0 2 11 f 1⁄

∆;bNc a − ∆;bNc d f-e8a⁄)0 2 =− − e8a⁄)0 12 f 1⁄

Where p0 represents the standard pressure. 3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties Physicochemical properties of ILs such as density, viscosity and thermal decomposition temperature play a significant role in screening suitable absorbents for gas separation. The density and viscosity of those ILs were determined in the temperatures range from 303.15 to 333.15 K as shown in Figure 3 (Table S2 and Table S3 in Supporting Information). As expected, both density and viscosity decrease with the increase of temperature, and the trend is in line with ILs reported by Huang’s work.33 Among these ILs, the [C4Py][NTf2] has highest densities and lowest viscosities than the others ILs. Cations and anions of ILs have great influence on physical properties. For the ILs with different alkyl chain length cations including [C4Py][SCN], [C6Py][SCN] and [C8Py][SCN], the orders of viscosity and density are opposite. Namely, the viscosity greatly decreases and the density increases with the increase of cations alkyl chain length. For the ILs with the different anions, the order of viscosity is: [C4Py][NO3] > [C4Py][BF4] > [C4Py][SCN] > [C4Py][NTf2], and the 11



viscosities of [C4Py][NO3] and [C4Py][NTf2] in 303.15K are 192.4850 and 48.1445 mPa s, respectively. The density of [C4Py][SCN] is the smallest, and the density of [C4Py][X] slightly increases from 1.1201 to 1.4498 g cm-3 at 303.15 K. Each experiment is repeated for at least three times, and the standard relative deviation of density and viscosity in this work are less than 1.0 %. In addition, the thermal stability of ILs is also very significant for gas separation. Therefore, the thermal decomposition temperatures of these pyridinium-based ILs were studied. As shown in Table 2, the thermal decomposition temperatures are all above 493.15 K, indicating that these ILs have good stability under the absorption and desorption conditions. The thermal stability of the ILs with [BF4]- and [NTf2]- anions are higher compared with the ILs with [SCN]- and [NO3]- anions, suggesting that anions have an important effect on the thermal stability. 3.2. H2S Solubility in Ionic Liquids H2S solubility in the pyridinium-based ILs at temperatures of 303.15, 313.15, 323.15 and 333.15 K and pressures up to 6.0 bar were shown in Figure 4-6, and the details data was summarized in Table S4. As shown in Figure 4-6, H2S solubility of six ILs under the same conditions follows the sequence: [C8Py][SCN] > [C6Py][SCN] > [C4Py][SCN] > [C4Py][NTf2] > [C4Py][NO3] > [C4Py][BF4], which has the same order as that of imidazolium-based ILs compared with literature data.34 The results showed that the cations and anions have a certain influence on H2S solubility. Hence, the effect of anions and cations on H2S absorption is discussed below. 3.2.1. Effect of Anions on H2S Solubility 12


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In order to obtain the effect of the anions of ILs, H2S solubility in four pyridinium-based ILs including [C4Py][SCN], [C4Py][NTf2], [C4Py][BF4] and [C4Py][NO3] at 313.15 K and pressure up to 6.0 bar were presented as shown Figure 5. H2S solubility in these four ILs decreases in the order of [C4Py][SCN] > [C4Py][NTf2] > [C4Py][NO3] > [C4Py][BF4]. For example, [C4Py][SCN] has the highest solubility of 0.10 molar fraction at 313.15 K and 1.0 bar, while [C4Py][BF4] has the lowest solubility of 0.07 molar fraction. Apparently, the anions play a slight role in the absorption process. In general, greater free volume that can accommodate more gas molecules is regarded to be favorable for increasing gas solubility in ILs. The molar densities of these four ILs followed the order: [C4Py][NO3] > [C4Py][SCN] > [C4Py][BF4] > [C4Py][NTf2]. As the molar density increases, the free volume decreases.35 However, the trend of H2S solubility in ILs with different anions cannot show good agreement with the free volume of ILs. In other words, H2S solubility in these ILs cannot be simply explained by the free volume, and the gas-IL interactions should be considered. In fact, Huang et al.23 have found that there is close relationship between the anion-H2S interaction and H2S solubility. Therefore, for the ILs with different anions, not only the void volume but also the interaction between H2S and ILs has an important influence on solubility. The H2S-IL interaction for these four ILs will be discussed based on thermodynamic properties in section 3.6. 3.2.2. Effect of Cations on H2S Solubility Considering the higher H2S solubility and lower viscosity of [C4Py][SCN], the effect of alkyl chain length on cations of ILs with the [SCN]- anion on H2S solubility 13



was studied. The molar fraction of absorbed H2S by ILs as a function of partial pressure is shown in Figure 6. It was found that when the anion is same, the solubility of H2S obviously increases with the alkyl length of cations increases. [C8Py][SCN] exhibits the highest absorption capacity of H2S, which might be caused by the fact that ILs with longer alkyl chains possess larger free volumes to accommodate more H2S molecules. Similar trends are coincided with the absorption of H2S and CO2 in other ILs.11, 23, 36, 37 3.3. CO2 Solubility in Ionic Liquids In order to further obtain the H2S/CO2 selectivity, the solubility of CO2 in the ILs was also systemically determined under the same condition of H2S absorption, and the results were presented in Figure 7 and Table S5. As shown in Figure 7, CO2 solubility of the four ILs decreases in order of [C4Py][NTf2] > [C4Py][BF4] > [C4Py][NO3] > [C4Py][SCN]. The amount of dissolved CO2 in ILs grow significantly with the increasing CO2 partial pressure, while decrease with the increasing temperature. Among the four ILs, [C4Py][NTf2] has the highest CO2 solubility of 0.39 molar fraction at 19.0 bar, while CO2 solubility in [C4Py] [SCN] is only 0.18 molar fraction at 20.0 bar, which is very different from H2S absorption in these two ILs. The reason may be that the [NTf2]- anion has high affinity toward CO2 due to the more fluorination numbers, and the fluoriation of anions can increase free volume of [C4Py][NTf2] to accommodate more CO2.32, 38 In addition, it was observed that the behaviors of CO2 absorption are very similar in the studied ILs. The solubility of CO2 increases almost linearly with pressure, 14


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displaying apparently physical absorption. In particular, the solubility of H2S in ILs is much higher than CO2 solubility. For example, [C4Py][NTf2] can only absorb 0.11 molar fraction (313.15 K and 5.0 bar) of CO2. The reason is mainly attributed to the fact that the absence of active protons in CO2 molecule unlike H2S molecule, so that ILs exhibit weak interaction with CO2. 39, 40 3.4. Effect of Pressure and Temperature on H2S and CO2 The temperature and the pressure play a great role in H2S and CO2 absorption in the pyridinium-based ILs. [C4Py][X] was taken as an example to elucidate the effect of temperature and pressure on H2S and CO2 solubilities, and the results were presented in Figure 4 and 7. It was found that with the increasing of temperature, H2S and CO2 solubilities in the ILs dramatically decrease. For example, the molar fraction of H2S in [C4Py][SCN] varies from 0.10 to 0.07 at 1.0 bar and CO2 solubility falls from 0.06 to 0.05 at 8.0 bar with an increase of temperature from 303.15 to 333.15 K. On the contrary, the increase in pressure leads to the increase in H2S and CO2 solubilities in the pyridinium-based ILs, which shows a similar behavior to the reported literature.18, 41, 42

Compared with the low pressure, the influence of the temperature under high

pressure is more obvious, which is in good agreement with Yokozeki’s work.43 This result indicated that it is favorable for H2S absorption at lower temperature and higher pressure, and desorption can be carried out at higher temperature. 3.5. Henry's Law Constants of H2S and CO2 and H2S/CO2 Selectivity Henry's constant is an important parameter to study gas absorption behavior. The Henry's law constants of H2S and CO2 are shown in Figure 8 for the ILs studied in 15



this work with standard relative deviations less than 3.0 %. It is seen that the Henry's law constants of H2S and CO2 increase with the increasing of temperature. The Henry's law constants of H2S in the pyridinium-based ILs at 303.15-333.15 K varies from 11.9 to 23.4 bar, with the lowest for [C4Py][SCN] and the largest for [C4Py][BF4]. However, the Henry’s law constants of CO2 ranges from 45.4 to 139.5 bar in these ILs under the same condition, which is much higher than that of H2S. Among these ILs, the Henry’s law constant of CO2 in [C4Py][SCN] is the highest, while that of H2S in [C4Py][SCN] is the lowest, which implies that [C4Py][SCN] has higher H2S solubility and lower CO2 solubility. Therefore, the [C4Py][SCN] is more effective for selective separation of H2S from CO2 compared with other ILs. Based on the equation (9), the selectivity of H2S/CO2 ([\ ]⁄^_ ) in the four ILs was

calculated and shown in Table 3. [\ ]⁄^_ in ILs varies 2.78 to 8.99, which is 1.5-4

times higher than that of some reported imidazolium-based ILs, implying that these ILs can selectively separate H2S from CO2. Apparently, [C4Py][SCN] exhibits the higher H2S/CO2 selectivity than others ILs, owing to its higher H2S solubility and lower CO2 solubility. Meanwhile, the effect of temperature on H2S /CO2 selectivity was also studied. It was found that H2S /CO2 selectivity gradually decreases with the increase of the temperature. The reason may be that the temperature has a smaller effect on H2S solubility than CO2. 3.6. Thermodynamic Properties of H2S and CO2 in Ionic Liquids In order to further understand the dissolution behaviors of H2S and CO2 in pyridinium-based ILs, the thermodynamic properties including Gibbs free energy 16


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∆solG, enthalpy ∆solH and entropy ∆solS were obtained and summarized in Table 4 and 5 with standard relative deviations less than 5.0 %. As shown in these two tables, the ∆solG is positive and increase with temperature in a similar manner for the solubilities of H2S and CO2 in ILs, which means that lower temperature is favorable for H2S and CO2 absorption. The different dissolution behaviors of H2S and CO2 in these ILs lead to different enthalpy and entropy effects. As we know, the enthalpy of absorption is a fundamental data for the estimation of the interaction between gas and ILs. From Table 4 and 5, the enthalpy value of H2S and CO2 is negative, indicating that the dissolution process is exothermal. The larger absolute value of enthalpy for gas absorption process indicates the stronger interaction between gas and ILs. For instance, H2S solubility in [C4Py][SCN] is greater than that in [C4Py][BF4], so the absolute value of H2S absorption enthalpy in [C4Py][BF4] is lower than that in [C4Py][SCN] (10.50 vs. 12.59 kJ mol-1). In addition, the absolute value of ∆solH for H2S absorption in the pyridinium-based ILs is slightly larger than CO2, implying a stronger interaction between H2S and ILs. The absorption entropy ∆solS reflects the ordering degree of the solvent-solute system. It was found that entropy changes are negative, and H2S and CO2 in ILs with different anions systems have very close values of ∆solS. For example, ∆solS of H2S and CO2 in [C4Py][SCN] is all -62 J mol-1 K-1. Consequently, H2S and CO2 absorption behavior are mainly affected by small positive ∆solG and large negative ∆solH. 3.7. Recycling of Pyridinium-based Ionic Liquids For potential applications, the pyridinium-based ILs should possess not only high 17



absorption capacity and selectivity but also good regeneration performance. In order to evaluate the recyclability of the pyridinium-based ILs for H2S absorption, [C4Py][SCN] was regenerated under a vacuum of 0.1 kPa at 353.15 K for 2 h at the end of absorption. The absorption-desorption process was repeated for five times, and the solubility of H2S at 313.15 K and 1 bar was shown in Figure 9. It was seen that the solubility of H2S has not obvious change, implying pyridinium-based ILs have excellent recyclability. 4. CONCLUSIONS A series of the pyridinium-based ILs including [C4Py][NTf2], [C4Py][SCN], [C4Py][NO3], [C4Py][BF4], [C6Py][SCN] and [C8Py][SCN] were synthesized and their physical properties (density, viscosity, thermal decomposition temperatures), as well as gas absorption performances (H2S and CO2 solubility and H2S/CO2 selectivity) were systematically investigated. The result showed that the solubilities of H2S and CO2 in ILs increase with increasing pressure and decrease with increasing temperature significantly, while slightly increase with the growing length of the alkyl chains on the cations. The Henry's law constants of CO2 in [C4Py][SCN] is the highest, while that of H2S in [C4Py][SCN] is the lowest, indicating that [C4Py][SCN] is more effective for selective separation of H2S compared with other ILs. Among these ILs with the same cation, [C4Py][SCN] has highest H2S/CO2 selectivity up to 8.99 at 303.15 K, which is 1.5 to 4 times higher than that of the reported conventional imidazolium-based ILs. Furthermore, consecutive absorption and desorption experiments suggested that these ILs can be regenerated and recycled. Owing to the efficient H2S/CO2 separation 18


Page 18 of 38

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performances,

excellent

recyclability

and

good

thermal

stability,

these

pyridinium-based ILs have great potential to separate H2S as effective absorbents. SUPPORTING INFORMATION FTIR spectra and NMR data of prepared pyridinium-based ILs; Physical properties of ILs and solubility data of H2S and CO2. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]; [email protected] Tel.: +86-010-62558174. Fax: +86-10-82625243. ACKNOWLEDGEMENTS This work was supported by National Key R&D Program of China (2017YFB0603401-03), the National Natural Science Foundation of China (No. 21425625, 21506219, U1662122, 21676271), and the Beijing hundreds of leading talents training project of science and technology (Z171100001117154). REFERENCES (1) Carvalho, P. J.; Coutinho, J. A. P., The Polarity Effect upon the Methane Solubility in Ionic Liquids: A Contribution for the Design of Ionic Liquids for Enhanced CO2/CH4 and H2S/CH4 Selectivities. Energy Environ. Sci. 2011, 4, 4614-4619. (2) Rivas, O. R.; Prausnitz, J. M., Sweetening of Sour Natural Gases by Mixed-solvent

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H2S

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and

Diffusion

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H2 S

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Ionic

Liquid

1-(2-Hydroxyethyl)-3-methylimidazolium tetrafluoroborate. J. Chem. Eng. Data 2010, 55, 1663-1668. (22) Huang, K.; Zhang, X.-M.; Hu, X.-B.; Wu, Y.-T., Hydrophobic Protic Ionic Liquids Tethered with Tertiary Amine Group for Highly Efficient and Selective Absorption of H2S from CO2. AIChE J. 2016, 62, 4480-4490. (23) Huang, K.; Cai, D.-N.; Chen, Y.-L.; Wu, Y.-T.; Hu, X.-B.; Zhang, Z.-B., Dual Lewis Base Functionalization of Ionic Liquids for Highly Efficient and Selective Capture of H2S. ChemPlusChem 2014, 79, 241-249. (24)Shang, D.; Zhang, X.; Zeng, S.; Jiang, K.; Gao, H.; Dong, H.; Yang, Q.; Zhang, S., Protic Ionic Liquid [Bim][NTf2] with Strong Hydrogen Bond Donating Ability for Highly Efficient Ammonia Absorption. Green Chem. 2017, 19, 937-945. (25) Zeng, S.; Gao, H.; Zhang, X.; Dong, H.; Zhang, X.; Zhang, S., Efficient and Reversible Capture of SO2 by Pyridinium-based Ionic Liquids. Chem. Eng. J. 2014, 251, 248-256. 22


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(26) Li, Z.; Zhang, X.; Dong, H.; Zhang, X.; Gao, H.; Zhang, S.; Li, J.; Wang, C., Efficient Absorption of Ammonia with Hydroxyl-functionalized Ionic Liquids. RSC Adv. 2015, 5, 81362-81370. (27) Zeng, S.; Wang, J.; Bai, L.; Wang, B.; Gao, H.; Shang, D.; Zhang, X.; Zhang, S., Highly Selective Capture of CO2 by Ether-functionalized Pyridinium Ionic Liquids with Low Viscosity. Energy Fuels 2015, 29, 6039-6048. (28) Zhang, J.; Jia, C.; Dong, H.; Wang, J.; Zhang, X.; Zhang, S., A Novel Dual Amino-functionalized Cation-tethered Ionic Liquid for CO2 Capture. Ind. Eng. Chem. Res. 2013, 52, 5835-5841. (29) Xu, F.; Gao, H.; Dong, H.; Wang, Z.; Zhang, X.; Ren, B.; Zhang, S., Solubility of CO2 in Aqueous Mixtures of Monoethanolamine and Dicyanamide-based Ionic Liquids. Fluid Phase Equilib. 2014, 365, 80-87. (30) Lei, Z.; Han, J.; Zhang, B.; Li, Q.; Zhu, J.; Chen, B., Solubility of CO2 in Binary Mixtures of Room-temperature Ionic Liquids at High Pressures. J. Chem. Eng. Data 2012, 57, 2153-2159. (31) Yokozeki, A.; Shiflett, M. B.; Junk, C. P.; Grieco, L. M.; Foo, T., Physical and Chemical Absorptions of Carbon Dioxide in Room-temperature Ionic Liquids. J. Phys. Chem. B 2008, 112, 16654-16663. (32) Aki, S.; Mellein, B. R.; Saurer, E. M.; Brennecke, J. F., High-pressure Phase Behavior of Carbon Dioxide with Imidazolium-based Ionic Liquids. J. Phys. Chem. B 2004, 108, 20355-20365. (33) Huang, K.; Lu, J.-F.; Wu, Y.-T.; Hu, X.-B.; Zhang, Z.-B., Absorption of SO2 in 23



Aqueous Solutions of Mixed Hydroxylammonium Dicarboxylate Ionic Liquids. Chem. Eng. J. 2013, 215, 36-44. (34) Zhou, X.; Cao, B.; Liu, S.; Sun, X.; Zhu, X.; Fu, H., Theoretical and Experimental Investigation on the Capture of H2S in a Series of Ionic Liquids. J. Mol. Graphics Modell. 2016, 68, 87-94. (35) Sakhaeinia, H.; Taghikhani, V.; Jalili, A. H.; Mehdizadeh, A.; Safekordi, A. A., Solubility of H2S in 1-(2-Hydroxyethyl)-3-methylimidazolium Ionic Liquids with Different Anions. Fluid Phase Equilib. 2010, 298, 303-309. (36) Shariati, A.; Peters, C. J., High-pressure Phase Equilibria of Systems with Ionic Liquids. J. Supercrit. Fluids 2005, 34, 171-176. (37) Blanchard, L. A.; Gu, Z. Y.; Brennecke, J. F., High-pressure Phase Behavior of Ionic Liquid/CO2 Systems. J. Phys. Chem. B 2001, 105, 2437-2444. (38)Shannon, M. S.; Tedstone, J. M.; Danielsen, S. P. O.; Hindman, M. S.; Irvin, A. C.; Bara, J. E., Free Volume as the Basis of Gas Solubility and Selectivity in Imidazolium-based Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51, 5565-5576. (39) Koech, P. K.; Rainbolt, J. E.; Bearden, M. D.; Zheng, F.; Heldebrant, D. J., Chemically Selective Gas Sweetening without Thermal-swing Regeneration. Energy Environ. Sci. 2011, 4, 1385-1390. (40) Xu, H. J.; Zhang, C. F.; Zheng, Z. S., Solubility of Hydrogen Sulfide and Carbon Dioxide in a Solution of Methyldiethanolamine Mixed with Ethylene Glycol. Ind. Eng. Chem. Res. 2002, 41, 6175-6180. (41) Anderson, J. L.; Dixon, J. K.; Maginn, E. J.; Brennecke, J. F., Measurement of 24


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SO2 Solubility in Ionic Liquids. J. Phys. Chem. B 2006, 110, 15059-15062. (42) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F., Solubilities and Thermodynamic Properties

of

Gases

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Ionic

Liquid

1-N-butyl-3-methylimidazolium

hexafluorophosphate. J. Phys. Chem. B 2002, 106, 7315-7320. (43) Yokozeki, A.; Shiflett, M. B., Ammonia Solubilities in Room-temperature Ionic Liquids. Ind. Eng. Chem. Res. 2007, 46, 1605-1610.

25



Page 26 of 38

Table Captions:

Table 1. Standard uncertainties in measurements

Table 2. Thermal decomposition temperatures of the pyridinium-based ILs

Table 3. H2S/CO2 selectivity in the pyridinium-based ILs

Table 4. Thermodynamic parameters of H2S absorption in the pyridinium-based ILs

Table 5. Thermodynamic parameters of CO2 absorption in the pyridinium-based ILs

Table 1. Standard uncertainties in measurements measurement

estimated uncertainty

density

0.0001 g·cm-3

viscosity

0.0001 mPa s

stainless steel chambers volume

0.1 ml

pressure transmitter

0.5 kPa

temperatures of water bath

0.1 K

water content / wt %

0.01

halide content / wt %

0.01

mass of ILs

0.001g

26


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Table 2. Thermal decomposition temperatures of the pyridinium-based ILs ILs

Td1 /℃

ILs

Td /℃

[C4Py][BF4]

365

[C4Py][NO3]

237

[C4Py][NTf2]

363

[C4Py][SCN]

220

1

Td is the thermal decomposition temperature, which is defined as the temperature corresponding

to the loss 5% mass fraction of ILs.

Table 3. H2S/CO2 selectivity in the pyridinium-based ILs T(K)

[C4Py][SCN]

[C4Py][ NTf2]

[C4Py][NO3]

[C4Py][BF4]

303.15

8.99±0.22

3.35±0.16

6.21±0.26

4.02±0.20

313.15

7.88±0.21

3.12±0.15

6.09±0.24

3.84±0.19

323.15

7.84±0.25

2.93±0.15

5.89±0.25

3.74±0.20

333.15

7.44±0.20

2.78±0.14

5.41±0.21

3.28±0.16

27



Page 28 of 38

Table 4. Thermodynamic parameters of H2S absorption in the pyridinium-based ILs T(K)

[C4Py][SCN]

[C4Py][NTf2]

[C4Py][NO3]

[C4Py][BF4]

303.15

6.23±0.08

6.57±0.10

6.74±0.09

7.03±0.08

313.15

7.04±0.08

7.11±0.10

7.25±0.09

7.61±0.08

323.15

7.58±0.09

7.78±0.11

7.92±0.10

8.34±0.10

333.15

8.12±0.10

8.42±0.12

8.48±0.11

8.74±0.11

-12.59±0.21

-12.31±0.24

-11.17±0.25

-10.50±0.22

303.15

-62 ± 1

-62 ±1

-59 ±1

-58 ±1

313.15

-62 ±1

-62 ±1

-58 ±1

-57 ±1

323.15

-62 ±1

-62 ±1

-59 ±1

-58 ±1

333.15

-62 ±1

-62 ±1

-58 ±1

-57 ±1

∆solG (KJ mol-1)

∆solH (KJ mol-1) 303.15 313.15 323.15 333.15 ∆solS (J mol-1 K-1)

28


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Table 5. Thermodynamic parameters of CO2 absorption in the pyridinium-based ILs T(K)

[C4Py][NTf2]

[C4Py][BF4]

303.15

9.61±0.06

10.56±0.07

11.34±0.06

11.77±0.05

313.15

10.07±0.06

11.12±0.07

11.95±0.07

12.42±0.06

323.15

10.61±0.07

11.88±0.08

12.68±0.07

13.11±0.06

333.15

11.25±0.07

12.47±0.08

13.16±0.07

13.68±0.07

-9.12±0.22

-7.74±0.27

-7.42±0.20

-7.07±0.26

303.15

-61 ± 1

-60 ± 1

-62 ± 1

-62 ± 1

313.15

-61 ± 1

-60 ± 1

-61 ± 1

-62 ± 1

323.15

-61 ± 1

-60 ± 1

-62 ± 1

-62 ± 1

333.15

-61 ± 1

-60 ± 1

-61 ± 1

-62 ± 1

[C4Py][NO3]

[C4Py][SCN]

∆solG (KJ mol-1)

∆solH (KJ mol-1) 303.15 313.15 323.15 333.15 ∆solS (J mol-1 K-1)

29



Figure Captions:

Figure 1. Structures of the pyridinium-based ILs used in this work Figure 2. CO2 solubility in [Bmim][BF4] at 298.15 K: ■, Yokozeki et al. 31; ●, Brennecke et al. 32; ∆, This work Figure 3. Densities and viscosity of the pyridinium-based ILs at different temperatures Figure 4. The solubility of H2S in [C4Py][BF4] (a), [C4Py][NTf2] (b), [C4Py][SCN] (c), and [C4Py][NO3] (d) at 303.15 K (■), 313.15 K (●), 323.15 K (▲), and 333.15 K (▼) Figure 5. Influence of anions on H2S absorption of in pyridinium-based ILs at 313.15 K Figure 6. Influence of cations on H2S absorption of in pyridinium-based ILs at 313.15 K Figure 7. The solubility of CO2 in [C4Py][BF4] (a), [C4Py][NTf2] (b), [C4Py][SCN] (c), and [C4Py][NO3] (d) at 303.15 K (■), 313.15 K (●), 323.15 K (▲), and 333.15 K (▼) Figure 8. Henry's constants of H2S (a) and CO2 (b) in the pyridinium-based ILs under different temperature Figure 9. Recycling of [C4Py][SCN] for H2S absorption at 313.15 K and 1 bar

30


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Figure 1. Structures of the pyridinium-based ILs used in this work

Figure 2. CO2 solubility in [Bmim][BF4] at 298.15 K: ■, Yokozeki et al. 31; ●, Brennecke et al. 32; ▲, This work

31



Figure 3. Densities and viscosity of the pyridinium-based ILs at different temperatures

32


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Figure 4. The solubility of H2S in [C4Py][BF4] (a), [C4Py][NTf2] (b), [C4Py][SCN] (c), and [C4Py][NO3] (d) at 303.15 K (■), 313.15 K (●), 323.15 K (▲), and 333.15 K (▼)

33



Figure 5. Influence of anions on H2S absorption of in pyridinium-based ILs at 313.15 K

Figure 6. Influence of cations on H2S absorption of in pyridinium-based ILs at 313.15 K

34


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Figure 7. The solubility of CO2 in [C4Py][BF4] (a), [C4Py][NTf2] (b), [C4Py][SCN] (c), and [C4Py][NO3] (d) at 303.15 K (■), 313.15 K (●), 323.15 K (▲), and 333.15 K (▼)

35



Figure 8. Henry's constants of H2S (a) and CO2 (b) in the pyridinium-based ILs under different temperature 36


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Figure 9. Recycling of [C4Py][SCN] for H2S absorption at 313.15 K and 1 bar

37



Abstract Graphic

38


Page 38 of 38

Selective Separation of Hydrogen Sulfide with Pyridinium-Based Ionic

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