Study on absorption and spectral properties of H2S in carboxylate protic ionic liquids with low viscosity

Article abstract of DOI:10.1016/j.molliq.2018.07.011

The exorbitant price and high viscosity are two major disadvantages for ionic liquids (ILs), which influence their practical applications in gas separation. Here we synthesized ten carboxylate protic ionic liquids (PILs), containing N-ethylmorpholine acetate, N-ethylmorpholine propionate, N-ethylmorpholine butyrate, N-ethylmorpholine methoxylacetate, 4-(2-hydroxyethyl) morpholine acetate, and 4-(2-hydroxyethyl) morpholine methoxyacetate, triethylamine acetate, triethylamine propionate, triethylamine propionate, and triethylamine methoxylacetate, with low cost and viscosity for absorption of H₂S. The densities and viscosities of these carboxylate PILs were measured in the temperature range of (298.2 to 333.2) K at atmospheric pressure. The solubility of H₂S in these PILs was determined using an isochoric saturation technique at 298.2–318.2 K and 0–1.096 bar. It was found that the solubility of H₂S in these PILs increased with the increasing pressure and length of the alkyl chains on the carboxylic acid. The absorption processes obeyed Henry's law within the given experimental conditions and Henry's constants were calculated from solubility data. In addition, the FTIR and NMR spectral properties in the absorption process of H₂S in N-ethylmorpholine butyrate [NEMH][Bu] also were investigated to present important information on absorption mechanism. The results indicated that [NEMH][Bu] demonstrated a highest absorption capacity among these PILs and absorption of H₂S is a physical behavior. Comparisons of PILs with the common ILs and organic solvents were also done to demonstrate the advantages of PILs.

Full text of DOI:10.1016/j.molliq.2018.07.011

Accepted Manuscript
Study on absorption and spectral properties of H2S in carboxylate
protic ionic liquids with low viscosity
Tianxiang Zhao, Ping Li, Xi Feng, Xingbang Hu, Youting Wu
PII:
DOI:
Reference:
S0167-7322(18)31965-2
doi:10.1016/j.molliq.2018.07.011
MOLLIQ 9332
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Journal of Molecular Liquids
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Accepted date:
13 April 2018
2 July 2018
3 July 2018
Please cite this article as: Tianxiang Zhao, Ping Li, Xi Feng, Xingbang Hu, Youting Wu ,
Study on absorption and spectral properties of H2S in carboxylate protic ionic liquids with
low viscosity. Molliq (2018), doi:10.1016/j.molliq.2018.07.011
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ACCEPTED MANUSCRIPT
Study on absorption and spectral properties of H2S
in carboxylate protic ionic liquids with low viscosity
Tianxiang Zhao, Ping Li, Xi Feng, Xingbang Hu* and Youting Wu*
School of Chemistry and Chemical Engineering, Separation Engineering Research Center, Key
Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing 210093 (P. R.
China). Corresponding author. E-mail: huxb@nju.edu.cn (X. Hu); ytwu@nju.edu.cn (Y. Wu)
ABSTRACT. The exorbitant price and high viscosity are two major disadvantages for ionic
liquids (ILs), which influence their practical applications in gas separation. Here we synthesized
ten carboxylate protic ionic liquids (PILs), containing N-ethylmorpholine acetate, N-
ethylmorpholine propionate, N-ethylmorpholine butyrate, N-ethylmorpholine methoxylacetate,
4-(2-hydroxyethyl) morpholine acetate, and 4-(2-hydroxyethyl) morpholine methoxyacetate,
triethylamine acetate, triethylamine propionate, triethylamine propionate, and triethylamine
methoxylacetate, with low cost and viscosity for absorption of H2S. The densities and viscosities
of these carboxylate PILs were measured in the temperature range of (298.2 to 333.2) K at
atmospheric pressure. The solubility of H2S in these PILs was determined using an isochoric
saturation technique at 298.2 ~ 318.2 K and 0 ~ 1.096 bar. It was found that the solubility of H2S
in these PILs increased with the increasing pressure and length of the alkyl chains on the
carboxylic acid. The absorption processes obeyed Henry’s law within the given experimental
conditions and Henry’s constants were calculated from solubility data. In addition, the FTIR and
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NMR spectral properties in the absorption process of H2S in N-ethylmorpholine butyrate
[NEMH][Bu] also were investigated to present important information on absorption mechanism.
The results indicated that [NEMH][Bu] demonstrated a highest absorption capacity among these
PILs and absorption of H2S is a physical behavior. Comparisons of PILs with the common ILs
and organic solvents were also done to demonstrate the advantages of PILs.
KEYWORDS. H2S capture, N-ethylmorpholine, Ionic liquids, Solubility, Absorption
mechanism.
1. Introduction
Hydrogen sulfide (H2S) is one of the notorious acid gases, thus controlling its concentration
has been a world issue to be urgently solved in the field of air pollution control [1]. The presence
of highly toxic H2S in natural gas, syngas, and hydro desulfurization gas imposes significant
threat to the safety and efficiency of energy utilization [2,3]. Consequently, the reduction,
capture, or utilization of H2S discharged from industrial processes have become the central focus
of current scientific and technological studies. An important technology for natural gas
sweetening is absorption in liquids, in which organic amines are widely used as the absorbents
[4-8]. Typically, aqueous solutions of amines, such as monoethanolamine (MEA),
diethanolamine (DEA), N-methyldiethanolamine (MDEA), and diisopropanolamine (DIPA), are
widely used for removal of H2S from gas streams with dilute H2S content in natural gas
sweetening. Some physical absorbents, including methanol, propylene carbonate, polyethylene
glycol dimethyl ether, and N-methylpyrrolidone are also used to remove H2S in some
commercial processes [4-8]. However, inherent disadvantages, such as volatile loss of solvents,
equipment corrosion, oxide deactivation of amines, high energy consumption of desorption
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process, are still difficult to overcome. Furthermore, traditional processes for natural gas
sweetening are not preferred from the viewpoint of green and sustainable chemistry.
The development of efficient absorbent for the capture of H2S is of critical importance in the
absorption process. Ionic liquids (ILs), a kind of typical green solvents, have attracted much
attention in the field of H2S separation due to their unique features such as high thermal stability,
negligible vapor pressure, and adjustable chemical properties [2,9,10]. Compared to other gases
(e.g. CO2 and SO2), the solubility of H2S are available in only a few conventional ILs [11,12],
such as [Bmim][PF6], [Bmim][BF4], [Bmim][Tf2N], [Hmim][PF6], [Hmim][BF4], and
[Hmim][Tf2N]. Although significant progresses have been made in improving the absorption of
H2S by ILs, the cost and viscosity of the room temperature ionic liquids (RTILs) reported in
literature are still relatively high [11,12], and the high viscosity is usually unfavorable for the
diffusion of H2S.
Protic ionic liquids (PILs) are a kind of ILs with low cost and low viscosity, which can be
easily prepared from simple materials through the direct neutralization between corresponding
acid and base. In this respect, our research group have paid great attention to PILs for H2S
removal based on their favorable properties and excellent selectivity to separate H2S and CO2
[13-15]. These important conclusions encourage us to develop high performance PILs for H2S
removal. Although designed PILs exhibit good H2S absorption capacity, the problem of high
viscosity of these PILs still needs improvement [13-15]. For instance, the viscosity of
methyldiethanolammonium acetate ([MDEAH][Ac]) exceeded 270 mPa·s at 303.2 K [13].
Herein, we prepared ten kinds of PILs with the low viscosity and inexpensive price containing
three cations [N-ethylmorpholine, 4-(2-hydroxyethyl) morpholine, and trimethylamine] and four
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anions (acetate, propionate, butyrate, and methoxy acetate). The density and viscosity of these
carboxylate PILs, and solubility of H2S in these PILs were measured, respectively. These PILs
showed both low viscosity and considerable loading capacity of H2S. Particularly, N-
ethylmorpholine butyrate ([NEMH][Bu]) demonstrated a highest absorption capacity among
these PILs. The absorption of H2S in [NEMH][Bu] is a physical behavior according to results of
absorption and FTIR and NMR spectral. These results could be used to provide important
information for the design and operation of the absorption and desorption processes of H2S with
potential industrial application for these PILs.
2. Experimental section
2.1. Materials
All chemicals are analytically pure agents purchased from Chemical Co. Ltd. with a
commercial purity and were used without further purification unless otherwise indicated. H2S
(99.9%) was obtained from Nanjing Messer Gas. (Nanjing, China). The deuterated solvents were
purchased from CIL (Cambridge Isotope Laboratories). Completely specification of all
chemicals is listed in Table 1.
Table 1.
2.2. Synthesis and characterization of carboxylate PILs
The investigated carboxylate PILs in this work, including N-ethylmorpholine acetate
([NEMH][Ac]), N-ethylmorpholine propionate ([NEMH][Pro]), N-ethylmorpholine butyrate
([NEMH][Bu]), N-ethylmorpholine methoxylacetate ([NEMH][MOAc]), 4-(2-hydroxyethyl)
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morpholine acetate ([NHEMH][Ac]), and 4-(2-hydroxyethyl) morpholine methoxyacetate
([NHEMH][MOAc]), triethylamine acetate ([TEAH][Ac]), triethylamine propionate
([TEAH][Pro]), triethylamine propionate ([TEAH][Bu]), and triethylamine methoxylacetate
([TEAH][MOAc]), were prepared by direct neutralization between corresponding acid and base
[13] without any solvents. Their chemical structures are shown in Figure 1. Typically, the 50
mmol of butyric acid was dropwise added to equimolar N-ethylmorpholine under ice bath. The
o
reaction was stirred for 2 h, and then mixture was warmed to 50 C and stirred for 24 h. The
resulting crude product was washed for three times with diethyl ether to remove unreacted
reactants (PILs are insoluble in diethyl ether). The obtained product was dried under vacuum for
24 h to remove trace of solvent, offering brown liquid of [NEMH][Bu]. Other PILs were
synthesized referring to the same method. The chemical structure of the PILs prepared in this
work was confirmed by NMR and FTIR spectra. The Bruker AV 400 spectrometer was used to
conduct the NMR experiments. FTIR spectra were recorded on a Nicolet iS50 infrared
spectrometer.
Figure 1.
[NEMH][Ac]: 1H NMR (400 MHz, CDCl3) δ = 13.35 (s, 1H), 3.84 – 3.76 (m, 4H), 2.82 – 2.57
(m, 6H), 2.01 (s, 3H), 1.16 (t, J = 7.3 Hz, 3H) ppm. 13C NMR (101 MHz, CDCl3) δ = 175.55 (s),
65.68 (s), 52.13 (s), 22.09 (s), 10.23 (s) ppm. IR 2968.5, 2860.2, 2806.0, 1709.1, 1361.5, 1255.4,
1113.2, 1020.7, 860.4, 770.2, 612.2, 451.9 cm-3.
[NEMH][Pro]: 1H NMR (400 MHz, CDCl3) δ = 13.43 (s, 1H), 3.80 (d, J = 4.8 Hz, 4H), 2.69 –
2.55 (m, 6H), 2.29 (q, J = 7.6 Hz, 2H), 1.14 (dt, J = 10.4, 7.4 Hz, 6H) ppm. 13C NMR (101 MHz,
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CDCl3) δ = 178.58 (s), 65.78 (s), 52.16 (s), 28.32 (s), 10.30 (s), 9.47 (s) ppm. IR 2982.1, 2864.7,
2810.5,1713.6, 1456.3, 1210.3, 1120.0, 1020.7, 853.7, 767.9, 606.9 cm-3.
[NEMH][Bu]: 1H NMR (400 MHz, CDCl3) δ = 12.01 (s, 1H), 3.80 – 3.75 (m, 4H), 2.69 – 2.55
(m, 6H), 2.25 (t, J = 7.4 Hz, 2H), 1.73 – 1.57 (m, 2H), 1.15 (t, J = 7.3 Hz, 3H), 0.96 (t, J = 7.4
Hz, 3H) ppm. 13C NMR (101 MHz, CDCl3) δ = 177.79 (s), 65.76 (s), 52.15 (s), 37.18 (s), 18.71
(s), 13.78 (s), 10.27 (s) ppm. IR 2961.8, 2869.2, 1718.1, 1451.8, 1255.4, 1205.8, 1120, 1029.7,
903.3, 860.4, 774.7, 609.9, 454.2 cm-3.
1
[NEMH][MOAc]: H NMR (400 MHz, CDCl3) δ = 13.35 (s, 1H), 3.84 – 3.76 (m, 4H), 2.82 –
13
2.57 (m, 6H), 2.01 (s, 3H), 1.16 (t, J = 7.3 Hz, 3H) ppm. C NMR (101 MHz, CDCl3) δ =
175.18 (s), 71.25 (s), 65.09 (s), 58.74 (s), 52.04 (s), 51.60 (s), 9.71 (s) ppm. IR 2873.7, 2815.0,
1596.2, 1447.3, 1260.0, 1196.7, 1111.0, 1025.2, 939.4, 695.7, 603.1, 445.1 cm-3.
[NHEMH][Ac]: 1H NMR (400 MHz, CDCl3) δ = 8.66 (s, 2H), 3.85 – 3.74 (m, 6H), 2.90 – 2.73
13
(m, 6H), 2.02 (s, 3H) ppm. C NMR (101 MHz, CDCl3) δ = 176.62 (s), 65.49 (s), 59.97 (s),
57.05 (s), 52.99 (s), 22.13 (s) ppm. IR 2871.5, 1709.1, 1564.6, 1399.9, 1266.7, 1115.5, 1009.4,
912.4, 858.2, 646.0, 449.7 cm-3.
[NHEMH][MOAc]: 1H NMR (400 MHz, CDCl3) δ = 8.78 (s, 2H), 3.95 (s, 2H), 3.93 – 3.82 (m,
6H), 3.43 (s, 3H), 3.07 – 2.84 (m, 6H) ppm. 13C NMR (101 MHz, CDCl3) δ = 176.15 (s), 71.21
(s), 64.88 (s), 60.00 (s), 58.86 (s), 56.64 (s), 52.93 (s) ppm. IR 2871.5, 1594.0, 1399.9, 1205.8,
1115.5, 903.3, 700.2, 600.9, 435.6 cm-3.
[TEAH][Ac]: 1H NMR (400 MHz, CDCl3) δ = 13.69 (s, 1H), 2.99 (q, J = 7.3 Hz, 6H), 1.99 (s,
3H), 1.24 (t, J = 7.3 Hz, 9H) ppm. 13C NMR (101 MHz, CDCl3) δ = 176.80 (s), 44.70 (s), 23.06
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(s), 8.63 (s) ppm. IR 2975.3, 1711.3, 1564.6, 1361.5, 1248.7, 1004.9, 833.4, 650.5, 609.9, 442.9
cm-3.
[TEAH][Pro]: 1H NMR (400 MHz, CDCl3) δ = 14.21 (s, 1H), 2.91 (q, J = 7.3 Hz, 6H), 2.26 (q,
J = 7.6 Hz, 2H), 1.15 (dt, J = 33.1, 7.4 Hz, 12H) ppm. 13C NMR (101 MHz, CDCl3) δ = 180.09
(s), 44.70 (s), 29.56 (s), 10.10 (s), 9.00 (s) ppm. IR 2979.8, 1715.9, 1562.4, 1348.0, 1246.4,
1007.1, 828.8, 657.3, 451.9 cm-3.
[TEAH][Bu]: 1H NMR (400 MHz, CDCl3) δ = 12.12 (s, 1H), 2.91 (q, J = 7.3 Hz, 6H), 2.21 (dd,
J = 8.5, 6.4 Hz, 2H), 1.71 – 1.57 (m, 2H), 1.19 (t, J = 7.3 Hz, 9H), 0.94 (t, J = 7.4 Hz, 3H) ppm.
13C NMR (101 MHz, CDCl3) δ = 179.33 (s), 44.72 (s), 38.63 (s), 19.24 (s), 13.95 (s), 9.05 (s)
ppm. IR 2970.8, 1715.9, 1569.1, 1463.0, 1399.9, 1201.3, 1036.5, 842.4, 774.7, 465.5 cm-3.
[TEAH][MOAc]: 1H NMR (400 MHz, CDCl3) δ = 13.26 (s, 1H), 3.91 (s, 2H), 3.42 (s, 3H), 3.09
13
(q, J = 7.3 Hz, 6H), 1.29 (t, J = 7.3 Hz, 9H) ppm. C NMR (101 MHz, CDCl3) δ = 175.54 (s),
71.74 (s), 58.45 (s), 44.69 (s), 8.34 (s) ppm. IR 2975.3, 2889.5, 2815.0, 1596.2, 1381.8, 1201.3,
1113.2, 939.4, 828.8, 695.7, 549.0 cm-3.
2.3. Physical properties of carboxylate PILs
The density and viscosity of these PILs were measured in the temperature range of (298.2 to
333.2) K. The densities were determined using an DM40/DM45 DeltaRange/DM50 automatic
densiometer with a precision of 0.00001 g·cm-3, which was calibrated using distilled water at
desired temperature. The relative uncertainties of density measurements are estimated to be ±
0.02%. Viscosities were measured on a HAAKE Rheostress 600 viscometer with an uncertainty
of ± 1% in relation to the full scale. The measurement temperatures were controlled by a bath
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with an uncertainty of ± 0.1 K. TG traces were recorded on a PerkinElmer Pyris 1 TGA from
o
o
room temperature to 400 C with a scanning rate of 10 C·min-1 under N2 atmosphere to
determine the decomposition temperatures of [NEMH][Bu] and NEM.
2.4. Absorption of H2S
Experimental apparatus for the measurement of H2S solubility is similar to our previous work
[16]. The experimental facility is shown in Figure 2.
Figure 2.
The whole device consists of two 316 L stain less steel chambers whose volume are 120.802
cm3 (V1) and 47.368 cm3 (V2), respectively. The bigger chamber, used as gas reservoir, isolates
the gas before it contacts with the absorbent in the smaller chamber. The smaller chamber,
named as equilibrium cell, is equipped with a magnetic stirrer. The temperature (T) of both
chambers are controlled using a water bath with an uncertainty of ± 0.1 K. The pressures in the
two chambers are monitored using two pressures transducers (WIDEPLUS Precision
Instruments) of ± 0.2 % uncertainty (in relation to the full scale). 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 absorbent was placed into the equilibrium cell, and the
air in the two chamber was evacuated to be p0 (< 0.1 kPa). H2S from cylinder was then fed into
the gas reservoir to pressure of p1. The needle valve between the two chambers was turned on to
let the gas be introduced to the equilibrium cell. Absorption equilibrium was thought to be
reached when the pressure of the two chambers remained constant for at least 2 h. The
equilibrium pressures were denoted as p2 for the equilibrium cell as p’1 for the gas reservoir. The
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gas partial pressure in the equilibrium cell was pg = p2 － p0. The H2S uptake, n(pg), could thus
be calculated using Eq. (1):
n(pg) = ρg(p1, T)V1 － ρg(p’1, T)V1 － ρg(pg, T)(V2 － W/ρa)
(1)
where ρg (pi, T) represents the density of the gas in mol·cm-3 at pi (i = 1, 2, s) and T, and is
obtained according to the NIST standard reference data. The ρa is the density of absorbent in
g·cm-3 at T. The volume increase of the ILs in the equilibrium cell as the result of gas solubility
is very small and neglected. V1 and V2 represent the volumes in cm3 of the two chambers,
respectively. Continual determinations of solubility data at elevated pressure performed by
introducing more H2S into the equilibrium. The solubility of H2S was defined in terms of mole
fraction (xH2S) in this work. After determinations, the gas remaining in the chambers were
introduced into an off-gas absorber containing aqueous solution of NaOH to prevent gas leakage
into atmosphere. Solubility of H2S in ten PILs was measured under desired temperature and
pressure. The average reproducibility of the solubility data in this work is well within ± 2.0 %.
The H2S release was done after the absorption, in which the H2S saturated [NEMH][Bu] was
heated to 80 °C in a pre-heated oil bath, refluxed for one hour in a 25 mL single necked round-
bottom flask and then cooled down for another absorption cycle. The regenerative process was
determined according to H2S loading capacity and system weight change. (Attention: Due to the
high toxicity of H2S gas, experiments must be performed under good ventilation conditions).
3. Results and discussion
3.1. Physical properties
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The density and viscosity of absorbents are requisite fundamental data for the process design
of gas separation. These physical properties can also influence the absorption of gas in liquid.
The detailed density and viscosity data of these PILs are still unknown. Therefore, we measured
the density and viscosity of these PILs in the temperature range of (298.2 to 333.2) K at
atmospheric pressure. The results are listed in Table 2 and plotted in Figure 3. It was found that
the densities decrease linearly with the increasing temperature and alkyl chain on anion, whereas
the viscosities decrease in an exponential manner for all PILs. As is known, viscosity is an
important parameter that determines the diffusion of gas in liquid. Particularly, the NEM and
TEA-based PILs exhibit much lower viscosity (< 40 mPa·s), which is regard to be helpful to the
mass transfer of H2S during the absorption. Compared with normal imidazolium-based ILs
[11,12] and MDEA-based PILs in our previous work [14], the PILs have lower viscosities under
same experimental conditions. However, the extremely high viscosity was observed due to the
strong hydrogen bond network in [NHEMH][Ac] and [NHEMH][MOAc]. Therefore, the NEM
and TEA-based PILs with lower viscosities are more applicable in the real case of H2S capture.
The dependence of densities and viscosities on temperatures are presented by the first-order
equation (2) and Arrhenius-like equation (3) [17], respectively. The correlations of temperatures
dependence for densities and viscosities could be used to predict densities or viscosities for PILs
especially when the temperatures are different from experimental extent, this method is useful
for theoretical study and industrial application.
(2)
0
1
Ea
 0eRT
(3)
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where a0, a1, and η0, are the undetermined parameters, Ea is activation energy, ρ and η are the
densities and viscosities of PILs, respectively. These parameters could be obtained by fitting the
experimental values to aforesaid equations.
Table 2.
Figure 3.
The thermal stability of absorbent is another important factor that identifies their practical
application in industry, and a higher thermal decomposition temperature is necessary for the
recycling circle. For organic acid-based PILs, the thermal decomposition temperatures (Td) are
usually low, and the substances of acid or amine with low boiling point might also evaporate in
the TGA instrument under the same temperatures. Figure 4 shows the TG curve of the N-
o
ethylmorpholine (NEM) and IL of [NEMH][Bu], the Td of NEM appears to be below 50 C
o
although its boiling point is 115 C under normal pressure. It is exciting to find that
o
[NEMH][Bu] has higher Td values (above 85 C). The results showed that the volatility of
amines could be effectively reduced by the formation of ionic liquids, which also helped to
improve the reusability of the absorbent.
Figure 4.
3.2. Solubility of H2S in carboxylate PILs
The solubility of H2S in ten carboxylate PILs was measured at temperature of 298.2 K as a
function of H2S partial pressure. These solubility data are listed in Table 3 and plotted in Figure
5. It can be seen that the solubility of H2S shows linear correlation with H2S partial pressure,
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which agreed well with characteristic of physical absorption. Particularly, the solubility of H2S is
highest in [NEMH][Bu] (Figure 5A), achieving 0.156 mole fraction at 1.056 bar and 298.2 K. In
comparison with those normal ILs [11,12,18], the PILs exhibit a much better absorption
capacity. For example, [C4Py][BF4] and [C4Py][SCN] have the solubility of 0.07 and 0.10
molar fraction at 1.0 bar and 313.15 K, respectively [18]. However, the high viscosity was
observed due to the strong hydrogen bond network in [NHEMH][Ac], leading to a long time of
absorption equilibration. The [NHEMH][Ac] showed a lowest absorption capacity of H2S under
same absorption conditions (Figure 5B). For the same cation (Figure 5C), the solubility of H2S
increases with the increasing alkyl chain on anion. The solubility of H2S decreases in the order
[TEAH][Bu] > [TEAH][Pro] > [TEAH][Ac], which is consistent with the order of the alkalinity
of the three anions. It is worth mentioning that the introduction of methoxy group shows positive
effect on the H2S solubility ([TEAH][Ac] vs [TEAH][MOAc]). These results indicated that both
the alkalinity of anions and the ether oxygen atoms on the anions are beneficial to favoring the
absorption of H2S. The effect of different cations on the solubility of H2S was also discussed
(Figure 6). The solubility of H2S with the same anion follows the sequence [NEMH][Ac] >
[TEAH][Ac] > [NHEMH][Ac]. The champion solubility of H2S in MEM-based PILs may be due
to active sites of ether oxygen atom.
Table 3.
Figure 5.
Figure 6.
3.3. Henry’s law constant
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Gas solubility in the liquid is based on purely physical absorption and can be described
generally in terms of Henry’s law [19]. According to the result of the above H2S absorption in
carboxylate PILs, it can be described by Henry’s law. The solubility of H2S in these PILs can be
expressed with Henry’s constant based on mole fraction Hx as follows,
f H S(p,T, x)
2
p(p,T)
Hx (p,T)  lim

x0
x
x
(4)
where Hx is the Henry’s law constant based on the mole fraction, x is mole fraction of H2S
dissolved in PILs, f H S is the fugacity of H2S, p is the pressure of H2S at equilibrium, and
2
fugacity coefficient (φ) at relatively low pressure can be calculated using two-term Virial
equation. Thus, Hx were obtained by calculating the linear slope of fugacity vs mole fraction of
H2S and are summarized in Table 4. It can be seen that the Hx of H2S in these PILs at 298.2 K
scatter from 6.9 to 11.0 bar with the lowest for [NEMH][Bu] and the largest for [NHEMH][Ac].
Comparison with some RTILs or solvents reported in the literatures [11-13,18,20-22], the
present carboxylate PILs demonstrate competitive or superior absorption capacity of H2S (Table
4) based on their Henry’s law constant and molality concentrations. Therefore, owing to the low
cost, low viscosity, and the considerable performance in H2S absorption, carboxylate PILs are
believed to be potential absorbents in the sweetening of natural gas, even though more work
should be done before practical application.
Table 4.
3.4. Thermodynamic parameters
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To investigate the effects of temperature on the H2S absorption, [NEMH][Bu] was selected
due to its excellent absorption property of H2S. As shown in Figure 7, the solubility of H2S
decreases with the increasing temperature. The above results indicated that the absorbed H2S can
be stripped out from [NEMH][Bu] by heating or reducing of pressure. The thermodynamic
parameters, such as dissolution enthalpy changes (ΔH), dissolution entropy changes (ΔS), and
dissolution Gibbs free energy changes (ΔG), are important to understand and develop the H2S
capturing process. These data can reflect the dissolution behavior of H2S in ionic liquids from the
molecular level. Furthermore, these data are also crucial and essential for the design of process in
gas separation technology. Based on the solubility data of H2S at (298.2 to 318.2) K, three
thermodynamic parameters, including ΔH, ΔS, and ΔG, can be deduced according to the
following relationships [23,24]:
∆G = －RT ln(Hx/p0)
∆G = ∆H － T∆S
(5)
(6)
where R is the ideal gas constant (8.314 Pa·m3·mol-1·K-1), T is the absolute temperature, and
the standard pressure p0 = 1 bar. Here ΔH can reflect the intermolecular interaction between the
absorbents and H2S in the liquid phase, and ΔS is represents the change of solution order degree.
The thermodynamic parameters of ΔH and ΔS, and ΔG are given in Table 6. As for the H2S
dissolving in [NEMH][Bu] under specific conditions, the negative ΔH indicates the absorption
process is exothermic and that dissolution of H2S in [NEMH][Bu] is also favorable from the
enthalpy view, meanwhile, the low ΔH means easy desorption of the absorbents. The negative
ΔS indicates that the solution became a higher ordering degree after the dissolution of H2S from
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molecular level. It was found that ΔG are always less than zero, which reveals the absorption
processes of H2S in [NEMH][Bu] is spontaneous and enthalpy driving behavior under the given
conditions [23,24].
Figure 7.
Table 6.
Table 5.
The excellent absorption capacity of H2S in [NEMH][Bu] enables the PILs an attractive
substitute for the removal of H2S. However, for practical application, the reusability of
[NEMH][Bu] is very necessary. As expected, the absorbed H2S can be released completely by
o
bubbling N2 at 80 C in five absorption-desorption cycles, illustrating that the solvent can be
recycled and reused (Figure 8). The regenerated [NEMH][Bu] was then reused for the
measurement of gas solubility for the next cycle of H2S absorption-desorption experiment. After
five cycles of absorption and desorption processes, the absorption capacities of H2S decline
slightly within five cycles, which exhibit a relatively stable recyclability for H2S absorption.
Figure 8.
3.5. Spectral properties
All above these results are much valuable for [NEMH][Bu] as best candidate among
investigative PILs used for absorbing H2S from a gaseous stream. In this case, FTIR and NMR
were carried out to get the absorption mechanism of H2S in [NEMH][Bu]. Compared the FTIR
spectra before and after H2S absorption in [NEMH][Bu] (Figure 9), no new peaks or a significant
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shift of peaks can be observed, which means that the absorption process is physical absorptions
[25]. In addition, the chemical shifts of all hydrogen atoms of [NEMH][Bu] moved downfield
(Figure 10A), which is attribute to the solvent-induced shift based on the intermolecular
interactions between H2S and solvent molecule [26]. Notably, the chemical shift of protons (a +
a’) shifted from 3.72 to 3.58 ppm after H2S absorption, indicating that strong hydrogen bond
interactions have occurred between H2S and ether oxygen atom on N-ethylmorpholine.
Meanwhile, no obvious changes are found in 13C NMR (Figure 10B), suggesting that H2S
interacts physically with [NEMH][Bu]. From these results, it is evident that the H2S gas must be
physically absorbed in the [NEMH][Bu], providing no chemical bond between the IL and H2S.
Figure 9.
Figure 10.
4. Conclusion
In summary, ten kinds of carboxylate PILs were synthesized for absorption of H2S. Their
densities and viscosities were measured over the range of (298.2 to 333.2) K, and the density and
viscosity are found to decrease with the increasing temperature. The solubility of H2S in these
PILs increased with the increasing pressure and length of the alkyl chains on the carboxylic acid.
Based on the results of spectra and absorption experiments, the absorption processes obeyed
Henry’s law within the given experimental conditions and Henry’s constants were calculated
from solubility data. The [NEMH][Bu] demonstrated a highest absorption capacity among these
PILs and absorption of H2S is a physical behavior. The results illustrated that carboxylate PILs
are a kind of promising absorbents for separation of H2S.
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Acknowledgments
This work was supported by National Natural Science Foundation of China (No.21376115,
and 21676134) and Jiangsu Province Natural Science Foundation (BK20141311).
References and notes
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[16]K. Huang, Y. Chen, X. Zhang, S. Ma, Y. Wu, Fluid Phase Equilibr. 378 (2014) 21-33.
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Figure 1. Chemical structure of the PILs prepared in this work.
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Figure 2. Apparatus for the determination of H2S solubility. 1 H2S gas cylinder; 2 storage tank
of H2S; 3 equilibrium cell; 4 constant temperature bath; 5 pressure sensor; 6 real-time data
acquisition instrument; 7 PC; 8 microthermometer; 9 vacuum pump.
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1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
40
600
A
500
400
300
200
100
0
B
35
30
25
20
15
10
5
[NEMH][Ac]
[NEMH][Pro]
[NEMH][Bu]
[NEMH][Ac]
[NEMH][MOAc]
[NHEMH][Ac]
[NHEMH][MOAc]
[TEAH][Ac]
[NEMH][Pro]
[NEMH][Bu]
300
310
320
330
340
T (K)
[NEMH][MOAc]
[NHEMH][Ac]
[NHEMH][MOAc]
[TEAH][Ac]
[TEAH][Pro]
[TEAH][Bu]
[TEAH][MOAc]
[TEAH][Pro]
[TEAH][Bu]
[TEAH][MOAc]
0
300
310
320
330
T (K)
340
350
360
300
310
320
330
T (K)
340
350
360
Figure 3. The density (A) and viscosity (B) of carboxylate PILs from 298.2 to 338.2 K.
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100
80
60
40
20
0
Td
[NEMH][Bu]
NEM
0
40
80
120
160
200
240
280
T (oC)
Figure 4. The TG curves of the NEM and [NEMH][Bu], the decomposition temperatures Td is
defined as the temperature at which the IL loses 5% of its initial mass.
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0.18
0.15
0.12
0.09
0.06
0.03
0.00
0.12
A
B
0.10
0.08
0.06
0.04
[NEMH][Ac]
[NEMH][Pro]
0.02
[NHEMH][Ac]
[NHEMH][MOAc]
[NEMH][Bu]
[NEMH][MOAc]
0.00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
pH2S(bar)
pH2S(bar)
0.15
C
0.12
0.09
0.06
0.03
0.00
[TEAH][Ac]
[TEAH][Pro]
[TEAH][Bu]
[TEAH][MOAc]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
pH2S(bar)
Figure 5. Solubility of H2S in PILs at 298.2 K as a function of H2S partial pressure. (A):
NEM-based PILs; (B) NHEM-based PILs, (C): TEA-based PILs. lines: linear fit.
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0.15
0.12
0.09
0.06
[NEMH][Ac]
0.03
[TEA][Ac]
[NHEMH][Ac]
0.00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
pH2S(bar)
Figure 6. The effect of different cations on the solubility of H2S. lines: linear fit.
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0.16
0.14
0.12
0.10
0.08
0.06
298.2 K
308.2 K
318.2 K
0.04
0.02
0.00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
pH2S(bar)
Figure 7. The effect of temperature on the solubility of H2S in [NEMH][Bu]. lines: linear
fit.
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1.0
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
Number of cycles
Figure 8. Absorption/desorption cycles of H2S in [NEMH][Bu].
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[NEMH][Bu]
[NEMH][Bu] + H2S
4000
3500
3000
2500
2000
1500
1000
500
v cm-3
Figure 9. FTIR spectra for [NEMH][Bu] before and after capture of H2S.
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Figure 10. NMR spectra for [NEMH][Bu] before and after capture of H2S, A: 1H
NMR; B: 13C NMR..
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Table 1. Specification of chemicals.
Chemicals
N-ethylmorpholine
Source
Purity ( wt.%)
Purification method
Desiccation with 4A
molecular sieve
Desiccation with 4A
molecular sieve
Desiccation with 4A
molecular sieve
None
Energy Chemical,
Shanghai, China
Energy Chemical,
Shanghai, China
Aladdin Chemical, Beijing,
China
Sinopharm Chemical,
Shanghai, China
Sinopharm Chemical,
Shanghai, China
Sinopharm Chemical,
Shanghai, China
Adamas Chemical,
Shanghai, China
CIL (Cambridge
Isotope Laboratories)
≥ 99 %
4-(2-hydroxyethyl)
morpholine
≥ 99 %
≥ 99 %
≥ 99.5 %
≥ 99.5 %
≥ 99 %
≥ 98 %
≥ 99 %
Triethylamine
Acetic acid
Propanoic acid
Butyric acid
None
None
None
None
Methoxy acetic acid
CDCl3
29