Investigation of furoate-based ionic liquid as efficient SO2 absorbent

Article abstract of DOI:10.1039/c6nj03563a

A series of furoate anion-functionized ionic liquids (ILs) were synthesized and applied as new absorbents for SO₂. Results showed that these ILs possessed satisfactory performance for SO₂ absorption. At 293.15 K, the maximum absorption capacity was 0.69 and 0.24 g SO₂ per g P₄₄₄₂[FA] under 1.0 and 0.1 bar, respectively. The SO₂ enriched ILs were easily regenerated by bubbling N₂ at 343.15 K and recycled five times without obvious loss of absorption performance. The influences of temperature, SO₂ partial pressure, and water content in ILs on SO₂ absorption performance were systematically explored. The absorption capacity of ILs is discussed from aspects of the structure of cations, anions, and the interaction of cation-anion. The absorption mechanism was analyzed using NMR and FTIR methods. The selectivities of SO₂/CO₂, SO₂/N₂ and SO₂/O₂ were studied and [P₄₄₄₂][FA] achieved relatively high selectivity for SO₂ compared to other gases. Present furoate ILs demonstrate potential application as a SO₂ absorbent owing to their good performance.

Full text of DOI:10.1039/c6nj03563a

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Investigation of furoate-based ionic liquid
as efficient SO2 absorbent
Cite this:DOI: 10.1039/c6nj03563a
Dongshun Deng,* Yaotai Jiang and Xiaobang Liu
A series of furoate anion-functionized ionic liquids (ILs) were synthesized and applied as new absorbents
for SO2. Results showed that these ILs possessed satisfactory performance for SO2 absorption. At 293.15
K, the maximum absorption capacity was 0.69 and 0.24 g SO2 per g P4442[FA] under 1.0 and 0.1 bar,
respectively. The SO2 enriched ILs were easily regenerated by bubbling N2 at 343.15 K and recycled five
times without obvious loss of absorption performance. The influences of temperature, SO2 partial
pressure, and water content in ILs on SO2 absorption performance were systematically explored.
The absorption capacity of ILs is discussed from aspects of the structure of cations, anions, and the
interaction of cation–anion. The absorption mechanism was analyzed using NMR and FTIR methods.
The selectivities of SO2/CO2, SO2/N2 and SO2/O2 were studied and [P4442][FA] achieved relatively high
selectivity for SO2 compared to other gases. Present furoate ILs demonstrate potential application as a
SO2 absorbent owing to their good performance.
Received 15th November 2016,
Accepted 22nd January 2017
DOI: 10.1039/c6nj03563a
rsc.li/njc
functionalized IL of 1,1,3,3-tetramethylguanidinium lactate
([TMG][L]) for chemisorption of SO2,9 many anion-functionalized
Introduction
Sulfur dioxide (SO2), largely emitted into the atmosphere from ILs have been developed for efficient and reversible SO2 capture.
fossil-fuel-fired power plants, is widely regarded as a vital For example, Shang et al.10 combined 1,1,3,3-tetramethyl-
component of acid rain and smog, which cause serious harm guanidinium with phenolate, 2,2,2-trifluoroethanoxide, or
to ecology, environment, health, etc.1 Ways to control and imidazolate to form ILs as efficient SO2 absorbents. Wang’s
reduce SO2 release at low cost have attracted extensive attention group11–14 designed a series of azole-based, phenol-based, and
because of stringent policies for environmental protection.2 acylamido-based ILs as efficient materials to capture SO2
To date, many commercial Flue Gas Desulfurization (FGD) through multiple-site chemical and physical absorption, with
technologies have been utilized to remove SO2 from exhaust the highest absorption capacity of 0.95 g SO2 per g IL at
flue gas with aqueous slurries of limestone as the most efficient 293.15 K and 0.1 MPa. Huang et al.15 reported six dicarboxylic
and typical absorbent.3 However, a valuable sulfur resource is acid salt ILs as reversible absorbents for SO2 with the best
transferred into a by-product of CaSO4 and wastewater is performance from 0.456 g SO2 per g IL at 1.0 bar and 313.15 K.
produced in abundance during that process. Moreover, some Cui et al.16 developed halogenated carboxylate ILs to capture SO2.
organic solvents have also been reported to absorb SO2, but Although ILs demonstrated satisfactory performance as SO2 absor-
secondary pollution caused from their volatility is a simultaneous bents, the abovementioned functionalized anions still encountered
obstacle.4 Thus, exploring a renewable and efficient absorbent for limitations of uncertain toxicity and poor biodegradability.
removal and recovery of SO2 is still highly in demand.
The emergence of Green Chemistry, replacing hazardous
Recently, Ionic Liquids (ILs) have manifested efficacy as reagents by ‘‘green’’ chemicals, has become important,17 and
absorbents for acidic gas because of their good absorption the preparation of ILs from renewable (‘‘green’’) materials is
capacity as well as superior properties of high thermal stability, completely in accordance with the concept of Green Chemistry.
chemical inertness, negligible volatility, and structure diversity.5–8 By developing a bio-based anion to form functionalized IL,
Generally, the absorption capacity of SO2 in conventional ILs is 2-furoic acid is a good alternative to traditional carboxylic acids
not high. Fortunately, the most attractive feature of ILs is the and can be conveniently obtained by oxidation of furaldehyde,
introduction of functional groups into cations or anions to adjust which is a typical platform chemical from biomass; this was
performance of gas absorption. Since Wu et al. reported the first highlighted by the United States Department of Energy in
2004 and again in 2010 as a promising building block for
chemistry.18 2-Furoic acid has a pKa value of 3.16. It was reported
Zhejiang Province Key Laboratory of Biofuel, College of Chemical Engineering,
that an organic acid with a larger pKa value than sulfurous acid
(pKa = 1.81 at 298 K) would be chosen and neutralized to
Zhejiang University of Technology, Hangzhou 310014, China.
E-mail: dengdsh@zjut.edu.cn
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synthesize a functional IL for chemically capturing SO2.19 [N2222]Br, respectively. [P4442][FA] was obtained by direct neutral-
Furthermore, 2-furoic acid has the merit of a high normal ization of equimolar FA and [P4442][OH] according to the litera-
boiling point of 403–405 K.20 For example, even if there is some ture method.1 [N2222][FA], [N2222][BFA], and Ch[FA] presented
water exited in the furoated IL or flue gas, the derived 2-furoic as solids; the other ILs with different combinations of cations
acid, due to a neutralization reaction, can still be kept in the and anions were clear, bright, and viscous liquids at room
system during the regeneration of SO2-saturated ILs by heating. temperature.
Based on above considerations, here we describe a new strategy
The water content of the ILs was measured by Karl Fischer
to build an anion-functionalized furoate-based IL. Previous analysis (SF-3 Karl–Fischer Titration, Zibo Haifen Instrument
research indicated that a halo atom in carboxylates is helpful Co., Ltd) with a mass fraction of r0.3% in all cases. An
for improving their absorption capacity with SO2.16 Therefore, electronic balance (Mettler-Toledo ME204E) with an accuracy
5-bromo-2-furoic acid was also selected to prepare anion- of Æ0.0002 g was used to determine weights of the materials.
functionalized ILs for a comparative study. To investigate the Thermogravimetric analysis (TGA) was performed on a PerkinElmer
relationship between a structure and absorption performances, instrument Q50 under N2 atmosphere with a heating rate of
and to design a more competitive absorbent for SO2, a serial 10 K minÀ1, and the decomposition temperatures (Td) were
group of two furoate-based chemicals with four different cations determined according to the TGA curves. Bruker AVANCE III
were synthesized and their physicochemical properties, as well (500, 126, 202 MHz) and Nicolet 6700 FT-IR spectrometers were
as the performances of SO2 absorption, were systematically used to characterize the structures of ILs, and no obvious
investigated for the first time. Meanwhile, effects of temperature, impurity peaks were found. Electrospray ionization-MS (ESI-MS)
partial pressure of SO2 and the water content in ILs on the characterization was performed on a Waters 2695 Thermo LCQ
absorption capacity, and recyclability of the P4442[FA] were Deca XP plus LCMS spectrometer. The samples were stored in a
further investigated.
desiccator to avoid any effect of atmospheric humidity. The
1H NMR chemical shift data for the ILs were listed as follows.
1
[Ch][FA]. H NMR (500 MHz, DMSO-d6, TMS), d/ppm: 7.74
(1H, dd, O–CHQCH), 6.98–6.97 (1H, dd, CH–CHQC), 6.54–
6.53(1H, t, CHQCH–CH), 3.86–3.84 (2H, t, OCH2), 3.44–3.42
(2H, t, NCH2), 3.13 (9H, s, 3 Â NCH3), 2.51–2.50 (1H, s, COH).
13C NMR (126 MHz, CDCl3, TMS), d/ppm: 147.32, 145.20,
116.50, 111.65, 68.16, 56.21, 54.50. ESI-MS: m/z = 103.9
([Ch]+), 318.2 ([Ch][FA] + [Ch]+).
Experimental
Materials
SO2 and N2 (0.999, mass fraction purity, with the same below)
were supplied by Jingong Special Gas Co., Ltd (Hangzhou,
China). 2-Furoic acid (FA), tributylphosphine (P444, 0.95), bromo-
ethane (0.99), and tetraethylammonium bromide ([N2222]Br, 0.98)
were purchased from Aladdin Industrial Co., Ltd (Shanghai,
China). 5-Bromo-2-furoic acid (BFA, 0.99) and trihexyl(tetradecyl)
phosphonium bromide ([P66614]Br, 0.95) were obtained from J&K
Scientific Ltd (Beijing, China). Choline hydroxide (ChOH, 0.99)
was supplied by Shouguang Jinyu Industrial Co., Ltd (Jinan,
China). 201 Â 7(OH) Anion-exchange resin was obtained from
the Chemical Plant of NanKai University. The exchange capacity
of anion-exchange resin was 3.41 mmol gÀ1 (dry resin). All the
reagents were used without further purification.
[N2222][FA]. 1H NMR (500 MHz, DMSO-d6, TMS), d/ppm: 7.46
(1H, dd, O–CHQCH), 6.53 (1H, dd, CH–CHQC), 6.35–6.34
(1H, t, CHQCH–CH), 3.23–3.19 (8H, m, 4 Â NCH2), 1.17–1.13
(12H, t, 4 Â CH3). 13C NMR (126 MHz, CDCl3, TMS), d/ppm:
164.08, 153.17, 142.24, 111.78, 110.77, 52.42, 52.40, 7.49.
ESI-MS: m/z = 130.2 ([N2222]+), 369.8 ([N2222][FA] + [N2222]+).
[P66614][FA]. 1H NMR (500 MHz, DMSO-d6, TMS), d/ppm: 7.40
(([P66614]+), 1077.7 ([P66614][FA] + [P66614]+) 1H, dd, O–CHQCH),
6.45–6.44 (1H, dd, CH–CHQC), 6.31–6.30 (1H, t, CHQCH–CH),
2.23–2.17 (8H, m, 4 Â PCH2), 1.49–1.24 (48H, m, 24 Â CH2),
0.89–0.84 (12H, m, 4 Â CH3). 13C NMR (126 MHz, CDCl3, TMS),
d/ppm: 163.76, 153.56, 141.37, 110.92, 110.21, 31.57, 30.77,
Synthesis and characterization of ionic liquids
All ILs were synthesized by neutralizing an equimolar corres- 30.49, 30.37, 30.17, 30.06, 29.33, 29.30, 29.27, 29.17, 29.01,
ponding base with FA or BFA. Here, the detailed procedure is 28.96, 28.64, 22.34, 22.00, 21.55, 21.52, 18.76, 18.38, 13.78,
described by taking [P4442][FA] as an example. The intermediate of 13.59. 31P NMR (202 MHz, CDCl3, H3PO4), d/ppm: 33.07. ESI-MS:
[P4442]Br was first prepared according to the literature procedure.12 m/z = 483.9 ([P66614]+), 1077.7 ([P66614][FA] + [P66614]+).
In a typical run, P444 (2.02 g, 10 mmol) in MeCN (10 cm3) was
[P4442][FA]. 1H NMR (500 MHz, CDCl3, TMS), d/ppm: 7.41
charged into a 100 cm3 flask. Subsequently, bromoethane (1.20 g, (1H, dd, O–CHQCH), 6.66 (1H, dd, CH–CHQC), 6.33 (1H, t,
11 mmol) in MeCN (10 cm3) was added dropwise into the flask via CHQCH–CH), 2.61 (2H, t, PCH2), 2.52 (6H, t, 3 Â PCH2), 1.69–
a syringe within 30 min at room temperature. The mixture was 1.43 (12H, d, 6 Â CH2), 1.26-1.15 (3H, t, CH3), 0.95–0.89 (9H, t,
then heated to 60 1C in a water bath and stirred for 24 h. MeCN 3 Â CH3). 13C NMR (126 MHz, CDCl3, TMS), d/ppm: 162.11,
was distilled off under reduced pressure at 70 1C. Finally, the 156.15, 141.13, 110.27, 110.00, 24.29, 24.19. 24.02, 23.90, 23.65,
product was washed with ethyl acetate (3 Â 20 mL) and dried at 23.62, 18.92, 18.03, 17.65, 13.72, 13.46, 12.30, 11.91, 5.95, 5.91.
80 1C under high vacuum for 24 h. The product of [P4442]Br was 31P NMR (202 MHz, CDCl3, H3PO4), d/ppm: 34.95. ESI-MS:
obtained as white solid with a yield of 87%.
m/z = 231.4 ([P4442]+), 573.1 ([P4442][FA] + [P4442]+).
[P4442][OH], [P66614][OH], and [N2222][OH] were obtained
[P4442][BFA]. 1H NMR (500 MHz, DMSO-d6, TMS), d/ppm:
using an anion-exchange resin from [P4442]Br, [P66614]Br, and 6.47–6.45 (1H, d, CH–CHQC), 6.41–6.40 (1H, d, CBrQCH–CH),
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2.52–2.50 (2H, t, PCH2), 2.27–2.18 (6H, t, 3 Â PCH2), 1.49–1.37
(12H, d, 6 Â CH2), 1.16–1.04 (3H, t, CH3), 0.92 (9H, t, 3 Â CH3).
13C NMR (126 MHz, CDCl3, TMS), d/ppm: 162.29, 155.58,
120.70, 113.30, 112.26, 23.91, 23.72, 23.60, 23.48, 23.45, 23.40,
23.36, 18.21, 18.14, 17.76, 13.32, 13.14, 12.45, 12.07, 5.77, 5.73.
31P NMR (202 MHz, CDCl3, H3PO4), d/ppm: 35.07. ESI-MS:
m/z = 231.2 ([P4442]+), 652.3 ([P4442][BFA] + [P4442]+).
[P66614][BFA]. 1H NMR (500 MHz, DMSO-d6, TMS), d/ppm:
6.47–6.45 (1H, d, CH–CHQC), 6.40 (1H, d, CBrQCH–CH), 2.28–
2.17 (8H, m, 4 Â PCH2), 1.50–1.26 (48H, m, 24 Â CH2), 0.89–
0.86 (12H, m, 4 Â CH3). 13C NMR (126 MHz, CDCl3, TMS),
d/ppm: 162.09, 154.15, 121.94, 114.69, 112.64, 31.80, 31.00,
30.73, 30.61, 30.40, 30.28, 29.56, 29.53, 29.50, 29.42, 29.23,
29.20, 28.88, 22.56, 22.23, 21.79, 21.75, 19.02, 18.65, 13.99,
13.82. 31P NMR (202 MHz, CDCl3, H3PO4), d/ppm: 33.11. ESI-MS:
m/z = 483.8 ([P66614]+), 1157.2 ([P66614][BFA] + [P66614]+).
Scheme 1 Chemical structures of cations and anions used in present ILs.
A similar effect of cations on IL’s properties was also reported in
Seo’s work.21
The thermal stability of the ILs
TGA curves of six ILs based on the furoate anion are illustrated
in Fig. 1. It can be seen from the curves that all the ILs are
stable below 195 1C, and no obvious loss of weight was
observed with heating. When the temperature was raised to
240 1C, the four phosphonium-based ILs began to decompose
rapidly. In all, the Td (thermal decomposition temperatures)
were 289, 281, 198, 216, 243, and 240 1C for [P4442][FA],
[P66614][FA], [N2222][FA], Ch[FA], [P4442][BFA], and [P66614][BFA],
respectively. Cations have an evident effect on the thermal
Experiments for SO2 absorption and desorption
Absorption and desorption experiments were carried out using
a bubble method under atmospheric pressure, in which the
former processes were carried out at various temperatures and
the latter were carried out at 343.15 K. The gas mixtures under
experimental conditions were assumed as ideal gases; namely,
the partial pressure of SO2 is proportional to its volume fraction
in the gas mixtures. Therefore, by changing the flow rates of
SO2 and N2 with rotometers (Changzhou Shuanghuan Thermo-
Technical Instrument Co., Ltd, LZB-3WB), the SO2 at various
partial pressures were obtained. In the release experiment, pure
N2 flowed through a 4A molecular sieve column and then
bubbled through the SO2 saturated ILs phase.
stability of the ILs. For example, the ILs with quaternary
+
ammonium cations of [N2222
]
and Ch+ are less stable than
+
those with quaternary phosphonium of [P4442
]
and [P66614]+,
while the influence of anions was relatively small. A high thermal
stability gives our ILs good operation flexibility for the absorp-
tion/desorption of SO2.
Effect of water on SO2 absorption
Absorption of SO2 by furoate ILs
To the best of our knowledge, water existing in either ILs liquid
phase or SO2 gas phase may change the SO2 absorption capacity
and mechanism of ILs. Accordingly, two glass tubes containing
ILs and water, respectively, were immersed in the same thermo-
static water bath at 30 1C. Pure N2 was then bubbled through
water and ILs successively. The amount of water loaded into the
ILs was determined at regular intervals with an electronic
balance (Mettler-Toledo ME204E). After 30 min of wet N2
absorption, the ILs was exposed to dry SO2 and the amount
of SO2 captured by the ILs was determined in the same manner.
To investigate relationships between the structure of cations
and anions as well as absorption performance, time-dependent
absorption profiles of SO2 in six ILs at 293.15 K under atmo-
spheric pressure are illustrated in Fig. 1, with a mass-based
scale (gram SO2 per gram IL) selected. Similar profiles based on
molar ratio were omitted due to the overlap of several curves.
We point out that the performance of [N2222][FA] was obtained
Results and discussion
In this work, the chemical structures of four cations and two
anions in the furoate ILs we used are depicted in Scheme 1.
The choice of cation has evident influence on several physical
properties for ILs. For example, the ILs of [N2222][BFA], Ch[BFA],
and [N2222][FA] are solid states at room temperature. Therefore, the
present study does not include the ILs of [N2222][BFA] and Ch[BFA].
However, [N2222][FA] still remained to compare the influence of
different cations on absorption performance in spite of the fact
that [N2222][FA] was also a solid at experimental temperatures.
Fig. 1 TGA graphs of six ILs. Black, [P4442][FA]; red, [P66614][FA]; blue,
Ch[FA]; orange, [N2222][FA]; magenta, [P4442][BFA]; cyan, [P66614][BFA].
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Fig. 3 Effect of temperature (at 1.0 bar, a) and partial pressure (at 303.15 K, b)
on [P4442][FA] performance for SO2 absorption.
Fig. 2 SO2 absorption profiles in six furoate ILs as a function of time at
293.15 K and atmospheric pressure. $, green, [N2222][FA]; J, blue,
[P4442][FA]; &, cyan, [P4442][BFA];B, black, Ch[FA]; n, red, [P66614][FA]; ,,
magenta, [P66614][BFA].
and low viscosity among the six ILs. As shown in Fig. 3a, the
SO2 absorption capacity decreased from 0.711 to 0.332 g SO2
as the deviation of absorption capacity between [N2222][FA]– per g IL when the temperature increased from 293.15 to
PEG-200 solution and PEG-200 because [N2222][FA] was a solid 333.15 K. Fig. 3b shows the pressure dependence of SO2 absorp-
at 293.15 K. Generally, the absorption process was basically tion capacity at 303.15 K. Absorption capacity increased signifi-
completed within thirty minutes for all the ILs. However, the cantly as the pressure increased. When pressure increased from
absorption capacity demonstrated evident differences, along 0.096 bar to 1.0 bar, the SO2 absorption capacity increased from
with the structure of the cations, for each kind of ILs. When 0.209 to 0.565 g SO2 per g IL. This result means that low
absorption capacity was compared with molecular structure, temperature is favorable for higher absorption of SO2, while a
the molar ratio provided a direct insight. The changing high temperature is helpful for stripping out the absorbed SO2
sequence was [P4442][FA] (3.72, molar ratio of SO2 to IL, and and regeneration of the absorbents. Moreover, pressure swelling
the same below) 4 [P66614][FA] (3.65) 4 [P4442][BFA] (3.58) 4 is also an alternative way to regenerate SO2-saturated IL.
[N2222][FA] (2.87) 4 [P66614][BFA] (2.80) 4 Ch[FA] (1.49). For the
Effect of water on the performance of ILs for SO2 absorption
same anion of furoate, the abovementioned sequence means
that the cation–anion interaction plays a more critical role than It is well known that water as steam is an accompanying
the SO2-anion in capturing SO2. This phenomenon was also component with SO2 in fuel gas emissions and that it will also
reported in CO2 capture by several anion-functioned ILs.21,22 If be absorbed by hydrophilic ILs. Thus, it is necessary to explore
the mass-based scale (gram SO2 per gram IL) was selected, then the influence of water on the absorption performance of the
the sequence of absorption capacity for furoate based IL changed ILs. In the present study, the capture performances of dry and
to be [N2222][FA] 4 [P4442][FA] 4 Ch[FA] 4 [P66614][FA]. This wet SO2 (100% humidity) by [P4442][FA] under 1.0 bar and
order included the comprehensive effects of molecular mass and 303.15 K were systematically compared. Results showed that
cation–anion interactions. Among the ILs, [P4442][FA] demon- the SO2 absorption capacity decreased when water was added to
strated the best performance for SO2 absorption from the the IL, presumably due to re-protonation of the anion. For
absorption rate as well as capacity. When a Br atom was example, SO2 absorption capacity of [P4442][FA] decreased from
introduced into the furan ring at the position of C-5, the derived 0.565 to 0.530 g SO2 per g IL when dry SO2 was saturated with
BFA had lower pKa value (pKa = 2.80) than FA (pKa = 3.27).23 Then, water to 100% humidity SO2, which means that the addition of
the furoate possessed higher basicity than 5-bromofuroate. water depressed the absorption capacity by possibly changing
Although the addition of a halo atom into the anion in an the absorption mechanism or its solution structure. This result
ordinary chain carboxylate or benzoate will enhance their absorp- is also similar to other carboxylate based ILs in the literature.16
tion capacity for SO2, because of the additional effect of halo atom
and SO2 interaction,16,24 the additional effect of a Br atom in
Selectivity of SO2 to CO2 or N2
the furan ring was discounted by the decreased basicity of the Gas absorption is generally related to gas separation in prac-
carboxylate anion, and the formed IL of [P4442][BFA] as demon- tical applications. Gas separation technology always involves
strated by a slightly lower absorption capacity than [P4442][FA] the gas mixture. For example, SO2 and CO2 are two major acidic
(Fig. 2).
gases coexisting in real flue gas. Thus, selectivity of different
gases in a selected gas absorbent is an essential parameter with
high priority for consideration. It is important to remove SO2
Effect of temperature and partial pressure on SO2 absorption
The temperature and pressure dependence of SO2 absorption prior to carbon capture with flue gas because SO2 is a relatively
capacity was also investigated. [P4442][FA] was selected as the stronger acidic gas than CO2, and what’s worse is that the
best candidate owing to its relatively high absorption capacity absorbed SO2 can hardly be released with the regeneration of
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Fig. 5 The density (a) and dynamic viscosity (b) of virgin and SO2 satu-
rated [P4442][FA] at various temperatures. &, ’: before absorption; J,
:
after absorption.
Fig. 4 Different gas absorption behaviours in [P4442][FA] as a function of
time at 293.15 K and 1.0 bar. J, SO2; n, CO2; &, N2.
Regeneration and recycling of ILs
Reusability is a critical property for IL as gas absorbents in
practical applications, because it directly relates to operation,
cost, and economic efficiency and thus determines the frequency
an absorbent for CO2.25 To investigate the potential suitability
of our ILs for the above applications, the absorption capacity of
CO2 and N2 in [P4442][FA] at 293.15 K and 1.0 bar were also
determined, as illustrated in Fig. 4. It is evident that the
absorption capacity of SO2 is much higher than that of CO2
and N2 with [P4442][FA]. Furthermore, the selectivity of SO2/CO2
and SO2/N2 (defined as the absorption capacity ratio of SO2 to
CO2 or N2) are calculated to be 37 and 114, respectively. The
selectivity of SO2/N2 is higher than that of N-butylpyridinium
thiocyanate ([C4Py][SCN]) and N-butylpyridinium tetrafluoro-
borate ([C4Py][BF4]).26 The selectivity of SO2/CO2 is similar with
that of 1-(2-diethylaminoethyl)-3-methyl-imidazolium tetrazolate
([Et2NEMim][Tetz]),27 while slightly lower than that of 1-{2-[2-(2-
methoxyethoxy)ethoxy]ethyl}pyridinium chloride ([E3Py]Cl).28
These results indicated that furoate based ILs have the potential
to capture or be useful with pretreatment of SO2 from CO2 in
flue gas.
of
a
IL’s replacement.30 Therefore, five SO2 absorption/
desorption cycles by [P4442][FA] was investigated, and the results
are presented in Fig. 6. The absorption/desorption procedures
were carried out at 303.15 K, 1.0 bar, SO2 of 30 cm3 minÀ1 and
343.15 K, 1.0 bar, and N2 at 60 cm3 minÀ1, respectively. As shown
from Fig. 6, the regeneration process was completed within
thirty minutes. After 5 absorption/desorption cycles, the absorp-
tion ability remained at almost the same level, indicating that
[P4442][FA] is satisfactory for repeated SO2 absorption/desorption
uses. If an anion with stronger basicity relative to furoate was
used to capture SO2, then the absorption capacity may be higher,
but the desorption of SO2 would become more difficult and the
neat absorption capacity would be even lower than with a
furoate-based IL. For example, as [P66614][C5H11COO] was men-
tioned, the desorption of SO2 was not complete even at 393.15 K
and the real absorption capacities were depressed during the
recycling of IL due to its strong interactions with SO2.16
The changes of physical property before and after absorption
It is well known that physical property data are important and
essential for industrial applications and engineering designs
with ILs. In our present study, the density and dynamic
viscosity data of virgin and SO2 saturated [P4442][FA] at various
temperatures were also determined, as shown in Fig. 5. As can
be seen from Fig. 5, the dissolution of SO2 increased the density
of the ILs at each temperature, and the density of pure and
mixture systems decreased with increasing temperature. Two
possibilities may explain this phenomenon. On one hand, the
absorbed SO2 would reinforce the attractive interactions
between ion pairs. On the other hand, the pores between
cations and anions would just accommodate the gas molecule.
It is noteworthy that the dynamic viscosity of [P4442][FA] changed
to be half of the initial value after saturation by SO2 at each
temperature. For example, the initial and final viscosity values
of [P4442][FA] at 303.15 K was 117.5 and 60.6 mPa, respectively.
For functionalized ILs, the behaviour of viscosity along with
the absorption of SO2 was complicated.6,29 Thus, changes of
dynamic viscosity of [P4442][FA] with increasing absorption
amount of SO2 needs further research.
Spectroscopic investigations comparing SO2 and ILs
Spectroscopy is a useful approach to get an insight of the
structure of a solution in situ.31 To investigate the interaction
Fig. 6 Five consecutive SO2 absorption (303.15 K, 1 atm, 30 cm3 minÀ1
and desorption (343.15 K, N2, 60 cm3 minÀ1) cycles by [P4442][FA]. K,
absorption; J, desorption.
)
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new band at 2441 cmÀ1 was assigned to the SQO/H–C hydro-
gen bonding interactions between furoate anion and SO2.
When water is present in ILs or gas, SO2 will react with it to
from H2SO3. Tracking [P4442][FA] as an example, since 2-furoic
acid (pKa = 3.16) is weaker than H2SO3 (pKa1 = 1.81), the anion
of [FA] would be transferred to 2-furoic acid in the presence of
H2SO3. Then, [P4442][FA] would react with H2SO3 to form
[P4442][HSO3]. As seen from the FTIR spectrum of [P4442][FA] +
100% humidity SO2, another new band at 1032 cÀm1À41 should be
attributed to the O–S stretching mode of HSO3
,
indicating
the rationality of the proposed mechanism. Moreover, as illu-
strated in Fig. 8 of the 1H NMR spectra for free and SO2 treated
[P4442][FA], the chemical shifts of the hydrogen atom in the ring
moved down-field from 7.41, 6.66, and 6.33 ppm to 7.92, 7.31,
and 6.82 ppm, respectively. The typical peak of the carboxyl
group on the anion of furoate in 13C NMR spectra moved
up-field from 162.1 ppm to 161.8 ppm, indicating the inter-
action of SO2 with the carboxyl group in the anion. For the
carbon atom adjacent to carboxyl group and oxygen atom, a
larger 13C NMR shift from 156.1 ppm to 148.1 ppm means the
synergistic absorption of SO2 by the carboxyl group and oxygen
atom in the furan ring. The pÁ Á ÁS interaction11 between the
furan ring and SO2 results in down-field movement of 13C NMR
Fig. 7 FT-IR spectra of [P4442][FA] (black), [P4442][FA] + SO2, (red) and
[P4442][FA] + 100% humidity SO2 (blue).
Table 1 Comparison of SO2 absorption capacities in different functiona-
lized ILs
SO2 absorption capacityb
Ionic liquidsa
T/K
At 1 bar
At 0.1 bar
Ref.
[P4442][FA]
[P66614][FA]
[N2222][FA]
293.15 0.69 (3.70) 0.24 (1.31)
293.15 0.39 (3.64) 0.15 (1.38)
293.15 0.76 (2.87) 0.34 (1.29)
293.15 0.37 (1.24) 0.10 (0.33)
293.15 0.55 (3.59) 0.19 (1.27)
293.15 0.27 (2.80) 0.12 (1.30)
313.15 0.46 (1.88) 0.15 (0.61)
293.15 0.49 (4.47) 0.18 (1.60)
This work
This work
This work
This work
This work
This work
15
14
16
16
24
33
34
34
34
35
Ch[FA]
[P4442][BFA]
[P66614][BFA]
[N2224][dimalonate]
[P66614][DAA]
[P66614][6-BrC5H10COO] 293.15 0.41 (4.34) 0.15 (1.61)
[P66614][C5H11COO] 293.15 0.41 (3.82) 0.18 (1.70)
[P66614][4-BrC6H4COO] 293.15 0.39 (4.12) 0.16 (1.66)
[P66614][Tetz]
[P66614][Im]
[Emim][SCN]
[Emim][C(CN)3]
[P444E3][Tetz]
[TMG]L
293.15 0.43 (3.72) 0.18 (1.54)
293.15 0.46 (4.00) 0.24 (2.07)
293.15 1.13 (2.99) 0.37 (0.98)
293.15 0.74 (2.33) 0.07 (0.23)
293.15 0.75 (4.87) 0.29 (1.87)
Fig. 8 1H NMR (upper) and 13C NMR (below) spectra of [P4442][FA] before
and after absorption of SO2.
293.15 0.62 (1.96)c 0.32 (1.03)d 36
between SO2 and IL in [P4442][FA], FTIR and NMR spectro-
scopy were used to characterize the structures of virgin and
SO2-saturated [P4442][FA], with the results illustrated in Fig. 7
and 8. By comparing the two FT-IR spectra in Fig. 7, two new
absorption peaks at 1266 and 1098 cmÀ1 were assigned to the
asymmetric stretching and symmetric vibration of SQO bonds,
respectively, and a peak at 969 cmÀ1 attributable to S–O
stretches, formed via the absorption of SO2. Furthermore, the
band at 1604 cmÀ1 was assigned to CQO vibration in the
carboxyl group (–COOÀ) of fresh IL [P4442][FA]; this same mode
was blue-shifted by 101 cmÀ1 in SO2-saturated [P4442][FA] to
1705 cmÀ1 because of the electrophilic environment due to
SO2.15,16 Another new peak at 527 cmÀ1 was regarded as a
scissor bending vibration (d) of dissolved SO2.32 Furthermore, a
[MEA]L
[N2222]L
[Bmim]L
293.15 0.42 (0.99)c 0.21 (0.53)d 36
313.15
313.15
—
—
0.23 (0.79)e 37
0.19 (0.67)e 37
a
[N2224][dimalonate], triethylbutylammoniumdimalonate; [P66614][DAA],
trihexyl(tetradecyl)phosphonium diacetamide; [P66614][6-BrC5H10COO],
trihexyl(tetradecyl)phosphonium6-bromo-n-hexanoic acid; [P66614]-
[C5H11COO], trihexyl(tetradecyl)phosphonium hexanoic acid; [P66614]-
[4-BrC6H4COO], trihexyl(tetradecyl)phosphonium 4-bromobenzoic acid;
[P66614][Tetz], trihexyl(tetradecyl)phosphonium tetrazole; [P66614][Im],
trihexyl(tetradecyl)phosphonium imidazole; [Emim][SCN], 1-ethyl-3-
methylimidazolium thiocyanate; [Emim][C(CN)3], 1-ethyl-3-methyl-
imidazolium tricyanomethanide; [P444E3][Tetz], tri-n-butyl{2-[2-(2-
methoxyethoxy)ethoxy]ethyl}phosphonium tetrazolate; [TMG]L, 1,1,3,3-
tetramethylguanidinium lactate; [MEA]L, monoethanolaminium lactate;
[N2222]L, tetraethylammonium lactate; [Bmim]L, 1-butyl-3-methylimidazolium
b
lactate. Values given in units of g SO2 per g IL (mol SO2 per mol IL).
c
d
e
At 101.3 kPa. At 6.08 kPa. At 0.03 bar.
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112.2 ppm for other carbon atoms in the furan ring, respec-
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Comparison with other functionalized ILs
Throughout investigations of present furoate based ILs as a SO2
absorbent, the absorption capacity of SO2 was compared with
those in other functionalized ILs reported in the literature, with
results listed in Table 1 based on molality and molarity scales.
Present furoate-based ILs demonstrate competitive or superior
absorption capacity of SO2 on a molality scale compared with
fatty acid salt based ILs. However, their performance was slightly
inferior to thiocyanate, lactate, imidazole, and tetrazolate-based
ILs. It should be noted that functionalized [Emim][SCN] is
susceptible to oxidization by SO2, which possesses relatively
strong oxidization ability, especially in the presence of water.
Besides, the alkalinity of [Tetz]- and [Im]-based ILs is very strong,
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furoate-based ILs are regarded as more promising SO2 absorbents
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Conclusions
In this work, a new kind of furoate-based functionalized ILs
were prepared and evaluated for SO2 capture from absorption
performance, physical property, gas selectivity, absorption
mechanism, and comparisons with other ILs. After comprehensive
consideration, [P4442][FA] is regarded as the most satisfactory and
competitive anion-functionalized IL with a maximum absorption
capacity of 0.69 and 0.24 g SO2 per g IL at 293.15 K under 1.0 bar
and 0.1 bar, respectively. Furthermore, the high selectivity of
SO2/CO2 in [P4442][FA] provides an attractive way to selectively
separate SO2 from CO2 in flue gas. The high efficiency, reason-
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the present furoate-based ILs very promising absorbents for SO2.
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Acknowledgements
The research was supported by the Natural Science Foundation
of Zhejiang Province (LY17B060010).
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