Highly efficient SO2 capture by dual functionalized ionic liquids through a combination of chemical and physical absorption

Article abstract of DOI:10.1039/c2cc16457d

Two kinds of dual functionalized ionic liquids with ether-functionalized cations and tetrazolate anions were designed, prepared, and used for SO ₂ capture, which exhibit an extremely high SO₂ capacity and excellent reversibility through a combination of chemical and physical absorption. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc16457d

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COMMUNICATION
Highly efficient SO2 capture by dual functionalized ionic liquids through
a combination of chemical and physical absorptionw
Guokai Cui,a Congmin Wang,*a Junjie Zheng,a Yan Guo,a Xiaoyan Luoa and Haoran Li*ab
Received 17th October 2011, Accepted 22nd December 2011
DOI: 10.1039/c2cc16457d
Two kinds of dual functionalized ionic liquids with ether-functio-
nalized cations and tetrazolate anions were designed, prepared,
and used for SO2 capture, which exhibit an extremely high SO2
capacity and excellent reversibility through a combination of
chemical and physical absorption.
SO2, one of the acid gases, has drawn much more attention
because it is a significant source of atmospheric pollution that
threatens the environment and human health. To remove SO2
released from the burning of fossil fuels, several processes,
such as limestone scrubbing, ammonia scrubbing, and organic
solvents absorption have been developed. Unfortunately, there
Scheme 1 Structures of dual functionalized ILs and azole-based ILs.
ILs, such as hydroxyl ammonium ILs,7 imidazolium ILs,5a,8
are some obvious disadvantages with these removal processes:
for example, they truly result in lots of byproducts such as
polymer ILs,2b,9 and supported ionic liquid membranes (SILMs)10
are also used to absorb SO2. Recently, we reported a new strategy
calcium sulfate (CaSO4) and wastewater, which is not consistent
for SO2 absorption by tunable azole-based ILs with multiple sites
of action.11
In continuation of our previous work, this communication seeks
with the sustainable principles. Accordingly, new materials that
can efficiently, reversibly, and economically capture SO2 must be
developed and are of critical importance for environmental
to develop a new method for SO2 capture by dual functionalized
protection.
ILs to enhance the absorption capacity through introducing a
Recently, ionic liquids (ILs) have received much more
combination of functionalized cations and anions in the ILs. Thus,
attention in the field of gas absorption for acid gases such as
CO2,1 SO2,2 NOx,3 and so on. The ILs seem to be a promising
alternative to conventional organic liquids as gas absorbents
tri-n-butyl{2-[2-(2-methoxyethoxy)ethoxy]ethyl}phosphonium tetra-
zolate ([P444E3][Tetz]) and 1-{2-[2-(2-methoxy)ethoxy] ethoxy}ethyl-
3-methyl imidazolium tetrazolate ([E3mim][Tetz]) (Scheme 1) were
due to their unique properties, such as extremely low vapor
prepared and applied in the capture of SO2. These ILs with ether-
high thermal and chemical stability, and tunable properties.4
functionalized cations and multiple-site anions were found to
pressure, wide liquid temperature range, non-flammability,
readily achieve a high capacity of SO2 (B5.0 moles per mole
of IL) and an excellent reversibility.
SO2 has a high solubility in some ILs through physical
absorption.2c,5 For example, ether-functionalized ILs exhibit
efficient and reversible absorption of SO2.6 Another strategy
for SO2 capture is chemical absorption by anion-functionalized
ILs. For example, Han and co-workers have reported that
The ILs [P444E3][Tetz] and [E3mim][Tetz] were synthesized
by the acid–base neutralization between tetrazole and a
solution of phosphonium or imidazolium hydroxide in ethanol
or deionized water, which was prepared by the anion-exchange
method12 from the chlorination of triethylene glycol mono-
methyl ether with tri-n-butylphosphine or 1-methylimidazole
(see ESIw).13 To investigate the effect of the ether group in the
IL on the capture of SO2, ILs [P44410][Tetz] and [C10mim][Tetz]
were also prepared.
1,1,3,3-tetramethylguandinium
lactate
([TMG][L])
could
chemically absorb B1.0 mole SO2 per mole IL at 1 bar with 8%
SO2 in a gas mixture of SO2 and N2.2a Subsequently, other kinds of
a Department of Chemistry, Zhejiang University, Hangzhou 310027,
China. E-mail: chewcm@zju.edu.cn, lihr@zju.edu.cn;
Fax: +86-571-8795-1895
The effect of different ILs on the capture of SO2 was
investigated (Fig. 1). It was seen that [P444E3][Tetz] and
[E3mim][Tetz] absorbed 5.0 and 4.43 moles of SO2 per mole
of IL at 20 1C and 1 bar, respectively, while [P44410][Tetz] and
[C10mim][Tetz] exhibited 4.0 and 3.37 moles of SO2 per mole
of IL, respectively. It indicates that SO2 absorption capacity
increased significantly due to the interaction between an
b State Key Laboratory of Chemical Engineering, Department of
Chemical and Biological Engineering, Zhejiang University, Hangzhou
310027, China
w Electronic supplementary information (ESI) available: Experimental
details of ionic liquid synthesis, absorption and desorption experi-
ments and NMR and FT-IR data of ionic liquids and SO2 absorbed
ionic liquids. See DOI: 10.1039/c2cc16457d
c
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Chem. Commun., 2012, 48, 2633–2635 2633

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The effect of the relatively low partial pressure such as 0.2%
SO2 on the absorption was also investigated (see Table S1 in ESIw).
As can be seen, the absorption capacity of SO2 by ILs such as
[E3mim][Tetz] reduced to 1.16 and 0.98 moles of SO2 per mole IL
when the content of SO2 decreased to 1.0% and 0.2%, respec-
tively. Furthermore, the effect of water in flue gas on the SO2
absorption and the stability of ILs in the presence of water were
investigated, which are shown in Table S2 and Fig. S1 (ESIw).
Clearly, the effect of water on the SO2 absorption was weak, and
the stability of ILs in the presence of water was good.
Fig. 1 Comparison of the SO2 absorption processes by the ILs as a
function of time at 20 1C and 1 bar. [P444E3][Tetz], ; [P44410][Tetz],
[E3mim][Tetz], ; and [C10mim][Tetz],
During the absorption of SO2 in this study, these ILs reacted
with SO2 to form a liquid sulfate salt, which was verified by
FT-IR and NMR spectroscopy (Fig. 3). For example, compared
with the FT-IR spectra of the fresh [E3mim][Tetz] (Fig. 3a), new
absorption bands at 1323 and 938 cmÀ1 attributable to sulfate
SQO and S–O stretches, respectively, formed via the absorption
of SO2. To further study the interaction mode of the SO2
absorption by [E3mim][Tetz] and [C10mim][Tetz], FT-IR spectra
of [C10mim][Br]–SO2, [E3mim][Cl]–SO2, [C10mim][Tetz]–SO2 and
[E3mim][Tetz]–SO2 were investigated (Fig. 4). Clearly, no new
band formed for [E3mim][Cl]–SO2 because of the physical inter-
action between the ether chain in the IL and acid SO2. However,
a new peak at 938 cmÀ1 produced by [E3mim][Tetz]–SO2
indicates the presence of the strong chemical interaction between
the anion [Tetz] and acid SO2. Similarly, after the absorption of
SO2, the typical peaks in the 1H NMR spectra of the anion [Tetz]
for [E3mim][Tetz] (Fig. 3b) moved to the low-field from 8.04 ppm
;
.
electronegative oxygen atom in the ether-based cation and
acid SO2, which is in agreement with the results of Kim and
co-workers.6c Another important feature of SO2 absorption by
these ILs is that the absorption is almost complete within the
first 6 min of SO2 exposure. This rapid absorption rate may be
related to the absence of strong hydrogen-bonded networks by
these azole-based ILs.11
Fig. 2a shows the effect of pressure on SO2 absorption by
these ILs. It was seen that the molar ratios of SO2 to IL for
[E3mim][Tetz] and [C10mim][Tetz] decreased from 4.43 and
3.37 to 1.58 and 1.53, respectively, when the SO2 partial
pressure decreased from 1.0 to 0.1 bar. Fig. 2b shows the
temperature dependence of the SO2 absorption at 1.0 bar. As
can be seen, the SO2 absorption capacity of [E3mim][Tetz] and
[C10mim][Tetz] decreased to 1.21 and 1.15 moles of SO2 per
mole of IL when the temperature increased to 100 1C. Similarly,
[P444E3][Tetz] exhibits a higher absorption capacity at relatively
low temperature or high pressure than [P44410][Tetz], while the
absorption capacity of these two ILs is very close at high
temperature or low pressure. The difference between these two
kinds of ILs is that the dual functionalized ILs have ether groups
in the cations, which results in the higher absorption capacity at
atmospheric pressure or lower temperature. When the SO2
partial pressure is reduced to 0.1 bar or the temperature is
increased to 100 1C, all these ILs exhibit a close chemical
absorption capacity because of the presence of the same anion
[Tetz], also indicating that the interaction between the ether
group and acid SO2 is physical absorption. All these results
indicate that there are the combinations of physical absorption
and chemical absorption in these dual functionalized ILs.
to 8.99 ppm, but up-field from 149.2 ppm to 144.8 ppm in the 13
C
NMR (Fig. 3c). Therefore, based on previous reports6c,11 and
Fig. 2 The effect of (a) pressure and (b) temperature on SO2 absorption
Fig. 3 FT-IR, 1H NMR and 13C NMR spectra of [E3mim][Tetz]
before and after the reaction with SO2. FT-IR, (a); 1H NMR, (b); and
13C NMR, (c).
by ILs: [P444E3][Tetz],
[C10mim][Tetz],
; [P44410][Tetz], ; [E3mim][Tetz],
; and
.
c
2634 Chem. Commun., 2012, 48, 2633–2635
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20990221) and the Fundamental Research Funds of the
Central Universities.
Notes and references
1 (a) E. D. Bates, R. D. Mayton, I. Ntai and J. H. Davis, J. Am.
Chem. Soc., 2002, 124, 926; (b) X. Y. Li, M. Q. Hou, Z. F. Zhang,
B. X. Han, G. Y. Yang, X. L. Wang and L. Z. Zou, Green Chem.,
2008, 10, 879; (c) B. E. Gurkan, J. C. de la Fuente, E. M. Mindrup,
L. E. Ficke, B. F. Goodrich, E. A. Price, W. F. Schneider and
J. F. Brennecke, J. Am. Chem. Soc., 2010, 132, 2116; (d) J. E. Bara,
T. K. Carlisle, C. J. Gabriel, D. Camper, A. Finotello, D. L. Gin
and R. D. Noble, Ind. Eng. Chem. Res., 2009, 48, 2739; (e) S. Choi,
J. H. Drese and C. W. Jones, ChemSusChem, 2009, 2, 796;
(f) C. M. Wang, H. M. Luo, D. E. Jiang, H. R. Li and S. Dai,
Angew. Chem., Int. Ed., 2010, 49, 5978.
Fig. 4 FT-IR spectra of [C10mim][Br]–SO2, [E3mim][Cl]–SO2,
[C10mim][Tetz]–SO2, and [E3mim][Tetz]–SO2 at 20 1C and 1 bar.
2 (a) W. Z. Wu, B. X. Han, H. X. Gao, Z. M. Liu, T. Jiang and
J. Huang, Angew. Chem., Int. Ed., 2004, 43, 2415; (b) D. An,
L. B. Wu, B. G. Li and S. P. Zhu, Macromolecules, 2007, 40, 3388;
(c) J. Huang, A. Riisager, P. Wasserscheid and R. Fehrmann,
Chem. Commun., 2006, 4027.
3 (a) A. L. Revelli, F. Mutelet and J. N. Jaubert, J. Phys. Chem. B, 2010,
114, 8199; (b) M. B. Shiflett, A. M. S. Niehaus and A. Yokozeki,
J. Phys. Chem. B, 2011, 115, 3478; (c) S. Shunmugavel, S. Kegnaes,
J. Due-Hansen, T. A. Gretasdottir, A. Riisager and R. Fehrmann,
ECS Trans., 2010, 33, 117.
4 (a) B. L. Bhargava, Y. Yasaka and M. L. Klein, Chem. Commun.,
2011, 47, 6228; (b) P. Pollet, C. A. Eckert and C. L. Liotta, Chem.
Sci., 2011, 2, 609; (c) J. Dupont, R. F. de Souza and
P. A. Z. Suarez, Chem. Rev., 2002, 102, 3667; (d) T. L. Merrigan,
E. D. Bates, S. C. Dorman and J. H. Davis, Chem. Commun., 2000,
2051; (e) J. F. Huang, H. M. Luo, C. D. Liang, I. W. Sun,
G. A. Baker and S. Dai, J. Am. Chem. Soc., 2005, 127, 12784;
(f) J. E. Bara, T. K. Carlisle, C. J. Gabriel, D. Camper,
A. Finotello, D. L. Gin and R. D. Noble, Ind. Eng. Chem. Res.,
2009, 48, 2739; (g) M. D. Soutullo, C. I. Odom, B. F. Wicker,
C. N. Henderson, A. C. Stenson and J. H. Davis, Chem. Mater.,
2007, 19, 3581.
5 (a) J. L. Anderson, J. K. Dixon, E. J. Maginn and J. F. Brennecke,
J. Phys. Chem. B, 2006, 110, 15059; (b) J. Huang, A. Riisager,
R. W. Berg and R. Fehrmann, J. Mol. Catal. A: Chem., 2008,
279, 170.
6 (a) J. E. Bara, D. E. Camper, D. L. Gin and R. D. Noble, Acc.
Chem. Res., 2010, 43, 152; (b) J. E. Bara, C. J. Gabriel,
S. Lessmann, T. K. Carlisle, A. Finotello, D. L. Gin and
R. D. Noble, Ind. Eng. Chem. Res., 2007, 46, 5380;
(c) S. Y. Hong, J. Im, J. Palgunadi, S. D. Lee, J. S. Lee,
H. S. Kim, M. Cheong and K. D. Jung, Energy Environ. Sci.,
2011, 4, 1802.
7 (a) X. L. Yuan, S. J. Zhang and X. M. Lu, J. Chem. Eng. Data,
2007, 52, 1150; (b) L. Z. Zhai, Q. Zhong, C. He and J. Wang,
J. Hazard. Mater., 2010, 177, 807.
8 (a) A. Yokozeki and M. B. Shiflett, Energy Fuels, 2009, 23, 4701;
(b) M. B. Shiflett and A. Yokozeki, Energy Fuels, 2010, 24, 1001;
(c) M. B. Shiflett and A. Yokozeki, Ind. Eng. Chem. Res., 2010,
49, 1370.
9 L. B. Wu, D. An, J. Dong, Z. M. Zhang, B. G. Li and S. P. Zhu,
Macromol. Rapid Commun., 2006, 27, 1949.
10 (a) Y. Y. Jiang, Y. T. Wu, Z. Zhou, Z. Jiao, L. Li and Z. B. Zhang,
J. Phys. Chem. B, 2007, 111, 5058; (b) P. Luis, L. A. Neves,
C. A. M. Afonso, I. M. Coelhoso, J. G. Crespo, A. Garea and
A. Irabien, Desalination, 2009, 245, 485.
Fig. 5 Six consecutive cycles of SO2 absorption and release by
[E3mim][Tetz]. SO2 absorption was carried out at 20 1C and 1 bar
under SO2 (60 mlÀ1), and desorption was performed at 80 1C and 1 bar
under N2 (60 mlÀ1). Absorption, ; desorption,
.
observed products, there are physical and chemical interactions
between these dual functionalized ILs and acid SO2.
Fig. 5 shows the effect of time on the release of SO2 upon heating
[E3mim][Tetz]. It can be seen that the release of SO2 was essentially
complete within 15 min at 80 1C under bubbling N2. Fig. 5 also
reveals that SO2 absorption/desorption can be repeatedly recycled
without any loss of absorption capability, indicating that the
process of SO2 absorption by dual functionalized ILs is highly
reversible. Compared with other functionalized ILs such as
[TMG][lactate], the desorption of SO2 is not easy at 80 1C, and
the absorption capacity decreases during the recycling due to its
low stability and its strong interaction with SO2.6c
In summary, two kinds of dual functionalized ILs with
ether-functionalized cations and multiple-site tetrazole anions
have been developed to capture SO2 through a combination of
chemical and physical absorption. Thus, an extremely high
SO2 capacity was achieved via the multiple-site interactions
between the nitrogen atom in the tetrazole anion and the
oxygen atom in the ether chain of the cation with the sulfur
atom in the acid SO2. Furthermore, the captured SO2 was easy
to release by heating or bubbling N2 through these ILs. This
method using dual functionalized groups with multi-sites
interaction opens a door to achieve high capacity as well as
excellent reversibility to capture gases such as SO2, H2S, and
CO2 by ILs. We believe that this high, rapid, and reversible
process by dual functionalized ILs can provide a potential
alternative for SO2 capture.
11 C. M. Wang, G. K. Cui, X. Y. Luo, Y. J. Xu, H. R. Li and S. Dai,
J. Am. Chem. Soc., 2011, 133, 11916.
12 (a) C. M. Wang, X. Y. Luo, H. M. Luo, D. E. Jiang, H. R. Li and
S. Dai, Angew. Chem., Int. Ed., 2011, 50, 4918; (b) Y. Q. Zhang,
S. J. Zhang, X. M. Lu, Q. Zhou, W. Fan and X. P. Zhang,
Chem.–Eur. J., 2009, 15, 3003; (c) K. Fukumoto, M. Yoshizawa
and H. Ohno, J. Am. Chem. Soc., 2005, 127, 2398.
We acknowledge the support of the National Natural
Science Foundation of China (20976151, 21176205, and
13 J. Kviesis, J. Leicunaite, I. Klimenkovs, D. Zacs and
J. P. Kreismanis, C. R. Chimie, 2010, 13, 1335.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2633–2635 2635