Capture of sulphur dioxide by hydroxyl ammonium ionic liquid-DBU mixtures

Article abstract of DOI:10.14233/ajchem.2014.17962

Three hydroxyl ammonium ionic liquids containing the alcoholic hydroxy group were synthesized that mixed with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to form integrated sorption systems with the mole ratio of 1:1. At ambient temperature and atmospheric pressure, the absorption of SO2 by integrated ionic liquid-DBU systems was investigated. The saturated absorption mole ratio of SO 2/ILs was 0.30, 0.50 and 0.66, respectively. And that of SO2/ILs-DBU was 2.63, 2.83 and 2.96, respectively. The net increase in the saturated absorption mole ratio of three mixed absorption systems was close to 2.3 after deducting the SO2 saturated absorption capacity of pure ionic liquids. The results indicated that SO2 was absorbed fastly and efficiently by the mixed systems and the absorption could be completed in 0.5 h. In addition, the mixed systems had a strong regenerative ability. It's absorption capacity and rate were hardly changed after four SO2 absorption-desorption cycle experiments.

Full text of DOI:10.14233/ajchem.2014.17962

Asian Journal of Chemistry; Vol. 26, No. 22 (2014), 7823-7827
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.17962
Capture of Sulphur Dioxide by Hydroxyl Ammonium Ionic Liquid-DBU Mixtures
*
GUANGFEI QU, RONGLONG JIAN, JUN ZHANG, JIANGNA LI and PING NING
Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Yunnan, P.R. China
*Corresponding author: E-mail: qgfljy@sina.com
Received: 7 May 2014;
Accepted: 11 July 2014;
Published online: 6 November 2014;
AJC-16243
Three hydroxyl ammonium ionic liquids containing the alcoholic hydroxy group were synthesized that mixed with 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) to form integrated sorption systems with the mole ratio of 1:1. At ambient temperature and
atmospheric pressure, the absorption of SO2 by integrated ionic liquid-DBU systems was investigated. The saturated absorption mole
ratio of SO2/ILs was 0.30, 0.50 and 0.66, respectively. And that of SO2/ILs-DBU was 2.63, 2.83 and 2.96, respectively. The net increase
in the saturated absorption mole ratio of three mixed absorption systems was close to 2.3 after deducting the SO2 saturated absorption
capacity of pure ionic liquids. The results indicated that SO2 was absorbed fastly and efficiently by the mixed systems and the absorption
could be completed in 0.5 h. In addition, the mixed systems had a strong regenerative ability. It’s absorption capacity and rate were hardly
changed after four SO2 absorption-desorption cycle experiments.
Keywords: Hydroxyl ammonium ionic liquids, 1,8-Diazabicyclo[5.4.0]undec-7-ene, Sulphur dioxide, Absorption.
ammonium-based ionic liquids in these ionic liquids are low
cost than others, their absorption capacity is usually lower than
other ionic liquids and the time required for reaching equili-
brium is relatively longer.
INTRODUCTION
With the rapid development of the economic globalization,
the demand and consumption of fossil fuels are soaring. The
SO2 pollution, which would lead to the formation of acid rain
and smog, caused by coal-combustion has become the most
pollution factor in atmospheric environment and achieves
extensively attention all over the world. Up to now, flue gas
desulphurization is considered as one of the most effective
ways to control emissions of SO2. Although there are some
commercial flue-gas desulphurization (FGD) ways including
wet FGD, semidry FGD and dry FGD processes which are
widely used in industrial desulphurization. There still exist
some inherent drawbacks in desulphurization processes, such
as the waste byproducts, corrosion of equipment, the difficulty
to recycle SO2 and the secondary pollution. So the most
attractive approach for the separation of SO2 from the flue gas
stream is that SO2 can be absorbed selectively and then released
from saturated absorption solution by some measures. In recent
years, many room temperature ionic liquids (RTILs) have been
proposed as alternative absorbent for SO21-10 for their negligible
vapor pressure, miniscule flammability, high thermal and
chemical stability. The room temperature ionic liquids used
so far for SO2 separation mainly including the hydroxyl-
ammonium-based ionic liquids5,6 and tetramethylguanidinium-
based ionic liquids (ILs) such as [TMG]L1, PTMGA10,
[TMG][BF4]2,3 and [TMG][BTA]2. Although the hydroxyl-
Recently, a novel class reversible of CO211,12 and SO213 binding
organic liquids (CO2BOL and SO2BOL) which are liquid
mixture of an hydroxyl compounds and 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) (amidine, guanidine) based on the
reactions of CO2, SO2 and DBU reported. In this mixed
absorption system, adding DBU in hydroxyl ammonium ionic
liquids can not only largely increase the absorption capacity
of absorbent to target gas, but also remain the original equili-
brium time. Herein, we report a new organic sulphur-bearing
ionic liquid system which is composed of the relative low cost
hydroxyl ammonium ionic liquids and DBU.
EXPERIMENTAL
1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was purchased
from Aladdin reagent Co., Ltd. Sulphur dioxide (99.9 %) was
supplied by Dalian Date Special Gas Co., Ltd. Triethanolamine
(AR grade, purity ≥ 98 %) was purchased from Tianjin Bodi
Chemical Reagent Co., Ltd. Diethanolamine (AR grade, purity
≥ 99 %) was purchased from Chengdu Kelong Chemical
Reagent Co., Ltd. Sulfamic acid (AR grade, purity ≥ 99.5 %)
was purchased from Tianjin Guangfu Fine Chemical Research
Institute. Co., Ltd. 2-Aminoethanol was purchased from Tianjin
Fengfan Chemical Reagent Co., Ltd.All other chemicals were

7824 Qu et al.
Asian J. Chem.
purchased from commercial suppliers and were of the highest
purity available.
Synthesis of ionic liquids: Hydroxyl ammonium ionic
liquids were prepared by direct neutralization of alkanolamine
and sulfamic acid. As an example, the synthesis procedure of
tris-(2-hydroxyethylammonium) sulphamate is proposed here:
9.71 g (0.1 mol) sulfamic acid was added into a 150 mL flask
and then a slight excess of an equimolar triethanolamine was
transferred to the flask. The solution was stirred vigorously to
mix well by a magnetic stirrer for approximately 24 h at 298.15 K
and 101 kPa. The resulting liquid was collected and washed
with ethanol (3 × 20 mL). A colourless product was obtained
after evaporation and dried under vacuum at 353.15 K for 24 h.
The synthesized reaction for ionic liquids is expressed in
Scheme-I.
Time (min)
Fig. 1. Mole ratio of SO2/ILs as a function of time
O
S
O
S
R1
R2
R3
R1, R2, R3: H or HO-CH2-CH2
R1
R2
R3
H2N
O
NH
+
H2N
OH
N
O
O
Scheme-I: Synthesized reaction of hydroxyl ammonium ionic liquids
Characterization of ionic liquids: The structure of synthe-
sized ionic liquids was determined by IR and NMR charac-
terization. IR spectra was obtained on a BioRad FTS-40
infrared spectrophotometer, 1H NMR spectra was recorded on
a Bruker DRX 500-MHz spectrometer, using DMSO as solvent
with TMS as internal standard.
Time (min)
Fig. 2. Mole ratio of SO2/ILs-DBU as a function of time
Absorption capacity determination: The selected mixed
adsorption systems were composed of DBU and different ionic
liquids with the molar ratio of 1:1. The pure ionic liquids and
ILs-DBU absorption solution were loaded into absorber, while
dry SO2 was passed into the reactor under atmospheric pressure.
Effect of temperature on absorption capacity: The varia-
tion of adsorption capacity of ionic liquids with temperature
was presented in Fig. 3. It could be seen that the solubility of
SO2 in three ionic liquids decreased with the increase of
temperature. The variation of adsorption capacity of ILs-DBU
with temperature was presented in Fig. 4. It could be seen that
the solubility of SO2 in three mixed absorption systems decreased
sharply with increasing temperature. This phenomenon indi-
cated that the SO2 desorption could be released by heating.
RESULTS AND DISCUSSION
Effect of time on absorption capacity: Sulphur dioxide
was absorbed by pure ionic liquids. Three reactions with diffe-
rent ionic liquids all reached equilibrium state after about 0.5 h
and the saturated absorption mole ratio of SO2/ILs was 0.30,
0.50 and 0.66, respectively. Fig. 1 showed, the order of mixed
absorption systems on the SO2 adsorption capacity was: tris-
(2-hydroxyethylammonium)sulfamate > bis-(2-hydroxyethyl-
ammonium)sulfamate > 2-hydroxyethylammonium sulfamate
and the absorption capacity increased with the basicity increase
in the cation.
Sulphur dioxide was absorbed by ILs-DBU mixed absor-
ption solution. Three mixed absorption systems reached the
equilibrium state about 0.5 h and the saturated absorption
mole ratio of SO2/ILs-DBU was 2.63, 2.83 and 2.96, respec-
tively. Fig. 2 showed the order of mixed absorption systems
on the SO2 adsorption capacity was: tris-(2-hydroxyethyl-
ammonium)sulfamate-DBU > bis-(2-hydroxyethylammonium)-
sulfamate –DBU > 2-hydroxyethylammonium sulfamate-
DBU. The result showed that adding DBU can not only remain
the time to reach equilibrium at 0.5 h, but also increase the
absorption capacity greatly.
Temperature (°C)
Fig. 3. Mole ratio of SO2/ILs as function of temperature
Due to the hydroxyl ammonium ionic liquids with a little
solubility for SO2, the absorption mole ratio of SO2 which has
been absorbed by alcoholic hydroxyl and DBU in mixed

Vol. 26, No. 22 (2014)
Capture of Sulphur Dioxide by Hydroxyl Ammonium Ionic Liquid-DBU Mixtures 7825
Wavenumber (cm–1)
Fig. 6. IR spectrumof the 2-hydroxyethylammonium sulfamate ionic liquid
before and after capturing SO2
Temperature (°C)
Fig. 4. Mole ratio of SO2/ILs-DBU as a function of temperature
absorption systems was equal to that of SO2/ILs-DBU subtrac-
ting the SO2 absorption capacity of pure ionic liquids. Fig. 5
showed that after deducting the SO2 saturated absorption
capacity of pure ionic liquids, it could be calculated that the
saturated absorption mole ratio of SO2/ILs-DBU separately
was 2.33, 2.33, 2.30 and the absorption capacity didn’t have
any visibly growth as the increase of alcoholic hydroxyl group
in ionic liquids.
8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 ppm
10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 ppm
Fig. 7. 1H NMR spectra of the 2-hydroxyethylammonium sulfamate ionic
liquid before and after capturing SO2
ammonium sulfamate for capturing SO2 was purely physical
absorption.
2-Hydroxyethylammonium sulfamate-DBU was charac-
terized by NMR spectrum before and after capturing SO2. The
1H NMR spectrum in Fig. 8 showed that three new resonance
peaks were found in δ= 7.73, 9.54 and 9.89 ppm and the chemical
shift at δ = 9.89 ppm was generated by the heterocyclic hydrogen
Time (min)
Fig. 5. Mole ratio of SO2/ILs-DBU deducting the mole ratio of SO2/ILs as
a function of time
1
(H-N=C) in DBU. Combined with the H NMR spectrum
of 2-hydroxyethylammonium sulfamate-DBU (Fig. 9), the
resonance peak at δ = 4.24 ppm which was produced by -NH2
and -OH was disappeared. When SO2 was passed into the
absorption system, the H in alcoholic hydroxyl would disso-
ciate and integrate with N atom in the DBU ring where contains
the isolated electron pair. So it leads to the formation of
conjugate-structure in DBU ring. Thus, the new resonance peak
at δ = 9.89 ppm was consistent with nitrogen atom in the
protonated DBU heterocyclic. The dissociation of H in
alcoholic hydroxyl weakened the effect of intra molecular
hydrogen bond and the chemical shift in NH2 downfield shifted
to δ = 7.73 ppm. Because the active hydrogen atom in -NH3+
shifted to N atom in the DBU ring, resulting in the formation
of N-H bond and the new resonance peak at 9.54 ppm.
Based on previous reports and above results, we proposed
the reaction mechanism between SO2 and the mixed absorption
system consisted of the alcoholic hydroxyl group and DBU,
which was shown in Scheme-II.
Mechanism of absorption: The saturated absorption
mole ratio of three mixed absorption systems was close to 2.3
after deducting the SO2 saturated absorption capacity of pure
ionic liquids. It was similar to the phenomenon that sulphur
dioxide absorbed by n-hexyl alcohol-DBU absorption solution
reported in literature13. It indicated that the chemical reaction
occurs in the ILs-DBU mixed system which including sulphur
dioxide with alcoholic hydroxyl and DBU.
In order to gain a further understanding of the absorption
mechanism, 2-hydroxyethylammonium sulfamate was charac-
terized by IR and NMR spectrum before and after capturing
SO2. From Fig. 6, it can be seen that IR spectrum of 2-hydroxy-
ethylammonium sulfamate didn’t generate new absorption
peaks before and after capturing SO2. 1H NMR spectrum (Fig. 7)
of 2-hydroxyethylammonium sulfamate didn’t emerge new
chemical shifts except 3.3 ppm water peak before and after
capturing SO2. So the results indicated that 2-hydroxyethyl-

7826 Qu et al.
Asian J. Chem.
ppm
10.510.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0
Fig. 8. 1H NMR spectrum of the 2-hydroxyethylammonium sulfamate ionic
liquid-DBU-SO2
Fig. 10. TGA and DTA curves of 2-hydroxyethylammonium sulfamate ionic
liquid. The curve of TGA is (a) and the curve of DTA is (b)
hydroxyl and formed new ionic liquids. From Fig. 11, it could
be seen that SO2 absorption capacity and absorption rate were
hardly changed in four SO2 absorption-desorption cycle
experiments. It indicated that the new ionic liquid had good
stability and could be recycled.
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm
Fig. 9. 1H NMR spectrum of the 2-hydroxyethylammonium sulfamate ionic
liquid-DBU
HO
O
N
H2N
S
O
+
N
O
NH3
SO2
-SO2(heat)
O
S
Time (min)
O
N
H
Fig. 11. Four consecutive cycles of SO2 absorption and desorption in
2-hydroxyethylammonium sulfamate ionic liquid
NH3
O
NH2
S
N
O
O
O
Comparison of SO2 absorption performance for diffe-
rent ionic liquids: In recent years, it was discovered that some
new alcohol amine ionic liquids have better effectiveness for
SO2 gas absorption and desorption from a large number of
experiments and research. The 1,1,3,3-tetramethyl guanidine
lactic acid salt ionic liquid was synthesized by Wu, et al.1 and
it was applied for absorbing SO2 at atmospheric pressure and
40 ºC, meanwhile SO2 could be desorbed after heating or
vacuum desorption. Wu et al.14 have prepared the novel porous
cross-linking particles with 1,1,3,3-tetramethyl guanidine
lactate salt and N,N-methylene bisacrylamide successfully and
the study for absorption and desorption of SO2 indicated that
it possessed higher efficiency and absorption capacity for SO2.
A series of 1,1,3,3-tetramethyl guanidine ionic liquids were
synthesized and studied by Riisager et al.7 and the result
showed that SO2 can be fully desorbed under the conditions
of high temperature and low pressure. A series of alkaline
liquids was synthesized by the reaction of alcohol amine with
acid, lactic acid or structure similar acids at room temperature
by Zhang et al.5,15-17, using the pure ionic liquids or supported
on the carrier have possessed more high absorption efficiency
Scheme-II: Reaction mechanism between the hydroxyl ammonium ionic
liquid-DBU and SO2
Thermal stability: The thermal behaviour of 2-hydroxy-
ethylammonium sulfamate was determined by DTA and TGA.
The heating rate and final temperature of is 5 ºC/min and
800 ºC, respectively. Fig. 10 shows the DTA and TGA curves
of 2-hydroxyethylammonium sulfamate from 25-800 ºC. It
indicated that 2-hydroxyethylammonium sulfamate was stable
up to 280 ºC and decomposed rapidly over 300 ºC.
Recycling of ionic liquids: In order to investigate the
stability of SO2-saturated mixed absorption systems which
were composed of hydroxyl ammonium ionic liquids and DBU,
we chose one SO2-saturated mixed absorption system which
was composed of 2-hydroxyethylammonium sulfamate ionic
liquid and DBU, to conduct SO2 absorption-desorption cycle
experiments.After the desorption cycle experiments, it showed
that it was difficult to absorb 1 mole SO2 from absorption
solution, because SO2 molecules were bonded with alcohol

Vol. 26, No. 22 (2014)
Capture of Sulphur Dioxide by Hydroxyl Ammonium Ionic Liquid-DBU Mixtures 7827
1
and less short equilibration time at ambient temperature and
atmospheric pressure. The principle of the absorption and
desorption SO2 of the alcohol amine ionic liquids were charac-
Combined previous researches with the ILs-DBU H NMR
spectrum data before and after capturing SO2, it is found that
the SO2 reacted with alcoholic hydroxyl firstly in ionic liquids
and DBU when SO2 was absorbed into ILs-DBU and it resulted
in new complex sulphur-bearing ionic liquid. Then, the new
sulphur-bearing ionic liquid could capture more SO2 by physical
means. The absorbed SO2 could be released by heating, there-
fore the absorption system could be recycled. Further appli-
cation of hydroxyl ammonium ionic liquids-DBU should be
enhanced for future large-scale SO2 scrubbing processes due
to its high absorption capacity, good regeneration and low cost.
1
terized by H NMR and the recycling study of ionic liquids
was carried out.
This study indicated that the mechanism of alcohol amine
ILs-DBU absorption for SO2 existed both physical and
chemical absorption, because SO2 was absorbed in the ionic
liquid by physical absorption, then the amino group active
positions reacted with ethanolamine cation easily. The result
showed that the alcohol amine ILs-DBU system had better
absorption capacity for SO2. We raised an assumption that the
mechanism was typical chemical absorption, exactly showed
in Scheme-II.
ACKNOWLEDGEMENTS
This research was funded by the National Natural Science
Foundation of China (51008147) and theYunnan Natural Sci-
ence Foundation (2009ZC026M).
However, this chemical bond was relatively weak, which
can be easily broken at high temperatures. The physical
adsorption of SO2 can be desorbed at high temperatures easily.
Thus, the desorption of SO2 will be easier and more complete
at the higher temperature, which results in shorter desorption
time and higher desorption rate.
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