Efficient absorption of ammonia with hydroxyl-functionalized ionic liquids

Article abstract of DOI:10.1039/c5ra13730f

Ammonia (NH₃) emitted from the ammonia synthesis process is a kind of waste chemical resource and a major environmental pollutant. The traditional water scrubbing method suffers from high energy consumption due to the concentrated NH₃ from aqueous ammonia. Therefore, it is desirable to develop novel absorbents for the efficient, reversible and environmentally-friendly recovery of NH₃. In this paper, a series of hydroxyl-functionalized imidazolium ILs ([EtOHmim]X, X = [NTf₂], [PF₆], [BF₄], [DCA], [SCN] and [NO₃]) were designed and prepared. Their physical properties and NH₃ absorption capacities under different temperatures and pressures were systematically investigated. The effects of hydroxyl cation, anionic structures, pressure and temperature on absorption performance were sufficiently studied. In addition, the absorption mechanism was investigated in detail by spectral analysis and quantum chemistry calculations. Compared with conventional IL [Emim]X, a higher absorption capacity was achieved by introducing the hydroxyl group on the imidazolium cation. The mechanism results showed the fascinating absorption performance of the task-specific ILs was attributed to the stronger hydrogen bonding interaction between NH₃ and the H atom of the hydroxyl group. Considering the excellent absorption performance, high thermal stability, and super reversibility, this type of IL provides great improvement over conventional IL and shows their enormous potential in NH₃ recovery.

Full text of DOI:10.1039/c5ra13730f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Efficient absorption of ammonia with hydroxyl-
functionalized ionic liquids†
Cite this: RSC Adv., 2015, 5, 81362
ab
a
a
a
a
Zhijie Li, Xiangping Zhang,* Haifeng Dong, Xiaochun Zhang, Hongshuai Gao,
a
b
c
Suojiang Zhang,* Jianwei Li and Congmin Wang
Ammonia (NH
major environmental pollutant. The traditional water scrubbing method suffers from high energy
consumption due to the concentrated NH from aqueous ammonia. Therefore, it is desirable to develop
novel absorbents for the efficient, reversible and environmentally-friendly recovery of NH . In this paper,
a series of hydroxyl-functionalized imidazolium ILs ([EtOHmim]X, X ¼ [NTf ], [PF ], [BF ], [DCA], [SCN]
and [NO ]) were designed and prepared. Their physical properties and NH absorption capacities under
3
) emitted from the ammonia synthesis process is a kind of waste chemical resource and a
3
3
2
6
4
3
3
different temperatures and pressures were systematically investigated. The effects of hydroxyl cation,
anionic structures, pressure and temperature on absorption performance were sufficiently studied. In
addition, the absorption mechanism was investigated in detail by spectral analysis and quantum
chemistry calculations. Compared with conventional IL [Emim]X, a higher absorption capacity was
achieved by introducing the hydroxyl group on the imidazolium cation. The mechanism results showed
the fascinating absorption performance of the task-specific ILs was attributed to the stronger hydrogen
Received 13th July 2015
Accepted 16th September 2015
bonding interaction between NH
absorption performance, high thermal stability, and super reversibility, this type of IL provides great
improvement over conventional IL and shows their enormous potential in NH recovery.
3
and the H atom of the hydroxyl group. Considering the excellent
DOI: 10.1039/c5ra13730f
www.rsc.org/advances
3
as negligible vapor pressure, wide liquid temperature range,
high thermal stability and adjustability and have been paid
1
. Introduction
2
3
4
–11
12–19
6,20–23
As a typical environmental pollutant, the increase of ammonia great attentions on separation of CO
,
SO
absorption by ionic liquids
3
and H S.
2
2
2
(NH
3
) release is leading to different environmental problems On the contrast, the reports on NH
including eutrophication of ecosystems and the formation of are scare. In fact, the results reported have shown that ionic
1

ne particulate matter and seriously threatens human health. liquids have great potential for NH
3
absorption. Yokozeki
demonstrated the conventional ionic liquids such as
], [Hmim][Cl] had high NH absorption perfor-
fertilizer and so on. Therefore, removal and recovery of NH is of mance and the anion had little effect on NH solubility. Li
solubility increased when the length of
24,25
Meanwhile, NH
3
is one of the important chemical raw materials et al.
and widely applied in producing nitric acid, nitrogenous [Bmim][PF
6
3
3
3
26
great signicance. Up to now, water scrubbing has been the et al. reported NH
most common method due to its high NH
absorption capacity. cation's alkyl increased. Shi et al. testied the cation played
However, this method suffers from several inherent drawbacks the leading role in determining NH solubility and they found a
such as high energy consumption, large amounts of waste water strong hydrogen bonding could be formed between NH
3
27
3
3
3
and difficult reclamation of NH
develop environmentally-friendly, recyclable absorbents for ular simulation.
3
. Hence, it is desirable to molecule and the ring H atom of the [Emim] cation by molec-
efficient NH absorption.
Although great efforts had been made to improve the NH
3
3
It is increasing recognized that ionic liquids (ILs) are absorption, the solubility of NH
3
in these conventional ILs was
promising absorbents because of their special properties such relatively low. What is more, the research on the mechanism of
IL–NH system was very scanty and not comprehensive. Shi
3
27
et al. investigated the NH3 absorption mechanism just by
a
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of
molecular simulation. As reported, hydrogen bonding interac-
Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of
Sciences, Beijing, 100190, China. E-mail: xpzhang@home.ipe.ac.cn
28
3
tion between IL and NH was benecial to enhance solubility,
b
thus in this work, hydroxyl, as the hydrogen bonding donor
group, was introduced into the imidazolium cation for further
improving the absorption. NH absorption capacities of these
3
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing, 100029, China
c
Department of Chemistry, Zhejiang University, Hangzhou, 310027, China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra13730f
hydroxyl-functionalized ILs at different temperatures and
1
81362 | RSC Adv., 2015, 5, 81362–81370
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
pressures were systematically investigated. Furthermore, the
absorption mechanism was deeply explored by experimental
spectra analysis and quantum chemistry calculations.
2. Experimental section
2
.1. Materials
NH (99.999%) was supplied by Beijing Beiwen Gas Factory.
-Methylimidazole (99.0%), NaBF (98.0%), LiNTf (98.5%) and
3
1
4
2
NaSCN (98.5%) were purchased from Sinopharm Chemical
Reagent Co., Ltd. 2-Chloroethanol (99.0%) was purchased from
Xiya Chemical Co., Ltd. KPF (98.0%) and NaNO (99.0%) were
6
3
purchased from Beijing Chemical Works Co., Ltd. All the
chemicals above were used without further purication. 1-Ethyl-
3-methylimidazolium chloride was obtained from Linzhou Fig. 2 Schematic diagram of NH3 absorption apparatus. (a) NH3
Keneng Materials Technology Co., Ltd. 1-2(-Hydroxyethyl)-3- cylinder; (b
1
–b ) valve; (c) storage tank; (d and d ) pressure sensor; (e)
absorption vessel; (f) thermostatic bath; (g) vacuum pump; (h)
magnetic stirrer.
4
1
2
methylimidazolium
salt
[EtOHmim]X
and
1-ethyl-3-
methylimidazolium salt [Emim]X were synthesized in our
laboratory.
2.3. Characterization and physical properties
1
13
2
.2. Synthesis of ionic liquids
H NMR and C NMR spectra were recorded on a Bruker
spectrometer (600 Hz) in deuterated dimethyl sulfoxide
[
EtOHmim][NTf ] was synthesized by the method shown in
2
2
9
(DMSO-d ) with tetramethylsilane as the internal standard. The
NMR data of ILs were presented in ESI.† FT-IR spectra were
Scheme S1 (seeing ESI†) according to the literature. Prepa-
ration of [EtOHmim][Cl]: 1-methylimidazole (41.00 g, 0.5 mol)
was put in a round-bottom ask (250 mL) and 2-chloroethanol
6
ꢀ1
obtained in the range of 400–4000 cm on a Thermo Nicolet
80 spectrometer. The water contents of the ILs were measured
by Karl Fisher coulometers C20 and had been reduced less than
00 ppm. The chlorine contents were tested by ioniza chlorine
3
(
48.31 g, 0.6 mol) was added slowly. The mixture was stirred
for 24 hours at 353.15 K. Then the product was washed several
times with ethyl acetate. The [EtOHmim][Cl] white solid was
obtained aer ltration and vacuum drying for 48 hours at
4
analyzer (Leici PXSJ-226). The densities of ionic liquids were
measured by a density meter (Anton Paar DMA 5000) with an
accuracy of ꢁ0.000005 g cm . The viscosities were measured by
an automated micro viscometer (Anton Paar AMVn). The
thermal decomposition temperatures were tested by TGA
3
43.15 K. Preparation of [EtOHmim][NTf ]: [EtOHmim][Cl]
2
ꢀ3
(
2
16.252 g, 0.1 mol) and LiNTf (28.69 g, 0.1 mol) were mixed
and some water was added as solvent. Then the mixture was
stirred for 24 hours at ambient temperature. By separating the
mixture, the supernate was removed and ionic liquid was
washed by deionized water for several times until there was no
sediment formed when AgNO3 was added in water. At last,
(Q5000 V3.15 Build 263) from room temperature to 773.15 K
ꢀ
1
2
with a heating rate of 10 K min under N atmosphere. The
glass transition temperatures were measured on a Mettler
Toledo DSC1 between 123.15 and 298.15 K at a heating rate of
high-purity [EtOHmim][NTf ] was obtained aer rotary evap-
oration and vacuum drying at 373.15 K for 64 hours. The
2
ꢀ
1
1
2
0 K min under N atmosphere.
ꢀ
synthesis methods of other ILs [EtOHmim]X (X ¼ [PF
6
] ,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
[
[
BF
BF
4
] , [DCA] , [SCN] , [NO
3
] ) and [Emim]X (X ¼ [NTf
2
] ,
2.4. Apparatus and procedures
solubility was measured by the gas–liquid equilibrium
structures of [EtOHmim] cation and [Emim] cation were pre- apparatus (shown in Fig. 2) which was similar to that in our
4
] , [NO ] ) were similar to that of [EtOHmim][NTf ]
3
2
except the solvent was replaced by acetone. The different NH
3
17,30
sented in Fig. 1.
previous work.
In a typical experiment, the temperature was
+
+
Fig. 1 Schematics of [EtOHmim] (a) and [Emim] (b), with atom labels.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 81362–81370 | 81363

View Article Online
RSC Advances
Paper
ꢄpffiffi
ꢅ

xed by a thermostatic water bath with an uncertainty of ꢁ0.1 K
pV
pðV ꢀ bÞ
a
V þ 2 þ 1 b
ln 4 ¼
ꢀ 1 ꢀ ln
ꢀ 2
ln
ꢄpffiffi
ꢅ
and the pressure was measured by pressure transmitters with
an accuracy of 0.0001 kPa. About 5.0 g IL was placed into the
absorption vessel (30 mL), then the gas in absorption vessel was
1:5bRT
RT
RT
V ꢀ 2 ꢀ 1 b
(
5)
removed by vacuum pump. The NH
was charged into the absorption vessel slowly through the valve
). Then the magnetic stirrer was opened to enhance the
dissolution. It is supposed the equilibrium is reached aer the
pressure keeps unchanged for 30 min. The solubility of NH was
3
in storage tank (500 mL)
q
q
/p )
D
solG ¼ RT ln(k
H
(6)
(7)
(b
2
q
2vlnðk
H
=p Þ
q
D
solH ¼ ꢀ RT
vT
3
q
q
calculated by Peng–Robinson (P–R) eqn (1) through the pres-
sure variation in storage tank and absorption vessel:
DsolH ꢀ DsolG
q
D
solS ¼
(8)
T
RT
a
1 3 1
where f (T, p) represents the fugacity of NH and 4 (T, p) is the
p ¼ V
ꢀ b
ꢀ V
ðV
þ bÞ þ bðV
ꢀ bÞ
(1)
eq
m
m
m
m
3
fugacity coefficient of NH solute in the gas phase. p is the
q
equilibrium partial pressure of NH
standard pressure.
3
and p represents the
a ¼ a
c
a
(1.1)
(1.2)
2
ðRT
c
Þ
a
c
¼ 0:457235
3
. Results and discussion
p
c
3
.1. Physical properties of ILs
RT
c
b ¼ 0:077796 p
(1.3)
Density and viscosity of IL were fundamental properties for gas
separation process, hence these properties were measured at
different temperatures from 298.15 K to 343.15 K. Fig. 3 shows
the densities of the functionalized ILs decrease linearly with the
increasing of temperature and they fall in the range from 1.15 to
1.58 g cm . It can be found the densities are greatly affected by
the different anions and [EtOHmim][DCA] shows the lowest
density.
c
0
a
.5
2
0.5
r
¼ 1 + (0.37464 + 1.54226u ꢀ 0.26992u )(1 ꢀ T
)
(1.4)
(1.5)
T
T
r
¼ T
ꢀ
1
c
ꢀ
ꢁ
ꢀ
ꢁ
V
S
V
A
ꢀ VIL
n
NH3 ¼ D
ꢀ
(2)
V
m;s
V
m;A
Fig. 4 shows the viscosities of the functionalized ILs with
different anions range from 19.10 to 609.98 mPa s and decrease
n
NH3
xNH3 ¼ n
(3) in an exponential manner with the increasing of temperature,
NH3 þ nIL
31
which is in agreement with the other imidazolium ILs.
where p, V
m
a
and T are denoted to pressure, molar volume and Viscosity activation energy (E ) of IL is estimated by a tting of
32
temperature, respectively. R is the gas constant and a, b, a , a the measured viscosity to Arrhenius expression using the
c
are the parameters of the cubic equation. Tc is the critical following equation:
temperature, pc is the critical pressure and Tr is relative
h ¼ h
N
exp(ꢀE
a
/RT)
(9)
temperature. nNH3 is the amount of NH
amount of ionic liquid. V , V and VIL represent the volumes of
storage tank, absorption vessel and ionic liquid, respectively.
m,S and Vm,A are the molar volume of storage tank and
3
absorbed and nIL is the
S
A
V
absorption vessel. xNH3 represents the mole fraction of NH
3
absorbed.
2.5. Thermodynamic analysis
Henry's constant (k
H
) is an important parameter to present the
in ILs. The Henry's constants of
absorbed in ionic liquids at different temperatures could
be obtained by the following eqn (4) and the fugacity coefficient
4) was calculated according to the P–R equation. Subsequently,
absorption behavior of NH
NH
3
3
(
q
the changes of standard Gibbs free energy (D G ), standard
sol
q
q
enthalpy (DsolH ) and standard entropy (D S ) could be esti-
sol
mated by the following eqn (6)–(8).
ꢂ
ꢃ
eq
f
1
ðT; pÞ
p 4 ðT; pÞ
1
k
H
ðTÞ ¼ lim
¼
(4)
p/0
x
x
1
1
Fig. 3 Densities of task-specific ILs at different temperatures.
81364 | RSC Adv., 2015, 5, 81362–81370
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 5 NH
literatures
3
5,26
absorption capacity of [Bmim][BF
at 298.15 K.
4
] in this work and the
Fig. 4 Viscosities of task-specific ILs at different temperatures.
In this equation, h is the viscosity at any temperature and h
2
N
available literatures in order to validate the reliability of data in
this work. Fig. 5 shows the solubility is similar to that in liter-
is the apparent viscosity at innite temperature. The values of E
for [EtOHmim]X shown in Table S1† and they range from 31 to
a
25,26
atures,
which indicates that the experiment apparatus is
ꢀ1
4
5 kJ mol . Generally, the larger value will indicate the higher
reliable and the data is credible. Then, a series of vapor liquid
equilibrium measurements were performed at pressures from
viscosity. The calculated viscosity activation energies of the
task-specic ILs are larger than those of the typical solutions
0
3
to 0.6 MPa (A (absolute)) and temperatures from 298.15 to
43.15 K. The solubility data of pressure–temperature–compo-
ꢀ
ꢀ1
1
ꢀ1
(
Ea water ¼ 17.0 kJ mol , Ea benzene ¼ 10.4 kJ mol , and
33
Ea acetone ¼ 7.1 kJ mol ). Although [EtOHmim][DCA] shows
sition (p–T–x) of the task-specic ILs are presented in ESI Table
S2.†
the lowest viscosity, the introducing of hydroxyl group results in
ꢀ
1
the larger viscosity than that of [Emim][DCA] (18.4 kJ mol ),
which indicates that cations also inuence the viscosity.
In addition, thermal stability of IL is also very signicant for
gas separation, because high thermal stability is benecial for
reversible absorption and desorption. The TGA curves are shown
in Fig. S2† and the results of DSC and TGA are summarized in
3.2.1 Effect of hydroxyl group cation. To assess the effect of
hydroxyl group cation on NH solubility, the absorption
capacity of functionalized ILs [EtOHmim]X were compared with
that of conventional ILs [Emim]X at 333.15 K. The results show
that the former captures higher amount of NH . Fig. 6 shows
that when the anion is same, an obvious increase of solubility is
found in [EtOHmim]X. For example, at 102.7 kPa, the solubility
3
3
Table 1. The thermal decomposition temperatures (T ) of the six
d
functionalized ILs are all above 500 K and [EtOHmim][NTf
2
]
of [EtOHmim][BF
the mole fraction is 0.56. For [Emim][BF
4
] is 0.27 in mole fraction, and at 547.0 kPa,
], the solubility at
shows the highest decomposition temperature. Pablo's group
34
4
d
reported the T of [Emim][SCN] was 538.6 K, which was similar
to that of [EtOHmim][SCN]. It shows the introducing of hydroxyl
is almost no inuence on the thermal stability.
3
.2. NH
3
absorption performance of [EtOHmim]X
absorption capacity of [Bmim][BF ] was investigated
First, NH
3
4
at 298.15 K and compared with the values reported from the
Table 1 The melting points (T
m
), glass transition temperatures (T
g
) and
d
the thermal decomposition temperatures (T ) of the functionalized ILs
[
EtOHmim]X
T
m
(K)
T
g
(K)
d
T (K)
[
[
[
[
[
[
NTf
PF
BF
DCA]
SCN]
2
]
—
308.32
—
—
286.32
281.32
190.02
187.67
187.98
185.28
185.64
183.35
643.75
564.11
542.81
507.18
6
]
]
4
543.91
NO
3
]
3
546.42 Fig. 6 Comparison of NH solubilities in [EtOHmim]X and [Emim]X (X
ꢀ ꢀ
ꢀ
3
¼
2 4
[NTf ] , [BF ] , [NO ] ) at 333.15 K.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 81362–81370 | 81365

View Article Online
RSC Advances
03.1 kPa is 0.10 and at 548.2 kPa the mole fraction is 0.40. It
Paper
1
suggests that the absorption capacities of imidazolium ILs are
distinctly enhanced due to the introducing of hydroxyl group. A
possible explanation is that hydrogen bonding interaction
brings about the improved absorption performance. The acid
hydroxyl, regarded as the hydrogen bonding donor group, could
3
form stronger hydrogen bonding with the N atom of NH than
the conventional [Emim]X. In addition, because the viscosities
of [EtOHmim]X are slightly larger than [Emim]X due to the
introducing of hydroxyl group, the equilibrium time for NH3
absorption in [EtOHmim][NTf ] and [Emim][NTf ] have been
2
2
also measured respectively using the same apparatus. The
variations of pressure with time are shown in Fig. S3.† We can

nd the equilibrium time of [EtOHmim][NTf
slightly longer than [Emim][NTf ] (about 4 min). In conclusion,
hydroxyl-functionalized ILs may be more suitable to be the
solvents for NH recovery than conventional ILs.
.2.2 Effect of anions. With the comparison of NH solu-
2
] (about 6 min) is
2
Fig.
3 2
8 The solubilities of NH in [EtOHmim][NTf ] at different
3
temperatures.
3
3
bilities in ILs with different anions, it is observed that anions
also have an obviously effect on NH absorption. Fig. 7 pres-
3
2
highest solubility of [EtOHmim][NTf ]. Because of the highest
ꢀ
ꢀ
ents the ILs of anions containing uoride ([NTf
2
ꢀ
] , [PF
6
] ,
absorption capacity of [EtOHmim][NTf
investigated as a typical example.
2
], this ionic liquid is
ꢀ
ꢀ
[
[
BF
NO
4
] ) have higher solubility than others ([DCA] , [SCN] ,
ꢀ
3
] ). The absorption capacities of these ILs decrease in the
3.2.3 Effects of temperature and pressure. The effects of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
following order: [NTf
2
]
> [PF
6
]
> [BF
4
]
> [SCN] > [NO
3
] .
3
temperature and pressure on NH absorption in [EtOHmim]
Especially, for [DCA] anion, when the pressure is less than
[
2
NTf ] are displayed in Fig. 8. It can be seen that with the
ꢀ
1
50 kPa, its absorption capacity is lower than that of [NO ] ;
3
increasing of temperature, the absorption capacity of
hydroxyl-functionalized ILs dramatically decreases. Besides,
the molar fraction of NH3 increases continuously with the
increasing of pressure. It suggests that low temperature is
but when the pressure is between 150 to 400 kPa, its absorp-
ꢀ
ꢀ
tion capacity falls in that between [SCN] and [NO ] . The
3
similar conclusion can be obtained at other temperatures. The
reason for the higher absorption capacities of ILs containing
3
better for NH absorption and the desorption can be carried

uoride may be that the F atom of the anions can also form
out at high temperature, which is in good agreement with the
2
7
hydrogen bonding interaction with the H atom of NH
3
.
From
24
literature.
a molecular perspective, there are three hydrogen bonding
donors on NH , four acceptors on [BF ], six acceptors on [PF ]
3
4
6
3.3. Desorption and recycling
and 10 acceptors on [NTf ]. Therefore, the relatively stronger
2
hydrogen bonding interaction may be formed between the H Recyclability is another important factor to evaluate an ionic
atom of NH and [NTf
] anion, which is consistent with the liquid solvent and a key issue for potential industrial appli-
cation, thus the absorption cycles of [EtOHmim][NTf ] have
3
2
2
Fig. 7 The solubilities of NH
3
2 3
in task-specific ILs with different anions Fig. 9 Recycling of [EtOHmim][NTf ] for NH absorption at 298.15 K
at 313.15 K.
and 100 kPa.
81366 | RSC Adv., 2015, 5, 81362–81370
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
3
Fig. 10 Henry's constants of NH in task-specific ILs versus
temperature.
Fig. 11 FT-IR spectra of [EtOHmim][NTf
of NH
2
] before and after absorption
3
.
been measured at 298.15 K and 100 kPa. First, the solubility
was investigated in the fresh IL, then NH -saturated IL was in the ILs containing uoride are smaller than those of others,
3
desorbed under a vacuum of 0.15 kPa for 1 hour at 353.15 K which accords with the higher NH absorption capacity of the
3
and reused for NH3 absorption. The absorption–desorption ILs containing uoride. Thermodynamic parameters including
experiments were performed for ve cycles. Fig. 9 shows that standard enthalpy, standard Gibbs free energy, and standard
the solubility keeps almost unchanged aer ve cycles. It is entropy were also calculated and the results were presented in
revealed that [EtOHmim][NTf
2
] can be reused with almost Table 2. Solution entropy represents the direction of disorder in
same capacity, which suggests the functionalized ILs have the absorption process. Table 2 shows standard Gibbs free
good recyclability.
energy changes are positive and entropy changes are negative.
Namely, at standard pressure, ammonia solution in [EtOHmim]
X is a process of entropy decreasing. The value of DG increases
with the increasing of temperature, which means higher
temperature is unfavorable for NH absorption. This conclusion
3
is consistent with the experimental results. Solution enthalpy is
3
.4. Thermodynamic properties of NH
Henry's constants were calculated according to the solubility of
NH . In Fig. 10, with the increasing of temperature, Henry's
constants increase for each ionic liquid. The Henry's constants
3
in ILs
3
an important parameter to evaluate the gas–liquid interaction.
Table 2 Thermodynamic parameters of NH
3
in [EtOHmim]X
T (K)
[EtOHmim][NTf
2
]
[EtOHmim][PF
6
]
[EtOHmim][BF
4
]
[EtOHmim][SCN]
3
[EtOHmim][NO ]
q
ꢀ1
D
solG (kJ mol
)
2
3
3
3
3
98.15
13.15
23.15
33.15
43.15
4.09
5.03
5.55
5.97
6.35
4.24
5.16
5.60
6.02
6.41
4.28
5.22
5.70
6.18
6.67
4.64
5.45
5.97
6.64
7.08
4.73
5.62
6.09
6.82
7.34
q
ꢀ1
D
solH (kJ mol
)
298.15
313.15
323.15
333.15
343.15
ꢀ9.39
ꢀ10.35
ꢀ11.03
ꢀ11.72
ꢀ12.43
ꢀ9.46
ꢀ10.44
ꢀ11.11
ꢀ11.81
ꢀ12.53
ꢀ9.83
ꢀ10.84
ꢀ11.55
ꢀ12.27
ꢀ13.02
ꢀ10.27
ꢀ11.33
ꢀ12.07
ꢀ12.83
ꢀ13.61
ꢀ11.01
ꢀ12.15
ꢀ12.94
ꢀ13.75
ꢀ14.59
q
ꢀ1 ꢀ1
D
solS (J mol
K )
298.15
313.15
323.15
333.15
343.15
ꢀ45.21
ꢀ49.14
ꢀ51.31
ꢀ53.09
ꢀ54.75
ꢀ45.94
ꢀ49.80
ꢀ51.72
ꢀ53.52
ꢀ55.20
ꢀ47.33
ꢀ51.30
ꢀ53.39
ꢀ55.38
ꢀ57.37
ꢀ50.02
ꢀ53.58
ꢀ55.82
ꢀ58.43
ꢀ60.29
ꢀ52.80
ꢀ56.75
ꢀ58.88
ꢀ61.74
ꢀ63.91
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 81362–81370 | 81367

View Article Online
RSC Advances
Paper
but the absolute value of enthalpy in [EtOHmim][NO
3 3
]–NH
system is higher than that in [EtOHmim][NTf ]–NH system.
2
3
Therefore, for the ionic liquids with different anions, not only
the interaction between NH and ILs but also the free volume of
3
anions plays important inuence on solubility.
3.5. Mechanism of NH
3
absorption
The mechanism of NH
comparison of FT-IR and H NMR spectra of [EtOHmim]X
3
absorption was investigated by the
1
before and aer NH
no obvious change of FT-IR spectra for [EtOHmim][NTf
before and aer NH absorption. Furthermore, in situ FT-IR
shown in Fig. S4†) is applied to monitor the spectral varia-
3
absorption. Fig. 11 shows that there is
2
]
3
(
tion of NH3 absorption process for one hour, which also
shows no distinct changes. Considering the absorption
performance, the easy desorption behavior and no formation
of new functional group, a possible explanation is that these
3
novel ionic liquids absorb NH through hydrogen bonding
interaction, such as O–H/N.
1
Fig. 12 H NMR spectra of [EtOHmim][NTf
2
] before and after NH
3
1
Meanwhile, H NMR spectra of [EtOHmim]X before and
absorption.
aer absorption of NH
3
were further studied. First, by
1
comparing the
H NMR spectra of [Emim][NTf2] and
From Table 2, the enthalpy changes are negative, which indi- [EtOHmim][NTf ] (seeing ESI Fig. S5†), the peak at 5.16 ppm is
2
cates the solution at standard pressure is exothermal. And this conrmed to be the shi of H11 atom on the hydroxyl. Fig. 12
conclusion corresponds with the ordinary solution rules for indicates the peak at 5.16 ppm disappears aer NH absorp-
3
most gas. The enthalpy changes increases with the increasing of tion and reappears aer desorption. And the same phenom-
temperature, it demonstrates the heat release in this solution enon is also happened in the other functionalized ILs. That
process decreases with the increasing of temperature. Usually, a shows the shi of the H11 on hydroxyl may be dramatically
higher value of enthalpy signies a stronger gas–liquid inter- affected due to the strong interaction between the electro-
action. But for [EtOHmim]X, the order of solubility is not as negative N atom of NH and H of hydroxyl.
3
11
same as that of enthalpy. For example, the solubility of
EtOHmim][NTf ] is signicantly higher than [EtOHmim][NO ], NH molecule was further studied by the quantum chemistry
The interaction between hydroxyl-functionalized cation and
[
2
3
3
+
+
Fig. 13 Optimized structures and interaction energies for [EtOHmim] –NH
3 3
(a and b) and [Emim] –NH (c and d) systems. O, red; N, blue; H,
white; N, gray. Hydrogen bonds are indicated by dotted lines, and distances are in angstroms.
81368 | RSC Adv., 2015, 5, 81362–81370
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
calculations at the B3LYP/6-311++G** level with Gaussian Natural Science Fund for Distinguished Young Scholars (No.
+
0
[
9 soware. By comparing [EtOHmim] –NH system with 21425625), the Key Program of National Natural Science Foun-
3
+
Emim] –NH3 system, it can be found both the hydrogen dation of China (21436010), and the Science and Technology
bonding and interaction energy of [EtOHmim] –NH system are Innovation Team of Cross and Cooperation of Chinese Academy
+
3
+
stronger than those of [Emim] –NH
3
system. First, two repre- of Sciences.
have been
located around H11 atom, the other is
sentative structures of [EtOHmim] cation and NH
calculated. One is NH
NH located around H
shown in Fig. 13a and b and they are different from those in
3
3
References
3
1
atom. The optimized structures are
1 J. W. Erisman, A. Bleeker, J. Galloway and M. S. Sutton,
Environ. Pollut., 2007, 150, 140–149.
27
Shi's work. It can be found that the hydrogen bonding is
formed between N atom of NH3 and H11 with distance of
2 C. Maton, N. D. Vos and C. V. Stevens, Chem. Soc. Rev., 2013,
42, 5963–5977.
ꢀ1
˚
1
.810 A, and the relative interaction energy is ꢀ56.32 kJ mol
.
Similarly, the hydrogen bonding also can be formed between
3 C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. Aki and
J. F. Brennecke, J. Chem. Eng. Data, 2004, 49, 954–964.
4 C. N. Dai, W. J. Wei, Z. G. Lei, C. X. Li and B. H. Chen, Fluid
Phase Equilib., 2015, 391, 9–17.
5 H. S. Gao, S. J. Zeng, X. M. Liu, Y. Nie, X. P. Zhang and
S. J. Zhang, RSC Adv., 2015, 5, 30234–30238.
6 A. H. Jalili, M. Shokouhi, G. Maurer and M. Hosseini-Jenab, J.
Chem. Thermodyn., 2014, 74, 286.
˚
the N atom of NH
3
and H
1
with a distance of 2.131 A, and the
ꢀ
1
interaction energy is ꢀ47.42 kJ mol . The shorter bonding
length and higher interaction energy show that NH
interact with the H11 atom of hydroxyl than H
3
prefers to
atom for
1
[
EtOHmim] cation and NH system. For comparison, the same
calculation method is used for [Emim] cation and NH system.
The structures are shown in Fig. 13c and d. The interaction
3
3
energy between NH
3
and [Emim] cation around H
1
atom is
7 J. L. Anderson, K. D. Janeille and F. B. Joan, Acc. Chem. Res.,
2007, 40, 1208–1216.
8 T. Nonthanasin, A. Henni and C. Saiwan, RSC Adv., 2014, 4,
7566.
ꢀ
1
ꢀ1
ꢀ
46.16 kJ mol , while that around H
What's more, the hydrogen bonding length between H
1
is only ꢀ20.98 kJ mol
.
1
and the
˚
N atom of NH
3
(2.145 A) is shorter than that between H11 and
˚
the N atom of NH
3
(2.402 A). The results show that NH
3
is more
9 M. Ramdin, S. P. Balaji, J. Manuel Vicent-Luna,
J. J. Gutierrez-Sevillano, S. Calero, T. W. de Loos and
T. J. H. Vlugt, J. Phys. Chem. C, 2014, 118, 23599–23604.
inclined to interact with H atom of imidazole ring rather than
1
H11 of C H group for [Emim] cation and NH system. By the
2
5
3
comparison of the two systems, the higher NH3 absorption 10 L. C. Tome, M. Isik, C. S. R. Freire, D. Mecerreyes and
capacity of hydroxyl-functionalized ILs than that of conven-
I. M. Marrucho, J. Membr. Sci., 2015, 483, 155–165.
tional imidazolium ILs should be attributed to the strong 11 Z. Z. Yang, Y. N. Zhao and L. N. He, RSC Adv., 2011, 1, 545.
hydrogen bonding and interaction energy. 12 P. J. Carvalho and J. A. P. Coutinho, Energy Fuels, 2010, 24,
662–6666.
6
1
3 G. K. Cui, W. J. Lin, F. Ding, X. Y. Luo, X. He, H. R. Li and
C. M. Wang, Green Chem., 2014, 16, 1211–1216.
4
. Conclusion
A series of hydroxyl-functionalized imidazolium ILs were 14 G. K. Cui, C. M. Wang, J. J. Zheng, Y. Guo, X. Y. Luo and
synthesized and their physical properties as well as NH3
H. R. Li, Chem. Commun., 2012, 48, 2633–2635.
absorption performance were systematically investigated. 15 S. Y. Sun, Y. X. Niu, Q. Xu, Z. Sun and X. H. Wei, RSC Adv.,
Compared with the conventional imidazolium ILs, the intro- 2015, 5, 46564–46567.
ducing of hydroxyl group results into the slight increase of 16 Z. Z. Yang, L. N. He, Q. W. Song, K. H. Chen, A. H. Liu and
viscosity, good thermal stability and improved NH absorption X. M. Liu, Phys. Chem. Chem. Phys., 2012, 14, 15832–15839.
capacity. Among these ILs, [EtOHmim][NTf ] has the highest 17 S. J. Zeng, H. Y. He, H. S. Gao, X. P. Zhang, J. Wang, Y. Huang
NH solubility, the mole fraction is 0.56 at 298.15 K and 159 kPa. and S. J. Zhang, RSC Adv., 2015, 5, 2470–2478.
The absorption cyclic experiments suggest these ILs can be 18 S. J. Zeng, H. Y. He, H. S. Gao and X. P. Zhang, RSC Adv.,
regenerated. Furthermore, the absorption mechanism was
2015, 5, 2470–2478.
detailedly studied by spectra analysis and quantum chemical 19 Y. Zhao and G. X. Hu, RSC Adv., 2013, 3, 2234–2240.
3
2
3
calculations. The results demonstrate the excellent NH
absorption ability of these novel ILs is due to the strong
hydrogen bonding between the N atom of NH
of the hydroxyl on cation. Owing to the efficient absorption
performance, simple preparation, good recyclability and excel- 22 K. Huang, Y. L. Chen, X. M. Zhang, S. L. Ma, Y. T. Wu and
3
20 M. A. Ahmadi, R. Haghbakhsh, R. Soleimani and
M. B. Bajestani, J. Supercrit. Fluids, 2014, 92, 60–69.
3
and the H atom 21 C. A. Faundez, J. F. Diaz-Valdes and J. O. Valderrama, Fluid
Phase Equilib., 2014, 375, 152–160.
lent thermal stability, these hydroxyl-functionalized ILs could
X. B. Hu, Fluid Phase Equilib., 2014, 378, 21–33.
be promising candidates for NH recovery.
23 Y. Q. Ma and R. Wang, Chem. Res. Chin. Univ., 2014, 35,
3
1
515–1522.
4 A. Yokozeki and B. S. Mark, Ind. Eng. Chem. Res., 2007, 46,
605–1610.
2
Acknowledgements
1
The authors would like to acknowledge the National Basic 25 A. Yokozeki and M. B. Shiett, Appl. Energy, 2007, 84, 1258–
Research Program of China (No. 2014CB744306), the National
1273.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 81362–81370 | 81369

View Article Online
RSC Advances
Paper
26 G. H. Li, Q. Zhou, X. P. Zhang, L. Wang, S. J. Zhang and 31 M. Zhu, M. X. Zhu, Y. Song, W. Z. Liang, W. Han and
J. W. Li, Fluid Phase Equilib., 2010, 297, 34–39.
7 W. Shi and E. J. Maginn, AIChE J., 2009, 55, 2414–2421.
Y. N. Chen, Engineering and manufacturing technologies,
2014, 541–542, 78–82.
2
2
8 J. Palomar, G. Miquel-Maria, J. Bedia, F. Rodriguez and 32 P. Brown, B. E. Gurkan and T. A. Hatton, AIChE J., 2015, 61,
J. J. Rodriguez, Sep. Purif. Technol., 2011, 82, 43–52. 2280–2285.
9 S. K. Tang, G. A. Baker and H. Zhao, Chem. Soc. Rev., 2012, 33 L. J. Wazer JRV, K. Y. Kim and R. E. Colwell, Viscosity and
1, 4030–4066. Flow Measurement, New York: Wiley, 1963, 406.
0 J. Z. Zhang, C. Jia, H. F. Dong, J. Q. Wang, X. P. Zhang and 34 P. Navarro, M. Larriba, E. Rojo, J. Garc ´ı a and F. Rodr ´ı guez, J.
S. J. Zhang, Ind. Eng. Chem. Res., 2013, 52, 5835–5841. Chem. Eng. Data, 2013, 58, 2187–2193.
2
3
4
81370 | RSC Adv., 2015, 5, 81362–81370
This journal is © The Royal Society of Chemistry 2015