Experimental study on CO2 sorption capacity of the neat and porous silica supported ionic liquids and the effect of water content of flue gas

Article abstract of DOI:10.1016/j.molliq.2017.02.104

In this research, absorption of CO₂ in different ionic liquids (ILs) is studied. Thus four ILs were synthesized containing 1-butyl-3-methylimidazolium as the cation and nitrate [NO₃]^?, thiocyanate [SCN]^?, dicyanamide [N(CN)₂]^? and hydrogen sulfate [HSO₄]^? as the anions, respectively. The resulting ILs were then immobilized into activated silica support in a 1/1 IL/SiO₂ weight ratio via the impregnation-vaporization method. CO₂ sorption behavior of both neat and silica supported ILs (ILs-SiO₂) were investigated at different temperatures and flow rates under atmospheric pressure, while their desorption process were carried out under 20?mm?Hg at 70?°C. In both sorbents, the best results were obtained at 25?°C with a flow rate of 12?mL/min, where [bmim][N(CN)₂] with 1.85 (wt%) or 0.42?mmol CO₂ per gram of sorbent and [bmim][HSO₄]-SiO₂ with 2.33 (wt%) or 0.53?mmol CO₂ per gram of sorbent showed the highest sorption capacities. The effect of water on CO₂ absorption capacity of the neat and silica supported ILs were also studied by transmission of CO₂ gas flue containing 400?ppm water. The results indicated that the mass gain was higher when wet CO₂ was passed through the sorbents, opposed to passing dry CO₂. Because of the existence of a weak coulombic intraction between the sorbents and CO₂, desorption occurs rapidly and a readily reuse of the sorbents is therefore provided.

Full text of DOI:10.1016/j.molliq.2017.02.104

Accepted Manuscript
Experimental study on CO2 sorption capacity of the neat and
porous silica supported ionic liquids and the effect of water
content of flue gas
Mojtabai Mirzaei, Ali Reza Badiei, Babak Mokhtarani, Ali Sharifi
PII:
DOI:
Reference:
S0167-7322(16)31538-0
doi: 10.1016/j.molliq.2017.02.104
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23 February 2017
25 February 2017
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ACCEPTED MANUSCRIPT
Experimental study on CO2 sorption capacity of the neat and porous silica
supported ionic liquids and the effect of water content of flue gas
Mojtabai Mirzaei †, Ali Reza Badiei ,†, Babak Mokhtarani ,,‡ and Ali Sharifi ‡
†School of Chemistry, College of Science, University of Tehran, Tehran, Iran
‡ Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran,
Iran.
Abstract:
In this research, absorption of CO2 in different ionic liquids (ILs) is studied. Thus four ILs were
synthesized containing 1-butyl-3-methylimidazolium as the cation and nitrate [NO3]-, thiocyanate
[SCN]-, dicyanamide [N(CN)2]- and hydrogen sulfate [HSO4]- as the anions, respectively. The
resulting ILs were then immobilized into activated silica support in a 1/1 IL/SiO2 weight ratio via the
impregnation-vaporization method. CO2 sorption behavior of both neat and silica supported ILs (ILs-
SiO2) were investigated at different temperatures and flow rates under atmospheric pressure, while
their desorption process were carried out under 20 mmHg at 70oC. In both sorbents, the best results
were obtained at 25oC with a flow rate of 12 mL/min, where [bmim][N(CN)2] with 1.85 (wt%) or
0.42 mmol CO2 per gram of sorbent and [bmim][HSO4]-SiO2 with 2.33 (wt%) or 0.53 mmol CO2 per
gram of sorbent showed the highest sorption capacities. The effect of water on CO2 absorption
capacity of the neat and silica supported ILs were also studied by transmission of CO2 gas flue
containing 400 ppm water. The results indicated that the mass gain was higher when wet CO2 was
passed through the sorbents, opposed to passing dry CO2. Because of the existence of a weak
coulombic intraction between the sorbents and CO2, desorption occurs rapidly and a readily reuse of
the sorbents is therefore provided.
Key words: Carbon dioxide, Sorption, Desorption, Supportted ionic liquid, ILs-loaded silica

Corresponding author E-mail: abadiei@ut.ac.irTel: +98-2161112614. Fax: +98-2161113301

Corresponding author E-mail: mokhtaranib@ccerci.ac.irTel.: +98 21 44787770; Fax: +98 21 44787781.
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1.Introduction
With the beginning of the industrial revolution, the atmospheric CO2 has faced an unexpected
alarming growth and this event has been suggested to be a leading contributor to global climate
change. In the wake of reductions in anthropogenic CO2 emissions [1], and in large emission sources
such as fossil-fuel-based power generation facilities, achieving an appropriate way to remove CO2 as
a predominant element in greenhouse phenomena is an important issue [2].
The traditional way for CO2 absorption in natural gas sweetening is the use of aqueous alkanol
amines that suffer from some intrinsic drawbacks like sorbent loss during recovery step, equipment
corrosion together with high energy comsumption in desorption proccess [3].
ILs that are generally composed of a large organic cation and organic or inorganic anions, have got
good academically and industrially attention in recent decade. No flammability and corrosion, high
thermal stability, negligible vapor pressure, a wide liquid range, electric conductivity following with
their tunable physiochemical properties made them safer and more environmentally friendly than
conventional organic absorbents [4]. In addition, selection diverse anions and cations, ILs with
different chemical and physical properties could be produced.
ILs have been used as reaction media in separation [5], extraction processes [6], and furthermore,
they seem suitable for being used as gas absorbent solvent [7].
First, Blanchard et al. [8], reported the solubility of CO2 in 1-butyl-3-methylimidazolium
hexafluorophosphate [bmim][PF6]. The mole fraction of CO2 in IL at 80 bar was 0.6. After
Blanchard, a lot of research in absorption of CO2 were reported [9-11].For example, Anthony et al.
[12], considered the CO2 absorption of the two [bmim]-based ILs containing tetrafluoroborate [BF4]-
and bis(trifluoromethane)sulfonimide [Tf2N]- as the anions and concluded that ILs with [Tf2N]- anion
showed the better affinity for CO2 absorption.
Aki et al. investigated the effect of the length of alkyl chains located on cation on CO2 solubility and
acclaimed that by increase of the alkyl chain length, CO2 absorption capacity increases [13].
In another assessment the impact of the anions on CO2 absorption capacity have been investigated
and it was found that CO2 solubility increased as follow [14], [C7F15CO2]- >[TF2N]- > [PF6]- >
[DCA]- > [BF4]- >[MeSO4]- >[SCN]- >[NO3]-
Mokhtarani et al. [15] have studied the solubility of CO2 in three imidazolium-based ILs containig
nitrate [NO3]- and thiocyanate [SCN]- as the anions at 298.15 K to 333.15 K and pressure up to
45 bar. The studied ILs were 1-hexyl-3-methylimidazolium thiocyanate [hmim][SCN], 3-methyl-
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1-octylimidazolium thiocyanate [omim][SCN] and 3-methyl-1-octylimidazolium nitrate
[omim][NO3]. The result showed that the [omim][NO3] was more favorable for solubility of CO2
than the both thiocyanate based ILs.
Kazarian et al. [16] observed that there are weak Lewis acid-base interactions between CO2
molecules and the fluorinated anions in [bmim][BF4] and [bmim][PF6] and found that the interactions
between CO2 and anions have not played a decisive role in the solubility of CO2 in ILs. They
concluded that the CO2 only occupies empty cavities created by the ions in the ILs structure.
Dissolution of CO2 in ILs will be done physically or chemically. In conventional ILs, absorption can
be carried out physically, therfore an effective absorption will be obtained in high pressure and low
temprature [17]. Because of low gas absorption capacity in conventional ILs, researchers have looked
for designing ILs containing effective groups in gas absorption proccess.
In this regard, Bates et al. [18] synthesized the first imidazolium based IL containing an amine group
on the cation ring known as Task-Specific IL (TSIL) which improved the CO2 absorption capacity
compared to common ones. After 3 hours continuous passage of dry CO2 through the TSIL at room
temperature and 1 atmosphere of pressure, 7.4% weight gain was observed.
Phosphonium amino acid salts ILs with amino groups in anions [19], or in anions & cations
[20],have showed better absorption capacity compared to corresponding imidazolium ILs [21].
Regarding the impotrance of acid gas elimination from continuoues gas flue in industry, numerous
reports were presented in this field. For example, Brennecke and co-workers [22], have investigated
the ability of conventional IL 1-hexyl-3-methylimidazolium hexafluorophosphate [hmim][PF6] when
exposed to the continuous flue of CO2 at 1 bar. Finally, there was only a 0.0881% mass gain in IL.
In another work, they showed that the CO2 mole fraction in 1-butyl-3-methylimidazolium
bis(trifluoromethane) sulfonimide [bmim][Tf2N] at 1 bar was only 0.03, while the mole fraction of
SO2 in the same IL was 0.552 [12].
Elliott et al. [23], have reported the SO2 solubility in some [bmim]-based ILs at 323 K and 1 bar
pressure. Unexpected results demonstrated that, ILs with [Br]- and [NO3]- as the anions had the
highest SO2 absorption capacity ([Br]- ≈ [NO3]- > [Tf2N]- ≈ [BF4]- > [PF6]-).
Park et al. [24] have used the imidazolium-based ILs containing ether functional group on the alkyl
chain in CO2 absorption proccess. They have seen that one mole of the ethereal group containing ILs
were able to absorbe 0.9 mol CO2 at 30 oC.
The use of neat ILs in absorption process are not economically affordable beacause they are more
expensive than conventional solvents [25]. In addition, the release of ILs in the environment, may
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cause serious problems especially for the water sources. For example ILs with the fluoronated anions
as [bmim][BF4] and [bmim][PF6] may be decomposed in the presense of water to give hydrofluoric
and phosphoric acids. The toxicity of both ILs is the same as benzene [26].
so, researchers preferred to design new cost-effective sorbents containing a combination of small
amount of ILs and large amounts of porous solid supports [25].
Several reviews on CO2 capture by adsorption showed that adsorption with solid porous material as
acivated Carbon [27], Zeolites [28], Ordered mesoporous silica [29-31], has some benefit like low
cast raw material, low heat capacity, fast kinetics, high CO2 adsorption capacity and high CO2
selectivity.
Future development in more operational adsorbents depend on designing of solid surfaces with new
properties there with, immobilization of an IL on a solid support as a good solution,is a new approach
to achieve this target [32].
When an IL immobilizes into solid supports, actually gives its specific properties to the solid
substrate,which leads to a new sorbent with new efficiency. Because of large interfacial exchange
area in solid supported ILs, mass transfer enhances and consequently, the gas absorption capacity
improves [33].
Among solid supports, porous silica supports with regular pore structure attract a good attention and
the presence of hydroxyl groups on the surface of mesoporous silica supports makes them an
excellent candidate for the functionalization of the surface with the desired functional groups [34].
Wu and Zhu, et al. [35], considered SO2 sorption/desorption behavior of the porous silica supported
1,1,3,3-tetramethylguanidinium lactate [tmgh][Lac] and introduced them as a good alternate for SO2
removal. The IL was dispersed on silica particles via an incipient wetness technique. The SO2
sorption was carried out at 1 bar at 20oC but desorption was done at 90oC under 80 mmHg vacuum.
Wu et al. [36] reported CO2 adsorption capacity of N-(3-aminopropyl) aminoethyl
tributylphosphonium amino acid ILs [Ts-P][amino acid] supported into silica particles and observed
that in 1/1 weight ratio of TSIL and porous silica, IL containing Lysin as the anion with 1.55 (mmol
CO2)/(g TSIL) had the highest adsorption capacity, while adsorption capacity of the bare silica was
0.17 mmol CO2/g SiO2(0.75 wt%).
In other study, Zhu et al. [37] measured CO2 capture capacity of grafted phosphonium IL [P8883][BF4]
on silica support. In this work, the IL was grafted on alkoxysilyl modified silica and adsorption
carried out at 40oC at 1 bar. The adsorption capacity of the bare and modified silica were higher than
neat phosphonium-IL[P8883][BF4]. Although the adsorption capacity of the bare silica was more than
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IL-loaded silica, the CO2/N2 selectivity of IL-loaded silica and bare silica at 1 bar was too different,
8 and 3 respectively.
Jiang and co-workers [38], reported the solubility of SO2 in some supported imidazolium-based ILs
membranes at 298 K and 1 bar. They discerned that the [bmim][BF4] had the highest absorption
capacity. The mole fraction of adsorbed SO2 was 0.57.
[bmim][BF4] (0.57) > [bmim][NTf2] ≈ [emim][BF4] (0.551) > [bmim][PF6] (0.536)
In this study, to reach more practical solid IL-containing CO2 sorbent, it was tried to immobilize
[bmim]+ type cation with the varieties of anions into the activated amorphous silica via impregnation-
vaporization method [34] to do comparative evaluation between CO2 absorption capacities of the
neat and porous silica-supported ILs under continuous flue gas at 1 bar.
To this end, the CO2 absorption capacities of the four neat and silica supported ILs at 25oC, 40oC and
50oC and ambient pressure was examined. The four ILs consisted of the same cation along with four
different anions. The cation was 1-butyl-3-methylimidazolium [bmim]+ and the anions were nitrate
[NO3]-, thiocyanate [SCN]-, dicyanamide [N(CN)2]- and hydrogen sulfate [HSO4]-.
2. Materials & methods
2.1. Materials
N-methylimidazole (98%, merck), 1-chlorobutane(98%), Silica- 60 (0.02-0.04 mm), hydrochloric
acid (HCl, 37%), pottasium thiocyanate (KSCN, 98%), sodium dicyanamide (NaN(CN)2, 98%),
sodium nitrate (NaNO3, 98%, ), silver nitrate (AgNO3, 99.8%),phosphorus pentoxide (P2O5, 98%),
sulforic acid (98%), acetonitrile, ethyl acetate, ether, dichloromethane, acetone, and chloroform all
were obtained from Merck Company. carbon dioxide (CO2, ≥ 99.5%), methane (CH4, ≥ 99.995%),
nitrogen (N2,≥ 99.5%) were supplied by Pars Gas Company. The four used ILs and ILs-loaded silica
support were synthesized in our Lab.
2.2. Synthesis and Immobilization of ILs into porous silica support
The [bmim][X]-SiO2 were synthesized in a three-step process. Quaternized 1-butyl-3-
methylimidazolium chloride [bmim][Cl] produced in the first step, underwent ionic exchange with
the corresponding salt in the second stage to give desired ILs. Finally, the synthesized [bmim][X]
immobilized into porous silica via impregnation-vaporization method.(Fig. SI-1).
2.2.1. Synthesis of [bmim][Cl]
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1-butyl-3-methylimidazolium chloride [bmim][Cl] synthesized according to organic synthesis [39].
A mixture of 1-methylimidazole (1 eq) and 1-chlorobutane (1.3 eq) in dry acetonitrile was stirred at
75o C for 48 hours under nitrogen.The mixture was cooled to room temperature. The volatile material
was removed under reduced pressure and the remaining light-yellow oil dissolved in appropriate
acetonitrile and added dropwise to a flask containing dry ethyl acetate dry ethyl acetate containing
flask to give [bmim][Cl] as a white crystal in approximately 82% yeild.
2.2.2. Synthesis of [bmim][NO3], [bmim][SCN] and [bmim][N(CN)2]
According to literature procedures [40], NaNO3(1.1 eq), KSCN (1.2 eq) or NaN(CN)2 (1.1 eq) was
added to a solution of [Bmim][Cl] (1 eq) in dichloromethane and stirred for 24 h at room
temperature.
The suspension was filtered to remove the produced NaCl and unreacted NaNO3, KSCN and
NaN(CN)2 salts. The organic phase was repeatedly washed with small volume of water until no
precipitation of AgCl occurred in the aqueous phase on addition of AgNO3 solution. Then solvent
was removed in vacuo and the synthesized IL was stirred with activated charcoal for 6h, removing
the solvent gives [bmim][NO3], [bmim][SCN] and [bmim][N(CN)2] with 82%, 74% and 85% yields,
respectively.
2.2.3. Synthesis of [bmim][HSO4]
1-butyl-3-methylimidazolium hydrogen sulfate [bmim][HSO4] was synthesized according to the
reported procedures [41], by a dropwise addition of concentrated sulfuric acid (1 eq) to a cooled
solution of 1-butyl-3-methylimidazolium chloride (1 eq) in anhydrous methylene chloride. The
mixture was refluxed for 24h and the produced HCl was neutrilized by the aqueous solution of
NaOH. Then, the solution was cooled to room temperature,consequently removing of solvent and
then drying under vacuum at 75 ºC for 3h gave yellow viscous liquid with 95% yeild.
2.2.4. Immobilization of [bmim][x] into porous silica support
Silica-60 powder was activated [42], by refluxing and stirring in hydrochloric acid (HCl, 6 M). After
24 h, the SiO2 powder was filtered and washed with distilled water until to adjust the pH of the
solution to 6-7. Then, the obtained wet solid was dried in 120oC for 6 h to give activated porous silica
support. To avoid absorbing the air humidity the dried SiO2 was kept in a desiccator.
In this work, the four ILs were immobilized into activated silica support via impregnation-
vaporization method to produce ILs-loaded silica (ILs-SiO2) [34]. To do this, suspension of 5 g IL
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was mixed with 5 g of activated silica gel in ethanol and the mixture was stirred for 2 h. the solvent
was removed in 50oC for 2h. After removing most of the solvent, wet powder was dried at 80°C
under vacuum for 6h. The resulting [bmim][X]- SiO2 sorbents were kept in a desiccator to avoid
absorbing moisture before use (Fig. SI-1).
2.3. Characterization
1
The structures of the synthesized ILs were characterized by H NMR (500 MHz) spectroscopy
Bruker Advance 500 spectrometer in CD3Cl and DMSO-d6 at 25oC.
Density and viscosity of the neat ILs were measured with densitometer model DEM-5000-Anton
paar and Anton Paar SVM 3000, respectively.The approximate water contents of the ILs before
use were determined by Karl Fischer titration (KF-Coulometer 831). The water content of the
CO2 flue gas was measured by a vaisala humidity sensor (VAISALA HMP63)
A BELSORP-mini II Analyzer was used to measure the BET specific surface area and pore
volume of the pure and IL- loaded silica using nitrogen at 77 K. First samples were degassed
about 4 h at 10-3 bar and then the BET surface area was calculated from the nitrogen adsorption
isotherm assuming area of nitrogen molecule to be 0.162 nm2.
FTIR surface group chemistry analysis was performed using a Perkin-Elmer 400 spectrometer to
identify the difference between blank and CO2-rich sorbents.
Thermogravimetric analysis (TGA) of the neat and silica supported ILs were performed on a
NETZSCH TG 209 F1 Iris instrument under nitrogen flue at a heating rate of 20 oC / min to 600
oC. The total chloride content was determind potemtiometrically using a chloride ion selective
electrode (ISE) in conjunction with a pH meter (Philips PW 9420) with an expanded millivolt scale.
3. Results and discussion
3.1. 1H NMR, Density and viscosity, halide and water contents of synthesized ILs
The 1H NMR data is presented at supplementary information (Fig. SI-2). The results confirm the
structure of ILs synthesized. The water and chloride contents of the synthesized ILs were below 500
ppm except [bmim][HSO4] whose chloride content was about 3000 ppm (Fig. SI-2). Density and
viscosity of the neat ILs were summarized in Table 1[43-52].
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3.2. Characteristics of the bare and ILs-loaded silica
The pore size distribution, specific surface area and porosity of the bare and ILs-loaded silica were
identified by BET analysis (model:BELSORP-mini II). The activated amorphous silica used as
support had a medium BET specific surface area of 369.8 m2/g, total pore volume of 0.79 cm3/g and
average pore diameter of 8.53 nm. The pore size distribution (Fig. 1) showed that the majority of
pores having diameters of less than 10 nm, indicating that mesopores (2 nm< pore diameter <50 nm)
have formed a majority of the pore volume. The N2 adsorption desorption isotherm (Fig. 2) of the
activated silica was typical of a type IV material and the presence of Hysteresis type A
confirmed that the mesoporous silica contains cylindrical pores.
As expected, The BET analysis identified that the ILs have immobilized into solid support and the
specific surface area of the IL-loaded activated silica was significantly less than the bare activated
silica. The effect of impregnation on the porosity of the silica support particles is shown in Table 2. It
was obvious that the pore volume and surface area had a significant decrease. in the sorbent prepared
at feed ratios of 0.25/1 and 0.5/1 of [bmim][NO3]\SiO2, the total pore volume (Vp) respectively
decreased from 0.79 to 0.51 and 0.284 cm3/g and the specific surface area (Ap) reduced from 369.8
to 248.10 and 152.4 m2/g. while by impregnating of equal weight ratio of [bmim][NO3] into porous
silica, the total pore volume(Vp) will be decreased from 0.79 to 0.0004 cm3/g and the specific surface
area (Ap) be changed from 369.8 to < 1 m2/g indicating that the micro & meso pores were
completelly filled with ILs and the surface area is truly the external surface of the activated silica
support (Table 2). The pore size distribution of the bare and [bmim][NO3]-loaded silica in varities
weight ratios have shown in Fig. 1.
The ILs-loaded silica sorbents was prepared by impregnation of certain weight ratio of IL into porous
silica support, but as seen in Fig. 3, the actual amount of immobilized IL in sorbent was determined
by thermogravimetric analysis (TGA).
As summarized in Table 2, according to TGA chromatogram (Fig. 3), the actual weight ratio of
immobilized [bmim][NO3] to silica support in [bmim][NO3]-SiO2 instead of 0.25, 0.50 and 1.00 were
0.247, 0.497 and 0.91 in order which represented the immobilization was incomplete. However, in
[bmim][SCN]-SiO2 the actual weight ratio of IL instead of 1.00 was 1.04.
In addition thermal stability of the neat ILs and ILs-loaded silica will be determined by TGA.
Thermogravimetric analysis of the [bmim][NO3], [bmim][SCN], [bmim][HSO4], [bmim][NO3]-SiO2
and [bmim][SCN]-SiO2 showed that they have a thermal decomposition temperature (Td) above
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250oC that approved their good thermal stability. It will be noted that by decrease of the IL portion
in IL-loaded silica the thermal stability of the sorbent will be increased. For example thermal
o
degradation of the neat [bmim][NO3] begins from 250 C whilein [bmim][NO3]-SiO2 with
o
o
weight ratios of 1/1 and 0.5/1 destruction begins from 280 C and 350 C, respectively.This
approved that by immobilizing an IL into solid support, its thermal stability increases.
4. CO2 sorption/desorption
The experimental apparatus was the same as described in litrature [36], but for the neat ILs, instead
of a U-tube, the sorption/desorption were carried out in a T-shaped glass reactor with input and
output valves.(Fig. 4)
CO2 was passed through P2O5 column in order to remove any existed humidity in the gas. The flow
rate of CO2 was contorlled using a mass flow controller (Smart MFC Brooks, SLA5850), after
stabilization of the flow rate at 12 ml/min, CO2 enters into the T-shaped glass reactor and blowes
through the neat IL and goes out from the output valve for venting.The CO2 sorption and desorption
were determined gravimetrically. At preferred time, the glass reactor’s valves were closed and the
reactor was removed from water bath and weighted with an electronic balance (±0.0001 g) to
measure amount of adsorbed CO2. The T-shaped glass then inserted to water bath again and this
proccess has continued untill reached to a costant weight.
In a typical method, 5 ml of the predried IL was transferred to a preweghited T-shaped glass. The IL
in the T-shapped glass was further dried under vaccum at 70oC for 2 h in order to eleminate any
impurity.Then, the T-shaped glass was filled with nitrogen and inserted into a preheated water bath
(Open Bath\circulation Huber CC-K20) at desired temperature (25oC, 40oC or 50oC) for gas
absorption.
For desorption stage, the T-shaped glass containing CO2-rich IL was immersed into a preheated
water bath and degassed under 20 mmHg pressure at 70oC. At the given time, it was filled with
nitrogen, seperated from the system and then weighted.This proccess has continued untill no weight
change observed in T-shaped glass reactor.
To evaluate the repeatability of the absorption/desorption cycle of ILs, some of tests were repeated 6
times.the exprimental error of the repeated tests was ±6%.
For ILs-loaded silica (ILs-SiO2), the CO2 sorption\desorption experiments were carried out in a U-
tube [36]. The method was similar to that in neat ILs, however silica supported ILs ([bmim][X]-SiO2)
were used instead of the neat ILs.
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4.1. CO2 sorption/desorption of the neat ILs
The CO2 sorption experiments were carried out at 1 atm of pressure at 25oC, 40oC and 50oC while,
the desorption proccesses were performed under 20 mmHg at 70oC.
In order to obtain the best condition, the CO2 sorption\desorption of the neat [bmim][NO3] was
studied in a T-shaped glass reactor at different temperature and gas flow rates of 12, 50 and 100
mL/min. Consequentlly, the best results were achieved at 25oC in a flow rate of 12 mL/min.(Fig. 5)
As seen in Fig. 2, the CO2 sorption capacities of [bmim][NO3] at 25oC, 40oC and 50oC were
1.42(wt%), 0.7 (wt%) and 0.42(wt%), respectively. In addition, the time of absorption, decreased
from 1200 min at 25oC to 630 min at 40oC and 320 min at 50oC.
Gas absorption results of the four investigated neat ILs were shown in Fig. 6. Among them,
[bmim][N(CN)2] has the highest absorption capacity and the weight percent of absorbed CO2(wt%)
with a significant increase compared to [bmim][NO3] raised to 1.85(wt%) at 25oC, 1.30 (wt%) at
40oC and 0.82(wt%) at 50oC.
As the results reveal, due to the use of the bubbling method, the absorption stage is time-consuming,
and the temperature has a significant effect on the absorption capacity.
as summarized in Table 3, the CO2 solubility order of the four tested ILs could be written as below:
[bmim][N(CN)2] > [bmim][NO3] > [bmim][HSO4] > [bmim][SCN]
there is approximatly a linear dependencey between the gas absorption capacity of the neat ILs and
temperature i.e. an increase in temperature results in decreasing of gas absorption capacity. This
dependency is shown in Fig. SI-3 of supporting information.
Unlike the absorption, the desorption was completed in less than 30 minutes. Such a fast desorption
attributed to the absence of a specific functional group in ILs’ structure to give strong link with CO2.
Fig. SI-4 shows the desorption process of studied ILs.
4.2. CO2 sorption/desorption of the ILs-loaded silica
The CO2 sorption\desorption of the silica supported ILs sorbents were studied in a U-tube glass
reactor. To obtain the optimum conditions, the gas adsorption capacities of the [bmim][SCN]-SiO2
with a flow rate of 12 ml/min at 25oC, 40oC and 50oC temperatures were measured and finally the
best results were achieved in lower temperature. As shown in Fig. 7, the gas adsorption capacity
decreases with an increase in temperature. CO2 adsorption capacity of [bmim][SCN]-SiO2 at 25oC
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was 1.25 (wt%) but by increasing of temperature to 40oC and 50oC, the mass gain decreased to
0.66(wt%) and 0.37(wt%) respectively.
The CO2 sorption of the four solid sorbents are shown in Fig. 8 and their results are listed in Table 3.
According to the results, the CO2 solubility order of the four tested SiO2-ILs might be written as
below:
[bmim][HSO4]-SiO2 > [bmim][NO3]-SiO2 > [bmim][N(CN)2]-SiO2 > [bmim][SCN]-SiO2
Similar to the neat ILs, an approximatly linear correlation between temprature and gas adsorption
capasity of ILs-SiO2 is shown in Fig. SI-5. According to this Figure, with increasing of temperature
the gas absorption capacity of the solid sorbent are decreased.
As shown in Fig. SI-6, desorption stage of the ILs-SiO2 sorbents almost like neat ILs were occured in
less than 30 minute.
At the end, it is noted that [bmim][N(CN)2] with 1.85 (wt%) and [bmim][HSO4]-SiO2 with 2.33
(wt%) showed better sorption behaviour than the others.
4.3. CO2 Sorption Mechanism
The silica used in this study had a CO2 adsorption capacity of 0.82 (wt%). By impregnation of IL
into porous silica support, silica surface was filled by IL and the existed silanol groups on silica
surface have been engaged by ILs molecules. Thus, silica portion in gas absorption was reduced.
Then, by the increase of IL weight percent in IL-loaded silica sorbent, CO2 sorption capacity
increases as well.
Due to lack of a strong link between CO2 and the considered sorbents, adsorption could only be
carried out physically [16], thus they have low absorption capacity; however, the major advantage of
this weak linkage is the rapid desorption stage of the CO2-rich sorbent, so the saturated liquid and
solid sorbents can be easily recycled and are ready to use. FT-IR spectrum of the gas-treated sorbent
has peak characteristic of dissolved CO2 at 2340 cm-1 [53].
5. Further Assessments for Practical Application
Effect of existed water in flue gas on CO2 sorption
11

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All used ILs in this study were able to adsorbe the existed water in the flue gas, and since the CO2
containing gas stream often contains certain amount of water, the effect of water on CO2 absorption
capacities of the neat and silica supported ILs were considered by transmission of CO2 flue gas
containing 400 ppm water. This amount of water was added to the gas cylinder by the gas supplier
Company (Pars Gas Company).
Similar to the dry flue gas, by passing wet CO2 through the sorbents, their CO2 sorption capacities at
25oC, 40oC and 50oC in various flow rates was evaluated .
From a comprative view,after 1200 min passing the dry CO2 through the neat [bmim][NO3] at 25oC
with a flow rate of 12 ml/min, the adsorbed CO2 weight rise was 1.42 (wt%). However; at the same
temperature and flow rate, by the pass of the CO2 containing 400 ppm water, the observed mass gain
was 3.10 (wt%) after 1800 min.(Fig. 9)
The results reveal that in case of wet gas, water together with CO2 can be absorbed and finally reaches
equilibrium at a specified time (Fig. 10). As seen in dry gas, temperature rise led to a decrease in gas
adsorption capacity.
The full results of the wet gas absorption by neat ILs are presented in Table 3. According to the
results, contrary to what was observed with dry gas, [bmim][NO3] with 3.1(wt%) absorption had the
highest absorption capacity among the four studied ILs (Fig. 11) while by passing the dry gas its
absorption capacity was only 1.42 (wt%). As the studied ILs in this research are hydrophilic and the
CO2-containing about 400 ppm water, the ILs will absorb the water from flue gas and come into
equilibrium with CO2.
The wet gas capacity for studied ILs follows from this order: [bmim][NO3]> [bmim][HSO4]>
[bmim][N(CN)2]> [bmim][SCN],
however for dry gas the following order was obsereved.
[bmim][N(CN)2]> [bmim][NO3]> [bmim][HSO4]> [bmim][SCN]
The outcomes clarified that there are a linear dependency between mass increase of neat ILs by wet
CO2 passage and temprature where an increase in temprature resulted a decrease in gas absorption
capacity.(Fig. SI-7)
The impact of the existed water in flue gas on gas adsorption capacity of the ILs-loaded silica was
also investigated. The results are reported in Table 3. As the results are shown, the weight will be
additionally increased by passing the wet gas rather than the dry gas.
Wet gas
[bmim][NO3]-SiO2>[bmim][N(CN)2-SiO2]>[bmim][SCN]-SiO2>[bmim][HSO4]-SiO2
12

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Dry gas
[bmim][HSO4]-SiO2> [bmim][NO3]-SiO2>[bmim][N(CN)2-SiO2]> [bmim][SCN]-SiO2
Among the four solid sorbents, [bmim][NO3]-SiO2 had the highest absorption capacity by
transmission of wet CO2. (Fig. SI-8)
Effect of IL loading on CO2 sorption
The effect of weight percent of [bmim][NO3] in [bmim][NO3]-SiO2 on adsorption capacity was
determined. It was founded that an increase to the [bmim][NO3]/SiO2 ratio (фw,IL(g IL/g sorb) led to
the increase of the CO2 sorption capacity but much time needs to reach equilibrium (Table 4). The
фw,IL=0 in Table 5 relates to the bare silica support.
By charting CO2 sorption capacity of [bmim][NO3]-SiO2 in terms of weight fraction of [bmim][NO3]
in the solid sorbent (фw,IL(g IL/g sorb), an approximately linear dependency is obsereved where
CO2(wt%) increases linearly by the increase of фw,IL (Fig. SI-9).
Effect of gas flow rate on CO2 sorption
The effect of feed gas flow rate on gas absorption capacity of the neat and silica supported ILs were
considered. As shown in Table 5, in similar condition, by the increase of the gas flow rate, the CO2
sorption capacity reduces [54]. In higher gas flow rate the CO2 residence time on the solid or liquid
sorbent will be shortend that causes an uneffective intraction between CO2 molecules and sorbents.
For example in [Bmim][N(CN)2] by the pass of the wet CO2 with a flow rate of 12 ml/min, the
absorption capacities at 25°C and 40°C were 2.73(wt%) and 1.82(wt%), respectively, but with a rise
in CO2 flow rate to 50 ml/min the sorption capacities reduced to 1.71(wt%) at 25oC and 0.8(wt%) at
40oC.
Comparing the CH4 and CO2 sorption capacity of the bare and IL-loaded silica
13

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The bare and ILs-loaded silica can be used in natural gas sweetening. However, in order to
investigate which one is better, it is necessary to measure the absorption capacity for CH4. The CH4
absorption capacity of the bare silica and [bmim][NO3]-loaded silica which was prepared in 1/1
o
weight ratio of [bmim][NO3] and silica were measured at 25 C and 1 bar pressure. The results are
summarized in Table 6. As the Table is shown by passing dry pure CH4 in a flow rate of 12 ml/min
through [bmim][NO3]-SiO2 about 0.162 (wt%) mass gain was observed, while the obtained mass
rising from the dry CO2 passage was 1.55 (wt%); i.e. the [bmim][NO3]-loaded silica is able to absorb
CO2 about 9.68 times more than CH4. In contrast, by passing dry CH4 through the bare activated
silica, the weight gain was 0.51(wt %) which has no big difference with CO2 absorption capacity of
bare silica (0.82 wt %) at the same condition. It was founded by decreasing the [bmim][NO3] weight
fraction from 0.50 to 0.334, the CH4 absorption capacity raised from 0.16 (wt%) to 0.22 (wt%) and
consequently the selectivity decreased from 9.68 to 6.36. It is true to say that ILs plays a key role in
CO2 absorption from natural gas stream.
Conclusion
In this study, four [bmim][X]-based ILs were synthesized and immobilized into porous silica
supports in 1/1 weight ratio of SiO2 and ILs. They showed good thermal stability up to 300oC. After
charactrizing the sorbents, their CO2 adsorption\desorption were considered in diverse tempratures
and flow rates. The best results for CO2 adsorption by neat and silica supported ILs were obtained at
25oC with a flow rate of 12 mL/min, where [bmim][N(CN)2] and [bmim][HSO4]-SiO2 with 1.85
(wt%) and 2.33 (wt%), respectively had the highest absorption capacities.
Despite the decline of ILs contribution in [bmim][X]-SiO2, their CO2 absorption capacities were
more than those of neat ILs. For instance, by transmission of dry CO2 with a flow rate of 12 ml/min
through neat [bmim][NO3] , it indicated 1.42 (wt%) mass gain, while [bmim][NO3]-loaded silica in
1/1 weight ratio of [bmim][NO3] and SiO2, the CO2 absorption capacity raised to 1.55(wt%) that it
is related to the synergic effect of the [bmim][NO3] and porous silica support.
Under the same conditions, the gas flow rate significantly influenced the CO2 absorption capacity of
sorbents, low flow rates led to the higher absorption capacity.
The effect of the existed water in flue gas on the absorption capacities of the sorbents was also
examined. It was discerened that the water together with CO2 would be absorbed by neat and silica
supported ILs. The results revealed that by transition of CO2 containig 400 ppm water through the
14

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sorbents, [bmim][NO3] and [bmim][NO3]-SiO2 with 2.33(wt%) and 3.10 (wt%) had the highest
absorption capacities.
According to the achieved outcomes, an increase in the IL/SiO2 weight ratio has led to the CO2
sorption capacity increase.
Meanwhile the CH4 adsorption capacities of the bare and [bmim][NO3]-loaded silica prepared in 1/1
weight ratio of [bmim][NO3] and silica were measured. The result showed the high ability of
[bmim][NO3]-SiO2 in absorbing CO2 about 9.6 times compared to CH4, while the bare silica
absorbed CO2 twice as much as CH4. So, the ILs-loaded silica have better efficiency in CO2
separation from natural gas. Due to the lack of appropriate functional groups on the neat and silica
supported ILs, they could be easily recycled and reused in less than 30 minutes.
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Table1. Density and viscosity of the synthesized ILs at different tempratures.
Density (kg/m3)
40oC
Viscosity (mPa.s)
40oC 50oC
Ionic Liquid
25oC
50oC
25oC
[bmim][NO3]
1154.77a 1145.47a 1138.00a 0.2107a
0.09302a 0.05495a
1156.543
114743
114444
1140.743
113744
0.165343
0.082843
0.08544
0.044643
0.054144
[bmim][SCN]
1073.02 a 1063.7 a
1071.0445 1062.2145 1056.3945 0.059845
1057.7 a
0.0622 a
0.03224 a 0.02281a
0.032245 0.0229745
[bmim][N(CN)2] 1059.76a 1050.29a 1044.05a 0.03005a 0.01788a 0.01338a
105146
1053.748
1044.846
1047.548
0.029947
0.031848
0.017747
0.018548
0.013347
0.0137848
1063.148
1060.4649 1050.9749 1044.7249 0.0300549 0.0177749 0.013349
[bmim]HSO4
1282.2a
1220.150
1270.3a
1200.650
126951
1258.5a
1187.350
1263.451
120852
0.355a
0.166a
0.114a
122452
121452
0.43952
0.18452
0.112452
athis work
Table 2. Pore characteristicsa of the bare SiO2, [bmim][NO3]- SiO2 and [bmim][SCN]- SiO2 sorbents.
b
c
Sample
IL/SiO2
0/1
IL/SiO2
0
Vp (cm3/g)
0.7891
0.51
Ap (cm2/g)
369.8
248.10
152.4
< 1
rp(nm)
8.53
7.93
7.46
< 1
SiO2
[bmim][NO3]- SiO2
[bmim][NO3]- SiO2
[bmim][NO3]- SiO2
[bmim][SCN]- SiO2
0.25/1
0.5/1
1/1
0.247
0.497
0.91
0.2843
0.0004
0.0031
1/1
1.04
< 2
< 1
b
aVp, Ap and rp are total intrusion volume, specific surface area and average pore radiuse, respectively. IL/SiO2 ratio
in feed, cIL/SiO2 in solid sorbent.
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Table 3: CO2 sorption capacity of the neat ILs and ILs-loaded silica by the pass of the wet and dry CO2 with a flow
rate of 12 mL/min.
CO2(w%)
40oC
25 oC
Wet
50oC
Sorbent
Dry
Dry
Wet
Dry
Wet
[bmim][NO3]- SiO2
[bmim][NO3]
1.55
2.33
3.10
1.95
1.40
2.07
2.73
1.65
3.00
0.79
1.47
0.53
1.20
1.42
1.25
1.10
1.32
1.85
2.33
1.34
0.70
0.66
0.69
0.86
1.31
1.41
1.13
1.78
1.22
1.24
0.87
1.82
1.10
2.10
0.43
0.37
0.55
0.84
0.82
0.94
0.78
1.31
0.85
0.68
[bmim][SCN]- SiO2
[bmim][SCN]
[bmim][N(CN)2]- SiO2
[bmim][ N(CN)2]
[bmim][HSO4]- SiO2
[bmim][ HSO4]
1.25
0.88
1.42
Table 4. The effect of the [bmim][NO3] weight fraction on CO2 absorption capacity of SiO2-[bmim][NO3].
[bmim][NO3]/SiO2
фw,IL(g IL/g sorb)
CO2(w%)
0
0
1/3
0.25
1.1
1/2
0.334
1.4
1/1
0.5
0.82
1.55
Table 5. The effect of wet CO2 flow rate on absorption capacity of [bmim][N(CN)2] and [bmim][NO3]-SiO2.
CO2 (wt%)
sorbent
12 mL/min
50 mL/min
100 mL/min
25 oC 40 oC 50 oC 25 oC
40 oC
50 oC
0.55
25 oC
0.60
40 oC
50 oC
[bmim][N(CN)2]
[bmim][NO3]-SiO2
2.73
2.33
1.82
1.47
1.25
0.97
1.71
1.93
0.80
0.84
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Table 6: CH4 sorption capacity of the bare and [bmim][NO3]-loaded silica by the pass of the dry CH4 with a flow
rate of 12 mL/min.
Sorbent
g IL/ gIL-SiO2
0
CO2 (wt %)
0.82
CH4 (wt %)
Selectivity
1.60
SiO2
0.51
-
[bmim][NO3]-SiO2
[bmim][NO3]-SiO2
[bmim][NO3]-SiO2
0.25
1.10
-
0.334
0.50
1.40
0.22
0.16
6.36
1.55
9.68
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Figure 1. Pore size distribution of the activated bare silica (♦), [bmim][NO3]-SiO2 at weight ratios of 1/3
(■), 1/2 (▲) and 1/1 (●).
Figure 2. N2 Adsorption/desorption isotherm of the activated bare silica support.
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Figure 3. TGA curves of the [bmim][SCN] ( ), [bmim][NO3] (♦), [bmim][HSO4] (●),
,[bmim][SCN]-SiO2 in feed ratio of 1/1(ж) [bmim][NO3]-SiO2 in feed ratio of 1/1(■), 1/2
(▲),bare SiO2 (+) (N2 atmosphere, heating rate of 20 oC/min).
Figure 4. Experimental setup for measuring the adsorbed CO2 during sorption proccess (1) CO2 cylinder;
(2) P2O5 column; (3-6) Valves; (7) T-shaped glass reactor; (8) water bath; (9) CO2 outlet for venting.
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ACCEPTED MANUSCRIPT
Figure 5. Absorption capacity of [bmim][NO3] at 25oC(♦), 40oC(■) and 50oC(▲) by passing dry CO2
with a flow rate of 12 mL/min.
Figure 6. Absorption capacity of [bmim][N(CN)2] (●), [bmim][NO3] (♦), [bmim][HSO4] (■) and
[bmim][SCN] (▲) by passing dry CO2 at 25oC with a flow rate of 12 mL/min.
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Figure 7. Absorption capacity of [bmim][SCN]-SiO2 at 25oC(♦),40oC(■) and 50oC(▲) by passing dry
CO2 with a flow rate of 12 mL/min.
Figure 8.Absorption capacity of [bmim][HSO4]-SiO2 (♦), [bmim][NO3]-SiO2 (●), [bmim][N(CN)2]-SiO2
(■), and [bmim][SCN]-SiO2 (▲) by transition of dry CO2 at 25 oC with a flow rate of 12 mL/min.
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Figure 9. Absorption capacity of the neat [bmim][NO3] by transition of wet (400 ppm water,▲) and dry
(♦) CO2 at 25 oC with a flow rate of 12 mL/min.
Figure 10. Absorption capacity of [bmim][NO3] at 25oC(♦),40oC(■) and 50oC(▲) by passing the wet CO2
(400 ppm water ) with a flow rate of 12 mL/min.
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Figure 11.Absorption capacity of [bmim][NO3] (♦), [bmim][HSO4] (●), [bmim][N(CN)2] (▲) and
[bmim][SCN] (■) by transition of wet CO2 (400 ppm water ) at 25 oC with a flow rate of 12 mL/min.
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