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Desulfurization Performance of Ether-Functionalized Imidazolium-Based Ionic Liquids Supported on Porous Silica Gel Ying Zhao, Jianying Wang, Haichao JIANG, and Yongqi Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502682v • Publication Date (Web): 30 Jan 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Energy & Fuels
Desulfurization Performance of Ether-Functionalized Imidazolium-Based Ionic Liquids Supported on Porous Silica Gel Ying Zhao1,2 , Jianying Wang2, Haichao Jiang2 , Yongqi Hu1,2* 1. Chemical Engineering Institute, Tianjin University, Tianjin, 300072, China. 2. College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang, 050018, China. Tel/Fax: (+) 86-0311-81668302, E-mail:
[email protected] ,
[email protected] of both ILs and synthetic porous silica gel, which will result in their good performances in desulfurization. A number of papers have been published on the use of ILs-SiO2 as SO2 absorbents. Zhang et al. [10] compared the sorption capacity for SO2 of pure TMGL with TMGL-SiO2. The sorption capacity for SO2 was maintained after immobilization. Wang et al.[11] indicated that immobilized [1-methylimidazole][Cl] silica gel with SiO2/ILs = 9.5:0.5 and 9:1 showed higher sulfur removal as compared to SG under the experimental conditions. Godwin Severa et al.[12] showed that the 1-ethyl-3-methylimidazolium acetate loaded activated carbon exhibited the highest SO2 sorption capacity performance among the nine ionic liquid sorbents.
ABSTRACT: Ether-functionalized imidazolium-based ionic liquids (EFIILs) [C3O1Mim][H3CSO3], [C5O2Mim][H3CSO3], and [C7O3Mim][H3CSO3] were supported onto porous silica gel particles via a facile impregnation method. The synthesized EFIILs-SiO2 were characterized by FT-IR, SEM, TGA and BET. Results show that the EFIILs-SiO2 have good thermal stability, high porosity and large specific surface area. The ability of both of the EFIILs and EFIILs-SiO2 to reversibly absorb gaseous sulfur dioxide (SO2) was experimentally investigated, and high capacity and rate for SO2 sorption were confirmed. The capacities reached 2.621 mol SO2 per mol of [C3O1Mim][H3CSO3], 3.106 mol SO2 per mol of [C5O2Mim][H3CSO3] and 3.453 mol SO2 per mol of [C7O3Mim][H3CSO3], which indicate that SO2 sorption capacity increases with the increase of ether chain length in the cation at 25℃. The EFIILs-SiO2 have better adsorption properties than pure EFIILs, because the supporting materialsporous silica gel played an important role in the adsorption. The EFIILs-SiO2 system could be reused for several adsorption/desorption cycles without loss of its initial capacity. 1.
The object of this paper is to study the desulfurization performances of EFIILs supported on synthetic porous silica gel (EFIILs-SiO2). FT-IR, SEM and TGA were used to characterize the properties of the samples. The experiments on the desulfurization properties of EFIILs-SiO2 were performanced and the comparison of EFIILs and EFIILs-SiO2 on desulfurization were discussed. 2.
Experimental
2.1. Materials and apparatus 2-Methoxyethanol (99%) and diethylene glycol monomethyl ether (99%) were obtained from Tianjin Yongda Chemical Reagent Co., Ltd., China; triethylene glycol monomethyl ether (96%) were from Tokyo Chemical Industry Co., Ltd., Japan; methanesulfonyl chloride (99%) were from Chengdu Gracian Chemical Technology Co., Ltd., China; N-methylimidazole (99%) were from Shanhai Chengjie Chemical Co., Ltd., China; SO2 cylinder gas (99.95%) were from Beijing Beiyang Special Institute Co. Ltd.; porous silica gel (40-60 mesh) were from Shanghai Shangchen Industrial Co., Ltd., China.
Introduction
In recent years, room-temperature ionic liquids (ILs) have attracted more attention, because ILs possess negligible vapor pressures, high absorption capacity, wide liquid temperature ranges, high thermal stabilities, designable structure, and no secondary pollution, etc.[1, 2] ILs exhibit efficient and reversible absorption of SO2, and can also reasonably be used as a solvent to scrub SO2 from flue gases. [3, 4, 5] For example, Huang et al. [6] reported that TMG-based (1,1,3,3-tetramethylguanidine) and BMIM-based (1-butyl-3-methylimidazolium) ionic liquids could absorb SO2 reversibly. The absorption capacity is up to 1.6 mole SO2 per mole of IL at 20 and 1 bar. Sung et al. [7] indicated that ether-functionalized ionic liquids (EFIILs) exhibit extremely high SO2 solubility. In our previous studies, a series of EFIILs were prepared and their thermodynamic properties were discussed
2.2 Synthesis and immobilization of EFIILs All the ether-functionalized imidazolium-based ILs (EFIILs), 1ethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate ([C3O1Mim][H3CSO3]), 1-diethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate ([C5O2Mim][H3CSO3]), and 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate ([C7O3Mim][H3CSO3]), were synthesized by a typical two-step process as published in papers [7,13,14] by our research group.
systematically. [8] However, ionic liquids have several disadvantages, such as high cost, high viscosity and their complex purification procedure, which limit their commercial application. [9] To solve these issues, Ils can be immobilized on porous materials. Thus, the amount of ILs needed will significantly decrease and the rate of sorption will increase due to the enlarged contact interface between gas and ILs, which will reduce the costs and raise the efficiency. [10] Synthetic porous silica gel possesses the excellent characteristics such as stable porous structure, large surface area, controllable pore size, good mechanical strength, and thermal stability. Synthetic porous silica gel immobilizing ILs (ILs-SiO2) will possess the advantages
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by nitrogen adsorption at -196 °C using BET and BJH methods (Surface Area and Porosity TriStar II 3020, Micromeritics Instrument Corporation). S-4800-1 scanning electron microscope (SEM, HITACHI Co. Ltd. Japan) was employed to gather the information on the microstructure of SiO2 and the surface coverage with EFIILs.
100
water content--1.66%
80
Weight /%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
2.4 SO2 adsorption and desorption The experiments on adsorption and desorption of SO2 were performed with pure SO2 and their quantities were determined by gravimetry. The experimental apparatus was shown in Figure 2. The experiments on SO2 absorption were determined at 25 ℃ and atmospheric pressure. The desorption experiments were performed by flowing N2 through a fixed bed with the particles of SO2saturated EFIILs-SiO2 at 100 ℃. To estimate the experimental errors with this method, the repeating experiments both for sorption and for desorption were made. The results showed that the error was in the range of ± 5%.
60
40
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0
100
200
300
400
500
600
Temperature / °C
Figure 1. TGA of original silica-gel.
The porous silica gel samples (40-60 mesh) were pretreated by calcining at 450 ℃ for 24 hours，and the water content of thFe obtained silica gel was lower than 2.0 wt.% after calcining, as shown in Figure1. 0.5 g EFIIL was dissolved in 10 mL ethanol, and 5 g porous silica gel was then added to the above solution. The mixture was stirred for 50 min and then was shaken for 24 h in thermostat oscillator. Finally, the ethanol in the mixture was slowly evaperated at room temperature for 1 h. The obtained solution was heated to 45 ℃ and maintained for 4 h, and then dried at 80 ℃ for 12 h. The product EFIILs -SiO2 were stored in a desiccator for further use. The weight ratio of immobilized EFIIL to silica support (EFIIL-SiO2) was ajusted in the range from 0.1/1 to 1/1 as shown in Table 1.
Figure 2: Experimental apparatus for SO2 sorption and desorption. 1. SO2/N2 steel cylinder 2. SO2 adsorption tube 3. Water bath 4. NaOH absorption equipment 5. Triangle funnel 6. Flow meter
3.
Table 1. Raw materials ratio of [C3O1Mim][H3CSO3]-SiO2 samples(ILs with SiO2 for [C3O1Mim][H3CSO3]-SiO2 Samples ） prepared by the impregnation process.
Results and discussion
3.1 Characterization of silica gel supported ether-functionalized imidazolium-based Ionic Liquids. 3.1.1 FT-IR
weight ratio (EFIIL/SiO2) 0.1/1
m[C3O1Mim] [H3CSO3]/g 0.5
Vethanol/mL
0.2/1
1.0
10
5g
0.3/1
1.5
10
5g
0.4/1
2.0
10
5g
0.5/1
2.5
10
5g
1/1
5
10
5g
msilica gel/g
To confirm the immobilization of [C3O1Mim][H3CSO3] on the support silica–gel, FT-IR spectroscopic analysis was carried out. The results show that the characteristic peak around 3151.62cm-1, 2946.09cm-1, 1638.53cm-1, 1575.39cm-1, 1455.24cm-1, -1 -1 -1 1329.11cm , 1194.48cm , and 1110.74cm could be clearly observed, as shown in Figure 1S, which is attributed to the existance of [C3O1Mim][H3CSO3] IL. This indicates that the [C3O1Mim][H3CSO3] had been loaded onto the porous silica gel.
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3.1.2 Textural properties The textural properties (BET surface area, pore volume, and pore size) of original SiO2 and [C3O1Mim][H3CSO3]-SiO2 samples are summarized in Table 2. Compared with the original SiO2, all the samples of [C3O1Mim][H3CSO3]-SiO2 give smaller specific surface area (SBET) and total pore volume (V). In addition, their specific surface area and total pore volume decreased with the increase of the immobilized amount of EFIILs. This may be explained by that the loading of IL on the external suface of silica gel causes pore plugging and leading to the decrease of the specific surface area and total pore volume. As shown in Table 2, the total porous volume shows a slight variation with different feed ratios.
2.3 Characterization FT-IR spectra analysis was conducted on type FTS135 Fourier transform infrared spectroscopy (FT-IR) instrument (PE, USA); 1H NMR spectra were measured by AVANCF 500 MHz spectrometer (Bruker Co. Ltd.), using CDCl3 as a solvent and TMS as the internal standard. The thermal properties were examined by a NETZSCH Q600 thermo gravimetric and differential scanning calorimetry (TG/DSC) analyzer (TA，USA) with a heating rate of 10 °C per minute under dynamic N2 atmosphere. The temperature range was from 25 °C to 600 °C. The surface area, pore size distributions, and porosities of the porous silica gel (SiO2) and EFIILs-SiO2 particles were measured
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Table 2. Pore characteristics [C3O1Mim][H3CSO3]-SiO2 samples .
of
original
SiO2
The pore size distributions for original silica-gel and [C3O1Mim][H3CSO3]-SiO2 (0.1/1, 0.3/1) are shown in Figure 4. There is a single peak of pore size distribution for every samples, and a majority of pores were in the size range of 0-200Å. For the original silica-gel (SiO2) particles, nearly half of its pores are mesopores, while loading onto IL, its porous structure is mainly mesopores. This indicated that the mesopore was packed by IL in the process of impregnation. After EFIILs immobilization, the mesopores remained. And the average pore size decreased with increasing [C3O1Mim][H3CSO3]/SiO2 ratio. Zhang et al. [10] compared the pore characteristics and the average pore size of TMGL with TMGL/SiO2. After TMGL immobilization, the micropores remained but the nanoscale pores diminished gradually and finally disappeared with further increase in the TMGL/SiO2 ratio. The differences in the pore characteristics lead to different SO2 adsorption behaviors.
and
Feed ratio
Sepecific surface area SBET(m2/g)
Total pore volume V(cm³/g)
Average pore size r(A)
0/1
355.43
1.15
106.10
0.1/1
271.52
0.95
98.47
0.2/1
220.94
0.83
100.93
0.3/1
181.02
0.70
101.26
0.4/1
145.16
0.57
104.26
0.5/1
117.70
0.48
110.23
1/1
62.62
0.28
116.92
3.1.3. SEM
The curves of nitrogen adsorption-desorption isotherms of original porous silica gels, [C3O1Mim][H3CSO3]-SiO2(0.1/1) and [C3O1Mim][H3CSO3]-SiO2(0.3/1) are presented in Figure 3. All samples exhibit type IV isotherms with H1 type hysteresis loops.
Figure 5 shows the SEM images of the original silica gel and EFIILs-SiO2. The morphology of silica gel are illustrated in Figure 5A and 5B, which were magnified 90 thousand times. The morphology of the samples with [C3O1Mim][H3CSO3]/SiO2 0.5/1,
[12]
[C5O2Mim][H3CSO3]/SiO2 1/1, and [C7O3Mim][H3CSO3]/SiO2 1/1 are shown in Figure 5C, 5D, and 5E, respectively. Compared with
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Quantity Adsorbed (cm /g STP)
800 700
silica gel [C3O1Mim][H3CSO3]-SiO2 0.1/1
600
[C3O1Mim][H3CSO3]-SiO2 0.3/1
the original silica gel, the external surface of EFIILs-SiO2 becomes rougher, and there is a trend that the size of the particles increases with increasing ether chain length of the EFIILs. This phenomenon demonstrates that EFIILs had been immobilized on the surface of silica gel. This result is in accordance with that of the IR analysis.
500 400 300
A
200
B
100 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po) C
D
E
Figure 3. The nitrogen adsorption-desorption isotherms of silica gel, [C3O1Mim][H3CSO3]-SiO2(0.1/1) and [C3O1Mim][H3CSO3]SiO2(0.3/1). Figure 5. SEM images of the external morphology of silica gel particles before (A, B) and after immobilization (C, D and E); C, [C3O1Mim][H3CSO3]-SiO2, 0.5/1; D, [C5O2Mim][H3CSO3]-SiO2, 1/1; E, [C7O3Mim][H3CSO3]-SiO2, 1/1.
3.0 3
dV/dlog(w) Pore Volume (cm /g)
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2.5 2.0
silica gel [C3O1Mim][H3CSO3]-SiO2 0.1/1 [C3O1Mim][H3CSO3]-SiO2 0.3/1
3.1.4 TG/DSC
1.5
TGA analysis was used to investigate the thermal stability of silica gel, EFIILs and EFIILs immobilized silica gel, as shown in Figure 6. There is a weight loss at the temperature below 150 ℃, which is attributed to the loss of water and residual solvent absorped on the samples[9]. In the temperature range of 150 to 300 ℃ , there is a slowly decrease of weight. When the temperature increased to 400 °C, the weight of EFIIL([C3O1Mim][H3CSO3]) decreased rapidly ， its residual weight was only about 10% around 400 ℃. The weight loss of [C3O1Mim][H3CSO3]-SiO2 at 300-400 ℃ is due to the decomposition of [C3O1Mim][H3CSO3]. And the weight loss of immobilized samples increased with increasing feed ratio of [C3O1Mim][H3CSO3]. Compared with the pure
1.0 0.5 0.0 -200 0
200 400 600 800 100012001400160018002000
Pore Width ()
Figure 4. Pore size distribution [C3O1Mim][H3CSO3]-SiO2(0.1/1) and SiO2(0.3/1).
of
original silica-gel, [C3O1Mim][H3CSO3]-
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SO2 solubility up to 2.01 moles SO2 per mole of IL. Ren et al. [15] reported the equilibrium absorption capacity of SO2 for different ILs at atmospheric pressure and 45.0 ℃. The SO2 absorbility for [BMIM][BF4] is 0.713 mol, for [BMIM][PF6] is 0.532 mol, for [TMG][BF4] is 0.482 mol, for [TMG]L is 1.62mol, and for [MEA]L is 0.903 mol SO2 per mole of IL at equilibrium state. Compared with the above ILs, pure EFIILs showed a preferable absorption capacity of SO2 and could absorb up to 1.933 mol pure SO2 at 45.0 ℃and atmospheric pressure. This result indicates that oxygen atom of ether in EFIIL is quite effective to SO2 dissolution. EFIILs could be recycled at least up to 5 cycles without any loss of initial performance, which demonstrates that SO2 is absorbed in physical state with EFIILs. [13]
[C3O1Mim][H3CSO3], the [C3O1Mim][H3CSO3] on silica gel shows a higher decomposition temperature. That indicates the [C3O1Mim][H3CSO3] loaded on silica gel exhibited a better thermal stability.
100
Weight/%
80 60 40 Siliga gel [C3O1Mim][H3CSO3] [C3O1Mim][H3CSO3]-SiO20.3/1
20
Figure 7 shows the adsorption curves of original SiO2 and the EFIILs-SiO2 samples with [C3O1Mim][H3CSO3]/SiO2 from 0.1/1 to 0.5/1 at 25 ℃ and under atmospheric pressure. After immobilization, the sorption capacity of SO2 increased and maintained steady with increasing ratios.
[C3O1Mim][H3CSO3]-SiO20.5/1 [C3O1Mim][H3CSO3]-SiO21/1
0 0
100 200 300 400 500 600 700 800 900 C
Tempureture/°
Figure 6. TGA curves of silica gel, [C3O1Mim][H3CSO3] , [C3O1Mim][H3CSO3]-SiO2 0.3/1, 0.5/1 and 1/1.
1.0
0.8
3.2 SO2 sorption and desorption behaviors. 0.6
The SO2 sorption behaviors of three typical immobilized ionic liquids [C3O1Mim][H3CSO3]-SiO2, [C5O2Mim][H3CSO3]-SiO2, [C7O3Mim][H3CSO3]-SiO2 were investigated, and the experiments were carried out at different temperatures under atmospheric pressure. SO2 sorption behaviors of EFIILs-SiO2 were performed at 25 ℃，35 ℃，45 ℃and 55 ℃, and the SO2 desorption were conducted at 100 ℃.
mSO2/g
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.4
Siliga gel 0.1:1 0.2:1 0.3:1 0.4:1 0.5:1
0.2
0.0
-10 0
10 20 30 40 50 60 70 80 90 100 110
Table 3. The SO2 absorption capacity of pure EFIILs. EFIILs [C3O1Mim][H3CSO3] (mol SO2/mol IL)
25℃ ℃
35℃ ℃
45℃ ℃
t/min
55℃ ℃
2.621mol
2.420
1.827
1.477
(0.762g
mol
mol
mol
3.106 mol
2.510
1.980
1.507
(0.709g
mol
mol
mol
3.453 mol
2.625
2.075
1.686
(0.680 g
mol
mol
mol
Figure 7. Adsorption behaviors of silica [C3O1Mim][H3CSO3]-SiO2 from 0.1/1 to 0.5/1 for SO2.
(mol SO2/mol IL)
SO2/g IL)
[C7O3Mim][H3CSO3] (mol SO2/mol IL)
and
The immobilized IL materials were used in multiple adsorption/desorption cycles. Figure 8 shows a typical set of 阿 adsorption/desorption cycle data for EFIILs-SiO2 0.1/1. The materials were stable and maintained their adsorption and desorption properties in five cycles at 25 ℃. The EFIILs-SiO2 system could be reused for several adsorption/desorption cycles without change in its capacity. 0.11 g SO2 was absorbed per gram of [C3O1Mim][H3CSO3]-SiO2 (0.1/1), which is equivalent to 1.1g SO2 per gram of [C3O1Mim][H3CSO3]. Comparatively, only 0.762g SO2 could be absorbed per gram of pure EFIILs [C3O1Mim][H3CSO3]. [C3O1Mim][H3CSO3]-SiO2 manifested better adsorption property than pure [C3O1Mim][H3CSO3], which demonstrates that the support materials-porous silica gel playes an important role in adsorption property. Zhang et al. [10] have shown that the sorption capacity for SO2 of pure TMGL reached 0.88 g/g TMGL (2.8 mol/mol TMGL). After immobilization, the absorption capacity for SO2 was maintained (0.87 g/g TMGL). Comparatively speaking, EFIILs and EFIILs-SiO2 all show the better sorption capacity.
SO2/g IL) [C5O2Mim][H3CSO3]
gel
SO2/g IL)
The experimental results shows that the synthesized EFIILs exhibited extremely high SO2 solubility. The capacities of SO2 absorption for pure EFIILs are list in Table 3. As seen from this table, with the increase of ether chain length of the cation, the ability to absorb gaseous SO2 increases in the order of [C3O1Mim][H3CSO3], [C5O2Mim][H3CSO3], and [C7O3Mim][H3CSO3] at all the experimental temperatures. And, the absorption capacity of EFIILs decreases with raising temperature. Zhang et al. [10] pointed out that the sorption capacity for SO2 of pure TMGL reached 0.88 g/g TMGL (2.8 mol/mol TMGL). Jun Huang et al. [13] found that [TMGHPO]Tf2N had high
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SiO2(0.3/1); Pore size distribution of blank silica-gel, [C3O1Mim][H3CSO3]-SiO2(0.1/1) and [C3O1Mim][H3CSO3]SiO2(0.3/1); SEM image of the external morphology of silica gel particles before (A, B) and after im-mobilization (C, D and E); B, [C3O1Mim][H3CSO3]-SiO2, 0.5/1; C, [C5O2Mim][H3CSO3]-SiO2, 1/1; D, [C7O3Mim][H3CSO3]-SiO2, 1/1; The TGA curve of blank siliga gel, [C3O1Mim][H3CSO3] , [C3O1Mim][H3CSO3]-SiO2 0.3/1, 0.5/1 and 1/1; The SO2 absorption capacity of pure EFIILs; Sorption behaviors of silica gel and [C3O1Mim][H3CSO3]-SiO2 from 0.1/1 to 0.5/1 for SO2; Absorption/desorption cycles of [C3O1Mim][H3CSO3]-SiO2 0. 1/1.
[C 3O 1Mim][H 3CSO 3]-SiO 2 0.1:1
0.5 0.4
mSO2/g
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0.3 0.2 0.1 0.0 -50
0
50
100 150 200 250 300 350 400 450 t/min
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Figure 8. Adsorption/desorption cycles of [C3O1Mim][H3CSO3]SiO2 0. 1/1.
4. Conclusions EFIILs were immobilized into porous silica particles by a simple impregnation method. The pore characteristics and SO2 adsorption/desorption properties of the immobilized IL (EFIILsSiO2) samples were investigated. EFIILs-SiO2 have high porosity, large specific surface area, and good mechanical properties. The pure EFIILs exhibit extremely high SO2 solubility. EFIILs-SiO2 have better absorption property than pure EFIILs, because support materials porous silica gel played an important role in absorption property. EFIILs-SiO2 system could be reused for five sorption/desorption cycles without loss of SiO2 absorption capacity, which demonstrated the potential application of this type of immobilized EFIILs as SO2 sorbent in gas desulfurization. ACKNOWLEDGMENT This work is supported by the National Nature Science Foundation of China (21206030) and Nature Science Foundation of Hebei Province (B2012208084). and Hebei research center of pharmaceutical and chemical engineering, Hebei University of Science and Technology, China. Supporting Information Available: TGA of blank silica-gel; Raw materials ratio of [C3O1Mim][H3CSO3]-SiO2 samples(ILs with SiO2 for [C3O1Mim][H3CSO3]-SiO2 Samples） prepared by the Impregnation Process; Experimental apparatus for SO2 adsorption and desorption of Silica gel supported etherfunctionalized imidazolium-based Ionic Liquids; Pore Characteristics of Original SiO2 and [C3O1Mim][H3CSO3]-SiO2 Samples Prepared by the Impregnation Process with Various [C3O1Mim][H3CSO3]-SiO2 Ratios; SBET of SiO2 and [C3O1Mim][H3CSO3]-SiO2 Samples Prepared by the impregnation process with various [C3O1Mim][H3CSO3]-SiO2 ratios; The nitrogen adsorption-desorption isotherms of silica gel, [C3O1Mim][H3CSO3]-SiO2(0.1/1) and [C3O1Mim][H3CSO3]-
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