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Property of diethanolamine glycinate ionic liquid and its performance for CO2 capture
Journal of Molecular Liquids 211 (2015) 1–6
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Property of diethanolamine glycinate ionic liquid and its performance for CO2 capture Jian-Gang Lu a,b,⁎, Zhen-Yu Lu a,b, Liu Gao b, Shuang Cao a, Jia-Ting Wang b, Xiang Gao b, Yin-Qin Tang a, Wen-Yi Tan c a b c
Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science and Technology, Nanjing 210044, China Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science and Technology, Nanjing 210044, China Department of Environment Engineering, Nanjing Institute of Technology, Nanjing 211167, China
a r t i c l e
i n f o
Article history: Received 24 May 2015 Received in revised form 15 June 2015 Accepted 21 June 2015 Available online xxxx Keywords: Ionic liquid Diethanolamine glycinate Physical property CO2 absorption
a b s t r a c t A protic ionic liquid (IL), diethanolamine glycinate ([DEA][GLY]), was synthesized and characterized. Its physical properties, including density, viscosity, surface tension, conductivity and refractive index, were determined. CO2 solubility in [DEA][GLY] IL was also measured. Performances of [DEA][GLY] IL in absorbing CO2 in a bubble column were evaluated. The synthesized [DEA][GLY] is an amino acid-functionalized IL with dual groups of ammonium and amino. There was a good CO2 solubility in the IL at room temperature and atmospheric pressure, much better than the traditional ILs. Pure and aqueous [DEA][GLY] ILs are good CO2 absorbents. CO2 partial pressure and IL concentration have a significant impact on their absorption performance. [DEA][GLY] had an obvious advantage over the conventional ILs for CO2 absorption. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Excessive CO2 has been discharged into atmosphere as a result of combustion of large amounts of fossil fuels such as coal, petroleum and natural gas for the energy industry. CO2 concentration of the atmosphere has continued to rise [1]. There is an increasing consensus that excessive CO2 has been significantly contributing to global warming. People throughout the world are suffering from a series of serious environmental issues arising from global warming [2]. CO2 emission has become a subject of worldwide concern. It is quite necessary to explore a cost-effective, low energy-consumption and available technique for CO2 capture. Ionic liquids (ILs) as an interesting and novel category of fluids or molten salts increasingly attract researchers' attention [3]. They have been widely used in catalyses [4], syntheses [5], electrochemistry [6], biotechnology [7], materials [8], pollutant controls and separation processes [9,10]. ILs have been also extensively accepted as green chemical solvents for the chemical processes because of their unique physicochemical properties which are good chemical and thermal stability, almost no vapor pressure, nonflammable, larger temperature range of liquid state, and favorable solubility, especially adjustable and designable structure [11–15]. ILs can be designed as specific structures ⁎ Corresponding author at: Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science and Technology, Nanjing 210044, China. E-mail address: [email protected] (J.-G. Lu).
http://dx.doi.org/10.1016/j.molliq.2015.06.059 0167-7322/© 2015 Elsevier B.V. All rights reserved.
for specific properties and particular applications taking advantage of structure–property relationships [16]. ILs can be functionally synthesized to form a class of task-specific ones based on the structure–property relationship [17]. Functional ILs were generally used in specific applications such as gas absorbents, solvents, and heat transfer fluids [18,19]. Amino-functionalized ILs as the task-specific ILs, which cations or anions contain reactive amino groups, can be applied to industrial flue gases for CO2 capture [19,20]. Solubility and absorption rate of CO2 into the amino-functionalized ILs were larger than that of other traditional ILs [21,22]. It demonstrates that amino acids are a very useful and significant category as functionalized materials for amino-functionalized ILs [23]. An amino acid molecule consists of at least an amino group and a carboxyl group. Amino acids can play the roles of both preparation and functionalization taking advantage of the process of hydroxyl ammonium (PHA) for protic IL preparation [24,25]. The carboxyl group of amino acids can achieve the PHA for protic IL preparation. The amino group of amino acids can realize amino-functionalized anion of the protic IL. Therefore, a novel amino acid-functionalized IL can be obtained as an alternative absorbent for efficient CO2 capture. In this work, a protic ionic liquid, diethanolamine glycinate ([DEA][GLY]) as an amino acid-functionalized IL, was synthesized by the PHA. Its structure was characterized by 1H NMR, FT-IR spectrometer and element analysis. Some basal physical properties, e.g., densities, viscosities, surface tension, conductivity and refractive index of [DEA][GLY] IL were measured at atmospheric pressure. Solubility of CO2 in [DEA][GLY] IL was also determined. Furthermore, performances in CO2
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absorption of the pure and aqueous [DEA][GLY] ILs were investigated in a bubble column.
2. Experimental 2.1. Materials Diethanolamine (DEA) (AR grade, ≥ 99.0%) was obtained from Shanghai Ling Feng Chemical Reagent Co., Ltd., China. Glycine (GLY) (AR grade, ≥99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Ethanol (AR grade, ≥ 99.9%) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. N2 and CO2 were commercial cylinder gases and their purity was more than 99.99% (Nanjing Real Special-Gas Co., China). H2SO4 was an analytical reagent (Nanjing Chemical Reagent Co., Ltd., China). DEA was purified by distillation under reduced pressure before use. Glycine, a biochemical reagent, was used as received. All reagents were more than 99.5% pure before use.
2.2. Synthesis Diethanolamine glycinate ([DEA][GLY], [(HOCH 2CH2 )2 NH2] [OOCCH2NH2]) IL was prepared by the PHA. The synthesis procedure for [DEA][GLY] IL preparation was carried out in three steps: 1) Neutralization: metered DEA (e.g., 0.25 mol) was dissolved in anhydrous ethanol. The DEA solution was loaded into a three-necked, roundbottomed flask as a reactor that immersed in a constant temperature water bath (X-II, Nanjing Instrument Factory, China) in which the temperature was controlled by a digital thermometer system with a platinum resistance probe within ± 0.01 K precision. The flask was equipped with a vigorous stirrer, a reflux condenser and a funnel. Metered glycine (e.g., 0.20 mol) was loaded into the funnel and gradually and slowly added into the reactor under vigorous stirring and room temperature (19–24 °C) within 60 min. Then the funnel was removed and the port of the flask was stoppered. The water bath was heated to 60 °C and maintained at the temperature. The reflux condenser was started. The reaction lasted for more than 24 h under vigorous stirring. An excess of DEA (e.g., 0.05 mol) was used in the reaction in order to ensure the complete reaction of glycine. 2) Purification: the resulting liquid of the first step was distilled under vacuum to remove the ethanol and excess DEA. A small quantity of unreacted GLY can be crystallized into solid residue as the solvent of ethanol was distilled out of the liquid. 3) Filtration: after the two steps, the crude product was filtered to remove the solid residue that contained unreacted GLY. The final product [DEA][GLY] IL, [(HOCH2CH2 )2 NH2][OOCCH2 NH2] was gained. [DEA][GLY] was stored in a glass vial that was sealed with a silicone septum. An analytical balance (FA2004, Shanghai Shang Tian Precision Instrument Co., Ltd., China) with precision and accuracy of ± 0.0001 g was used for weighting during the preparation process. Uncertainty for the concentration was ± 0.01%.
2.3. Structural characterization The chemical structure of the [DEA][GLY] IL was characterized using the following analytical instruments. 1H NMR spectrum was measured on a Bruker DXM 300 MHz spectrometer, using dimethyl sulfoxide (DMSO) as solvent with TMS as internal standard. Elemental analysis was determined on an Elementar Vario EL III. FT-IR spectrum was performed by an Ava-Tar 360 IR spectrometer. The water content of the IL was determined by Karl-Fischer measurement. The purity of [DEA][GLY] was also measured by an Agilent 6890 GC.
2.4. Physical properties Density, viscosity, surface tension, conductivity and refractive index of the [DEA][GLY] IL were determined at atmospheric pressure by the following instruments and analytical methods. Density was measured by a Gay–Lussac pycnometer (5 ml). Results of measurement were the average values of five samples. Uncertainty of measurement for density was estimated to be ±0.05%. A suspended level Ubbelohde viscometer (R200, Yuyao Precise Instrument Co., Ltd., Zhejiang, China) was used to determine the viscosity of the IL. Flow time of samples was recorded by a stop-watch with an accuracy of ± 0.01 s. Measurements were repeated several times until no more than 0.2 s deviation was found. The uncertainty of the viscosity measurements was estimated to be ±0.5%. Surface tension was determined using a surface tension meter (CAM 200, KSV Instruments Co., Ltd., Finland). Surface tension was measured with a ±1.7% uncertainty. The measurement procedures in detail for the density, viscosity and surface tension can be also found in our previous work [26,27]. Conductivity was measured by a DDS-307 conductivity meter (Precision & Scientific Instrument Co., Ltd., Shanghai, China). Uncertainty of conductivity was ±0.1%. Refractive index was determined with a WAY-2 W Abbe refractometer (Precision & Scientific Instrument Co., Ltd., Shanghai, China) [28]. The refractometer was calibrated by measuring the refractive indexes of deionized water and glycerol before determination. The measurement uncertainty for refractive index is ±0.03%. The temperature control system for density and viscosity measurements was in a well-stirred constant temperature water bath (X-II, Nanjing Instrument Factory, China) in which the temperature was controlled by a digital thermometer system with a platinum resistance probe within ±0.01 K precision. 2.5. CO2 solubility Measurement of CO2 solubility in the [DEA][GLY] IL was carried out in a sealed apparatus at atmospheric pressure, as shown in Fig. 1. The equilibrium cell (No. 4 in Fig. 1) was a stainless steel cylinder with ⌀ 30 mm × 100 mm (internal diameter × height) that was placed in a thermostatic water bath to maintain a constant temperature. A gas sparger with a porous cross-type tube (No. 9 in Fig. 1) was set up in the cell to disperse uniform gas bubbles into the IL. The foam-film flowmeter (No. 8 in Fig. 1, LZB-I, Yutao Automation Instrument Co., Ltd., Zhejiang, China) at the gas outlet of the cell was used as the measure
8 10
7 6
2 P
5 1 4
3
9
Fig. 1. Schematic diagram for determination of CO2 solubility in IL, 1 — gas cylinder, 2 — gas flowmeter, 3 — spiral heat exchanger, 4 — equilibrium cell (absorption reactor), 5 — thermostatic water bath, 6 — thermometer, 7 — sample tube, 8 — soap-bubble flowmeter, 9 — porous cross-type gas-tube, 10 — gas analyzer.
J.-G. Lu et al. / Journal of Molecular Liquids 211 (2015) 1–6
R1R2-NH + HOOC-R3 = [R1R2-NH2]+[OOC-R3]Scheme 1. The process of hydroxyl ammonium for protic IL preparation.
of outlet gas flowrate and calculation of mass balance. An IR gas analyzer (No. 10 in Fig. 1, MB154S, Varian Co., USA) was used to analyze the content of CO2 in the inlet and outlet of gas phase as a gas mixture (N2/CO2) used. When a gas mixture (N2/CO2) was used in the experiment, the mixture was beforehand prepared in a gas-prepared system (a selffabricated system in the lab based on the partial-pressure principle) to a given concentration. Equilibrium point for CO2 solubility was identified by the indication of the outlet equal to the inlet gas flowrates or a constant outlet CO2 concentration equal to that of the inlet of gas stream. The experimental method and procedure were described in our previous work in detail [29]. The CO2 content in the IL was determined by the method of chemical analysis. A known volume of the IL sample (e.g., 1 ml) was acidified with a diluted H2SO4 aqueous solution (volume ratio of H2SO4:H2O was 1:4), and the volume of the evolved gas was measured with a gas burette. After temperature and pressure corrections, the CO2 content of the sample was calculated. The gas burette precision was ±0.02 ml. Measurement for each datum was repeated at least three times. Deviation of the solubility was no more than ±0.05%. 2.6. CO2 absorption performance Performance of the [DEA][GLY] IL in absorbing CO2 was studied in a bubble column. The apparatus (Fig. 1) as a bubble column can be also used to investigate the performance of the IL in absorbing CO2. The equilibrium cell here was an absorption reactor with a characteristic of gas–liquid direct contact. 3. Results and discussion 3.1. Synthesis process for [DEA][GLY] IL [DEA][GLY] was synthesized to form a protic IL by the PHA based on the neutralization mechanism of DEA as a base with GLY as an acid [24]. The PHA is described in Scheme 1. R1 and R2 are any alkyl or alkyl groups with hydroxyl, e.g., R1 = R2 = –CH2CH2OH, and R3 is any alkyl or alkyl groups with amino, e.g., R3 = CH3, –CH2NH2. The amine may be primary, secondary or tertiary. The carboxylic acid can be an amino acid when R2 is an alkyl group with amino, e.g., glycine. The PHA for IL synthesis shows the favorable characteristics: a simple process, a single step reaction and no byproducts. Conventional IL preparations are usually quite complicated, e.g., a transition of intermediates [16]. Synthetic processes of most traditional ILs were designed to use organic solvents (e.g., acetone, benzene, xylene and dichloromethane) [16,30]. The traces of organic solvents would remain in ILs resulting in the formation of unwelcome impurities and the complicated purificatory steps. These solvents in the synthetic processes were also disagreeable because of volatility, safety and environmental issues. The preparation of [DEA][GLY] IL here was developed to use only ethanol as solvent. Scheme 2 is the chemical reaction to prepare the [DEA][GLY] IL following the PHA. [DEA][GLY] as an amino acid-functionalized IL can structurally possess dual groups of ammonium in the cation and amino in the
HOCH2CH2
O Alcohol NH + HO-C-CH2-NH2
HOCH2CH2
3
anion, different from other hydroxyl ammonium ILs with only the ammonium group in the cation [24,25]. These groups are significant for CO2 and SO2 capture [22,24]. Because the synthetic process of [DEA][GLY] IL was simple, effect on the process was primarily both reaction temperature and reaction completeness. The temperature involved initial neutralization and subsequent neutralization in the first step of synthesis procedure. It was found experimentally that a lower initial neutralization temperature and a higher subsequent neutralization temperature were favorable for the synthetic process, e.g., the initial neutralization and subsequent neutralization temperatures were 20 °C and 60 °C, respectively. The neutralization process of DEA with GLY was an exothermic reaction. A lower initial neutralization temperature can quickly remove the reaction heat and facilitate neutralization. A higher successive neutralization temperature can increase the internal energy of reaction system to facilitate molecular collisions and make the reaction proceed to completion. However, the initial neutralization temperature cannot be too low, taking into account the solubility of glycine in ethanol. Because of zwitterion of GLY in solution, amino group of GLY may be partially protonated to affect reaction completeness. In order to ensure complete reaction, prolongation of reaction time and an excess of DEA were implemented in the experiments. A small quantity of unreacted GLY was remained in the solid residue after distillation. Reaction completeness was verified by the approach of mass balance of distillates and reactants. It was experimentally found that there were a minimal amount of distillates and residue when the reaction time and the excess of DEA were no less than 24 h and 0.05 mol, respectively. Therefore, the successive neutralization time was not less than 24 h and DEA exceeded 0.05 mol of stoichiometry in synthesis process for [DEA][GLY] IL. The [DEA][GLY] IL was prepared with yields of 84.8–86.6%. Excessive DEA and unreacted GLY were eliminated by the ways of distillation and filtration of solid residue. There were also no by-products in the chemical reaction. A pure [DEA][GLY] IL can be obtained accordingly. It displayed a colorless, transparent, sticky and oily liquid at room temperature (see Fig. 1 of the Supporting material).
3.2. Characterization of [DEA][GLY] IL The synthesized [DEA][GLY] IL was characterized as follows. 1H NMR and FT-IR spectra of [DEA][GLY] IL are shown in Figs. 2 and 3 of the Supporting material. The water content of the IL was found to be less than 300 ppm. The purity of [DEA][GLY] was found to be more than 99.9%. [DEA][GLY] IL is found to be in good agreement with its corresponding chemical structure. 1 H NMR (300 MHz, DMSO, TMS), δ: 4.62 ppm (broad peak, H, OH), 3.46–3.49 ppm (triplet, 4H, –CH2–NH+ 2 ), 2.72–2.74 ppm (triplet, 2H, – NH2), 2.49 ppm (single, 2H, N–CH2–), and 2.–2.38 ppm (quartet, 2H, – O–CH2–). FT-IR spectrum: 3362.2 cm− 1 (hydroxyl stretching and N–H stretching vibration), 2162.9 cm− 1 (primary amine salt ion weak stretching), 1570.8 cm−1 (carboxyl anti-symmetric stretching and N– H stretching bonding vibrations), 1402.3 cm−1 (carboxyl symmetrical stretching vibration), and 1074.1 cm−1 (C–N stretching vibration). Elemental analysis: calculated for C6H16N2O4: C 39.99%, H 8.95%, and N 15.55%; Found: C 40.12%, H 8.98%, and N 15.41%.
HOCH2CH2
-
+
NH2 HOCH2CH2
Scheme 2. Synthetic route of [DEA][GLY] IL in single step reaction.
O O-C-CH2-NH2
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Temperature K
Density g·cm−3
Viscosity mPa·s
Surface tension mN m−1
Conductivity mS cm−1
Refractive index
IL. In order to decrease the viscosity and increase the dissolution rate, aqueous [DEA][GLY] may be appropriate for CO2 absorption. For aqueous [DEA][GLY], the ion–dipole interaction of water molecule weakened the hydrogen bonds and decreased the viscosity of [DEA][GLY] IL.
293.15 303.15 313.15 323.15
1.0990 1.0824 1.0786 1.0781
11.93 7.346 4.779 3.307
58.43 58.31 57.63 57.73
8.19 10.4 13.2 16.5
1.417a
3.5. Performance of [DEA][GLY] IL in absorbing CO2 in a bubble column
Table 1 Physical properties of [DEA][GLY] IL.
a
At 283.15 K.
3.3. Physical properties of [DEA][GLY] IL Experiments were performed for the determination of density, viscosity, surface tension, conductivity and refractive index of [DEA][GLY] IL at 293.15–323.15 K and atmospheric pressure. Results are listed in Table 1. The results indicate that density, viscosity, and surface tension of the [DEA][GLY] IL decreased with the increase of temperature. Conductivity of the [DEA][GLY] IL increased as temperature increased. Temperature had a slight effect on density and surface tension of the IL. It significantly influenced on both viscosity and conductivity of the [DEA][GLY]. Compared with organic salts such as glycinate and citrate [26,27], the viscosity of [DEA][GLY] IL was much greater at room temperature. High viscosity is one of the characteristics of pure ILs. The surface tension of [DEA][GLY] IL was less than that of the organic salts.
3.4. CO2 solubility in [DEA][GLY] IL [DEA][GLY] IL is an anion-amino-functionalized IL according to the molecular structure of [DEA][GLY]. [DEA][GLY] IL absorbing CO2 is a chemical process. The amino (–NH2) of anion [GLY]− is a reactive group here. Owing to the amino of DEA that has been protonated to form an ammonium in the PHA, the ammonium (= NH+ 2 ) of cation [DEA]+ is ineffective as CO2 was absorbed by [DEA][GLY] IL. Therefore, anion [GLY]− of the IL plays a decisive role during the [DEA][GLY] IL absorption of CO2 [20]. The zwitterion mechanism can explain the reaction mechanism of CO2 with [DEA][GLY] IL [31]. Scheme 3 gives the reaction mechanism. The zwitterion product was [−OOCH2N+ CH2COO]− as an intermediate and subsequently was deprotonated by [GLY]− to form a carbamate product [−OOCHN CH2COO]−. Solubility of CO2 in [DEA][GLY] IL was measured at 293.15–323.15 K and at atmospheric pressure using pure CO2. Results are shown in Table 2. It is found that solubility of CO2 in [DEA][GLY] IL decreased as temperature increased. It indicates that solubility of CO2 in [DEA][GLY] IL, similar to [P4444][AA] IL [32], can reach near 0.5 mol CO2/mol IL at room temperature and atmospheric pressure. The solubility of CO2 in [DEA][GLY] IL was much better than in traditional ILs such as [bmim][BF4] IL [3,13,16]. Solubility of CO2 in [bmim][BF4] was only 0.02 mol CO2/mol. The process of [DEA][GLY] absorbing CO2 took about 50 min to reach the equilibrium state. CO2 gas was slowly diffused into the IL leading to a slower dissolution rate due to the high viscosity of [DEA][GLY] IL. Cation ([(HOCH2CH2)2NH2]+) of [DEA][GLY] IL is with a diol structural ion. Polyols themselves are a class of high-viscosity compounds [33]. The characteristics of molecule structure of the [DEA][GLY] IL determined its property of high viscosity. In addition, hydroxyls possibly led to a hydrogen-bonding interaction between the molecules of [DEA][GLY] IL [34]. This also increased the viscosity of the
[H2NCH2COO]- + CO2
[-OOCH2N+CH2COO]- + [H2NCH2COO]-
Pure ILs usually represent a higher viscosity, for instance, pure [bmim][BF4] with a high viscosity of 132 mPa·s at 293.15 K and 0.1 MPa [35]. Although viscosity of [DEA][GLY] is not comparatively very high, it would withstand a big resistance as flowing through pipes. Diffusion of gas in [DEA][GLY] also suffers from a big resistance. Slow diffusion would decrease the absorption velocity consequently. As [DEA][GLY] IL is mixed with water, the viscosity would decrease. For example, the viscosity of 50% mass fraction of aqueous [DEA][GLY] decreased to 2.075 mPa·s from 11.93 mPa·s of the pure [DEA][GLY] at 293.15 K and 0.1 MPa. Experiments were carried out to evaluate the performances of pure [DEA][GLY] and aqueous [DEA][GLY] in absorbing CO2 in a bubble column. The effect of concentration of aqueous [DEA][GLY] solution on CO2 loading of the IL was studied. Experimental conditions were: gas flowrate in 1 l min−1, absorption temperature at 293.15 K, and gas pressure at atmospheric pressure. Results are shown in Fig. 2. Concentration of [DEA][GLY] in aqueous solution was in mass fraction as shown in Fig. 2. The results indicate that the CO2 loading of the IL rapidly increases with time, and then reaches a plateau to achieve an absorption capacity (the maximal CO2 loading) at a certain operation time. The IL has achieved about 70–80% of the absorption capacity in 20–25 min from the start of operation. The absorption capacity of [DEA][GLY] IL is larger than that of the amine DEA, but slightly lower than that of the amine piperazine and amino acid salt glycinate [36]. Although [DEA][GLY] mixed with water can decrease the viscosity and improve gas diffusibility, absorption capacity of the IL was also sharply decreased due to the decrease of the IL concentration. Absorption capacity of the IL was decreased to 0.24 from 0.41 mol mol−1 as the IL concentration decreased from 100% to 50%. Water in the IL has a significant influence on the capacity of [DEA][GLY] in absorbing CO2. Absorption capacity of the IL was much higher than that of the traditional [bmim][BF4] IL. The capacity of traditional [bmim][BF4] IL was only 0.012 mol mol−1 at 293.15 K and 0.1 MPa [13,17]. It demonstrates that amino-functionalized ILs have a distinct advantage over the traditional ILs. Effect of CO2 concentration of gas mixture on CO2 loading of the IL was also studied. Experimental condition was the same as above. Results are shown in Fig. 3. Concentration of CO2 in mixed gas was in mole fractions as shown in Fig. 3. CO2 concentration (i.e., CO2 partial pressure) of gas mixture is crucial for the absorption performance [20, 25]. Results in Fig. 3 show that absorption capacity decreased with the decrease of CO2 partial pressure. Absorption capacity of the IL decreased to 0.21 from 0.24 mol mol−1 as the CO2 partial pressure decreased from 100% (0.1 MPa) to 50% mole fraction (0.05 MPa) using 50% mass fraction of aqueous [DEA][GLY]. CO2 partial pressure has a significant impact on the performance of [DEA][GLY] IL in absorbing CO2. Experiments were performed for CO2 loading of [DEA][GLY] IL compared with that of the traditional [bmim][BF4] IL and conventional alkanolamine DEA. Experimental conditions were: gas flowrate at 1 l min−1, absorption temperature at 293.15 K, solution concentrations in 50% mass fraction, pure CO2, and gas pressure at atmospheric pressure. Experimental data are given in Fig. 4. Results show that the CO2
[-OOCH2N+CH2COO]-
[-OOCHNC H2COO]- + [H3N+CH2COO]-
Scheme 3. The reaction mechanism of CO2 with [DEA][GLY] IL.
J.-G. Lu et al. / Journal of Molecular Liquids 211 (2015) 1–6
0.30 303.15 0.3983
313.15 0.3849
323.15 0.3658
loading of aqueous [DEA][GLY] was higher than that of the aqueous DEA and much higher than that of the aqueous [bmim][BF4]. Under the same experimental conditions, the CO2 loading of aqueous [DEA][GLY] achieved 0.248 mol mol−1, while the CO2 loading of aqueous DEA as a conventional alkanolamine reached 0.216 mol mol−1 and that of the [bmim][BF4] as a traditional IL was only 0.011 mol mol−1. It demonstrates that functionalized ILs have a distinct advantage over the conventional ILs as well as conventional alkanolamines.
0.25
-1
293.15 0.4039
CO2 loading in IL (mol mol )
Table 2 CO2 solubility in [DEA][GLY] IL. Temperature/K Solubility/mol mol−1
0.20 0.15 0.10
50% IL pure CO2
0.05
50% CO2 0.00
30% CO2
-0.05 0
4. Conclusions
20
30
40
50
60
70
Fig. 3. Effect of CO2 concentration on CO2 loading of [DEA][GLY] IL at 293.15 K and 0.1 MPa.
0.30
-1
CO2 loading (mol mol )
0.25 0.20 0.15 0.10 0.05 0.00
[DEA][GLY]
DEA
[bmim][BF4]
Fig. 4. CO2 loadings in [DEA][GLY], [bmim][BF4] and DEA at 293.15 K and 0.1 MPa.
Acknowledgments We gratefully acknowledge financial support from the Open Project by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control & Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC) (KHK1412), the University Graduate Scientific Research and Innovation Planning Project of Jiangsu Province (KYLX-0857), the Priority Academic Program Development of Jiangsu Higher Education
-1
10
Absorption time (min)
A protic IL, [DEA][GLY] IL as an amino acid-functionalized IL with dual groups of ammonium and amino, was successfully synthesized by the PHA. Its chemical structure was characterized by 1H NMR, FT-IR and elemental analysis. Density, viscosity, surface tension, conductivity and refractive index of the [DEA][GLY] were determined. CO2 solubility in the IL was measured at atmospheric pressure. Performance of [DEA][GLY] IL in absorbing CO2 was evaluated. Several significant conclusions have been found out. Synthetic method for [DEA][GLY] IL by the PHA was a simple, no-byproduct and single step reaction. [DEA][GLY] IL exhibited the representative physical properties of an amino acid-functionalized IL. CO2 solubility in [DEA][GLY] can experimentally achieve near 0.5 mol CO2/mol IL at room temperature and atmospheric pressure. It was much more than the solubility of the traditional ILs. [DEA][GLY] IL and its aqueous IL can give a good performance for CO2 absorption by evaluation of the bubble column. CO2 partial pressure and IL concentration have a significant effect on the absorption performance. The performance of [DEA][GLY] in absorbing CO2 was much more efficient than the traditional ILs and conventional alkanolamine DEA.
CO2 loading in IL (mol mol )
5
Institutions (PAPD), the Students Practice and Innovation Project of Jiangsu Province (201510300078) and the Laboratory Opening Project of NUIST (2015). Appendix A. Supplementary data
0.5
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2015.06.059.
0.4
References
0.3 0.2 pure CO2 0.1
pure IL 50% IL 30% IL
0.0 0
10
20
30
40
50
60
70
80
Absorption time (min) Fig. 2. Effect of concentration of aqueous [DEA][GLY] on CO2 loading of IL at 293.15 K and 0.1 MPa.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Property of diethanolamine glycinate ionic liquid and its performance for CO2 capture Jian-Gang Lu a,b,⁎, Zhen-Yu Lu a,b, Liu Gao b, Shuang Cao a, Jia-Ting Wang b, Xiang Gao b, Yin-Qin Tang a, Wen-Yi Tan c a b c
Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science and Technology, Nanjing 210044, China Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science and Technology, Nanjing 210044, China Department of Environment Engineering, Nanjing Institute of Technology, Nanjing 211167, China
a r t i c l e
i n f o
Article history: Received 24 May 2015 Received in revised form 15 June 2015 Accepted 21 June 2015 Available online xxxx Keywords: Ionic liquid Diethanolamine glycinate Physical property CO2 absorption
a b s t r a c t A protic ionic liquid (IL), diethanolamine glycinate ([DEA][GLY]), was synthesized and characterized. Its physical properties, including density, viscosity, surface tension, conductivity and refractive index, were determined. CO2 solubility in [DEA][GLY] IL was also measured. Performances of [DEA][GLY] IL in absorbing CO2 in a bubble column were evaluated. The synthesized [DEA][GLY] is an amino acid-functionalized IL with dual groups of ammonium and amino. There was a good CO2 solubility in the IL at room temperature and atmospheric pressure, much better than the traditional ILs. Pure and aqueous [DEA][GLY] ILs are good CO2 absorbents. CO2 partial pressure and IL concentration have a significant impact on their absorption performance. [DEA][GLY] had an obvious advantage over the conventional ILs for CO2 absorption. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Excessive CO2 has been discharged into atmosphere as a result of combustion of large amounts of fossil fuels such as coal, petroleum and natural gas for the energy industry. CO2 concentration of the atmosphere has continued to rise [1]. There is an increasing consensus that excessive CO2 has been significantly contributing to global warming. People throughout the world are suffering from a series of serious environmental issues arising from global warming [2]. CO2 emission has become a subject of worldwide concern. It is quite necessary to explore a cost-effective, low energy-consumption and available technique for CO2 capture. Ionic liquids (ILs) as an interesting and novel category of fluids or molten salts increasingly attract researchers' attention [3]. They have been widely used in catalyses [4], syntheses [5], electrochemistry [6], biotechnology [7], materials [8], pollutant controls and separation processes [9,10]. ILs have been also extensively accepted as green chemical solvents for the chemical processes because of their unique physicochemical properties which are good chemical and thermal stability, almost no vapor pressure, nonflammable, larger temperature range of liquid state, and favorable solubility, especially adjustable and designable structure [11–15]. ILs can be designed as specific structures ⁎ Corresponding author at: Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science and Technology, Nanjing 210044, China. E-mail address: [email protected] (J.-G. Lu).
http://dx.doi.org/10.1016/j.molliq.2015.06.059 0167-7322/© 2015 Elsevier B.V. All rights reserved.
for specific properties and particular applications taking advantage of structure–property relationships [16]. ILs can be functionally synthesized to form a class of task-specific ones based on the structure–property relationship [17]. Functional ILs were generally used in specific applications such as gas absorbents, solvents, and heat transfer fluids [18,19]. Amino-functionalized ILs as the task-specific ILs, which cations or anions contain reactive amino groups, can be applied to industrial flue gases for CO2 capture [19,20]. Solubility and absorption rate of CO2 into the amino-functionalized ILs were larger than that of other traditional ILs [21,22]. It demonstrates that amino acids are a very useful and significant category as functionalized materials for amino-functionalized ILs [23]. An amino acid molecule consists of at least an amino group and a carboxyl group. Amino acids can play the roles of both preparation and functionalization taking advantage of the process of hydroxyl ammonium (PHA) for protic IL preparation [24,25]. The carboxyl group of amino acids can achieve the PHA for protic IL preparation. The amino group of amino acids can realize amino-functionalized anion of the protic IL. Therefore, a novel amino acid-functionalized IL can be obtained as an alternative absorbent for efficient CO2 capture. In this work, a protic ionic liquid, diethanolamine glycinate ([DEA][GLY]) as an amino acid-functionalized IL, was synthesized by the PHA. Its structure was characterized by 1H NMR, FT-IR spectrometer and element analysis. Some basal physical properties, e.g., densities, viscosities, surface tension, conductivity and refractive index of [DEA][GLY] IL were measured at atmospheric pressure. Solubility of CO2 in [DEA][GLY] IL was also determined. Furthermore, performances in CO2
2
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absorption of the pure and aqueous [DEA][GLY] ILs were investigated in a bubble column.
2. Experimental 2.1. Materials Diethanolamine (DEA) (AR grade, ≥ 99.0%) was obtained from Shanghai Ling Feng Chemical Reagent Co., Ltd., China. Glycine (GLY) (AR grade, ≥99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Ethanol (AR grade, ≥ 99.9%) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. N2 and CO2 were commercial cylinder gases and their purity was more than 99.99% (Nanjing Real Special-Gas Co., China). H2SO4 was an analytical reagent (Nanjing Chemical Reagent Co., Ltd., China). DEA was purified by distillation under reduced pressure before use. Glycine, a biochemical reagent, was used as received. All reagents were more than 99.5% pure before use.
2.2. Synthesis Diethanolamine glycinate ([DEA][GLY], [(HOCH 2CH2 )2 NH2] [OOCCH2NH2]) IL was prepared by the PHA. The synthesis procedure for [DEA][GLY] IL preparation was carried out in three steps: 1) Neutralization: metered DEA (e.g., 0.25 mol) was dissolved in anhydrous ethanol. The DEA solution was loaded into a three-necked, roundbottomed flask as a reactor that immersed in a constant temperature water bath (X-II, Nanjing Instrument Factory, China) in which the temperature was controlled by a digital thermometer system with a platinum resistance probe within ± 0.01 K precision. The flask was equipped with a vigorous stirrer, a reflux condenser and a funnel. Metered glycine (e.g., 0.20 mol) was loaded into the funnel and gradually and slowly added into the reactor under vigorous stirring and room temperature (19–24 °C) within 60 min. Then the funnel was removed and the port of the flask was stoppered. The water bath was heated to 60 °C and maintained at the temperature. The reflux condenser was started. The reaction lasted for more than 24 h under vigorous stirring. An excess of DEA (e.g., 0.05 mol) was used in the reaction in order to ensure the complete reaction of glycine. 2) Purification: the resulting liquid of the first step was distilled under vacuum to remove the ethanol and excess DEA. A small quantity of unreacted GLY can be crystallized into solid residue as the solvent of ethanol was distilled out of the liquid. 3) Filtration: after the two steps, the crude product was filtered to remove the solid residue that contained unreacted GLY. The final product [DEA][GLY] IL, [(HOCH2CH2 )2 NH2][OOCCH2 NH2] was gained. [DEA][GLY] was stored in a glass vial that was sealed with a silicone septum. An analytical balance (FA2004, Shanghai Shang Tian Precision Instrument Co., Ltd., China) with precision and accuracy of ± 0.0001 g was used for weighting during the preparation process. Uncertainty for the concentration was ± 0.01%.
2.3. Structural characterization The chemical structure of the [DEA][GLY] IL was characterized using the following analytical instruments. 1H NMR spectrum was measured on a Bruker DXM 300 MHz spectrometer, using dimethyl sulfoxide (DMSO) as solvent with TMS as internal standard. Elemental analysis was determined on an Elementar Vario EL III. FT-IR spectrum was performed by an Ava-Tar 360 IR spectrometer. The water content of the IL was determined by Karl-Fischer measurement. The purity of [DEA][GLY] was also measured by an Agilent 6890 GC.
2.4. Physical properties Density, viscosity, surface tension, conductivity and refractive index of the [DEA][GLY] IL were determined at atmospheric pressure by the following instruments and analytical methods. Density was measured by a Gay–Lussac pycnometer (5 ml). Results of measurement were the average values of five samples. Uncertainty of measurement for density was estimated to be ±0.05%. A suspended level Ubbelohde viscometer (R200, Yuyao Precise Instrument Co., Ltd., Zhejiang, China) was used to determine the viscosity of the IL. Flow time of samples was recorded by a stop-watch with an accuracy of ± 0.01 s. Measurements were repeated several times until no more than 0.2 s deviation was found. The uncertainty of the viscosity measurements was estimated to be ±0.5%. Surface tension was determined using a surface tension meter (CAM 200, KSV Instruments Co., Ltd., Finland). Surface tension was measured with a ±1.7% uncertainty. The measurement procedures in detail for the density, viscosity and surface tension can be also found in our previous work [26,27]. Conductivity was measured by a DDS-307 conductivity meter (Precision & Scientific Instrument Co., Ltd., Shanghai, China). Uncertainty of conductivity was ±0.1%. Refractive index was determined with a WAY-2 W Abbe refractometer (Precision & Scientific Instrument Co., Ltd., Shanghai, China) [28]. The refractometer was calibrated by measuring the refractive indexes of deionized water and glycerol before determination. The measurement uncertainty for refractive index is ±0.03%. The temperature control system for density and viscosity measurements was in a well-stirred constant temperature water bath (X-II, Nanjing Instrument Factory, China) in which the temperature was controlled by a digital thermometer system with a platinum resistance probe within ±0.01 K precision. 2.5. CO2 solubility Measurement of CO2 solubility in the [DEA][GLY] IL was carried out in a sealed apparatus at atmospheric pressure, as shown in Fig. 1. The equilibrium cell (No. 4 in Fig. 1) was a stainless steel cylinder with ⌀ 30 mm × 100 mm (internal diameter × height) that was placed in a thermostatic water bath to maintain a constant temperature. A gas sparger with a porous cross-type tube (No. 9 in Fig. 1) was set up in the cell to disperse uniform gas bubbles into the IL. The foam-film flowmeter (No. 8 in Fig. 1, LZB-I, Yutao Automation Instrument Co., Ltd., Zhejiang, China) at the gas outlet of the cell was used as the measure
8 10
7 6
2 P
5 1 4
3
9
Fig. 1. Schematic diagram for determination of CO2 solubility in IL, 1 — gas cylinder, 2 — gas flowmeter, 3 — spiral heat exchanger, 4 — equilibrium cell (absorption reactor), 5 — thermostatic water bath, 6 — thermometer, 7 — sample tube, 8 — soap-bubble flowmeter, 9 — porous cross-type gas-tube, 10 — gas analyzer.
J.-G. Lu et al. / Journal of Molecular Liquids 211 (2015) 1–6
R1R2-NH + HOOC-R3 = [R1R2-NH2]+[OOC-R3]Scheme 1. The process of hydroxyl ammonium for protic IL preparation.
of outlet gas flowrate and calculation of mass balance. An IR gas analyzer (No. 10 in Fig. 1, MB154S, Varian Co., USA) was used to analyze the content of CO2 in the inlet and outlet of gas phase as a gas mixture (N2/CO2) used. When a gas mixture (N2/CO2) was used in the experiment, the mixture was beforehand prepared in a gas-prepared system (a selffabricated system in the lab based on the partial-pressure principle) to a given concentration. Equilibrium point for CO2 solubility was identified by the indication of the outlet equal to the inlet gas flowrates or a constant outlet CO2 concentration equal to that of the inlet of gas stream. The experimental method and procedure were described in our previous work in detail [29]. The CO2 content in the IL was determined by the method of chemical analysis. A known volume of the IL sample (e.g., 1 ml) was acidified with a diluted H2SO4 aqueous solution (volume ratio of H2SO4:H2O was 1:4), and the volume of the evolved gas was measured with a gas burette. After temperature and pressure corrections, the CO2 content of the sample was calculated. The gas burette precision was ±0.02 ml. Measurement for each datum was repeated at least three times. Deviation of the solubility was no more than ±0.05%. 2.6. CO2 absorption performance Performance of the [DEA][GLY] IL in absorbing CO2 was studied in a bubble column. The apparatus (Fig. 1) as a bubble column can be also used to investigate the performance of the IL in absorbing CO2. The equilibrium cell here was an absorption reactor with a characteristic of gas–liquid direct contact. 3. Results and discussion 3.1. Synthesis process for [DEA][GLY] IL [DEA][GLY] was synthesized to form a protic IL by the PHA based on the neutralization mechanism of DEA as a base with GLY as an acid [24]. The PHA is described in Scheme 1. R1 and R2 are any alkyl or alkyl groups with hydroxyl, e.g., R1 = R2 = –CH2CH2OH, and R3 is any alkyl or alkyl groups with amino, e.g., R3 = CH3, –CH2NH2. The amine may be primary, secondary or tertiary. The carboxylic acid can be an amino acid when R2 is an alkyl group with amino, e.g., glycine. The PHA for IL synthesis shows the favorable characteristics: a simple process, a single step reaction and no byproducts. Conventional IL preparations are usually quite complicated, e.g., a transition of intermediates [16]. Synthetic processes of most traditional ILs were designed to use organic solvents (e.g., acetone, benzene, xylene and dichloromethane) [16,30]. The traces of organic solvents would remain in ILs resulting in the formation of unwelcome impurities and the complicated purificatory steps. These solvents in the synthetic processes were also disagreeable because of volatility, safety and environmental issues. The preparation of [DEA][GLY] IL here was developed to use only ethanol as solvent. Scheme 2 is the chemical reaction to prepare the [DEA][GLY] IL following the PHA. [DEA][GLY] as an amino acid-functionalized IL can structurally possess dual groups of ammonium in the cation and amino in the
HOCH2CH2
O Alcohol NH + HO-C-CH2-NH2
HOCH2CH2
3
anion, different from other hydroxyl ammonium ILs with only the ammonium group in the cation [24,25]. These groups are significant for CO2 and SO2 capture [22,24]. Because the synthetic process of [DEA][GLY] IL was simple, effect on the process was primarily both reaction temperature and reaction completeness. The temperature involved initial neutralization and subsequent neutralization in the first step of synthesis procedure. It was found experimentally that a lower initial neutralization temperature and a higher subsequent neutralization temperature were favorable for the synthetic process, e.g., the initial neutralization and subsequent neutralization temperatures were 20 °C and 60 °C, respectively. The neutralization process of DEA with GLY was an exothermic reaction. A lower initial neutralization temperature can quickly remove the reaction heat and facilitate neutralization. A higher successive neutralization temperature can increase the internal energy of reaction system to facilitate molecular collisions and make the reaction proceed to completion. However, the initial neutralization temperature cannot be too low, taking into account the solubility of glycine in ethanol. Because of zwitterion of GLY in solution, amino group of GLY may be partially protonated to affect reaction completeness. In order to ensure complete reaction, prolongation of reaction time and an excess of DEA were implemented in the experiments. A small quantity of unreacted GLY was remained in the solid residue after distillation. Reaction completeness was verified by the approach of mass balance of distillates and reactants. It was experimentally found that there were a minimal amount of distillates and residue when the reaction time and the excess of DEA were no less than 24 h and 0.05 mol, respectively. Therefore, the successive neutralization time was not less than 24 h and DEA exceeded 0.05 mol of stoichiometry in synthesis process for [DEA][GLY] IL. The [DEA][GLY] IL was prepared with yields of 84.8–86.6%. Excessive DEA and unreacted GLY were eliminated by the ways of distillation and filtration of solid residue. There were also no by-products in the chemical reaction. A pure [DEA][GLY] IL can be obtained accordingly. It displayed a colorless, transparent, sticky and oily liquid at room temperature (see Fig. 1 of the Supporting material).
3.2. Characterization of [DEA][GLY] IL The synthesized [DEA][GLY] IL was characterized as follows. 1H NMR and FT-IR spectra of [DEA][GLY] IL are shown in Figs. 2 and 3 of the Supporting material. The water content of the IL was found to be less than 300 ppm. The purity of [DEA][GLY] was found to be more than 99.9%. [DEA][GLY] IL is found to be in good agreement with its corresponding chemical structure. 1 H NMR (300 MHz, DMSO, TMS), δ: 4.62 ppm (broad peak, H, OH), 3.46–3.49 ppm (triplet, 4H, –CH2–NH+ 2 ), 2.72–2.74 ppm (triplet, 2H, – NH2), 2.49 ppm (single, 2H, N–CH2–), and 2.–2.38 ppm (quartet, 2H, – O–CH2–). FT-IR spectrum: 3362.2 cm− 1 (hydroxyl stretching and N–H stretching vibration), 2162.9 cm− 1 (primary amine salt ion weak stretching), 1570.8 cm−1 (carboxyl anti-symmetric stretching and N– H stretching bonding vibrations), 1402.3 cm−1 (carboxyl symmetrical stretching vibration), and 1074.1 cm−1 (C–N stretching vibration). Elemental analysis: calculated for C6H16N2O4: C 39.99%, H 8.95%, and N 15.55%; Found: C 40.12%, H 8.98%, and N 15.41%.
HOCH2CH2
-
+
NH2 HOCH2CH2
Scheme 2. Synthetic route of [DEA][GLY] IL in single step reaction.
O O-C-CH2-NH2
4
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Temperature K
Density g·cm−3
Viscosity mPa·s
Surface tension mN m−1
Conductivity mS cm−1
Refractive index
IL. In order to decrease the viscosity and increase the dissolution rate, aqueous [DEA][GLY] may be appropriate for CO2 absorption. For aqueous [DEA][GLY], the ion–dipole interaction of water molecule weakened the hydrogen bonds and decreased the viscosity of [DEA][GLY] IL.
293.15 303.15 313.15 323.15
1.0990 1.0824 1.0786 1.0781
11.93 7.346 4.779 3.307
58.43 58.31 57.63 57.73
8.19 10.4 13.2 16.5
1.417a
3.5. Performance of [DEA][GLY] IL in absorbing CO2 in a bubble column
Table 1 Physical properties of [DEA][GLY] IL.
a
At 283.15 K.
3.3. Physical properties of [DEA][GLY] IL Experiments were performed for the determination of density, viscosity, surface tension, conductivity and refractive index of [DEA][GLY] IL at 293.15–323.15 K and atmospheric pressure. Results are listed in Table 1. The results indicate that density, viscosity, and surface tension of the [DEA][GLY] IL decreased with the increase of temperature. Conductivity of the [DEA][GLY] IL increased as temperature increased. Temperature had a slight effect on density and surface tension of the IL. It significantly influenced on both viscosity and conductivity of the [DEA][GLY]. Compared with organic salts such as glycinate and citrate [26,27], the viscosity of [DEA][GLY] IL was much greater at room temperature. High viscosity is one of the characteristics of pure ILs. The surface tension of [DEA][GLY] IL was less than that of the organic salts.
3.4. CO2 solubility in [DEA][GLY] IL [DEA][GLY] IL is an anion-amino-functionalized IL according to the molecular structure of [DEA][GLY]. [DEA][GLY] IL absorbing CO2 is a chemical process. The amino (–NH2) of anion [GLY]− is a reactive group here. Owing to the amino of DEA that has been protonated to form an ammonium in the PHA, the ammonium (= NH+ 2 ) of cation [DEA]+ is ineffective as CO2 was absorbed by [DEA][GLY] IL. Therefore, anion [GLY]− of the IL plays a decisive role during the [DEA][GLY] IL absorption of CO2 [20]. The zwitterion mechanism can explain the reaction mechanism of CO2 with [DEA][GLY] IL [31]. Scheme 3 gives the reaction mechanism. The zwitterion product was [−OOCH2N+ CH2COO]− as an intermediate and subsequently was deprotonated by [GLY]− to form a carbamate product [−OOCHN CH2COO]−. Solubility of CO2 in [DEA][GLY] IL was measured at 293.15–323.15 K and at atmospheric pressure using pure CO2. Results are shown in Table 2. It is found that solubility of CO2 in [DEA][GLY] IL decreased as temperature increased. It indicates that solubility of CO2 in [DEA][GLY] IL, similar to [P4444][AA] IL [32], can reach near 0.5 mol CO2/mol IL at room temperature and atmospheric pressure. The solubility of CO2 in [DEA][GLY] IL was much better than in traditional ILs such as [bmim][BF4] IL [3,13,16]. Solubility of CO2 in [bmim][BF4] was only 0.02 mol CO2/mol. The process of [DEA][GLY] absorbing CO2 took about 50 min to reach the equilibrium state. CO2 gas was slowly diffused into the IL leading to a slower dissolution rate due to the high viscosity of [DEA][GLY] IL. Cation ([(HOCH2CH2)2NH2]+) of [DEA][GLY] IL is with a diol structural ion. Polyols themselves are a class of high-viscosity compounds [33]. The characteristics of molecule structure of the [DEA][GLY] IL determined its property of high viscosity. In addition, hydroxyls possibly led to a hydrogen-bonding interaction between the molecules of [DEA][GLY] IL [34]. This also increased the viscosity of the
[H2NCH2COO]- + CO2
[-OOCH2N+CH2COO]- + [H2NCH2COO]-
Pure ILs usually represent a higher viscosity, for instance, pure [bmim][BF4] with a high viscosity of 132 mPa·s at 293.15 K and 0.1 MPa [35]. Although viscosity of [DEA][GLY] is not comparatively very high, it would withstand a big resistance as flowing through pipes. Diffusion of gas in [DEA][GLY] also suffers from a big resistance. Slow diffusion would decrease the absorption velocity consequently. As [DEA][GLY] IL is mixed with water, the viscosity would decrease. For example, the viscosity of 50% mass fraction of aqueous [DEA][GLY] decreased to 2.075 mPa·s from 11.93 mPa·s of the pure [DEA][GLY] at 293.15 K and 0.1 MPa. Experiments were carried out to evaluate the performances of pure [DEA][GLY] and aqueous [DEA][GLY] in absorbing CO2 in a bubble column. The effect of concentration of aqueous [DEA][GLY] solution on CO2 loading of the IL was studied. Experimental conditions were: gas flowrate in 1 l min−1, absorption temperature at 293.15 K, and gas pressure at atmospheric pressure. Results are shown in Fig. 2. Concentration of [DEA][GLY] in aqueous solution was in mass fraction as shown in Fig. 2. The results indicate that the CO2 loading of the IL rapidly increases with time, and then reaches a plateau to achieve an absorption capacity (the maximal CO2 loading) at a certain operation time. The IL has achieved about 70–80% of the absorption capacity in 20–25 min from the start of operation. The absorption capacity of [DEA][GLY] IL is larger than that of the amine DEA, but slightly lower than that of the amine piperazine and amino acid salt glycinate [36]. Although [DEA][GLY] mixed with water can decrease the viscosity and improve gas diffusibility, absorption capacity of the IL was also sharply decreased due to the decrease of the IL concentration. Absorption capacity of the IL was decreased to 0.24 from 0.41 mol mol−1 as the IL concentration decreased from 100% to 50%. Water in the IL has a significant influence on the capacity of [DEA][GLY] in absorbing CO2. Absorption capacity of the IL was much higher than that of the traditional [bmim][BF4] IL. The capacity of traditional [bmim][BF4] IL was only 0.012 mol mol−1 at 293.15 K and 0.1 MPa [13,17]. It demonstrates that amino-functionalized ILs have a distinct advantage over the traditional ILs. Effect of CO2 concentration of gas mixture on CO2 loading of the IL was also studied. Experimental condition was the same as above. Results are shown in Fig. 3. Concentration of CO2 in mixed gas was in mole fractions as shown in Fig. 3. CO2 concentration (i.e., CO2 partial pressure) of gas mixture is crucial for the absorption performance [20, 25]. Results in Fig. 3 show that absorption capacity decreased with the decrease of CO2 partial pressure. Absorption capacity of the IL decreased to 0.21 from 0.24 mol mol−1 as the CO2 partial pressure decreased from 100% (0.1 MPa) to 50% mole fraction (0.05 MPa) using 50% mass fraction of aqueous [DEA][GLY]. CO2 partial pressure has a significant impact on the performance of [DEA][GLY] IL in absorbing CO2. Experiments were performed for CO2 loading of [DEA][GLY] IL compared with that of the traditional [bmim][BF4] IL and conventional alkanolamine DEA. Experimental conditions were: gas flowrate at 1 l min−1, absorption temperature at 293.15 K, solution concentrations in 50% mass fraction, pure CO2, and gas pressure at atmospheric pressure. Experimental data are given in Fig. 4. Results show that the CO2
[-OOCH2N+CH2COO]-
[-OOCHNC H2COO]- + [H3N+CH2COO]-
Scheme 3. The reaction mechanism of CO2 with [DEA][GLY] IL.
J.-G. Lu et al. / Journal of Molecular Liquids 211 (2015) 1–6
0.30 303.15 0.3983
313.15 0.3849
323.15 0.3658
loading of aqueous [DEA][GLY] was higher than that of the aqueous DEA and much higher than that of the aqueous [bmim][BF4]. Under the same experimental conditions, the CO2 loading of aqueous [DEA][GLY] achieved 0.248 mol mol−1, while the CO2 loading of aqueous DEA as a conventional alkanolamine reached 0.216 mol mol−1 and that of the [bmim][BF4] as a traditional IL was only 0.011 mol mol−1. It demonstrates that functionalized ILs have a distinct advantage over the conventional ILs as well as conventional alkanolamines.
0.25
-1
293.15 0.4039
CO2 loading in IL (mol mol )
Table 2 CO2 solubility in [DEA][GLY] IL. Temperature/K Solubility/mol mol−1
0.20 0.15 0.10
50% IL pure CO2
0.05
50% CO2 0.00
30% CO2
-0.05 0
4. Conclusions
20
30
40
50
60
70
Fig. 3. Effect of CO2 concentration on CO2 loading of [DEA][GLY] IL at 293.15 K and 0.1 MPa.
0.30
-1
CO2 loading (mol mol )
0.25 0.20 0.15 0.10 0.05 0.00
[DEA][GLY]
DEA
[bmim][BF4]
Fig. 4. CO2 loadings in [DEA][GLY], [bmim][BF4] and DEA at 293.15 K and 0.1 MPa.
Acknowledgments We gratefully acknowledge financial support from the Open Project by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control & Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC) (KHK1412), the University Graduate Scientific Research and Innovation Planning Project of Jiangsu Province (KYLX-0857), the Priority Academic Program Development of Jiangsu Higher Education
-1
10
Absorption time (min)
A protic IL, [DEA][GLY] IL as an amino acid-functionalized IL with dual groups of ammonium and amino, was successfully synthesized by the PHA. Its chemical structure was characterized by 1H NMR, FT-IR and elemental analysis. Density, viscosity, surface tension, conductivity and refractive index of the [DEA][GLY] were determined. CO2 solubility in the IL was measured at atmospheric pressure. Performance of [DEA][GLY] IL in absorbing CO2 was evaluated. Several significant conclusions have been found out. Synthetic method for [DEA][GLY] IL by the PHA was a simple, no-byproduct and single step reaction. [DEA][GLY] IL exhibited the representative physical properties of an amino acid-functionalized IL. CO2 solubility in [DEA][GLY] can experimentally achieve near 0.5 mol CO2/mol IL at room temperature and atmospheric pressure. It was much more than the solubility of the traditional ILs. [DEA][GLY] IL and its aqueous IL can give a good performance for CO2 absorption by evaluation of the bubble column. CO2 partial pressure and IL concentration have a significant effect on the absorption performance. The performance of [DEA][GLY] in absorbing CO2 was much more efficient than the traditional ILs and conventional alkanolamine DEA.
CO2 loading in IL (mol mol )
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Institutions (PAPD), the Students Practice and Innovation Project of Jiangsu Province (201510300078) and the Laboratory Opening Project of NUIST (2015). Appendix A. Supplementary data
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Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2015.06.059.
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References
0.3 0.2 pure CO2 0.1
pure IL 50% IL 30% IL
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Absorption time (min) Fig. 2. Effect of concentration of aqueous [DEA][GLY] on CO2 loading of IL at 293.15 K and 0.1 MPa.
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