Article pubs.acs.org/jced
Influence of Water on the Carbon Dioxide Solubility in [OTf]- and [eFAP]-Based Ionic Liquids Małgorzata E. Zakrzewska* and Manuel Nunes da Ponte LAQV, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ABSTRACT: The solubility of high-pressure carbon dioxide in mixtures of ionic liquid and water was determined experimentally for 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-alkyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ionic liquids (where alkyl is ethyl, butyl, or hexyl). Experiments were performed at 313.15 K and/or 318.15 K, at pressures up to 10 MPa, and for various compositions (dry ionic liquids, 0.1 or 10 wt % of water), depending on the ionic liquid under investigation. It was found that the solubility of carbon dioxide in tris(pentafluoroethyl)trifluorophosphate-based ionic liquids is not affected by the water content, whereas for 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, the carbon dioxide sorption capacity is reduced by up to 10%, when 10 wt % of water is present in the system, at the same temperature and pressure.
1. INTRODUCTION Ionic liquids (ILs) and supercritical carbon dioxide (scCO2), or mixtures of both of them (scCO2/ILs system), are examples of systems that may lead to improvements in reaction and processing technologies. In recent years, one important area of the research related to these versatile solvents has been focused on the usage of ionic liquids as solvents for reactions involving permanent gases, gas separation/storage applications, and the use of gases (mainly carbon dioxide) to separate solutes from ionic liquid solutions. The solubility of carbon dioxide in pure ionic liquids is an active area of investigation.1 From the point of view of any large-scale application involving ionic liquids, the presence of common impurities, water in particular, must be taken into account, as they may have a large effect on many physicochemical properties of ionic liquids.2,3 Since many ionic liquids are hygroscopic,4,5 water can be easily present in a system as moisture absorbed from the air or as a residual contamination from ionic liquid synthesis. It can also be deliberately introduced in order to change particular characteristics, performance of ionic liquid medium (e.g., viscosity reduction, enzymatic activity, co/antisolvent effect), or to lower an overall cost associated with the usage of pure ionic liquid. Blanchard et al.6 observed that a small amount of water in an ionic liquid may have a large effect on the phase behavior of mixtures with CO2. They measured a 75% reduction in carbon dioxide solubility in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) when the water content was increased from 0.15 to 2.3 wt %. Literature data on this water effect is, however, still limited,6−20 and most of the reported results indicate much smaller effects than those given by Blanchard et al. © XXXX American Chemical Society
The addition of even a small mass of water to a binary (IL + CO2) system will introduce a large quantity, in moles, because water is a small molecule and its molar mass is, in any case, at least an order of magnitude smaller than the molar mass of the ionic liquid. Mole fractions of carbon dioxide and the ionic liquid, calculated as the ratio of number of moles of the substance to the total number of moles present in the system (sum of moles of CO2, IL, and H2O), will vary significantly. Using as an example the work of Blanchard et al.,6 varying from 0.15 wt % to 2.3 wt % water in [bmim][PF6] changes the IL mole fraction from 0.977 to 0.729. Total mole fractions are therefore not adequate to evaluate the effect of water on solubility. This explains why most of the literature references in this area use mole fractions of carbon dioxide calculated on a water-free basis.8−10,13,17 In recent work carried out in our laboratory,21 mixtures of water and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (also known as triflate), [emim][OTf], were selected as electrolyte for the electrochemical reduction of high pressure carbon dioxide to syngas. Water was chemically needed, for hydrogen production, but it was also important to reduce viscosity and thus enhance electrical conductivity. Also considered for the same purpose were 1-alkyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([cnmim][eFAP]) (n = 2, 4, 6) ionic liquids, as they display high CO2 solubilities. Special Issue: In Honor of Cor Peters Received: June 8, 2017 Accepted: September 14, 2017
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DOI: 10.1021/acs.jced.7b00521 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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amount of CO2 dissolved in ionic liquid is determined by an expansion into a previously calibrated volume at subatmospheric pressure. The assumption of a pure CO2 vapor phase is made, since ionic liquids have negligible volatility and water shows very low solubility in carbon dioxide, even at high pressures.29 For each temperature and pressure conditions, the measurement is repeated at least once, and the average result is taken, giving the overall experimental uncertainty less than 0.02 mole fraction.
Data on the solubilitv of high-pressure carbon dioxide in the dry ionic liquids were already reported,22−26 but no information is available on the effect of water on their capacity to dissolve carbon dioxide. On the other hand, as explained above, there is no reliable method available to predict that effect. As a precise knowledge of the quantity of CO2 in the liquid electrolyte is an important parameter for the interpretation of the electrochemical reduction results, in this work measurements of the solubility of carbon dioxide in mixtures of water and the abovementioned ionic liquids were undertaken, and their results are presented.
3. RESULTS AND DISCUSSION 3.1. Experimental Results. Table 2 and Table 3 present high-pressure solubility data for carbon dioxide in (IL + H2O)
2. EXPERIMENTAL SECTION 2.1. Materials. The ionic liquids used in this study were 1ethyl-3-methylimidazolium trifluoromethanesulfonate, supplied by Iolitec, and (1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, and 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate), supplied Merck KgaA, Germany. They were used without further purification. Carbon dioxide (99.98% purity) was supplied by Air Liquide and was used as received. Dichloromethane of ≥99.9 mass % purity (Sigma−Aldrich) was used in solubility measurements only as a solvent to wash the lines and dissolve the salts precipitated after CO2 expansion from the sample loop. Distilled water (Diwer Technologies Water max station Type I) was used to prepare the desired (IL + H2O) samples. Their concentration was determined by Karl Fischer coulometric titration (Metrohm 831 KF coulometer). Chemical names, abbreviations, purities, and sources for all chemicals used in this work are presented in Table 1.
Table 2. Experimental Results for the Solubility of CO2 in (99.9 wt % [cnmim][eFAP] (n = 2, 4, 6) + 0.1 wt % H2O) at 313.15 Ka
Table 1. Chemicals Used in This Study name
abbreviation
purity (mass %)
IL
wH2O/%
p/MPa
xCO2
x′CO2
mCO2/mol kg−1
[emim][eFAP]
0.1
[bmim][eFAP]
0.1
[hmim][eFAP]
0.1
2 4 6 8 10 2 4 6 8 10 2 4 6 8 10
0.43 0.59 0.68 0.74 0.77 0.42 0.60 0.70 0.78 0.79 0.47 0.64 0.72 0.79 0.82
0.42 0.58 0.67 0.74 0.76 0.41 0.59 0.70 0.78 0.79 0.46 0.63 0.71 0.79 0.81
1.3 2.5 3.8 5.2 5.9 1.3 2. 6 4.1 6.1 6.6 1.4 2.9 4.2 6.2 7.3
a
xCO2 and x′CO2 are defined by eq 1 and 2, respectively; mCO2 is expressed as moles of CO2 per kilogram of solvent mixture (IL + H2O). Standard uncertainties u are u(T) = 0.1 K, u(p) = 0.07 MPa, u(xCO2) = 0.02.
source
1-ethyl-3-methylimidazolium trifluoromethanesulfonate 1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate 1-butyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate 1-hexyl3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate dichloromethane
[emim] [OTf] [emim] [eFAP]
>99
Iolitec
>99
Merck
[bmim] [eFAP]
>99
Merck
[hmim] [eFAP]
>99
Merck
CH2Cl2
99.8
carbon dioxide
CO2
99.98
water
H2O
distilled
Sigma− Aldrich Air Liquide this work
mixtures with different solvent compositions. The experiments were performed at 313.15 and/or 318.15 K, pressures up to 10 MPa, and for various water concentrations [dry IL sample, 0.1 or 10 wt % of water in the (gas-free) IL], depending on the ionic liquid under investigation. The majority of the research on carbon dioxide solubility in ionic liquids to date reports results in the mole fraction units, as it can reflect the molecular interaction between gas molecules and ionic liquid structure. As referred in the Introduction, when the effect of water on solubility is being accounted for, most literature references use mole fraction on a water-free basis. However, mole fraction units do not account for the asymmetry of the molar masses of the substances present, where the ionic liquid always has a much larger value than the gas, that is, carbon dioxide, as pointed out by Carvalho et al.30 From an engineering perspective, it is often more convenient to express gas solubility in molality (quantity of gas absorbed per mass of solvent). In this work, the solubility of carbon dioxide, for the convenience of comparison, is expressed in both concentration units, mole fraction, and molality (moles of CO2 per kilogram of solvent). The mole fraction of carbon dioxide in the liquid phase is calculated both as the relative, water-free, molar composition:
2.2. Apparatus and Experimental Procedure. The detailed description of the experimental apparatus and methodology used to determine the solubility of carbon dioxide in ionic liquids has already been described.27,28 Briefly, the apparatus consist of a 3.5 cm3, fixed volume, movableposition high-pressure cell, with a sapphire window allowing for a visual observation of the fluid. The cell is placed inside an air bath, but equipped with a system of special holders, built in our laboratory, which allow handling from outside. Carbon dioxide is introduced under pressure from a reservoir tank, and the whole content is vigorously stirred to ensure equilibrium. Equilibrated samples are collected in a closed loop, and the B
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water in each of the three ionic liquids has no distinguishable effect on solubility. In Figure 2, the results reported in Table 3 for carbon dioxide solubility in dry [emim][OTf] are compared with data previously reported in the literature.22,23 Shin et al.22 measure the bubble point pressure of mixtures of carbon dioxide and ionic liquid with a known composition and at a fixed temperature, while Soriano et al.23 determine the solubility of carbon dioxide for temperatures ranging from 303.2 to 343.2 K and pressures up to 5.9 MPa using a thermogravimetric microbalance. Our results seem to be in agreement, within combined uncertainties, with both Shin et al. and Soriano et al., although the same might not be said of the agreement between those two sets of values. Figure 3 shows the results of the solubility of carbon dioxide in mixtures of [emim][OTf] and water, with three different solvent compositions, wH2O = 0, 0.1%, and 10%. As with the previously described hydrophobic ionic liquids of the [eFAP]family, at low water concentrations, the differences measured are within the combined uncertainties, and no effect can actually be determined, although visually a slight increase in carbon dioxide solubility may be detected. At 10 wt % water, the carbon dioxide solubility decreases approximately 10% reduction for the mixture of (10 wt % H2O + 90 wt % [emim][OTf]) composition, at 313.15 K. The results obtained in this work are consistent with the outcomes of the majority of the literature on the subject. A small effect of water on the carbon dioxide solubility in ionic liquids is commonly noticed,7−15 but with different deviations being observed. For example, Fu et al.9 studied the ([bmim][PF6] + CO2 + H2O) system and reported an average decrease in carbon dioxide solubility of about 4% mole fraction on a water free basis, with the water mass fraction ranging from 0.0067% to 1.6 wt %. The largest carbon dioxide solubility difference between the same ionic liquids, but with different water contents, observed by Lim et al.10 was 7−8%. The reduction of the carbon dioxide solubility in ionic liquids is most likely due to the competition between H2O and CO2 molecules for the same solvation sites. As the concentration of water increases, its molecules start to position themselves closer to the anions, thus shielding the access of carbon dioxide molecules to the ionic liquid.32,33 Water is also expected to effectively dilute the ionic liquid solvent and decrease the capacity of the mixed solvent for absorption, as a result of weak affinity between water and carbon dioxide.29 Yet, there exist studies that demonstrate a marked enhancement of the carbon dioxide absorption in the mixed IL−H2O system compared to the dry ILs.17,18 Romanos et al.18 argued that most of the published reports had been performed for ionic liquids with fluorinated anions and for low water concentrations, where a rather weak influence of water on the ionic liquid network was established. Authors argued that, to break the strong anion−cation interactions of the ionic liquid, higher water amounts are necessary. A greatly enhanced CO2 load, which is 4−5 higher compared to the dry ionic liquid, was reported for the binary system composed of tricyanomethanidebased ionic liquid and water, at mole fractions above 0.12. The authors explained it by the increase of the free volume through the formation of hydrogen-bonded IL−H2O complexes.
Table 3. Experimental Results for the Solubility of CO2 in ([emim][OTf] + H2O) Systems with Different Water Contentsa T/K
wH2O/wt %
p/MPa
xCO2
313.15
“0”
2 4 6 8 10 2 4 6 8 10 2 4 6 8 10 2 4 6 8 1 2 3 4 6 8
0.25 0.39 0.51 0.56 0.65 0.25 0.40 0.52 0.58 0.66 0.22 0.35 0.44 0.52 0.58 0.25 0.39 0.48 0.53 0.16 0.22 0.29 0.36 0.45 0.53
a
0.1
10
318.15
“0”a
10
x′CO2
0.25 0.40 0.52 0.57 0.65 0.11 0.19 0.27 0.34 0.39
0.08 0.12 0.15 0.20 0.27 0.33
mCO2/mol kg−1 1.3 2.5 4.0 5.0 7.2 1.3 2.6 4.2 5.3 7.3 1.1 2.1 3.1 4.4 5.4 1.3 2. 5 3.6 4.4 0.7 1.1 1.5 2.1 3.2 4.3
a
xCO2 and x′CO2 are defined by eq 1 and 2, respectively; mCO2 is expressed as moles of CO2 per kilogram of solvent mixture (IL + H2O); “0” denotes dried samples with water content less than 320 ppm. Standard uncertainties u are u(T) = 0.1 K, u(p) = 0.07 MPa, u(xCO2) = 0.02.
xCO2 = nCO2 /(nCO2 + nIL)
(1)
and as the absolute mole fraction: x′CO2 = nCO2 /(nCO2 + nIL + nH2O)
(2)
where nCO2, nIL, and nH2O are number of moles of carbon dioxide, ionic liquid, and water, respectively. The molality (mCO2) is presented as the quantity of carbon dioxide in one kilogram of the solvent mixture of ionic liquid and water, as the important information is how much carbon dioxide dissolves in a given amount of the mixture of solvents used. In Tables 2 and 3, the results of the solubility of carbon dioxide in the indicated mixtures of water with [cnmim][eFAP] (n = 2,4,6) and [emim][OTf] + H2O, respectively, are presented. 3.2. Discussion. Imidazolium tris(pentafluoroalkyl)trifluorophosphate ionic liquids are hydrophobic, with water solubility limits below 2000 ppm.31 In recently published carbon dioxide solubility data in dried samples of three ionic liquids, where the alkyl in the anion is ethyl and the imidazolium cations are [emim], [bmim], and [hmim],26 obtained in the same apparatus as used for this work, the water concentration in the ionic liquids was below 250 ppm, as measured by Karl Fischer analysis. A comparison of the results obtained in this work, for each ionic liquid + 0.1 wt % water, with the previously presented results,26 is shown in Figure 1. As can be seen, within experimental error, the small quantity of
4. CONCLUSIONS The aim of this work was to evaluate the influence of water on the sorption capacity of trifluoromethanesulfonate- and trisC
DOI: 10.1021/acs.jced.7b00521 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 1. Solubility of CO2 in [eFAP]-based ionic liquids at 313.15 K, for dry (geometric symbols) [ref 26], or 0.1% water (asterisks) samples [this work], (a) [emim]; (b) [bmim]; (c) [hmim].
Figure 2. Solubility of CO2 in [emim][OTf] at (a) 313.15 K and (b) 318.15 K: ●, [this work]; ■, [ref 23]; ▲, [ref 22] (bubble point data); empty symbols denote interpolated data between 313.15 and 323.15 K.
Figure 3. Solubility of CO2 in [emim][OTf] with different water contents at (a) 313.15 K and (b) 318.15 K: ○, dried IL samples; ▲, 0.1 wt % H2O; ■, 10 wt % H2O.
D
DOI: 10.1021/acs.jced.7b00521 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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(5) Di Francesco, F.; Calisi, N.; Creatini, M.; Salvo, B. M. P.; Chiappe, C. Water sorption by anhydrous ionic liquids. Green Chem. 2011, 13, 1712−1717. (6) Blanchard, L. A.; Gu, Z.; Brennecke, J. F. High-pressure phase behavior of ionic liquids/CO2 systems. J. Phys. Chem. B 2001, 105, 2437−2444. (7) Aki, S. N. V. K.; Mellein, B. R.; Saurer, E. M.; Brennecke, J. F. High-pressure phase behavior of carbon dioxide with imidazoliumbased ionic liquids. J. Phys. Chem. B 2004, 108, 20355−20365. (8) Scovazzo, P.; Camper, D.; Kieft, J.; Poshusta, J.; Koval, C.; Noble, R. Regular solution theory and CO2 gas solubility in room-temperature ionic liquids. Ind. Eng. Chem. Res. 2004, 43, 6855−6860. (9) Fu, D.; Sun, X.; Pu, J.; Zhao, S. Effect of water content on the solubility of CO2 in the ionic liquid [bmim][PF6]. J. Chem. Eng. Data 2006, 51, 371−375. (10) Lim, B.-H.; Choe, W.-H.; Shim, J.-J.; Ra, C. S.; Tuma, D.; Lee, H.; Lee, C. S. High-pressure solubility of carbon dioxide in imidazolium-based ionic liquids with anions [PF6] and [BF4]. Korean J. Chem. Eng. 2009, 26, 1130−1136. (11) Baltus, R. E.; Culbertson, B. H.; Dai, S.; Luo, H.; DePaoli, D. W. Low-pressure solubility of carbon dioxide in room-temperature ionic liquids measured with a quartz crystal microbalance. J. Phys. Chem. B 2004, 108, 721−727. (12) Andanson, J.-M.; Jutz, F.; Baiker, A. Investigation of binary and ternary systems of ionic liquids with water and/or supercritical CO2 by in situ attenuated total reflection infrared spectroscopy. J. Phys. Chem. B 2010, 114, 2111−2117. (13) Bermejo, M. D.; Montero, M.; Saez, E.; Florusse, L. J.; Kotlewska, A. J.; Cocero, M. J.; van Rantwijk, F.; Peters, C. J. Liquidvapor equilibrium of the systems butylmethylimidazolium nitrate-CO2 and hydroxypropylmethylimidazolium nitrate-CO2 at high pressure: influence of water on the phase behavior. J. Phys. Chem. B 2008, 112, 13532−13541. (14) Kumełan, J.; Pérez-Salado Kamps, Á .; Tuma, D.; Maurer, G. Solubility of carbon dioxide in liquid mixtures of water + [bmim][CH3SO4]. J. Chem. Eng. Data 2011, 56, 4505−4515. (15) Husson, P.; Pison, L.; Jacquemin, J.; Costa Gomes, M. F. Influence of water on the carbon dioxide absorption by 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)amide. Fluid Phase Equilib. 2010, 294, 98−104. (16) Kerlé, D.; Ludwig, R.; Paschek, D. The influence of water on the solubility of carbon dioxide in imidazolium based ionic liquids. Z. Phys. Chem. 2013, 227, 167−176. (17) Taib, M. M.; Murugesan, T. Solubilities of CO2 in aqueous solutions of ionic liquids (ILs) and monoethanolamine (MEA) at pressures from 100 to 1600 kPa. Chem. Eng. J. 2012, 181−182, 56−62. (18) Romanos, G. E.; Zubeir, L. F.; Likodimos, V.; Falaras, P.; Kroon, M. C.; Iliev, B.; Adamova, G.; Schubert, T. J. S. Enhanced CO2 capture in binary mixtures of 1-alkyl-3-methylimidazolium tricyanomethanide ionic liquids with water. J. Phys. Chem. B 2013, 117, 12234−12251. (19) Mirzaei, M.; Badiei, A. R.; Mokhtarani, B.; Sharifi, A. Experimental study on CO2 sorption capacity of the neat and porous silica supported ionic liquids and the effect of water content of flue gas. J. Mol. Liq. 2017, 232, 462−470. (20) Ventura, S. P. M.; Pauly, J.; Daridon, J. L.; Lopes da Silva, J. A.; Marrucho, I. M.; Dias, A. M. A.; Coutinho, J. A. P. High pressure solubility data of carbon dioxide in (tri-iso-butyl(methyl)phosphonium tosylate + water) systems. J. Chem. Thermodyn. 2008, 40, 1187−1192. (21) Pardal, T.; Messias, S.; Sousa, M.; Reis Machado, A. S.; Rangel, C. M.; Nunes, D.; Pinto, J. V.; Martins, R.; Nunes da Ponte, M. Syngas production by electrochemical CO2 reduction in an ionic liquid basedelectrolyte. J. CO2 Util. 2017, 18, 62−72. (22) Shin, E.-K.; Lee, B.-C. High-pressure phase behavior of carbon dioxide with ionic liquids: 1-alkyl-3-methylimidazolium Trifluoromethanesulfonate. J. Chem. Eng. Data 2008, 53, 2728−2734. (23) Soriano, A. N.; Doma, B. T., Jr.; Li, M.-H. Carbon dioxide solubility in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. J. Chem. Thermodyn. 2009, 41, 525−529.
(pentafluoroethyl)trifluorophosphate-based ionic liquids. It was found that, regardless of the ionic liquid investigated, the small amounts of water added to the ionic liquid, here 0.1 wt %, do not change the solubility of carbon dioxide in the mixed solvent. When more water was added to the hydrophilic 1ethyl-3-methylimidazolium trifluoromethanesulfonate, the approximately 10% reduction in carbon dioxide solubility was reported for 10 wt % of water, at the same temperature and pressure.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Małgorzata E. Zakrzewska: 0000-0002-8346-5932 Manuel Nunes da Ponte: 0000-0003-4499-4521 Funding
This work was supported by the Associate Laboratory for Green Chemistry LAQV which is financed by national funds from FCT/MEC (UID/QUI/50006/2013) and cofinanced by the ERDF under the PT2020 Partnership Agreement (POCI01-0145-FEDER-007265). M. E. Zakrzewska is thankful to FCT for her postdoctoral fellowship SFRH/BPD/122655/ 2016. Notes
The authors declare no competing financial interest.
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ABBREVIATIONS IL, ionic liquid CO2, carbon dioxide scCO2, supercritical carbon dioxide H2O, water [cnmim], 1-alkyl-3-methylimidazolium [emim], 1-ethyl-3-methylimidazolium [bmim], 1-butyl-3-methylimidazolium [hmim], 1-hexyl-3-methylimidazolium [PF6], hexafluorophosphate [OTf], trifluoromethanesulfonate [eFAP], tris(pentafluoroethyl)trifluorophosphate wH2O, mass percentage (wt %) xCO2, relative, water-free, mole fraction of carbon dioxide in a liquid phase x′CO2, absolute mole fraction of carbon dioxide in a liquid phase nCO2, number of moles of carbon dioxide in a liquid phase nH2O, number of moles of water nIL, number of moles of ionic liquid mCO2, molality of carbon dioxide expressed as moles of CO2 per kilogram of solvent mixture (IL + H2O)
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REFERENCES
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