Capture of Dioxins by Ionic Liquids - American Chemical Society

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Environ. Sci. Technol. 2008, 42, 2570–2574

Capture of Dioxins by Ionic Liquids P R A S H A N T S . K U L K A R N I , * ,† LUÍS C. BRANCO,† JOÃO G. CRESPO,‡ A N D C A R L O S A . M . A F O N S O * ,† CQFM, Departamento de Engenharia Química e Biológica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, and REQUIMTE, Departamento de Química, FCT-Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal

Received October 24, 2007. Revised manuscript received December 22, 2007. Accepted December 27, 2007.

Dioxins are highly toxic compounds that mainly originate from incineration and combustion sources. In this work, a new, simple, and efficient approach for the absorption of dioxins from gaseous streams using thermally stable ionic liquids is proposed. The absorption process of nonchlorinated and chlorinated dibenzo-p-dioxin compounds was studied in the temperature range 100-200 °C. Imidazolium-, ammonium-, and guanidinium-based ionic liquids were designed for this specific purpose. It was observed that imidazolium cations having long alkyl side chains exhibit the highest absorption capacities, whereas the anion dicyanoamide [DCA] possesses higher absorption capacity than other anions studied. In a typical experiment, it was found that the ionic liquid 1-n-octyl3-methyl imidazolium dicyanoamide [C8mim][DCA] can absorb more than 14% by weight of dibenzo-p-dioxin, 2-chlorodibenzo-p-dioxin, and 1,2,3,4-tetrachlorodibenzo-p-dioxin from a gaseous stream. A process for desorption of dioxins from the ionic liquid was tested, revealing that complete desorption can be achieved under a high vacuum. Additionally, the feasibilityoftheprocesswasexaminedbycarryingoutexperiments under real operating conditions of incineration and combustion processes. The success of the method heavily relies upon the design and selection of specific ionic liquids having enhanced affinity for the aromatic compound functionality present in dioxins and, simultaneously, possessing extremely low volatility and high chemical and thermal stability.

Introduction Dioxins are considered to be among the most dangerous pollutants on earth (1). They originate from incineration and combustion sources and are also formed as unintentional byproduct of several chemical processes. Dioxins usually occur as a mixture of congeners and are a class of structurally and chemically related polyhalogenated aromatic hydrocarbons that mainly include polychlorinated dibenzodioxins (PCDDs), dibenzofurans (PCDFs), and the “dioxin-like” biphenyls (PCBs) (2). There are 75 possible congeners of PCDD, 135 possible congeners of PCDF, and 209 possible congeners of PCB. They constitute a group of persistent environmental chemicals and, hence, are toxic in nature. The toxicity of dioxins is expressed as toxic equivalent * Corresponding author phone: +351-218417627; fax: +351218464455; e-mail: [email protected] (P.S.K.), [email protected] (C.A.M.A.). † Instituto Superior Técnico. ‡ FCT-Universidade Nova de Lisboa. 2570

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quantities (TEQs). Several adverse health effects, such as soft tissue sarcomas, lymphomas, skin lesions (chloracne), stomach cancer, immune system, and neurological effects are associated with dioxins (3). Therefore, many organizations such as the World Health Organization (WHO), the United States Environmental Protection Act (US-EPA), the United Kingdom Committee on Toxicity (UK-COT), and the European Union (EU) nations have labeled them as the most potent carcinogenic compounds and have defined stringent actions for their disposal in the environment. A variety of treatment processes for dioxin abatement has been proposed by scientists and engineers (4). These processes include adsorption on carbon (5), catalytic destruction (6), thermal and hydrothermal treatment (7), radiolytic degradation (8), biodegradation (9), and the use of nonthermal plasma (10), among others. Recently, the use of catalytic filters with membranes was proposed for the destruction of dioxins (11). Evaluation and selection of an appropriate dioxin remediation technology depends on the concentration, source, and nature of these compounds and on other factors, such as safety and economical considerations. A cleaner and energy efficient technology for the dioxin capture, concentration and destruction from gaseous streams is always in demand to meet its stringent discharge standard limit of 0.1 ng/m3-TEQ (12, 13). Dioxins may be captured by ionic liquids, which are a class of organic salts that exist as liquids at temperatures lower than 100 °C (14–17). A significant characteristic of ionic liquids is their immensely low vapor pressure (18, 19). Therefore, most of them are generally regarded as “green” solvents because they do not contaminate the atmosphere, contrary to conventional volatile organic compounds (VOC) (20). Ionic liquids also exhibit various attractive properties, such as good thermal and chemical stability, nonflammability, high ionic conductivity, and a large electrochemical window. They may be in the liquid state even at temperatures as high as 400 K, and their density can be greater than that of water. Therefore, they have been extensively investigated as solvents or cocatalysts in many green chemical synthesis and separation processes (21), extraction with organic solvents and scCO2 (22), pervaporation (23), and extraction with supported liquid membranes (24). This work demonstrates the absorption of dioxin compounds using high-temperature stable ionic liquids as a selective and versatile separation process able to quantitatively capture dioxins from gaseous streams. It may offer a unique separation solution for the removal and concentration of dioxins from gaseous effluents, formed during incineration and combustion operations, because these processes work at high temperatures. Ionic liquids may withstand the high temperature of these gaseous effluents and, simultaneously, exhibit an extremely low vapor pressure (18). Therefore, the safety and environmental friendliness of absorptive separation techniques can be improved with the use of ionic liquids. It was previously observed that ionic liquids solubilize gases, such as SO2, CO2, ethylene, and ethane, especially at high pressure (25–27). Recently, we have shown the possibility to absorb organic vapors using ionic liquids (28).

Materials and Methods 2-Chlorodibenzo-p-dioxin and 1,2,3,4-tetrachlorodibenzop-dioxin were purchased from AccuStandard Inc., Europe; dibenzo-p-dioxin was purchased from TCI, Europe; and trin-octyl methyl ammonium chloride (Aliquat 336) was purchased from Aldrich. The salts, 1-benzyl 3-methyl imi10.1021/es702687x CCC: $40.75

 2008 American Chemical Society

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FIGURE 1. Schematic of the apparatus used for absorption of dioxin compounds using ionic liquids. dazolium chloride, 1,3-dibenzyl imidazolium chloride, 1-noctyl 3-methyl imidazolium chloride, 1-n-decyl 3-methyl imidazolium chloride, tetra-n-hexyl-dimethylguanidinium chloride, and the other salts resulted of anion exchange were prepared in the laboratory. Detailed experimental procedures and characterization of the ionic liquids used have been described elsewhere (29–31). All other chemicals were obtained from Fluka and were used as received. Flash chromatography was carried out on silica gel 60 M from Macherey-Nagel (MN) (ref. 815381) or on aluminum oxide basic from MN (ref. 815010, Brockmann activity 1). Information about spectral data and thermal properties of ionic liquids and representative gas chromatograms of selective absorption is available in the Supporting Information. General Procedure for Absorption Experiments. A scheme of the apparatus used for absorption of dioxin FIGURE 2. Structures of the dioxin compounds (a) and ionic compounds is presented in Figure 1. A specially devised glass liquids (b) used for the absorption studies. chamber (absorption cell) with a capacity of 30 mL was used for the absorption studies (Figures S1–S3). A glass vial with The difference between the weight of the dioxin presenting a capacity of 2 mL was used for the ionic liquids and dioxins in the glass vial before and after the experiment gave the compounds. Dioxin compounds (50–100 mg) under invesvalue of the concentration of dioxin present in the vapor tigation were placed in one of the vials, and the ionic liquid phase at a particular temperature. The vapor phase was also (500–1000 mg) was placed in the other vial, with a magnetic analyzed by collection of vapors from the glass assembly stirrer. The glass chamber and vials were weighed accurately using a 100 mL syringe, which was subsequently dissolved before the start of the experiment. The whole equipment into the eluent containing internal standard (benzophenone). was kept inside a sand bath. The sand bath containing the The results were obtained using GLC with the prior calibration absorption cell was held at constant temperature using a measurements. Both methods provided comparable results, forced air circulation oven, which acted as an incineration and the concentration of dioxin in the vapor phase was found tool. The temperature knob of the oven was calibrated to to be in the range of 0.1-0.12 µg/mL. attain the desired temperature. Absorption runs were perGeneral Procedure for Desorption Experiments. After formed for 4-100 h at temperatures ranging from 100 to 200 the absorption experiment, the vial containing ionic liquid °C. Fluctuation in temperature was in the range of and absorbed dioxin compound was placed into the vacuum (3 °C. The control experiments of each ionic liquid, in the apparatus system. It consisted of a glass chamber, where the absence of dioxin, have not shown any changes in the vial was introduced, that is connected to a receiving trap at chemical and thermal stability and in the weight of the ionic low temperature (using liquid nitrogen) (Figure S4). The glass liquids. Utmost care was taken to maintain the system chamber was kept in the oil bath at a constant temperature droplet-free to avoid formation of any sorbed dioxin on the of 100 °C, under vacuum (0.2 mbar), for 24 h. The receiving glass surface. Finally, the toxic gas generated was absorbed trap was washed with dichloromethane, and the percentage in the ionic liquid. The ionic liquid containing the absorbed absorption of dioxin in the ionic liquids was determined using dioxin compound was weighed in an analytical balance to GLC. The recovered ionic liquid was free from the dioxin find out the absorption gain. In addition, it was passed compound. through a column of silica (eluent: ethyl acetate and ether, 40:60) to remove the ionic liquid; then, the eluent was Results and Discussion analyzed using gas chromatography (GLC) with benzopheThe dioxin compounds used for the absorption studies are none as an internal standard. depicted in Figure 2a. Thermally stable ionic liquids having General Procedure for Analysis of Dioxin in the Vapor degradation temperatures higher than 200 °C were prepared, Phase. Concentration of dioxin in the vapor phase was based on the combination of the cations containing tri-nanalyzed by gravimetric measurement and GLC; a control octyl methyl ammonium [Aliquat]+, 1-benzyl-3-methylexperiment was performed at 100–200 °C without ionic liquid. VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Absorption of dibenzo-p-dioxin by ionic liquid [Aliquat][DCA] as a function of time. imidazolium [Bzmim]+, 1,3 dibenzyl-imidazolium [BzimBz]+, 1-n-octyl-3-methyl-imidazolium [C8mim]+, 1-n-decyl-3methyl-imidazolium [C10mim]+, and dimethyl-tetra-hexyl guanidinium [DMG]+ units and the anions bis-trifluoromethanesulfonimide [Tf2N]-, trifluoro methane sulfonate [TfO]-, trifluoro acetate [TFA]-, 2-sulfobenzoic acid imide or saccharine [SAC]-, and dicyanamide [DCA]- (Figure 2b). Their degradation temperature, along with density and water content data, are reported in Table S1 of the Supporting Information (31). Absorption studies were performed by measuring the amount of dioxin compounds present in the ionic liquid after constant exposure, above room temperature (100–200 °C), to a saturated air stream containing a synthetically simulated gaseous dioxin stream. A control experiment in the absence of ionic liquid was performed to measure the concentration of dioxin in the vapor phase at a given temperature. Method Validation. The system was validated by measuring the amount of dibenzo-p-dioxin absorption in the ionic liquid [Aliquat][DCA] at 100 °C. Equilibrium was achieved within 48 h. The measured dibenzo-p-dioxin capacity was found to be 15.6% by weight at equilibrium (Figure 3). The gain in mass of the ionic liquid, due to absorption, was measured by weighing and by GLC. At this equilibrium, the molar ratio of dibenzo-p-dioxin to [Aliquat][DCA] was 0.39:1 (0.156 g of dioxin g-1 of IL). The 1H NMR spectrums of the dibenzo-p-dioxin treated [Aliquat][DCA] and untreated are presented in Figure S5 of the Supporting Information. Integration of the NMR spectra also exhibited the same molar ratios. The assignment of the resonances of the [Aliquat][DCA] is shown in detail, elsewhere (31). Two new resonances were observed at δ ) 6.85 ppm (multiplet, 4H) and 6.81 ppm (multiplet, 4H), and the 13C NMR spectra shows three new resonances at δ ) 116.21, 123.68, and 142.03 ppm, corresponding to the structures of dibenzo-p-dioxin (Figure S6). Similarly, FTIR spectra of the dibenzo-p-dioxin-absorbed [Aliquat][DCA] and dibenzo-pdioxin-free ionic liquid [Aliquat][DCA] showed only the new additional absorption bands characteristic of dibenzo-pdioxin alone (Figure S7). These observations may suggest that there is no considerable specific bond formed between the ionic liquid [Aliquat][DCA] and dibenzo-p-dioxin during absorption. Probably, the absorption of dioxin in the ionic liquids is merely governed by weak van der Waals forces of attraction and/or via the formation of inclusion type compounds (32). This impressive absorption capacity of the ionic liquid, in one equilibrium stage, clearly demonstrates the enormous potential of this media for the efficient capture of dioxins present in the environment at several contaminated sources, with diverse concentrations (4). Effect of Ionic Liquid Cations and Anions on the Absorption Process of Dioxins. Screening for an optimal solution also involved the design and synthesis of high2572

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FIGURE 4. Effect of ionic liquid cations on the absorption of dibenzo-p-dioxin. (T ) 100 °C.)

FIGURE 5. Effect of ionic liquid anions on the absorption of dibenzo-p-dioxin. (T ) 100 °C.) temperature stable ionic liquids. Several types of cations and anions were tried in order to obtain ionic liquids having degradation temperatures higher than 200 °C. The cations were imidazolium, ammonium, and guanidinium based, and the anions were fluorine, nitrogen, and sulfur based. From these combinations, 10 best candidates having absorption capacities equal to or higher than 1% by weight of dioxin were selected. First, the effect of cations on the absorption process of dibenzo-p-dioxin was studied by keeping the anion [DCA] constant. Figure 4 shows that, at equilibrium, the ionic liquid took up to 4.1% [DMG][DCA], 4.8% [Bzmim][DCA], 6.2% [BzimBz][DCA], 15.6% [Aliquat][DCA], 18% [C8mim][DCA], and 19.4% [C10mim][DCA] by weight of dibenzo-p-dioxin. It indicates that, within the imidazolium group, ionic liquids containing a long alkyl chain favors the absorption process over the aromatic ones. It is interesting to note that absorption further increases from the C8 to C10 alkyl side chain within the imidazolium group. However, the guanidinium cation has shown the lowest absorption capacity for dibenzo-p-dioxin. The absorption ability of the ammonium ionic liquid was greater than that of the imidazolium compounds having an aromatic ring and that of the guanidinium compounds. The effect of the anions on the absorption process of dibenzo-p-dioxin was studied by selecting the cation [C8mim] and varying the anions, such as [DCA], [SAC], [SCN], [TfO], and [Tf2N]. Figure 5 shows that the ionic liquid [C8mim][SAC] has the lowest absorption capacity of 1%, whereas the ionic liquid [C8mim][DCA] has the highest. In this series, the absorption of dibenzo-p-dioxin increases in the following order: [C8mim][SAC] < [C8mim][Tf2N] < [C8mim][SCN] < [C8mim][TfO] < [C8mim][DCA]. These studies demonstrate

FIGURE 6. Effect of temperature on the absorption of dibenzo-p-dioxin in the ionic liquid [C8mim][Tf2N]. that the absorption ability of the ionic liquids is extremely dependent on the selected combination of cation and anion structure, in which the ionic liquid anion-cation and ionic liquid-dioxin balance interactions play an important role. Role of Dioxin Type. Dioxins are polychlorinated aromatic compounds whose toxicity depends on the number and position of chlorine atoms attached to the aromatic ring. It was, therefore, thought desirable to study the effect of dioxin type on the process of absorption. Two chlorinated dioxin compounds, namely, 2-chlorodibenzo-p-dioxin and 1,2,3,4tetrachlorodibenzo-p-dioxin, were selected for the test. Experiments were performed under similar conditions of temperature (100 °C) and time (48 h) using the ionic liquid [C8mim][DCA]. The results obtained for these chlorinated dioxins were compared with those for dibenzo-p-dioxin. The absorption capacity of the ionic liquid [C8mim][DCA] was found to be of 18% by weight of dibenzo-p-dioxin, 19.2% by weight of 2-chlorodibenzo-p-dioxin, and 14.8% by weight of 1,2,3,4-tetrachlorodibenzo-p-dioxin. These results indicate that absorption of 2-chlorodibenzo-p-dioxin is the highest in comparison with the other two dioxins. We presume that the lowest absorption of 1,2,3,4-tetrachlorodibenzo-p-dioxin in the ionic liquid [C8mim][DCA] is due to the high molecular weight (321.97 g mol-1) of the compound, which may have a lower vapor equilibrium. These studies highlight that the process is equally applicable to any chlorinated dioxin congeners. Effect of Temperature. In authentic conditions, the release of dioxins from incineration and combustion sources occurs at temperatures higher than 100 °C. Hence, studies were carried out to verify the effect of temperature. This effect was studied using dibenzo-p-dioxin and the ionic liquid [C8mim][Tf2N]. Figure 6 shows that with the increase in temperature from 100 to 200 °C the absorption capacity of the ionic liquid [C8mim][Tf2N] slowly decreases from 7.5 to 6%. However, this decrease is found to be much lower in comparison with the absorption behavior of SO2 in ionic liquids (25). This may be attributed to the organic nature of the dioxin gas. However, as expected, the absorption kinetics of the dioxin was found to be faster with the increase in temperature; the equilibrium time decreased from 48 h at 100 °C to 4 h at 200 °C. Further, there was no loss or degradation of the ionic liquid [C8mim][Tf2N] at 200 °C for 4 h. These results demonstrate that this process may also be conducted at temperatures as high as 200 °C. Simulation Experiments. Under real operating conditions the dioxins are formed in technical incineration processes by “de novo” synthesis on fly ashes in a temperature range of 200–400 °C by metal catalysis (2). Usually, they occur adsorbed on these solid particles. To test the applicability of

the present method to real conditions, a wood-burned fly ash was used and 10% of dibenzo-p-dioxin was physically immobilized to it. The immobilization procedure was done by dissolving both components in dichloromethane solvent; then, the solvent was evaporated to dryness. The ionic liquid [C8mim][Tf2N] was used for this absorption study at 200 °C, for 4 h. It was found that the ionic liquid [C8mim][Tf2N] shows the same absorption capability as that previously observed under neat conditions (fly ash, 5.98% by weight; neat, 6% by weight). These findings mimic the actual remediation conditions of dioxin formation in incineration and combustion processes. Desorption of Dioxins from the Ionic Liquids. A process for desorption of dioxins, back from the ionic liquids, is also necessary in order to accomplish the overall process of dioxin remediation. Desorption of dioxins from ionic liquids was found to be prompt compared to the absorption process. The desorption method was validated using the ionic liquid [Aliquat][DCA] absorbed dibenzo-p-dioxin, under vacuum at a constant temperature of 100 °C. A quantitative amount of dibenzo-p-dioxin was found to be desorbed from the ionic liquid [Aliquat][DCA] by applying vacuum of 0.2 mbar for 24 h. Additionally, 1H NMR analysis of the [Aliquat][DCA] was performed and, as expected, it has not shown any traces of dibenzo-p-dioxin, which also confirmed that a complete desorption of dibenzo-p-dioxin was achieved. Therefore, it is possible to reuse the same ionic liquid repeatedly for the absorption of dioxins. In conclusion, the unique properties of the ionic liquids designed and selected, which exhibit a high capability to absorb the dangerous dioxin compounds from a vapor phase above room temperature, together with their minimal volatility and thermal stability (in some case above 300 °C), opens new perspectives to academia, the environmental scientist, and industry (33). This process may be used to develop efficient and specifically designed equipment (e.g., ionic liquids immobilized or incorporated in inorganic supports, such as particles and membranes) for absorption of dioxins directly from gaseous dioxin sources at high temperatures. Captured dioxins can then be safely recovered by desorption from the ionic liquid media, concentrated, and destroyed using the most appropriate available technologies. This process may demand significantly less energy than current remediation technologies of dioxins.

Acknowledgments This work was supported by the Fundação para a Ciência e a Tecnologia and FEDER (ref SFRH/BPD/14848/2003). We gratefully acknowledge Solchemar company (http://www. solchemar.com) for providing some of the ionic liquids.

Supporting Information Available Supporting Information for this article, containing analytical tools used, thermal properties of the ionic liquids, photographs of the absorption and desorption apparatus, typical spectral data (1H and 13C NMR, FTIR), and gas chromatogram, is available free of charge via the Internet at http:// www.pubs.acs.org.

Literature Cited (1) Stanmore, B. R. The formation of dioxins in combustion systems. Combust. Flame 2004, 136, 398–427. (2) McKay, G. Dioxin characterization, formation and minimization during municipal solid waste (MSW) incineration: review. Chem. Eng. J. 2002, 86, 343–368. (3) Giesy, J. P.; Kannan, K. Dioxin-like and non-dioxin-like toxic effects of polychlorinated biphenyls (PCBs): Implications for risk assessment. Crit. Rev. Toxicol. 1998, 28, 511–569. (4) Kulkarni, P. S.; Crespo, J. G.; Afonso, C. A. M. Dioxins Sources and Current Remediation Technologies—A review. Environ. Int. 2008, 34, 139–153. VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

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(5) Liljelind, P.; Unsworth, J.; Maaskant, O.; Marklund, S. Removal of dioxins and related aromatic hydrocarbons from flue gas streams by adsorption and catalytic destruction. Chemosphere 2001, 42, 615–623. (6) Everaert, K.; Baeyens, J. Catalytic combustion of volatile organic compounds. J. Hazard. Mat. 2001, 109, 113–139. (7) Gerasimov, G. Y. Degradation of dioxins in electron-beam gas cleaning of sulphur and nitrogen dioxides. Radiation Chem. 2001, 35, 427–431. (8) Buekens, A.; Huang, H. Comparative evaluation of techniques for controlling the formation and emission of chlorinated dioxins/furans in municipal waste incineration. Chemosphere 1998, 62, 1–33. (9) Bunge, M.; Adrian, L.; Kraus, A.; Opel, M.; Lorenz, W. G.; Andreesen, J. R.; Görisch, H.; Lechner, U. Reductive dehalogenation of chlorinated dioxins by an anaerobic bacterium. Nature 2003, 421, 357–360. (10) Zhou, Y. X.; Yan, P.; Cheng, Z. X.; Nifuku, M.; Liang, X. D.; Guan, Z. C. Application of non-thermal plasmas on toxic removal of dioxin-contained fly ash. Powder Technol. 2003, 135, 345–353. (11) GORE, REMEDIA: Catalytic filtration systems 2000. Available at http://www.gore.com/remedia. Accessed August 2007. (12) U.S. Environmental Protection Agency, Office of Health and Environmental Assessment. Health Assessment document for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. August 1994; EPA: Washington, DC. Available at http:// www.epa.gov/ncea/pdfs/dioxin/. Accessed August 2007. (13) WHO European Centre for Environmental and Health. International Program on Chemical Safety (Assessment of the health risk of dioxins), WHO Consultation, May, 1998, Geneva. European Dioxin Inventory—Vol. 1, Part A, 2000. Office of Science and Technology: http://www.netl.doe.gov/; SRI International Corporation—Available at http://www.sri.com/psd/ technologies/jetrempi_dio.html/. (14) Rogers, R. D.; Seddon, K. R. Ionic Liquids: Industrial Applications for Green Chemistry; American Chemical Society: Washington DC, 2002. (15) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; WileyVCH: Weinheim, 2003. (16) Freemantle, M. BASF’s smart ionic liquid. Chem. Eng. News 2003, 81, 9–9. (17) Rogers, R. D.; Seddon, K. R. Ionic liquids—Solvents of the future. Science 2003, 302, 792–793. (18) Earle, M. J.; Esperança, J. M. S. S.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The distillation and volatility of ionic liquids. Nature 2006, 439, 831– 834. (19) Wasserscheid, P. Chemistry—Volatile times for ionic liquids. Nature 2006, 439, 797–797. (20) Zhao, H.; Xia, S.; Ma, P. Use of ionic liquids as “green” solvents for extractions. J. Chem. Technol. Biotechnol. 2005, 80, 1089– 1096.

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9

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(21) Dupont, J. Ionic Liquids: Properties and major applications in extraction/reaction technology. In Green Separation Processes: Fundamentals and Applications; Afonso, C. A. M., Crespo, J. P. S. G., Eds.; Wiley-VCH: Weinheim, 2005. (22) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Green processing using ionic liquids and CO2. Nature 1999, 399, 28–29. (23) Schäfer, T.; Rodrigues, C. M.; Afonso, C. A. M.; Crespo, J. G. Selective recovery of solutes from ionic liquids by pervaporation — A novel approach for purification and green processing. Chem. Commun. 2001, 1622–1623. (24) Branco, L. C.; Crespo, J. G.; Afonso, C. A. M. Highly selective transport of organic compounds by using supported liquid membranes based on ionic liquids. Angew. Chem., Int. Ed. 2002, 41, 2771–2773. (25) Wu, W.; Han, B.; Gao, H.; Liu, Z.; Jiang, T.; Huang, J. Desulphurization of Flue Gas: SO2 Absorption by an Ionic Liquid. Angew. Chem., Int. Ed. 2004, 43, 2415–2417. (26) Huang, J.; Riisager, A.; Wasserscheid, P.; Fehrmann, R. Reversible physical absorption of SO2 by ionic liquids. Chem. Commun. 2006, 4027–4029. (27) Shiflett, M. B.; Yokozeki, A. Solubility and diffusivity of hydrofluorocarbons in room-temperature ionic liquids. AIChE J. 2006, 52, 1205–1219. (28) Kulkarni, P. S.; Branco, L. C.; Crespo, J. P. G.; Afonso, C. A. M. A comparative study on absorption and selectivity of organic vapors using ionic liquids based on imidazolium, quaternary ammonium and guanidinium cations. Chem.—Eur. J. 2007, 13, 8470–8477. (29) Branco, L. C.; Rosa, J. N.; Ramos, J. J. M.; Afonso, C. A. M. Preparation and Characterization of New Room Temperature Ionic Liquids. Chem.—Eur. J. 2002, 8, 3671–3677. (30) Mateus, N. M. M.; Branco, L. C.; Lourenço, N. M. T.; Afonso, C. A. M. Synthesis and properties of tetra-alkyl-dimethylguanidinium salts as a potential new generation of ionic liquids. Green Chem. 2003, 5, 347–352. (31) Kulkarni, P. S.; Branco, L. C.; Crespo, J. G.; Nunes, M. C.; Raymundo, A.; Afonso, C. A. M. Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations. Chem.—Eur. J. 2007, 13, 8477–8488. (32) Holbrey, J. D.; Reichert, W. M; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R.D. Liquid clathrate formation in ionic liquid-aromatic mixtures. Chem. Commun. 2003, 47, 6–477. (33) Kulkarni, P. S.; Branco, L. C.; Crespo J. P. G.; Afonso C. A. M. Removal of Dioxins and Analogous Compounds Using Ionic Liquids and Membrane Systems. Patent pending, PT 103717 (13/04/2007).

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