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Flammability and Thermal Analysis Characterization of Imidazolium-Based Ionic Liquids Douglas M. Fox,*,† Jeffrey W. Gilman,‡ Alexander B. Morgan,§ John R. Shields,‡ Paul H. Maupin,| Richard E. Lyon,⊥ Hugh C. De Long,# and Paul C. Trulove∇ Department of Chemistry, American UniVersity, Washington, D.C. 20016, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, UniVersity of Dayton Research Institute, Dayton, Ohio 45469, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Federal AViation Administration, Atlantic City International Airport, New Jersey 08405, Air Force Office of Scientific Research, Arlington, Virginia 22203, and Chemistry Department, U.S. NaVal Academy, Annapolis, Maryland 21402
Room-temperature ionic liquids have been identified as nonvolatile, nonflammable compounds with a wide range of applications. However, numerous thermal studies have identified volatile decomposition products and a source for fuel, raising questions regarding the fire hazard of ionic liquids. To address these questions, the flammability properties of imidazolium-based ionic liquids have been measured using cone calorimetry and microscale combustion calorimetry. The combustion data are compared to flashpoints estimated from thermal gravimetric analysis data. The resulting flammability properties of ionic liquids are comparable to aliphatic hydrocarbon plastics (polyethylene and polyamide) and lower than high boiling organic solvents (ethyl lactate and dimethyl sulfoxide). Several structure-property relationships are observed, including alkyl chain length and anion type. Introduction Over the past decade, interest in ionic liquids, which are salts that melt below 100 °C, has grown exponentially1–10 because they exhibit unique properties that make them candidates for use in many applications. Ionic liquids have a wide spectrum of potential applications, including use as alternative solvents for many types of chemical transformations,1–4 as catalysts,5–8 and to make nanoparticles compatible with polymers.9–11 Ionic liquids are typically composed of organic cations and inorganic anions. Some of the more common ions that can be combined to form ionic liquids are shown in Figure 1, although these represent only a small fraction of the possible ionic liquids. The fundamental physical properties of ionic liquids have only just begun to be studied. Previously, we have reported on the thermal analysis and flammability properties (flashpoint) of imidazoliumbased ionic liquids.11–13 Recently, ionic liquids have been accurately subcharacterized as “combustible”, class IIIB liquid.14–16 (The general definition of “flammable” is “capable of being easily ignited and of burning rapidly”. The National Fire Protection Association (NFPA), the U.S. Department of Transportation (DOT), and the U.S. Occupational Safety and Health Association (OSHA) agreed upon more specific classifications to remove inconsistencies in the term “flammable”. According to OSHA Standard No. 1910.106, a “flammable liquid” means any liquid having a flashpoint below 100 °F (37.8 °C), whereas a “combustible liquid” means any liquid having a flashpoint at or above 100 °F (37.8 °C). Both “flammable liquids” and “combustible liquids” must adhere to similar additional fire safety protocols.) However, the flammability * To whom correspondence should be addressed. E-mail: dfox@ american.edu. † American University. ‡ National Institute of Standards and Technology. § University of Dayton Research Institute. | U.S. Department of Energy. ⊥ Federal Aviation Administration. # Air Force Office of Scientific Research. ∇ U.S. Naval Academy.
properties and potential fire hazards of a material cannot be defined by a single flammability test.16,17 Indeed, when considering fire protection and emergency response procedures, the heat release characteristics are essential.17–19 This is especially important when considering the industrial and commercial applications of ionic liquids, such as their use as solvents, battery electrolytes, or lubricants. So, the question of whether ionic liquids are “flammable” or “combustible liquids” is not as important as their overall flammability properties. In the case of an industrial fire or a fire involving materials containing ionic liquids, the ionic liquids will act as fuels and emit combustible decomposition products. With the growing interest in ionic liquids, it would be very useful to have quantifiable flammability and fire testing data on ionic liquids. Here we report on the cone calorimetry and microscale combustion calorimetry of the family of ionic liquids shown in Figure 2. Experimental (This work was carried out by the National Institute of Standards and Technology (NIST) and other agencies of the U.S. Government (DOT, DOE, DoD) and by statute is not subject to copyright in the United States. Certain commercial equipment, instruments, materials or companies are identified in this paper in order to adequately specify the experimental procedure. This in no way implies endorsement or recommendations by any government agencies contributing to this work.) Materials. All ionic liquids were synthesized using previously published methods. Structural characterization data on these have also been published previously.11–13,20–22 Ethyl lactate (Aldrich), dimethyl sulfoxide/DMSO (Aldrich), and polypropylene (PP, extrusion grade, DuPont) were used as received. All gases were high purity reagent grade. TGA. Thermal stabilities were measured using a TA Instruments Q-500 Thermogravimetric Analyzer. Samples of 5.0 ( 0.2 mg were placed in open platinum pans and heated at a scan rate of 10 °C/min while purged with 100 mL/min N2. The mean
10.1021/ie800665u CCC: $40.75 2008 American Chemical Society Published on Web 07/09/2008
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Figure 1. Selection of the more common cations and anions used to make ionic liquids.
Figure 2. Imidazolium ionic liquid structures.
of three replicate measurements is reported. The temperatures of both the onset (5% mass fraction loss) and peak mass loss rate have an uncertainty of σ ) ( 2 °C. SDT. Simultaneous DSC and TGA measurements were carried out using a TA Instruments SDT600 Simultaneous DSC/ TGA. Samples of 3.0 ( 0.2 mg were placed in open alumina pans and heated at a scan rate of 10 °C/min while purged with 100 mL/min of ultra pure nitrogen. Data were collected during the first scans. Cone Calorimetry. A high-throughput modification of a standard fire (cone) calorimetry method was used to evaluate the flammability of ionic liquids in this study. Cone calorimetry is based on the principle of oxygen consumption calorimetry,23 whereby the depletion of oxygen in the combustion gases is measured and used to calculate the heat released in flaming combustion. Flammability parameters such as heat release rate (HRR), mass loss rate (MLR), and heat of combustion (HOC) are measured. In the cone calorimeter, the sample sits on a load cell while being exposed to a radiant heat flux from a coneshaped electric heater. A spark igniter is used to ignite the pyrolysis products at the surface as they mix with air and initiate combustion. The standard cone calorimeter procedure was modified in the present study. In particular, sample holders were pans made of brass foil, 3 cm in diameter and 1 cm deep. Two pans were used, together with a layer of insulating material between them, as the sample holder. In a typical experiment, ionic liquid (2.5
g) was placed in a brass foil pan and the pan was placed on the load cell under the conical heater in the calorimeter (calibrated for a radiant heat flux of 35 kW/m2). To ignite the specimen, an electric spark was brought within 2.5 cm of the sample surface. As the sample burned, its mass loss and the oxygen consumed from the combustion gases were measured continuously. When the sample had burned completely, new samples were placed on the load cell for the next experiment, and data were acquired continuously. This semicontinuous operation allowed the flammability of up to 12 samples to be measured each hour. The semicontinuous experimental approach was undertaken because of our interest in developing high-throughput flammability measurement methods24 and to allow characterization of ionic liquids that were only available in small quantities. Microscale Combustion Calorimetry (MCC). Pyrolysiscombustion flow calorimetry (PCFC)25 is a relatively new technique that measures thermal combustion properties of milligram samples using controlled heating and oxygen consumption calorimetry in nonflaming mode. Because the PCFC sample size is 10-6 times the amount required for cone calorimetry, PCFC is also called microscale combustion calorimetry (MCC). In the present work, the sample was thermooxidatively decomposed in synthetic air (20 volume percent oxygen in nitrogen) at a heating rate of 1 °C/s between 100 and 650 °C, depending on thermal stability, and the gaseous thermal oxidative decomposition products were completely oxidized at 900 °C in a combustion tube. The oxygen depletion of the combustion gas stream was used to calculate the specific combustion rate Q (W/g-K) as a function of sample temperature, and the total heat released for the test Hc (J/g) was determined from the time integral of Q(t). Experiments were conducted on 12 samples (6 liquids, 6 solids) in a commercial MCC (MCC1, The Govmark Organization, Inc.). A general schematic of the instrument is shown in Figure 3. Each sample was tested in triplicate, and the typical relative error was ( 5%. Results and Discussion Thermal Analysis and Flashpoints. The decomposition temperatures of imidazolium compounds from thermal gravimetric analysis have been studied extensively.11,12,26–28 We have previously shown that this data can be used to approximate the
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Figure 4. Heat flow from SDT data of several ionic liquids under N2 flow. Only the dicyanamide ionic liquid exhibits an exothermic decomposition. Figure 3. Schematic drawing of the MCC.
flashpoints of the imidazolium salts with a slight overestimation.12,13 This is a clear indication that ionic liquids are, in fact, combustible and not nonflammable as often reported in the literature.4,29–31 For comparison to the fire tests discussed in the next sections, the estimated flashpoints based on TGA data for the fuels used in this study are reported in Table 1. The combustion studies conducted previously by Smiglak et al.15 were conducted for ionic liquids with high nitrogen content exhibiting exothermic decompositions and potential uses as energetic materials. However, the majority of ionic liquids currently under investigation have relatively low nitrogen contents and typically exhibit endothermic decompositions. Heat flow from simultaneous differential scanning calorimeter-thermal gravimetric analyzer (SDT) data for the decomposition of the ionic liquids used in this study, shown in Figure 4, illustrates this point. The dicyanamide ionic liquid exhibits an exothermic decomposition in the first half of the decomposition temperature range (250-300 °C), and is therefore the exception. The use of cone calorimetry and microscale combustion calorimetry experiments can elucidate the combustibility of materials whether the decompositions are exothermic or endothermic. Cone Calorimetry. The flammability properties of imidazolium-based ionic liquids have been measured using cone
calorimetry and microscale combustion calorimetry. The ionic liquids evaluated are shown in Table 1. Typical HRR curves obtained by semicontinuous data acquisition for small-scale samples are shown in Figure 5. These data show the repeatability of the heat release rate measurement using this technique. Typical repeatability of the maximum heat release rate was ( 10% (σ). The HRR data comparing two ionic liquids (BMI-BF4 and BMI-PF6) to dimethyl sulfoxide (DMSO) and ethyl lactate (EtLac) are shown in Figure 7. BMI-PF6 has a peak HRR less than one-quarter of that for EtLac and DMSO. We have previously shown that during the thermal decomposition of imidazolium-based ionic liquids the alkyl-imidazolium parent compound can be found in the pyrolysis gases at temperatures above the onset of decomposition.11 The organic imidazolium cation contains combustible carbon, nitrogen, and hydrogen atoms, which are the sources of the fuel and the HRR. BMI-PF6 has a peak HRR less than one-third of that for BMI-BF4 and has an ignition delay time twice that of BMI-BF4. The presence of phosphorus in addition to fluorine in the gas phase during the decomposition of PF6 may inhibit gas phase reactions that lead to ignition. Thus, the heat released is reduced in subsequent flaming combustion of BMI-PF6 compared to BMI-BF4. Since the onset decomposition (ignition) temperatures in nitrogen and
Table 1. Estimated Flashpoints for Fuels Used in this Study sample abbreviation BMI-Cl BMI-Br BMI-BF4 BMI-PF6 DMBI-Cl DMBI-Br DMBI-BF4 DMBI-PF6 DMBI-TFSI DMBI-N(CN)2 DMHDI-Cl DMHDI-Br DMHDI-BF4 DMHDI-PF6 PP DMSO EtLac a
d
R butyl butyl butyl butyl butyl butyl butyl butyl butyl butyl hexadecyl hexadecyl hexadecyl hexadecyl
H H H H methyl methyl methyl methyl methyl methyl H H H H
flashpointa (°C)
Xs
name
Cl BrBF4PF6ClBrBF4PF6N(SO2CF3)2N(CN)2ClBrBF4PF6-
1-butyl-3-methylimidazolium chloride 1-butyl-3-methylimidazolium bromide 1-butyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium hexafluorophosphate 1-butyl-2,3-dimethylimidazolium chloride 1-butyl-2,3-dimethylimidazolium bromide 1-butyl-2,3-dimethylimidazolium tetrafluoroborate 1-butyl-2,3-dimethylimidazolium hexafluorophosphate 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfoyl)imide 1-butyl-2,3-dimethylimidazolium dicyanamide 1-hexadecyl-2,3-dimethylimidazolium chloride 1-hexadecyl-2,3-dimethylimidazolium bromide 1-hexadecyl-2,3-dimethylimidazolium tetrafluoroborate 1-hexadecyl-2,3-dimethylimidazolium hexafluorophosphate polypropylene dimethyl sulfoxide (-)-ethyl L-lactate
R′ -
Flashpoints are reported as TGA estimates (ASTM D92-05a Cleveland Open Cup flashpoints). Data taken from ref 13.
b
Data taken from ref 11.
c
260b n/a (224d) 432 398c 283c 304c (234d) 460b 458c 467c 295 265 460c 430c 433 79 (89c) 54 (46c)
Data taken from ref .
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Figure 7. HRR data for ethyl lactate, DMBI-Br, DMBI-N(CN)2, BMI-BF4, and BMI-PF6.
Figure 5. Heat release rate data for three replicate experiments from burning BMI-BF4 in the cone calorimeter.
Figure 8. Voluminous char structure from combustion of DMBI-N(CN)2.
Figure 6. HRR data for DMSO, ethyl lactate, BMI-BF4, and BMI-PF6.
decomposition endotherms for both compounds are similar (see Figure 4), thermal criterion for ionic liquid ignition would predict that times to ignition should be the same. These observations suggest a gas phase criterion for the ignition of ionic liquids.32 In addition to the ignition delay associated with P- and F-anions, the peak HRR of the BMI-PF6 ionic liquid is reduced by a factor of over four compared to conventional solvents suggesting a significant fire hazard reduction. The effect of changing from a BF4 to a PF6 anion, observed above (Figure 6) for the dialkyl-imidazolium salts, is also observed for trialkylimidazolium salts. The HRR data for both dialkyl- and trialkylimidazolium salts as well as ethyl lactate are shown in Figure 7. Surprisingly, the dicyanamide salt (DMBI-N(CN)2) has a peak HRR three times that of the ethyl lactate and 10 times that of the BMI-PF6. Presumably, this effect is not due to the additional methyl group at the C-2 position on the imidazolium ring but is associated with the dicyanamide anion, since the trialkyl-imidazolium bromide (DMBI-Br) does not exhibit such a high peak HRR. Figure 8 shows photographs of three samples of the dicyanamide ionic liquid following combustion in the
Figure 9. SDT data for DMBI-N(CN)2 under nitrogen.
cone calorimeter. The formation of pyrolysis char has also been noted by others for pyrrolidinium dycyanamides (but not quaternary phosphonium dicyanamides).28 This corroborates their findings that polymerization occurs most readily when all ions present contain nitrogen atoms. These black foam char samples represent a 10% char yield, which usually would be expected to provide a reduced heat release rate due to the insulating properties of such a low density foam char. However, from the SDT data (Figure 9), we found that the pyrolysis of the dicyanamide ionic liquid is initially exothermic. Schnick and co-workers have found that this pyrolysis reaction is exothermic because of the formation of the aromatic compound melamine and its higher oligomers.33 It is possible that the
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Figure 11. MCC data for specific combustibility (Q) of BMI (8-carbons), DMBI (9-carbons), and DMHDI (21-carbons) ionic liquids.
Figure 10. HRR data for DMBI-Br, DMBI-N(CN)2, DMHDI-Cl, DMHDI-PF6, and PP.
exothermic reaction produces internal heating which, in addition to the effect of the external heat flux from the cone heater, also increases the mass loss and heat release rates. Moreover, the char formation process from dicyanamide does not occur until the very end of the combustion process, so the char forms too late to act as a protective thermal barrier. The effect of increasing the alkyl chain length on the flammability of the imidazolium salts was also evaluated. Figure 10 shows HRR data for the two ionic liquids with hexadecyl alkyl chains attached to the imidazolium ring, DMHDIM-Cl and DMHDIM-PF6, and two with butyl alkyl chains, DMBIM-Br and DMBIM-N(CN)2. Comparison of the HRR data for the PF6 salts in Figure 10 and Figure 7 shows that the introduction of the hexadecyl alkyl chain almost doubles the peak HRR of ionic liquid. As discussed above, the presence of high-fuelcontent organic groups in ionic liquids produces pyrolysis products upon heating which are combustible. Increasing the alkyl content in the imidazolium salt increases the fuel fraction and heat of combustion of the ionic liquid. This is also shown in the comparison of heat flow during decomposition of DMBI-Br to DMHD-Cl (Figure 4). The 3-fold increase in peak HRR for the chloride as compared to the PF6 salt is due to both the thermal stability of these two salts (the onset of degradation for the chloride salt is approximately 250 °C, whereas that for the PF6 salt is close to 400 °C),11 the inhibiting effect of the P and F on gas phase combustion for the PF6 salt, as discussed above for BMI-PF6, and the larger mole ratio of these flame inhibiting elements (P, F) to combustable elements (C, H, and N) in BMI-PF6 (5:25) as compared to the mole ratio of Cl to C, H, and N in DMHD-Cl (1:64). The HRR data and decomposition onset vs ignition times for the ILs are compared to that for polypropylene (PP), a commodity plastic, in Figure 10. In most cases, comparable ignition times, but higher HRR and lower decomposition onset temperatures, are observed for the ILs over that of PP. Comparison with data shown in Figure 6 reveals that PP has a peak HRR similar to solvents like ethyl lactate and DMSO, but a longer ignition time. This confirms that the ILs flammability properties are comparable to aliphatic hydrocarbon polymers but more flammable than aromatic hydrocarbon polymers. Furthermore, the data indicate the profound affect the anions have on the combustion of ILs, with PF6- anions inhibiting flammability and nucleophilic anions, such as N(CN)2- and halide ions, enhancing flammability.
Figure 12. Comparison of specific combustibility Q and heat of combustion Hc, from MCC and peak HRR in the cone calorimeter (peak HRR) for ionic liquids.
Microscale Combustion Calorimetry. The MCC was used to evaluate the 11 ionic liquids from Table 1. The specific combustibilities, Q expressed in J/(g · K), of this set of ionic liquids (Figure 11) reveals the same trends found in the cone calorimeter (see Figure 12) with the exception of DMBI-N(CN)2. The cone peak HRR for this sample is much higher than would be expected if the Q results were used as an indication. This is an effect of the different combustion conditions (complete versus incomplete) or different sample size (5 mg versus 2500 mg) of the MCC and cone calorimeter, respectively. With regard to the latter, the heat of exothermic pyrolysis of the dicyanamide anion may not contribute significantly in the small MCC sample, whereas it may in the larger cone sample. Conclusions Cone calorimetry and MCC reveal that most imidazoliumbased ionic liquids are less flammable than the common molecular organic solvents ethyl lactate and DMSO. Several structure-property relationships are also evident from the data gathered here for imidazolium-based ionic liquids: (a) thermally stable anions and those with potential for gas phase combustion inhibition increase flashpoints and ignition time and decrease heat release rate in flaming combustion (HRR) and nonflaming combustion (Q). (b) Exothermic decomposition is associated with increased heat release (HRR). (c) An increase in the combustible mass fraction of an ionic liquid will increase its heat release rate (HRR and Q). The correlation between HRR and Q indicates that MCC is an excellent alternative to assess fire hazards of materials in instances where the sample size
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available is limited and the heat of combustion is not too exothermic, where self-heating becomes significant. According to the results presented here, the flammability characteristics of ionic liquids are similar to those of aliphatic hydrocarbon plastics such as polypropylene. Consequently, whether ionic liquids are defined as “combustible liquids” or “flammable”, they possess significant fire hazards and will act as a fuel source in large-scale fire situations. It would therefore be more appropriate to describe ionic liquids as having “low or reduced flammability hazards”, rather than identifying them as “nonflammable” materials. Furthermore, ionic liquids like aliphatic hydrocarbon plastics may benefit from the addition of flame retardant additives to improve fire safety. Acknowledgment We would like to thank the Air Force Office of Scientific Research (F1ATA06300J001) and the Federal Aviation Administration (ISSA-DTFA0003-92-Z-0018) for partial funding of this work. Literature Cited (1) Wasserscheid, P.; Welton, T., Eds. Ionic Liquids in Synthesis; WileyVCH: Weinheim, Germany, 2003. (2) Rogers, R. D.; Seddon, K. R., Eds. Ionic Liquids: Industrial Applications for Green Chemistry; American Chemical Society: Washington, DC, 2002. (3) Borodkin, G. I.; Shubin, V. G. Electrophilic reactions of aromatic and heteroaromatic compounds in ionic liquids. Russ. J. Org. Chem. 2006, 42, 1745. (4) El Seoud, O. A.; Koschella, A.; Fidale, L. C.; Dorn, S.; Heinze, T. Applications of ionic liquids in carbohydrate chemistry: A window of opportunities. Biomacromolecules 2007, 8, 2629. (5) Wasserscheid, P.; Keim, W. Ionic Liquids s New “solutions” for transition metal catalysts. Angew. Chem., Int. Ed. 2000, 39, 3773. (6) Malhotra, S. V.; Kumar, V.; Parmar, V. S. Asymmetric catalysis in ionic liquids. Curr. Org. Synth. 2007, 4, 370. (7) Nara, S. J.; Naik, P. U.; Harjani, J. R.; Salunkhe, M. M. Potential of ionic liquids in greener methodologies involving biocatalysis and other synthetically important transformations. Ind. J. Chem. B-Org. Chem. 2006, 45, 2257. (8) Harjani, J. R.; Naik, P. U.; Nara, S. J.; Salunkhe, M. M. Enzyme mediated reactions in ionic liquids. Curr. Org. Synth. 2007, 4, 354. (9) Aida, T.; Fukushima, T. Soft materials with graphitic nanostructures. Phil. Trans. R. Soc. A-Math. Phys. Eng. Sci. 2007, 365, 1539. (10) Bellayer, S.; Gilman, J. W.; Eidelman, N.; Bourbigot, S.; Flambard, X.; Fox, D. M.; De Long, H. C.; Trulove, P. C. Preparation of homogeneously dispersed multiwalled carbon nanotube/polystyrene nanocomposites via melt extrusion using trialkyl imidazolium compatibilizer. AdV. Funct. Mater. 2005, 15, 910. (11) Awad, W. H.; Gilman, J. W.; Nyden, M.; Harris, R. H., Jr.; Sutto, T. E.; Callahan, J.; Trulove, P. C.; De Long, H. C.; Fox, D. M. Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites. Thermochim. Acta 2004, 409, 3. (12) Fox, D. M.; Awad, W. H.; Gilman, J. W.; Maupin, P. H.; De Long, H. C.; Trulove, P. C. Flammability, thermal stability, and phase change characteristics of several trialkylimidazolium salts. Green Chem. 2003, 5, 724. (13) Fox, D. M.; Awad, W. H.; Gilman, J. W.; Maupin, P. H.; Trulove, P. C.; De Long, H. C. Thermal and kinetic studies of trialkylimidazolium
salts. Ionic Liq. IIIA: Fundam., Progress, Challenges, Opportunities, Prop. Struct. 2005, 901, 193. (14) http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id) 9752&p_table)STANDARD; http://www.nfpa.org/faq.asp?categoryID)920. (15) Smiglak, M.; Reichert, M.; Holbrey, J. D.; Wilkes, J. S.; Sun, L.; Thrasher, J. S.; Kirichenko, K.; Singh, S.; Katritzky, A. K.; Rogers, R. D. Combustible ionic liquids by design: is laboratory safety another ionic liquid myth. Chem. Commun. 2006, 2554. (16) Apte, V. B., Ed. Flammability Testing of Materials Used in Construction, Transport, and Mining; CRC Press: Boca Raton, FL, 2006. (17) Kittle, P. A., Ed. Plastics and Polymers Used as Alternate Daily CoVers: A Summary of Technical Information from the Public Domain; Rusmar Inc.: West Chester, PA, 1993. (18) Babrauskas, V.; Grayson, S. J. Heat Release in Fires; Chapman and Hall: London, UK, 1995. (19) Burke, R. Hazardous Materials Chemistry for Emergency Responders, 2nd ed.; CRC Press: Boca Raton, FL, 2002. (20) Wilkes, J. S.; Zaworotko, M. J. Air and Water Stable 1-Ethyl-3Methylimidazolium Based Ionic Liquids. J. Chem. Soc., Chem. Commun. 1992, 965. (21) Chauvin, Y.; Mussman, L.; Olivier, H. A Novel Class of Versatile Solvents for Two-Phase Catalysis: Hydrogenation, Isomerization, and Hydroformylation of Alkenes Catalyzed by Rhodium Complexes in Liquid 1,2-Dialkylimidazolium Salts. Angew. Chem., Int. Ed. 1995, 34, 2698. (22) Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; De Souza, R. F.; Dupont, J. The Use of New Ionic Liquids in Two-Phase Catalytic Hydrogenation Reaction By Rhodium Complexes. Polyhedron 1996, 15, 1217. (23) Test Method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter, ASTM E 1354, American Society for Testing and Materials International: West Conshohockent, PA. (24) Wilkie, C. A.; Chigwada, G.; Gilman, J. W.; Lyon, R. E. HighThroughput Techniques for the Evaluation of Fire Retardancy. J. Mater. Chem. 2006, 16, 2023. (25) Lyon, R. E.; Walters, R. N.; Stoliarov, S. I. A Thermal Analysis Method for Measuring Polymer Flammability. J. ASTM Int. 2006, 3, 1. (26) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermal properties of imidazolium ionic liquids. Thermochim. Acta 2000, 357, 97. (27) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. Thermophysical properties of imidazolium-based ionic liquids. J. Chem. Eng. Data 2004, 49, 954. (28) Wooster, T. J.; Johanson, K. M.; Fraser, K. J.; MacFarlane, D. R.; Scott, J. L. Thermal degradation of cyano containing ionic liquids. Green Chem. 2006, 8, 691. (29) Keskin, S.; Kayrak-Talay, D.; Akman, U.; Hortacsu, O. A review of ionic liquids towards supercritical fluid applications. J. Supercrit. Fluids 2007, 43, 150. (30) Hagiwara, R.; Lee, J. S. Ionic liquids for electrochemical devices. Electrochem. 2007, 75, 23. (31) Seki, S.; Ohno, Y.; Kobayashi, Y.; Miyashiro, H.; Usami, A.; Mita, Y.; Tokuda, H.; Watanabe, M.; Hayamizu, K.; Tsuzuki, S.; Hattori, M.; Terada, N. Imidazolium-Based Room-Temperature Ionic Liquid for Lithium Secondary Battery. J. Electrochem. Soc. 2007, 154, A173. (32) Lyon, R. E. Plastics and Rubber, In Handbook of Building Materials for Fire Protection; Harper, C. A., Ed.; McGraw-Hill: New York, 2004; Chapter 3, pp 3.1-3.51. (33) Lotsch, B. V.; Schnick, W. Towards novel C-N materials: crystal structures of two polymorphs of guanidinium dicyanamide and their thermal conversion into melamine. New J. Chem, 2004, 28, 1129.
ReceiVed for reView April 23, 2008 ReVised manuscript receiVed May 30, 2008 Accepted June 14, 2008 IE800665U
