Ionic Liquids as Green Solvents - American Chemical Society

Downloaded by UNIV MASSACHUSETTS AMHERST on July 26, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch005

Chapter 5

New Ionic Liquids Based on Alkylsulfate and Alkyl Oligoether Sulfate Anions: Synthesis and Applications 1

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Peter Wasserscheid , Roy van Hal , Andreas Bösmann , Jochen Eβer , and Andreas Jess 1

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Institut für Technische Chemie und Makromolekulare Chemie, University of Technology at Aachen, Worringer Weg 1, D-52074 Aachen, Germany Lehrstuhl für Chemische Verfahrenstechnik, Universität Bayreuth, Universitätsstrasse 30, D-95440 Bayreuth, Germany 2

Typical ionic liquids consist of halogen containing anions such as [AlCl ] , [PF ] , [BF ] , [CF SO ] or [(CF SO ) N] . However, for many technical applications the presence of halogen atoms in the ionic liquid's anion may cause concerns if the hydrolytic stability of the anion is poor (e. g. for chloroaluminate and hexafluorophosphate systems) or if a thermal treatment of the spent ionic liquid is desired. In this contribution, synthesis, properties and application of several new alkylsulfate and alkyloligoethersulfate ionic liquids are presented. The described systems are easily available from technical raw materials. Some candidates combine low melting points with high hydrolytic stability and acceptable viscosity. Some of the new ionic liquids have been tested as catalyst layer in the Rh-catalyzed hydroformylation of 1-octene. -

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© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Why develop new, halogen-free ionic liquids? The historical development of ionic liquids can be structured according to the different classes of anions that were found to form low melting salts in combination with imidazolium, pyridinium, ammonium and phosphonium cations. Low melting chloroaluminate salts can be regarded as the "first generation ionic liquids". They were described as early as in 1948 by Hurley and Wier at the Rice Institute in Texas as bath solutions for electroplatinating aluminum (7). Later in the seventies and eighties, these systems were intensively studied by the groups of Osteryoung (2), Wilkes (3), Hussey (4) and Seddon (4hj). In 1992, ionic liquid methodology received a substantial boost when Wilkes and Zaworotko described the synthesis of non-chloroaluminate, room temperature liquids (e. g. low melting tetrafluoroborate melts) which may be regarded as "secondgeneration ionic liquids" (6). Nowadays, tetrafluoroborate and [the slightly later published(7)] hexafluorophosphate ionic liquids are among the "working horses" in ionic liquid research. However, their use in many technical applications is still limited by their relatively high sensitivity vs. hydrolysis. The tendency of anion hydrolysis is of course much less pronounced than for the chloroaluminate melts but still existent. The [PF ]' anion of 1-butyl-3methylimidazolium ([BMIM]) hexafluorophosphate - for example - has been found in our laboratories to completely hydrolyze after addition of excess water when the sample was kept for 8h at 100°C. HF (toxic and highly corrosive) and phosphoric acid was formed. Under the same conditions hydrolysis of the tetrafluoroborate ion of [BMIM][BF ] was observed as well, however to a much smaller extent (S). Consequently, the application of tetrafluoroborate and hexafluorophosphate ionic liquids is effectively restricted- at least under a technical scenario - to those applications where water-free conditions can be realized at acceptable costs. In 1996, Grâtzel, Bonhôte and coworkers published synthesis and properties of ionic liquids with anions containing CF - groups and otherfluorinatedalkyl groups (9). These do not show the same sensitivity towards hydrolysis than [BF ]" and [PF ]" containing systems. In fact, heating [BMIM][(CF S0 ) N] with excess of water to 1G0°C for 24h did not reveal any hint for anion hydrolysis (9). However, despite the very high stability of these salts against hydrolysis and a number of other very suitable properties (e. g. low viscosity, high conductivity, high thermal stability, easy preparation in halogen-free form due to miscibilitygap with water etc.) the high price of [(CF S0 ) N]" and of related anions may be a major problem for their practical application in larger quantities. Moreover, the 6

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In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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presence of fluorine in the anion may still be problematic even if hydrolysis is not an issue. Besides the elevated price of the anion (which is also related to the presence of fluorine), the relatively obvious idea to dispose technical amounts of spent ionic liquid by thermal treatment becomes complicated with these ionic liquids. Additional efforts to avoid the liberation of toxic and highly corrosive HF during the combustion of these systems is needed. In the last two years, an interesting process can be observed in the research aiming for the development of new ionic liquids. Depending on the complexity of the combination of properties required and the amount of ionic liquid consumed for a given application, the recently developed ionic liquids can be divided in two groups: The first group falls under the definition of "bulk ionic liquids". This means a class of ionic liquids that is designed to be produced, used and somehow consumed in larger quantities. Applications for these ionic liquids are expected to be solvents for organic reactions, homogeneous catalysis, biocatalysis and other synthetic applications with some ionic liquid consumption as well as non synthetic applications such as the application as heat carriers, lubricants, additives, surfactants, phase transfer catalysts, extraction solvents, solvents for extractive distillation, antistatics etc. Cation and anion of these "bulk ionic liquids" are chosen to make a relatively cheap (expected price on a multihundred litre scale: ca 30€/litre) and toxologically well-characterized liquid [a preliminary study about the acute toxicity of a non-chloroaluminate ionic liquid has been recently published(iO)]. The second group comprises highly specialised, task-specific ionic liquids that of course - will be used in much smaller quantities. Fields of applications for the latter are expected to be special solvents for organic synthesis, homogeneous catalysis, biocatalysis and all other synthetic applications with very low ionic liquid consumption (e. g. due to very efficient multiphasic operation). Nonsynthetic applications for these materials are analytic applications (stationary or mobile phases for chromatography, matrices for MS etc.), sensors, batteries etc. These ionic liquids are designed and optimised for the best performance in highvalue-adding applications. Concerning the first group of "bulk ionic liquids" it can be expect that only a very limited number of candidates will be selected for an industrial use on larger scale. However, these candidates will become well characterised and - due to their larger production quantities - readily available. From the above mentioned considerations concerning price, hydrolysis stability and disposal options we anticipate that all future "bulk ionic liquids" will contain no halogen atoms.



60 A number of halogen-free ionic liquids are already known from the literature. However, none of these systems fulfils the complex combination of properties that is - according to our experiences - required for such a technically suitable "bulk ionic liquid": a) melting point or glass point below 40°C; b) thermal stability > 250 °C; c) stability vs. hydrolysis in neutral aqueous solution up to 80°C; d) possible disposal by combustion without formation of highly corrosive gases; e) possible biodégradation of the used anion in ordinary waste water treatment; f) synthesisfromcheap, technical available raw materials e. g. alkali salts. Imidazolium salts with nitrate (6), nitrite (6), sulfate (6), benzene sulfonate (77) and phosphonium salts with toluene sulfonate (72) anions are described in the literature but their reported melting points are usually higher then 40 °C. Hydrogensulfate and hydrogenphosphate ionic liquids abstract their protons in aqueous solution to form acidic solutions (73) Ionic liquids with methylsulfate and ethylsulfate anions show significant hydrolysis in aqueous solution at 80°C. Hydrogensulfate is formed together with the corresponding alcohol (14).

In this contribution we like to give a brief description of our recent research on new, halogen-free ionic liquids which fulfil the above mentioned technical criteria in a more promising manner. The synthesis and some properties of selected, new alkylsulfate and alkyloligoethersulfate ionic liquids will be presented together with preliminary results on their physico-chemical properties. Some of the described systems have been applied as catalyst solvent in the Rhcatalyzed hydroformylation of 1-octene.

Synthesis of Octylsulfate Ionic Liquids Na[w-C H| 0-S0 ] is a commercial chemical which is produced in a multithousand ton scale as detergent and ingredient for cosmetics [e. g.fromCognis GmbH, DUsseldor&Germany (75) in >87% purity;main impurities are inorganic water soluble salts e. g. Na S0 ]. From the technical application of this salt it becomes quite clear that hydrolysis stability, biological degradation and toxicity of the octylsulfate anion are very well documented. For the ionic liquid synthesis, this anion was combined with the chloride salt of the desired cation in a metathesis reaction either by precipitation of NaCl (in dry acetone) or by extraction with CH C1 from an aqueous solution containing Na[fi-C Hi 0-S0 ]. The latter method is preferred because it tolerates water in the sodium salt and allows as well the removal of all ionic impurities originating from the technical quality of the used Na[n-C H O-S0 ]. The synthesis route is displayed in Scheme 1. 8

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[BMIM] Cl + C H O S 0 N a 8

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Scheme 1: Synthesis of 1 -buty1-3-methyl ([BMIM]) octylsulfate. It is well known that impurities in an ionic liquid can have large effects on the physico-chemical properties of the material under investigation (16). For the synthesis of l-butyl-3-methylimidazolium([BMIM]) octylsulfate the following potential impurities were identified, a) organic volatiles (e. g. traces of methylimidazol from the synthesis of the chloride salt); b) halide impurities from incomplete metathesis reaction; c) other ionic impurities resulting from the technical grade of the applied Na[n-C Hi OS0 ] or from some solubility of Na[#i-C Hi OS0 ] in the ionic liquid product; d) water. In order to obtain reliable data for physico-chemical properties of [BMIM][nC Hi OS0 ] we took maximum care to either eliminate the impurities completely during synthesis and purification [in case of a)-c)] or to investigate a material with a clearly defined amount of the impurity (in case of water). To achieve this we checked for volatile impurities (e.g. methylimidazol) in the [BMIM]C1 prior to its application in synthesis, e.g. using the known methods described earlier by Holbrey, Seddon and Wareing (17). The amount of chloride in the final product was checked with AgN0 from an acidic aqueous solution ([BMIM][n-C H OS0 ] is water soluble). Using the above described extraction of [BMIM][n-C Hi OS0 ] from an aqueous solution with CH C1 [+ some additional washing of the CH C1 phase with small portions of water (18)] the product could be easily obtained in chloride-free quality. A chloride-free quality of the ionic liquid is not only important for the determination of physicochemical data but also crucial for the application of the material in hydroformylation catalysis. Apart from chloride impurities, one has to consider that the synthesis of [BMIM][w-C H, OS0 ] from technical [BMIMJC1 and Na[w-C H OS0 ] may result in contamination of the final products with other ionic impurities. Two main sources of such a potential contamination have to be considered here namely the fact that Na[n-C H OS0 ] shows significant solubility in [BMIM][«-C H OS0 ] and the fact that the Na[n-C H OS0 ] in the applied technical quality contains inorganic salts (mainly Na S0 ) in considerable amounts. While ionic impurities such as Na or S0 ~ may not be a problem for some catalytic applications of the ionic liquid (such as e.g. in Rh- catalyzed hydroformylation) it is of great relevance for the determination physico-chemical properties of the melt. 8

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62 Fortunately, the applied synthetic method by extraction of [BMIM][nCgHi OS0 ] from water with CH C1 allows to remove all ionic impurities due to their high water solubility. Concerning the amount of water in [BMIM][n-C Hi OS0 ] we found it very difficult to reduce the latter in large samples to less than 200 ppm by evaporation at 80°C under high vacuum (Iff bar). This reflects the highly hygroscopic nature of the "dry" octylsulfate salt and gives rise to the question whether anybody would use such an ionic liquid under absolute dry condition. Therefore, we decided to determine physico-chemical data for [BMIM][it-C H OS0 ] of a defined, low water content rather than for the absolute "dry" material. We found that the water content in [BMIM][/i-C H OS0 ] can be adjusted quite reproducibly to 1000± 100 ppm (0.1±0.01 mass%) after a defined drying procedure of the material [80°C; 3h, high vacuum (10* bar)]. This water content was checked by coulometric Karl-Fischer titration prior to the measurements and catalytic experiments. 7

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Synthesis of alkyloligoethersulfate ionic liquids Alternatively to the application of higher alkylsulfate anions, we were interested to synthesize and investigate ionic liquids with alkyloligoethersulfate anions as well. The motivation for this research stem from the fact that we had found in earlier research that the replacement of alkyl groups by oligoether groups at the ionic liquid's cation can decrease the ionic liquid's viscosity to a significant extent (8) [thesefindingshave been very recently confirmed by Afonso and coworkers (19)]. Therefore we were interested to look for similar effects on the anion side by replacing the alkyl group in higher alkylsulfate anions by a alkyloligoether group. A very promising access to alkyloligoethersulfate ionic liquids uses technically available oligoethylenglycol monoalkylethers of the general type R-(0-CH CH ) -OH as starting materials. Sulfation of these alcohols can be readily achieved with pyridine-S0 (at room temperature) or with N H S 0 H (80-160 °C) yielding quantitatively the corresponding pyridinium or ammonium sulfate salts respectively. For a later cation exchange by metathesis (to obtain e.g. the corresponding imidazolium salts) the synthesis of the ammonium salt is preferred since the ammonium cation allows a simple metathesis reaction with all chloride salt in dry CH C1 under precipitation of NH4CI. The overall route for the synthesis of alkyloligoethersulfate ionic liquids and two examples prepared by this method are presented in Scheme 2. 2

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[BMIM][E(EG) OS0 ] Downloaded by UNIV MASSACHUSETTS AMHERST on July 26, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch005

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Ό Scheme 2: General synthesis route for the preparation of alkyloligoethersulfate ionic liquids and two examples that have been prepared following this route. It is noteworthy, that the here described synthesis method for the new alkyloligoethersulfate ionic liquids may be more generally applied to prepare a whole range of different ionic liquids with funetionalized alkylsulfates from their correspondung alcohols. Concerning the quality of the ionic liquids obtained after the here described method, the main problems occur during the metathesis step. While the sulfatation reaction proceeds smoothly to quantitative yield, it is not easy to dry the resulting, very hygroscopic ammonium salt to very low levels of water. Consequently, during the metathesis reaction it is not easy to obtain a fully halide-free product. Therefore all physico-chemical data for alkyloligoethersulfate ionic liquids discribed in the contribution will be related to the amount of chloride detected in the sample under investigation.

Selected physico-chemical properties of alkylsulfate and alkyloligoethersulfate ionic liquids

Thermal properties [BMIM][n-C Hi70S0 ] is often obtained as a sub-cooled melt which slowly crystallizes only below room temperature. The crystalline material has a melting point of 34-35°C (according to DSC) and the heat of fusion was determined to be 12.7 kJ/mol. [BMIM][n-C HnOS0 ] has a thermal decomposition temperature of 341°C (determined by TGA) and is stable vs. hydrolysis in neutral solution at 80°C for at least 8 hours. 8

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64 In contrast to [BMIM][n-C Hi OS0 ] we never observed so far the crystallization of [BMIM][E(EG)OS0 ] or [BMIM][E(EG) OS0 ]. The determination of a complete set of thermal properties of these two new ionic liquids is actually on-going in our group. 8

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Viscosity As mentioned earlier, the viscosity of an ionic liquid is very dependent on the amount of impurities in the ionic liquid material. While water and other solvents present in the ionic liquid are known to reduce the viscosity drastically, chloride impurities have been found to increase the ionic liquid's viscosity with increasing concentration in the liquid (16). To produce reliable viscosity date we routinely correlate all viscosity data to the specific impurities that are found in the specific sample under investigation. The remaining water was analyzed by coulometric Karl-Fischer titration using a Metrohm 756 K F Coulometer with a Hydranal® Coulomat A G reagent. The chloride content was determined as earlier described by Seddon and coworkers (16). For practical reasons, we decided to determine our viscosity data for [BMIM][nC H OS0 ] for a material 1000± 100 ppm water (0.1±0.01 mass%) (for more details see above). The investigated samples of [BMIM][E(EG)OS0 ] and [BMIM][E(EG) OS0 ] contained less water (166 ppm in case of [BMIM][E(EG)OS0 ] and 127 ppm in case of [BMIM][E(EG) OS0 ]).No chloride was detected in the [BMIM][n-C Hi OS0 ] under investigation, while the amount of chloride in [BMIM][E(EG)OS0 ] and [BMIM]fE(EG) OS0 ] was about SOOppm in each sample (from incomplete metathesis).The viscosity of all samples were determined using RS 100 viscometer from Haake. 8

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The viscosity of [BMIM][n-C H OS0 ] was found to be 874.5 cP at 20°C (as sub-cooled liquid) and 152.3 cP at 50 °C. To compare the viscosity of [BMIM][n-C H| OS0 ] to other known solvent systems it may be of interest to note that [BMIM][n-C Hi OS0 ] reaches at about 45°C the viscosity of [BMIM][PF ] at room temperature [which is 207 cP according to a paper by Bright et al.(20)] and comes at 100°C close to the room temperature viscosity of ethanediol [16.1 cP according to reference(2i)]. The viscosities of [BMIM][E(EG)OS0 ] and [BMIM][E(EG) OS0 ] are surprisingly low. The viscosity of [BMIM][E(EG) OS0 ] allows a direct comparison with [BMIM][n-C Hi OS0 ] since the number of chain members at the substituted sulfate is identical and only two CH -groups are replaced by ether bridges in the case of [BMIM][E(EG) OS0 ]. For this ionic liquid in the above described quality we determined a viscosity of 568 cP (20°C) which clearly demonstrates the effect of the ether functionalities on the viscosity. In fact, the 8

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65 difference is supposed to be even more significant for a comparison of very pure materials since the viscosity for dry [BMIM][n-CgH| OS0 ] is supposed to be higher than 873 cP (the water in the investigated sample has obviously a viscosity decreasing effect) while the viscosity of halide free [BMIM] [E(EG) OS0 ] can be expected to be even lower than 568 cP (due to the known viscosity increasing effect of chloride impurities (16). Finally, for [BMIM][E(EG)OS0 ] a viscosity of 92cP was determined at 20°C. This value indicates that viscosity is decreasing with shorter substituents at the sulfate ion even for alkyloligoethersulfate systems. This value represents - to our best knowledge - the lowest reported viscosity for a halogen-free ionic liquid based on alkylsulfate, alkylsulfonate or related anions, so far. 7

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Catalytic Application in the Rh-catalyzed hydroformylation of 1-octene Since the pioneering work of Chauvin et al. who described in 1995 the first Rhcatalyzed biphasic hydroformylation using room temperature liquid ionic liquids (22,23) the research efforts in this field were largely dominated by attempts to improve the immobilization of phosphine ligands in the ionic liquid solvent. The use of phosphine ligands with cobaltocenium(24), guanidinium(25,26), imidazolium and pyridinium(27) or sulfonate(22)were described by our group and others. Thus coming closer to an industrial realization of hydroformylation reactions using ionic liquids, potential users of this technology have drawn our attention to the fact that the application of ionic liquids containing fluorine atoms [which have been almost exclusively used in the earlier work; for one exception see (12)] gives rise to serious concerns with regard to the ionic liquid's price, stability and disposal options (see earlier in this contribution). Therefore it appeared highly interesting to test our new halogen-free systems as solvent for the Rh-catalyzed hydroformylation of 1-octene. So far, only the octylsulfate systems have been evaluated in the hydroformylation catalysis in more detail since the residual chloride contaminants in the alkyloligoethersulfate ionic liquids act as a catalyst poison for the Rh-catalyst. All catalytic experiments were carried out with [BMIM][n-C Hi OS0 ] synthesised after the above described method. As catalyst system we used a Rh(acac)(CO)2 precursor in combination with the earlier described (25) phenylguadinium modified triphenylphosphine ligand precursor 1. Under reaction conditions the anion attached to 1 is readily exchanged by the anion of the ionic liquid. 8

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The results of the hydroformylation experiments are given in Table 1.



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Table 1: Rh-catalyzed hydroformylation of 1-octene in different ionic liquids. Ionic liquid [BMIM] [Octylsulfate] [BMIM] [Octylsulfate] + C Hi 6

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TOF(h") 892 862

n;/-ratio (Sel.) 2.86/74% 2.53/72%

For comparison [BMIM] [PF ] biphasic 276 2.00/67% [BMIM] ÏBF4I biphasic 317 2.60/72% conditions: 25-28 barCO/H (1:1), 100°C, lh, l-octene/Rh= 1000, 5 ml IL, Rhprecursor: Rh(acac)(CO) ,2 eq. of ligand precursor 1 as hexafluorophosphate salt. 6

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67 Remarkably, the reaction mixture with [BMIM][/î-C Hi OS03] does not form a biphasic system any more at high 1-octene conversion. Consequently the reaction mixture becomes monophasic during reaction. However, with cyclohexane added as extraction solvent the biphasic reaction system is maintained even at very high 1-octene conversion and the ionic catalyst solution can be easily separated from the colorless product layer by simple décantation. Interestingly, the activity of the Rh-catalyst is significantly higher with [BMIM][ii-C H| OS0 ] being the solvent in comparison to commonly used hexafluorophosphate and tetrafluoroborate ionic liquids with the same cation. This may be due to the higher 1-octene solubility in [BMIM]fn-C H OS03] (600 mmol/mol IL at 25°C) vs. [BMIM][PF ] (25 mmol/mol IL at 25°C). Another advantage may arise from the fact that the fluoride anion is a wellknown catalyst poison for the Rh-catalyst and the formation of traces of fluoride during the reaction conditions can not be excluded absolutely if hexafluorophosphate ionic liquids are used. For all examples displayed in Table 1 the ratio between linear and branched hydroformylation products is between 2 and 3 which is the expected range for triphenylphosphine derived ligands. 8


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Conclusion For good reasons, ionic liquids are often discussed as 'green solvents*. Besides their negligible vapor pressure which prevents solvent evaporation into the atmosphere, two additional options are of interest for transition metal catalysis. Firstly, the special solubility characteristics of the ionic reaction medium enables often a biphasic operation mode of the reaction allowing effective separation of the catalyst from the product and catalyst recycling. Secondly, the non-volatile nature of ionic liquids allows a more effective product isolation by distillation. However, the use of typical ionic liquids consisting of halogen containing anions (such as [A1C1 ]*, [PF ]\ [BF ]\ [CF3SO3]" or [(CF S0 )2N]") restrict in some regard their 'greenness'. The presence of halogen atoms may cause serious concerns if the anion hydrolyzes under the reaction conditions or if thermal or biological treatment of a spent ionic liquids is desired. In this context, we propose halogen-free octylsulfate and alkyloligoethersulfate ionic liquids as promising candidates for many catalytic, biocatalytic and engineering applications. Although it is still much too early for a final evaluation of the usefulness of these systems (mainly due to missing physico-chemical data), their synthesis from technically available, cheap starting materais and their halogenfree character make these systems especially promising for many of the expected, future "bulk" application of ionic liquids. 4

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69 22 Chauvin, Y.; Muβmann, L.; Olivier, H. Angew. Chem. 1995, 107, 29412943; Angew. Chem. Int. Ed. Engl. 1995, 34, 2698-2700. 23 Chauvin, Y.; Olivier, H.; Muβmann, L. EP 0776 880 A1, 1996, to IFP. 24 Brasse, C. C.; Englert, U.; Salzer, Α.; Waffenschmidt, H.; Wasserscheid, P. Organometallics 2000, 19, 3818-3823. 25 Wasserscheid, P.; Waffenschmidt, H.; Machnitzki, P.; Kottsieper, K. W.; Stelzer, O. Chem. Commun. 2001, 451-452. 26 Favre, F.; Olivier-Bourbigou, H.; Commereuc, D.; Saussine, L . Chem. Commun. 2001, 1360-1361. 27 Brauer, D. J.; Kottsieper, K. W.; Liek, C.; Stelzer, O.; Waffenschmidt, H.; Wasserscheid, P. J. Organomet. Chem. 2001, 630, 177-184.


Ionic Liquids as Green Solvents - American Chemical Society

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