離子液體當作溶劑的應用
近年來，離子液體在化學上的應用非常廣泛。由於離子液體具有極低蒸氣壓、低熔點、高極性、不可燃性、耐強酸、高熱穩定性、高導電度、電化學性佳及較廣的液體溫度範圍( -96 ~ 400 ℃)等特殊性質，可替代一般所用之揮發性有機溶劑(volatile organic compounds VOCs)，應用在化學合成，而且離子液體可在常壓下操作，不但可降低操作成本，且可消除VOCs對環境的污染，並可避免操作人員暴露於VOCs的風險，可再回收使用，所以離子液體有時被認為是一種新的綠色溶劑“green solvent”[1]。
離子液體是由陰離子及陽離子所組成的有機熔鹽，依不同組合方式，可超過一兆種。鹽類的熔點可高達 801 ℃或低到–96 ℃，所以為方便與高溫熔鹽做區分，通常把熔點低於 100 ℃的熔鹽稱為室溫離子液體(room temperature ion liquids-RTILs)，簡稱為離子液體(IL)，目前所發現的離子液體已超過 200 多種，常用的離子液體結構如圖一[2]。離子液體的陽離子包含有1-alkyl-3-methylimidazolium ([CnMIM]+, n為線性烷基碳的數目)、N-alkylpyridinium([CnPY]+)、tetraalkylammonium及tetraalkylphosphonium等陽離子，這些陽離子可結合不同的有機或無機的陰離子形成數目龐大的離子液體[3]，常見的陰離子有hexafluorophosphate(PF6-)、tetrafluoroborate(BF4-)、trifluoromethylsulfonate(CF3SO3-)、bis[(trifloromethyl)sulfonyl]amide [(CF3SO2)2N]-、trifluoroethanoate(CF3CO2-)、ethanoate(CH3CO2-)及halide(Br-,Cl-,I-)。
雖然早在 1914 年Walden即首先報導低溫的離子液體ethylammonium nitrate[4]，接下來於 1951 年Hurley首先合成室溫離子液體N-ethylpyridinium bromide- aluminium chloride[5]，但一直到 1970 年代，Osteryong和Willkers成功製備出chloroaluminate melts[6]，從此離子液體被大量應用於電化學、反應介質及催化劑。 1992 年，Wilkes等人發展出一係列咪唑(imidazolium)陽離子及BF4-、PF6-等陰離子組成的離子液體[7]，此類離子液體在空氣及水中相當穩定，使得這些類離子液體的應用引起廣大重視[8]，之後離子液體的發展大多以咪唑鹽類為主，進而發展出含DNA離子液體[9]、適合電化學的兩性離子液體[10]、磁性離子液體[11]及以胺基酸作為陰離子的離子液體等特殊功能的離子液體[12]，近年來離子液體的研究趨勢往功能性上發展。
離子液體的物理性質:
親水性：離子液體的親水性主要是取決於陰離子的結構，對水溶解度趨勢如圖一[3]，另外陽離子碳鏈愈長親水性愈差。
酸鹼性：一般離子液體可由陰離子部分判斷其酸鹼性，如表一[13]，因此可藉由陰離子的部分來調控溶劑的酸鹼度，而不必再加入額外的酸或鹼。
熔點：陽離子的對稱性愈低，會影響晶體的堆疊性，使熔點降低，而分子間的氫鍵會使熔點提高，常用雙烷基咪唑鹽類(dialkylimidazolium)的離子液體熔點，如表二。[14]
黏度：由於正負離子的作用力，使得離子液體黏度通常比水的黏度大很多，離子液體黏度的大小受分子間的氫鍵及凡得瓦作用力影響，陽離子碳鏈愈長，凡得瓦力愈強則黏度愈高。對於相同種類的陽離子，不同陰離子所形成的離子液體其黏度高低順序為Cl->PF6->BF4->NO3->(CF3SO3)2N-，如表二。
密度：大部分的離子液體是密度在 1 到 1.6 g/cm3之間，隨著溫度增加密度會降低。
離子液體在化學領域上有許多其特殊的物理性質，例如可溶解許多的無機和有機物質，但與部分的有機溶劑不互溶，可形成兩相反應系統，其優點是反應與分離可同時進行。不具揮發性，在高度真空下操作不易流失。不可燃性及有高的熱穩定性，加上其液體範圍廣，使其可應用的反應溫度範圍廣。另外，還可改變陰離子及陽離子的組成以調控其特性，所以離子液體亦被認為是“designer solvent”。離子液體的這些特殊物理性質，離子液體做為溶劑除了在電化學當作“nonaqueous elelctrolyte”外，其做為溶劑已被廣泛應用，其應用簡述如下：
a.有機合成的應用: 由於離子液體可溶解有機或無機物質，可當有機反應之溶劑取代傳統有機溶劑，不祇可減少傳統揮發性溶劑的危害，有時還可提高反應之選擇性和產率[16]，Jaeger等人首先以離子液體[EtNH3][NO3]進行Diels-Alder反應[17]，與有機溶劑甲苯及四氫夫喃比較[18]，可提高產率及Endo/Exo選擇性。Wasserscheid[19]等人及Ishida[20]等人合成不同對掌性陽離子型式的離子液體，此種對掌性離子液體於不對稱合成時可增加產物的立體選擇性，亦可作為對掌性分離中所填充靜相異構物。使用離子液體於合成時溶劑的應用，在過去十年呈現快速的發展。
b.催化反應的應用：離子液體可與催化劑形成共催化劑，增加催化劑的活性、選擇性及穩定性。例如Friedel–Crafts reaction在傳統條件下須加入固體的AlCl3作為催化劑，而[emim][AlCl3]可以代替催化劑及溶劑增加反應速率及選擇性[21]。離子液體在催化反應上的應用，包含hydrogenation [22]、hydroformylation[23]、olefin dimerisation[22]、Heck reaction[24]、alkoxycarbonylation[25]、catalytic oxidation[26]等反應；在酵素催化方面，離子液體可提高酵素功能[27]，以親水性的離子液體[C4MIM][BF4]不但可溶解有機反應物與生成物，而且可溶解酵素，且酵素的觸媒在其溶液仍具有活性且相當穩定，與在傳統的有機溶劑中酵素容易失去活性兩相比較，具有相當的優勢。
c.雷射脫附基質的應用：由於離子液體的不揮發性及可溶解生物樣品寡醣、蛋白質和高分子，可作為較軟性(soft)的離子源，將其吸收雷射之能量轉移至分析物中，除了可提升基質輔助雷射脫附的離子化效果[28,29]，亦可解決MALDI再現性的問題。
d.氣相層析管柱固定相的應用：在 1986 年Poole等人曾經使用alkylammonium和tetraalkylammonium鹽類作為氣相層析(GC)管柱的固定相[30,31]，但由於這種鹽類操作溫度的限制，降低其實用性。而Armstrong等人使用1-benzyl-3-methylimidazolium trifluoromethanesulfonate及1-(4-methoxyphenyl)-3-methylimidazolium Trifluoromethanesulfonate的離子液體的作為GC液相固定相(GLC)，分析揮發性或半揮發性有機物，分離效果相當好[32]，並利用[C4MIM][Cl]溶解25%(w/w) 的β-Cyclodextrins (β-CDs)[33]及合成具對掌性的離子液體[34]作為GC的對掌性固定相，分離對掌性化合物；由於這類的離子液體可耐高溫的特性，大大提高其商業化的潛力。
e.萃取的應用：疏水性的離子液體可被利用疏水性來萃取水中金屬離子[35,36]與染料萃取[37]，也利用其在柴油中氧化/萃取達成脫硫的目的[38]，Stepnowski使用固相萃取及Liu等人使用液相微萃取來濃縮水中的有機物[39,40]，Andre等人使用頂空式(head space)方法萃取分析物後再以GC分析[41]，離子液體具有特殊溶解性可萃取水溶液中球狀及棒狀金奈米[42]。
f.電化學的應用: 由於離子液體具導電性，可取代傳統的電解液，且有電化學視窗較廣的優點，可改善電化學過程中使用溶劑的偵測限制。使用離子液體在電化學的研究，開啟了離子液體在綠色化學領域之重視，另外也推展其在鋰離子電池[43]、燃料電池[44]、太陽能電池[45]、電容[46]及可偵測O2、CO2、SO2氣體的薄膜電極[47-51]等方面應用。
g.其它功能上的應用：離子液體可溶解纖維素[52]、作為溶膠-凝膠(Sol-Gel)的溶劑[53]及潤滑劑[54]等應用，所以離子液體的應用隨著新的離子液體發現而陸續增加，加上離子液體已突破實驗室的限制，已初步應用於商業發展上[55]，另外，離子液體可結合超臨界CO2於反應、萃取及分離相關應用[56]。
基本上，隨著離子液體的研究發展，其應用將更深入、更廣泛，使其在綠色化學的重要性相對增加，但其邁向廣泛工業上應用仍有許多問題需要克服[57-58]，此有待進一步研究來達成。
http://gc.chem.sinica.edu.tw/new-no-ionic.html
2008年5月16日 星期五
綠色化學的 12 基本法則
1. 預防廢棄物的產生。
2. 充分利用反應物的所有原子。
3. 設計合成方法時，儘量考慮反應物與生成物的毒性。
4. 設計低毒性的化學品。
5. 少用或使用安全的溶劑與輔助物。
6. 為節省能源、降低環境衝擊，反應條件以常溫常壓狀態為主。
7. 使用永續資源為原料。
8. 簡化反應步驟，減少非必要性衍生物的產生。
9. 盡可能使用高選擇性的催化劑。
10. 設計可分解的化學品。
11. 污染物的及時偵測。
12. 慎選製程中的化學物質，以減少意外災害的發生。
http://www.nsc.gov.tw/_newfiles/popular_science.asp?add_year=2005&popsc_aid=140
2. 充分利用反應物的所有原子。
3. 設計合成方法時，儘量考慮反應物與生成物的毒性。
4. 設計低毒性的化學品。
5. 少用或使用安全的溶劑與輔助物。
6. 為節省能源、降低環境衝擊，反應條件以常溫常壓狀態為主。
7. 使用永續資源為原料。
8. 簡化反應步驟，減少非必要性衍生物的產生。
9. 盡可能使用高選擇性的催化劑。
10. 設計可分解的化學品。
11. 污染物的及時偵測。
12. 慎選製程中的化學物質，以減少意外災害的發生。
http://www.nsc.gov.tw/_newfiles/popular_science.asp?add_year=2005&popsc_aid=140
Ionic liquid(From Wikipedia)
An ionic liquid is a liquid that contains essentially only ions. Some ionic liquids, such as ethylammonium nitrate are in a dynamic equilibrium where at any time more than 99.99% of the liquid is made up of ionic rather than molecular species. In the broad sense, the term includes all molten salts, for instance, sodium chloride at temperatures higher than 800 °C. Today, however, the term "ionic liquid" is commonly used for salts whose melting point is relatively low (below 100 °C). In particular, the salts that are liquid at room temperature are called room-temperature ionic liquids, or RTILs. There also exist mixtures of substances which have low melting points, called Deep eutectic solvent, or DES, that have many similarities with ionic liquids.
History
The date of discovery, as well as discoverer, of the "first" ionic liquid is disputed. Ethanolammonium nitrate (m.p. 52-55 °C) was reported in 1888 by Gabriel.[1] However, one of the earlier known truly room temperature ionic liquids was [EtNH3]+ [NO3]- (m.p. 12 °C), the synthesis of which was published in 1914.[2] Much later, series of ionic liquids based on mixtures of 1,3-dialkylimidazolium or 1-alkylpyridinium halides and trihalogenoaluminates, initially developed for use as electrolytes, were to follow.[3][4] An important property of the imidazolium halogenoaluminate salts was that they were tuneable – viscosity, melting point and the acidity of the melt could be adjusted by changing the alkyl substituents and the ratio of imidazolium or pyridinium halide to halogenoaluminate.[5]
A major drawback was their moisture sensitivity and, though to a somewhat lesser extent, their acidity/basicity, the latter which can sometimes be used to an advantage. In 1992, Wilkes and Zawarotko reported the preparation of ionic liquids with alternative, 'neutral', weakly coordinating anions such as hexafluorophosphate ([PF6]-) and tetrafluoroborate ([BF4])-, allowing a much wider range of applications for ionic liquids.[6] It was not until recently that a class of new, air- and moisture stable, neutral ionic liquids, was available that the field attracted significant interest from the wider scientific community.
More recently, people have been moving away from [PF6]- and [BF4]- since they are highly toxic, and towards new anions such as bistriflimide [(CF3SO2)2N]- or even away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium salts (such as choline) being just as flexible a scaffold as imidazole.
Characteristics
Ionic liquids are electrically conductive and have extremely low vapor pressure. (Their noticeable odours are likely due to impurities.) Their other properties are diverse. Many have low combustibility, excellent thermal stability, a wide liquid range, and favorable solvating properties for diverse compounds. Many classes of chemical reactions, such as Diels-Alder reactions and Friedel-Crafts reactions, can be performed using ionic liquids as solvents. Recent work has shown that ionic liquids can serve as solvents for biocatalysis [7]. The miscibility of ionic liquids with water or organic solvents varies with sidechain lengths on the cation and with choice of anion. They can be functionalized to act as acids, bases or ligands, and have been used as precursor salts in the preparation of stable carbenes. Because of their distinctive properties, ionic liquids are attracting increasing attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering; see for instance magnetic ionic liquid.
Despite their extremely low vapor pressures, some ionic liquids can be distilled under vacuum conditions at temperatures near 300 °C.[8] Some ionic liquids (such as 1-butyl-3-methylimidazolium nitrate) generate flammable gases on thermal decomposition. Thermal stability and melting point depend on the components of the liquid. Thermal stability of various RTILs are available. The thermal stability of a task-specific ionic liquid, protonated betaine bis(trifluoromethanesulfonyl)imide is of about 534 K and N-Butyl-N-Methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide was thermally stable up to 640 K [9]
The solubility of different species in imidazolium ionic liquids depends mainly on polarity and hydrogen bonding ability. Simple aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas olefins show somewhat greater solubility, and aldehydes can be completely miscible. This can be exploited in biphasic catalysis, such as hydrogenation and hydrocarbonylation processes, allowing for relatively easy separation of products and/or unreacted substrate(s). Gas solubility follows the same trend, with carbon dioxide gas showing exceptional solubility in many ionic liquids, carbon monoxide being less soluble in ionic liquids than in many popular organic solvents, and hydrogen being only slightly soluble (similar to the solubility in water) and probably varying relatively little between the more popular ionic liquids. (Different analytical techniques have yielded somewhat different absolute solubility values.)
Room temperature ionic liquids
Room temperature ionic liquids consist of bulky and asymmetric organic cations such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and ammonium ions. A wide range of anions is employed, from simple halides, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate and to large organic anions like bistriflimide, triflate or tosylate. There are also many interesting examples of uses of ionic liquids with simple non-halogenated organic anions such as formate, alkylsulfate, alkylphosphate or glycolate. As an example, the melting point of 1-butyl-3-methylimidazolium tetrafluoroborate or [bmim][BF4] with an imidazole skeleton is about -80 °C, and it is a colorless liquid with high viscosity at room temperature.
It has been pointed out that in many synthetic processes using transition metal catalyst, metal nanoparticles play an important role as the actual catalyst or as a catalyst reservoir. It also been shown that ionic liquids (ILs) are an appealing medium for the formation and stabilization of catalytically active transition metal nanoparticles. More importantly, ILs can be made that incorporate co-ordinating groups,[10], for example, with nitrile groups on either the cation or anion (CN-IL). In various C-C coupling reactions catalyzed by palladium catalyst, it has been found the palladium nanoparticles are better stabilized in CN-IL compared to non-functionalized ionic liquids; thus enhanced catalytic activity and recyclability are realized.
Low temperature ionic liquids
Low temperature ionic liquids (below 130 kelvins) have been proposed as the fluid base for an extremely large diameter spinning liquid mirror telescope to be based on the earth's moon.[12] Low temperature is advantageous in imaging long wave infrared light which is the form of light (extremely red-shifted) that arrives from the most distant parts of the visible universe. Such a liquid base would be covered by a thin metallic film that forms the reflective surface. A low volatility is important for use in the vacuum conditions present on the moon.
Food science
Ionic liquids have been used in food science. [bmim]Cl for instance is able to completely dissolve freeze dried banana pulp and the solution with an additional 15% DMSO lends itself to Carbon-13 NMR analysis. In this way the entire banana compositional makeup of starch, sucrose, glucose, and fructose can be monitored as a function of banana ripening.
Applications
Nowadays ionic liquids find a number of industrial applications which vary greatly in character. A few of their industrial applications are briefly described below; more detailed information can be found in a recent review article.
BASIL
The first major industrial application of ILs was the BASIL (Biphasic Acid Scavenging utilizing Ionic Liquids) process by BASF, in which a 1-alkylimidazole was used to scavenge the acid from an existing process. This then results in the formation of an IL which can easily be removed from the reaction mixture.[15] But the easier removal of an unwanted side-product (as an IL rather than as a solid salt) is not the only advantage of the IL based process. By using an IL it was possible to increase the space/time yield of the reaction by a factor of 80,000. It should, however, be kept in mind that improvements of such scale are rare.
Cellulose Processing
Occurring at a volume of some 700 billion tons, cellulose is the earth’s most widespread natural organic chemical and, thus, highly important as a bio-renewable resource. But even out of the 40 billion tons nature renews every year, only approx. 0.2 billion tons are used as feedstock for further processing. A more intensive exploitation of cellulose as a biorenewable feedstock has to date been prevented by the lack of a suitable solvent that can be used in chemical processes. Robin Rogers and co-workers at the University of Alabama have found that by means of ionic liquids, however, real solutions of cellulose can now be produced for the first time at technically useful concentrations [16]. This new technology therefore opens up great potential for cellulose processing.
For example, making cellulosic fibers from so-called dissolving pulp currently involves the use, and subsequent disposal, of great volumes of various chemical auxiliaries, esp. carbon disulfide (CS2). Major volumes of waste water are also produced for process reasons and need to be disposed of. These processes can be greatly simplified by the use of ionic liquids, which serve as solvents and are nearly entirely recycled. The “Institut für Textilchemie und Chemiefasern” (ITCF) in Denkendorf and BASF are jointly investigating the properties of fibers spun from an ionic liquid solution of cellulose in a pilot plant setup.
Eastman chemical’s DHF plant
Eastman operated an ionic liquid-based plant for the synthesis of 2,5-dihydrofuran from 1996 to 2004. However, the plant is now defunct because demand for the product has ceased.
Dimersol - Difasol
The dimersol process is a traditional way to dimerise short chain alkenes into branched alkenes of higher molecular weight. Nobel laureate Yves Chauvin and Hélène Olivier-Bourbigou at IFP (France) have developed an ionic liquid-based add-on to this process called the Difasol process. However, while may be licensed it has as yet not been put into commercial practice.
Petrochina
Petrochina have announced the implementation of an ionic liquid-based process called Ionikylation. This process, the alkylation of C4 olefins with iso-butane, is retrofitted into a 65,000 tonne per year alkylation plant, making it the biggest industrial application of ILs to date.
Degussa paint additives
Ionic liquids can enhance the finish, appearance and drying properties of paints. Degussa are marketing such ILs under the name of TEGO Dispers. These products are also added to the Pliolite paint range.
Air products - ILs as a transport medium for reactive gases
Air products make use of ILs as a medium to transport reactive gases in. Reactive gases such as trifluoroborane, phosphine or arsine, BF3, PH3 or AsH3, respectively, are stored in suitable ILs at sub-ambient pressure. This is a significant improvement over pressurised cylinders. The gases are easily withdrawn from the containers by applying a vacuum.
Linde's IL 'piston'
Whereas Air Product’s Gasguard system relies on the solubility of some gases in ILs, Linde are exploiting other gases’ insolubility in ILs. As mentioned above, the solubility of Hydrogen in ILs is very low. Linde now make use of this insolubility by using a body of ionic liquid to compress Hydrogen in filling stations; and in so doing they reduced the number of moving parts from about 500 in a conventional piston pump engine down to 8.
Nuclear industry
RTILs are extensively explored for various innovative applications in nuclear industry. It includes application of ionic liquid as extractant/diluent in solvent extraction systems, as alternate electrolyte media for the high temperature pyrochemical processing, etc. Fundamental studies on the extraction cum electrodeposition of fission products like uranium, palladium etc., from spent nuclear fuel using RTILs as extractants are reported. Reports on employing using Ionic liquids as non-aquoues electrolyte media for the recovery of uranium [18]and useful fission products like palladium [19] and rhodium [20] from spent nuclear fuel are also available.Studies on the electrochemical behavior of uranium(VI) in ionic liquid, 1-butyl-3-methylimidazolium chloride and also the recovery of valuable fission products from tissue paper waste was studied in room temperature ionic liquids.
Safety
Due to their non-volatility, effectively eliminating a major pathway for environmental release and contamination, ionic liquids have been considered as having a low impact on the environment and human health, and thus recognized as solvents for green chemistry. However, this is distinct from toxicity, and it remains to be seen how 'environmentally-friendly' ILs will be regarded once widely used by industry. Research into IL aquatic toxicity has shown them to be as toxic or more so than many current solvents already in use [22]. A review paper on this aspect has been published in 2007.[23] Available research also shows that mortality isn't necessarily the most important metric for measuring their impacts in aquatic environments, as sub-lethal concentrations have been shown to change organisms' life histories in meaningful ways. According to these researchers balancing between zero VOC emissions, and avoiding spills into waterways (via waste ponds/streams, etc.) should become a top priority. However, with the enormous diversity of substituents available to make useful ILs, it should be possible to design them with useful physical properties and less toxic chemical properties.
With regard to the safe disposal of ionic liquids, a 2007 paper has reported the use of ultrasound to degrade solutions of imidazolium-based ionic liquids with hydrogen peroxide and acetic acid to relatively innocuous compounds.[24]
Despite their low vapor pressure many ionic liquids have also found to be combustible and therefore require careful handling [25]. Brief exposure (5 to 7 seconds) to a flame torch will ignite these IL's and some of them are even completely consumed by combustion.
http://en.wikipedia.org/wiki/Ionic_liquid
History
The date of discovery, as well as discoverer, of the "first" ionic liquid is disputed. Ethanolammonium nitrate (m.p. 52-55 °C) was reported in 1888 by Gabriel.[1] However, one of the earlier known truly room temperature ionic liquids was [EtNH3]+ [NO3]- (m.p. 12 °C), the synthesis of which was published in 1914.[2] Much later, series of ionic liquids based on mixtures of 1,3-dialkylimidazolium or 1-alkylpyridinium halides and trihalogenoaluminates, initially developed for use as electrolytes, were to follow.[3][4] An important property of the imidazolium halogenoaluminate salts was that they were tuneable – viscosity, melting point and the acidity of the melt could be adjusted by changing the alkyl substituents and the ratio of imidazolium or pyridinium halide to halogenoaluminate.[5]
A major drawback was their moisture sensitivity and, though to a somewhat lesser extent, their acidity/basicity, the latter which can sometimes be used to an advantage. In 1992, Wilkes and Zawarotko reported the preparation of ionic liquids with alternative, 'neutral', weakly coordinating anions such as hexafluorophosphate ([PF6]-) and tetrafluoroborate ([BF4])-, allowing a much wider range of applications for ionic liquids.[6] It was not until recently that a class of new, air- and moisture stable, neutral ionic liquids, was available that the field attracted significant interest from the wider scientific community.
More recently, people have been moving away from [PF6]- and [BF4]- since they are highly toxic, and towards new anions such as bistriflimide [(CF3SO2)2N]- or even away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium salts (such as choline) being just as flexible a scaffold as imidazole.
Characteristics
Ionic liquids are electrically conductive and have extremely low vapor pressure. (Their noticeable odours are likely due to impurities.) Their other properties are diverse. Many have low combustibility, excellent thermal stability, a wide liquid range, and favorable solvating properties for diverse compounds. Many classes of chemical reactions, such as Diels-Alder reactions and Friedel-Crafts reactions, can be performed using ionic liquids as solvents. Recent work has shown that ionic liquids can serve as solvents for biocatalysis [7]. The miscibility of ionic liquids with water or organic solvents varies with sidechain lengths on the cation and with choice of anion. They can be functionalized to act as acids, bases or ligands, and have been used as precursor salts in the preparation of stable carbenes. Because of their distinctive properties, ionic liquids are attracting increasing attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering; see for instance magnetic ionic liquid.
Despite their extremely low vapor pressures, some ionic liquids can be distilled under vacuum conditions at temperatures near 300 °C.[8] Some ionic liquids (such as 1-butyl-3-methylimidazolium nitrate) generate flammable gases on thermal decomposition. Thermal stability and melting point depend on the components of the liquid. Thermal stability of various RTILs are available. The thermal stability of a task-specific ionic liquid, protonated betaine bis(trifluoromethanesulfonyl)imide is of about 534 K and N-Butyl-N-Methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide was thermally stable up to 640 K [9]
The solubility of different species in imidazolium ionic liquids depends mainly on polarity and hydrogen bonding ability. Simple aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas olefins show somewhat greater solubility, and aldehydes can be completely miscible. This can be exploited in biphasic catalysis, such as hydrogenation and hydrocarbonylation processes, allowing for relatively easy separation of products and/or unreacted substrate(s). Gas solubility follows the same trend, with carbon dioxide gas showing exceptional solubility in many ionic liquids, carbon monoxide being less soluble in ionic liquids than in many popular organic solvents, and hydrogen being only slightly soluble (similar to the solubility in water) and probably varying relatively little between the more popular ionic liquids. (Different analytical techniques have yielded somewhat different absolute solubility values.)
Room temperature ionic liquids
Room temperature ionic liquids consist of bulky and asymmetric organic cations such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and ammonium ions. A wide range of anions is employed, from simple halides, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate and to large organic anions like bistriflimide, triflate or tosylate. There are also many interesting examples of uses of ionic liquids with simple non-halogenated organic anions such as formate, alkylsulfate, alkylphosphate or glycolate. As an example, the melting point of 1-butyl-3-methylimidazolium tetrafluoroborate or [bmim][BF4] with an imidazole skeleton is about -80 °C, and it is a colorless liquid with high viscosity at room temperature.
It has been pointed out that in many synthetic processes using transition metal catalyst, metal nanoparticles play an important role as the actual catalyst or as a catalyst reservoir. It also been shown that ionic liquids (ILs) are an appealing medium for the formation and stabilization of catalytically active transition metal nanoparticles. More importantly, ILs can be made that incorporate co-ordinating groups,[10], for example, with nitrile groups on either the cation or anion (CN-IL). In various C-C coupling reactions catalyzed by palladium catalyst, it has been found the palladium nanoparticles are better stabilized in CN-IL compared to non-functionalized ionic liquids; thus enhanced catalytic activity and recyclability are realized.
Low temperature ionic liquids
Low temperature ionic liquids (below 130 kelvins) have been proposed as the fluid base for an extremely large diameter spinning liquid mirror telescope to be based on the earth's moon.[12] Low temperature is advantageous in imaging long wave infrared light which is the form of light (extremely red-shifted) that arrives from the most distant parts of the visible universe. Such a liquid base would be covered by a thin metallic film that forms the reflective surface. A low volatility is important for use in the vacuum conditions present on the moon.
Food science
Ionic liquids have been used in food science. [bmim]Cl for instance is able to completely dissolve freeze dried banana pulp and the solution with an additional 15% DMSO lends itself to Carbon-13 NMR analysis. In this way the entire banana compositional makeup of starch, sucrose, glucose, and fructose can be monitored as a function of banana ripening.
Applications
Nowadays ionic liquids find a number of industrial applications which vary greatly in character. A few of their industrial applications are briefly described below; more detailed information can be found in a recent review article.
BASIL
The first major industrial application of ILs was the BASIL (Biphasic Acid Scavenging utilizing Ionic Liquids) process by BASF, in which a 1-alkylimidazole was used to scavenge the acid from an existing process. This then results in the formation of an IL which can easily be removed from the reaction mixture.[15] But the easier removal of an unwanted side-product (as an IL rather than as a solid salt) is not the only advantage of the IL based process. By using an IL it was possible to increase the space/time yield of the reaction by a factor of 80,000. It should, however, be kept in mind that improvements of such scale are rare.
Cellulose Processing
Occurring at a volume of some 700 billion tons, cellulose is the earth’s most widespread natural organic chemical and, thus, highly important as a bio-renewable resource. But even out of the 40 billion tons nature renews every year, only approx. 0.2 billion tons are used as feedstock for further processing. A more intensive exploitation of cellulose as a biorenewable feedstock has to date been prevented by the lack of a suitable solvent that can be used in chemical processes. Robin Rogers and co-workers at the University of Alabama have found that by means of ionic liquids, however, real solutions of cellulose can now be produced for the first time at technically useful concentrations [16]. This new technology therefore opens up great potential for cellulose processing.
For example, making cellulosic fibers from so-called dissolving pulp currently involves the use, and subsequent disposal, of great volumes of various chemical auxiliaries, esp. carbon disulfide (CS2). Major volumes of waste water are also produced for process reasons and need to be disposed of. These processes can be greatly simplified by the use of ionic liquids, which serve as solvents and are nearly entirely recycled. The “Institut für Textilchemie und Chemiefasern” (ITCF) in Denkendorf and BASF are jointly investigating the properties of fibers spun from an ionic liquid solution of cellulose in a pilot plant setup.
Eastman chemical’s DHF plant
Eastman operated an ionic liquid-based plant for the synthesis of 2,5-dihydrofuran from 1996 to 2004. However, the plant is now defunct because demand for the product has ceased.
Dimersol - Difasol
The dimersol process is a traditional way to dimerise short chain alkenes into branched alkenes of higher molecular weight. Nobel laureate Yves Chauvin and Hélène Olivier-Bourbigou at IFP (France) have developed an ionic liquid-based add-on to this process called the Difasol process. However, while may be licensed it has as yet not been put into commercial practice.
Petrochina
Petrochina have announced the implementation of an ionic liquid-based process called Ionikylation. This process, the alkylation of C4 olefins with iso-butane, is retrofitted into a 65,000 tonne per year alkylation plant, making it the biggest industrial application of ILs to date.
Degussa paint additives
Ionic liquids can enhance the finish, appearance and drying properties of paints. Degussa are marketing such ILs under the name of TEGO Dispers. These products are also added to the Pliolite paint range.
Air products - ILs as a transport medium for reactive gases
Air products make use of ILs as a medium to transport reactive gases in. Reactive gases such as trifluoroborane, phosphine or arsine, BF3, PH3 or AsH3, respectively, are stored in suitable ILs at sub-ambient pressure. This is a significant improvement over pressurised cylinders. The gases are easily withdrawn from the containers by applying a vacuum.
Linde's IL 'piston'
Whereas Air Product’s Gasguard system relies on the solubility of some gases in ILs, Linde are exploiting other gases’ insolubility in ILs. As mentioned above, the solubility of Hydrogen in ILs is very low. Linde now make use of this insolubility by using a body of ionic liquid to compress Hydrogen in filling stations; and in so doing they reduced the number of moving parts from about 500 in a conventional piston pump engine down to 8.
Nuclear industry
RTILs are extensively explored for various innovative applications in nuclear industry. It includes application of ionic liquid as extractant/diluent in solvent extraction systems, as alternate electrolyte media for the high temperature pyrochemical processing, etc. Fundamental studies on the extraction cum electrodeposition of fission products like uranium, palladium etc., from spent nuclear fuel using RTILs as extractants are reported. Reports on employing using Ionic liquids as non-aquoues electrolyte media for the recovery of uranium [18]and useful fission products like palladium [19] and rhodium [20] from spent nuclear fuel are also available.Studies on the electrochemical behavior of uranium(VI) in ionic liquid, 1-butyl-3-methylimidazolium chloride and also the recovery of valuable fission products from tissue paper waste was studied in room temperature ionic liquids.
Safety
Due to their non-volatility, effectively eliminating a major pathway for environmental release and contamination, ionic liquids have been considered as having a low impact on the environment and human health, and thus recognized as solvents for green chemistry. However, this is distinct from toxicity, and it remains to be seen how 'environmentally-friendly' ILs will be regarded once widely used by industry. Research into IL aquatic toxicity has shown them to be as toxic or more so than many current solvents already in use [22]. A review paper on this aspect has been published in 2007.[23] Available research also shows that mortality isn't necessarily the most important metric for measuring their impacts in aquatic environments, as sub-lethal concentrations have been shown to change organisms' life histories in meaningful ways. According to these researchers balancing between zero VOC emissions, and avoiding spills into waterways (via waste ponds/streams, etc.) should become a top priority. However, with the enormous diversity of substituents available to make useful ILs, it should be possible to design them with useful physical properties and less toxic chemical properties.
With regard to the safe disposal of ionic liquids, a 2007 paper has reported the use of ultrasound to degrade solutions of imidazolium-based ionic liquids with hydrogen peroxide and acetic acid to relatively innocuous compounds.[24]
Despite their low vapor pressure many ionic liquids have also found to be combustible and therefore require careful handling [25]. Brief exposure (5 to 7 seconds) to a flame torch will ignite these IL's and some of them are even completely consumed by combustion.
http://en.wikipedia.org/wiki/Ionic_liquid
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