Recycling of Ionic Liquids

Ionic liquids operate in true biphasic mode. While the recovery and recyclability of ionic liquid was found to be more efficient than with the conventional AICI3 catalyst (red oil), the selectivity for the monoalkylated aromatic hydrocarbon was lower. In this gas-liquid-liquid reaction, the solubility of the reactants in the ionic phase (e.g. the benzene/ethene ratio in the ionic phase) and the mixing of the phases were probably critical. This is an example in which the engineering aspects are of the utmost importance. 

Fig. 11.16 Recycling of ionic liquid (1 ChCl 2 ethylene glycol) used to electropolish stainless steel (a) used liquid containing Fe Cr and Ni salts, (b) as (a) with 1 equiv. v/v added water, (c) as (b), after gravity filtration and subsequent removal of residual water by distillation.

Kraft lignin is soluble in alkaline media or some organic solvents but is not as easily degraded as cellulose. Ionic liquids have also been reviewed for their dissolution capability, but no processes are commercialized yet. This is mainly due to the high prices of ionic liquids compared to regular solvents and the challenging separation and recycling of ionic liquids [7]. 

Precipitation using anti-solvent Miscibility of anti-solvent with IL, low solubility of extract in anti-solvent Cellulose precipitation from [C,mim][Cl] by addition of water [117, 118] Easy removal, control of product form Removal of anti-solvent required for recycling of ionic liquid... 

Distillation of ionic liquid Volatile ionic liquid, non-volatile product, thermal stability of product Macrocyclic compounds and monoarylidene ketones from DIMCARB [72, 73] Ready recycling of ionic liquid High energy use, possible incomplete removal of ionic liquid... 

This may be attributed due to the ability of [MOEMIM][TFA] to hydrogen bond with aromatic/heterocyclic/l,2-phenylenediamine. Smdies for recyclability of the regenerated ionic liquids cleared that the yield of the products decreases in various cycles, yet ionic liquid can be reused with significant success. The absence of catalyst and recyclability of ionic liquid make this procedure cleaner and promising for scale-up. 

To summarize, several procedures for the recycling of ionic liquids have been reported. Depending on the ionic liquid used and the reaction performed in it, a variety of methods for recycling are possible. By picking the right purification steps, an individually optimized work-up procedure can be obtained. 

Recycling of ionic liquids is easy, if protonated cations are used. In this case the ionic liquids can be switched offby deprotonation (Scheme 9-12). The resulting amine or imidazole is a conventional neutral molecule that can be distilled for recycling or purification purposes. 

Niralwad et al. (2010) reported an efficient synthesis of octahydroquinazolinone derivatives (112) under ultrasound irradiation via the reaction of dimedone (60), aromatic aldehyde (111), and ureas (98 or 99) using [tbmim]Cl2/AlCl3 as an acidic ionic liquid catalyst (Scheme 8.37). This protocol has advantages in terms of (i) a short reaction time, (ii) a solvent-free reaction, (iii) high yield, (iv) easy workup, (v) being environmentally friendly, and (vi) recyclability of ionic liquid. 

Beste, Y, Schoenmakers, H., Arlt, W, Seiler, M. and Jork, C., Recycling of ionic liquids with extractive distillation, WO Pat. 2005016484 (2005). 

The final part will summarise the key issues from each of the two subsections and will also briefly discuss strategies and techniques for the sustainable reuse and recycling of ionic liquids in industrial processes, with the aim of reducing exposure as well as making sensible economical use of them... 

Table 10. Recyclability of Ionic Liquids in the Acylation of Benzene and Acetic anhydride [Xiao, 2006]. Numbers in parenthesis isolated yields...

Recycle of ionic liquid after the radical graft polymerization... 

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

To be applied industrially, performances must be superior to those of existing catalytic systems (activity, regioselectivity, and recyclability). The use of ionic liquid biphasic technology for nickel-catalyzed olefin dimerization proved to be successful. 

The possibility of recycling the catalyst was also studied. In order to decrease the amount of ethyl maleate and fumarate, the addition rate of ethyl diazoacetate was reduced and the reaction temperature was kept low during the addition. Furthermore, the catalyst concentration was reduced by doubling the volume of ionic liquid. Under these conditions the catalyst was reused seven times, although both the yield and the enantioselectivity decrease from the fourth reuse on (entry 6 in Table 6). 

Scheme 5.16. In some instances, e.g. the aza-Diels-Alder reaction illustrated, Lewis acid catalysts are additionally required but use of ionic liquids greatly enhanees their ease of recovery and recycle.

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

Since the focus of this contribution is clearly on catalysis and catalyst recycle using the ionic liquid methodology it is not possible to go into more detail on the materials science aspects of ionic liquids. However, it should be clearly stated that at least some understanding of the ionic liquid material is a prerequisite for its successful use as a liquid catalyst support in catalysis. Therefore, the interested reader is strongly encouraged to explore the more specialized literature [28],... 

Recently, Dupont and coworkers described the use of room-temperature imi-dazolium ionic liquids for the formation and stabilization of transition-metal nanoparticles. The potential interest in the use of ionic liquids is to promote a bi-phasic organic-organic catalytic system for a recycling process. The mixture forms a two-phase system consisting of a lower phase which contains the nanocatalyst in the ionic liquid, and an upper phase which contains the organic products. Rhodium and iridium [105], platinum [73] or ruthenium [74] nanoparticles were prepared from various salts or organometallic precursors in dry 1-bu-tyl-3-methylimidazolium hexafluorophosphate (BMI PF6) ionic liquid under hydrogen pressure (4 bar) at 75 °C. Nanoparticles with a mean diameter of 2-3 nm... 

Interestingly, the dimeric Cr(salen) catalyst 64 supported on silica showed enhanced activity for ARO of 1,2-epoxyhexane and cyclohexene oxide in the presence of ionic liquids particularly with [BMIM][PF6] (64-IL) [86] (Table 6). A significant increase in the product selectivity was also observed with silica supported ionic liquid (64-SILP) for ARO of 1,2-epoxyhexane and cyclohexene oxide (ee, of 87% and 75% respectively) as eompared to silica supported catalyst minus the ionie liquid (Table 6, entries 5,6). However, after repeated recycling, the silica support material deteriorates due to the abrasive forees in the stirred reactor. As a result, silica material was non-recoverable, but the expensive dimeric Cr(salen) catalyst 64 and the ionic liquid was recovered quantitatively by Soxhlet extraction with acetone. SILP-catalyst system was also used in a eontinuous-flow reactor. 

There are many examples showing the possibilities for designing catalysts specifically for sustained recyclability in ionic liquids. A recent example is the synthesis of an alkene ring-closing metathesis (RCM) catalyst for the RCM of dienes (Scheme 21) 188). 

An excellent demonstration of the tunability of ionic liquids for catalysis is provided by an investigation of the dimerization of 1-butene (235). A Ni(cod)(hfacac) catalyst (Scheme 23) was evaluated for the selective dimerization of 1-butene after it was dissolved in various chloroaluminate ionic liquids. Earlier work on this reaction with the same catalyst in toluene led to the observations of low activity and difficult catalyst separation. In ionic liquids of varying acidity, little catalytic activity was found. However, a remarkable activity was achieved by adding a weak buffer base to an acidic ionic liquid. The reaction took place in a biphasic reaction mode with facile catalyst separation and catalyst recycling. A high selectivity to the dimer product was obtained because of a fast extraction of the Cg product from the ionic liquid phase, with the minimization of consecutive reaction to give trimers. Among a number of weak base buffers, a chinoline was chosen. The catalyst performance was compared with that in toluene. The catalyitc TOF at 90°C in toluene was... 

The hydrophobicity of ionic liquids was found to be particularly beneficial for lipase PS-C-catalyzed transesterification of 2-hydroxymethyl-1,4-benzodioxane in the presence of vinyl acetate (277). The hydrophobic [BMIMJPFg functioned as a better promotional medium than methylene chloride and hydrophilic [BMIM]BF4, with either supported or unsupported enzyme for the catalytic transesterifications. The ionic liquid not only acted as a medium but also as a permanent host for the enzymes, so that the enzyme-ionic liquid system could be recycled several times without substantial diminution in lipase activity. 

The application of ionic liquids as a reaction medium for the copper-catalyzed aerobic oxidation of primary alcohols was reported recently by various groups, in attempts to recycle the relatively expensive oxidant TEMPO [150,151]. A TEMPO/CuCl-based system was employed using [bmim]PF6 (bmim = l-butyl-3-methylimodazolium) as the ionic liquid. At 65 °C a variety of allylic, benzylic, aliphatic primary and secondary alcohols were converted to the respective aldehydes or ketones, with good selectiv-ities [150]. A three-component catalytic system comprised of Cu(C104)2, dimethylaminopyridine (DMAP) and acetamido-TEMPO in the ionic liquid [bmpy]Pp6 (bmpy = l-butyl-4-methylpyridinium) was also applied for the oxidation of benzylic and allylic alcohols as well as selected primary alcohols. Possible recycling of the catalyst system for up to five runs was demonstrated, albeit with significant loss of activity and yields. No reactivity was observed with 1-phenylethanol and cyclohexanol [151]. 

The double carbonylation of iodobenzene with diethylamine catalyzed by Pd(OAc)2-PPh3 was carried out in l-butyl-3-methylimidazolium tetrafluoroborate 315 as reaction medium at 80 °C and 38 atm of CO to give phenyl-glyoxamide 314 as the predominant product (83%) accompanied by benzamide 313 (17%) (Equation (29)). The use of ionic liquids showed the same reactivity and product selectivity as those using diethylamine as solvent for this reaction, while separation of products and recycling of the catalyst was easier. ... 

Gutowski, K.E., Broker, G.A., Willauer, H.D., Huddelston, R.R, Swatloski, J.D., Holbrey, J.D., Rogers, R.D., Gontrolling the aqueous miscibility of ionic liquids Aqueous biphasic systems of water-miscible ionic liquids and waterstructuring salts for recycle, metathesis and separations, /. Am. Chem. Soc., 125, 6632-6633, 2003. 

The use of ionic liquids in most applications is stiU in development. The chemical industry in Europe is showing increasing interest in them, particularly for olefin dimerizations and Friedel-Crafts reactions. A two-phase loop reactor has been designed for large-scale preparations which allows for continuous reaction, separation of the product, and recycling of the ionic liquid (Chauvin and Helene, 1995). 

---