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Keywords:

  • Biodiesel;
  • ionic liquid;
  • transesterification;
  • triglyceride;
  • alcoholysis;
  • microalgae

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES

During the past decade, ionic liquids (ILs) have gained tremendous attention in nearly every branch of the chemical and physical sciences as designer (task-driven) and budding ‘green’ solvent alternatives to conventional volatile organics. In particular, with a more in-depth understanding of their physicochemical properties, the active exploration of ILs as alternative solvents and/or catalysts in the chemical or enzymatic (biocatalytic) production of biodiesel has gained momentum. Most excitingly, very recent developments in the science of deep eutectic solvents (DESs) have initiated potentially more cost-effective approaches to biodiesel synthesis. At this stage, there is sufficient research completed to provide an important opportunity to stand back and assess the progress in the field, critically examining the strengths and limitations for IL and DES technology in biodiesel synthesis. No such comprehensive evaluation exists. This work, therefore, seeks to bridge this gap by systematically reviewing the reported methods for biodiesel production which make use of ILs, either as (co)solvent components or catalysts, highlighting existing problems and limitations, with an emphasis placed on the future research required to bypass the hurdles to employing ILs in commercial biodiesel production. Copyright © 2012 Society of Chemical Industry


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES

In the face of diminishing oil supplies, national security issues arising from reliance on imported crude oil, and the climate implications of the continued burning of fossil fuels (coal, oil, and natural gas), there has been global demand for the development of renewable energy sources that are not based on crude petroleum oil. This demand becomes more urgent with the environmental hazards of drilling and the risk of severe environmental disasters such as the recent Gulf of Mexico BP blowout/leak. As one of the alternative biological sources, biodiesel is becoming an attractive, renewable, ‘CO2 neutral’, and biodegradable fuel for diesel engines and heating systems. Biodiesel refers to an oil-based diesel fuel consisting of long-chain alkyl esters and is typically made by chemically reacting lipids with an alcohol to produce fatty acid monoesters. The National Biodiesel Board (USA) also has a technical definition of ‘biodiesel’ as a mono-alkyl ester. Biodiesel can be prepared from abundant vegetable oils or animal fats (tallow),[1-3] as well as other renewable sources of lipids including oleaginous microbial biomass (i.e. molds, yeasts, algae),[4, 5] soybean oil deodorizer distillate (SODD),[6] and pine trees.[7] It is important to point out that not all biodiesel is sustainable, particularly those relying on resources imported from other areas and using rain forestlands. Biodiesel has a comparable fuel economy as petroleum-based diesel, and can also reduce the emissions of polluting substances, such as particulate matter (PM), carbon monoxide, and hydrocarbon.[8] Conventional chemical methods for preparing biodiesel have a number of limitations such as corrosion and emulsification problems, saponification of fatty acids, energy intensive operations, and high waste treatment. Therefore, because of these limitations, there is a lack of a commercially viable, efficient means for biodiesel production to make it a long-term viable substitute for crude oil energy sources in the quantities required to sustain our 21st century energy needs. As an alternative method to conventional chemical routes to biodiesel synthesis, the enzymatic approach seems very attractive due to its mild conditions, the use of ‘green’ catalysts (mostly lipases), and low waste treatment. However, the enzymatic synthesis of biodiesel has itself been challenged by a number of difficulties, one of which is the lack of lipase-compatible non-aqueous solvents. Now, fast-growing research on a new type of non-volatile solvent, named ionic liquids (ILs), has demonstrated their potential for effective enzyme stabilization and activation.[9, 10] There promises to be great benefit from the development of low-cost ILs as alternative solvents for biodiesel synthesis. Despite numerous studies focusing on the application of ILs in the preparation of biodiesel and limited discussion on the subject,[11] no in-depth review on this viable alternative technique exists to date.

Many review articles on the subject of biodiesel have been published recently, including general reviews on chemical and enzymatic methods,[12-16] and biodiesel production from different biomass feedstock[17, 18] (particularly from microalgae[5, 19-22]). Therefore, to minimize the redundancy, we only briefly outline some advantages and disadvantages of current technologies for biodiesel production.

A common synthetic route to biodiesel production is the transesterification (alcoholysis) of vegetable oils (or animal fats) with a primary aliphatic alcohol (methanol or ethanol). Typically, biodiesel comprises long-chain fatty acid methyl esters (FAMEs) obtained by the methanolysis of triacylglycerides (e.g. triolein), as summarized in Scheme 1. This reaction can be catalyzed by acids, alkaline metal hydroxides, alkoxides, and non-ionic bases (such as amines and amidines).[23] However, these methods have several drawbacks: (1) acid/base processes are often related to corrosion and emulsification problems; (2) acid-catalyzed reactions are usually much slower than base-catalyzed processes,[24, 25] and may require a large excess of alcohol and high pressure (such as 170–180 kPa);[17] (3) the base-catalysis technology may cause unnecessary saponification of fatty acids,[26] thus requiring a low content of free fatty acids in feedstock;[17, 27] (4) a large excess of alcohol is required to drive the equilibrium to the ester formation and to achieve the facile separation of biodiesel from the glycerol; furthermore, it creates recycling problems (such as the cost and the recovery of methanol); (5) practically, oils and fats are not soluble in alcohols, resulting in barriers for triglyceride conversions; and (6) other issues such as being energy intensive, alkaline waste-water treatment, and interference of free fatty acids and water.[1] Therefore, a number of new approaches have been vigorously pursued to circumvent these problems, such as heterogeneous catalysis,[28-34] alcoholysis in supercritical methanol,[35-38] ILs-catalyzed transesterification,[39-41] and lipase-catalyzed transesterification.[1, 6, 42, 43]

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Scheme 1. Schematic illustration of the chemo- or bio-catalyzed synthesis of biodiesel by methanolysis of triolein (glyceryl trioleate).

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By contrast, the enzymatic transesterification method offers many advantages over the chemical methods. The advantages include mild reaction conditions, low energy demand, low waste treatment, the reusability of enzymes (lipases in most cases), flexibility in choosing different enzymes for different substrates, and allowing a small amount of water in substrates, etc.[42, 44] Therefore, the enzymatic synthesis of biodiesel is a greener alternative to chemical methods. However, the current lipase-catalyzed methods have some downsides that prevent this promising approach from being commercialized. These disadvantages include the high cost of enzymes, lipase inactivation by acyl acceptors such as methanol, and lipase inactivation by impurities in crude and waste oils.[42] In addition, due to the poor miscibility between oils/fats and methanol, many enzymatic transesterification reactions are heterogeneous systems involving a complicated liquid–liquid interface.[42] To address the issue of methanol inhibition of lipases, a number of approaches have been studied, for example, a stepwise addition of methanol during the reaction,[45] the use of other acyl acceptors such as methyl and ethyl acetate,[46, 47] enzyme immobilization,[42, 48] the use of other organic solvents such as t-butanol, hexane, n-heptane and 1,4-dioxane,[42, 48, 49] the use of fatty acid-containing feedstock,[6] and the genetic modification of lipases for higher methanol tolerance.[42] In addition to causing possible enzyme inactivation, the use of volatile organic solvents in biodiesel production creates potential environmental concerns and safety issues. Therefore, there is a great need for developing an alternative solvent for biodiesel production, such as ionic liquids (ILs).

ILs consist of ions and remain liquid at temperatures lower than 100°C. Typical IL cations are nitrogen-containing (such as alkylammonium, N, N′-dialkylimidazolium, N-alkylpyridinium and pyrrolidinium), or phosphorous containing (such as alkylphosphonium). The common choices of anions include halides, BF4, PF6, CH3CO2, CF3CO2, NO3, Tf2N [i.e., (CF3SO2)2N], [RSO4], and [R2PO4]. Some typical cations and anions are illustrated in Scheme 2. As a new generation of non-aqueous solvents, ILs have many favorable properties such as low vapor pressure, a wide liquid range, low flammability, high ionic conductivity, high thermal conductivity, high dissolution capability toward many substrates, high thermal and chemical stability, and a wide electrochemical potential window.[50] Because of these unique properties, ILs have been widely recognized as solvents or (co-)catalysts in a variety of applications including organic catalysis,[50-53] inorganic synthesis,[54] biocatalysis,[9, 10, 53] polymerization,[55] and engineering fluids.[56, 57] More importantly, physical properties of ILs (such as polarity, hydrophobicity and hydrogen-bond basicity) can be finely tuned through the judicious selection of cations and anions. All these adjustable properties are crucial to the enzyme stabilization and activation; therefore, numerous enzymatic reactions have been investigated in different types of ILs.[9, 10, 58, 59]

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Scheme 2. Structures of representative cations and anions in ILs.

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This review focuses on the reaction medium role of ILs for chemical and enzymatic preparation of biodiesel, the catalyst role of ILs in acid/base-catalyzed transesterification of triglycerides, and the extraction solvent role of ILs for biodiesel production processes. We also discuss the use of deep eutectic solvents for biodiesel synthesis since these solvents share some common properties with regular ILs. Finally, we lay out the challenges of employing ionic solvents in biodiesel preparation.

ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES

There are only limited studies involving ILs as co-solvents for chemical preparation of biodiesel. DaSilveira Neto et al.[33] used a tin-based catalyst [Sn(3-hydroxy-2-methyl-4-pyrone)2(H2O)2] for the methanolysis of soybean oil to biodiesel in [BMIM][InCl4], achieving a biodiesel yield of 83% in 4 h under reflux condition. ESI-MS measurements suggest a cationic species is formed during the reaction via the substitution of pyrone ligand by alcohol, followed by the coordination of carboxylate compound to tin. Lapis et al.[34] carried out the transesterification of soybean oil to prepare biodiesel using [BMIM][Tf2N] as the co-solvent under acidic or basic conditions: acid catalysis with H2SO4 resulted in >93% biodiesel yields; base catalysis with K2CO3 (40 mol%) led to >98% biodiesel yields. Both types of reactions proceeded as multiphase systems where biodiesel formed the top layer and glycerol accumulated in the alcohol-IL-acid/base phase. The phase separation not only favorably drives the reaction equilibrium to the product side, but also simplifies the biodiesel purification and glycerol recovery.

The enzymatic transesterification of vegetable oils in ILs has been demonstrated by many groups in producing biodiesel.[60] Ha et al.[61] screened 23 different ILs for the methanolysis of soybean oil catalyzed by the immobilized Candida antarctica lipase (Novozym® 435), and identified the hydrophilic ionic liquid (i.e. [EMIM][OTf])1 as the best solvent for achieving the highest yield (80%) of fatty acid methyl esters at 12 h. On the other hand, Sunitha et al.[62] obtained 98–99% yields of fatty acid methyl esters within 10 h of methanolysis of sunflower oil in hydrophobic [BMIM][PF6] and [EMIM][PF6] when catalyzed by Novozym® 435; however, the same reaction in hydrophilic BF4- based ILs gave very poor yields. Gamba et al.[63] also utilized several ILs (i.e. [BMIM][Tf2N], [BMIM][BF4] and [BMIM][PF6]) as solvents for the enzymatic transesterification of soybean oil, achieving over 90% biodiesel yields in 48 h. Arai et al.[64] investigated the fungus whole-cell catalyzed production of biodiesel from soybean oil in two ILs ([EMIM][BF4] and [BMIM][BF4]), and observed that ionic solvents enabled the lipase to be more tolerant to methanol inhibition and allowed quantitative synthesis of biodiesel under optimized conditions. Yang et al.[65] carried out the methanolysis of corn oil in [BMIM][PF6] catalyzed by Penicillium expansum lipase and obtained a 69.7% yield in 25 h. The Zhao group[66] demonstrated that ether-functionalized ILs based on acetate (Scheme 3) were capable of dissolving oils and afforded high enzymatic activities in biodiesel synthesis.

image

Scheme 3. Imidazolium and ammonium based ILs consisting of alkyloxyalkyl-substituted cation and acetate anion (abbreviated as [CH3(OCH2CH2)n-Et-Im][OAc] and [CH3(OCH2CH2)n-Et3N][OAc], respectively) (n = 2, 3,…).

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Ruzich and Bassi[67] studied the Novozym 435-catalyzed transesterification of triolein with methyl acetate in [BMIM][PF6], and achieved 80% biodiesel yield under optimal conditions (i.e. 1:1 volume ratio of IL/oil, 14:1 molar ratio of methyl acetate/oil, 20% wt. of Novozym 435/wt. of oil, and 48–55°C). The same group[68] further conducted Novozym 435-catalyzed synthesis of biodiesel from triolein and waste canola oil using methyl acetate as the acyl acceptor and [BMIM][PF6] as the co-solvent. In a small-scale shake flask, 83% biodiesel yield was observed from triolein and 72% from waste canola oil; in a jacketed conical reactor, lower yields were obtained (54% from triolein and 30% from waste canola oil) due to poor mixing of two phases. Furthermore, this group[69] suggested that the above enzymatic transesterification reaction followed the Ping–Pong Bi–Bi mechanism, and the use of [BMIM][PF6] actually lowered the initial reaction rates due to mass transfer limitations around the immobilized lipase. Ha et al.[70] demonstrated a continuous operation (0.375–0.75 mL h-1) of Novozym 435-catalyzed transesterification of vinyl laurate and 1-butanol in [OMIM][OTf] through a double-layer type continuous stirred tank reactor (Fig. 1). The advantage of this process is in situ removal of the product (butyl laurate), pushing the equilibrium to the product side. The highest product concentration of 2.71 mol L-1 was observed at a steady state of 0.375 mL h-1 flow rate.

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Figure 1. Illustration of a double-layer type continuous stirred tank reactor for continuous production and in situ separation of butyl laurate: (1) feed reservoir, (2) pump, (3) reactor, (4) product phase, (5) inner reactor, (6) IL phase, (7) magnetic stirrer, and (8) product reservoir[70] (Reproduced by permission of Elsevier).

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Lozano et al.[71] indicate that triolein is soluble in [C18MIM][Tf2N] (m.p. 53°C); as a result, the Novozym 435-catalyzed transesterification of triolein and methanol resulted in 96% biodiesel yield in 6 h. In another study, De Diego et al.[72] found that triolein was soluble in hydrophobic ILs such as [C16MIM][Tf2N] and [C18MIM][Tf2N], and thus carried out a homogeneous transesterification of triolein with methanol in these two ILs catalyzed by Novozym 435, producing quantitative yields for at least six continuous operation cycles. Interestingly, at the completion of reaction, a three-layer mixture was formed: a top layer containing biodiesel, a middle layer containing glycerol and excess methanol, and a bottom layer containing IL and enzyme. This enables simple separation and purification of biodiesel. In addition, De Diego et al.[73] systematically investigated the lipase-catalyzed transesterification of triolein in a series of imidazolium ILs with different alkyl chain lengths (C10 to C18) and different anions (Tf2N, PF6, and BF4), and found the lipase activity increased with the alkyl chain length to a maximum (the highest synthetic activity in [C16MIM][Tf2N]) and then decreased; the biodiesel yields of Novozym 435-catalyzed conversions of different oils in [C16MIM][Tf2N] after 24 h were in the range 93–97%. Lozano et al.[74] further developed a continuous enzymatic reactor, using Novozym 435 coated with hydrophobic ILs (i.e. [C18MIM][Tf2N]), for biodiesel synthesis from triolein in supercritical CO2 at 60°C and 180 bar, showing high operational stability of the lipase (up to 82% biodiesel yield after 12 cycles of 4 h). In addition, they observed that the lipase activity was improved by the increase in the IL anion's hydrophobicity (Tf2N > PF6 > BF4).

Yu et al.[75] suggest a synergistic effect of microwave irradiation and ionic solvent (such as [EMIM][PF6]) on Novozym 435-catalyzed transesterification of soybean oil with methanol; under microwave irradiation, a 92% yield of biodiesel was achieved in [EMIM][PF6] at optimized conditions, while only 70% yield was obtained under conventional heating. The Yang group[76] achieved a higher biodiesel yield (86%) in [BMIM][PF6] than in t-butanol (52%) for Penicillium expansum lipase-catalyzed transesterification of corn oil with methanol; they also observed negligible conversions when hydrophilic ILs (containing anions such as MeSO4, OAc, NO3, and H2PO4) were used instead. Liu et al.[77] examined the enzymatic transesterification of soybean oil with methanol in 19 ILs catalyzed by Burkholderia cepacia lipase, and suggested the lipase activity decreased with the order of anions PF6 > Tf2N > OTf > BF4 > CH3SO3 ∼ Cl. The highest biodiesel yield of 82% was observed in [OmPy][BF4], and it was argued that a low α-helix content of the lipase in ILs (from FT-IR analysis) was responsible for the high enzyme activity. de los Ríos et al.[78] investigated Novozym 435-catalyzed conversion of sunflower and waste cooking oils to biodiesel in 10 different ILs. This group confirmed that the lipase activity was improved by decreasing the anion's nucleophilicity (CALB activity in the order of PF6, Tf2N > OTf > BF4) and increasing the cation's hydrophobicity. The triglyceride conversion was also affected by the methanol/substrate molar ratio, water content and IL viscosity. The highest sunflower oil conversion of 60% was observed in [OMIM][PF6] at optimized conditions (methanol/sunflower oil = 12:1 molar ratio, 1% water, 900 rpm, 40°C and 24 h). The Xu group[79] evaluated the biodiesel synthesis by the enzymatic methanolysis of rapeseed oil in a number of ILs, and reported a 98% biodiesel yield in Ammoeng 102; the computer modeling study indicated that the amphiphilic property of Ammoeng 102 promotes efficient interactions between IL molecules and immiscible substrates, while different partition behaviors of biodiesel and glycerol in ionic solvent drive the reaction equilibrium to the product (biodiesel) side. The Yang group[80] extracted about 41% (w/w) lipids from microalgae Chlorella vulgaris and also identified the main fatty acid composition; furthermore, they performed the enzymatic synthesis of biodiesel from microalgal oil in [BMIM][PF6] catalyzed by Penicillium expansum lipase or Novozym 435, resulting in yields of 91% and 86%, respectively (only 49% and 44%, respectively, in t-butanol). Lozano et al.[81] immobilized CALB onto new nanostructured supports (via a covalent attachment of 1-decyl-2-methyimidazolium cations to a polystyrene divinylbenzene porous matrix), and demonstrated the high performance of immobilized lipase in catalyzing the methanolysis of triolein in both t-butanol and supercritical scCO2 (18 MPa and 45°C) with high yields (up to 95%). They also indicate that the presence of t-butanol in scCO2 is essential to prevent the blockage of enzyme active sites by polar glycerol.

ILS AS CATALYSTS IN BIODIESEL SYNTHESIS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES

There are many acidic ILs being prepared and used as catalysts for biodiesel synthesis. Wu et al.[39] studied the transesterification of cottonseed oil with methanol catalyzed by Brønsted acidic ILs carrying sulfonic acid group in cations (Scheme 4). The catalytic efficiency was correlated with the Brønsted acidity of IL catalysts, and thus 1-(4-sulfonic acid) butylpyridinium hydrogen sulfate (Scheme 4(b)) was identified as the best catalyst. A 92% biodiesel yield was obtained under an optimized condition (methanol:oil:IL = 12:1:0.057 (molar ratio) at 170°C for 5 h). Zhang et al.[82] also prepared a number of Brønsted acidic ILs and applied them as catalysts for the esterification of long-chain fatty acids (such as oleic acid, stearic acid, myristic acid, palmitic acid and mixed acids); they further chose N-methyl-2-pyrrolidonium methyl sulfonate ([NMP][CH3SO3]) as the best and recyclable catalyst giving 94–95% yields at 70°C for 8 h under optimum conditions. Han et al.[40] obtained up to 94% biodiesel yield when the transesterification of waste oils with methanol was catalyzed by a sulfonic acid group-containing IL at 170°C for 4 h. Liang et al.[41] performed the conversion of soybean oil to biodiesel catalyzed by a chloroaluminate type of IL, [Et3NH]Cl-AlCl3 (x(AlCl3) = 0.7), and achieved a 98.5% yield with no saponification under the optimum reaction conditions.

image

Scheme 4. Structures of Brønsted acidic ILs.

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Liang and Yang[83] prepared a multi [BOND]SO3H functionalized Brønsted acidic IL (Scheme 5) as the catalyst for biodiesel synthesis from rapeseed oil and methanol, achieving a 98% biodiesel yield under optimized conditions. Li et al.[84] found that acidic N-((4-sulfo)butyl)pyridinium trifluoromethylsulfonate was capable of catalyzing the transesterification of Jatropha oil with methanol at 100°C to obtain up to 92% biodiesel yields. Guo et al.[85] found that the mixture of [BMIM][CH3SO3] and FeCl3 could be an effective catalyst for the conversion of Jatropha oil with a high-acid value (13.8 mg KOH/g) to biodiesel, where metal ions in ILs acted as Lewis acidic sites. A high biodiesel yield of 99.7% was achieved at an optimal condition of 45 g Jatropha oil, 9 mmol [BMIM][CH3SO3]-FeCl3 (mole fraction of FeCl3: 0.7), methanol/oil molar ratio 6/1, 120°C, and 5 h. Due to a high free fatty acids (FFA) value in the crude palm oil, Elsheikh et al.[86] suggest a two-step process: esterification of FFA catalyzed by Brønsted acidic ILs (i.e. [BMIM][HSO4]), and KOH-catalyzed transesterification of triglycerides. For the first step, a 91% FFA conversion was obtained under optimal conditions (4.4 wt% [BMIM][HSO4], methanol/crude palm oil = 12:1 molar ratio, 160°C, 600 rpm and 2 h); the second step was carried out at 1.0 wt% KOH, methanol/oil = 6:1 molar ratio, 60°C, 600 rpm and 1 h, achieving an overall biodiesel yield of 98%. Ghiaci et al.[87] prepared a number of Brønsted acidic ILs derived from 1-benzyl-1H-benzimidazole, and found that one of them [3,3'-(hexane-1,6-diyl)bis(6-sulfo-1-(4-sulfobenzyl)-1H-benzimidazolium) hydrogensulfate] produced the highest catalytic activity (95% yield in 5 h) and best recyclability in the transesterification of canola oil with methanol. This acidic IL could also catalyze the transesterification of other vegetable oils and alcohols, achieving satisfactory yields (85–95%). The production separation was straightforward as three layers were formed after the reaction: upper biodiesel layer, middle glycerol layer and bottom IL catalyst layer. The same group[88] further demonstrated that the bentonite organoclay modified by 3,3'-(butane-1,6-diyl)bis(6-sulfo-1-(4-sulfobenzyl)-1H-benzimidazolium) hydrogensulfate was effective in catalyzing the esterification of long-chain fatty acids (such as oleic acid, lauric acid, palmitic acid and stearic acid) to their methyl esters in 6 h with yields of 86–98%.

image

Scheme 5. A multi -SO3H functionalized Brønsted acidic IL.

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Zhou et al.[89] reported that five Brønsted acidic ILs (carrying propyl sulfonic acid groups on their cations) showed high catalytic activities and reusability toward the biodiesel preparation from tung oil. Fang et al.[90] studied the esterification of free fatty acids with low alcohols (from methanol to butanol) catalyzed by dicationic acidic ILs (Scheme 6), and found that the reaction mixture became homogeneous one-phase upon heating to 70°C and the product settled as a separate layer after cooling. High conversions (93–96%) were obtained with this new reaction system, and the catalyst could be reused at least six times with little loss in activity. The same system was also applied to soybean for biodiesel production, achieving up to 94% conversions.

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Scheme 6. Structures of dicationic acidic ILs.

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Although basic ILs are not commonly used as catalysts in biodiesel synthesis, Zhou et al.[91] investigated several imidazolium hydroxides (such as [BMIM][OH]) as recyclable catalysts for the transesterification of glycerol trioleate with methanol, and obtained a biodiesel yield of 87% at 120°C for 8 h (methanol/triglyceride = 9:1 molar ratio).

BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES

Recently, the Abbott group92-94 demonstrated that a mixture of solid organic salt and a complexing agent can form a liquid at temperatures below 100°C, so-called ‘deep eutectic solvents’ (DES), or ‘deep eutectic ILs’ (DEILs). The mechanism is that the complexing agent (typically a H-bond donor) interacts with the anion and increases its effective size, which in turn reduces the anion interaction with the cation and thus decreases the freezing point (Tf) of the mixture. A classic example is the mixture of choline chloride (m.p. = 302°C, 2-hydroxyethyl-trimethylammonium chloride, Scheme 7) and urea (m.p. = 133°C) in a 1:2 molar ratio resulting in a room-temperature liquid (Tf = 12°C).[92] Although the research community is ambivalent on whether these deep eutectic solvents should be formally classified as ILs because they contain a significant molecular component, they definitely share a number of attractive solvent features with regular ILs. The major advantage of this approach is that inexpensive and non-toxic compounds can be used and the properties of the liquid can be finely tuned with different combinations of organic salts and complexing agents. Considering that many inexpensive quaternary ammonium salts are available and there is a wide choice of amides, amines, carboxylic acids, alcohols and metal salts that can be used as complexing agents,[93-97] the possibility of forming new and inexpensive eutectic ILs is enormous. In particular, choline chloride, so called vitamin B4, is produced on the scale of Mtonne (million metric tons) per year as an additive for chicken feed and many other applications. This ammonium salt is not only cheap and easy to make, but also non-toxic and biodegradable. Therefore, eutectic ILs based on cholinium can be biodegradable and inexpensive. The Domínguez de María group[98] formed new eutectic ILs by pairing choline chloride with renewable levulinic acid or sugar-based polyols, and suggest that the addition of glycerol leads to lower viscosities. In addition, eutectic ILs can dissolve many metal salts, aromatic acids, amino acids, glucose, citric acid, benzoic acid and glycerol.[92, 93, 99-101] Also, choline chloride is an essential micronutrient and human nutrient,[102] and cholinium alkanoates (including acetate) are environmentally benign and biodegradable.[103] One major application of choline-based eutectic solvents is electrodeposition and electropolishing of metals,[99, 104, 105] which has been reviewed for several types of eutectics.[106-108] In addition, these new solvents are being actively exploited as benign solvents for a number of chemical and enzymatic reactions.[59] The following section focuses on their utilizations as catalysts or (co-)solvents in biodiesel preparation.

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Scheme 7. Structures of cholinium salts (choline chloride and choline acetate).

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The eutectic mixture of choline chloride and zinc chloride (1:2 molar ratio) was used as a Lewis acidic catalyst (Zn2Cl5 as predominant species) for the transesterification of soybean oil with methanol, resulting in a 55% conversion under optimum conditions (methanol/oil molar ratio 16:1, 10% catalyst, 70°C and 72 h).[109] A similar transesterification of palm oil catalyzed by choline chloride/ZnCl2 (1:2) or choline chloride/FeCl3 (1:2) along with 95 vol% H2SO4 produced up to 92% methyl ester.[110] However, these eutectic mixtures involving metal halides usually have relatively high freezing points (usually > 25°C) and very high viscosities (such as 281 Pa s at 25°C for choline chloride/ZnCl2 at 1:2).[96, 109]

More recently, Zhao et al.[111] developed biocompatible eutectic ILs (such as choline chloride/glycerol at 1:2 and choline acetate/glycerol at 1:1.5) that were capable of maintaining high CALB activity and stability, advocating their promising application in the enzymatic preparation of biodiesel. In addition, those eutectic solvents derived from cholinium salts and glycerol have lower viscosities (∼80 mPa s at 50°C) than that of choline chloride/urea 1:2 (120 mPa s at 50°C).[111] Furthermore, choline chloride/glycerol (1:2 molar ratio) was evaluated as a co-solvent in Novozym 435-catalyzed transesterification of soybean oil with methanol to achieve up to 88% triglyceride conversions in 24 h.[112]

ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES

ILs have also been used as extraction solvents in biodiesel production for different purposes: (a) lipid extraction from biomass; (b) extraction of free fatty acids (FFA) before reaction; (c) extraction of glycerol from biodiesel after reaction; and (d) extraction of unsaturated fatty acid esters from biodiesel.

Lipid extraction from biomass

Young et al.[113] discovered that a mixture of methanol and [EMIM][MeSO4] (mass ratio 1.2:1) was effective for extracting lipids out of microalgae and different oil seeds; instead of dissolving the lipids, the solvent mixture allowed the auto-partition of lipids to a separate immiscible phase for easy harvesting. A similar study by Kim et al.[114] indicates that 1:1 (v/v) mixtures of methanol and ILs (such as [BMIM][CF3SO3] and [EMIM][MeSO4]) are capable of dissolving algal biomass, leaving lipids insoluble as a separate layer. The total lipids recovered by some IL mixtures were higher than the with Bligh and Dyer's method, and such an improvement could be attributed to the dipolarity/polarizability and hydrogen-bond acidity of ILs (rather than their hydrogen-bond basicity) as confirmed by a multi-parameter regression based on the linear solvation energy relationship.

Extraction of free fatty acids

Manic et al.[115] examined the use of ILs (i.e. Ammoeng 100 and [BMIM][dca]) and PEGs (Mw = 200, 400, 2000 and 4000) as alternative solvents for removing FFA (linoleic acid as a model acid) from soybean oil. The liquid–liquid phase equilibrium data suggest that ILs and PEGs are fully miscible with linoleic acid, but not miscible with soybean oil. In particular, the highest values of linoleic acid distribution coefficient were observed in Ammoeng 100. That study implied the high potential of using these solvents for biodiesel deacidification.

Extraction of glycerol from biodiesel

Interestingly, some deep eutectic solvents were found capable of removing glycerol from biodiesel product (Scheme 8). A 1:1 mixture of quaternary ammonium salt–glycerol was used to extract glycerol from biodiesel product mixtures, and salts including choline chloride, [ClEtMe3N]Cl, and [EtNH3]Cl were most effective in glycerol removal.[101] The mixture of choline chloride/glycerol (1:1) was found effective in extracting glycerol from biodiesel, resulting in 51 wt% of glycerol removal when the biodiesel/eutectic mixture ratio was kept at 1:1 (molar ratio).[116] In addition, choline chloride/ethylene glycol (1:2.5 molar ratio) and choline chloride/2,2,2-trifluoroacetamide (1:1.75, molar ratio) were shown effective in glycerol removal from palm oil based biodiesel.[117] Shahbaz et al.[118] indicate that eutectic solvents formed between methyltriphenylphosphunium bromide and ethylene glycol (or triethylene glycol) are effective in removing glycerol, as well as monoglycerides and diglycerides, from palm oil-based biodiesel.

image

Scheme 8. Extraction of glycerol from biodiesel via forming DES.

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Extraction of unsaturated fatty acid esters from biodiesel

To reduce NOx emissions and to improve the oxidation stability of biodiesel, Li et al.[119] extracted unsaturated fatty acid esters (i.e. methyl linolenate (18:3) and methyl linoleate (18:2)) from soy-derived biodiesel using π-complexing sorbents. The new sorbents were constructed by covalently attaching ILs (the imidazolium type of PF6 and BF4) onto silica followed by coating these silica-supported ILs with silver salts (AgBF4 or AgNO3). One of the sorbents (AgBF4/SiO2· Im+·PF6) exhibited a higher sample capacity and selectivity in extracting 18:3 ester than conventional sorbents. A similar study by this group[120] reported the high selectivity of a new adsorbent based on mesoporous silica (SBA-15) (i.e. AgBF4/SBA-15·HPSiOEtIM+PF6) for separating polyunsaturated triacylglycerols such as linolenin.

SUMMARY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES

In biodiesel production, ILs can perform several different vital roles, acting variously as acid/base catalyst, (co)solvent for chemical or enzymatic reaction, and as extraction solvent. Many ILs have already demonstrated high potential in biodiesel production, with substantial benefits, such as high product yield, compatibility with enzyme catalysts, and facile post-synthetic separation (i.e. biodiesel generally forms as an upper layer whilst glycerol and ILs form other layers), bringing some added values to classic biodiesel synthesis. When selecting or designing an IL intended for biodiesel applications, several factors must be considered, keeping these goals in mind: (1) as catalysts or solvents, the ideal system must achieve a high biodiesel yield under mild conditions within a relatively short reaction time; (2) during enzymatic preparation, the solvent should allow the maintenance of high lipase activity and confer stability in the presence of methanol and glycerol; (3) the components should demonstrate low toxicity and high biodegradability, as well as being (4) relatively inexpensive.

Although great strides have been made on the laboratory bench, several obstacles remain to employing ILs in industrial-scale biodiesel production, including: (1) the high cost of ILs, particularly those based on the imidazoliums and pyridiniums. One alternative is to develop low-cost ILs such as those derived from less expensive alkylammonium and piperidinium salts, or the use of DESs based on inexpensive choline salts; (2) the high viscosities common to most ILs, which may limit mass-transfer kinetics, causing poor operability. The judicious incorporation of ether chains in the cations or the selection of ‘viscosity-reducing’ anions (e.g. dicyanamide, tricyanomethanide, tetracyanoborate, thiocyanate) may aid in lowering IL viscosities;[59, 121] and (3) concerns over IL toxicity[122-124] and biodegradability.[123-125] In particular, as promising and inexpensive alternatives to conventional ILs, DESs deserve more attention as benign reaction media for the chemical and enzymatic preparation of biodiesel.

Another important aspect of future work in this area will include extending these approaches to non-edible oils (e.g. castor, jatropha, rubber seed, polanga, karanja) which do not compete with potential food sources. The economic viability of biodiesel production can also be enhanced by implementing used cooking oils, which are otherwise wasted and are available throughout the world, especially in developed countries (e.g. fast food restaurant chains). An additional advisable path forward is the greater consideration of alternative inexpensive lipases such as Penicillium expansum lipase, as well as the stimulating but grander challenge of designing artificial lipase mimics for use in ILs. Studies directed at devising nanoscale or porous enzyme supports to enhance the thermal, solvent, and operational stability of lipases are also warranted. Finally, accomplishing the enhanced and expedient recovery of ILs with minimal energy expenditure so that they may be recycled and reused is an imposing problem seeking solution, both within the area of biodiesel synthesis and ubiquitously within the wider field of IL research itself.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ILS AS (CO-)SOLVENTS FOR CHEMICAL AND ENZYMATIC PREPARATIONS OF BIODIESEL
  5. ILS AS CATALYSTS IN BIODIESEL SYNTHESIS
  6. BIODIESEL SYNTHESIS IN DEEP EUTECTIC SOLVENTS
  7. ILS AS EXTRACTION SOLVENTS IN BIODIESEL PRODUCTION
  8. SUMMARY
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  • 1
    Fukuda H, Kondo A and Noda H, Biodiesel fuel production by transesterification of oils. J Biosci Bioeng 92:405416 (2001).
  • 2
    Meher LC, Sagar DV and Naik SN, Technical aspects of biodiesel production by transesterification - a review. Renew Sustain Energy Rev 10:248268 (2006).
  • 3
    Marchetti JM, Miguel VU and Errazu AF, Possible methods for biodiesel production. Renew Sustain Energy Rev 11:13001311 (2007).
  • 4
    Liu B and Zhao Z, Biodiesel production by direct methanolysis of oleaginous microbial biomass. J Chem Technol Biotechnol 82:775780 (2007).
  • 5
    Chisti Y, Biodiesel from microalgae. Biotechnol Adv 25:294306 (2007).
  • 6
    Du W, Wang L and Liu D, Improved methanol tolerance during Novozym435-mediated methanolysis of SODD for biodiesel production. Green Chem 9:173176 (2007).
  • 7
  • 8
    Wang WG, Lyons DW, Clark NN and Gautam M, Emissions from nine heavy trucks fueled by diesel and biodiesel blend without engine modification. Environ Sci Technol 34:933939 (2000).
  • 9
    van Rantwijk F and Sheldon RA, Biocatalysis in ionic liquids. Chem Rev 107:27572785 (2007).
  • 10
    Moniruzzaman M, Nakashima K, Kamiya N and Goto M, Recent advances of enzymatic reactions in ionic liquids. Biochem Eng J 48:295314 (2010).
  • 11
    Liu C-Z, Wang F, Stiles AR and Guo C, Ionic liquids for biofuel production: opportunities and challenges. Appl Energy 92:406414 (2012).
  • 12
    Vasudevan PT and Briggs M, Biodiesel production - current state of the art and challenges. J Ind Microbiol Biotechnol 35:421430 (2008).
  • 13
    Ganesan D, Rajendran A and Thangavelu V, An overview on the recent advances in the transesterification of vegetable oils for biodiesel production using chemical and biocatalysts. Rev Environ Sci Biotechnol 8:367394 (2009).
  • 14
    Bajaj A, Lohan P, Jha PN and Mehrotra R, Biodiesel production through lipase catalyzed transesterification: an overview. J Mol Catal B: Enzym 62:914 (2010).
  • 15
    Bisen PS, Sanodiya BS, Thakur GS, Baghel RK and Prasad GBKS, Biodiesel production with special emphasis on lipase-catalyzed transesterification. Biotechnol Lett 32:10191030 (2010).
  • 16
    Gog A, Roman M, Toşa M, Paizs C and Irimie FD, Biodiesel production using enzymatic transesterification – current state and perspectives. Renewable Energy 39:1016 (2012).
  • 17
    Zhang Y, Dubé MA, McLean DD and Kates M, Biodiesel production from waste cooking oil. 1. Process design and technological assessment. Bioresource Technol 89:116 (2003).
  • 18
    Canakci M and Sanli H, Biodiesel production from various feedstocks and their effects on the fuel properties. J Ind Microbiol Biotechnol 35:431441 (2008).
  • 19
    Li Y, Horsman M, Wu N, Lan CQ and Dubois-Calero N, Biofuels from microalgae. Biotechnol Prog 24:815820 (2008).
  • 20
    Posten C and Schaub G, Microalgae and terrestrial biomass as source for fuels - a process view. J Biotechnol 142:6469 (2009).
  • 21
    Taher H, Al-Zuhair S, Al-Marzouqi AH, Haik Y and Farid MM, A review of enzymatic transesterification of microalgal oil-based biodiesel using supercritical technology. Enzyme Res Article ID 468292 (2011).
  • 22
    Halim R, Danquah MK and Webley PA, Extraction of oil from microalgae for biodiesel production: a review. Biotechnol Adv 30:709732 (2012).
  • 23
    Schuchardt U, Sercheli R and Vargas RM, Transesterification of vegetable oils: a review. J Braz Chem Soc 9:199210 (1998).
  • 24
    Formo MW, Ester reactions of fatty materials. J Am Oil Chem Soc 31:548559 (1954).
  • 25
    Freedman B, Pryde EH and Mounts TL, Variables affecting the yields of fatty esters from transesterified vegetable oils. J Am Oil Chem Soc 61:16381643 (1984).
  • 26
    Vicente G, Martínez M and Aracil J, Integrated biodiesel production: a comparison of different homogeneous catalysts systems. Bioresource Technol 92:297305 (2004).
  • 27
    Canakci M and Van Gerpen J, Biodiesel production from oils and fats with high free fatty acids. Trans ASAE 44:14291436 (2001).
  • 28
    Liu Y, Lotero E, Goodwin JG and Lu C, Transesterification of triacetin using solid Brønsted bases. J Catal 246:428433 (2007).
  • 29
    Chai F, Cao F, Zhai F, Chen Y, Wang X and Su Z, Transesterification of vegetable oil to biodiesel using a heteropolyacid solid catalyst. Adv Synth Catal 349:10571065 (2007).
  • 30
    Abreu FR, Alves MB, Macêdo CCS, Zara LF and Suarez PAZ, New multi-phase catalytic systems based on tin compounds active for vegetable oil transesterificaton reaction. J Mol Catal A Chem 227:263267 (2005).
  • 31
    Abreu FR, Lima DG, Hamú EH, Einloft S, Rubim JC and Suarez PAZ, New metal catalysts for soybean oil transesterification. J Am Oil Chem Soc 80:601604 (2003).
  • 32
    Toda M, Takagaki A, Okamura M, Kondo JN, Hayashi S, Domen K and Hara M, Green chemistry: biodiesel made with sugar catalyst. Nature 438:178 (2005).
  • 33
    DaSilveira Neto BA, Alves MB, Lapis AAM, Nachtigall FM, Eberlin MN, Dupont J and Suarez PAZ, 1-n-Butyl-3-methylimidazolium tetrachloro-indate (BMI.InCl4) as a media for the synthesis of biodiesel from vegetable oils. J Catal 249:154161 (2007).
  • 34
    Lapis AAM, de Oliveira LF, Neto BAD and Dupont J, Ionic liquid supported acid/base-catalyzed production of biodiesel. ChemSusChem 1:759762 (2008).
  • 35
    Demirbas A, Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Prog Energy Combust Sci 31:466487 (2005).
  • 36
    Saka S and Kusdiana D, Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 80:225231 (2001).
  • 37
    Kusdiana D and Saka S, Kinetics of transesterification in rapeseed oil to biodiesel fuel as treated in supercritical methanol. Fuel 80:693698 (2001).
  • 38
    He H, Wang T and Zhu S, Continuous production of biodiesel fuel from vegetable oil using supercritical methanol process. Fuel 86:442447 (2007).
  • 39
    Wu Q, Chen H, Han M, Wang D and Wang J, Transesterification of cottonseed oil catalyzed by bronsted acidic ionic liquids. Ind Eng Chem Res 46:79557960 (2007).
  • 40
    Han M, Yi W, Wu Q, Liu Y, Hong Y and Wang D, Preparation of biodiesel from waste oils catalyzed by a Bronsted acidic ionic liquid. Bioresource Technol 100:23082310 (2009).
  • 41
    Liang X, Gong G, Wu H and Yang J, Highly efficient procedure for the synthesis of biodiesel from soybean oil using chloroaluminate ionic liquid as catalyst. Fuel 88:613616 (2009).
  • 42
    Akoh CC, Chang SW, Lee G-C and Shaw J-F, Enzymatic approach to biodiesel production. J Agric Food Chem 55:89959005 (2007).
  • 43
    Zhao X, El-Zahab B, Brosnahan R, Perry J and Wang P, An organic soluble lipase for water-free synthesis of biodiesel. Appl Biochem Biotechnol 143:236243 (2007).
  • 44
    Shimada Y, Watanabe Y, Sugihara A and Tominaga Y, Enzymatic alcoholysis for biodiesel fuel production and application of the reaction to oil processing. J Mol Catal B: Enzym 17:133142 (2002).
  • 45
    Shimada Y, Watanabe Y, Samukawa T, Sugihara A, Noda H, Fukuda H and Tominaga Y, Conversion of vegetable oil to biodiesel using immobilized Candida antarctica lipase. J Am Oil Chem Soc 76:789793 (1999).
  • 46
    Du W, Xu Y, Liu D and Zeng J, Comparative study on lipase-catalyzed transformation of soybean oil for biodiesel production with different acyl acceptors. J Mol Catal B: Enzym 30:125129 (2004).
  • 47
    Modi MK, Reddy JRC, Rao BVSK and Prasad RBN, Lipase-mediated conversion of vegetable oils into biodiesel using ethyl acetate as acyl acceptor. Bioresource Technol 98:12601264 (2007).
  • 48
    Iso M, Chen B, Eguchi M, Kudo T and Shrestha S, Production of biodiesel fuel from triglycerides and alcohol using immobilized lipase. J Mol Catal B: Enzym 16:5358 (2001).
  • 49
    Royon D, Daz M, Ellenrieder G and Locatelli S, Enzymatic production of biodiesel from cotton seed oil using t-butanol as a solvent. Bioresource Technol 98:648653 (2007).
  • 50
    Wasserscheid P and Welton T, Ionic Liquids in Synthesis. Wiley-VCH, Weinheim (2008).
  • 51
    Jain N, Kumar A, Chauhan S and Chauhan SMS, Chemical and biochemical transformations in ionic liquids. Tetrahedron 61:10151060 (2005).
  • 52
    Zhao H and Malhotra SV, Applications of ionic liquids in organic synthesis. Aldrichimica Acta 35:7583 (2002).
  • 53
    Domínguez de María P, Ionic Liquids in Biotransformations and Organocatalysis: Solvents and Beyond. Wiley, New York (2012).
  • 54
    Endres F and Welton T, Inorganic synthesis, in Ionic Liquids in Synthesis, ed by Wasserscheid P and Welton T. Wiley-VCH Verlag, Weinheim, 289318 (2008).
  • 55
    Kubisa P, Application of ionic liquids as solvents for polymerization processes. Prog Polymer Sci 29:312 (2004).
  • 56
    Brennecke JF, and Maginn EJ, Purification of gas with liquid ionic compounds. US 6,579,343 (2003).
  • 57
    Zhao H, Innovative applications of ionic liquids as ‘green’ engineering liquids. Chem Eng Commun 193:16601677 (2006).
  • 58
    Zhao H, Methods for stabilizing and activating enzymes in ionic liquids - a review. J Chem Technol Biotechnol 85:891907 (2010).
  • 59
    Tang S, Baker GA and Zhao H, Ether- and alcohol-functionalized task-specific ionic liquids: attractive properties and applications. Chem Soc Rev 41:40304066 (2012).
  • 60
    Earle MJ, Plechkova NV and Seddon KR, Green synthesis of biodiesel using ionic liquids. Pure Appl Chem 81:20452057 (2009).
  • 61
    Ha SH, Lan MN, Lee SH, Hwang SM and Koo Y-M, Lipase-catalyzed biodiesel production from soybean oil in ionic liquids. Enzyme Microbiol Technol 41:480483 (2007).
  • 62
    Sunitha S, Kanjilal S, Reddy PS and Prasad RBN, Ionic liquids as a reaction medium for lipase-catalyzed methanolysis of sunflower oil. Biotechnol Lett 29:18811885 (2007).
  • 63
    Gamba M, Lapis AAM and Dupont J, Supported ionic liquid enzymatic catalysis for the production of biodiesel. Adv Synth Catal 350:160164 (2008).
  • 64
    Arai S, Nakashima K, Tanino T, Ogino C, Kondo A and Fukuda H, Production of biodiesel fuel from soybean oil catalyzed by fungus whole-cell biocatalysts in ionic liquids. Enzyme Microbiol Technol 46:5155 (2010).
  • 65
    Yang Z, Zhang K-P, Huang Y and Wang Z, Both hydrolytic and transesterification activities of Penicillium expansum lipase are significantly enhanced in ionic liquid [BMIm][PF6]. J Mol Catal B: Enzyme 63:2330 (2010).
  • 66
    Zhao H, Song Z, Olubajo O and Cowins JV, New ether-functionalized ionic liquids for lipase-catalyzed synthesis of biodiesel. Appl Biochem Biotechnol 162:1323 (2010).
  • 67
    Ruzich NI and Bassi AS, Investigation of enzymatic biodiesel production using ionic liquid as a co-solvent. Canadian J Chem Eng 88:277282 (2010).
  • 68
    Ruzich NI and Bassi AS, Investigation of lipase-catalyzed biodiesel production using ionic liquid [BMIM][PF6] as a co-solvent in 500 ml jacketed conical and shake flask reactors using triolein or waste canola oil as substrates. Energy Fuels 24:32143222 (2010).
  • 69
    Ruzich NI and Bassi AS, Proposed kinetic mechanism of biodiesel production through lipase catalysed interesterification with a methyl acetate acyl acceptor and ionic liquid [BMIM][PF6] co-solvent. Canadian J Chem Eng 89:166170 (2011).
  • 70
    Ha SH, Lan MN and Koo Y-M, Continuous production and in situ separation of fatty acid ester in ionic liquids. Enzyme Microbiol Technol 47:610 (2010).
  • 71
    Lozano P, Bernal JM, Piamtongkam R, Fetzer D and Vaultier M, One-phase ionic liquid reaction medium for biocatalytic production of biodiesel. ChemSusChem 3:13591363 (2010).
  • 72
    De Diego T, Manjón A, Lozano P and Iborra JL, A recyclable enzymatic biodiesel production process in ionic liquids. Bioresource Technol 102:63366339 (2011).
  • 73
    De Diego T, Manjón A, Lozano P, Vaultier M and Iborra JL, An efficient activity ionic liquid-enzyme system for biodiesel production. Green Chem 13:444451 (2011).
  • 74
    Lozano P, Bernal JM and Vaultier M, Towards continuous sustainable processes for enzymatic synthesis of biodiesel in hydrophobic ionic liquids/supercritical carbon dioxide biphasic systems. Fuel 90:34613467 (2011).
  • 75
    Yu D, Wang C, Yin Y, Zhang A, Gao G and Fang X, A synergistic effect of microwave irradiation and ionic liquids on enzyme-catalyzed biodiesel production. Green Chem 13:18691875 (2011).
  • 76
    Zhang K-P, Lai J-Q, Huang Z-L and Yang Z, Penicillium expansum lipase-catalyzed production of biodiesel in ionic liquids. Bioresource Technol 102:27672772 (2011).
  • 77
    Liu Y, Chen D, Yan Y, Peng C and Xu L, Biodiesel synthesis and conformation of lipase from Burkholderia cepacia in room temperature ionic liquids and organic solvents. Bioresource Technol 102:1041410418 (2011).
  • 78
    de los Ríos AP, Hernández Fernández FJ, Gómez D, Rubio M and Víllora G, Biocatalytic transesterification of sunflower and waste cooking oils in ionic liquid media. Process Biochem 46:14751480 (2011).
  • 79
    Devi BLAP, Guo Z and Xu X, Characterization of ionic liquid-based biocatalytic two-phase reaction system for production of biodiesel. AIChE J 57:16281637 (2011).
  • 80
    Lai J-Q, Hu Z-L, Wang P-W and Yang Z, Enzymatic production of microalgal biodiesel in ionic liquid [BMIm][PF6]. Fuel 95:329333 (2012).
  • 81
    Lozano P, García-Verdugo E, Bernal JM, Izquierdo DF, Burguete MI, Sánchez-Gómez G and Luis SV, Immobilised lipase on structured supports containing covalently attached ionic liquids for the continuous synthesis of biodiesel in scCO2. ChemSusChem 5:790798 (2012).
  • 82
    Zhang L, Xian M, He Y, Li L, Yang J, Yu S and Xu X, A Brønsted acidic ionic liquid as an efficient and environmentally benign catalyst for biodiesel synthesis from free fatty acids and alcohols. Bioresource Technol 100:43684373 (2009).
  • 83
    Liang X and Yang J, Synthesis of a novel multi –SO3H functionalized ionic liquid and its catalytic activities for biodiesel synthesis Green Chem 12:201204 (2010).
  • 84
    Li K-X, Chen L, Yan Z-C and Wang H-L, Application of pyridinium ionic liquid as a recyclable catalyst for acid-catalyzed transesterification of Jatropha oil. Catal Lett 139:151156 (2010).
  • 85
    Guo F, Fang Z, Tian X-F, Long Y-D and Jiang L-Q, One-step production of biodiesel from Jatropha oil with high-acid value in ionic liquids. Bioresource Technol 102:64696472 (2011).
  • 86
    Elsheikh YA, Man Z, Bustam MA, Yusup S and Wilfred CD, Brønsted imidazolium ionic liquids: synthesis and comparison of their catalytic activities as pre-catalyst for biodiesel production through two stage process. Energy Conversion Manage 52:804809 (2011).
  • 87
    Ghiaci M, Aghabarari B, Habibollahi S and Gil A, Highly efficient Brønsted acidic ionic liquid-based catalysts for biodiesel synthesis from vegetable oils. Bioresource Technol 102:12001204 (2011).
  • 88
    Ghiaci M, Aghabarari B and Gil A, Production of biodiesel by esterification of natural fatty acids over modified organoclay catalysts. Fuel 90:33823389 (2011).
  • 89
    Zhou J, Lu Y, Huang B, Huo Y and Zhang K, Preparation of biodiesel from tung oil catalyzed by sulfonic-functional Brønsted acidic ionic liquids. Adv Mater Res 314–316:14591462 (2011).
  • 90
    Fang D, Yang J and Jiao C, Dicationic ionic liquids as environmentally benign catalysts for biodiesel synthesis. ACS Catal 1:4247 (2011).
  • 91
    Zhou S, Liu L, Wang B, Xu F and Sun RC, Biodiesel preparation from transesterification of glycerol trioleate catalyzed by basic ionic liquids. Chinese Chem Lett 23:379382 (2012).
  • 92
    Abbott AP, Capper G, Davies DL, Rasheed RK and Tambyrajah V, Novel solvent properties of choline chloride/urea mixtures. Chem Commun 7071 (2003).
  • 93
    Abbott AP, Boothby D, Capper G, Davies DL and Rasheed RK, Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids. J Am Chem Soc 126:91429147 (2004).
  • 94
    Abbott AP, Capper G and Gray S, Design of improved deep eutectic solvents using hole theory. ChemPhysChem 7:803806 (2006).
  • 95
    Abbott AP, Capper G, Davies DL, Munro HL, Rasheed RK and Tambyrajah V, Preparation of novel, moisture-stable, Lewis-acidic ionic liquids containing quaternary ammonium salts with functional side chains. Chem Commun 20102011 (2001). Data missing
  • 96
    Abbott AP, Capper G, Davies DL and Rasheed R, Ionic liquids based upon metal halide/substituted quaternary ammonium salt mixtures. Inorg Chem 43:34473452 (2004).
  • 97
    Wang H, Jing Y, Wang X, Yao Y and Jia Y, Ionic liquid analogous formed from magnesium chloride hexahydrate and its physico-chemical properties. J Mol Liq 163:7782 (2011).
  • 98
    Maugeri Z and Domínguez de María P, Novel choline-chloride-based deep-eutectic-solvents with renewable hydrogen bond donors: levulinic acid and sugar-based polyols. RSC Advances 2:421425 (2012).
  • 99
    Abbott AP, Capper G, Swain BG and Wheeler DA, Electropolishing of stainless steel in an ionic liquid. Trans Inst Metal Finishing 83:5153 (2005).
  • 100
    Hou Y, Gu Y, Zhang S, Yang F, Ding H and Shan Y, Novel binary eutectic mixtures based on imidazole. J Mol Liq 143:154159 (2008).
  • 101
    Abbott AP, Cullis PM, Gibson MJ, Harris RC and Raven E, Extraction of glycerol from biodiesel into a eutectic based ionic liquid. Green Chem 9:868872 (2007).
  • 102
    Blusztajn JK, Choline, a vital amine. Science 281:794795 (1998).
  • 103
    Petkovic M, Ferguson JL, Gunaratne HQN, Ferreira R, Leitão MC, Seddon KR, Rebelo LPN and Pereira CS, Novel biocompatible cholinium-based ionic liquids - toxicity and biodegradability. Green Chem 12:643649 (2010).
  • 104
    Abbott AP, Capper G, McKenzie KJ and Ryder KS, Electrodeposition of zinc–tin alloys from deep eutectic solvents based on choline chloride. J Electroanal Chem 599:288294 (2007).
  • 105
    Haerens K, Matthijs E, Chmielarz A and Van der Bruggen B, The use of ionic liquids based on choline chloride for metal deposition: a green alternative? J Environ Manage 90:32453252 (2009).
  • 106
    Abbott AP and McKenzie KJ, Application of ionic liquids to the electrodeposition of metals. Phys Chem Chem Phys 8:42654279 (2006).
  • 107
    Abbott AP, Ryder KS and König U, Electrofinishing of metals using eutectic based ionic liquids. Trans Inst Metal Finishing 86:196204 (2008).
  • 108
    Endres F, MacFarlane DR and Abbott AP, Electrodeposition from Ionic Liquids. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (2008).
  • 109
    Long T, Deng Y, Gan S and Chen J, Application of choline chloride·xZnCl2 ionic liquids for preparation of biodiesel. Chin J Chem Eng 18:322327 (2010).
  • 110
    Isahak WNRW, Ismail M, Jahim JM, Salimon J and Yarmo MA, Transesterification of palm oil by using ionic liquids as a new potential catalyst. Trends Appl Sci Res 6:10551062 (2011).
  • 111
    Zhao H, Baker GA and Holmes S, New eutectic ionic liquids for lipase activation and enzymatic preparation of biodiesel. Org Biomol Chem 9:19081916 (2011).
  • 112
    Zhao H, Zhang C and Crittle TD, Choline-based deep eutectic solvents for enzymatic preparation of biodiesel from soybean oil. J Mol Catal B: Enzym: doi: 10.1016/j.molcatb.2012.09.003 (2013).
  • 113
    Young G, Nippgen F, Titterbrandt S and Cooney MJ, Lipid extraction from biomass using co-solvent mixtures of ionic liquids and polar covalent molecules. Sep Purif Technol 72:118121 (2010).
  • 114
    Kim Y-H, Choi Y-K, Park J, Lee S, Yang Y-H, Kim HJ, Park T-J, Kim YH and Lee SH, Ionic liquid-mediated extraction of lipids from algal biomass. Bioresource Technol 109:312315 (2012).
  • 115
    Manic MS, Najdanovic-Visak V, Nunes da Ponte M and Visak ZP, Extraction of free fatty acids from soybean oil using ionic liquids or poly(ethyleneglycol)s. AIChE J 57:13441355 (2011).
  • 116
    Hayyan M, Mjalli FS, Hashim MA and AlNashef IM, A novel technique for separating glycerine from palm oil-based biodiesel using ionic liquids. Fuel Process Technol 91:116120 (2010).
  • 117
    Shahbaz K, Mjalli FS, Hashim MA and ALNashef IM, Using deep eutectic solvents for the removal of glycerol from palm oil-based biodiesel. J Appl Sci 10:33493354 (2010).
  • 118
    Shahbaz K, Mjalli FS, Hashim MA and AlNashef IM, Using deep eutectic solvents based on methyl triphenyl phosphunium bromide for the removal of glycerol from palm-oil-based biodiesel. Energy Fuels 25:26712678 (2011).
  • 119
    Li M, Pham PJ, Wang T, Pittman CU and Li T, Solid phase extraction and enrichment of essential fatty acid methyl esters from soy-derived biodiesel by novel π-complexing sorbents. Bioresource Technol 100:63856390 (2009).
  • 120
    Pham PJ, Pittman CU, Li T and Li M, Selective extraction of polyunsaturated triacylglycerols using a novel ionic liquid precursor immobilized on a mesoporous complexing adsorbent. Biotechnol Prog 25:14191426 (2009).
  • 121
    Zhao H, Baker GA, Song Z, Olubajo O, Crittle T and Peters D, Designing enzyme-compatible ionic liquids that can dissolve carbohydrates. Green Chem 10:696705 (2008).
  • 122
    Pham TPT, Cho C-W and Yun Y-S, Environmental fate and toxicity of ionic liquids: a review. Water Res 44:352372 (2010).
  • 123
    Ranke J, Stolte S, Störmann R, Arning J and Jastorff B, Design of sustainable chemical products - the example of ionic liquids. Chem Rev 107:21832206 (2007).
  • 124
    Petkovic M, Seddon KR, Rebelo LPN and Pereira CS, Ionic liquids: a pathway to environmental acceptability. Chem Soc Rev 40:13831403 (2011).
  • 125
    Coleman D and Gathergood N, Biodegradation studies of ionic liquids. Chem Soc Rev 39:600637 (2010).
  1. 1

    EMIM + = 1-ethyl-3-methylimidazolium; BMIM + = 1-butyl-3-methyl-imidazolium.