An overview of the role of ionic liquids in biodiesel reactions

http://dx.doi.org/10.1039/c4ra15922e

http://pubs.rsc.org/en/journals/journal/RA

http://crossmark.crossref.org/dialog/?doi=10.1039/C4RA15922E&domain=pdf&date_stamp=2015-02-13

N. Muhammad et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 3

G Model

JIEC-1888; No. of Pages 10

formation of many undesired and unsafe compounds if the oil isused continually [42].

The amount of waste cooking oil generated in each countryvaries depending on the use of vegetable oil by households and thestyle of cooking adopted by the people. There is a significantvariation in the cooking style between the Asians and theEuropeans. Generally, the waste cooking oils can be an effectivefeedstock for biodiesel production as it will not be seen ascompeting with the food source material in the case where virginoil is used. Although the waste cooking oil is available locally and inabundance, the major issue is associated with the massive logisticarrangement required for collecting the waste oil as the sourcelocation is widespread within the locality in order to generatesufficient quantities. The production of biodiesel from wastecooking oil is one of the better ways to utilize it efficiently and cost-effectively.

The general synthetic route for biodiesel production is thetransesterification of vegetable oils (or animal fats) with alcoholwhich is normally either methanol or ethanol. This reaction can becatalyzed by acids, alkaline metal hydroxides, alkoxides, and non-ionic bases (such as amines and amidines) [43]. However, thesemethods have several drawbacks:

(i) The acid/base processes are often related to decay andemulsification problems

(ii) The acid-catalyzed reactions are usually much slower than thebase-catalyzed processes [44,45]. However, the base-cata-lyzed process may cause unnecessary saponification of thefatty acids feed [46].

(iii) A significantly large excess of alcohol is required to drive theequilibrium to the ester formation and to achieve the facileseparation of biodiesel from the produced glycerol by thereaction hence creating recycling problem i.e., the investmentand operating cost required to recover the alcohol used.

(iv) Practically, the oils and fats are not soluble in alcoholsresulting in barriers for the triglyceride conversions.

(v) Other issues such as energy intensive, requirement of alkalinewaste-water treatment, and interference of free fatty acids andwater [47].

Therefore, a number of new approaches have been vigorouslypursued to circumvent these problems, such as the developmentof heterogeneous catalysts [48,49] conducting the alcoholysis insupercritical methanol [50], use of ILs-catalyzed transesterifica-tion [51,52] and use of the lipase-catalyzed transesterification[53,54].

For product separation, there were several methods usedto separate FAMEs i.e., the biodiesel produced, from theother components. However, there are only two generallyaccepted methods to purify the biodiesel namely wet and drywashing.

The more traditional wet washing method is widely used toremove excess contaminants and available production chemicalsfrom the biodiesel. Since both glycerin and methanol are highlysoluble in water, water washing is very effective in removing bothcontaminants and until recently was the most common method ofpurification. It also has the advantage of removing any residualsodium salts and soaps, the latter being a byproduct of high freefatty acids (FFA) feeds, due to their water solubility. However, theinclusion of additional water to the process offers manydisadvantages, including increased operating cost and productiontime. A highly polluting liquid effluent is also generated thusrequiring special waste treatment. Significant product losses wasalso seen due to their carried over in the water phase. Furthermore,emulsion formation was also observed during the processing of

Please cite this article in press as: N. Muhammad, et al., J. Ind. Eng.

waste cooking oils or other feeds that contain high FFA content dueto soap formation [55,56].

Dry washing has replaced the water washing where an ionexchange resin or a magnesium silicate powder was used toneutralize the impurities. Both of the dry washing methods arebeing used in industrial plants currently. Other possible methodsthat can be used to purify the produced biodiesel comprised ofmembrane reactors and the addition of lime and phosphoric acid[57–59]. Nonetheless, there were numerous problems reportedespecially on the costs and the complications faced in operating theprocess at industrial scale.

3. Role of ionic liquid as a catalyst for preparation of biodiesel

ILs have been widely accepted as a new green chemical that iscapable of revolutionizing chemical processes due to theirinteresting properties. The subject has created tremendousinterest within both the academia and the chemical industries.ILs is basically a molten salt which exist in liquid state attemperature below 100 8C. Often they are also called roomtemperature ionic liquids (RTILs) [60]. For a start, the ionic liquidspossessed greener properties such as very low relative volatilityi.e., close to zero, wide liquids temperature and significantly lesstoxic compared to the organic solvent. More important is theirability to selectively dissolve various organic, inorganic, andorganometallic materials due to their tuneable polarity which canbe designed for specific purposes. They can also be made miscibleor immiscible with organic solvents and water. Both could beachieved by simply varying their anion–cation combinations.Because of these, ILs possesses numerous advantages over theconventional organic solvents, besides being more environmen-tally compatible [61,62].

In industrial practice, ILs has also been proven through fewchemical manufacturing processes to give significant advantagesuch as in the production of alkoxyphenolphospines which is ageneric precursor for photoinitiator by BASF (BASIL process) wherethe reaction time was reduced considerably resulting in tremen-dous increase in capacity. In addition, the separation of the productfrom the catalyst and ionic liquids was also easily attainedcompared to the earlier process. Generally for industrial applica-tions, considerable efforts have been made on designing acidicionic liquids to replace homogeneous and heterogeneous acids in avariety of chemical applications [63], such as catalysis insupporting enzyme catalyzed reactions [64,65] homogeneousand heterogeneous catalysis [66] purification [67] photo-isomeri-zation [68] and electrochemistry [69]. In view of the environmen-tal friendly nature of ILs, the catalyst developed was also dubbed asgreen catalyst [64,65].

Among the major impetus in reaction catalysis developmentsinvolved, is the development of processes in which easy separationof products and reuse of catalyst is made viable along with highreactivity and selectivity [70]. In this context, ILs are viewed asprospective catalysts for biodiesel production where the technical,economic and environmental aspects were addressed throughtheir inherent characteristics of being less corrosive, ease ofseparation, recyclable, applicable for continuous process and lessproduction of waste water [71,72]. Applying ILs based catalystcould reduce the number of reactions and purification stepsrequired in the biodiesel preparation and separation hencerendering the process to be more economically competitive. Atypical path way for biodiesel preparation while using ionic liquidas a catalyst has been depicted in Fig. 2.

The Brønsted acidic ILs possesses several advantageous inwhich the uniqueness of combining and designing the solid andmineral acids to substitute the traditional mineral hazardousliquid acids, such as H2SO4 [73]. Its applications are widely known

Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.01.046

http://dx.doi.org/10.1016/j.jiec.2014.01.046

C

O

O

O C

O

O C R3

R2

R1

O

3 ROH

C

O

OR R1

R O C

O

R2

R O C R3

O

OH

OH

OH

+Ionic liquid

+

Fig. 2. Mechanism of biodiesel preparation via ionic liquid catalyzed process.

N. Muhammad et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx4

G Model


in the area of esterification with excellent yields and selectivity[74,75] following its first synthesis made in 2002 by Cole and co-workers [76]. However, most of the reported works dealt withaliphatic or arylic esters. Only little information on preparation offatty acid alkyl esters using acidic ILs was found in the literature[51,77].

ILs has also been looked at in addressing the drawbacksassociated with product separation and catalyst recycling [78] inreactions where for most cases it was impossible to achieve 100%reaction conversion with 100% selectivity due to the thermody-namic limitations and the competition from parallel reactions [79].As such, the use of ILs as excellent substitute for traditionalsolvents or catalyst in organic synthesis has been growingconsiderably along with the prospect of developing green catalysts[70]. Within the last decade, the potential of ILs applications in thechemical processes have taken a new uprising turn [79]. Many ofthe reported studies have demonstrated the successful applicationof ILs as catalysts in hydrogenation, isomerization, C–C and C–Ocleavage reactions such as catalytic cracking of polyalkenes and C–C coupling reactions such as Friedel–Crafts reaction, producingexcellent yields and selectivity [79].

3.1. Ionic liquid coupled with inorganic supporting material as

catalyst for biodiesel reaction

Abreu et al. [80] studied the activity of two multi-phasesystems, for soybean oil alcoholysis based on tin compounds. Thetwo systems were prepared from the complex Sn(3-hydroxy-2-methyl-4-pyrone)2(H2O2) by dissolving it in 1-butyl-3-methyli-midazolium hexafluorophosphate ([BMIM][PF6]) and supporting iton acidic resin. On the contrary, their recyclability run resultsshowed that the multi-phase systems failed to achieve more than1.0% yield.

Later, DaSilveira et al. [78] immobilized a complex Sn(3-hydroxy-2-methyl-4-pyrone)2(H2O)2 in 1-n-butyl-3-methylimi-dazolium tetrachloro-indate (BMIM+�InCl4

�), and compared theirperformance against different catalysts. Their idea in preparing thebiphasic systems in ILs seemed to be based on the expectedadvantages for both ILs and heterogeneous catalysis. The variouscatalysts tested in their work were lanthanide(III) chloride,cerium(III) chloride, barium(II), titanium(III), zirconium(IV), cop-per(II), magnesium(II), zinc(II), cadmium(II), platinum(IV), plati-num(II), nickel(II), tin(II) and indium(III) chlorides, aluminum(III)and germanium(IV) oxides, copper(II) bromide, copper(I) iodide,Na2O5(H2O), Na2SiF6, and BF3�OEt2. However, all of these catalystsshowed very low efficiency or almost failed to promote thereaction, giving only traces of biodiesel. But when the tin complexwas used in replacement of these catalysts, the highest conversionobtained was reported to be 83% after 4 h of reaction time using1.0% catalyst. However, the reusability of the catalyst in these twosystems was poor since the yield of biodiesel reduces close to zero


after three cycles. On the other hand, a high biodiesel yield wasalso produced from vegetable oils (93–98.5%) when an organicacid/base and a Lewis acid were combined with [Et3NH]Cl-ALCl3

and H2SO4, and are immobilized in [BMIM]NTf2 but there was littlechange in the yield shown even after six cycles of re-utilisation[81]. From the work of Abreu et al. [80] and DaSilveira Neto et al.[78], it seems that the use of biphasic systems is expensive becauseof the number of chemicals used for synthesizing the ILs and for thepreparation of the complex mixtures.

Zhang et al. [82] evaluated the efficiency of Brønsted acidic ionicliquid supported onto Fe-incorporated SBA-15 (Fe-SBA-15) foresterification of oleic acid with short-chain alcohols. The resultantcatalyst showed high efficiency because of the synergistic effect ofLewis site of Fe and Brønsted acidic sites of ionic liquid. Theesterified product obtained was 87.7% under the optimizedreaction conditions of temperature 363 K, molar ratio of methanolto oleic acid 6:1, and catalyst amount 5 wt%, and reaction time 3 h.Yuan et al. [83] evaluated a novel tungstophosphoric anion basedionic liquid [BsAIm]3[PW12O40], grafted on the surface of silica foresterification of oleic acid to biodiesel. The yield of 48.49% wasachieved for product.

Zhang et al. [82] evaluated the efficiency of Brønsted acidic ionicliquid supported onto Fe-incorporated SBA-15 (Fe-SBA-15) foresterification of oleic acid with short-chain alcohols. The resultantcatalyst showed high efficiency because of the synergistic effect ofLewis site of Fe and Brønsted acidic sites of ionic liquid. Theesterified product obtained was 87.7% under the optimizedreaction conditions of temperature 363 K, molar ratio of methanolto oleic acid 6:1, and catalyst amount 5 wt%, and reaction time 3 h.Yuan et al. [83] evaluated a novel tungstophosphoric anion basedionic liquid [BsAIm]3[PW12O40], grafted on the surface of silica foresterification of oleic acid to biodiesel. The yield of 48.49% wasachieved for product.

Feng et al. [21] evaluated 1-butyl-3-methylimidazoliumtosylate ([BMIm][CH3SO3]) coupled with FeCl3 for transesterifica-tion of un-pretreated Jatropha oil containing larger amount of oleicacid with high-acid value (13.8 mg KOH/g). The yield of biodieselwas noted to increase from 12% to 93% with raise in temperaturefrom 120 to 140 8C, when only ionic liquid was used as a catalyst.Moreover a yield of 99.7% for biodiesel was obtained when FeCl3

was added to the reaction mixture at 120 8C. The high yield at lowtemperature was attributed to the metal ions having Lewis acidicsites, and more of the sites could be provided by trivalent metallicions than those of bivalent ones. The mixture of [BMIm][CH3SO3]and FeCl3 was easily recycled for another turn of esterification.

Young et al. [84] investigated the role of ionic liquid as a co-solvent for acid(HCl)-catalyzed transesterification of biomasscontaining intracellular lipids. A yield of 100% was obtained forall biomass except using Chlorella microalgae. The ionic liquid wasidentified to enhance the separation of product from by-products.It was assumed that ionic liquid containing hydrophobic regions




G Model


showed some association with the alkyl side chains on the lipid, aswell as provide energy to push the FAME product out of phase withthe co-solvent, and into its own separate and immiscible phase self,as the ionic liquid anions and cations continued to self-associate.

Zhao et al. [85] synthesized a new type of ether-functionalizedionic liquids (ILs) carrying anions of acetate or formate; which arecapable of dissolving a variety of substrates and are also lipase-compatible. These ILs are capable of dissolving oils at the reactiontemperature (50 8C); meanwhile, lipases maintained high catalyticactivities in these media even in high concentrations of methanol(up to 50%, v/v). The study on the transesterification of soybean oilin IL/methanol mixtures further confirmed the potential of usingoil-dissolving and lipase-stabilizing ILs in the efficient productionof biodiesels.

Tao et al. [86] synthesized Lewis-acidic ionic liquid i.e., cholinechloride � ZnCl2 for transesterification of soybean oil to biodiesel.The use of this ionic liquid has the advantage of performing thereaction at mild conditions and also lower cost of synthesizing theionic liquid from cheap material.

3.2. Ionic liquid solely as a catalyst for biodiesel reaction

Certain ILs may also be used as catalysts directly for oilesterification. Wu et al. [51] reported the first work on direct useof ILs as transesterification catalyst particularly the acidic ones suchas 1-(4-sulfonic acid) propylpyridiniumhydrogensulfate [HSO3-PPyr][HSO4], 1-(4-sulfonic acid) butylpyridiniumhydrogensulfate[HSO3-BPyr][HSO4], 1-(4-sulfonic acid) propyl-3-methylimidazo-lium hydrogensulfate [HSO3-PMIm][HSO4], 1-(4-sulfonic acid)butyl-3-methylimidazolium hydrogensulfate [HSO3-BMIm][HSO4],and N-(4-sulfonic acid) propyl triethylammoniumhydrogensulfate[HSO3-PEt3Am][HSO4]. A well refined cottonseed oil was used andthe reactions were catalyzed by various Brønsted acidic ILscomprising of pyridinium, imidazolium and ammonium as thecation with butyl and propyl sulfonic acid group as its side chains,and hydrogen sulfate as the anion (Fig. 3). The ILs 1-(4-sulfonic acid)butylpyridiniumhydrogensulfate showed the best catalytic perfor-mance with a 92% conversion after 5 h at an optimum reactionconditions of 12:1 oil to ILs molar ratio, catalyst concentration of5.7 wt%, and temperature at 170 8C.

[BSPy]CF3SO3 showed consistent activity after seven successivecycles in the transesterification reaction of Jatropha oil [87]. Morerecently, Han et al. [77] prepared biodiesel from waste oil usingacidic [HSO3-BPyr][HSO4] as a catalyst. The yield was found to be93.6% after 4 h of reaction time using 6:1 oil to ILs molar ratio, 6%catalyst concentration and temperature at 170 8C.

Fig. 3. Effect of IL type on the FFA conversion to methyl esters (methanol/CPO molar

ratio 21:1, temperature 170 8C, and rate of stirring 600 rpm for 4 h) [88].


Anions play a pivotal role in the acidic nature of ILs [82], and thesubstituents on cations may have an important influence on theaccessibility of the acid active sites. Ghiaci et al. [87] studiedthe effect of benzyl side chain (attached to cation) on acidity ofionic liquid for transesterification of canola oil with methanol. Theresults show that the 4B ionic liquid has the highest catalyticactivity and best recyclability under the optimized reactionconditions.

Elsheikh et al. [88] evaluated three imidazolium-based ionicliquids containing different size of alkyl side chains on cation andfixed acidic anion HSO4. These three ionic liquids were BMIMHSO4,BIMHSO4 and MIMHSO4. The BMIMHSO4 ionic liquid due to longeralkyl side chain was found more efficient than the other two ionicliquids (Fig. 3). The high catalytic activity of BMIMHSO4 wascorrelated to its acidity which was attributed to the sterichindrance of alkyl side chains on cation moiety. The optimumconditions for highest yield was 4.5 wt.% loading of BIMHSO4,methanol/CPO molar ratio of 12:1, a temperature of 160 8C, andagitation speed of 600 rpm.

Similarly Liu et al. [89] employed 1-(3-sulfonic acid) propyl-3-methylimidazolehydrosulfate–[HO3S-pmim]HSO4 for transester-ification of waste oil. The yield of biodiesel was more than 96% forthe reaction under the optimum conditions: oil/methanol ratio1:12, waste oil 15.0 g, ionic liquid 2.0 g, reaction temperature120 8C and pretreatment time 8 h. The ionic liquid was recycledfive times with more than 93% of biodiesel yield (Fig. 4).

Elsheikh et al. [90] also studied pyrazoliums based ionic liquidsi.e., sulfoalkyl-pyrazoliumhydrogensulfate and alkylsulfo-alakyl-pyrazoliumhydrogensulfate for esterification of bitter apple oil.The results showed that 2-(4-sulfobutyl) pyrazoliumhydrogen-sulfate (SBPHSO4) ionic liquid due to strong acidity, was good in itscatalytic activity. A yield of 89.5% was obtained when reaction wascarried under the condition of 5.2 wt% of SBPHSO4, molar ratio ofmethanol to BAO of 15:1, 170 8C, and 800 rpm for 6 h.

Liu et al. [91] investigate the ionic liquid [HO3S–(CH2)3–NEt3]Cl–FeCl3 containing both Brønsted and Lewis acidic sites forsynthesis of biodiesel from waste cooking oil. Due to dualfunctionality in the ionic liquid the yield of biodiesel was obtainedeven more than 95% at 120 8C for 4 h. Man et al. [92] usedtriethylammoniumhydrogensulfate (Et3NHSO4) as pretreatmentsolvent followed by KOH for transesterification of crude palm oil.The biodiesel in the yield of 82.1% was obtained when only 5.2 wt.%(with respect to reaction mixture) of Et3NHSO4 was used at 170 8Cfor 3 h. The end product obtained after KOH treatment was 96.9%.

Fang et al. [93] compared a basic ionic liquid with conventionalcatalysts for transesterification of castor oil with methanol andhigh yield of biodiesel i.e., 96% was achieved under optimum

Fig. 4. Reusability of IL [89].




G Model


reaction conditions. Zhang et al. [94] investigated the N-methyl-2-pyrrolidonium methyl sulfonate ([NMP][CH3SO3]) as a catalystwithout addition of other organic acids for transesterificationreaction of fatty acids. The yield of fatty acid alkyl esters of 93.6%and 95.3% was obtained at 70 8C for 8 h the ionic liquids waseffective in catalysis even recycled after eight times.

Liang et al. [30] synthesized acidic-based solid ionic liquidpolymer by coupling acidic ionic liquid oligomers with divinyl-benzene. The synthesized ionic liquid was used as a catalyst fortransesterification of triglycerides, a total yield of >99% ofbiodiesel was attained in one spot reaction under mild conditions.The high acidity of synthesized ionic liquid was attributed to itshigh hydrophobic surface area, high acidity, and high stability.

Guo et al. [23] investigated the effect of ultrasound irradiationon transesterification of soybean oil with methanol in the presenceof a Brønsted acidic ionic liquid. Under predetermined conditionsof methanol/oil molar ratio 9:1, 1.0 wt.% catalyst in oil, ultrasoundpower of 200 W, and temperature of 60 8C, the yield was about93.2% within the reaction time of 60 min. Cui et al. [95] studied thekinetic behavior of ionic liquid catalyzed transesterification ofmethyl acetate with n-butanol. Two different kinetic models, theideal homogeneous (IH) model and the non-ideal homogeneous(NIH) model, were used to correlate the kinetic data. The ionicliquid catalysis was compared with conventional catalysts such asion-exchange resin catalysts sulfuric acid and Amberlyst 15. Theresults showed that ionic liquid catalysis was effective than thetwo conventional catalysts.

Liang et al. [27] reported dication ionic liquids of imidazoliumbased with fix Brønsted basic anion of OH� for transesterificationof cottonseed oil. The best result of biodiesel preparation with theyield of 98.5% was obtained for bis-(3-methyl-1-imidazolium)-ethylene dihydroxide at 55 8C, 0.4% catalyst and 12:1 methanol tocottonseed oil molar ratio after 4 h of reaction time. He et al. [96]synthesis and characterize a long-chain Brønsted acid ionic liquid(IL), 3-(N,N-dimethyldodecylammonium)propanesulfonic acid p-toluenesulfonate ([DDPA][Tos]) for synthesis of biodiesel from freefatty acids (FFAs). The product of biodiesel was obtained with goodyield of 92.5–96.5% under optimum condition of: molar ratio ofalcohols to FFA at 1.5:1, mole fraction of ionic liquid at 10%, 60 8C,and 3 h. The long-chain Brønsted acidic ionic liquid was good incatalysis and could be recycled up to nine times.

3.3. Ionic liquid as deep eutectic solvents (DES) for biodiesel reaction

Recently, a special class of ILs known as the deep eutecticsolvents (DES), was introduced and they are believed to be a newgeneration of ionic liquids. The DES has received increasinginterest due to their potential to be even more environmentallybenign compared to the earlier traditional ILs besides alsopossessing equally good if not better solvation properties.

Zhao and Baker [97] critically addressed the possibility ofcombining the conventional ILs with deep eutectic solvents forbiodiesel production. The DES comprise of mixtures of organichalide salts, such as choline chloride with an organic compoundthat is a hydrogen bond donor (HBD) capable of forming ahydrogen bond with the halide ion, such as amides, amines,alcohols, carboxylic acids and many more [98]. The liquid state ofthe DES is produced through freezing point depression, wherebyhydrogen-bonding interactions between an anion and an HBD aremore energetically favored relative to the lattice energies of thepure constituents [99]. In another study, Huang et al. [100]identified a straightforward and energy efficient method toactivate commercial CaO for biodiesel production without anypretreatment by the addition of a novel DES which could easilyremove the inactive layers of calcium carbonate and calciumhydroxide on the surface of the commercial CaO during the


reaction to achieve a high FAME yield, while the reaction resultedin an extremely low yield without the addition of DES. Among themain advantages of the DES over the traditional ILs are that theyare easier to be prepared in pure state and they are not reactive towater. Studies on its toxicology reveal that quite a significantnumber of them are biodegradable [101].

Hayyan et al. [102] used a phosphonium-based deep eutecticsolvent (P-DES) followed by alkali treatment for esterification oflow grade crude palm oil (LGCPO). The P-DES was added in therange from 0.25 to 3.5% (wt/wt) of reaction mixture and highcatalytic activity in the pre-treatment of LGCPO was noted. Hayyanet al. [103] also evaluated the ammonium-based deep eutecticsolvent which consisted of hydrogen bond donor (i.e., p-toluenesulfonic acid monohydrate) (PTSA) and salt (i.e., N,N-diethylenethanol ammonium chloride) as a novel recyclablecatalyst (DEAC-DES) for transesterifcation of LGCPO. The free fattyacid content of LGCPO was reduced from 9.5% to 1% using optimumconditions, could be efficiently converted to biodiesel after alkalitreatment.

ILs provide an ideal medium for the separation of the biodieseland for the removal of the glycerol by-product. However, there is alack of long-term and/or continuous biodiesel production systemsbased on ILs, and the accumulation of glycerol and the recovery ofthe ILs are likely to pose future challenges. In addition, theenvironmental fate and any potential toxicity issues for most ILsare still largely unknown and up to the present, only a fewpreliminary reports on the toxicological properties of them wereavailable.

4. Role of ionic liquid as a solvent for biocatalyzedtransesterification process

The enzymatic transesterification method have many advan-tages over chemical methods such as mild reaction conditionsleading to low energy consumption, flexibility in choosingdifferent enzymes for various types of substrates, reusability ofenzymes, reduced water requirement in substrates hence loweringthe waste treatment, etc. [54]. Unfortunately, the current lipase-catalyzed method exhibits several downsides which prevent itfrom being commercialized. Among others, these include the highenzymes cost and lipase inactivation by acyl acceptors such asmethanol and by impurities in crude and waste oils. In addition,due to the poor miscibility between oils/fats with methanol, manyof the enzymatic transesterification reactions have to be conductedin heterogeneous systems involving complicated liquid–liquidinterface [54].

On the issue of methanol inhibition of lipases, severalapproaches have been introduced such as stepwise addition ofmethanol during reaction [104], use of other acyl acceptors such asmethyl and ethyl acetate [105], enzyme immobilization, use ofother organic solvents such as t-butanol, hexane, n-heptane, and1,4-dioxane, use of fatty acid-containing feedstock [105], andgenetic modification of lipases for higher methanol tolerance [54].

Nevertheless, some of the alternative solvents are found to betoxic to the lipase enzyme resulting in poor enzyme activity andstability [106]. Besides, it was also observed that the lipase can bedeactivated by glycerol which is a byproduct from the transester-ification reaction through its adsorption onto the enzyme surface[107]. Thus, in order to produce biodiesel using a truly greenprocess, a combination of catalysts and solvents that are bothenvironmentally friendly are needed.

Due to their unique and greener properties, ILs has beenidentified as a promising option to meet the requirement. Theenzymatic transesterification of vegetable oils in ionic liquids hasbeen demonstrated by several groups in producing biodiesel[107,108,39,109]. Placing the enzymes in the imidazolium type ILs




G Model


has been found to generally produce more active and stablecatalysts as the ILs were able to protect the lipase fromdeactivation induced by methanol [110]. A schematic representa-tion of enzymatic biodiesel process in ILs is shown in Fig. 5.

Typically, the ILs containing shorter chain 1,3-dialkylimidazo-lium cation (e.g., [BMIM]PF6 or [BMIM]NTf2) were used and thereaction was performed in a biphasic system which requires acertain amount of water. The ILs [BMIM]NTf2, when used incombination with lipase for biodiesel production, was reported toproduce a biodiesel yield of 96.3% from soybean oil [111].

In another study conducted by Yang et al. [108] involving theaddition of a salt hydrate directly to the non-aqueous system, wasfound to be unsuitable to meet the water requirement of thelipase-catalyzed reaction in [BMIM]PF6. The salt hydrate mayaffect both the water buffering and the specific ion effect on theenzyme activities but the latter was found to be more dominant inthe IL system [92]. When the water is added to the originallybiphasic system, it causes a formation of a third phase. Normally,after transesterification, the water phase will help in capturing theby-products and the acyl acceptor, thereby facilitating the recoveryand reuse of the ILs. However, it was shown in a study that thelipase activity demonstrated a decline in its efficiency after thesixth cycle of reused [112].

Contrary to the use of shorter alkyl chain cation, theImidazolium ILs containing longer alkyl chains (e.g., [C16MIM]NTf2

and [C18MIM]NTf2) have been used in a homogeneous one-phasesystem for lipase-catalyzed biodiesel production. The benefit ofthis is that it protects the enzyme and avoids direct interactionswith methanol thus allowing for the stable reuse of lipase in the ILs[11,110]. These long chain and lipophilic ILs create a nonaqueoussystem for oil transesterification, and a triphasic system at the endof the reaction, facilitates biodiesel extraction [11,110]. Similar to

Fig. 5. Enzymatic biodiesel pro


the influence of ILs on cellulase activity, increasing the viscosity ofthe ILs with [CnMIM] cations produced a negative effect on thelipase-catalyzed transesterification, in an anion order that wasconsistent with increasing viscosity: [PF6] > [BF4] > [NTf2] [110].

Ha et al. [113] has screened 23 ILs for methanolysis of soybeanoil catalyzed by immobilized Candida antarctica lipase (Novozym1

435), and identified the hydrophilic IL (i.e., [EMIM][OTf]) as thebest solvent for achieving the highest yield (80%) of fatty acidmethyl esters based on 12 h reaction time. On the other hand,Sunitha et al. [114] obtained 98–99% yields of fatty acid methylesters within 10 h of methanolysis of sunflower oil in hydrophobic[BMIM][PF6] and [EMIM][PF6] when catalyzed by Novozym 435(Table 1). Zhaoa et al. [115] introduced choline-based deep eutecticsolvents (such as choline chloride/glycerol at 1:2 molar ratio) asinexpensive, non-toxic, biodegradable and lipase-compatiblesolvents for the enzymatic preparation of biodiesel from soybeanoil. Through the evaluation of different eutectic solvents anddifferent lipases, as well as the study of reaction parameters (i.e.,methanol concentration, Novozym 435 loading and reaction time),they achieved up to 88% triglyceride conversions in 24 h.

Gamba et al. [111] also utilized several ILs (i.e., [BMIM] [Tf2N],[BMIM][BF4], and [BMIM][PF6]) as solvents in the enzymatictransesterification of soybean oil, achieving over 90% biodieselyield in 48 h. However, hydrophobic ILs usually does not dissolvetriglycerides and the lipase, resulting in a multi-phase reaction[111,113]. On the other hand, hydrophilic ILs often causes enzymedissolution and denaturation [15,116].

Rhizopusoryzae and Aspergillusoryzae were used as a whole-cell biocatalyst for biodiesel production in ionic liquids due to theirlipase-production ability. In this system, the whole-cells can beused as a lipase-immobilized matrix, and ionic liquids play a role inavoiding deactivation of the biocatalysts. The use of ILs, which is

cess in ionic liquids [17].



Table 1Effect of concentrations and types of ionic liquids on Candida antarctica-catalyzed methanolysis of sunflower oila [114].

Ionic liquidb Oil:ionic liquid (w/w) Relative product composition (% w/w)c

FAME TG DG MG

BMIm PF6 1:2 97 � 0.8 1 � 0.8 2 � 0.3 –

1:1 98 � 0.6 – 1 � 0.2 –

4:3 95 � 1 3 � 1 – –

2:1 38 � 1.5 50 � 2 8 � 0.81 –

4:1 – 98 � 0.8 1 � 0.8 –

EMIm PF6 1:2 98 � 0.8 – 1 � 0.8 –

1:1 98 � 1.6 – 1 � 1.6 –

4:3 96 � 1.4 2 � 1 1 � 1.4 –

2:1 46 � 3 40 � 3 6 � 1 1 � 0.2

4:1 – 99 � 1 – –

HMIm BF4 1:2 10 � 2.4 80 � 2.5 3 � 2 1 � 0.2

BMIm BF4 1:2 – 94 � 1 5 � 1 –

a Reaction conditions: sunflower oil (1 g, average equivalence 1.3 mM), methanol (0.4 ml, 10.4 mM), Candida antarctica (10%, 0.1 g) at 60 8C for 4 h. Values are reported as the

mean of three independent reactions � SD.b [BMIm][BF4], 1-butyl-3-methyl imidazolium tetrafluoroborate; [BMIm][PF6], 1-butyl-3-methyl imidazolium hexafluorophosphate; [EMIm][PF6], 1-ethyl-3-methyl

imidazolium hexafluorophosphate; [HMIm][BF4], 3-methyl imidazolium tetrafluoroborate.c FAME, fatty acid methyl ester; TG, triacylglycerol; DG, diacylglycerol; MG, monoacylglycerol.

Fig. 6. Fractionated reaction mixture of solid IL, glycerol and pure biodiesel [124].


G Model


totally composed of salt, make the recovery of methyl ester easy asit does not dissolve the methyl esters and the unreacted oil thusforming a biphasic system. Moreover, ionic liquids can also beexpected to act as an extracting phase for the by-product glycerol,which often has a negative effect on lipase activity in biodieselproduction [117,118].

Lipase PS i.e., Burkholderiacepacia lipase (BCL) has been widelyapplied for biotransformation reactions both in aqueous and non-aqueous phases [119,120]. BCL has been immobilized on NKA resinto produce a high performance biocatalyst which could be used forsynthesizing biodiesel in isooctane and solvent free systems[121,122]. Similarly when BCL is coated with RTILs, it was notedthat the enantioselectivity for various substrates was tremendous-ly improved [123].

Lozano et al. [124] investigated the ILs N-octadecyl-N0,N00,N000-trimethylammoniumbis(trifluoromethylsulfonyl)imide([C18tma][NTf2]) coupled with lipase for methanolysis of triolein toobtain biodiesel. The reaction was divided into liquid and solidphases to evaluate the yield and recycling of reaction components.The reaction carried out in liquid phases has better results and theyield obtained was 100% at temperature of 60 8C with pre-treatment time of 8 h. Enhanced enzyme stability of up to 1370days half-life time was observed for the ionic liquid at 60 8C. In caseof solid phases the reaction mixture was completely separated intosolid IL, glycerol and pure biodiesel via centrifugation at controlledtemperature (Fig. 6).

Diego et al. [125] used hydrophobic ionic liquids (containingCation; [C10-C18MIM], Anion; [BF4], [PF6] or [NTf2]) as reactionmedia for biocatalysis resulting in the biodiesel production. Twotypes of lipases i.e., (C. antarctica lipase B and Pseudomonas

fluorescens lipase AK) were analyzed. The highest activity wasnoted when coupling the Novozym 435 (immobilized C. antarctica

lipase with [C16MIM] [NTf2] ILs). The activity in the presence of theILs was three times more as compared to the solvent-free systemand the biodiesel yield after 3 h of reaction time was 90.29% and27.3% respectively for the case with and without the ILs.

Pan et al. [126] evaluated the esterification activity of Novozym435 and Lypozyme RM IM in ten different ionic liquids for thesynthesis of tricaprylin. Novozym 435 was observed to have highercatalytic activity compared to Lypozyme RM IM for all the studiedILs. Among the ILs used, those containing Tf2N� and PF6

� anionsshown higher catalytic activity than BF4

� anion. Subsequently,they also possessed higher activity compared to the system usinghexane as the solvent. The higher catalytic activity of Novozym 435observed in [C4MIM] Tf2N and [C4MIM] PF6 was argued to be due to


the reduction in a-helix-secondary structure of the lipase whichwas supported through analysis conducted on FTIR. A higher yieldof 92.4% for tricaprylin was obtained under the optimum reactioncondition i.e., the lipase amount of 6.1% substrate mass at a caprylicacid/glycerol molar ratio of 4.5:1 and temperature of 66.7 8C.

Lai et al. [25] extract lipids from microalgae Botryococcus braunii

strains, Chlorella vulgaris (CV), and Chlorella pyrenoidosa (CP) andwere esterified by two immobilized lipases, Penicillium expansum

lipase (PEL) and C. antarctica lipase B (Novozym 435) in solventsystems of 1-butyl-3-methylimidazolium hexafluorophosphate,[BMIm][PF6] ionic liquid and tert-butanol. In the predeterminedconditions, both enzymes induced significantly higher yields in theIL (90.7% and 86.2%) relative to that obtained in tert-butanol (48.6%and 44.4%). In comparison, PEL results slightly in higher yield thanNovozym 435.

The range of examples demonstrated shown the fact that ILs hasthe potential of a supporting and good reaction medium forbiodiesel production. However, it has to be highlighted here thatthe Novozym 435 has mainly always been used as the catalyst forall these applications.

5. Conclusion

Biodiesel production from non-edible oil is one of the potentialalternative energy sources which would reduce the worlddependency on petroleum based diesel for generation of energy.The ionic liquids especially those having acidic nature show goodcatalytic activity for biodiesel production. The ionic liquidpretreatment has the advantage over other techniques as theresulted biodiesel and glycerol could be easily separated. Also, it




G Model


has been shown that the ionic liquids could be recycled manytimes without decrease in efficiency. Ionic liquids have also beentested as solvent for catalytic reactions of biodiesel production.Both inorganic and organic catalysts were added to enhance thecatalytic activity of the ionic liquid. Generally, the ILs especially thehydrophobic ones were found to be good solvent for biocatalysis asit will not denature the enzyme.

Acknowledgement

The authors wish to thank Higher Education Commission,Government of Pakistan for providing financial support for thecurrent study under the National Research Program for Universi-ties (NRPU).

References

[1] Y. Zhang, M.A. Dube, D.D. McLean, M. Kates, Bioresource Technology 89 (2003) 1.[2] A.K. Endalew, K. Kiros, R. Zanzi, Biomass and Bioenergy 35 (2011) 3787.[3] W.J. Ting, C.M. Huang, N. Giridhar, W.T. Wu, Journal of the Chinese Institute of

Chemical Engineers 39 (2008) 203.[4] M. Verziu, B. Cojocaru, J. Hu, R. Richards, C. Ciuculescu, P. Filip, V.I. Parvulescu,

Green Chemistry 10 (2008) 373.[5] M.M. Soumanou, U.T. Bornscheuer, European Journal of Lipid Science and

Technology 105 (2003) 656.[6] Z. Masoud, A.W.D. Wan Mohd, K. Mohamed, Fuel Processing Technology 90

(2009) 770.[7] N. Jain, A. Kumar, S. Chauhan, S.M.S. Chauhan, Tetrahedron 61 (2005) 1015.[8] F. van Rantwijk, R.A. Sheldon, Chemical Reviews 107 (2007) 2757.[9] T. De Diego, P. Lozano, S. Gmouh, M. Vaultier, J.L. Iborra, Biotechnology and

Bioengineering 88 (2004) 916.[10] J.L. Kaar, A.M. Jesionowski, J.A. Berberich, R. Moulton, A.J. Russell, Journal of the

American Chemical Society 125 (2003) 4125.[11] P. Lozano, E. Garcıa-Verdugo, R. Piamtongkam, N. Karbass, T. De Diego, M.I.

Burguete, S.V. Luis, J.L. Iborra, Advanced Synthesis & Catalysis 349 (2007) 1077.[12] K.-W. Kim, B. Song, M.-Y. Choi, M.-J. Kim, Organic Letters 3 (2001) 1507.[13] H.-P. Zhu, F. Yang, J. Tang, M.-Y. He, Green Chemistry 5 (2003) 38.[14] H. Pfruender, M. Amidjojo, U. Kragl, D. Weuster-Botz, Angewandte Chemie

International Edition 43 (2004) 4529.[15] R. Madeira Lau, M.J. Sorgedrager, G. Carrea, F. van Rantwijk, F. Secundo, R.A.

Sheldon, Green Chemistry 6 (2004) 483.[16] http://www.palmoilhq.com.[17] T.D. Diego, A. Manjon, P. Lozano, J.L. Iborra, Bioresource Technology 102 (2011)

6336.[18] Y.A. Elsheikh, Industrial Crops and Products 49 (2013) 822.[19] M. Fan, J. Huang, J. Yang, P. Zhang, Applied Energy 108 (2013) 333.[20] M.M. Fan, J.J. Zhou, Q.J. Han, P.B. Zhang, Chinese Chemical Letters 23 (2012) 1107.[21] F. Guo, Z. Fang, X.-F. Tian, Y.-D. Long, L.-Q. Jiang, Bioresource Technology 102

(2011) 6469.[22] F. Guo, Z. Fang, X.-F. Tian, Y.-D. Long, L.-Q. Jiang, Bioresource Technology 140

(2013) 447.[23] W. Guo, H. Li, G. Ji, G. Zhang, Bioresource Technology 125 (2012) 332.[24] M. Hayyan, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, Fuel Processing Technology

91 (2010) 116.[25] J.-Q. Lai, Z.-L. Hu, P.-W. Wang, Z. Yang, Fuel 95 (2012) 329.[26] D.Y.C. Leung, Y. Guo, Fuel Processing Technology 87 (2006) 883.[27] J.-h. Liang, X.-q. Ren, J.-t. Wang, m. Jinag, Z.-j. Li, Journal of Fuel Chemistry and

Technology 38 (2010) 275.[28] M. Di Serio, R. Tesser, M. Dimiccoli, F. Cammarota, M. Nastasi, E. Santacesaria,

Journal of Molecular Catalysis A: Chemical 239 (2005) 111.[29] X. Liang, Applied Catalysis A: General 455 (2013) 206.[30] X. Liang, H. Xiao, C. Qi, Fuel Processing Technology 110 (2013) 109.[31] Y. Liu, D. Chen, Y. Yan, C. Peng, L. Xu, Bioresource Technology 102 (2011) 10414.[32] T. Long, Y. Deng, S. Gan, J. Chen, Chinese Journal of Chemical Engineering 18

(2010) 322.[33] M.J. Haas, A.J. McAloon, W.C. Yee, T.A. Foglia, Bioresource Technology 97 (2006)

671.[34] J.V. Gerpen, Fuel Processing Technology 86 (2005) 1097.[35] E. Crabbe, C. Nolasco-Hipolito, G. Kobayashi, K. Sonomoto, A. Ishizaki, Process

Biochemistry 37 (2001) 65.[36] M.A. Kalam, H.H. Masjuki, Biomass and Bioenergy 23 (2002) 471.[37] B.S. Souza, D.M.M. Pinho, E.C. Leopoldino, P.A.Z. Suarez, F. Nome, Applied

Catalysis A: General 433–434 (2012) 109.[38] Q. Wu, H. Wan, H. Li, H. Song, T. Chu, Catalysis Today 200 (2013) 74.[39] K.-P. Zhang, J.-Q. Lai, Z.-L. Huang, Z. Yang, Bioresource Technology 102 (2011)

2767.[40] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce, J.G. Goodwin, Industrial

& Engineering Chemistry Research 44 (2005) 5353.[41] M.C. Math, S.P. Kumar, S.V. Chetty, Energy for Sustainable Development 14

(2010) 339.[42] M.K. Lam, K.T. Lee, A.R. Mohamed, Biotechnology Advances 28 (2010) 500.


[43] U. Schuchardt, R. Sercheli, R.M. Vargas, Journal of the Brazilian Chemical Society9 (1998) 199.

[44] M. Formo, Journal of the American Oil Chemists Society 31 (1954) 548.[45] B. Freedman, E.H. Pryde, T.L. Mounts, Journal of the American Oil Chemists

Society 61 (1984) 1638.[46] G. Vicente, M. Martınez, J. Aracil, Bioresource Technology 92 (2004) 297.[47] H. Fukuda, A. Kondo, H. Noda, Journal of Bioscience and Bioengineering 92 (2001)

405.[48] Y. Liu, E. Lotero, J.G. Goodwin Jr., C. Lu, Journal of Catalysis 246 (2007) 428.[49] A.A.M. Lapis, L.F. dejOliveira, B.A.D. Neto, J. Dupont, ChemSusChem 1 (2008) 759.[50] H. He, T. Wang, S. Zhu, Fuel 86 (2007) 442.[51] Q. Wu, H. Chen, M. Han, D. Wang, J. Wang, Industrial & Engineering Chemistry

Research 46 (2007) 7955.[52] X. Liang, G. Gong, H. Wu, J. Yang, Fuel 88 (2009) 613.[53] H. Fukuda, S. Hama, S. Tamalampudi, H. Noda, Trends in Biotechnology 26 (2008)

668.[54] C.C. Akoh, S.W. Chang, G.-C. Lee, J.-F. Shaw, Journal of Agricultural and Food

Chemistry 55 (2007) 8995.[55] M. Berrios, R.L. Skelton, Chemical Engineering Journal 144 (2008) 459.[56] M. Canakci, J.V. Gerpen, Transactions of the ASAE 44 (2001) 1429.[57] S. Cooke, S. Abrams, B. Bertram, Purification of biodiesel with adsorbent materi-

als, US, 2003.[58] C.W. Chiu, M.A. Dasari, W.R. Sutterlin, G.J. Suppes, Industrial and Engineering

Chemistry Research 45 (2006) 791.[59] J.C. Yori, S.A. D’Ippolito, C.L. Pieck, C.R. Vera, Energy and Fuels 21 (2007) 347.[60] T. Welton, Chemical Reviews 99 (1999) 2071.[61] Shariati, K. Gutkowski, C.J. Peters, AIChE Journal 51 (2005) 1532.[62] J.F. Brennecke, E.J. Maginn, AIChE Journal 47 (2001) 2384.[63] H. Xing, T. Wang, Z. Zhou, Y. Dai, Molecular Catalysis A: Chemical 264 (2007) 53.[64] J. Shen, H. Wang, H. Liu, Y. Sun, Z. Liu, Journal of Molecular Catalysis A: Chemical

280 (2008) 24.[65] J. Gui, X. Cong, D. Liu, X. Zhang, Z. Hu, Z. Sun, Catalysis Communications 5 (2004)

473.[66] J. Gorman, Science News 160 (2001) 156.[67] S.G. Cull, J.D. Holbrey, V. Vargas-Mora, K.R. Seddon, G.J. Lye, Biotechnology and

Bioengineering 69 (2000) 227.[68] X. Li, D. Zhao, Z. Fei, L. Wang, Science in China Series B 49 (2006) 385.[69] T. Tsuda, C.L. Hussey, Interface-Electrochemical Society 16 (2007) 42.[70] M. Picquet, D. Poinsot, S. Stutzmann, I. Tkatchenko, I. Tommasi, P. Wasserscheid,

J. Zimmermann, Topics in Catalysis 29 (2004) 139.[71] M.J. Earle, K.R. Seddon, Pure and Applied Chemistry 72 (2000) 1391.[72] C. Xie, H. Li, L. Li, S. Yu, F. Liu, Journal of Hazardous Materials 151 (2008) 847.[73] D. Fang, K. Gong, Q. Shi, Z. Liu, Catalysis Communications 8 (2007) 1463.[74] D.C. Forbes, K.J. Weaver, Journal of Molecular Catalysis A: Chemical 214 (2004)

129.[75] Z. Zhang, W. Wu, B. Han, T. Jiang, B. Wang, Z. Liu, The Journal of Physical

Chemistry B 109 (2005) 16176.[76] A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weaver, D.C. Forbes, J.H. Davis, Journal

of the American Chemical Society 124 (2002) 5962.[77] M. Han, W. Yi, Q. Wu, Y. Liu, Y. Hong, D. Wang, Bioresource Technology 100

(2009) 2308.[78] B.A. DaSilveira Neto, M.B. Alves, A.A. Lapis, F.M. Nachtigall, M.N. Eberlin, J.

Dupont, P.A. Suarez, Journal of Catalysis 249 (2007) 154.[79] D. Zhao, M. Wu, Y. Kou, E. Min, Catalyst Today 74 (2002) 157.[80] F.R. Abreu, M.B. Alves, C.S. Macedo, L.F. Zara, P.A.Z. Suarez, Molecular Catalysis A:

Chemical 227 (2005) 263.[81] Teresa De Diego, Arturo Manjon, Pedro Lozano, J.L. Iborra, Bioresource Technol-

ogy 102 (2011) 6336.[82] L. Zhang, Y. Cui, C. Zhang, L. Wang, H. Wan, G. Guan, Industrial & Engineering

Chemistry Research 51 (2012) 16590.[83] L. Yuan, Z. Bin, L. Hansheng, Industrial Catalysis 19 (2011) 27.[84] G. Young, F. Nippen, S. Titterbrandt, M.J. Cooney, Biofuels 2 (2011) 261.[85] H. Zhao, Z. Song, O. Olubajo, J.V. Cowins, Applied Biochemistry and Biotechnol-

ogy 162 (2010) 13.[86] L. Tao, Y. Deng, S. Gan, C. Ji, Chinese Journal of Chemical Engineering 18 (2010)

322.[87] M. Ghiaci, B. Aghabarari, S. Habibollahi, A. Gil, Bioresource Technology 102

(2011) 1200.[88] Y.A. Elsheikh, Z. Man, M.A. Bustam, S. Yusup, C.D. Wilfred, Energy Conversion and

Management 52 (2011) 804.[89] S. Liu, Z. Wang, S. Yu, C. Xie, Bulletin of the Chemical Society of Ethiopia 27 (2013)

289.[90] Y.A. Elsheikh, Process Safety and Environmental Protection (2013).[91] S. Liu, Z. Wang, K. Li, L. Li, S. Yu, F. Liu, Z. Song, Renewable Sustainable Energy 5

(2013) 1.[92] Z. Man, Y.A. Elsheikh, M.A. Bustam, S. Yusup, M.I.A. Mutalib, N. Muhammad,

Industrial Crops and Products 41 (2013) 144.[93] D. Fang, C. Jiang, J. Yang, Energy Technology 1 (2013) 135.[94] L. Zhang, M. Xian, Y. He, L. Li, J. Yang, S. Yu, X. Xu, Bioresource Technology 100

(2009) 4368.[95] X. Cui, J. Cai, Y. Zhang, R. Li, T. Feng, Industrial & Engineering Chemistry Research

50 (2011) 11521.[96] L. He, S. Qin, T. Chang, Y. Sun, X. Gao, Catalysis Science & Technology 3 (2013)

1102.[97] H. Zhao, G.A. Baker, Journal of Chemical Technology & Biotechnology 88

(2013) 3.


http://refhub.elsevier.com/S1226-086X(14)00085-9/sbref0005
























http://www.palmoilhq.com/































































































































G Model


[98] L. Liu, Y. Kong, J.P.L.H. Xu, Z.L.J.X. Dong, Microporous and Mesoporous Materials115 (2008) 624.

[99] C.A. Nkuku, R.J. LeSuer, Journal of Physical Chemistry 111 (2007) 13271.[100] W. Huang, S. Tang, H. Zhao, S. Tian, Industrial & Engineering Chemistry Research

52 (2013) 11943.[101] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, Journal of the

American Chemical Society 126 (2004) 9142.[102] A. Hayyan, M. Ali Hashim, F.S. Mjalli, M. Hayyan, I.M. AlNashef, Chemical

Engineering Science 92 (2013) 81.[103] A. Hayyan, M.A. Hashim, M. Hayyan, F.S. Mjalli, I.M. AlNashef, Industrial Crops

and Products 46 (2013) 392.[104] Y. Shimada, Y. Watanabe, A.T.Y. Sugihara, Journal of Molecular Catalysis B:

Enzymatic 17 (2002) 133.[105] W. Du, Y. Xu, D. Liu, J. Zeng, Journal of Molecular Catalysis B: Enzymatic 30

(2004) 125.[106] X.Q. Chen, C.J. Liu, J. Wang, Y. Li, Chinese Journal of Organic Chemistry 29 (2009)

128.[107] A.P. de los Rıos, F.J.H. Fernandez, D. Gomez, M. Rubio, G. Vıllora, Process

Biochemistry 46 (2011) 1475.[108] Z. Yang, K.-P. Zhang, Y. Huang, Z. Wang, Journal of Molecular Catalysis B:

Enzymatic 63 (2010) 23.[109] N.-W. Li, M.-H. Zong, H. Wu, Process Biochemistry 44 (2009) 685.[110] T. De Diego, P. Lozano, S. Gmouh, M.I.,J.L. Vaultier, Biomacromolecules 6 (2005)

1457.[111] M. Gamba, AAM.D.J. Lapis, Advanced Synthesis and Catalysis 350 (2008) 160.


[112] N.I. Ruzich, A.S. Bassi, The Canadian Journal of Chemical Engineering 88 (2010)277.

[113] H.S.H.M.N. Lan, S.H. Lee, S.M. Hwang, Y.-M. Koo, Enzyme and Microbial Tech-nology 41 (2007) 480.

[114] S. Sunitha, S. Kanjilal, P.S. Reddy, R.B.N. Prasad, Biotechnology Letters 29 (2007)1881.

[115] H. Zhao, C. Zhang, T.D. Crittle, Journal of Molecular Catalysis B: Enzymatic 85–86(2013) 243.

[116] H. Zhao, G.A. Baker, S. Holmes, Organic & Biomolecular Chemistry 9 (2011) 1908.[117] A.P. Abbott, P.M. Cullis, M.J. Gibson, R.C. Harris, E. Raven, Green Chemistry 9

(2007) 868.[118] V. Dossat, D. Combes, A. Marty, Enzyme and Microbial Technology 25 (1999) 194.[119] L.H. Andrade, L.P. Rebelo, C.G.C.M. Netto, H.E. Toma, Journal of Molecular

Catalysis B: Enzymatic 66 (2010) 55.[120] Z.G. Chen, R.X. Tan, M. Huang, Process Biochemistry 45 (2010) 415.[121] T. Liu, Y. Liu, X. Wang, Q. Li, J. Wang, Y. Yan, Journal of Molecular Catalysis B:

Enzymatic 71 (2011) 45.[122] Y. Liu, T. Liu, X. Wang, L. Xu, Y. Yan, Energy and Fuels 25 (2011) 1206.[123] P. Hara, J.-P. Mikkola, D.Y. Murzin, L.T. Kanerva, Journal of Molecular Catalysis B:

Enzymatic 67 (2010) 129.[124] P. Lozano, J.M. Bernal, G. Sanchez-Gomez, G. Lopez-Lopez, M. Vaultier, Energy &

Environmental Science 6 (2013) 1328.[125] T. De Diego, A. Manjon, P. Lozano, M. Vaultier, J.L. Iborra, Green Chemistry 13

(2011) 444.[126] Q. Pan, L. Yang, X. Meng, Journal of the American Oil Chemists’ Society (2013) 1.





















































An overview of the role of ionic liquids in biodiesel reactions

Documents

Transcript of An overview of the role of ionic liquids in biodiesel reactions