GreenSolventsfor Biocatalysis
Editedby
INAMUDDIN
AssistantProfessor,DepartmentofAppliedChemistry,Facultyof EngineeringandTechnology,AligarhMuslimUniversity,Aligarh,India
RAJENDERBODDULA
CASKeyLaboratoryofNanosystemsandHierarchicalFabrication, NationalCenterforNanoscienceandTechnology,Beijing,China
MOHDIMRANAHAMED
DepartmentofChemistry,FacultyofScience,AligarhMuslimUniversity, Aligarh,India
ABDULLAHM.ASIRI
Professor,DepartmentofChemistry,FacultyofScience, KingAbdulazizUniversity,Jeddah,SaudiArabia
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Contributors
1.Biocatalysisinindustrialbiodieselandbioethanolproduction1
DipeshKumar,AyanBanerjee,andBhaskarSingh
1. Introduction 1
2. Biodieselproduction1
3. Biocatalystsinlignocellulosicethanolproduction18
4. Conclusion 23 References 23
2.Anticancerpotentialofgreensolvents29
D.JiniandA.Anitha
1. Introduction 29
2. Greensolvents30
3. Anticancerpotentialofgreensolvents33
4. Useofgreensolventsintheextractionofoilanditsanticanceractivity43
5. Conclusionandfuturedirections44
References 46
3.SupercriticalCO2 forbiocatalysis55
ShokufehBagheri,HamidrezaBagheri,MohammadAminSedghamiz,and MohammadRezaRahimpour
1. Introduction 55
2. Kindsofnonaqueoussolventsusedforbiocatalysis58
3. Supercriticalfluid59
4. Applicationsofsupercriticalcarbondioxide(SC-CO2)solventsinbiocatalysts65
5. Conclusionsandfuturetrends69 References 70
4.Biocatalysis:Fundamentalsandsolventparameters73
HilalAcidereli,EdaGokırmakSogut,SibelDemirogluMustafov, MehmetGulcan,andFatihSen
1. Introduction 73
2. Biocatalysis 74
3. Futureperspectives82
5.Biosolventsforbiocatalysis85
MahmoudEl-Maghrabey,MohamedAmin,AbdelazizElgaml,and RaniaEl-Shaheny
1. Introduction 85
2. Classificationofbiosolventsaccordingtotheirchemicalstructure87
3. Applicationofbiosolventsinbiocatalysis95
4. Conclusions 103 References
6.Immobilizedionicliquidsforbiocatalysis109
M.J.Salar-GarcíaandV.M.Ortiz-Martínez
1. Introduction
Hydrolases
Conclusions
7.Biocatalysisinnonaqueousmedia125
MohdImranAhamed,NimraShakeel,andNaushadAnwar
1. Introduction 125
2. Selectionofnonaqueoussolvents127
3. Propertiesofnonaqueoussolventmedia128
4. Biocatalysisinsupercriticalfluids(enzymeactivity)129
5. Biocatalysisinnonaqueousorganicsolvents132
6. Biocatalysisinhydrophobicionicliquids133
7. Conclusion 136 References 136
8.Greenfluorosolventsforbiocatalysis143
MuhammadFaisal
1. Introduction 143
2. Applicationofgreenfluorosolventsinbiocatalyzedresolutionandsynthesis147
3. Conclusion 159 References 159
9.Stateoftheartandperspectivesofgreensolventsinbiocatalysis163
KrishnamoorthyLalitha,Y.SivaPrasad,andSubbiahNagarajan
1. Introduction 164
2. Reactionmediumforbiocatalysis165
3. Supercriticalfluids167
4. ApplicationsofscCO2 169
5. Othergreensolvents185
6. Conclusion 185
7. Futureperspectives185 Acknowledgments187 References 187
10.Ionicliquidsasagreensolventsfordrugsorasanactive pharmaceuticalingredient193
FarahBashir,NawshadMuhammad,NajmulHassanKhan,AbdurRahim, PervaizAhamad,AmirSadaKhan,ZahoorUllah,andMuhammadSamie
1. Introduction193
2. Properties 195
3. Pharmaceuticalapplications195
4. ILsasactivepharmaceuticalingredients196
5. SynthesisofAPI-ILs199
6. ThefutureofILsasAPIs201
7. ILsassolvents202
8. Ionicliquidsinproductionofpharmaceuticaldrugsasanalternativemedium202
9. Ionicliquidsindrugdelivery204
10. Conclusion 206 References 206
11.Switchablesolventsforbiocatalysis211
RaniaEl-Shaheny,MahmoudEl-Maghrabey,andFathallaBelal
1. Introduction 211
2. Classificationofswitchablesolvents211
3. MechanismofthereactionofCO2 withaminesandalcohols217
4. Applicationsofswitchablesolvents219
5. (Un)greennessofbiocatalysis:Facts,solutions,andmotivesforusingswitchable solvents 220
6. Realapplicationsofswitchablesolventsinbiocatalysis224
7. Conclusions 231 References 231
12.Ionicliquidsforbiocatalysis235
NaushadAnwarandMohdImranAhamed
1. Introduction 236
2. Selectionofthecomponents239
3. ILsasmediaforbiocatalyticreactions240
4. Conclusions 248 References 248
13.Organiccarbonateasagreensolventforbiocatalysis253
CongChienTruong,DineshKumarMishra,andVivekMishra
1. Greensolvents253
2. Organiccarbonates253
3. Syntheticroutes254
4. Carbonatesasthesolvents257
5. Applicationsofgreenorganiccarbonatesinchemicaltransformation258
6. Applicationinbiocatalysis265 References 269
14.Ionicliquid-mediatedbiocatalyzedorganictransformations277
BubunBanerjeeandAditiSharma
1. Introduction 278
2. Ionicliquid-mediatedorganictransformationsusingbiocatalysts278
3. Conclusions 295
15.Applicationofsupercriticalwaterinbiocatalyticprocesses301
SetarehHeidari,ErfanSadatshojaei,JalalForoozesh,andMohammadLatifi
1. Introduction 301
2. Choiceofsolventforenzymaticreactions302
3. ParametersaffectingSCF-biocatalyticsystems304
4. Applicationofsupercriticalwatertechnologiesinbiocatalyticprocesses315
5. Challengesandlimitations318
6. Conclusion 319 References 319
16.ThepotentialuseofsupercriticalCO2 asasustainablesolvent inbiocatalyticreactions325
ErfanSadatshojaei,SetarehHeidari,andReihanehHaghniaz
1. Introduction
2. BiocatalysisinscCO2
3. Biocatalysisreactions327
4. Challengesandopportunities337
17.Sustainableapproachinbiocatalyticpreparationofantibioticpeptide345
ErfanSadatshojaei,SetarehHeidari,andDavidA.Wood
1. Introduction
2. Antibioticpeptides:Characteristicsandfunctionality347
3. Cyclicpeptidesandrelatedpeptidemolecularcomplexities350 4. Peptidesynthesis352 5. Conclusions
18.Ionicliquidsandtheirbeneficialcontributionstoenzyme-catalyzed reactions,catalyticbiomassconversionandenergyconversion andstoragesystems369
SetarehHeidari,ErfanSadatshojaei,andDavidA.Wood
1.
Contributors
HilalAcidereli
SenResearchGroup,DepartmentofBiochemistry,FacultyofScience,Universityof Dumlupınar,Kutahya,Turkey
PervaizAhamad DepartmentofPhysics,UniversityofAzadJammu&Kashmir,Muzaffarabad,Pakistan
MohdImranAhamed DepartmentofChemistry,FacultyofScience,AligarhMuslimUniversity,Aligarh,Uttar Pradesh,India
MohamedAmin DepartmentofBiochemistry,FacultyofPharmacy,MansouraUniversity,Mansoura,Egypt
A.Anitha
DepartmentofChemicalEngineering,HindustanInstituteofTechnologyandScience,Chennai, India
NaushadAnwar
DepartmentofChemistry,FacultyofScience,AligarhMuslimUniversity,Aligarh,Uttar Pradesh,India
HamidrezaBagheri
DepartmentofChemicalEngineering,FacultyofEngineering,ShahidBahonarUniversityof Kerman,Kerman,Iran
ShokufehBagheri
DepartmentofChemicalEngineering,ShirazUniversity,Shiraz,Iran
AyanBanerjee
MaterialResourceEfficiencyDivision,CSIR-IndianInstituteofPetroleum,Dehradun,India
BubunBanerjee DepartmentofChemistry,IndusInternationalUniversity,Una,HimachalPradesh,India
FarahBashir
InterdisciplinaryResearchCentreinBiomedicalMaterials(IRCBM)COMSATSUniversity Islamabad;PharmaceuticalChemistry,FacultyofPharmacy,TheUniversityofLahore, Lahore,Pakistan
FathallaBelal
DepartmentofPharmaceuticalAnalyticalChemistry,FacultyofPharmacy,MansouraUniversity, Mansoura,Egypt
AbdelazizElgaml
DepartmentofMicrobiologyandImmunology,FacultyofPharmacy,MansouraUniversity, Mansoura;DepartmentofMicrobiology,FacultyofPharmacy,HorusUniversity, NewDamietta,Egypt
MahmoudEl-Maghrabey
DepartmentofPharmaceuticalAnalyticalChemistry,FacultyofPharmacy,MansouraUniversity, Mansoura,Egypt;DepartmentofAnalyticalChemistryforPharmaceuticals,Courseof PharmaceuticalSciences,GraduateSchoolofBiomedicalSciences,NagasakiUniversity, Nagasaki,Japan
RaniaEl-Shaheny
DepartmentofPharmaceuticalAnalyticalChemistry,FacultyofPharmacy,MansouraUniversity, Mansoura,Egypt;DepartmentofHygienicChemistry,CourseofPharmaceuticalSciences, GraduateSchoolofBiomedicalSciences,NagasakiUniversity,Nagasaki,Japan
MuhammadFaisal
DepartmentofChemistry,Quaid-i-AzamUniversity,Islamabad,Pakistan
JalalForoozesh
ChemicalEngineeringDepartment,UniversitiTeknologiPETRONAS,SeriIskandar,Perak, Malaysia
MehmetGulcan
DepartmentofChemistry,FacultyofScience,UniversityofVanY
ReihanehHaghniaz
DepartmentofBioengineering;CenterforMinimallyInvasiveTherapeutics(C-MIT),California NanoSystemsInstitute(CNSI),UniversityofCalifornia,LosAngeles,LosAngeles,CA, UnitedStates
SetarehHeidari
FacultyofChemicalEngineering,SahandUniversityofTechnology,Tabriz,Iran
D.Jini
DepartmentofChemicalEngineering,HindustanInstituteofTechnologyandScience,Chennai, India
AmirSadaKhan
DepartmentofChemistry,UniversityofScienceandTechnologyBannu,Bannu,Pakistan
NajmulHassanKhan
PharmaceuticalChemistry,FacultyofPharmacy,TheUniversityofLahore,Lahore,Pakistan
DipeshKumar
CentreforEnergy,IndianInstituteofTechnologyGuwahati,Assam,India
KrishnamoorthyLalitha
DepartmentofChemistry,SchoolofChemicalandBiotechnology,SASTRADeemed University,Thanjavur,TamilNadu,India
MohammadLatifi
ProcessEngineeringAdvancedResearchLab(PEARL),ChemicalEngineeringDepartment, PolytechniqueMontreal,Montreal,QC,Canada
DineshKumarMishra
GreenMaterialandProcessR&DGroup,KoreaInstituteofIndustrialTechnology,Cheonan, SouthKorea
VivekMishra
AmityInstituteofClickChemistryResearchandStudies,AmityUniversity,Noida,Uttar Pradesh;GreenChemistryNetworkCentre,DepartmentofChemistry,UniversityofDelhi, Delhi,India
NawshadMuhammad
InterdisciplinaryResearchCentreinBiomedicalMaterials(IRCBM)COMSATSUniversity Islamabad,Lahore,Pakistan
SibelDemirogluMustafov
SenResearchGroup,DepartmentofBiochemistry,FacultyofScience,Universityof Dumlupınar,K € utahya,Turkey
SubbiahNagarajan
DepartmentofChemistry,SchoolofChemicalandBiotechnology,SASTRADeemed University,Thanjavur,TamilNadu;DepartmentofChemistry,NationalInstituteofTechnology Warangal(InstituteofNationalImportance),Warangal,Telangana,India
V.M.Ortiz-Martı ´ nez
DepartmentofChemicalandEnvironmentalEngineering,TechnicalUniversityofCartagena, Murcia,Spain
Y.SivaPrasad
DepartmentofChemistry,SchoolofChemicalandBiotechnology,SASTRADeemed University,Thanjavur,TamilNadu,India
AbdurRahim
InterdisciplinaryResearchCentreinBiomedicalMaterials(IRCBM)COMSATSUniversity Islamabad,Lahore,Pakistan
MohammadRezaRahimpour
DepartmentofChemicalEngineering,ShirazUniversity,Shiraz,Iran
ErfanSadatshojaei
DepartmentofChemicalEngineering,ShirazUniversity,Shiraz,Iran
M.J.Salar-Garcı ´ a
DepartmentofChemicalandEnvironmentalEngineering,TechnicalUniversityofCartagena, Murcia,Spain
MuhammadSamie
InterdisciplinaryResearchCentreinBiomedicalMaterials(IRCBM)COMSATSUniversity Islamabad,Lahore,Pakistan
MohammadAminSedghamiz
DepartmentofChemicalEngineering,ShirazUniversity,Shiraz,Iran
FatihSen
SenResearchGroup,DepartmentofBiochemistry,FacultyofScience,Universityof Dumlupınar,Kutahya,Turkey
NimraShakeel
DepartmentofChemistry,FacultyofScience,AligarhMuslimUniversity,Aligarh,Uttar Pradesh,India
AditiSharma
DepartmentofChemistry,IndusInternationalUniversity,Una,HimachalPradesh,India
BhaskarSingh
DepartmentofEnvironmentalSciences,CentralUniversityofJharkhand,Ranchi,India
EdaGokırmakSogut
VanSecurityVocationalSchool,UniversityofYuzuncuYil,Van,Turkey
CongChienTruong
GreenProcessandSystemEngineering,KoreaUniversityofScienceandTechnology;Green MaterialandProcessR&DGroup,KoreaInstituteofIndustrialTechnology,Cheonan, SouthKorea
ZahoorUllah
DepartmentofChemistry,BalochistanUniversityofIT,EngineeringandManagementSciences (BUITEMS),Quetta,Pakistan
DavidA.Wood
DWAEnergyLimited,Lincoln,UnitedKingdom
Biocatalysisinindustrialbiodiesel andbioethanolproduction
DipeshKumara,AyanBanerjeeb,andBhaskarSinghc,* aCentreforEnergy,IndianInstituteofTechnologyGuwahati,Assam,India bMaterialResourceEfficiencyDivision,CSIR-IndianInstituteofPetroleum,Dehradun,India cDepartmentofEnvironmentalSciences,CentralUniversityofJharkhand,Ranchi,India *Correspondingauthor.e-mailaddress:bhaskar.singh@cuj.ac.in
1.Introduction
Fossilfuelshavedriventhewheelsofoursocioeconomicstructurestothecurrentlevels ofluxury.However,thesamehasledtoirreversibleenvironmentaldegradation,global warming,climatechange,andhasalsogivenrisetoenergyinsecurity.Tosupportand sustaintheviablelevelsofeconomicdevelopment,alternativesourcesofenergyare urgentlyrequired.Despitethefactthatsignificantprogresshasbeenmadetoincrease theshareofalternativeenergysourcesintheenergymix,thetransportsectorisstillpredominantlypoweredbyliquidfossilfuels.Battery-poweredvehiclesareveryappealing, butdespiteasignificantsurgeindemand,theshareofe-vehiclesplyingontheroadis negligible.Wecannoteasilydoawaywiththevehiclespoweredbyinternalcombustion enginesastheelectricvehicleswillonlyplayapivotalroleinmidtolongtermdueto severalstrategic,policy,technological,economicandinfrastructurerelatedlimitations. Tothisend,liquidbiofuelsareanattractivechoiceowingtotheirrenewability,cleaner emissionprofile,nontoxicity,miscibilityinpetrol/diesel,biodegradability,andmost importantlytheircompatibilitywiththeexistinginfrastructureofinternalcombustion engines.Biodieselandbioethanolhaveremainedthemostpopularchoicesofliquidbiofuelsforthetransportationindustry.However,theoverallsustainabilityofsuchbiofuelsis muchdependentontheprocessesandinputsthatgointotheirproduction.Conventional catalyticapproacheshaveseveralimportantlimitationandasaresultthereisagrowing bodyofknowledgeonenvironmentfriendlybiocatalysticapporoachesintheproduction ofbiofuels.Thepresentstudyreviewsthestate-of-the-artproductiontechnologyfor thesebiofuelswithaparticularemphasisonthebiocatalyticroute.
2.Biodieselproduction
Biodieseliscurrentlyoneofthemostattractivealternativefuelchoicesintransportation owingtoitsmiscibility(indiesel),renewability,cleaneremissionprofile,local
productionpotential,biodegradability,nontoxicity,andhigherlubricity [1].Thepasttwo decadeshavewitnessedanunprecedentedaugmentationinindustrialbiodieselproduction capacityinlinewiththegrowingdemandforalocallyproducedandcleanerautomotive fuel,biodieselmandates,andcommitmenttoclimatechangemitigation [2–4]
Theideaofusingvegetableoilasafuelfordieselenginedatesbacktoearly1900s whentheinventorofthedieselengineDr.RudolphDieselhadtestedpeanutoilfor thepurpose [5].However,thecurrentdesignoftheengineonlyallowsfortheuseof fuelshavingviscositycomparabletothatofdiesel.Ithaslimitedtheutilityofstraight vegetableoilinadieselengineasvegetableoilsareinvariablywaymoreviscous (10–20 diesel) [6]
Therenewedinterestinvegetableoilasafuelstemsfromthechallengesofenergy insecurity,environmentalpollution,andlimitedreservesofpetroleum [7].Tocircumventthechallengesofhighviscosityofvegetableoils,fivestrategieshavebeenputforth, andtheseinclude:blending(indiesel),pyrolysis,formationofmicroemulsion(with alcoholicsolvents),transesterification(tobiodiesel),andhydro-processing(torenewable diesel) [8].
Transesterificationofvegetableoil(orotheroleaginousmattersuchasanimalfat, recycledcookingoil,grease,etc.)remainsthemostcommonapproachtoproducefuels withviscositycomparabletodiesel [9–11].Intransesterification,thetriglyceride(TAG) tri-esterisconvertedtothreemoleculesofmono-alkylestersoffattyacids(biodiesel)and thealcoholicbackboneofTAG(glycerol)isreleasedasaby-productoftheprocess [12]. Theprocessinvolvesanacylacceptor(short-chainalcohol,methyl/ethylacetate,or dimethylcarbonate)andisgenerallysupportedbyacatalyst [9].Short-chainalcohols(primarilymethanolandethanol)arethemostcommonlyusedacylacceptors.
Transesterificationofoleaginousfeedstockstobiodieselisoftenacatalyzedprocess. Conventionally,homogeneousalkali/acidservesthepurposeofacatalyst,butthese catalystshaveseveralinherentdisadvantages(e.g.,nonreusablenature,productcontamination,downstreampurificationchallenges,andtherequirementofwaterwashand consequentgenerationofwastestreams),whichhavespurredresearchinterestsinthe developmentofnovelheterogeneouscatalysts [13,14].Severaltypesofheterogeneous (basic/acidic)materialshavebeenexaminedastransesterificationcatalysts,andeachof themhavetheirownsetofmeritsandlimitations [15,16].
Mostoftheindustrialfacilitiescurrentlyusethealkali-catalyzedprocessasitisefficientandoperatesundermildreactionconditions [17].TheUnitedStatesofAmericaand theUnitedKingdomarethetoptwobiodieselproducers [18].Thecurrentsuppliesof biodieselarepredominantlysourcedfromediblevegetableoil(rapeseedoilintheUnited Kingdom,soybeanoilintheUnitedStates,palmoilinMalaysiaandIndonesia)forwhich alkali/base-catalyzedtransesterificationisconventionallyappropriate [18].
Theutilityofnonediblefeedstocksliesinthefactthatbarringafewcountries,mostare netimportersofedibleoilsandhenceforthem,thediversionofediblesuppliestoward
biofuelproductioncannotbejustified(foodvs.fueldilemma) [19,20].Besides,mostofthe promisingnonedibleoilseedplants(suchas Jatrophacurcas, Pongamiapinnata,etc.)arehardy, havelimiteddemandformoistureandfertilizers,cangrowonmarginalland,oftenpestand diseaseresistant,andcantoleratewiderangesofenvironmentalstress.Consideringthese advantages,India’sNationalPolicyonBiofuels(2009and2018)explicitlystatesthatonly nonedibleoils(andrecycledcookingoil:RCO)beusedfortheproductionofbiodiesel [2, 21].Sincethecostofthefeedstock( 70%ofthetotalcost)hasbeenidentifiedtobethe mostsignificantdetrimentinthecost-competitiveproductionofbiodiesel,utilizationof RCOpresentsnovelcostreductionopportunities [22,23]
Theutilityofalkali/base-catalyzedtransesterificationfornonedibleandrecycled (used)cookingoilislimitedasthesetypicallycontainhighlevelsoffreefattyacids (FFAs)andmayalsocontainmoistureinexcessoftheprescribedlimits [10,24].The presenceoftheformerduringthealkali/base-catalyzedprocesssupportssaponification whilethelatermaycausehydrolysisofTAG [24].Bothofthesesidereactionsareundesirableasitnotonlyreducestheoverallyield,butalsocomplicatesproductpurification andseparationoperations.Forthesaidschemetooperateefficiently,therecommended limitsofFFAandmoisturearesetat 4mgKOHg 1 oiland0.5wt.%,respectively [25]. Unfortunately,forthenonedibleandrecycledfeedstocks,bothofthesespecificationsare rarelysatisfied [26]
Theacid-catalyzedprocess,ontheotherhand,isindependentofthefeedqualityasit cansimultaneouslyfacilitateesterification(ofFFA)andtransesterification(ofTAG)to biodiesel [27].Moreover,suchcatalystscantolerateexceedinglyhighlevelsofmoisture [28].Thus,acid-catalyzedconversionofRCOoffersaneconomicallyappealingperspective,assuggestedbyseveralofthetechno-economicstudiesonthesubject [29].Despite theadvantages,therequirementoflongreactiontime,hightemperature,high alcohol-to-oilmolarratios(andconsequentrecoveryinfrastructuredemand),neutralization,water-washsetup,andacid-resistantinfrastructurehavehamperedthelarge-scale adoptionoftheprocess [23,29,30].
Thechallengesofalkali/acid-catalyzedtransesterification,asstatedabove,haveforced researchanddevelopmentaleffortstowardthedeploymentofheterogeneous(basic/ acidic)transesterificationcatalysts.Numerousheterogeneousmaterialsofbasic(such asalkaliandalkalineearthmetals,mixedmetaloxides,andbasiczeolites) [31],acidic (heteropolyacids,sulfatedzirconia,andtungstatedzirconia) [32],andbifunctionalnature [33,34] havebeenexaminedforthepurpose.Although,unliketheirconventional homogeneouscounterparts,theheterogeneouscatalysts(bothacidandbase)offerreusability,productpurity,andavoidanceofwaterwash,thesematerialsstillsufferfrom therestofthechallenges(suchaspoorFFAandmoisturetoleranceforbasesandrequirementofhighlydemandingreactionconditionsforacids) [35].Asreported,bifunctional catalystscanfacilitatesimultaneousesterificationandtransesterificationreactions.Such catalystsholdpromiseinimprovingtheenvironmentappealandcost-competitiveness
ofbiodieselproduction,butsincebifunctionaltransesterificationcatalystsisarelatively newdevelopment,researchonsuchmaterialsstillhastogoalongwaybeforetheiractual performancecanbeassessed [33]
Anotherinterestingapproachisthenoncatalytictransformationofoleaginousmatter tobiodiesel [36].Theprocessoperatesundersupercriticalconditionsofmethanol(temperatureandpressureinexcessof300°Cand15MPa,respectively).Theprocessoffers severalvaluableadvantagesoverthecatalyzedprocess,includingveryshortreactiontime, completeconversion,simultaneoustransformationofFFAandTAG,highproduct purity,andeaseofseparation [37].However,therequirementofhighacylacceptor tooilratio,exceedinglyhightemperatureandpressureandrelatedinfrastructuraldemand hampersthetechno-economicviabilityofthefacility [23].
Anothercatalyticapproachtotransesterificationistheutilizationofbiocatalysts (enzyme).Enzymatictransesterificationhasemergedasoneofthemostattractiveopportunitiestoovercomethelimitationsofconventionalcatalysts [38].Environmentfriendliness,easeofrecovery,reusability,benignreactionconditions,high-qualityproduct,and avoidanceofawater-washingsteparetheprominentadvantagesofenzymatictransesterification [39].Enzymatictransesterificationisbyfarthemostenvironmentallyappealing approachfortheproductionofbiodiesel.
2.1Lipase-catalyzedtransesterification
Enzymatictransesterificationinvolvestheutilizationoflipase(TAGacylhydrolase,EC 3.1.1.3) [40].Theseenzymesaretypicallysourcedfromyeast,fungi,andbacteria.Lipases sourcedfromtheseorganismshaveshowndifferentregioselectivity,andthesamehas beenusedtoclassifylipaseintothefollowingfourgroups:(i)fattyacid-specificlipase, (ii)nonselectivelipase,(iii)sn-1-,sn-3-specificlipase,and(iv)sn-2-specificlipase [41].
Thefattyacid-specificlipasebringsaboutthehydrolysisofesterlinkageinTAG betweenthe9thand10thcarbonatomsinthefattyacidchain.Thesn-1-,sn-3-,and sn-2-specificlipasesperformthesamefunctionforesterlinkagesatsn-1,sn-3,and sn-2positionsinTAG.Thenonspecificformoflipaseiscapableofhydrolyzingester linkagesatanypositionintheparentTAGmolecule.Clearly,thenonspecificlipaseoffers significantadvantagesovertheircounterpartsastheycanbringaboutcompletetransesterificationofamoleofTAGtothreemolesoffattyacidalkylester(FAAE;biodiesel)and amoleofglycerol [42]
Thefirstinvestigationonlipase-catalyzedtransesterificationdatesbacktothe1990s whenlipasesourcedfrom Pseudomonasfluorescens and Mucormiehei wereutilizedforthe productionofbiodieselfromsunfloweroil [42].Sincethenavastliteraturehasaccumulatedonthesubjectcoveringdifferentdimensionsofenzymatictransesterificationincludinglipasesourcedfromnovelstrains,theirutilityfordifferentfeedstocks,recombinant lipase,lipaseimmobilization,whole-cellcatalyst,pre/posttreatmentofenzymes,effect ofdifferentacylacceptors,andtheirdosageoradditionstrategies,effectofreaction
temperatureandsolvents,processintensification,andtechno-economicandlife-cycle simulationstudies [40].
Enzymatictransesterificationisproposedtooperateviathe ping-pongbi-bimechanism inwhichtwosubstrates(TAGandmethanol)reacttoproducetwoproducts(FAMEand glycerol)withtheformationofanunstableenzyme-substrateintermediatecomplex [40]. Threekineticpathwayshavebeenputforth.Theseincludethe(i)directconversionof tri/di/monoglyceridestoFFAE,(ii)two-stepconversion(hydrolysisofTAGfollowed byesterification),and(iii)simultaneoustransesterificationofTAGandhydrolysisof TAGtoindividualfattyacidswhichisfollowedbyitsesterificationtobiodiesel [42]
Thegeneralizedmechanismofatwo-stepenzymatictransesterificationispresented below(Eqs.1and2).TheoverallreactioncanbedepictedintheformofEq.(3).The processinvolvesasequentialtransformationofTAGtoamoleculeoffattyacidanda moleculeofdiacylglycerol(DAG)inwhichthelaterproduct(DAG)isbrokendown tomonoacylglycerol(MAG)andthecorrespondingfattyacidchain.MAGreactsin anidenticalmannertoreleasetheglycerolbackbone,andthecorrespondingfattyacid chainisreleased.Thefattyacids(03molecules)thenreactwiththeacylacceptor(03molecules)inanesterificationsteptoproducethecorrespondingFAAEs(03molecules)and H2O(03molecules) [40]:
where En istheenzyme(lipase), Es istheEstersubstrate(TAG,DAG,orMAG), Fa isthe correspondingfattyacid, Gl isGlyceride(DAG,MAG)orglycerol, Al istheacylacceptor (methanol/ethanol), Ep istheproductester(FAAE),and H isH2O.
Unliketheconventionalcatalyticsystemsinwhichtherateofreactionincreaseswith anincreaseinreactiontemperature(untiloptimumtemperature),therateofincreasein enzymatictransesterificationislimitedbythetendencyoflipasedenaturationathigher temperatures.Theoptimumtemperatureforlipase-catalyzedtransesterificationisusually lowerthanthatforothercatalyst-basedtransesterification.Thereactiontemperaturefor enzymatictransesterificationhasbeenanalyzedintherangeof20–60°C [43].Theoptimumrangeformostofthelipaseshasbeenfoundtoliebetween30°Cand50°C [44].
Table1 presentsacomparisonofalternativecatalystsandsupercriticaltechnologyinthe transesterificationofoleaginousfeedstocks.
Enzymatictransesterificationoffersseveraltechnical,economic,andenvironmental advantagesovertheconventionallycatalyzedsystems.Theseprimarilyincludethe following:
(i) SimultaneousesterificationandtransesterificationofFFAandTAG,andthusappropriateforlow-costRCO.
(ii) Benignreactionconditions(lowtemperature,lowmethanoltooilmolarratio).
Table1 Comparisonofdifferentalternativecatalyticandsupercriticaltransesterification.
TypeHomogeneous [45] Heterogeneous [35] Enzymatic [40] Supercritical [46]
AlkaliAcidBaseAcidLipaseNoncatalytic
AcidtolerancePoorUnaffectedPoorUnaffectedUnaffectedUnaffected
Water tolerance PoorUnaffectedPoorUnaffectedUnaffectedUnaffected
Feedstock quality RefinedCrudeRefinedCrudeCrudeCrude
PressureAmbientAmbientAmbientAmbientAmbient10–30MPa
Time1–1.5h4–8h1–3h4–8h12–24h 1h
Molarratio1:6–1:101:30–1:501:10–1:301:30–1:501:3–1:61:30–1:50
Catalyst loading 0.5–1.5%5–10%1–5%5–10%5–10%Notrequired
Catalyst reusability PoorPoorBetterBetterBetterGreen
ProductpurityPoorPoorHighHighHighVeryhigh Downstream purification WaterwashWaterwashEco-friendlyEco-friendlyEcofriendly Notrequired
Product inhibition NoNoNoNoYesNo
Methanol inhibition NoNoNoNoYesNo
Contribution totheoverall cost
HighHighLow-refined feedstock Low,highfor crude feedstock
Lowforwastefeedstock, highforrefined feedstocks
High-cost ofenzyme High-running, operationand maintenance
Overall environmental footprint HighHighLowHighLowHigh
(iii) Theenzymecanretainitsactivityoverseveralcycles.
(iv) Veryhigh-purityproducts(thatconformtoASTMD6751andEN14214 specificationsofbiodieselfornonfeedstock-dependentpropertiessuchasthecontentofgroupIandIImetals,ediblegradeglycerol).
(v) Avoidanceofwaterwashforcrudebiodiesel.
2.1.1Effectofreactionconditions
Acylacceptor
Conventionallyshort-chainalcohols(primarilymethanol)areusedastheacylacceptorin transesterification.Othershort-chainalcoholssuchaspropanol,butanol,octanol,and branched-chainalcoholssuchasisopropanol, tert-butanol,etc.havebeenusedinsome oftheresearchinvestigations [42].Moreover,fewstudieshavealsoanalyzedtheutilityof methyl/ethylacetateanddimethylcarbonateforthesaidpurpose [41].Acetatesare attractiveacylacceptorsforbothrefinedandcrudefeedstocks,asthesedonotcauselipase inactivationandproducetriacetylglycerol(triacetin)asaby-product,whichhasfound applicationsinfood,cosmetic,pesticide,andseveralotherindustries [47].Inaddition tothese,dimethylcarbonatehasalsobeenusedinafewstudiesowingtoitsnontoxic andheat-stableproperties,andthesamecanalsobeusedasanextractionsolvent [40] Owingtoitscheaperproductioncost,methanolisthepreferredchoiceoftheacyl acceptor.Sincethecurrentsuppliesofmethanolaremajorlysourcedfromfossilfuels, itsutilizationcompromisestherenewablenatureofthefuel.Ethanol,ontheotherhand, ispredominantlyderivedfromalcoholicfermentationofbiomassandaccordinglywhen ethanolservesthepurposeofacylacceptorintransesterification(producesfattyacidethyl ester),biodieselcanbeconsideredasentirelyrenewable [48].Stoichiometrically,anoilto alcoholmolarratioof3:1issufficientforthecompleteconversionofeachfattyacidchain inTAGtobiodiesel,butusuallyanexcessofalcoholissuppliedtodrivetheforward reaction(TAGtobiodiesel)towardcompletion.Therequirementofanexcessofalcohol necessitatestheinstallationofrequisiterecoveryinfrastructuretypicallyintheformof vacuumdistillation.Thechoiceoftheacylacceptoralsodictatessomeofthevitalpropertiesofthefuelsuchasitscold-flowproperties,cetanenumber,viscosity,density,calorificvalue,etc. [49].Highpolarityandshort-chainlengthsupporthighconversion whenmethanolisusedastheacylacceptor,butitspoormiscibilityintheoleaginousmatter(nonpolar)maygiverisetomasstransferchallenges.Numerousinvestigationshave reportedtheutilityofcosolventsforimprovedmiscibilityofthereactingspecies.Ethanol, unlikemethanol,hasbettermiscibilityintheoleaginousmatterandproducesafuelwitha higherflashpoint,butitsutilityishamperedduetotheeffectofsterichindrance,which checksthekineticsofthereaction,andasaresult,theyieldofthebiodieselis compromised [50]
Oneofthechallengesinenzymatictransesterificationliesinthetoxicnatureofmethanol,whichcanaltertheactivityoflipasethroughitsinhibition,deactivation,or
denaturationand,therefore,precisecontroloverthemethanoltooilmolarratioand methanoladditionstrategyiscritical [24].
Molarratio
ForthecompleteconversionofamoleofTAG,aminimumof3:1molarequivalentof methanolisrequired;however,theliteraturesuggeststhatenzymedeactivationmay resultfromavalueaslowas 1:1.BlockingofTAGentry,enzymeunfolding,conformationalchanges,poormiscibilityofoilinmethanol,andadsorptionofmethanolon immobilizationsupportfortheenzymearesomeofthefactorsthatmaychecktheconversionoffeedstocktobiodiesel [51].Toovercomechallengesarisingfromthetoxic natureofmethanol,thefollowingfourstrategieshavefoundwidespreadappeal,andthese include(i)continuousadditionofmethanol,(ii)stepwiseadditionofmethanol,(iii)using ablendofethanolandmethanol,and(iv)usingsilicagelswelledinmethanol.Thesestrategiesenvisagelimitingtheconcentrationofmethanolinthereactionmediabelowthe toxiclevelsatanymomentintime [52]
Theoriginoflipaseaffectsitsabilitytotoleratedifferentlevelsofmethanolinthe reactionmedium.SoumanouandBornscheuer [41] reportedthatlipasesourcedfrom Pseudomonas couldbettertoleratehighlevelsandhavehigheroptimumconcentration formethanolthanthatsourcedfrom Rhizomucormiehei or Thermomyceslanuginosus Theoptimumconcentrationforthelipase(from P.cepacia)-catalyzedmethanolysisof soybeanoilwasfoundtobe8.2:1(methanoltooilmolarratio).Thelipasesourcedfrom P.cepacia couldtoleratethehighestlevelsofmethanoloverlipasederivedfromeight othersourcesunderidenticalexperimentalconditions [41].
Lipaseimmobilization
Onthebasisofthepositionofapolypeptidechain(lid),twodifferentconformationsof lipasehavebeenreported.Theseincludetheclosedinactiveconformation(blocked activesite)andtheopen,activeconformation(exposedactivesite) [41].Lipaseimmobilizationisastrategytophysicallyconfinetheenzymeoninactivesupportinitsactive conformation [53].Lipaseimmobilizationisessentialforthedevelopmentofacontinuousindustrial-scaleproductionchain.Asopposedtosmall-scalebatchoperations, whichusuallyinvolvelipaseinfreeform,thecontinuousoperationsofferhighefficiency perunitofreactorconfigurationandacomparativelyhigherrateofreturn [54].Useof immobilizationimprovesthereusability,thermal,andchemicalstability,andreducesthe chancesofproductcontaminationduetotheresiduallipase.Ithasbeenreportedthatthe incrementalcostofusinglipaseinitsfreestateis 20 higherthantheimmobilized lipase,whichisprimarilyattributedtotherequirementofregularinputsofnonreusable lipaseintheformercase [54].
However,thepossibilityofactivityloss,reducedsubstratediffusion,andleakageof theenzymefromtheimmobilizedsupportaresomeofthedisadvantagesofusingimmobilizedlipase [55]
Adsorption,ionicbonding,covalentbonding,entrapment,encapsulation,and cross-linkingarethemostcommonlyadoptedmethodsofenzymeimmobilization [54].Adsorptioninvolvestheattachmentoflipasetocarriersupport(suchasacrylicresin, textilemembrane,diatomaceousearth,celite,andpolypropylene)throughweakvander Waalsanddispersionforcesorbyhydrophobicinteractions [56].Theprocessiseasy, reversible,operatesundermildconditions,andincurslowcost.Moreover,theprocess doesnotleadtoanymajoractivitylossoflipase.Enzymeleakage,thepossibilityofsteric hindrancebythecarriersupport,andnonspecificbindingaresomeofthedisadvantages ofadsorption-basedimmobilization [57].
Numerousresearchinvestigationsonimmobilizedlipase(mainlythroughadsorption) areavailableintheliterature.MostofthesehaveanalyzedtheutilityofNovozym435. Novozym435isthetradenameforanonspecificlipasesourcedfrom Candidaantarctica Bandimmobilizedonacrylicresin [42].Amajorityofstudieshavereportedconversion of 90%forNovozym435-catalyzedtransesterificationofvirginandrecycledvegetable oils [51,58].Novozym435hasshowngoodcatalyticpropertiesandreusabilityin tertbutanolsolvent [59].InadditiontoNovozym435, Candida sp.99–125lipasesupported oncheaptextilemembranehasalsobeenusedinseveralinvestigationsasapotential immobilizationcostreductionstrategy.Immobilized Candida sp.99–125lipasehas showngoodcatalyticproperties( 85%conversion)intheconversionoflard,virginvegetableoils,andRCO [60].
Lipaseimmobilizationthroughcovalentbondingoffersthepossibilityofirreversible bindingoftheenzymeonthesupportandthuscancheckthepossibilityofenzymeleakage [53]. T.lanuginosus lipasewascovalentlyimmobilizedontohydrophobicmicroporouspoly-glutaraldehyde-activatedstyrene-divinylbenzenecopolymerandwasusedto catalyzethetransesterificationofCanolaoil.Underoptimizedconditionsof50°Cand 24h,97%conversionofthefeedstockwasobtained,andunderbatchmodeoperation, theimmobilizedlipasecouldretainitsactivityof10cycles [57]
Inentrapment-basedimmobilization,theenzymeisretainedinthematrixofapolymer,butintheory,theenzymedoesnotattachtothepolymer,andasaresult,itessentiallyremainsfreeinthesolution.Polymerization(bychemicalorphotochemical reaction),temperature-inducedgelation,andionotropicgelationarecommonmethods ofcellimmobilization(byentrapment) [61].Theprocessofentrapmentiseasy,fast, cheap,andusuallyoperatesundermildconditions.Thecatalyticeffectof P.cepacia entrappedinahydrophobicsol-gel(chemicallyinertgelpreparedbypolycondensation oftetraethoxysilaneandisobutyltrimethoxysilane)forthetransesterificationofsoybean oilwasreportedbyNoureddinietal. [62].Thestudyexaminedtheeffectofenzyme dosages,temperature,water,andalcohol.Attheoptimallevelsoftheexaminedvariables
Table2 Advantagesanddisadvantagesofalternativeimmobilizationstrategies.
MethodAdvantagesDisadvantages
AdsorptionSimplestmethod,doesnotleadtoany chemicalchanges,theprocesscanbe easilyreversed,costeffective
EntrapmentCanbeappliedtobroadrangeoflipase andsupport,operatesundermild conditions
Covalent bonding
Unlikeadsorptiontheattachmentis strongandpreventsenzymeleakage
EncapsulationSupportsco-immobilizationoftwoor morelipasesorcellsfortargeted applications,couldprovetobecost effective
Possibilityofenzymeleakage, nonspecificbindingandsteric hindrancebycarrier
Supportactsasabarrierformass transfer
Enzymeconformationand activityarestronglyinfluencedin theprocess
Diffusionchallenges
Ionic bonding SameasadsorptionEnzymeconformationisaffected
formethanolysis(0.5gwater,475mglipase,35°C,7.5:1molarratioofmethanoltosoybeanoil),aconversionof67%within1hwasobtained.Whilefortheethanolysis,aconversionof65%wasattainedwithin1hundertheoptimizedsetofconditionsforthe examinedvariables(0.5gwater,475mglipase,15.2:1molarratioofethanoltosoybean oiland35°C).Althoughthelipasecouldtolerateahighdegreeofstress(intheformof alcohols),onlyamodestdegreeofconversioncouldbeattained.Thelowconversion couldbeattributedtomasstransferlimitationsduetothesupport,whichisoneofthe mostsignificantdisadvantagesofentrapment [61] Table2 presentssomeoftheprominentadvantagesanddrawbacksofdifferentlipaseimmobilizationstrategies.
Effectofasolvent
Numerousstudiesontheutilityoforganicsolventsintransesterificationareavailablein theliteraturebothforconventional(homogeneous)andunconventional(heterogeneous)catalyticsystems.Theutilityofsolventsliesintheirabilitytoimprovethemiscibility(ofmethanolandoleaginousfeedstock)andtherebytoimprovethecontact betweenthereactants.Moreover,solventshavealsoshowneffectivenessinlowering thetimeandtemperaturerequiredtoattainequilibrium.Inenzymatictransesterification, solventsproduceanotherfavorableeffectintermsofloweredinhibitionoflipaseactivity duetomethanol,asthesolventessentiallydilutesthereactionmixture.Solventssuchas diesel,kerosene,n-hexane,benzene,toluene,cyclohexane,carbontetrachloride,chloroform,ethylacetate, tert-butanol,ethanol,etc.havebeenusedforthepurpose [42].The useofdieselandkeroseneassolventsfacilitatedcompleteconversionoffeedstockwithin
3and7h,respectively [63].Duringtransesterificationofglyceroltrioleateusing Candida sp.99–125(immobilized),Luetal. [60] analyzedtheeffectof12differentsolventsand foundthathydrophobicsolvents(suchascyclohexane,benzene, n-hexane,etc.)facilitate higheryields.ThefindingsofthestudyweresubstantiatedbyQinetal. [64] whotested ninesolventsandKojimaetal. [63] whotested18solvents.However,therearecertain challengesintheuseofsolvents,andtheseprimarilyincludetheincreasedcostandenergy requirement(fortherecoveryofsolvents).However,thesechallengesarenotapplicable todiesel(asasolvent)asbiodieselisoftenusedintheblendedform(blendofdieseland biodiesel),butadetailedanalysisofitseffectontheactivityoflipaseandotherdownstreampurificationoperationsislacking.
2.2Otherstrategiestoimprovetheefficiencyofenzymatic transesterification
Lipasessourcedfromdifferentorganisms/species/strainsexhibitadifferentdegreeof feedstockspecificity,regioselectivity,andactivityandthususingacombinationofthese presentsopportunitiesinimprovingtheoperationandcost-competitivenessoftheprocessinvolved.Thecapabilityofdifferentlipasestoselectivelytargetdifferentregionsand subsequentlycatalyzeaspecificmain/sidereactionallowsformoretargetedapplications. Utilizationofacheaplipasewithhigh-gradenonspecificandhighlyactivelipase(suchas Novozym435)canhelpreducethedemandofthelaterandaccordinglyassistincost reductionwithoutcompromisingtheyieldoftheproduct.Moreover,usingacombinationoflipasescancomplementtheactivityofindividualenzymes [51].Forinstance,the rate-limitingstepforlipasesourcedfrom T.lanuginosus isthetransformationofTAGto DAG,whilethatfor C.antarctica istheconversionofDAGtoMAG,andthereforeusing thesecancomplementeachothertoachievetheequilibriuminashorterspanoftime. Arecombinant P.pastoris celldevelopedbyYanetal. [65] couldexpress T.lanuginosus and C.antarctica lipasesbothindividuallyandinsynergy.Individually,for C.antarctica lipasetheconversionwaslimitedto61.58%,whilethatfor T.lanuginosus lipase,the reportedconversionwasonly60%.Thecombinedexpressionanduseofbothofthese ledtoaconversionof95.4%underoptimumconditionsforreactionvariables.Theutilizationoftwocommercialbrandsforthesaidpurpose(Novozym435andLipozymTL IM)(sn-1-,sn-3-specificlipasefrom T.lanuginosus immobilizedonnoncompressiblesilicagel)ledtoahighlyencouragingresult(conversion97.3%).Inasimilarinvestigation, Guanetal. [66] couldexpressboth R.miehei (sn-1,3)and P.cyclopium (nonspecific)lipase in P.pastoris andwhenbothofthesewereuseda95.1%conversionofsoybeanoilwas attainedin12h.Similarresultswereobtainedinotherstudiesonmixinglipasesfromtwo differentsources [67–69].Theapproachishighlyappealing,andthereisagrowingbody ofknowledgeonthesubject,whichwillhelpusbetterunderstandthescalabilityofthe process.
