Contents lists available atScienceDirect
Catalysis Today
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d
Catalytic insights into the production of biomass-derived side products methyl levulinate, furfural and humins
Layla Filiciotto
a, Alina M. Balu
a,∗, Jan C. Van der Waal
b, Rafael Luque
aaDepartamentodeQuimicaOrganica,UniversidaddeCordoba,CampusdeRabanales,EdificioMarieCurie(C-3),CtraNnalIV-A,km396,Cordoba,Spain
bAvantiumChemicals,Zekeringstraat29,1014BV,Amsterdam,TheNetherlands
a r t i c l e i n f o
Articlehistory:
Received2December2016
Receivedinrevisedform15February2017 Accepted4March2017
Availableonlinexxx
Keywords:
Biomassvalorization Side-products Methyllevulinate Furfural Humins
a b s t r a c t
Biomassconversionintousefulchemicals,materialsandfuelsemergedasapromisingalternativetoward replacingthecurrentproductionofmostofthesecommoditiesandspecialtyproductsfrompetroleum feedstocks.Interestingly,notonlyendproductsbutalsoside-productsfrombiomassvalorizationhavea significantpotentialforfutureresearchandfurtherconversionintonovelfamiliesofusefulderivatives.
Basedonsuchpotential,theproposedcontributionhasbeenaimedtofocusoncatalyticinsightsintothe productionofthreeparticularbiomass-derivedsideproductsfromthehemicellulosicfraction,namely methyllevulinate(MeL),FurfuralandHuminsusingbothhomogeneousandheterogeneouslycatalyzed processes.
©2017ElsevierB.V.Allrightsreserved.
1. Introduction
Fossil-basedresourcescurrentlyusedforenergyandmaterial productionarenotendlessandwillrunoutinduetime.Thison- goingdepletiontogetherwiththedetrimentalaugmentation of greenhouseeffectshavedriventheworldwidescientificcommu- nitytofindeconomicallyvalid,environmental-friendly,sustainable alternatives tocurrent petroleum-based industry. Novel routes employingsustainableC-sourceswillneedtobesought,especially inthecaseofmaterialsandforsometypesoftransportationfuels.
Thedevelopmentofhigh-outputtechnologiesinagronomics[1]
hasyieldedasurplusofoneofthemostpromisingundepletable sources:vegetablebiomass.Infact,plant-derivedfeedstockshave beenproposedasthemajorrenewableC-sourcealternativethat canmatchesfossilresourcesinthechemicalsand specificfuels production[2].Thevarietyofbiologicaland/orchemocatalyticcon- versionstrategiestodrop-infuels,highaddedvaluechemicalsand novelmaterialsmakebiomassanattractiveandversatileresource [1–5]. In particular, the plentiful abundance of lignocellulosic biomass,alongwithitspotentialfastrenewability,accommodate theneedtofindacheapandreadilyavailablegreenresource.
Lignocelluloseis themain structuralconstituent of allland- based plants [4], found in agricultural and forestry residues,
∗ Correspondingauthor.Tel.:+34957211050.
E-mailaddresses:[email protected],[email protected](A.M.Balu).
herbaceous and woody crops alike. This makes lignocellulosic biomassareadilyavailablenon-fossilcarbonsourcewithoutbeing in competition with the food and feed industries [6]. A clear challengeintheconversionoflignocelluloseintovaluable-added chemicalsrelatestoitsstructuralcomplexity.Inpart,thisisdue tothenatureofthethreemajorcomponentsinbiomass,namely:
(i)lignin(15–30%)whichconfersstructuralrigiditytotheplant, comprisingarecalcitrantpolyaromatic/phenolicspolymer;(ii)cel- lulose(35–50%),aninsoluble,crystalline,linearglucosepolymer linkedby-1,4glycosidicbondswhosecrystallinitygivestensile strengthtotheplantcellwalls;and(iii)hemicellulose(20–35%), a cross-linked fiberous amorphous polymer, mostly comprised ofdifferentpentoseswithsomehexosesanduronicacidssugar monomersoftenfunctionalizedwithmethoxyandacetoxygroups.
Asawholefeedstock,lignocellulosicbiomasscanbedirectlycon- vertedinto syngasandbio-oilby gasificationandpyrolysis[7].
However, more promising routes seem to be thefractionation oflignocelluloseintoitsindividualconstituentsbythemeansof chemicaland/orenzymatichydrolysis[2,4].
Lignocellulosic biomass canbe converted intoa plethora of possible compounds, ranging from biofuels to fine chemicals and specialty materials, which include both commercial and unknowncompounds[2].Inordertolimittheresearch’stargeted molecules, the2004US DOEreport [8]listedthe topbiomass- derivedchemicalcompounds(laterrevisitedbyBozellandPetersen [9]),whichcomprisedofthe10mostpromisingcompoundsthat couldachievemassproductionwiththecurrentbiorefinerytech- http://dx.doi.org/10.1016/j.cattod.2017.03.008
0920-5861/©2017ElsevierB.V.Allrightsreserved.
nologies. Several new industrial scale productions of biomass derivatives have already been achieved, for example with the two-stepacid-catalyzedproductionofLevulinicAcid(LA)intheso- calledBiofineProcess[10],whichrecentlyhasbeenrevitalizedby GFBiochemicals.AdvancementsofAvantium’sYXYtechnology[11]
havefurtherplansforlargescaleproductioninAntwerp,Belgium forthesustainableconversionofplant-basedfeedstocksinto2,5- furandicarboxylicacid(FDCA),platformchemicalforthecomplete substitutionofpollutingterephthalateplastics.Itis,however,clear fromtheDoEreportandexemplifiedbytheseprocessesthatthe preferredfeedstockforselectivechemicalproductionaresugars fromthe(hemi-)cellulosicfractionofbiomass.
Chemocatalyticsugarconversionsoftenleadstonumerousside- productswhichlimittargetedproductyields,increaseseparation costsand rendersoverallproduction uneconomical.Fine-tuning ofthecurrentprocesseswiththeaimtominimizewastehasso farprovendifficult,andthus,thevalorizationofside-productsand wastestreamshastobestudiedindepth.
Thepresentcontributionaimstoprovideageneraloverview of theproduction of three majorproducts fromacid catalyzed sugarconversionprocesses:LevulinicacidanditsMethylester(LA, resp.,MLA),Furfural(FF),andHumins(HUM).Exploringnewways oftheirutilizationasa feedstockfor novelprocessmayleadto improvedefficienciesinnewbiorefineryconcepts.
2. Methyllevulinate
Levulinateestersareapromisingclassofbiomassderivatives withlow toxicity, highlubrification andmoderate flowproper- ties at low temperatures [12–15]. Due to their relative simple production,theypossesswideapplicationpotentialasfragrances [16],herbicides[17],cancertherapeutics[18],andasoxygenated additivesforgasolineanddieseltransportationfuels.Intheirpro- duction,huminformation viapolymerizationofformed furanic compoundsuponhemicellulosedehydrationissuppressedathigh alcohol/waterratios, makingalkyl levulinatesamore attractive platformchemicalascomparedtoitshydrolyzedcounterpart,lev- ulinicacid.
Methyl levulinate, in particular, is employed as a flavoring agent,and,moreimportantly,possessesgreatpotentialasplatform molecule,duetoitstransformableketoandestergroup.Alkyllev- ulinatesmaybeproducedby(i)alcoholesterificationoflevulinic acid[19],(ii)conversionoffurfurylalcoholanditsethers[20],(iii) alcoholreactiveextraction[21–23],(iv)carbohydratesconversion inalcoholmedia[24],or(v)asaco-productbydirectsynthesisfrom biomass(Scheme1).
Forexample,methyllevulinateisaside-producttoAvantium’s methanolic fructose dehydration to 5-methoxymethylfurfural (MMF)[11],aswellasaby-productinthearomaticproductionfrom liquefactionandfractionationoflignocellulosicbiomassundersub- criticalmethanolconditions[25].
2.1. HomogeneouslycatalyzedMeLproduction
Producing levulinic esters from levulinic acid has been attemptedby esterification of levulinicacid, produced from an aqueousproductmixtureofhomogeneouslyacid-catalyzedcon- versionofbiomass[19]whichcomprisesalsofurfural,formicacid andthemineralacidcatalyst(thelattertwocatalyzetheesterifica- tionaswell).Asummaryofthedifferentemployedhomogeneous catalyticsystemsfor MeLproduction fromvarioussubstratesis giveninTable1.
Homogeneousacidcatalystshavebeenextensivelyusedinthe synthesisofmethyllevulinate.Brønstedacidsareknowntocat- alyzefructosedehydration,whilealdoseisomerization,required
Table1
Overallcomparison ofhomogeneous catalysts employedinmethyl levulinate production.
Catalyst Substrate T(◦C) t(min) Yield(%) Refs.
HCl Glucose 160 150 12 [31]
H2SO4 Glucose 160 150 13 [31]
H2SO4 Fructose 160 150 73 [31]
H2SO4 Cellulose 183 12 38 [32,33]
H2SO4 Cellulose 190 300 55 [34]
H2SO4 Glucose 200 120 49 [32]
Al2(SO3)4 Glucose 160 150 64 [36]
Al2(SO3)4 Glucose 200 120 54 [35]
PTSA Cellulose 180 300 20 [42]
In(OTf)3 Cellulose 180 300 52 [42]
In(OTf)3+PTSA Cellulose 180 300 70 [42]
In(OTf)3+2-NSA Cellulose 180 300 75 [42]
whenusingglucose,iscatalyzedbybasesorLewisacids[26–30].
Forthisreason,mineralacidssuchasH2SO4andHClyielded>70%
methyllevulinate from fructose, but low yields(ca. 12%)were observedfromglucose[31](Table1).
Asforcellulose,yieldsof46%[32,33]and55%ofmethyllevuli- nate[34]wereobtainedinthepresenceofconcentratedsulfuric acidat179◦Candinnear-criticalconditions,respectively.Garves etal.[33]investigatedtheefficiencyofextremelylowacid(ELA) concentrations(upto0.01molL–1),obtaining49%methyllevuli- natefromglucoseafter2hat200◦CatthehighestELA.ELAswere alsoinvestigated in themethanolysis ofcellulose, yielding 50%
methyllevulinateat210◦Cusing0.01molL-1sulfuricacid.
Homogeneousmixedacids,particularlyAl2(SO4)3,sparkedsci- entific interest thanks to the co-presence of both Lewis and Brønsted acidity.Yields of 54% [35] and of 64% [36] fromglu- cosewerereportedduetothemetalcationabilitytocatalyzethe isomerizationof glucose to fructose, and thelatter consequent dehydration.
Although highyields might beachieved, homogeneous acid catalystspresentmajorprocessingdrawbacks(e.g.equipmentcor- rosion,inefficientseparabilityandreusability,pollutingwaste)as wellasalcoholdehydrationtoitsdiether.Apreferentialfocuswas shiftedtowardheterogeneouscatalyticsystems.
2.2. HeterogeneouslycatalyzedMeLproduction
Forinstance,methyllevulinateyieldsof88%wereobtainedover a methylatedpolymer–supported triazene(Me-PST)[36],while
>95%methyllevulinateyieldswereachievedinmesoporousstruc- tures.Inparticular,Meleroetal.[38]reporteda95%yieldofthe levulinateesterinmethanolat117◦CoverSBA 15 (CH2)3 SO3H, whichdecreasedtoonly93%afterfourconsecutiveruns.Thiswas attributedtoanenhancedmassdiffusionoflevulinicacidandrel- ativeesterinthemesopores.Similartrendswerealsoobservedin theuseofsolidheteropolyacids(HPA).HPApossesshigherintrinsic aciditycomparabletoH2SO4butthesecatalyticsystemsoftendis- playlimitedsurfaceareaandoftenhighsolubilityinwaterand/or polarsolvents,which leadstoleachingof theactivephase.The useof a porouscatalyst supportis oftenemployed in orderto avoidtheseproblems.Yieldsof73to99%ofmethyllevulinatewere reportedforlevulinicacidconversionforH4SiW12O40incorporated onmesoporousSiO2[39]andH3PW12O40–ZrO2withbenzenemoi- eties[40].Theremarkableactivityofthesecatalystsisgivenby(i) thepresenceofhydrophobicmoietiesfacilitatingfasterformation ofmethyllevulinatethankstotheircapacitiesoffacilitatinglev- ulinicacidadsorptionoverH2O,and(ii)thelargerporestructure, ratherthanacidsitesdensity,type,andactivity,factorsthatinnon- porouscatalysts(e.g.sulfonatedoxides)arekeyfortheobserved activity[41].
Scheme1.Differentpathwaysintheformationofmethyllevulinate.
Tominagaetal.[42]employedLewisacidIn(OTf)3incombina- tionwithBrønstedp-toluenesulfonicacid(PTSA)andobtained70%
methyllevulinateat180◦CwithanoptimumLewis/Brønstedacid ratioof0.2.SubstitutionofPTSAwith2-naphtalenesulfonicacid improvedalkyllevulinateyieldsto75%,suggestingthatsulfonic acidsarehighlyactiveinthehydrolysisofcellulosetosugars,while metaltriflatescatalyzemethyllevulinateformation.Anoptimum acidity(Brönsted+Lewis)wasrequiredforthesetypesofreactions.
Infact,methyllevulinateyieldsdroppedinpureBrønsted(20%),or pureLewis(52%)acidcatalyzedreactions.
Shapeselectivityandtunableacidityarewellknowncharac- teristicsofzeolites.Theiruseascatalystswasinvestigatedinthe formationof methyllevulinatefromglucose.Conventionalzeo- litessuchasH-Beta,H-mordenite,orH-ZMS-5resultedinlower methyllevulinate yieldsthan those typically reported for their homogeneouscounterparts [32]. OnlyH-USY reportedby Sara- vanamuruganandRiisager[43]yielded49%methyllevulinate.Acid sitedensitiesof300molg−1andoptimumstrong/weakacidratio of0.8(0.3–1.1range)werebestsuitedforquantitativeconversions ofglucoseintoMeL.ThesamegroupalsoreportedMeLyieldsof 13,51, and53% fromcellulose,maltose,and cellobiose,respec- tively.Pengetal.[22]obtainedyieldsof10,33,43,and59mol%of MeLfromcellulose,glucose,sucrose,andfructose,respectively,in near-criticalmethanolusingsulfatedTiO2ascatalyst,withmuch reducedyieldsobservedfortheuseofZSM-5(25and36),NaY, H-mordenite,sulfated ZrO2,and pristineTiO2 ascatalysts.Zeo- litesperformedcomparativelyworsethansulfatedtitania(which exhibitedmainlystrongacidsites)despitetheirlargeramountsof acidsites,stressingtheimportanceofstrictcatalysttuning.Inaddi- tion,5-methoxymethylfurfural(MMF)andmethylglucosideswere foundasbyproductsundertheinvestigatedreactionconditions.
Recently, zirconia–zeolite hybrid ZrY6(0.5) was found tobe remarkablyactiveintheconversionofavarietyofsubstrates,yield- ingto68%methyllevulinatefromglucose,70%frommannose,73%
fromgalactose,78%fromsucrose,46%fromcellobiose,and53and 27%fromstarchandcellulose,respectively[44].Inparticular,the acid–basebifunctionalityofthecatalystwaseffectiveinglucose isomerizationandconsequentdehydration.Njagietal.[45]further investigatedmesoporoussulfatedTiO2-basedcatalysts,comparing pristineTiO2andco-precipitatedsulfatedTiO2–ZrO2(TZ)(Zrcon- tent=10–50%).Sulfatedmetaloxideswiththehighestpercentage ofZrexhibitedthehighestcatalyticactivityintheproductionof methyllevulinateinmethanolat200◦Cafter1h.Significantyields
ofmethyllevulinatewereobtainedfromfructose(71%),sucrose (54%)andglucose(23%).SulfatedTZ50presentedlargemesopores thatfavoredinternaldiffusion.However,thetypeofacidsitetypes and strengthwerenotreportedinthis work,perhaps ofstrong nature due tothe claimed low catalyst stability (and theneed ofthermaltreatmentforreactivation)probablyduetoinsoluble huminsadsorbedonthecatalystsurfaceoriginatedonthestrong Brönstedacidsitesofthesulfatedcatalyst.Ontheotherhand,sul- fonatedcarbonsderivedfrombiocharhavebeenproveneffectivein theconversionofpolysaccharides,yielding30%methyllevulinate [46].
In order toprovideenvironment-friendly andcheap hetero- geneous alternatives to those reported, Xu et al. [47] tested inexpensive montmorillonite (MMT) in the conversion of car- bohydrates at 200◦C. The highest yield was obtained with 20-SO42−/MMT,i.e.65%methyllevulinatefromfructose,whileglu- coseandcelluloseconversionyielded65and24%,respectively.A substantialdropincatalyticactivitycouldbeobserveduponcat- alystrecycling,withdeactivationcorrelatedtohumindeposition oncatalytic surface.However,calcinationtreatmentswereinef- fectiveinrecoveringanycatalyticactivity,possiblyduetosulfur loss,which,atindustriallevel,wouldincreaseproductioncostsof wastedisposal.
Huetal.[24] investigatedtheconversionoflevoglucosanin methanol rich medium in order to hinder humins formation, obtaining>90%MeLyield,withaca.95%selectivity(whilelevulinic acidselectivityisattestedatca.25%).Thesamegroupalsopro- posedareactionmechanismfromlevoglucosanwhichundergoes methanolysis to a glucose acetal (methyl ␣-d-glucopyranoside, MGP)atlow temperatures(<130◦C),followedbytheformation of intermediate 2-(dimethoxymethyl)-5-(methoxymethyl)furan (5-MMF, acetal and ether of HMF) at higher temperatures (150–170◦C), eventuallydecomposinginto methylformate and levulinate.
Amorerecentmechanisticstudy[48]confirmedthereaction pathwayproposedbyHuetal.(videsupra)andstressedtheinflu- enceofLewisandBrønstedacidsites.Infact,Brønstedacidswere furtherreportedtofavormethylglucosideconversioninto5-MMF, whileLewisacidspromotetheformationofC Ccleavageproducts (e.g.1,1,2-trimethoxymethane)andpreventfurtherconversionof 5-MMFtoMeL[48].However,Lewisacidspromotedcellulosealco- holysistomethylglucoside,consequentlyimprovingoverallMeL yields.Inconclusion,improvedyieldsofmethyllevulinatecould
beachievedutilizingsolid acidcatalystsfeaturing(i)fine-tuned Lewis/Brønstedacidsitesratios,(ii)mediumstrengthacidsites, and(iii)regularporesizedistributiontoavoiddiffusionalissues.
Promisingyieldstoalkyllevulinates(52%)couldbeobtained fromfurfurylalcohol conversioninmethanol underacidiccon- ditions[49].Similarlytolevulinicacidconversionintoitsesters, furfurylalcoholconversiontoalkyllevulinatesalsoshowedastrong dependenceontheaccessibilityofthereactanttotheacidsites ratherthanacidsitestrengthitself.Infact,acidsitesdensityand strengthonlyplayaminorroleintheconversiontoalkylesters ascomparedtosubstrateinternaldiffusionlimitations,although suitableacidsitesmustexist.
Yieldscloseto90%wererecentlyobtainedbypreparingNb2O5 hybridbifunctionalcatalystscontainingH3PW12O4-typeHPA,with phenyl-bridgedorganosilicamoieties[50].Thesehybridcatalysts combinedhydrophobicmoietiesandstrongBrønstedwithmod- erateLewisaciditiesthatfavored thereaction.Interestingly,the highlyorderedstructuresusedinthisstudyshowedsystematically higheractivitieswithrespecttotheirdisorderedcounterparts.
Hengneetal.[51]alsoreportedtheuseoffunctionalizedionic liquidsinthealcoholysisoffurfurylalcoholto␥-valerolactoneand alkyllevulinates.Inparticular,95%selectivitytomethyllevulinate with99%conversionoffurfurylalcoholwasgivenusingionicliquid [BMIM-SH][HSO4]. Combination of thelatter with5%Ru/C cat- alyzedringcyclizationofthealkyllevulinateledtotheformation of␥-valerolactone.Althoughattractive,thisstrategyishighlycost ineffectivebecauseoftheexpensivetechnologycurrentlyneeded forionicliquidsmassproduction.Table2summarizesacompre- hensiveoverview ofresultsin MeL production reportedin this reviewfromdifferentheterogeneouscatalysts.
3. Furfural
Different from5-hydroxymethylfurfural (HMF) [52], another member of thefuranfamily (furfural) wasidentified as one of thetop10biomass-derivedchemicalsbytheUS Departmentof Energy[8].Firstdiscoveredin1821byDöbereiner[53],furfural hastakenholdofthescientificcommunityasapotentialchemi- calplatform,thankstoitshighversatility.Infact,furfural(andits derivatives)arehighlyversatilemolecules,findingnumerousappli- cationsincludingtheiruseasselectivesolventforextractionsof conjugatedmolecules,non-toxicfungicidesandnematicides,food anddrinksflavorenhancersandasbuildingblocksfortransporta- tionfuels,gasolineadditives,resins,biopolymers,pharmaceuticals, andsimilar[54,55].
Furfuralhasbeensynthesizedboth instand-alone processes from C5-rich carbohydrates (i.e. xylose, arabinose content in hemi-cellulose[54])andasaco-productofavarietyofbiomass valorizationprocessesincludingtheBiofineprocess forlevulinic acidproduction (vide supra)as wellasHMFproduction viathe dehydrationoflignocellulosicbiomass[56,57].Currentindustrial productionoffurfuralinvolvesmineralacid-catalyzedprocessing ofxylose-richlignocellulosicresidues(e.g.bagasse,corncobs,oat hulls,almondhusks)inaqueousmediaunderhighsteamstrip- pingconditionsforproductrecovery[58,59].Thoughthisparticular processutilizesagreenresource,ithassignificantdrawbacksdue toequipmentcorrosion,highamountsofacidicandtoxicwaste, energyintensiveness,homogeneouscatalystloss,and,lastbutnot least,anelevatednumberofpossibleofinevitablesidereactions (e.g.degradation,condensation)thatgeneratelowvaluebyprod- uctsandlimitfurfuralyieldstoca.50%[60](theoreticalyieldof 73wt%frompentose,64wt%frompentosan[61]).
Inordertoimproveoperationconditions,abetterunderstand- ingofthereactionmechanismisrequired.Themechanismofthis dehydrationstephasnotyetbeenfullyelucidated.Owingtodif-
ferentreactionpathwaysproposed,itisstillanowadaysmatterof debate.Neutralmediaconversionspresentlowfurfuralproduction rateswhichfindaslightincreaseattheoccurrenceofautocataly- sisbymeansofthebiomassdegradationproductaceticacid[62], thereforeindicatingapositiveeffectofthepresenceofBrønsted- typeacids.
3.1. Homogeneouslycatalyzedproductionoffurfural
Danonetal.[59]extensivelyaddressedmechanisticandkinet- icsaspectsofhomogeneousacidcatalysisdehydration.Different mechanismshavebeenproposedforxyloseconversioninaqueous acidicmedia,comprisingofopenringenolizationor-elimination, andcyclicintermediatespathways.Inacidicmediumsuchassulfu- ricorhydrochloricacid(upto2.5wt%[63]),pentosanshydrolysis comparablyproceedsatafasterratetothatofthedehydrationof theC5-carbohydratesintofurfural,thereforemakingthelatterthe ratedeterminingprocessandthefocusofmechanisticinsights[64].
Byanalyzing isotope-labeledexperimentalevidence andactiva- tionenergies,acomprehensivereactionmechanismthatinvolves onlyacyclic intermediateswasproposed, excluding theclosed- ringmechanismproposedbyAntaletal.[65],duetoaparticularly lowactivationenergyfortheO-5protonationandsubsequentring openingofthecyclicintermediate2,5-anhydroxylose.
Inparticular,itissuggestedthatxyloseundergoes(i)aldose- ketose isomerization via 1,2-hydride shift to xylulose that eventuallyleadstofurfuralviathe1,2-enediolorside-reactionsand humins,predominantatpH<1withhigherconversionrates,(ii) enolizationto1,2-enediolfollowedbytwoconsecutivedehydra- tionreactionstofurfuralviathe2,3-(␣,)-unsaturatedaldehyde, catalyzed by halides anions and favored at pH ca. 3, (iii) - elimination that follows the pattern via 2,3-(␣,)-unsaturated aldehyde(vide supra),and(iv)equilibriumwithd-xylopyranose thatleadstounspecifiedsideproducts,asshowninScheme2.The proposedreactionpathwayvalidatedtheopenringmechanisms suggestedbyAhmad[66]andFeather[67]bycombiningthemboth.
Furtherexperimentalevidenceisrequiredinelucidatingthis complexmechanism.Nonetheless,itcanbedrawnoutthatthekey stepsthatrequirecatalysisinordertoderivehigherfurfuralyields areisomerization/enolizationanddehydrationreactionswhichare catalyzedunderbasicandacidmedia,respectively.
Currentindustrialproductionoffurfuralstillreliesonhomo- geneousacidcatalysts.Thefirstfurfuralproductionwasreported byQuakerOats,basedonoathullsconversioninthepresenceof sulfuricacidat153◦Cfor5h.Heatingoftheprocesswasachieved bycontinuoussteaminjection,whichalsostrippedfurfuralfrom thereactionenvironment,withyieldsofca.50%[64].Thebiggest furfuralexporterofmoderntimes(accountingfor70%offurfural production),China,hasbasedtheirownprocessonthe1921Quaker Oats,employingsulfuricacid(3–4wt%)andsteamstrippinginthe conversionofcorncobs,withasimilarfurfuralyieldoftheformer process.Themoderateyieldsoffurfuralaremainlyduetothefor- mationofsolidhumins(videinfra),derivingfromtheoccurrenceof undesiredpolymerizationreactions.Asmentionedabove,furfural isproducedasco-chemicalinthelevulinicacidBiofineprocess.A furfuralyieldof70%canbeachievedfromcellobioseata20min residencetime, operatingat210–220◦Cwith3%ofsulfuricacid under25barpressureofsteam[68].
Alternatively, recent scientific progress provided significant advancesinfurfuralproduction.Severalmineralacidswerealso investigatedintheproductionoffurfuralfromavarietyofbiomass sources.Ricehuskshaveproventobeineffectiveintheproduction offurfural(9%yield[69]), whileolivestonesconversionyielded 65%furfuralat230◦Cwithsulfuricacid[63].Ascreeningofvarious mineralacids(i.e.HCl,H2SO4,HNO3,andH3PO4)alongwithorganic Trifluoroaceticacid(TFA)catalystsin theproductionof furfural
Table2
SummaryofliteraturereportedMeLyieldsfromvarioussubstratesusingheterogeneouscatalysts.
Catalyst Substrate T(◦C) t(min) Yield(%) Ref.
Me-PST Levulinicacid 25 120 88 [37]
SBA-15-(CH2)3-SO3H Levulinicacid 117 120 95 [36]
H4SiW12O40–SiO2 Levulinicacid 65 360 73 [38]
H3PW12O40/ZrO2–Si(Ph)Si Levulinicacid 65 180 99 [40]
H-USY Glucose 160 1800 49 [43]
H-USY Cellulose 160 1800 13 [43]
H-USY Maltose 160 1800 51 [43]
H-USY Cellobiose 160 1800 53 [43]
SO2−4/TiO2 Cellulose 200 120 10 [22]
SO2−4/TiO2 Glucose 200 120 33 [22]
SO2−4/TiO2 Sucrose 200 120 43 [22]
SO2−4/TiO2 Fructose 200 120 59 [22]
ZrY6(0.5) Glucose 180 3 68 [44]
ZrY6(0.5) Mannose 180 3 70 [44]
ZrY6(0.5) Galactose 180 3 73 [44]
ZrY6(0.5) Sucrose 180 3 78 [44]
ZrY6(0.5) Cellobiose 180 3 46 [44]
ZrY6(0.5) Starch 180 3 53 [44]
ZrY6(0.5) Cellulose 180 3 27 [44]
TZ50 Fructose 200 60 71 [45]
TZ50 Sucrose 200 60 54 [45]
TZ50 Glucose 200 60 23 [45]
Sulfonatedbiochar Cellulose 200 75 30 [46]
PW12/Nb2O5-Si(Ph)Si Levulinicacid 65 180 90 [50]
[BMIM-SH][H2SO4] Furfurylalcohol 130 120 94 [51]
Scheme2.FurfuralformationmechanismproposedbyDanonetal.adaptedfromRef.[59].
fromloblollypinewasconductedbyMarzialettietal.[70],finding reasonablefurfuralyields(andHMFandhexosesasby-products) at200◦CfortheTFAandsulfuricacidsystems(57and34%,respec- tively).Barbosaetal.[71]alsoscreenedmineralacidsHCl,H2SO4, andH3PO4inthehomogeneousliquidphasedehydrationdiffer- entbiomasses,andfoundthebestactivitywasgivenbychloridric acid,whichyielded30,26,and14%furfuralfromcornstalk,sugar-
canebagasse,andeucalyptuswood,respectively.Phosphoricacid wasfoundtobefairlyactiveintheconversionofsorghumstraw with furfural yield of 38% [72], while maleic acid yielded 55%
furfuralfromswitchgrassliquor[73].Heteropolyacidswerealso employedinthedehydrationofxylose.Forinstance,Diasetal.[74]
reportedfurfuralyieldsof58–67%at140◦CemployingH3PW12O40 (PW),andH4SiW12O40(SiW)after4h,whileH3PMo12O40showed
Table3
Overallviewofhomogeneouscatalystsemployedinfurfuralproduction.
Catalyst Substrate T(◦C) t(min) Yield(%) Ref.
H2SO4 Oathulls 153 300 50 [63]
H2SO4 Cellobiose 210 20 70 [68]
H2SO4 Ricehusks 230 288 9 [69]
H2SO4 Olivestone 230 2.5 65 [63]
H2SO4 Loblollypine 200 60 34 [69]
TFA Loblollypine 200 60 57 [69]
HCl Eucalyptuswood 170 180 14 [71]
HCl Sugarcanebagasse 170 180 26 [71]
HCl Cornstalk 170 180 30 [71]
H3PO4 Sorghumstraw 134 300 38 [72]
Maleicacid Switchgrassliquor 200 15 55 [73]
H3PW12O40 Xylose 140 240 67 [74]
H4SiW12O40 Xylose 140 240 58 [74]
HCl+NaCl Xylose 200 3.3 81 [75]
thelowestactivity.Acombinationofacid–basecatalystshasalso beenemployed,reaching76%furfuralyieldasreportedbyChoud- haryetal.[75].Additionofhalidesaltstomineralcatalystshas alsoprovedtheincreaseoffurfuralproduction.Inparticular,Mar- cotullioet al.screened NaCl,KCl, KBr,CaCl2,and FeCl3,finding sodiumchlorideasthemosteffective[76].Infact,halidesaltsnot onlypossesstheabilityofsalting-outfurfuralinbiphasicsystems [77,78], therefore reducing overall residence time and possible side-reactions,butalsoenhancefurfuralselectivitiestoupto90%
at200◦CasseenbyMarcotullioetal.[76]intheco-presenceofHCl, andNaCl.Higherselectivityisreportedtobegivenbyanacceler- ationinthe1,2-enediolformationbytheweakbasebehaviorof chlorineions.Althoughtheuseofmineralacidcatalystssimplifies operations,andfairlyhighyieldsoffurfuralcanbeobtained,these homogeneousacidsystemspresentnumerousdrawbacksinclud- ingcorrosion,acidicwastedisposal,andcatalystloss.Therefore, solidacidcatalysismightseemagreenerchoiceintheproduction offurfural.Anoverviewofthehomogeneouscatalystsreportedin thisreviewispresentedinTable3.
3.2. Heterogeneouslycatalyzedproductionoffurfural
Asforheterogeneouscatalysisisconcerned,adistinctivereac- tion pathway has yet to be described as just few mechanistic investigationsarebeingadvancedinthismatter.Inthepresence ofBrønsted-acidcatalysts,acyclo-dehydrationmechanismismost likelytooccurwhereapseudo-cycleundergoesring-openingand subsequentringclosingtofurfural,describedbyZeitsch[64].In particular,xyloseundergoesasasequenceoftwoproton-catalyzed 1,2-eliminationsthat openthepentosering, followedby a 1,4- eliminationthatclosestheringyieldingfurfural.Instead,inthecase ofBrønstedacidscombinedwithLewisacidsites(eitherinhomoge- neousorheterogeneousphase),allexperimentalevidencesuggest thatthesynergisticeffectofthetwotypesofacidspromotexylose isomerizationtoxylulose,andoverallincreasingfurfuralyieldsby acceleratingoverallxyloseconversionrate.Forinstance,Choud- haryetal.[75]reporteda10%increaseoffurfuralyieldsinwater, anda47% increaseina water/toluenemedium(tolueneactsas furfural-extractingsolvent)whenaLewisacid(i.e.CrCl3)isadded toahomogeneousBrønstedacid(i.e.HCl),suggestingamechanism viaisomerizationtoxylulose(andtheotherisomer,lyxose)onlyin thepresenceofaLewisacidcatalystbutwithoutproposingafull reactionmechanism.
Recently,Shirotorietal.[79]studiedthexyloseconversionin thepresenceofBrønstedacidAmberlyst-15andacid–basecatalyst Cr/hydrotalcites.Theresearchgroupproposedareactionmech- anismbased on ring openingand Lobry de Bruyn-Alberda van Ekensteintransformations(aldose-ketose).Thesuggestedmech- anismdoes not exclude a catalytic activity of dispersed Cr2O3
particlesin thedirecthydrideshiftfor xylose–xyluloseisomer- ization, however, a much lower furfural yield of solely Cr2O3
(4%)comparedtosolelyHTBrønstedbasecatalyst(19%)suggest thatthelatterisindeedthemainactivesite.Withthisinmind, afirstxylopyranose-Cr2O3 coordinationinitiatesthereactionby increasingthepositivechargeontheC1oxygenatomandfavors HT-catalyzeddeprotonationtoopen-chainxylose.Subsequently, anotherdeprotonationoccursattheC2Hofxylosebymeansof HT,generatingan1,2-enediolwhichcomplexeswiththeCr-Lewis acidthatundergoesprotonshiftfromC2toC1,generatingxylu- lose.Ultimately,itisreportedthatxylulosewillbedehydratedto furfuralthankstothecatalyticactivityofBrønstedacidAmberlyst- 15.However,thesemechanismsdonottakeintoconsiderationthe numeroussidereactionsthatlimitfurfuralyieldsincludingpoly- merizationordegradationreactionsthatleadtohumins(videinfra) and decompositionproducts(e.g. formic acid,lactic acid, pyru- valdehyde[65]),respectively.
Inconclusion,adefinedschemeofthiscomplexmechanismhas yettobeelucidated,butaco-existenceofLewisandBrønstedacids mayremarkablyimprovefurfuralyields.
Thetunableacidities,excellentthermalandchemicalstability and pore selectivity properties of zeolites make them attrac- tive catalysts in xylose dehydration to furfural. Moreau et al.
[80]first reportedtheuseofmicroporouszeolites intheselec- tive preparation of furfural, studying H-Y Faujasite (Si/Al=10 and 15), H-Mordenite (Si/Al=11 and 12) in water/toluene and water/methylisobutylketone(1:3byvolume)intheconversionof xyloseat170◦Cfor30min.Furfuralselectivityof96%wasachieved withH-Mordenite(Si/Al=11)inwater/toluenemedium(96%after 30minof reaction), while inwater/methylisobutyl ketonefur- furalselectivityforH-Mordenite(Si/Al=12)was51%.Thehighest xyloseconversionwasobservedforthecatalystwiththelowest Si/Al(HYFaujasite,Si/Al=10),duetotheaccessibilityofxyloseinto the3DstructureofHY–Faujasiteandporesofca.13 ˚A.However, largeporevolumesalsofavorfurtherrearrangementsoffurfural intodegradationand/orpolymerizationcompounds.Alongthese lines, Bruceet al. [81] investigated smallpore zeolite catalysts (bothsynthesizedandcommercial)intheconversionofxyloseina
␥-valerolactone/watersystem.Lessardetal.[82]testedhighertem- peratures(260◦C)forxyloseconversionoveravarietyofzeolites, findingthatphosphoricacidtreatmentonmordenite13catalysts couldincreasedfurfuralyieldsto98mol%withalmostquantitative xyloseconversion(99%).O’Neilletal.[83]obtained46%furfural yieldat200◦CoverprotonicH-ZSM-5(poresizeof1.2nm)inwater environment. Different zeoliteswereinvestigated by Kimet al.
[84]indifferentreactionmedia(i.e.H2O,H2O/toluene,andDMSO), reportinglowerconversionandfurfuralyieldsinwatermedium, andageneraldecreaseofcatalyticactivityinfurfuralproduction withincreasingSiO2/Al2O3 ratios,which iscloselycorrelatedto acidsitesaccessibility.
Thisindicatesthatfurfuralyieldscriticallydependonresidence timeinwatermedia(therefore,validatingbiphasicsystems),and onreagent/productdiffusionlimitationsinporoussystems.Asan attempttoavoidthisphenomenon,Limaetal.[85]attemptedto achievesinglecrystallinesheetsoflayeredaluminosilicatespre- cursors through swelling and ultrasonication. The delaminated zeolitesshowedaremarkableincreaseinsurfaceareaandaslight decrease of the Si/Al ratio, and yield to 47% furfural at 170◦C inawater/toluenesolution.Thesameresearchgroup,in alater paper[86],reported58%furfuralselectivityofthemedium-large poresizesilicoaluminophosphates(SAPOs5,11,40)withthehigh- estBrønstedacidcontentandsurfacearea.SAPOSareknownto possesslessbutstrongeracid-sites,thankstothephosphoroussub- stitutionwithSi.Overall,studiesemployingSAPO-5,-11,-40,and -44reportedfurfuralyieldsof40–65%inbiphasicsystems[86,87].
Similarly,Valenteetal.[85,87]prepareddelaminatedzeolitesby
swellingandultrasonicationfromferrierite,Nu-6(2),andMCM-22 (ITQ-2).Highersurfaceareaandporosityofthedelaminatedcom- poundsleadtohigherfurfuralyieldsbyfavoringinternaldiffusion.
Mesoporousacidcatalysts,suchasSBA-15andMCM-41have beenreceivingincreasingattentioninbiomassconversion.Xylose conversiontofurfuralwasachievedwitha68%yieldbyShietal.
[89]over SBA-15-SO3H inwater/tolueneenvironment. MCM-41 showedlowthermalstabilityoftheacidsitesandfurfuralyields lowerthan40%inDMSOorwater/butanol(48%inwater/butanol withthe additionof NaCl[90]), while modified MCM-41-SO3H showedincreasedactivitiesinwater/methylisobutylketoneand water/toluene,withfurfuralyieldsof51%and 76%,respectively [90,91].For thesecatalytic systems,thehigher activitywasdue tothenatureoftheMCMacidsites,andnotitsporousstructure.In fact,asmentionedbefore,Lewisacidsitesfavortheisomerization ofxylosetoxylulosethatpossessoverallfasterreactionkineticsto furfuralratherthanthedirectxylose-furfuralpathwayscatalyzed byBrønstedacidsites.Increasingacidstrengthmayincreasereac- tionrates,butalsofurthercatalyzepolymerizationside-reactions [61,75,86,92].AnoptimumLewis/Brønstedacidsitesratiohasbeen reportedbetween30and80%[93].Amongothermesoporousacid catalystsinvestigated,highfurfuralyieldsof82–86%wereobtained withpropylsulfonic-SBA[94]andarenesulfonic-SBA[95],respec- tively,thankstotheiroptimum BETsurface areaand poresize.
Yieldsof78%werereportedforflurosulfonicmodifiedMgF2cata- lysts(71wt%HF)inwater/tolueneat160◦C[95],andNb2O5/Cabosil inwaterat175◦C[96],withgoodLewis/Brønstedacidsitesratio.
Strong acidPTSA-POM catalyst also resulted into high furfural yields(ca.80%)at170◦Cfor 10mininGVL/water[97].Furfural yieldsreportedinliteraturefromxyloseconversionaresumma- rizedinTable4.
Heterogeneous conversion of whole biomass has been also attempted (Table 5). One-pot hydrolysis/dehydration of raw biomass(softwoodhemicelluloses)tofurfuralhasbeencarriedout byDhepeetal.[98]usingHUSY,H-Beta,andH-mordenite,how- everonlyca.12%offurfuralwasrecovered.Corncobconversion withmixedmetaloxideZrO2–TiO2at300◦Cyieldedonly10%fur- fural[99].Ahigherfurfuralyieldofca.56%wasinsteadachieved forbagasseconversionwithHUSYinwater/p-xyleneat140◦Cafter 300min[98,100].
Even though furfural traditionally is derived from C5- carbohydrates, recently its production from hexoses has also been attemptedusing microporous ion exchanged H-Betazeo- lites [101] (Table 5). A synergistic effect of textural properties (i.e.poresizecomparabletofurfural,andshorterporepathways), Lewis/Brønstedacidsitesratio(ca.1.3,withthepresenceofmainly weakacidsitesandsmallamountsofmedium-strength)madeSn- Betathemostactivecatalyst,reaching95%glucoseconversion,ca.
68%furfuralyieldwithaca.72%selectivityin25minat170◦C.
Comprehensively,astrictcorrelationofoptimumsurfaceareas, poresize,andacidtypeandstrengthexistsinthexylose/biomass conversioninfurfural.Limitationsofthisparticularheterogeneous processaregivenbyinternaldiffusionlimitations,andoptimized Lewis/Brønsted acidsitesratio. Comparablepore sizeto xylose andfurfuralmolecularsizes(6.8and5.7 ˚A,respectively)mustbe obtained, withease accessibilitygiven bya largersurfacearea;
moreover,therightacidcontentandstrengthmustbeachieved, limitedbythepresenceoftoostrongacidsitesthatcatalyzesec- ondaryreactions[93].
In conclusion, environment-friendlyheterogeneous catalysts areattractivepossiblealternativestocurrenthomogeneousmin- eralacidcatalystsemployedintheindustrialproductionoffurfural.
Peleteiroetal.[58]wrotearecentreviewontheapplicationof ionicliquidsinfurfuralproduction.Forsakeofcompleteness,avery generaloverviewisgivenherein.Theirhighboilingpoint,optimum dissolving capacity [103], chemical and thermal stability, recy-
clability,andnon-toxicnature(exceptforchloride-based)make ionicliquidsan attractivegreenalternativetoconventional sol- vents,however,theirindustrialapplicationisstilllimitedbytheir cost,andseparability.Nonetheless,remarkablehighyieldswere obtainedinthedehydrationofavarietyofbiomassesintofurfural.
Themostnotableexampleswillbegivenasfollows.Highactivi- tieswereobtainedwhen[BMIM]Cl(1-butyl-3-methylimidazolium chloride)was employedas solventover a variety of substrates (lyxose,ribose, arabinose, xylose,pine wood),combinedwitha widerangeofcatalysts(Amberlyst-15,H3PW12O40,PEG-OSO3H, CrCl3,AlCl3)[101–106].Remarkableyieldsofca.94%fromxylan werereportedwhen[BMIM]ClwascombinedtoH3PW12O40[103]
at160◦Cfor10min.[BMIM]ClandCrCl3 yieldedca.31%furfural frompinewood[105].Overall,moderate-highfurfuralyieldswere reportedforallsystems(48–84%fromxylose,75%fromlyxose,90%
fromribose,72%fromarabinose).
Thepossiblerelease ofhalogensin theenvironmentor con- sequences of its thermal decomposition, be it very slow, lead totheprevailingofscale-upstudiesfornon-halideionicliquids.
[BMIM]HSO4(1-butyl-3-methyl-imidazoliumhydrogensulphate) hasyieldedalsotopromisingresultswhenemployedasbothco- solventandacidcatalyst.Forinstance,Limaetal.[107]utilized [EMIM]HSO4 (1-ethyl-3-methylimidazolium hydrogensulphate) combined with toluene, obtaining 84% furfural from xylose.
Peleteiroetal.[108]reportedyieldsofupto82%when[BMIM]HSO4
wasinthepresenceofdioxane,andofupto80%withMIBK,in xylosedehydrationat140◦C.Attemptsonwholebiomassconver- sionwerealsoattempted,yieldingto33%furfuralfromMiscanthus giganteusat120◦Cfor22h[109],andyieldsof36%werereported fromwheatstrawdehydration,withupto3%co-yieldofHMF[110].
Ionicliquids,inconclusion, seemtohave optimumemployabil- ityforfuturefurfuralproductions.However,thescarcetechnology knowledgeofindustrialproductionandunknownconsequences oflong-termuseofionicliquids,alongsidetheirelevatedcostand yetinefficientrecoveryandrecycling,setsomemajordrawbacksin theuseofthesesystems.Anumberoffurfuralyieldsinionicliquid systemsarereportedinTable6[54].
In general, furfural industrial production is feasible when yieldsexceed50wt%,butmanytechnologicalaspectscanstillbe improved.
4. Humins
Thelimitedyieldsoftheproductsmentionedbefore(i.e.methyl levulinate,andfurfural),alongsideotherproductsofdehydration reactionsof biomass-derived carbohydrates(e.g.HMF, levulinic acid),areduetotheformationofwasteby-products.Amongthe latter,themostundesiredby-productsaregivenbytheformation ofso-calledhumins.Thesedark,tarrytosolid,insolublepolymeric structuresarefavoredbytheaqueousacidicmediaofmostligno- cellulosicbiomasshomogeneoustransformations,althoughmost likelyalsopresentalsowhenheterogeneouscatalytictreatments arecarriedoutinbothwaterand/ororganicsolvents(Scheme3).
Forinstance,37–39%yieldsofhuminswerereportedinthemin- eralaciddehydrationofaldose[111],whileyieldsofeven50wt%
ofhuminby-productscanderivefromHMFproduction[112],and thetypicalfurfuralyieldsdonotexceed55%becauseofthisside- reactions[113].Althoughnotyetunequivocallydetermined,the huminstructureis knowntodependonreactiontime, temper- ature, and feedstockstructure and itsconcentration[114–116].
Severalkineticstudiesonhuminformationwereperformed.Val- uesof85–130kJmol−1werereportedfortheactivationenergyfor huminformationfromHMF[52,117,118];loweractivationener- gieswerefoundforthehuminsformedfromfurfuralinthepresence ofeitherxyloseorglucose,comparedtofurfuralalone[119–121].
Table4
Overallviewofheterogeneouscatalystsemployedinfurfuralproductionfromxylose.
Catalyst Substrate Solvent T(◦C) t(min) Yield(%) Ref.
H-Mordenite Xylose Water/toluene 170 50 33 [78]
H-Mordenite Xylose Water/methylisobutylketone 170 50 20 [78]
HY-Faujasite Xylose Water/toluene 170 50 42 [78]
HY-Faujasite Xylose Water/methylisobutylketone 170 50 30 [78]
ZSM-5 Xylose Water/␥-valerolactone 190 15 70 [81]
Amberlyst-70 Xylose Water/␥-valerolactone 190 15 67 [81]
SAPO-34C Xylose Water/␥-valerolactone 190 360 40 [81]
SAPO-56 Xylose Water/␥-valerolactone 190 360 38 [81]
SAPO-34 Xylose Water/␥-valerolactone 190 360 31 [81]
H-Mordenite Xylose Water/toluene 260 3 98 [82]
H-ZSM-5 Xylose Water 200 30 46 [84]
H-ZSM-5 Xylose Water/toluene 140 240 42 [84]
Del-Nu-6 Xylose Water/toluene 170 240 47 [85]
SAPO-11 Xylose Water/toluene 180 240 51 [86]
H-MCM-22 Xylose Water/toluene 170 960 70 [88]
ITQ-2 Xylose Water/toluene 170 960 66 [88]
SBA-15-SO3H Xylose Water/toluene 160 240 68 [89]
MCM-41+NaCl Xylose Water/1-butanol 170 120 48 [90]
MCM-41-SO3H Xylose DMSO 140 1440 75 [91]
Propylsulfonic-SBA Xylose Water/toluene 160 1200 82 [94]
Arenesulfonic-SBA Xylose Water/toluene 160 1200 86 [95]
FluorosulfonicMgF2 Xylose Water/toluene 160 1200 78 [96]
Nb2O5/Cabosil Xylose Water(N2stripping) 175 180 78 [97]
PTSA-POM Xylose Water/␥-valerolactone 170 10 80 [98]
Table5
Overallviewofheterogeneouscatalystsemployedinfurfuralproductionfromsubstratesdifferentfromxylose.
Catalyst Substrate Solvent T(◦C) t(min) Yield(%) Ref.
Amberlyst-70 Switchgrass Water/␥-valerolactone 190 30 60 [81]
SAPO-34C Switchgrass Water/␥-valerolactone 190 1440 30 [81]
SAPO-56 Switchgrass Water/␥-valerolactone 190 1440 29 [81]
HUSY Softwoodhemicelluloces Water 170 180 12 [99]
ZrO2-TiO2 Corncob Water 300 5 10 [100]
HUSY Bagasse Water/p-xylene 140 300 56 [99,101]
Sn-Beta Glucose ␥-valerolactone 170 25 68 [102]
Table6
Overallviewoffurfuralproductionemployingionicliquids.
Catalyst Substrate Solvent T(◦C) t(min) Yield(%) Ref.
H3PW12O40 Xylan [BMIM]Cl 160 10 94 [105]
CrCl3 Pinewood [BMIM]Cl (400W) 3 34 [107]
[EMIM]HSO4 Xylose [EMIM]HSO4/toluene 100 360 84 [109]
[BMIM]HSO4 Xylose [BMIM]HSO4/dioxane 140 360 82 [110]
[BMIM]HSO4 Xylose [BMIM]HSO4/MIBK 140 360 80 [110]
[C4C1im][HSO4]80%and[C4C1im][MeSO4]80% Miscanthusgiganteus – 120 1320 33 [111]
[BMIM]HSO4 Wheatstraw – 160 157 36 [112]
Thekineticstudyofglucoseconversiontolevulinicacidconducted byGirisutaetal.[122]reportedhigheractivationenergyforthe formationofhumins(165kJmol−1)thanalloftheotheractiva- tionenergiesof glucoseconversion.However,both Weingarten etal.[121],andEifertandLiauw[123]obtainedloweractivation energiesforhuminformationcomparedtoxylosedehydrationto furfural, makinghumins thermodynamically favored.Moreover, EifertandLiauwobservedalinearcorrelationbetweenthereaction rateofhuminformationandtemperature.Evennowadays,struc- turalcharacterizationisthemainresearchfocus,andstillobject ofdebate.Oneofthefirststudiesonhuminstructureisgivenby F.A.H.Rice[111]in1958,whoperformedIRanalysisonhomoge- neouslydehydratedaldoses,reportingthepresenceofhydroxyl, carbonyland,possibly,carbon-carbondoublebonds,aswellas, highlysimilarspectraofthesolubleandunsolublehuminfractions.
Sumerskiietal.[113]studiedthemultistephuminformation inacidic mediumfrom monosaccharides.It wasnoted thatthe investigatedpentose(i.e.arabinose)yieldedmorehuminsthanthe hexosescounterparts.Thisfindingwassuggestedtobecausedby
thehighreactivityoffurfural,readilyformedfrompentosesugars, toundergofurthercondensationreactionstohumin,whilelower thermalstability wouldcauseHMF todecomposein twostable acids(levulinicandformicacid).13CNMRanalysisshowed35.1%
ofrelativeamountofcarbonsascarbonandhydrogensubstituted Catomsinfurancompounds;and27.3%asoxygen-substitutedC atomsinfurans.Amongtherest,inlowpercentages(2–4%)each, carboxylandesters,methoxy,-CH2O-fragments,andacetalcar- bonatomswerefound.Thesamegroup,proposesalsotworeaction pathwaysforhuminformationinpentosesandhexoses.Forxylose dehydration,furfuralpolycondensatetohuminsviaelectrophilic substitution,formingC Cbondsbetweenfuranrings.Ashexoses concern,huminformationfromHMFproceedsviaaromaticelec- trophilicsubstitution,leading toetheror acetalbondsbetween rings.
Recently,pyrolytichuminswerefoundtopossessmainlycar- bonyl functional groups [124]. Horvat [125] first proposedthe formationofhighlyreactive2,5-dioxo-6-hydroxyhexanal(DHH), crucialmoleculeregardinghuminformation,althoughDHH has
Scheme3. Pathwaysformationofhuminby-products.
Scheme4.HuminsformationmechanismasproposedbyLundetal.,adaptedfromRef.[126].
Table7
Huminstransformationsreportedinliterature.
Process Catalyst Conversion(%) T(◦C) t(min) Yield(%) Ref.
Gasification Na2CO3 100 750 30 81(H2)or75(syngas) [114]
Fastpyrolysis – – 500 12s 30(volatile) [143]
Hydrotreatment Ru/C 69 400 360 65(phenolics) [144]
Hydrotreatment Ru/Al2O3 59 400 360 70(phenolics) [144]
yettobeidentifiedspectroscopically.Thishypothesiswasfurther accreditedbyLundetal.[118,126]inthehuminformationfrom HMF.IRspectrashowedthepresenceofanextensiveconjugated network,andfuranringswithaldehydeorketonefunctionalgroups weresupposed. Theproposedmechanism involvesDHH as key intermediate,formedby2,3-additionofwatertoHMF.Inparticu- lar,thehighlyreactiveintermediateundergoesaldolcondensations withthecarbonylgroupofHMF,yieldingtohumins(Scheme4).
However,themechanismhasnotyetbeenunequivocallyestab- lished.
Self-condensationofHMForfurfuralhasbeenpostulated[127];
however, this possibility has been discarded by others [128].
MoleculessuchasHMF andfurfuralcannot undergocondensa- tionreactionsduetotheabsenceofan␣H-atom,pushingforward thetheorythathuminsareformedviacross-polymerizationreac- tionsbetweenHMFandeithersugarsorreactiveHMF-derivatives [52]. Dee et al. [129] reported up to 30% humin yield in the hydrolysisofcellulosetoglucoseinionicliquidsandsulfuricacid.
Theresearch groupobserved an increase of huminyieldswith increasingHMFconcentration,confirmingtheimportantrolethis moleculeplaysinhuminsformation.Itwaspostulatedthathumins are formed via polymerization reactionsof HMF with glucose, initiatedbyprotonationofHMFaldehydegroupfollowedbycon- densationwithasugarmolecule.Similarfindingswerereportedby Herzfeldetal.[130],whosuggestedthatfuranunitsacttocross-link sugarmolecules.Akienetal.[131]investigatedthedehydrationof fructosetoHMFwithinsituNMRand13Clabelingstudies,inter- mediatesthatarebelievedtopolymerizetohumins.Inparticular, itwasproposedthatthefuranose-typeintermediateleadtoHMF whilepyranoseformsleadtohumins.
Theformationandmorphologyofhuminsasafunctionoffeed- stockandoperatingconditionswasrecentlystudied[116].Itwas foundthathuminformationisstronglyinfluencedbyreactiontem- peraturesand acidconcentrationsbut littleeffect wasgivento sugarconcentrations. A dependence onthefeedstocktype was nonethelessobserved,withhigherhuminformationfromfructose duetothefastformationof HMF.Condensationreactionswere attributedtonucleophilic attacksresultingin linkages atthe␣ positionofHMF,orsubstitutionattheposition.
Combination of elemental analysis, IR, 13C ssNMR, and pyrolysis–GC–MS,leadtospeculateafuranicstructureformedvia dehydration,withalcohol, acid, ketoneand aldehyde groups.It wasalsonotedthatxylose-derivedhuminspresentedamorecon- jugatedstructure due tothereactivityof thefree5-position of furfural,asitiswell-knownintheformationofresins[132].1D and2DssNMRwereperformedbythesameresearchgroupon13C- labeledhumins,inalaterpublication[133].Thisstudyrevealedthat intheacidicdehydrationofglucose,themajorlinkagesinhumins include C␣ Caliphatic and C␣ C␣ bonds,while C linkages were minor.Moreover,reactivesolubilizationviaalkalinetreatmenton thehuminsformedamorearomaticstructure,attheexpenseof thefuranrings.
Tsilomelekis etal. [117]validates theproposed nucleophilic attack of a HMF carbonyl group tothe ␣ or position of the furanicring,andATR-FTIRindicatedthathuminformationinvolves amultitudeofparallelreactionstothealdolcondensation.Recent
workbyWangetal.[134]reportedFTIRand13CNMRanalysison glucose-andxylose-derivedhumins,findingca.50%ofcarbonin furfuralrings,andanabundanceofoxygenatedfunctionalgroup.
Xylose-derivedhumins,however,containedhigheraliphaticcon- tent.Analysisofthevolatilesfromthepyrolysisofthetwodifferent humins showedsimilar compositions,with furans and phenols asthemainproducts.Aromaticproductsandcarbonizationwere favoredbythehighpyrolysistemperatures.
Inconclusion,huminformationmechanismislargelyunknown, alongsidethevariousfeedstocksintermediates.Nonetheless,some attempts in upgradingthis humin wastehave beenattempted.
Traditionally,theminimizationofhumicsubstancesisusuallypre- ferred[135,136].For example,less huminswereobserved with tolueneextractionintheethanolysisofcarbohydrates[135],and additionof co-solvents suchasGVL andmethyl-THF [137,138].
However,differentapproaches havebeentaken intoconsidera- tion for upgradingthis unavoidable waste.Steam reforming of huminswasreportedbyHoangetal.[112]at750◦Coveravari- etyofalkali-andalkaline-earthbasedcatalysts,Na2CO3wasfound tobethemosteffectivegivingacompleteconversionofhumins witha75%selectivitytosyngas(81%H2yield).Byunetal.[139]
proposedtheintegrationofaheatexchangerthatutilizeslignin andhuminsascombustibleintheproductionofbuteneoligomers fromcellulose.Huminshavealsobeenintegratedinpolyfurfuryl alcohol(PFA)polymericstructure,creatinganewcompositewith lower brittleness than the pure PFA sample [140]. These vari- ousmaterialapplicationsmight seemdirect andsimple forthe undesired substances. Humins offer thepotential alsoto liber- atehighlyadded-valuechemicals.Rasrendraetal.[141]reported, infact,thatliquefactionofhuminsbyfastpyrolysisyieldedlow molecularfuranicssuchasfurfuraland2,5-dimethylfuran,which showapotentialofobtainingimportantplatformchemicalsfrom the humic waste. Wang et al. [142] reported the hydrocrack- ing and hydrodeoxygenation of low molecular weight humins, witha mixtureofformicacidandisopropanolover ruthenium- basedcatalystsat400–500◦C.Themainproductsobtainedwere naphthalenes, alkylphenolics, and cyclicoalkanes. Other prod- uctsincludedalkanes,aromatics,andpolyaromatichydrocarbons.
Remarkablehuminconversionsof69%wereobtainedonRu/Cwith formicacid,whileRu/Al2O3gaveonly59%huminconversion.These resultspointedoutthepotentialofhuminsasnewfeedstocksrather thantraditionalwaste(Table7).
5. Conclusionsandfutureprospects
Thismanuscriptwasaimedtoprovideageneraloverviewon thevariouspossibilitiestovalorizethreekeybiomass-derivedside products from hemicellulosic fractionprocessing. MeL, as well asingenerallevulinateesters,haveper-seremarkableproperties withpotentialapplicationsinthefragrancesandflavoringindus- triesandadditionalpossibilitiestobeconvertedintoanumberof usefulderivatives(e.g.gamma-valerolactone)andcanbesimply producedfromanumberofbiomassfeedstocksunderbothhomo- geneous and heterogeneous catalysis. Interestingly, fine-tuning Lewis/Brønstedacidsitesratios,thepresenceofmediumstrength acidsitesandaregularporesizedistributiontoavoiddiffusional