金属离子对离子液体中竹子酸解的影响(3)

Cu2+

9.23±0.7214.03±0.9718.11±0.7823.52±1.1827.22±0.9329.89±1.1133.14±0.96

Fe3+

2.52±1.284.21±1.077.52±1.5612.78±1.1819.21±1.3522.34±1.4625.72±1.11

Table3

Totalreducingsugaryieldsofthesampleshydrolyzedby0.23%(w/w)HClwithoutandwith0.13%(w/w)metalionadditionsatdifferentreactiontime.Time(h)

TRSyield(%)Blank

1.5234567

11.13±1.2116.02±1.5226.08±2.1037.34±1.6147.38±1.3755.87±1.8159.03±1.12

Na+

14.52±0.8322.13±1.7132.58±1.2445.23±1.7655.71±1.8558.34±1.6859.52±1.46

K+

13.64±1.7719.61±2.2630.28±1.8542.64±1.5553.17±1.1356.78±1.5159.22±1.18

Mg2+

14.13±1.3620.24±1.7731.59±1.2643.88±1.5154.12±1.5757.10±1.7859.33±1.86

Ca2+

13.88±1.0322.14±1.3631.82±1.0744.37±1.5655.31±1.1857.82±1.8659.11±1.45

Cu2+

13.78±0.7219.67±1.1039.31±0.8756.74±1.1766.62±1.0966.21±1.4569.38±1.01

Fe3+

9.11±0.6315.62±0.8930.34±1.0750.71±0.8258.03±1.0158.38±0.9159.83±1.07

Table4

Hydrolyisrateandmonosaccharidesyieldsof0.2gbamboocatalyzedby0.45%(w/w)HClwith0.13%(w/w)metalionsasco-catalystsin4g[C4mim]Clat100°C.Entry1234567

MetalionBlankNa+K+Mg2+Ca2+Cu2+Fe3+

Hydrolysistime(h)7776644

Hydrolysisrate(gLà1hà1)3.05±0.013.25±0.093.21±0.053.73±0.083.73±0.045.69±0.115.52±0.12

YieldTRS(%)60.82±0.1764.80±1.6664.00±1.1263.72±1.5463.72±0.8164.82±1.1663.04±1.27

Yieldglucose(%)32.32±1.0337.61±1.2336.59±0.9337.05±1.437.32±1.2538.56±1.5337.53±1.41

Yieldxylose(%)28.5±1.6227.20±1.5427.41±1.1226.70±1.026.40±1.2126.21±0.8125.43±1.33

Table5

0.2gbamboohydrolysisin4gwatercatalyzedby0.45%(w/w)HClwith0.13%(w/w)metalionsasco-catalystsat100°C.Entry1234567

MetalionBlankNa+K+Mg2+Ca2+Cu2+Fe3+

Hydrolysistime(h)7777777

Hydrolysisrate(gLà1hà1)0.37±0.020.45±0.030.48±0.020.51±0.040.50±0.030.56±0.030.51±0.02

YieldTRS(%)

8.54±0.4610.08±0.6010.83±0.3711.74±0.5311.52±0.4312.78±0.4611.62±0.35

was?xedas0.45%(w/w)inthissection.AsshowninFig.1,thelig-nocellulosedegradationisaffectedbytheCu2+ionconcentrationsigni?cantly.Whentheionadditionsconcentrationwas0.02%(w/w),theTRSyieldincreasedwiththereactiontime.After7h,theTRSyieldreachedapeakvalue,whichincreasedbyapproxi-mately6%comparedtothesampletreatedwithoutionaddition.WiththeincreaseofCu2+ionconcentration,thereleasingrateofreducingsugarimprovedobviously.Afterreactedfor4h,theTRSyieldreached67.1%withCu2+ionconcentrationof0.23%(w/w).Comparedwiththereactionwithoutmetaladditions,theTRSyieldincreasedbyabout7%,andthereactiontimeofpeakTRSyieldwasdecreasedfor3h.Thenasthehydrolysisreactionprogressed,theTRSyielddecreased.AscanbeseeninFig.1,thehighertheCu2+ionconcentration,thefasterthetotalreducingsugaryieldreachedthepeakvalue.Thisphenomenoncouldbeexplainedas:thehigher

theCu2+ionconcentration,thelowerthehydrolysisreactionacti-vationenergyrequired,thusthemaximumTRSyieldcouldbereachedafterashortreactiontime.However,asthereactionspro-ceed,thedegradationofmonosaccharidepredominated.

Ananalysisofthemassandcompositionoftheproductshasbeencarriedoutwhenusing0.2gofbambooin4gof[C4mim]Cl

N.Wangetal./BioresourceTechnology173(2014)399–405

Table6

Recoveryyieldsofsugars,ILsandmetalions.0.2gbamboowashydrolyzedby0.45%(w/w)HClwith0.23%(w/w)Cu2+ionasco-catalystin[C4mim]Clat100°C.

Massbeforereaction(g)

GlucoseXyloseIL

Cu2+ions

0.088±0.0010.045±0.0014.000±0.0020.011±0.002

Massafterreaction(g)

0.022±0.0030.009±0.0023.326±0.0020.005±0.001

Recoveryyield(%)

25.33±2.6119.46±3.3483.15±0.1546.53±1.13

403

withCu2+ionconcentrationof0.23%(w/w)andHClconcentrationof0.45%(w/w)(Table6andFig.2).0.022gglucoseand0.009gxylosewererecoveredinthealkaliphase,accountingforarecov-eryof25.33%and19.46%,respectively.Besides,83.15%oftheILpresentinthesystemand46.53%oftheCu2+ionwasregenerated.Whentherecoveryrecyclewasconductedfor3times,ahigherglu-coseyieldcouldbereachedas32.53%.Comparedtothemaximumrecoveryyieldsof53%glucosereportedbySunetal.(2013),theglucoserecoveryyieldinourstudywaslower,whichwasattrib-utedtothelowertemperatureandHClconcentration.3.3.XRDspectra,FTIRspectraandSEManalysis

Togainaninsightintothepossiblemechanismrevealingtheenhancementofthehydrolysisbymetalions,thestructuralfea-turesofregeneratedbambooafter2htreatmentbyHClwith/with-outCu2+ionin[C4mim]Clat100°CwereexaminedusingXRD,FTIRandSEDandcomparedtothecorrespondinguntreatedbamboosamples.

Fig.1intheSupplementarymaterialsshowedtheX-raydiffrac-tionspectraofuntreatedandtreatedbamboo.TheX-raydiffractionofuntreatedbamboosamplesshowedamainpeakofcrystallinecelluloseat22°withabroadshoulderofamorphouscelluloseandhemicelluloseandligninat15°.BambootreatedwithHClpre-sentedadecreaseofX-raydiffractionintensitycomparedwiththatofuntreatedbamboo.However,thesigni?cantdecreasesofinten-sitywasderivedfromusingHCl/Cu2+ionforbambootreatment,suggestingthatCu2+ionwasef?cienttodestroythecrystalstruc-tureofcellulose.

InFig.2intheSupplementarymaterials,severalFTIRbandswereusedtomonitorthechemicalchangesofligninandcarbohy-drates.ComparedtotheuntreatedandHCltreatedbamboo,theintensityofbandsat1510cmà1(aromaticskeletalfromlignin)and1329cmà1(syringylandguaiacylcondensedlignin)decreased

2+decreaseinthepeakat1098cmà1(referringtothecrystallinecel-lulose)wereobservedfortheHCl/Cu2+iontreatedbamboo,exhib-itingatransformationofthecrystallinecellulosetoamorphouscelluloseafterthecombinedtreatmentofbamboo.Thephenome-nonwasconsistentwiththeXRDpattern,whichindicatedadecreaseincellulosecrystallinity(Zhuetal.,2012).

Fig.3intheSupplementarymaterialswasacollageofSEMimagesobtainedfromsamplesofHCltreatedbambooandHCl/Cu2+iontreatedbamboo.HCltreatedandHCl/Cu2+iontreatedbamboosamplesbothfollowedasimilarstructuralmodi?cationinthevasculartissue.However,theinsideandoutsidesurfacesoftheHCltreatedsamplesappearsmoothandlitteredwithsmalltolargedropletsoflignin.Incontrast,thesurfacesofHCl/Cu2+iontreatedbamboosamplesappeartohavealargepercentageofthematrixingmaterial(hemicelluloses)removed,withthecellulosemacro?bersintactbut?oatingabovethesurfaceinadelicate,lace-likepatternwithinterspersedlignindroplets(Weietal.,2011).Fromaboveexperiments,athree-stepmechanismtodepoly-merizebamboointomonosaccharidesbymetalionsmightbededuced:?rstly,themetalionsreactedwiththelignintobreaktheintricatestructureofbiomassandreleasecellulose;secondly,thecellulosewasdissolvedintotheILsandthecrystalstructureofcellulosewasdestroyedbybothILsandmetalions;thirdly,themetalionspromotedthecleavageofglycosidicbond.ThemetalionsinteractedwiththeoxygenoftheC–O–CglycosidebondbetweentheD-glucoseunitsincellulose,formingacomplexinter-mediateandweakeningtheheterocyclicetherbondbetweenthesugarmonomers.AsWeietal.(2011)reportedthat,metalionsassociatedwithcelluloseinnatural,untreatedbiomass.Thentheweakenetherbondwasbrokenbyhydrogenion,producingsugarmonomersandoligomers.Asreportedbypreviousstudies(Salmietal.,2014),theattachmentofthehydroxoniumiontotheglyco-sidicbondpromotedthecleavageofC–Obond,followedbyfurtherdegradationofmonosaccharide,whichcouldbeprominentundermoresevereconditions.Inourhydrolysissystem,thehemicellu-losereactionmechanismmightfollowasimilarway.Themetalionsinteractedwiththeoxygenoftheglycosidebondinhemicel-lulose,formingacomplexintermediateandweakeningtheC–Obond.Thenthehydroxoniumionpromotedthecleavageoftheweakenbond,formingsugarmonomersandoligomers.Duetothedifferentabilitytoattractelectrons,differentmetalionsexhib-iteddifferenteffectsonthehydrolysisofbamboo.Theorderofabilitytoattractelectronsforthesixmetalionsstudiedwas:Fe3+>Cu2+>Mg2+>Na+>Ca2+>K+.Theoretically,theFe3+ionsshouldshowthebesteffectonthehydrolysis,whiletheresult

404N.Wangetal./BioresourceTechnology173(2014)399–405

etal.,2009;Zhangetal.,2012).Inthisstudy,anewmodelforbamboohydrolyzedwithmetalions,diluteacidinILsolventwasconstructedasfollows,basedona?rst-orderrandomchainscissionfollowedbya?rst-orderreducingsugardegradation:

bamboo!k1reducingsugar!k2

decompositionproducts

e4T

wherek1istherateconstantforbamboohydrolysisandk2istherateconstantforreducingsugardegradation.