金属离子对离子液体中竹子酸解的影响(2)

Thetotalreducingsugars(TRS)wasmeasuredusingDNSmethod(Miller,1959).Thecolortestsweremadewith1.5mLali-quotsofDNSregentaddedto1.5mLaliquotsofreactionsample.Themixtureswereheatedfor5mininaboilingwaterbathandthencooledtoambienttemperature.ThecolorintensitiesweremeasuredinaJASCOV-530Modelspectrophotometerat540nmwithaslitwidthof0.06mm.TheconcentrationofTRSinthereac-tionsamplewascalculatedbasedonastandardcurveobtainedwithglucosestandard.Themonomericsugarswereanalyzedbyhighperformanceliquidchromatography(HPLC)usinganAminexHPX-87Hion-exchangecolumn(Bio-Rad,USA)andAgilent1200chromatographyworkingstationsystem(AgilentTechnologies,USA)equippedwithUVabsorbancedetector(AgilentTechnologies,G1315D)andrefractiveindexdetector(AgilentTechnologies,G1362A).Sampleswerecentrifugedat10,000rpmfor10min.Eachsupernatantwasdilutedwith10volumesof5mMH2SO4,and20lLofthedilutedsamplewasinjected.Thecolumnwaselutedisocraticallyatarateof0.6mLminà1with5mMH2SO4under55°C.ThemassofTRSandtheyieldofTRSwerecalculatedasfollows:

MT?M1?eM0=MST

e1TM1?TRSconcentrationeg=LT?V1e2TTRSyield?MT=e200?A?1:1T?100%

e3T

inwhich,MTisthemassofTRS,M1isthemassofTRSinthereactionsample,M0isthetotalmassofthereactionsolution,MSisthemassofsample,200isthemassofbamboobiomass(mg),andAistheweightpercentageofthepolysaccharidescontainedinlignocellu-losicmaterials.Forbamboo,A=40.1%+20.3%=60.4%.2.4.XRDanalysis

Afterthehydrolysisreactionwascompleted,themixturewasdilutedwithcoldwater,neutralizedwith0.05MNaOH,andcentri-fugedat10,000rpmfor5min.Thesolidwascollected,re-sus-pendedindeionizedwater,andcentrifuged.Thepuri?cationprocesswasrepeatedfor5times.Therecoveredsolidmaterial

N.Wangetal./BioresourceTechnology173(2014)399–405401

wasdriedat108°Ctoaconstantweight,thensquashedonglasssolidwithasmoothsurface,andsubmittedtoXRDanalysis.TheX-raypowderdiffractionpatternoftheoriginalandregeneratedbamboopreparationswasmeasuredusinganXRD-6000instru-ment(Shimadzu,Japan)withaCuKaradiationsource(k=0.154nm)at40kVand30mA.Sampleswerescannedfrom2h=5–50°ataspeedof2°/min.2.5.FTIRspectroscopy

Fouriertransforminfraredspectroscopy(FTIR)wasconductedusingaNicoletiN10FT-IRmicroscope(ThermoNicoletCorpora-tion,Madison,WI).Thesampleswerepresseduniformlyagainstthediamondsurfacewithaspring-loadedanvil.Thesamplespec-trawasobtainedinduplicatesusinganaverageof64scansovertherange4000–500cmà1withaspectralresolutionof8cmà1(Bianetal.,2014).2.6.SEManalysis

ThemorphologyandporousfeaturesofbamboowereobservedbyJSM-6700F(JEOL,Japan)scanningelectronmicroscopy(SEM)operatedat20kVacceleratingvoltage.Priortoimaging,thesam-plesweresputter-coatedwithgoldtomakethe?bersconductive,avoidingdegradationandbuildingupthechargeofthespecimen.2.7.RecoveryofsugarsandILs

Afterthehydrolysisreactionwascompleted,1mLofthemix-turesolutionwasplacedintoa2mLeppendorftubeandanappro-priateamountof50%(w/w)NaOHwereaddedtogivethe?nalNaOHconcentrationof20%(w/w).Themixturewasagitatedat1400rpmfor0.5handthencentrifugedat14,000rpmforphaseseparate.TheupperILphaseandlowerNaOHphasewasseparatedandcollected.Themassanddensityofbothphaseswascalculatedandthesugarcontentwasquanti?ed(Sunetal.,2013).3.Resultsanddiscussion

3.1.Effectofdifferentmetalionsonbamboohydrolysis

Table1summarizedtheTRSyieldsofbamboobiomassin[C4mim]Clcatalyzedby0.45%(w/w)HClwithdifferentmetalionsasco-catalysts.AsshowninTable1,ittook7hforthebambootreatmentwithoutmetalionadditionstoachievethemaximumTRSyieldof60.82%.ThemaximumTRSyieldswere64.80%and64.00%,respectively,afterhydrolysisfor7hwithNa+andK+ions(alkalimetalions)asco-catalysts.WhenMg2+andCa2+ions(alkaliearthmetalions)wereaddedintothehydrolysissystem,thereactiontimewasreducedfor1hwithasimilarTRSproduction,indicatingthatalkaliearthmetalionsaremoreeffectivethanalkalimetalionsonbambooacidichydrolysis.Hydrolysisexperimentwithtransitionmetalions(Cu2+andFe3+)werealsoperformed.TRSyieldswere64.82%and63.04%afterhydrolysisfor4h,

respectively,withCu2+andFe3+asco-catalysts.Thereactiontimewasdecreasedfor2hcomparedtothecaseswithalkaliearthionsasco-catalystswhenthesimilarTRSproductionwasobtained.Theseresultsdemonstratedthatalowcontentofmetalionsastheco-catalystswasanef?cientmethodforbiomassacidichydro-lysis.Transitionmetalionsexhibitedbettereffectsthanthealkaliearthmetalionsandalkalimetalions.Amongallthesixmetalions,Cu2+ionsshowedthebesteffectsbothonthehydrolysisrateandTRSproduction.

Thehydrolysisexperimentwithgraduallyreducedacidloadingwasexecuted,andtheresultsweresummarizedinTables2and3.MetalionshadlittleeffectontheTRSyieldexceptforCu2+ionwhenHClconcentrationwas0.23%(w/w),demonstratingonceagainthatCu2+caneffectivelyimprovetheyieldofTRSandaccel-eratethehydrolysisthananyothermetalions.Furtherreducingtheacidconcentrationto0.05%(w/w)producedlowerTRSyieldsafter7h.SotheoptimalHClconcentrationforbamboobiomasshydrolysiswaschosenas0.45%(w/w).

Besides,themetalionswerefoundtohaveaslightin?uenceontheglucoseandxyloseproduction.Experimentswasconductedwith0.45%(w/w)HCland0.13%(w/w)metalionasco-catalysts,andtheresultswereshowninTable4.Theglucoseyieldwasincreasedwithmetalionsadditionwhilethexyloseyieldwasdecreased,whichindicatedthatmetalionscouldpromotethehydrolysisofcellulose,meanwhileacceleratethedegradationofxylosetofurfurals(Salmietal.,2014).AsreportedbyLiuandWyman(2006),theinorganicsaltsKCl,NaCl,CaCl2,MgCl2,andFeCl3signi?cantlyincreasedxylosemonomerdegradationat180°Cinwater.Theconditioninthissystemwasmoremoderate,thuslessdegradationofxylosewouldoccurduringthehydrolysis.Table5illustratedtheeffectsofwaterusedasthesolventonthesugaryieldandconversionwith0.45%(w/w)HClanddifferentmetalionsasco-catalysts.Ittookabout7htoachievetheTRSyieldof8.54%whenthebamboobiomasswastreatedbyHClwithoutmetalionadditions.Whenmetalionswereaddedintothehydrolysissystem,theTRSproductionwasincreasedbyabout1.54–4.24%comparedtothecontrol.Theresultshowedthatmetalionsalsoworkedwellontheacidichydrolysiswithwaterassolvent.However,theTRSyieldsweredecreasedwithwaterassol-ventunderotherwiseidenticalconditions,comparedtotheresultsobtainedwithILassolventinTable5,suggestingthatILassolventplayedanimportantroleinthisreactionsystem.Thepossiblerea-sonwasthatlignocellulosesandcelluloseinthebamboobiomasscouldbedissolvedin[C4mim]Clastheircomplex,andthehydroly-siswasconductedeasilycomparedwiththewatersystem,inwhichhydrolysisjustoccurredatthesurfaceoflignocellulosesandcellulose(LiandZhao,2007).

3.2.EffectofCu2+ionsconcentrationontotalreducingsugarproduction

TofurtherinvestigatetheeffectofCu2+ionontheacidichydro-lysiswith[C4mim]Classolvent,differentconcentrationsofCu2+ionwereusedinthehydrolysisprocess.TheHClconcentration

Table1

Totalreducingsugaryieldsofthesampleshydrolyzedby0.45%(w/w)HClwithoutandwith0.13%(w/w)metalionadditionsatdifferentreactiontime.Time(h)

TRSyield(%)Blank

1234567

10.78±1.8926.89±1.4542.21±2.2651.13±2.0555.72±1.7858.87±0.2060.82±0.17

Na+

14.88±1.2635.56±1.0653.02±1.2760.78±1.5863.89±1.3664.67±0.8764.80±1.66

K+

11.52±1.4528.89±1.2744.31±1.9656.91±1.7861.73±1.5663.44±1.2164.00±1.12

Mg2+

12.50±0.9731.32±1.1646.64±0.8957.04±1.3661.32±1.1863.72±1.5466.62±1.38

Ca2+

11.62±1.5831.31±1.4547.87±1.0658.22±0.9661.78±1.1563.72±0.8165.33±1.27

Cu2+

15.14±2.0645.67±1.6762.23±1.7864.82±1.1665.78±1.5759.21±0.9653.84±1.56

Fe3+

15.32±1.5946.42±1.0660.33±0.8963.04±1.2757.78±1.2153.82±1.6050.23±1.45

402N.Wangetal./BioresourceTechnology173(2014)399–405

Table2

Totalreducingsugaryieldsofthesampleshydrolyzedby0.05%(w/w)HClwithoutandwith0.13%(w/w)metalionadditionsatdifferentreactiontime.Time(h)

TRSyield(%)Blank

1.5234567

5.13±1.208.21±0.9814.46±1.1120.03±0.8321.89±0.9624.42±0.8727.67±1.15

Na+

5.33±0.909.64±1.1014.31±0.9717.78±0.7823.11±0.6223.82±1.2125.78±1.10

K+

4.23±1.367.62±1.1710.84±1.4616.42±1.2621.53±1.1722.67±1.0324.32±1.10

Mg2+

4.89±1.188.34±1.4512.21±0.7216.62±1.1321.73±1.0223.04±1.3626.32±1.58

Ca2+

4.56±1.468.42±0.9713.10±1.2117.23±1.5621.87±1.0823.21±0.7825.89±1.36

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