Carbon dioxide as a pH-switch anti-solvent for biomass fractionation and pre-treatment with aqueous hydroxide solutions

Article abstract of DOI:10.1039/c7gc00574a

Rice husks (or rice hulls) pre-treated with aqueous potassium hydroxide solutions showed excellent glucose yields during enzymatic saccharification. CO₂ addition to the hydroxide solutions precipitated the dissolved rice husk silica as a nanoporous powder, while Ca(OH)₂ addition regenerated the hydroxide solution and precipitated the dissolved lignin. Fractionation of the biomass was thus achieved using CO₂ addition as a reversible pH-switch, and the hydroxide could be repeatedly recycled while maintaining biomass pre-treatment and fractionation efficacy.

Full text of DOI:10.1039/c7gc00574a

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Green
Chemistry
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Kenny, S. Cheong and L. Aldous, Green Chem., 2017, DOI: 10.1039/C7GC00574A.
Volume 18 Number
7
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April 2016 Pages 1821–2242
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ISSN 1463-9262
CRITICAL REVIEW
G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
rsc.li/green-chem

Page 1 of 7
Pleas Ge ꢀ rd eo eꢀ nn o Ct ꢀ ha ed mj u si st ꢀt mr yarginsꢀ
View Article Online
DOI: 10.1039/C7GC00574A
JournalꢀNameꢀ
ꢀ
COMMUNICATIONꢀ
ꢀ
CarbonꢀdioxideꢀasꢀaꢀpH-switchꢀanti-solventꢀforꢀbiomassꢀ
fractionationꢀandꢀpre-treatmentꢀwithꢀaqueousꢀhydroxideꢀ
solutionsꢀ
Receivedꢀ00thꢀJanuaryꢀ20xx,ꢀ
Acceptedꢀ00thꢀJanuaryꢀ20xxꢀ
DOI:ꢀ10.1039/x0xx00000xꢀ
a
a
b
a,c
TrangꢀQ.ꢀTo, ꢀCeireꢀKenny, ꢀSoshanꢀCheong ꢀandꢀLeighꢀAldous *
www.rsc.org/ꢀ
Riceꢀ husksꢀ (orꢀ riceꢀ hulls)ꢀ pre-treatedꢀ withꢀ aqueousꢀ potassiumꢀ toothpaste.ꢀ
hydroxideꢀ solutionsꢀ showedꢀ excellentꢀ glucoseꢀ yieldsꢀ duringꢀ Highꢀ silicaꢀ contentꢀ inꢀ biomassꢀ isꢀ associatedꢀ withꢀ
enzymaticꢀ saccharification.ꢀ CO ꢀ additionꢀ toꢀ theꢀ hydroxideꢀ significantlyꢀreducedꢀdigestibility, ꢀexcessiveꢀteethꢀwear, ꢀandꢀ
solutionsꢀ precipitatedꢀ theꢀ dissolvedꢀ riceꢀ huskꢀ silicaꢀ asꢀ aꢀ evenꢀmachineꢀwearꢀduringꢀup-streamꢀmechanicalꢀprocess. ꢀAsꢀ
nanoporousꢀ powder,ꢀ whileꢀ Ca(OH) ꢀ additionꢀ regeneratedꢀ theꢀ such,ꢀriceꢀhusksꢀcanꢀbeꢀconsideredꢀ‘zeroꢀcalorie’ꢀwaste,ꢀsinceꢀ
hydroxideꢀ solutionꢀ andꢀ precipitatedꢀ theꢀ dissolvedꢀ lignin.ꢀ theirꢀhighꢀsilicaꢀcontentꢀ(15-25ꢀwt%)
FractionationꢀofꢀtheꢀbiomassꢀwasꢀthusꢀachievedꢀusingꢀCO ꢀadditionꢀ thanꢀtheꢀtypicalꢀlimitꢀforꢀlivestockꢀfeed.ꢀ
asꢀaꢀreversibleꢀpH-switch,ꢀandꢀtheꢀhydroxideꢀcouldꢀbeꢀrepeatedlyꢀ Dueꢀtoꢀaꢀlackꢀofꢀviableꢀalternatives,ꢀriceꢀhusksꢀareꢀtypicallyꢀ
recycledꢀ whileꢀ maintainingꢀ biomassꢀ pre-treatmentꢀ andꢀ disposedꢀ ofꢀ orꢀ burned. ꢀ Theꢀ resultingꢀ silica-richꢀ ashesꢀ causeꢀ
ꢀ
10
11
2
5b
2
5b,ꢀ7,ꢀ12
ꢀisꢀca.ꢀ40ꢀfoldꢀhigherꢀ
2
ꢀ
12
9,ꢀ 13
fractionationꢀefficacy.ꢀ
extensiveꢀ environmentalꢀ problems.
ꢀ Attemptsꢀ toꢀ useꢀ theꢀ
riceꢀ huskꢀ ashꢀ forꢀ otherꢀ applications,ꢀ e.g.ꢀ asꢀ waterꢀ glassꢀ forꢀ
construction,ꢀareꢀhinderedꢀbyꢀtheꢀrelativelyꢀlowꢀpurityꢀofꢀtheꢀ
silica,ꢀ containingꢀ inorganicꢀ contaminantsꢀ suchꢀ asꢀ metalsꢀ andꢀ
Inedibleꢀlignocellulosicꢀbiomassꢀrepresentsꢀaꢀprimeꢀcandidateꢀ
1
forꢀbiofuelꢀproduction. ꢀHowever,ꢀtheꢀinherentꢀrecalcitranceꢀofꢀ
14
thisꢀbiomassꢀtoꢀ(bio)chemicalꢀprocessingꢀnecessitatesꢀchemicalꢀ
2
pre-treatment. ꢀ Thisꢀ pre-treatmentꢀ addsꢀ significantlyꢀ toꢀ theꢀ
phosphates. ꢀ
ꢀ
Carbonꢀ dioxideꢀ (CO2)ꢀ isꢀ theꢀ mainꢀ wasteꢀ productꢀ ofꢀ
2a
cost, ꢀ andꢀ techno-economicꢀ analysisꢀ hasꢀ demonstratedꢀ thatꢀ
otherꢀ chemicalꢀ constituentsꢀ presentꢀ inꢀ theꢀ biomassꢀ (beyondꢀ
carbohydrates)ꢀ ideallyꢀ needꢀ toꢀ beꢀ utilisedꢀ asꢀ value-addedꢀ
combustion,ꢀandꢀasꢀaꢀgreenhouseꢀgasꢀitsꢀemissionꢀisꢀofꢀgreatꢀ
environmentalꢀconcern.ꢀCO2ꢀinꢀtheꢀformꢀofꢀsupercriticalꢀliquidꢀ
hasꢀbeenꢀemployedꢀinꢀmanyꢀapplications,ꢀsuchꢀasꢀaꢀsolventꢀforꢀ
3
15
materialsꢀ forꢀ suchꢀ processesꢀ toꢀ beꢀ economicallyꢀ viable. ꢀ Bio-
basedꢀ productsꢀ achievedꢀ ca.ꢀ 10%ꢀ penetrationꢀ intoꢀ theꢀ USAꢀ
chemicalꢀmarketꢀinꢀ2015,ꢀandꢀbyꢀ2025ꢀthisꢀisꢀexpectꢀtoꢀriseꢀtoꢀ
extractingꢀ herbalꢀ ingredientsꢀ inꢀ theꢀ foodꢀ industry, ꢀ anti-
16
solventꢀ forꢀ proteinꢀ formulationꢀ inꢀ biosciences ꢀ andꢀ
17
solvent/anti-solventꢀ forꢀ chemicalꢀ reactions ꢀ andꢀ materialꢀ
4
ca.ꢀ22ꢀ%; ꢀfullꢀuseꢀofꢀtheꢀchemicalꢀconstituentsꢀofꢀbiomassꢀareꢀ
18
synthesis, ꢀtoꢀnameꢀaꢀfew.ꢀꢀ
CO2ꢀ hasꢀ recentlyꢀ beenꢀ employedꢀ inꢀ biomassꢀ processing,ꢀ
requiredꢀfromꢀbothꢀeconomicꢀandꢀsustainabilityꢀstandpoints.ꢀꢀ
Riceꢀ husksꢀ (orꢀ riceꢀ hulls)ꢀ areꢀ anꢀ agriculturalꢀ cropꢀ waste;ꢀ
ꢀ
e.g.ꢀthroughꢀCO2-explosion, ꢀsupercriticalꢀCO2ꢀextractionꢀfromꢀ
19
ꢀ
theyꢀ areꢀ producedꢀ onꢀ aꢀ massiveꢀ scaleꢀ worldwide, ꢀ andꢀ areꢀ
5
20
biomass ꢀ andꢀ theꢀ supercriticalꢀ CO2-drivenꢀ precipitationꢀ ofꢀ
22
6
readilyꢀsuppliedꢀinꢀeasilyꢀhandledꢀca.ꢀ8ꢀmmꢀbyꢀ2ꢀmmꢀpieces. ꢀ
21
dissolvedꢀ cellulose ꢀ andꢀ lignin. ꢀ CO2ꢀ hasꢀ alsoꢀ beenꢀ reactedꢀ
withꢀ basesꢀ toꢀ formꢀ ‘switchableꢀ ionicꢀ liquids’,ꢀ whichꢀ canꢀ beꢀ
Asꢀ aꢀ lignocellulosicꢀ biomass,ꢀ theyꢀ areꢀ composedꢀ (onꢀ aꢀ dryꢀ
basis)ꢀofꢀca.ꢀ38ꢀwt%,ꢀcellulose,ꢀ24ꢀwt%ꢀhemi-cellulose,ꢀ20ꢀwt%ꢀ
23
employedꢀ for,ꢀ e.g.ꢀ celluloseꢀ dissolution ꢀ andꢀ biomassꢀ
7
silicaꢀ andꢀ 19ꢀ wt%ꢀ lignin. ꢀ Theꢀ celluloseꢀ andꢀ hemicelluloseꢀ
24
fractionation. ꢀNotably,ꢀtheseꢀprocessesꢀtypicallyꢀinvolveꢀhighꢀ
pressuresꢀ andꢀ temperatures,ꢀ andꢀ haveꢀ beenꢀ reviewedꢀ
2a
representꢀsourcesꢀofꢀfermentableꢀsugars, ꢀligninꢀisꢀaꢀpotentialꢀ
8
sourceꢀ ofꢀ aromaticꢀ feedstockꢀ chemicals ꢀ andꢀ silicaꢀ hasꢀ
25
elsewhere. ꢀ
ꢀ
9
numerousꢀ applications, ꢀ e.g.ꢀ fromꢀ solarꢀ technologyꢀ toꢀ
Theꢀ pre-treatmentꢀ ofꢀ recalcitrantꢀ lignocellulosicꢀ biomassꢀ
withꢀ aqueousꢀ alkalineꢀ solutionsꢀ representsꢀ aꢀ methodꢀ toꢀ
significantlyꢀ improveꢀ downstreamꢀ treatments.ꢀ Forꢀ example,ꢀ
pre-treatmentꢀofꢀswitchgrassꢀwithꢀaqueousꢀsodiumꢀhydroxideꢀ
26
(
resultsꢀinꢀaꢀlignin-richꢀalkalineꢀsolutionꢀ(‘blackꢀliquor’)ꢀthatꢀcanꢀ
NaOH)ꢀ improvesꢀ theꢀ resultingꢀ enzymaticꢀ sugarꢀ yield. ꢀ Thisꢀ
27
beꢀ reusedꢀ forꢀ furtherꢀ biomassꢀ pretreatment. ꢀ Theꢀ
effectivenessꢀofꢀNaOHꢀsolutionsꢀcanꢀbeꢀfurtherꢀsupplementedꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ1ꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ

Pleas Ge ꢀ rd eo eꢀ nn o Ct ꢀ ha ed mj u si st ꢀt mr yarginsꢀ
Page 2 of 7
COMMUNICATIONꢀ
JournalꢀNameꢀ
2
8
byꢀadditionꢀofꢀCaO. ꢀAlkalineꢀtreatmentꢀenhancesꢀyieldsꢀfromꢀ
down-streamꢀ(bio)chemicalꢀprocessesꢀprimarilyꢀdueꢀtoꢀpartialꢀ
View Article Online
(
a) 14.0
(b)14.0 DOI: 10.1039/C7GC00574A
NaOH
NaOH
28
ligninꢀremovalꢀandꢀsignificantꢀstructuralꢀmodifications. ꢀꢀ
ꢀAqueousꢀalkalineꢀsolutionsꢀareꢀalsoꢀableꢀtoꢀpre-treatꢀriceꢀ
husksꢀ effectivelyꢀ priorꢀ toꢀ chemicalꢀ (e.g.ꢀ acid)ꢀ orꢀ biochemicalꢀ
Na CO
3
ꢀ
2
1
0.5
10.5
7.0
3.5
0.0
NaOH +
eq.
2
CO2
7
(
e.g.ꢀcellulase)ꢀsaccharification, ꢀlargelyꢀdueꢀtoꢀtheirꢀabilityꢀtoꢀ
removeꢀ silica,ꢀ inꢀ additionꢀ toꢀ ligninꢀ removalꢀ andꢀ structuralꢀ
changes.ꢀAꢀrecentꢀcationꢀstudyꢀhasꢀinvestigatedꢀconcentratedꢀ
alkaliꢀsolutionsꢀforꢀpre-treatingꢀriceꢀhusks,ꢀwithꢀcationsꢀrangingꢀ
fromꢀ theꢀ relativelyꢀ smallꢀ lithiumꢀ cationꢀ upꢀ toꢀ theꢀ relativelyꢀ
7
3
0
.0
.5
.0
NaHCO3
NaOH +
xs CO2
6
largeꢀtetrahexylammoniumꢀcation. ꢀExcessiveꢀinteractionsꢀwithꢀ
celluloseꢀwereꢀfoundꢀtoꢀbeꢀdetrimental,ꢀwhileꢀtheꢀaboveꢀnotedꢀ
biomassꢀalterationsꢀwereꢀstillꢀkey.ꢀLithiumꢀhydroxideꢀsolutionsꢀ
(
withꢀ ca.ꢀ 11ꢀ toꢀ 23ꢀ equivalentsꢀ ofꢀ water)ꢀ wereꢀ foundꢀ toꢀ beꢀ
0
.0
0.5
1.0
0.0
0.5
1.0
optimum,ꢀalthoughꢀsodiumꢀandꢀpotassiumꢀhydroxideꢀsolutionsꢀ
maintainedꢀ70-95%ꢀofꢀtheꢀactivityꢀwhileꢀalsoꢀbeingꢀmoreꢀviableꢀ
Eq. HCl
Eq. HCl
Figureꢀ 1.ꢀ Titrationꢀ curvesꢀ forꢀ (a)ꢀ 0.34ꢀ Mꢀ ofꢀ (p)ꢀ NaOH,ꢀ (n)ꢀ Na
2
CO
3
3
ꢀ andꢀ (l)ꢀ NaHCO ꢀ
6
economically. ꢀꢀ
comparedꢀ withꢀ (b)ꢀ 0.34ꢀ Mꢀ NaOH,ꢀ andꢀ thisꢀ solutionꢀ afterꢀ theꢀ additionꢀ ofꢀ (n)ꢀ two
equivalentsꢀofꢀCO ꢀandꢀ(l)ꢀmoreꢀthanꢀtwoꢀequivalentsꢀofꢀCO ꢀ(asꢀdryꢀice),ꢀhighlightingꢀ
completeꢀconversionꢀfromꢀNaOHꢀtoꢀNaHCO .ꢀꢀ
ꢀ
Weꢀ haveꢀ thereforeꢀ employedꢀ aqueousꢀ sodiumꢀ hydroxideꢀ
NaOH)ꢀ andꢀ potassiumꢀ hydroxideꢀ (KOH)ꢀ inꢀ thisꢀ study,ꢀ andꢀ
2
2
3
(
investigatedꢀ usingꢀ CO2ꢀ toꢀ achieveꢀ fractionationꢀ ofꢀ theꢀ riceꢀ
husks,ꢀ enhanceꢀ enzymaticꢀ digestibility,ꢀ recycleꢀ ofꢀ theꢀ
hydroxideꢀ andꢀ isolationꢀ ofꢀ CO2ꢀ asꢀ aꢀ solidꢀ product.ꢀ Theꢀ fullꢀ
MaterialsꢀandꢀMethodsꢀsectionꢀcanꢀbeꢀfoundꢀinꢀtheꢀElectronicꢀ
processꢀ entirelyꢀ consistentꢀ withꢀ completeꢀ conversionꢀ toꢀ
NaHCO3ꢀ(Figureꢀ1).ꢀ
ꢀ
Theꢀ conversionꢀ ofꢀ NaHCO3ꢀ backꢀ toꢀ NaOHꢀ wasꢀ alsoꢀ
investigated,ꢀusingꢀsolidꢀCa(OH)2.ꢀTheꢀsolubilityꢀofꢀCa(OH)2ꢀinꢀ
waterꢀ isꢀ relativelyꢀ lowꢀ (<ꢀ 2ꢀ g/L),ꢀ henceꢀ anꢀ excessꢀ ofꢀ solidꢀ
Ca(OH)2ꢀwasꢀaddedꢀinꢀorderꢀtoꢀachieveꢀtheꢀmetathesisꢀshownꢀ
inꢀequationꢀ(4);ꢀ
SupplementaryꢀInformationꢀ(ESI).ꢀ
ꢀ
-
Whileꢀtheꢀchemistryꢀbehindꢀtheꢀreactionꢀwithꢀ[OH] ꢀtoꢀCO2ꢀ
2-
-
toꢀformꢀ[CO3] ꢀandꢀ[HCO3] ꢀisꢀstandardꢀchemistry,ꢀweꢀcarriedꢀ
outꢀ theꢀ experimentsꢀ firstlyꢀ toꢀ ensureꢀ thatꢀ theꢀ inter-
conversionsꢀ happenꢀ inꢀ aꢀ timelyꢀ manner,ꢀ andꢀ secondlyꢀ toꢀ
-
-
[
HCO3] (aq)ꢀ+ꢀCa(OH)2(s)ꢀ→ꢀCaCO3(s)ꢀ+ꢀ[OH] (aq)ꢀ+ꢀH2Oꢀꢀ(4)ꢀ
6-7
ensureꢀthatꢀitꢀisꢀpossibleꢀatꢀtheꢀhighꢀconcentrations ꢀrequiredꢀ
forꢀeffectiveꢀriceꢀhuskꢀpre-treatment.ꢀTheꢀback-conversionꢀofꢀ
ꢀ
Metathesisꢀ wasꢀ observedꢀ toꢀ beꢀ extremelyꢀ rapidꢀ withꢀ
associatedꢀ heatꢀ generation.ꢀ Someꢀ ofꢀ theꢀ Ca(OH)2(s)ꢀ dissolvedꢀ
andꢀ aꢀ finerꢀ whiteꢀ precipitateꢀ wasꢀ formed.ꢀ Theꢀ isolatedꢀ solidꢀ
wasꢀ confirmedꢀ toꢀ beꢀ composedꢀ ofꢀ aꢀ mixtureꢀ ofꢀ CaCO3ꢀ andꢀ
excessꢀ Ca(OH)2,ꢀ whileꢀ theꢀ titrationꢀ curveꢀ ofꢀ theꢀ treatedꢀ
solutionꢀ wasꢀ virtuallyꢀ identicalꢀ toꢀ thatꢀ ofꢀ pureꢀ NaOH.ꢀ Theseꢀ
observationsꢀ confirmedꢀ theꢀ facileꢀ inter-conversionꢀ betweenꢀ
-
carbonatesꢀtoꢀ[OH] ,ꢀwhileꢀsimultaneouslyꢀremovingꢀCO2ꢀasꢀaꢀ
solid,ꢀ wasꢀ alsoꢀ investigated.ꢀ Thisꢀ involvedꢀ theꢀ additionꢀ ofꢀ
calciumꢀ hydroxideꢀ (Ca(OH)2),ꢀ toꢀ yieldꢀ calciumꢀ carbonateꢀ
(
available,ꢀe.g.ꢀitꢀisꢀtheꢀmainꢀproductꢀuponꢀheatingꢀfoodꢀwasteꢀ
CaCO3)ꢀ asꢀ anꢀ innocuousꢀ by-product.ꢀ Ca(OH)2ꢀ isꢀ abundantlyꢀ
29
-
-
suchꢀasꢀeggꢀshellsꢀandꢀmolluscꢀshells. ꢀꢀ
ꢀ
[
respectively.ꢀ Thisꢀ couldꢀ beꢀ achievedꢀ usingꢀ bothꢀ NaOHꢀ andꢀ
OH] ꢀ andꢀ [HCO3] ꢀ madeꢀ possibleꢀ byꢀ CO2ꢀ andꢀ Ca(OH)2,ꢀ
Proof-of-conceptꢀ pHꢀ switches:ꢀ Aꢀ solutionꢀ ofꢀ NaOHꢀ (0.34ꢀ M)ꢀ
wasꢀtitratedꢀusingꢀHClꢀ(Figureꢀ1).ꢀAsꢀexpected,ꢀitꢀwasꢀinitiallyꢀ
highlyꢀ alkalineꢀ andꢀ displayedꢀ onlyꢀ oneꢀ stepꢀ inꢀ itsꢀ titrationꢀ
curve,ꢀasꢀshownꢀbyꢀequationꢀ(1).ꢀ
KOH.ꢀ
ꢀ
-
Solutionsꢀ moreꢀ concentratedꢀ thanꢀ 0.34ꢀ Mꢀ [OH] ꢀ areꢀ
requiredꢀ forꢀ effectiveꢀ biomassꢀ pre-treatmentꢀ (atꢀ roomꢀ
temperature).ꢀWithꢀincreasingꢀalkaliꢀconcentration,ꢀtheꢀdirectꢀ
additionꢀ ofꢀ dryꢀ iceꢀ eventuallyꢀ becameꢀ unfeasibleꢀ dueꢀ toꢀ theꢀ
relativeꢀquantityꢀofꢀsolidꢀtoꢀliquid.ꢀHowever,ꢀbyꢀplacingꢀtheꢀdryꢀ
+
-
ꢀ
H
(aq)ꢀ+ꢀ[OH] (aq)ꢀàꢀH2O
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(1)ꢀ
ꢀ
CO2ꢀ isꢀ readilyꢀ availableꢀ inꢀ solidꢀ formꢀ asꢀ dryꢀ ice.ꢀ Theꢀ iceꢀ inꢀ aꢀ sealedꢀ containerꢀ atꢀ roomꢀ temperature,ꢀ andꢀ allowingꢀ
-
stoichiometricꢀ quantityꢀ ofꢀ CO2ꢀ (twoꢀ CO2ꢀ perꢀ [OH] )ꢀ wasꢀ
weighedꢀoutꢀasꢀdryꢀice.ꢀThisꢀdryꢀiceꢀwasꢀthenꢀaddedꢀslowlyꢀtoꢀ
theꢀNaOHꢀsolution,ꢀinꢀorderꢀtoꢀachieveꢀtheꢀtwo-stepꢀreactionꢀ
shownꢀinꢀequationsꢀ(2)ꢀandꢀ(3);ꢀꢀ
theꢀevolvedꢀCO2ꢀtoꢀbubbleꢀthroughꢀtheꢀhydroxideꢀsolutionꢀviaꢀ
anꢀ immersedꢀ tube,ꢀ efficientꢀ andꢀ completeꢀ conversionꢀ toꢀ
-ꢀ
[
HCO3] couldꢀbeꢀreproduciblyꢀachieved,ꢀwithinꢀanꢀhourꢀforꢀca.ꢀ
-
ꢀMꢀ[OH] ꢀsamples. ꢀ
ꢀ
5
ꢀ
-
2-
ꢀ
ꢀ
2[OH] (aq)ꢀ+ꢀCO2(s)ꢀ→ꢀ[CO3] (aq)ꢀ+ꢀH2Oꢀꢀ ꢀ
ꢀ
ꢀ
(2)ꢀ
(3)ꢀ
2-
ꢀ
-
ꢀ
ꢀ
Inherentꢀ recalcitranceꢀ andꢀ theꢀ needꢀ forꢀ fractionationꢀ Theꢀ
directꢀ additionꢀ ofꢀ as-receivedꢀ riceꢀ husksꢀ toꢀ aꢀ standardꢀ
cellulase-basedꢀ enzymaticꢀ brothꢀ yieldedꢀ noꢀ detectableꢀ sugarꢀ
release,ꢀconfirmingꢀtheꢀextremeꢀrecalcitranceꢀofꢀtheꢀriceꢀhusks.ꢀ
Stirringꢀtheꢀriceꢀhusksꢀinꢀwaterꢀforꢀthreeꢀdaysꢀbeforeꢀsubjectingꢀ
themꢀ toꢀ enzymaticꢀ hydrolysisꢀ resultedꢀ inꢀ onlyꢀ extremelyꢀ lowꢀ
sugarꢀyieldsꢀ(22ꢀ±ꢀ1ꢀmgꢀglucoseꢀperꢀgꢀofꢀriceꢀhusks).ꢀ
[CO3] (aq) +ꢀH2Oꢀ+ꢀCO2ꢀ→ꢀ2[HCO3] (aq)
ꢀ
Titratingꢀ theꢀ solutionꢀ withꢀ HClꢀ demonstratedꢀ aꢀ two-stepꢀ
curveꢀ(Figureꢀ1)ꢀindicativeꢀofꢀaꢀmixtureꢀofꢀNa2CO3ꢀandꢀNaHCO3.ꢀ
Conversionꢀ wasꢀ thereforeꢀ near-quantitative;ꢀ itꢀ shouldꢀ beꢀ
notedꢀ thatꢀ thisꢀ wasꢀ allꢀ performedꢀ atꢀ roomꢀ temperatureꢀ andꢀ
openꢀ toꢀ theꢀ atmosphere,ꢀ henceꢀ aꢀ smallꢀ amountꢀ ofꢀ CO2ꢀ wasꢀ
lost.ꢀAdditionꢀofꢀanꢀexcessꢀofꢀCO2ꢀresultedꢀinꢀaꢀsingleꢀtitrationꢀ
2
ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
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DOI: 10.1039/C7GC00574A
Figureꢀ 3:ꢀ (a)ꢀ Low-ꢀ andꢀ (b)ꢀ high-magnificationꢀ TEMꢀ imagesꢀ showingꢀ theꢀ 3Dꢀ porousꢀ
structureꢀofꢀtheꢀCO -precipiatedꢀriceꢀhuskꢀsilicaꢀparticles.ꢀꢀ
2
hemicelluloseꢀfraction.ꢀHigherꢀglucoseꢀyieldsꢀwereꢀachievedꢀafterꢀ48ꢀ
hꢀ orꢀ moreꢀ pre-treatment,ꢀ butꢀ onlyꢀ ca.ꢀ 17%ꢀ higherꢀ thanꢀ thatꢀ
obtainedꢀ afterꢀ 2ꢀ hꢀ pre-treatment.ꢀ Precipitationꢀ issuesꢀ inꢀ Stepꢀ 2ꢀ
(
discussedꢀbelow)ꢀnecessitatedꢀtheꢀuseꢀKOHꢀoverꢀNaOH,ꢀandꢀaꢀKOHꢀ
concentrationꢀnotꢀexceedingꢀ2.4ꢀM.ꢀ
Afterꢀ2ꢀhꢀpre-treatment,ꢀca.ꢀ72%ꢀofꢀtheꢀinitialꢀmassꢀofꢀriceꢀ
ꢀ
huskꢀ wasꢀ recovered.ꢀ Theꢀ compositionꢀ ofꢀ theꢀ recovered,ꢀ pre-
treatedꢀriceꢀhusksꢀwasꢀfoundꢀtoꢀbeꢀ(onꢀaꢀdryꢀbasis)ꢀca.ꢀ80ꢀwt%ꢀ
holocellulose,ꢀ ca.ꢀ 7ꢀ wt%ꢀ silicaꢀ andꢀ ca.ꢀ 13ꢀ wt%ꢀ lignin.ꢀ Thisꢀ isꢀ
fromꢀanꢀinitialꢀcompositionꢀofꢀca.ꢀ62ꢀ±ꢀ6ꢀwt%ꢀholocellulose,ꢀ20ꢀ
Figureꢀ2:ꢀThree-stepꢀriceꢀhuskꢀpre-treatment,ꢀfractionationꢀandꢀrecyclingꢀprocessꢀꢀ
±
ꢀ
ꢀ2ꢀwt%ꢀsilicaꢀandꢀ19ꢀ±ꢀ2ꢀwt%ꢀlignin.ꢀ
Theꢀpre-treatedꢀriceꢀhusksꢀ(shownꢀdriedꢀinꢀtheꢀphotoꢀinsertꢀ
ꢀ Followingꢀbiomassꢀpre-treatment,ꢀtheꢀdilutionꢀorꢀadditionꢀ
ofꢀaꢀsolventꢀtoꢀtheꢀtreatmentꢀsolutionꢀisꢀcommonlyꢀemployedꢀ
inꢀFigureꢀ2)ꢀwereꢀphysicallyꢀisolatedꢀbyꢀfiltration,ꢀrinsedꢀthreeꢀ
timesꢀwithꢀwaterꢀandꢀenzymaticallyꢀhydrolysed.ꢀTreatmentꢀofꢀ
theꢀriceꢀhusksꢀinꢀthisꢀmannerꢀcouldꢀreproduciblyꢀyieldꢀca.ꢀ329ꢀ±ꢀ
30
toꢀ assistꢀ inꢀ separation. ꢀ Theꢀ directionꢀ additionꢀ ofꢀ CO2ꢀ wasꢀ
benchmarkedꢀ againstꢀ theꢀ additionꢀ ofꢀ methanolꢀ asꢀ anꢀ anti-
solvent,ꢀ andꢀ theꢀ enzymaticꢀ hydrolysabilityꢀ ofꢀ theꢀ riceꢀ husksꢀ
evaluated.ꢀHowever,ꢀone-potꢀtreatmentꢀinꢀtheꢀalkaliꢀsolutionꢀ
thenꢀCO2ꢀorꢀmethanolꢀadditionꢀcouldꢀnotꢀraiseꢀtheꢀenzymaticꢀ
glucoseꢀyieldsꢀaboveꢀ19ꢀ±ꢀ5ꢀmg/g,ꢀlargelyꢀdueꢀtoꢀvisibleꢀsilicaꢀ
depositionꢀoverꢀtheꢀsurfaceꢀofꢀtheꢀpre-treatedꢀriceꢀhusks.ꢀThisꢀ
demonstratedꢀthatꢀtheꢀpre-treatedꢀbiomassꢀhadꢀtoꢀbeꢀisolatedꢀ
priorꢀ toꢀ anyꢀ attemptꢀ toꢀ recycleꢀ theꢀ hydroxideꢀ solution,ꢀ andꢀ
promptedꢀ theꢀ three-foldꢀ fractionationꢀ andꢀ recycleꢀ routeꢀ
displayedꢀ inꢀ Figureꢀ 2.ꢀ Theseꢀ threeꢀ stepsꢀ areꢀ discussedꢀ
separatelyꢀbelow,ꢀandꢀfullꢀdescriptionsꢀofꢀeachꢀprocessꢀcanꢀbeꢀ
foundꢀinꢀtheꢀMaterialsꢀandꢀMethodsꢀsectionꢀinꢀtheꢀElectronicꢀ
Supplementaryꢀ Informationꢀ (ESI).ꢀ Aꢀ combinedꢀ mass-balanceꢀ
figureꢀandꢀflowꢀchartꢀcanꢀbeꢀfoundꢀinꢀtheꢀESIꢀasꢀFigureꢀS1.ꢀ
ꢀ
6
3
ꢀmgꢀglucoseꢀperꢀgꢀofꢀtheꢀstartingꢀmassꢀofꢀtheꢀriceꢀhusksꢀ(orꢀ
54ꢀmgꢀperꢀgꢀonꢀaꢀdryꢀbasis),ꢀconfirmingꢀeffectiveꢀdisruptionꢀofꢀ
theꢀ riceꢀ husk.ꢀ Acidꢀ hydrolysisꢀ optimisedꢀ forꢀ non-silicaꢀ
31
containingꢀlignocellulosicꢀbiomass ꢀcouldꢀonlyꢀreleaseꢀ172ꢀ±ꢀ3ꢀ
7
mg/gꢀ fromꢀ theꢀ sameꢀ batchꢀ ofꢀ riceꢀ husks, ꢀ highlightingꢀ theꢀ
superiorityꢀofꢀtheꢀhydroxideꢀrouteꢀinꢀpre-treatingꢀtheꢀsilica-richꢀ
riceꢀhusks.ꢀSimilarꢀKOH-treatmentsꢀofꢀriceꢀhusksꢀcouldꢀnotꢀbeꢀ
foundꢀinꢀtheꢀliterature,ꢀbutꢀKOHꢀtreatmentꢀofꢀentireꢀriceꢀplantsꢀ
followedꢀ byꢀ cellulaseꢀ hydrolysisꢀ resultedꢀ inꢀ ca.ꢀ 405ꢀ ±ꢀ 4ꢀ mgꢀ
reducingꢀsugarsꢀperꢀgꢀofꢀdryꢀriceꢀplant;ꢀthisꢀisꢀconsistentꢀwithꢀ
32
theꢀhigherꢀcarbohydrateꢀcontentꢀofꢀtheꢀplant. ꢀ
ꢀ
Stepꢀ2:ꢀCO2-treatmentꢀandꢀsilicaꢀrecoveryꢀAdditionꢀofꢀCO2ꢀasꢀ
anꢀ anti-solventꢀ toꢀ theꢀ pre-treatmentꢀ liquorꢀ wasꢀ semi-
optimised;ꢀuseꢀofꢀNaOHꢀwasꢀfoundꢀtoꢀbeꢀlessꢀthanꢀoptimalꢀdueꢀ
toꢀ theꢀ significantlyꢀ lowerꢀ solubilityꢀ ofꢀ NaHCO3.ꢀ Theꢀ higherꢀ
solubilityꢀofꢀKHCO3ꢀavoidedꢀthisꢀandꢀpromptedꢀitsꢀfurtherꢀuse,ꢀ
althoughꢀ onlyꢀ atꢀ aꢀ maximumꢀ concentrationꢀ ofꢀ 2.4ꢀ M.ꢀ Thisꢀ isꢀ
equivalentꢀtoꢀ22.5ꢀmolꢀH2OꢀperꢀmolꢀKOH,ꢀorꢀca.ꢀ96ꢀmol%ꢀH2O.ꢀꢀ
Stepꢀ1:ꢀPre-treatmentꢀandꢀfractionationꢀSemi-optimisedꢀconditionsꢀ
involvedꢀtheꢀroomꢀtemperatureꢀpre-treatmentꢀofꢀ2.5ꢀwt%ꢀriceꢀhuskꢀ
inꢀ 2.4ꢀ Mꢀ KOHꢀ forꢀ 2ꢀ h,ꢀ followedꢀ byꢀ physicalꢀ separationꢀ ofꢀ theꢀ
undissolvedꢀ materialꢀ fromꢀ theꢀ supernatant.ꢀ Longerꢀ treatmentꢀ (upꢀ
toꢀ8ꢀh)ꢀreducedꢀtheꢀglucoseꢀyield,ꢀasꢀshownꢀinꢀtheꢀESIꢀinꢀFigureꢀS2;ꢀ
thisꢀ isꢀ attributedꢀ toꢀ gradualꢀ removalꢀ ofꢀ theꢀ alkalineꢀ solubleꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ3ꢀ
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amorphousꢀ inꢀ nature,ꢀ andꢀ qualitativelyꢀ similarꢀ toꢀ
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35
(a)
After
Pre-treatment
After recycle
(b) 3.0
DOI: 10.1039/C7GC00574A
After
Pre-treatment
After recycle
colloidal/precipitatedꢀsilica. ꢀ
ꢀ
1
0
0
.0
.5
Stepꢀ 3:ꢀ KOHꢀ regeneration,ꢀ ligninꢀ removalꢀ andꢀ recycleꢀ Theꢀ
regenerationꢀ ofꢀ alkaliꢀ metalꢀ hydroxidesꢀ fromꢀ alkaliꢀ metalꢀ
carbonatesꢀ usingꢀ Ca(OH)2ꢀ (theꢀ so-calledꢀ ‘causticisationꢀ
reaction’)ꢀisꢀaꢀwell-studiedꢀreaction,ꢀandꢀusedꢀonꢀaꢀhugeꢀscaleꢀ
2
1
0
.0
.0
.0
36
inꢀ theꢀ Kraftꢀ pulpingꢀ industry. ꢀ Asꢀ anꢀ equilibriumꢀ process,ꢀ itꢀ
typicallyꢀhasꢀaꢀmaximumꢀefficiencyꢀofꢀca.ꢀ90%ꢀ(inꢀtheꢀabsenceꢀ
36b
ofꢀsulfides). ꢀAdditionꢀofꢀCa(OH)2ꢀpowderꢀtoꢀtheꢀ2.4ꢀMꢀKHCO3ꢀ
solution,ꢀpost-riceꢀhuskꢀtreatmentꢀandꢀCO2ꢀaddition,ꢀresultedꢀ
inꢀ facileꢀ metathesisꢀ toꢀ reformꢀ ca.ꢀ 2.4ꢀ Mꢀ KOH.ꢀ Althoughꢀ aꢀ
relativelyꢀ largeꢀ amountꢀ ofꢀ powderꢀ wasꢀ required,ꢀ theꢀ liquid-
solidꢀseparationꢀwasꢀeasilyꢀaccomplished.ꢀꢀ
.0
2
25 300 375 450 525
Wavelength / nm
1
2
3
4
5
6
(Re)cycle number
ꢀ
Havingꢀ completedꢀ theꢀ wholeꢀ cycleꢀ shownꢀ inꢀ Figureꢀ 2,ꢀ
Figureꢀ4:ꢀ(a)ꢀUV-Visꢀspectraꢀofꢀ2.4ꢀMꢀKOHꢀ(-ꢀ-ꢀ-)ꢀafterꢀpre-treatingꢀ2.5ꢀwt%ꢀriceꢀhusksꢀforꢀ
ꢀh,ꢀ highlightingꢀtheꢀdissolvingꢀlignin,ꢀandꢀ(—)ꢀafterꢀbeingꢀrecycledꢀonce,ꢀhighlighting
reducedꢀligninꢀcontentꢀafterꢀCa(OH) ꢀtreatment.ꢀ(b)ꢀAbsorbanceꢀatꢀ293ꢀnmꢀasꢀaꢀfunctionꢀ
ofꢀ(re)cycleꢀnumber,ꢀ(blue,ꢀdashed)ꢀdirectlyꢀafterꢀpre-treatmentꢀofꢀriceꢀhusksꢀandꢀ(red,
solid)ꢀafterꢀCa(OH) ꢀre-generation.ꢀAllꢀsolutionsꢀwereꢀdilutedꢀ30-foldꢀwithꢀfreshꢀ2.4ꢀM
KOHꢀbeforeꢀmeasurement.ꢀꢀ
recycleꢀ wasꢀ easilyꢀ achieved.ꢀ Oneꢀ solutionꢀ wasꢀ thereforeꢀ
utilisedꢀinꢀsixꢀcompleteꢀrecycles.ꢀAꢀfixedꢀbiomassꢀloadingꢀwasꢀ
usedꢀ(2.5ꢀwt%ꢀriceꢀhusks)ꢀandꢀtheꢀKOHꢀsolutionꢀ(initiallyꢀ40ꢀmL)ꢀ
wasꢀnotꢀtreatedꢀinꢀanyꢀmannerꢀbetweenꢀcyclesꢀbeyondꢀwhatꢀisꢀ
shownꢀinꢀFigureꢀ2,ꢀi.e.ꢀadditionalꢀKOHꢀorꢀwaterꢀwasꢀnotꢀadded,ꢀ
norꢀmadeꢀmoreꢀconcentratedꢀbyꢀwaterꢀevaporation.ꢀHowever,ꢀ
2
2
2
ꢀ
AdditionꢀofꢀsufficientꢀCO2ꢀtoꢀ2.4ꢀMꢀKOHꢀcouldꢀconvertꢀtheꢀ dueꢀ toꢀ someꢀ naturalꢀ volumeꢀ reductionꢀ throughꢀ theꢀ cycles,ꢀ
-
systemꢀtoꢀ[HCO3] ,ꢀandꢀadditionꢀofꢀCO2ꢀtoꢀtheꢀdarkꢀbrownꢀpre- eachꢀ(re)cycleꢀwasꢀperformedꢀonꢀaꢀslightlyꢀsmallerꢀscale.ꢀ
treatmentꢀ liquorꢀ alsoꢀ resultedꢀ inꢀ theꢀ precipitationꢀ ofꢀ aꢀ EachꢀcycleꢀinvolvedꢀtheꢀadditionꢀofꢀexcessꢀCa(OH)2ꢀpowder,ꢀ
ꢀ
significantꢀquantityꢀofꢀwhiteꢀflakes.ꢀTheꢀwhiteꢀprecipitateꢀwasꢀ andꢀrecoveryꢀofꢀaꢀCaCO3/Ca(OH)2ꢀpowder.ꢀAdditionally,ꢀasꢀtheꢀ
completelyꢀ solubleꢀ inꢀ tetramethylammoniumꢀ hydroxide,ꢀ andꢀ cyclesꢀprogressedꢀtheꢀCaCO3/Ca(OH)2ꢀpowderꢀwasꢀnotedꢀtoꢀbeꢀ
ICP-MSꢀanalysisꢀfoundꢀonlyꢀSiꢀandꢀK;ꢀafterꢀoneꢀrecycleꢀofꢀtheꢀ off-whiteꢀ (photoꢀ insertꢀ inꢀ Figureꢀ 2).ꢀ Ca(OH)2ꢀ willꢀ dissolveꢀ inꢀ
KOHꢀ solutionꢀ usingꢀ Ca(OH)2ꢀ (Stepꢀ 3,ꢀ discussedꢀ later)ꢀ theꢀ diluteꢀ HClꢀ toꢀ formꢀ aꢀ clearꢀ solution,ꢀ whileꢀ dissolutionꢀ ofꢀ theꢀ
precipitatedꢀsilicaꢀwasꢀconsistentlyꢀca.ꢀ80ꢀwt%ꢀSi,ꢀ17ꢀwt%ꢀKꢀandꢀ recoveredꢀ CaCO3/Ca(OH)2ꢀ inꢀ diluteꢀ HClꢀ resultedꢀ inꢀ significantꢀ
3
huskꢀ ashꢀ followedꢀ byꢀ neutralisationꢀ typicallyꢀ containsꢀ Na,ꢀ Kꢀ yellowꢀ (ESI,ꢀ Figureꢀ S3).ꢀ UV-Visꢀ analysisꢀ ofꢀ theꢀ solutionꢀ
ꢀwt%ꢀCa.ꢀSilicaꢀgelꢀpreparedꢀbyꢀtheꢀalkalineꢀdissolutionꢀofꢀriceꢀ brownꢀ solidꢀ remainingꢀ andꢀ theꢀ solutionꢀ beingꢀ darkꢀ brown-
33
andꢀCa; ꢀmineralꢀleachingꢀstepsꢀcanꢀextensivelyꢀremoveꢀtheseꢀ confirmedꢀtheꢀcharacteristicꢀpresenceꢀofꢀacid-solubleꢀligninꢀinꢀ
34
contaminants. ꢀSignificantly,ꢀtheꢀotherꢀcontaminantsꢀcommonꢀ theꢀsolution,ꢀandꢀtheꢀsolidꢀwasꢀdeterminedꢀtoꢀbeꢀacid-insolubleꢀ
31
toꢀriceꢀhuskꢀashꢀsuchꢀasꢀiron,ꢀmagnesium,ꢀmanganese,ꢀsulfurꢀ lignin,ꢀ asꢀ perꢀ NRELꢀ guidelines. ꢀ Theꢀ acid-solubleꢀ andꢀ acid-
14
andꢀ extensiveꢀ phosphate ꢀ wereꢀ allꢀ belowꢀ detectionꢀ limits.ꢀ insolubleꢀligninꢀwereꢀpresentꢀinꢀaꢀca.ꢀ20:80ꢀratioꢀbyꢀmass,ꢀandꢀ
TGAꢀanalysisꢀofꢀtheꢀdriedꢀprecipitateꢀrevealedꢀca.ꢀ2ꢀwt%ꢀmassꢀ theꢀtotalꢀligninꢀrecoveredꢀcorrespondedꢀtoꢀ64ꢀ±ꢀ4%ꢀ(byꢀmass)ꢀ
lossꢀ consistentꢀ withꢀ waterꢀ ofꢀ hydration,ꢀ andꢀ ca.ꢀ 6ꢀ wt%ꢀ ofꢀtheꢀligninꢀthatꢀwasꢀremovedꢀfromꢀtheꢀriceꢀhusks.ꢀꢀ
consistentꢀwithꢀcarbonateꢀdecomposition.ꢀStirringꢀinꢀ0.1ꢀMꢀHClꢀ
ꢀ
Theꢀligninꢀcontentꢀofꢀtheꢀliquorꢀwasꢀthereforeꢀmonitoredꢀasꢀ
forꢀ18ꢀhꢀresultedꢀinꢀtheꢀremovalꢀofꢀallꢀresidualꢀcarbonates,ꢀasꢀ aꢀ functionꢀ ofꢀ recycleꢀ stageꢀ andꢀ number.ꢀ Figureꢀ 4(a)ꢀ displaysꢀ
wellꢀasꢀtheꢀKꢀandꢀCa,ꢀtoꢀyieldꢀca.ꢀ100%ꢀpureꢀsilicaꢀ(confirmedꢀ UV-Visꢀ spectraꢀ ofꢀ theꢀ KOHꢀ solutionꢀ afterꢀ pre-treatingꢀ riceꢀ
byꢀTGAꢀandꢀICP-MS)ꢀasꢀaꢀfineꢀwhiteꢀpowder,ꢀasꢀshownꢀinꢀtheꢀ husks,ꢀdisplayingꢀtheꢀtwoꢀpeaksꢀatꢀca.ꢀ300ꢀnmꢀandꢀca.ꢀ325ꢀnmꢀ
photoꢀinsertꢀinꢀFigureꢀ2.ꢀꢀ
ꢀ
thatꢀ areꢀ characteristicꢀ ofꢀ ligninꢀ dissolvedꢀ inꢀ aqueousꢀ alkalineꢀ
7,ꢀ37
TheꢀresultingꢀCO2-precipitated,ꢀacid-washedꢀsilicaꢀcouldꢀbeꢀ solution. ꢀAfterꢀregenerationꢀofꢀtheꢀCO2-treatedꢀsampleꢀwithꢀ
readilyꢀ re-suspendedꢀ inꢀ water.ꢀ Transmissionꢀ electronꢀ excessꢀ Ca(OH)2ꢀ powder,ꢀ theꢀ recoveredꢀ KOH-solutionꢀ clearlyꢀ
microscopyꢀ (TEM)ꢀ characterisationꢀ showedꢀ thatꢀ theꢀ acid- hadꢀaꢀlowerꢀligninꢀcontent.ꢀFigureꢀ4(b)ꢀdisplaysꢀtheꢀabsorbanceꢀ
washedꢀsilicaꢀwasꢀcomposedꢀofꢀ3D,ꢀporousꢀnanostructuresꢀinꢀ acrossꢀ theꢀ sixꢀ recycles.ꢀ Theꢀ ligninꢀ absorbanceꢀ peakꢀ doubledꢀ
theꢀsizeꢀrangeꢀofꢀ400-800ꢀnmꢀ(Figureꢀ3a).ꢀHigh-magnificationꢀ afterꢀtheꢀsecondꢀrecycle,ꢀbutꢀgraduallyꢀreachedꢀaꢀplateauꢀwithꢀ
TEMꢀobservationsꢀshowedꢀthatꢀtheseꢀstructuresꢀwereꢀmadeꢀupꢀ repeatedꢀrecycling,ꢀindicatingꢀthatꢀtheꢀligninꢀlevelꢀinꢀtheꢀliquorꢀ
byꢀ relativelyꢀ uniformꢀ ribbon-likeꢀ componentsꢀ ofꢀ ~20ꢀ nmꢀ inꢀ wasꢀ approachingꢀ saturation,ꢀ althoughꢀ itꢀ wasꢀ stillꢀ graduallyꢀ
diameterꢀ thatꢀ hadꢀ formedꢀ intoꢀ 3Dꢀ networksꢀ (Figureꢀ 3),ꢀ increasingꢀpost-CO2-treatment.ꢀ
renderingꢀtheꢀsampleꢀhighlyꢀporous.ꢀTheꢀentireꢀstructureꢀwasꢀ
4
ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
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JournalꢀNameꢀ
Therefore,ꢀligninꢀwasꢀbeingꢀextractedꢀfromꢀtheꢀriceꢀhusksꢀ
COMMUNICATIONꢀ
SiO recovered
ꢀ
Glucose
View Article Online
DOI: 10.1039/C7GC00574A
2
byꢀ theꢀ KOHꢀ solutionꢀ andꢀ accumulatingꢀ throughꢀ theꢀ recycles,ꢀ
withꢀ someꢀ precipitatedꢀ byꢀ Ca(OH)2ꢀ treatmentꢀ ofꢀ theꢀ lignin-
containingꢀ KHCO3ꢀ solution;ꢀ thisꢀ potentiallyꢀ reachedꢀ aꢀ self-
1
00
300
80
60
th
limitingꢀvalueꢀbyꢀtheꢀ6 ꢀcycle.ꢀ
ꢀ
2
1
00
00
0
ꢀ
Evaluationꢀ ofꢀ pre-treatmentꢀ effectivenessꢀ asꢀ aꢀ functionꢀ ofꢀ
4
2
0
0
0
recyclesꢀꢀ
ꢀ
Theꢀpercentageꢀofꢀligninꢀremainingꢀinꢀtheꢀpre-treatedꢀriceꢀ
31
husksꢀ wasꢀ quantifiedꢀ usingꢀ aꢀ standardꢀ procedure, ꢀ andꢀ wasꢀ
foundꢀtoꢀbeꢀwithinꢀconsistentꢀwithinꢀerrorꢀ(32ꢀ±ꢀ13%ꢀremoved)ꢀ
acrossꢀtheꢀrecycles,ꢀwithꢀnoꢀsystematicꢀtrend.ꢀUV-Visꢀanalysisꢀ
withꢀ aꢀ freshꢀ KOHꢀ solutionꢀ demonstratedꢀ thatꢀ theꢀ ligninꢀ
contentꢀofꢀtheꢀalkalineꢀsolutionꢀdoubledꢀafterꢀstirringꢀforꢀ120ꢀhꢀ
insteadꢀofꢀ2ꢀhꢀ(ESI,ꢀFigureꢀS4)ꢀbutꢀthisꢀonlyꢀcorrelatedꢀwithꢀaꢀca.ꢀ
d
e
th
th
th
1
st
3rd
4
5
6
k
2nd
a
So
As received
(Re)cycle number
Figureꢀ5:ꢀGlucoseꢀyieldꢀbyꢀ enzymaticꢀhydrolysisꢀ(48ꢀh)ꢀandꢀtheꢀ%ꢀ ofꢀintroducedꢀsilica
recoveredꢀbyꢀCO ꢀaddition,ꢀacrossꢀsixꢀroundsꢀofꢀKOHꢀpre-treatmentꢀ(2.4MꢀKOH,ꢀ2ꢀhꢀpre-
treatmentꢀtime).ꢀAsꢀreceivedꢀriceꢀhusksꢀyieldedꢀnoꢀglucose,ꢀwhileꢀsoakingꢀinꢀwaterꢀforꢀ
2
1
7%ꢀ increaseꢀ inꢀ theꢀ overallꢀ glucoseꢀ hydrolysisꢀ yieldꢀ (cf.ꢀ ESI,ꢀ
FigureꢀS2),ꢀi.e.ꢀremovalꢀofꢀca.ꢀ32%ꢀofꢀtheꢀligninꢀwasꢀsufficient.ꢀ
Theꢀquantityꢀofꢀsilicaꢀremovedꢀwasꢀalsoꢀlargelyꢀinsensitiveꢀ
-
1
72ꢀhꢀonlyꢀboostedꢀtheꢀglucoseꢀyieldꢀtoꢀca.ꢀ22ꢀmgꢀg .ꢀꢀ
ꢀ
31
recommendedꢀ protocol ).ꢀ Highꢀ concentrationsꢀ ofꢀ KOHꢀ wereꢀ
usedꢀtoꢀensureꢀeffectiveꢀpre-treatmentꢀatꢀroomꢀtemperature.ꢀ
Movingꢀtoꢀhigherꢀtemperaturesꢀduringꢀpre-treatmentꢀinvolvesꢀ
additionalꢀenergyꢀexpenditure,ꢀbutꢀcouldꢀallowꢀequivalentꢀpre-
treatmentꢀeffectivenessꢀatꢀshorterꢀtimes,ꢀwithꢀhigherꢀbiomassꢀ
loadingsꢀandꢀwithꢀlowerꢀKOHꢀconcentrations.ꢀHigherꢀloadingsꢀ
duringꢀ theꢀ enzymaticꢀ hydrolysisꢀ stageꢀ areꢀ expectedꢀ toꢀ beꢀ
viable.ꢀꢀ
ofꢀ recycleꢀ number,ꢀ withꢀ 59ꢀ ±ꢀ 5%ꢀ consistentlyꢀ removed.ꢀ
However,ꢀ theꢀ amountsꢀ ofꢀ (crude)ꢀ silicaꢀ precipitatedꢀ byꢀ CO2ꢀ
additionꢀvariedꢀwithꢀrecycleꢀnumber;ꢀthisꢀisꢀshownꢀgraphicallyꢀ
inꢀFigureꢀ5ꢀasꢀ%ꢀofꢀsilicaꢀrelativeꢀtoꢀtheꢀamountꢀinitiallyꢀpresentꢀ
rd
th
inꢀtheꢀriceꢀhusks.ꢀFromꢀtheꢀ3 ꢀtoꢀ6 ꢀrecycles,ꢀca.ꢀ59%ꢀsilicaꢀwasꢀ
dissolvingꢀwhileꢀca.ꢀ40%ꢀofꢀtheꢀintroducedꢀsilicaꢀ(ca.ꢀ68%ꢀofꢀtheꢀ
dissolvedꢀ silica)ꢀ wasꢀ recoveredꢀ atꢀ theꢀ CO2ꢀ additionꢀ stage.ꢀ
ThereforeꢀSiꢀisꢀlikelyꢀstillꢀaccumulatingꢀinꢀtheꢀsystem,ꢀasꢀwasꢀ
foundꢀ toꢀ beꢀ theꢀ caseꢀ forꢀ lignin.ꢀ Someꢀ Ca(OH)2ꢀ wasꢀ alsoꢀ
potentiallyꢀ carriedꢀ ontoꢀ theꢀ nextꢀ cycle,ꢀ althoughꢀ itꢀ hasꢀ beenꢀ
notedꢀ thatꢀ combinationsꢀ ofꢀ NaOHꢀ andꢀ Ca(OH)2ꢀ canꢀ beꢀ moreꢀ
ꢀ
Envisagingꢀhowꢀthisꢀprocessꢀcouldꢀbeꢀexpandedꢀtoꢀoperateꢀ
onꢀanꢀindustrialꢀscale,ꢀCO2ꢀcaptureꢀfromꢀaꢀcombustionꢀstreamꢀ
couldꢀ beꢀ realised,ꢀ providingꢀ waterꢀ evaporationꢀ issuesꢀ areꢀ
resolved.ꢀAlternatively,ꢀtheꢀCO2ꢀcouldꢀbeꢀemployedꢀinꢀaꢀclosedꢀ
cycle,ꢀe.g.ꢀafterꢀreleaseꢀfromꢀtheꢀCaCO3ꢀeitherꢀthermallyꢀorꢀbyꢀ
acid.ꢀ Additionꢀ ofꢀ acidꢀ toꢀ theꢀ CaCO3-ligninꢀ compositeꢀ wouldꢀ
releaseꢀCO2ꢀasꢀaꢀgasꢀ(forꢀre-use),ꢀandꢀtheꢀacid-insolubleꢀligninꢀ
couldꢀ beꢀ isolatedꢀ asꢀ aꢀ solid.ꢀ However,ꢀ itꢀ wouldꢀ produceꢀ
calciumꢀwasteꢀwhichꢀwouldꢀbeꢀdifficultꢀtoꢀbeꢀreused,ꢀandꢀcouldꢀ
introduceꢀissuesꢀwithꢀcorrosion.ꢀConversely,ꢀthermalꢀprocessesꢀ
suchꢀasꢀtheꢀcombustionꢀofꢀligninꢀ(asꢀaꢀblackꢀliquor)ꢀforꢀenergyꢀ
28
effectiveꢀatꢀbiomassꢀpre-treatmentꢀthanꢀNaOHꢀalone. ꢀꢀ
Finally,ꢀ theꢀ enzymaticꢀ hydrolysisꢀ ofꢀ theꢀ pre-treatedꢀ riceꢀ
ꢀ
husksꢀ wasꢀ evaluatedꢀ asꢀ aꢀ functionꢀ ofꢀ recycleꢀ number;ꢀ theꢀ
resultingꢀ glucoseꢀ yieldsꢀ perꢀ gramꢀ ofꢀ startingꢀ riceꢀ huskꢀ vsꢀ
recycleꢀnumberꢀisꢀplottedꢀinꢀFigureꢀ5.ꢀTheꢀhighestꢀglucoseꢀyieldꢀ
-1
wasꢀ obtainedꢀ forꢀ theꢀ pristineꢀ KOHꢀ solutionꢀ (ca.ꢀ 325ꢀ mgꢀ g ),ꢀ
whileꢀtheꢀyieldsꢀfromꢀcyclesꢀ2ꢀtoꢀ6ꢀwereꢀlowerꢀbutꢀrelativelyꢀ
-1
consistentꢀ(ca.ꢀ275ꢀmgꢀg ).ꢀTheꢀsilicaꢀyieldꢀandꢀglucoseꢀyieldꢀ
showꢀ aꢀ degreeꢀ ofꢀ correlation,ꢀ withꢀ theꢀ pristineꢀ KOHꢀ ableꢀ toꢀ
achieveꢀ theꢀ bestꢀ pre-treatmentꢀ andꢀ fractionation;ꢀ theꢀ
accumulationꢀofꢀresidualꢀcomponentsꢀ(silica,ꢀligninꢀandꢀhemi-
cellulose)ꢀinꢀtheꢀsolutionꢀhadꢀaꢀinitialꢀminorꢀdetrimentalꢀeffectꢀ
uponꢀbothꢀtheꢀpre-treatmentꢀandꢀfractionationꢀprocesses,ꢀbutꢀ
wasꢀneverthelessꢀconsistentꢀacrossꢀ5ꢀsubsequentꢀrecycles.ꢀ
36a
releaseꢀandꢀinorganicꢀchemicalꢀrecovery, ꢀandꢀcalcinationꢀtoꢀ
36b
recoverꢀ CaO, ꢀ areꢀ bothꢀ industrialꢀ processesꢀ thatꢀ areꢀ
performedꢀinꢀparallelꢀinꢀtheꢀpaperꢀpulpingꢀindustry.ꢀIfꢀtheꢀtwoꢀ
areꢀcombined,ꢀthenꢀtheꢀexothermicꢀcombustionꢀofꢀligninꢀwouldꢀ
assistꢀinꢀtheꢀcalcinationꢀreactionꢀofꢀCaCO3,ꢀwouldꢀresultꢀinꢀCO2ꢀ
releaseꢀfromꢀbothꢀtheꢀligninꢀandꢀCaCO3,ꢀandꢀultimatelyꢀachieveꢀ
recoveryꢀofꢀbothꢀCa(OH)2ꢀ(asꢀCaO)ꢀandꢀCO2ꢀforꢀfurtherꢀuse.ꢀAꢀ
furtherꢀ fractionationꢀ possibilityꢀ isꢀ theꢀ lowerꢀ temperatureꢀ
ꢀ
Theꢀ fateꢀ ofꢀ theꢀ hemi-celluloseꢀ inꢀ theꢀ riceꢀ husksꢀ wasꢀ notꢀ
evaluatedꢀinꢀthisꢀpreliminaryꢀstudy.ꢀHowever,ꢀhemicelluloseꢀisꢀ
typicallyꢀ solubleꢀ inꢀ aqueousꢀ alkalineꢀ solutions,ꢀ evenꢀ atꢀ roomꢀ
36a
gasificationꢀ ofꢀ lignin ꢀ followedꢀ byꢀ higherꢀ temperatureꢀ
calcinationꢀofꢀtheꢀCaCO3,ꢀallowingꢀtheꢀligninꢀtoꢀbeꢀutilisedꢀasꢀaꢀ
sourceꢀofꢀchemicalsꢀ(e.g.ꢀsyngas)ꢀratherꢀthanꢀenergy.ꢀꢀꢀ
38
temperature. ꢀThereforeꢀitꢀisꢀexpectedꢀtoꢀhaveꢀbeenꢀpartiallyꢀ
removedꢀduringꢀpre-treatment,ꢀandꢀlikelyꢀaccumulatedꢀinꢀtheꢀ
hydroxideꢀ solution.ꢀ Hemi-celluloseꢀ canꢀ beꢀ precipitatedꢀ byꢀ
ꢀ
hydrolysisꢀisꢀundesirableꢀonꢀaꢀlargeꢀscale,ꢀbutꢀwasꢀrequiredꢀdueꢀ
Washingꢀ ofꢀ theꢀ pre-treatedꢀ riceꢀ husksꢀ priorꢀ toꢀ enzymaticꢀ
39
moreꢀ extensiveꢀ acidificationꢀ thanꢀ weꢀ employedꢀ here, ꢀ andꢀ
furtherꢀ workꢀ isꢀ requiredꢀ inꢀ orderꢀ toꢀ ensureꢀ isolationꢀ ofꢀ theꢀ
hemicelluloseꢀcanꢀbeꢀachieved.ꢀꢀ
-
toꢀresidualꢀ[OH] ;ꢀtheꢀuseꢀofꢀCO2ꢀatꢀthisꢀstageꢀtoꢀmoderateꢀtheꢀ
pHꢀisꢀalsoꢀbeingꢀinvestigated.ꢀꢀ
ꢀ
Toꢀ conclude,ꢀ weꢀ haveꢀ demonstratedꢀ thatꢀ CO2ꢀ canꢀ beꢀ
ꢀ Severalꢀvariablesꢀwereꢀfixedꢀthisꢀpreliminaryꢀstudy,ꢀsuchꢀasꢀ
aꢀbiomassꢀloadingꢀofꢀ2.5ꢀwt%ꢀduringꢀpre-treatmentꢀatꢀca.ꢀ293ꢀ
K,ꢀandꢀ20ꢀmgꢀpre-treatedꢀwetꢀbiomassꢀperꢀmlꢀenzymaticꢀbrothꢀ
introducedꢀ(asꢀaꢀsolidꢀorꢀasꢀaꢀgas)ꢀasꢀaꢀpH-switchꢀanti-solventꢀ
toꢀaidꢀinꢀbiomassꢀpre-treatment,ꢀfractionation,ꢀandꢀtheꢀrecycleꢀ
ofꢀ theꢀ alkalineꢀ pre-treatmentꢀ solution.ꢀ Enzymatically-
-1
duringꢀenzymaticꢀhydrolysisꢀwithꢀ25ꢀFPUꢀg ꢀcellulaseꢀ(asꢀperꢀaꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ5ꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ

Pleas Ge ꢀ rd eo eꢀ nn o Ct ꢀ ha ed mj u si st ꢀt mr yarginsꢀ
Page 6 of 7
COMMUNICATIONꢀ
JournalꢀNameꢀ
3
1.ꢀ A.ꢀSluiter,ꢀB.ꢀHames,ꢀR.ꢀRuiz,ꢀC.ꢀScarlata,ꢀJ.ꢀSluiter,ꢀD.ꢀTempletonꢀandꢀD.ꢀCrocker,ꢀ
View Article Online
hydrolysableꢀ cellulose,ꢀ high-purityꢀ nanostructuredꢀ silicaꢀ andꢀ
ligninꢀ canꢀ allꢀ beꢀ isolatedꢀ viaꢀ aꢀ three-stepꢀ cycle.ꢀ Theꢀ
Determinationꢀ ofꢀ structuralꢀ carbohydratesꢀ andꢀ ligninꢀ inꢀ biomass,ꢀ Nationalꢀ
DOI: 10.1039/C7GC00574A
RenewableꢀEnergyꢀLaboratory,ꢀ2008.ꢀ
regeneratedꢀpre-treatmentꢀmediaꢀandꢀsolidꢀcarbonꢀdioxideꢀ(asꢀ 32.ꢀ G.ꢀD.ꢀSarataleꢀandꢀM.ꢀK.ꢀOh,ꢀRSCꢀAdv.,ꢀ2015,ꢀ5,ꢀ97171-97179.ꢀ
3
3
3.ꢀ S.ꢀR.ꢀKamathꢀandꢀA.ꢀProctor,ꢀCerealꢀChem.ꢀJ.,ꢀ1998,ꢀ75,ꢀ484-487.ꢀ
4.ꢀ U.ꢀKalapathy,ꢀA.ꢀProctorꢀandꢀJ.ꢀShultz,ꢀBioresourceꢀTechnol.,ꢀ2000,ꢀ73,ꢀ257-262.ꢀ
CaCO3)ꢀ canꢀ alsoꢀ beꢀ isolated,ꢀ viaꢀ theꢀ applicationꢀ ofꢀ Ca(OH)2.ꢀ
Afterꢀ fiveꢀ roundsꢀ ofꢀ pre-treatmentꢀ usingꢀ recycledꢀ solutionsꢀ 35.ꢀ M.ꢀM.ꢀHessien,ꢀM.ꢀM.ꢀRashad,ꢀR.ꢀR.ꢀZaky,ꢀE.ꢀA.ꢀAbdel-AalꢀandꢀK.ꢀA.ꢀEl-Barawy,ꢀMat.ꢀ
Sci.ꢀEng.:ꢀB,ꢀ2009,ꢀ162,ꢀ14-21.ꢀ
thereꢀwasꢀnoꢀsignificantꢀlossꢀinꢀtheꢀactivityꢀofꢀtheꢀKOHꢀsolution.ꢀ
3
6.ꢀ (a)ꢀM.ꢀNaqvi,ꢀJ.ꢀYanꢀandꢀE.ꢀDahlquist,ꢀBioresourceꢀTechnol.,ꢀ2010,ꢀ101,ꢀ8001-8015;ꢀ
b)ꢀM.ꢀMahmoudkhaniꢀandꢀD.ꢀW.ꢀKeith,ꢀInt.ꢀJ.ꢀGreenh.ꢀGasꢀCon.,ꢀ2009,ꢀ3,ꢀ376-384.ꢀ
(
3
3
7.ꢀ W.ꢀE.ꢀS.ꢀHart,ꢀJ.ꢀB.ꢀHarperꢀandꢀL.ꢀAldous,ꢀGreenꢀChem.,ꢀ2015,ꢀ17,ꢀ214-218.ꢀ
8.ꢀ T.ꢀYokoyama,ꢀJ.ꢀF.ꢀKadlaꢀandꢀH.ꢀM.ꢀChang,ꢀJ.ꢀAgric.ꢀFoodꢀChem.,ꢀ2002,ꢀ50,ꢀ1040-
Acknowledgementsꢀ
1
044.ꢀ
3
9.ꢀ Q.ꢀ Luo,ꢀ H.ꢀ Peng,ꢀ M.ꢀ Zhou,ꢀ D.ꢀ Lin,ꢀ R.ꢀ Ruan,ꢀ Y.ꢀ Wan,ꢀ J.ꢀ Zhangꢀ andꢀ Y.ꢀ Liu,ꢀ
ThisꢀworkꢀwasꢀsupportedꢀbyꢀtheꢀAustralianꢀResearchꢀCouncilꢀ
throughꢀProjectsꢀDE130100770.ꢀPartꢀofꢀthisꢀresearchꢀusedꢀtheꢀ
BioResources,ꢀ2012,ꢀ7.ꢀ
facilitiesꢀatꢀtheꢀElectronꢀMicroscopeꢀUnitꢀatꢀUNSW.
ꢀ
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