Behaviour of activated carbons with different pore size

Behaviourofactivatedcarbonswithdifferentporesize

distributionsandsurfaceoxygengroupsforbenzene

andtolueneadsorptionatlowconcentrations

M.A.Lillo-Ro´denas*,D.Cazorla-Amoro´s,A.Linares-Solano

DepartamentodeQuı´micaInorga´nica,UniversidaddeAlicante,Apartado99,E-03080Alicante,Spain

Received28October2004;accepted17February2005Availableonline21March2005

Abstract

ThispaperdealswiththestudyoftheeffectthattheporosityandthesurfacechemistryoftheactivatedcarbonshaveontheadsorptionoftwoVOC(benzeneandtoluene)atlowconcentration(200ppmv).Inthissense,activatedcarbonswithverydifferentporositiesandcontentsinoxygensurfacegroupshavebeentested.Ourresultsregardingtheeffectoftheporosityshowthatthevolumeofnarrowmicropores(size<0.7nm)seemstogoverntheadsorptionofVOCatlowconcentration,speciallyforbenzeneadsorption.Regardingthesurfacechemistry,ACwithlowcontentinoxygensurfacegroupshavethebestadsorptioncapacities.AmongtheACtested,thosepreparedbychemicalactivationwithhydroxidesexhibitthehigheradsorptioncapacitiesforVOC.Theadsorptioncapacitiesachievedarehigherthanthosepreviouslyshownintheliteraturefortheseconditions,speciallyfortol-uene.Adsorptioncapacitiesashighas34gbenzene/100gACor64gtoluene/100gAChavebeenachieved.Ó2005ElsevierLtd.Allrightsreserved.

Keywords:Activatedcarbon;Adsorption;Porosity;Surfaceproperties

1.Introduction

VOCarepollutantspresentingasand/orliquid

streamsofmanyindustrialapplications.Theyarevery

harmfulforbothhumanhealthandenvironment,even

atverylowconcentrations[1–6].Itcanberemarkedthat

theyare:(1)agentsthatdestroytheozonestratospheric

layer,(2)precursorsofphotochemicaloxidants,(3)

agentsoftheacidrain,(4)elementsoftheclimatic

change,(5)agentsthataffectthenervoussystemand

(6)carcinogenicandmutagenicagents.

Forallthesereasons,aconsiderableefforthasbeen

dedicatedinthelastyearsregardingtheirremoval.ThemostimportantmethodsusedfortreatingVOC

gaseousstreamsare[3,7]:(i)absorption,(ii)adsorption,

(iii)condensation,(iv)thermaloxidationand(v)cata-

lyticoxidation.Allthesemethodsprovidegoodresults

whenusingtheappropriateconditionsofconcentration,

flowandtemperature.

Manyofthegaseousstreamsinvolvedinindustrial

processescontainVOCatverylowconcentrations.Both

legislationandhumanhealthdeterminethatthese

streamsmustbetreatedtoremovetheorganiccom-

pounds.Consideringthelowconcentrationofthe

VOCinthestreams,adsorptionisoneofthetechniques

whichprovidesbetterresults[2,3,7,8].

Theadsorptionoforganicpollutantscanbecarried

outemployingdifferentadsorbents.Amongthem,acti-

vatedcarbonsareoneofthebestoptionsduetotheir

hydrophobicpropertiesandtheirhighsurface

areaCarbon43(2005)

1758–1767

www.elsevier.com/locate/carbon

0008-6223/$-seefrontmatterÓ2005ElsevierLtd.Allrightsreserved.

doi:10.1016/j.carbon.2005.02.023*Correspondingauthor.Tel.:+34965909350;fax:+34965903454.E-mailaddress:mlillo@ua.es(M.A.Lillo-Ro´denas).andbecausetheyareusefulforadsorbingmolecules

withmolecularweightsbetween45and130[2,3,6].

Inthiswork,theadsorptionoftwoofthemostcom-

monVOC,benzeneandtoluene,atverydilutedconcen-

tration(i.e.200ppmvconcentration)willbeanalysed.

Therelativepressurefor200ppmvconcentrationis

1.6·10À3and5.4·10À3forbenzeneandtoluene,

respectively,at298K(saturationpressuresforbenzene

andtolueneat298Kare126.9and37.9mbar,respec-

tively,calculatedusingthecorrespondingAntoinecoef-

ficients[9]).

Manystudiesregardtheabatementofbenzeneand

tolueneatlowconcentration[2–8,10–23].Thesestudies

trytofindadsorbentswithhighadsorptioncapacities

andfastkinetics.Table1summarisestheresultsofthe

papersrelatedtotheadsorptionofbenzeneandtoluene

onAC.

Asitcanbeseeninthistable,alargevarietyofAC

havebeenused(granular,pelletsandactivatedcarbonfi-

bres).Unfortunately,inmostofthesestudieswecannot

findanappropriatecharacterizationoftheporosityof

theactivatedcarbonsused.Usually,theyonlyinclude

BETsurfaceareadataoftheactivatedcarbons;very

fewincludedataaboutthemicroporevolumecalculated

bynitrogenadsorptionandnoneofthemdealswiththe

characterisationofthevolumeofnarrowmicropores.It

mustbeunderlinedthatthevolumeofnarrowmicropores

(size<0.7nm,inwhichprimaryadsorptiontakesplace)

hasshowntobeveryimportantinmanygasadsorption

applications,suchasSO2adsorptionandmethanestor-

age[24–26].Thisnarrowmicroporositycanbeeasily

measuredbyCO2adsorptionat273K[27–30].

Selectingtheconcentrationusedinthispaper

(200ppmv)wecanobserve,fromTable1,thatthebest

activatedcarbonforadsorptionofbenzeneadsorbs29g

benzene/100gAC[11].Regardingtoluene,thereareless

studiesrelatedandfewdatawereobtainedinconditions

similartoourstudy.At151ppmvconcentrationand

298KYunetal.[22]achievedadsorptioncapacitiesof

30gtoluene/100gAC.Nohigheradsorptioncapacities

havebeenobtainedfortolueneat200ppmvinthe

literature.

Thus,thepurposeofthisworkistomakeasystem-

aticstudytodeterminewhatrangeofmicroporosity

hashigherinfluenceintheVOCadsorptionatlowcon-

centration.Forthatpurpose,theporositiesoftheAC

inthispaperwillbecharacterizedindetail.Also,the

effectofthecontentinsurfaceoxygengroupsinthe

VOCadsorptionwillbestudiedtooptimisetheacti-

vatedcarbonpropertiesfortheadsorptionofVOCat

lowconcentration.Thefinalaimofthisworkisto

knowtheactivatedcarbonparametersthatcontrol

theadsorptionofVOCatlowconcentration.Itwill

allowustoimprovethepreparationsteptodevelop

theACwiththebestcharacteristicsforlow-concentra-

tion-VOCadsorption.2.Experimental

Tenactivatedcarbonshavebeenselectedforthis

study.Theyhavebeenpreparedbydifferentactivation

techniquesandusingdifferentprecursors;twocommer-

cialactivatedcarbonsprovidedbyWestvaco,anAC

preparedbyphysicalactivationwithsteamandchemi-

callyactivatedcarbons,preparedbyeitherNaOHor

KOHactivation.Furtherdetailsofthechemicalactiva-

tionmethodemployedcanbeobtainedfromprevious

works[31,32].

Table2summarisesthenomenclatureofthesamples

employedincludingthecarbonprecursorused,the

methodofpreparationandthechemicalusedinthe

activation.

Thecharacterisationoftheporoustextureoftheacti-

vatedcarbonshasbeendoneusingphysicaladsorption

ofN2andCO2at77and273K,respectively,inan

Autosorb-6apparatus.Nitrogenadsorptionhasbeen

usedfordeterminingthetotalvolumeofmicropores

(poresizesmallerthan2nm)whereastheadsorption

ofCO2at273Kallowsustoassessthenarrowest

micropores(poresizesmallerthan0.7nm)[27–30].

Table3summarisestheporositycharacterisationof

theactivatedcarbons.ThemeanporesizeoftheAC

used,applyingtheDubininequationtotheCO2adsorp-

tionisothermsat273K[29],isintherange0.6–0.9nm.

Thepercentageofmicroporosityhasbeencalculatedby

meansofthefollowingexpression:

%microporosity¼V-DRN2ðVN2ÞP=P0¼0:95Á100ð1Þ

Tostudytheeffectofthesurfaceoxygengroupson

theVOCadsorption,someoftheseAChavebeenther-

mallytreatedininertatmosphere.AnadditionalTis

addedinthenomenclatureofthesamplesthermally

treated.Thethermaltreatmentconsistsonheatingthe

ACsamplesinafurnacewithaheliumflowof100ml/

minusingaheatingrateof20K/minupto1173K,

andthencoolinginhelium.Thistreatmentremoves

mostofthesurfaceoxygengroupsoftheAC,whereas

theporosityremainsnearlyunchanged.Thus,theper-

formanceofthepristineACandofthosewhosecontent

insurfaceoxygengroupshasbeenreducedhavebeen

analysed.Thecharacterisationofsurfaceoxygengroups

presentinAChasbeendonebyTPDexperiments.Dur-

ingthethermaldecompositionofthesurfacecomplexes,

CO2evolvesfromcarboxylicgroupsandtheirderiva-

tives,suchaslactonesandanhydrides,whileCOis

mainlyadecompositionproductofquinones,hydroqui-

nonesandphenols[33–36].

TheexperimentalsystemforTPDexperimentscon-

sistsofafurnacecoupledtoamassspectrometer(VG,

Quadrupoles).Aheliumflowof60ml/minisusedand

theheatinghasbeendoneat20K/min,upto1173K.M.A.Lillo-Ro´denasetal./Carbon43(2005)1758–17671759