K2MgSiO4 : a novel K+-trapped biodiesel heterogeneous catalystproduced from serpentinite Mg3Si2O5(OH)4.

Journal

of

Molecular

Catalysis

A:

Chemical

K

2

MgSiO

4

:

A

novel

K

+

-trapped

biodiesel

heterogeneous

catalyst

produced

from

serpentinite

Mg

3

Si

2

O

5

(OH)

4

Fabiane

Carvalho

Ballotin

_,

_Thérèse

_Ebambi

_Cibaka

_,

_Tatiana

_A.

_{Ribeiro-Santos}

_,

Eleonice

Moreira

Santos

_,

_Ana

_Paula

_de

_Carvalho

_Teixeira

_,

_Rochel

_Montero

_Lago

a_{UniversidadeFederaldeMinasGerais,InstitutodeCiênciasExatas,DepartamentodeQuímica,BeloHorizonte,MG31270-901,Brazil} b_{UniversidadeFederaldeOuroPreto,EscoladeNutric¸ão−ENUT,DepartamentodeAlimentos,OuroPreto,MG35400-000,Brazil}

a

r

t

i

c

l

e

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n

f

o

Articlehistory:

Received31October2015

Receivedinrevisedform1February2016 Accepted2February2016

Availableonline4February2016 Keywords:

K2MgSiO4

Serpentinite Biodiesel

a

b

s

t

r

a

c

t

Inthisstudy,anewcatalystforbiodieselsynthesis,basedonK2MgSiO4,wasproducedfromthemineral

serpentinite.TG,SEM,XRD,AA,BETanalysesshowedthatserpentiniteMg3Si2O5(OH)4impregnatedwith

KOH(5,10and20wt%),andthermallytreatedat500◦C,700and900◦C,canbeconvertedtothemain

crys-tallinephaseK2MgSiO4.AnalysesbyTPD-MS(CO2)andtitrationsuggestedthepresenceofweak/medium

basicsitesinrelativelyhighconcentrations.Biodieselproductionusingsoybeanoil(methanol:soybeanoil

ratioof1:12,1:9;1:6,60◦C)showedyieldshigherthan95%withcatalystat10wt%,whichcanbereused

forthreeconsecutivetimeswithoutsignificantdecreaseonthereactionyield.Theobtainedresultsare

discussedintermsofanewcatalyticphasebasedonK+_{ionstrappedinthecavitiesoftheMgSiO}

42−

structurecontainingnegativelychargedoxygenbasicsites.

©2016ElsevierB.V.Allrightsreserved.

1. Introduction

Basiccatalysts,suchasalkalinemetalhydroxidesand methox-idesarewidelyemployedashomogeneouscatalystsinthebiodiesel industrialprocess[1,2].However,theuseofhomogeneous cata-lysts,leadstosomeproblemssuchas:theneedofawashingstep, theformationofemulsions,largevolumesofwastewater,theneed ofadryingstepandlossofthecatalyst[2–4].Ontheotherhand, heterogeneouscatalystscanofferseveraladvantagessuchasthe possibilityofreuse[5],eliminationofsomestepsintheprocessand simpleseparationfromthefinalproduct[6],resultinginsignificant decreaseonproductioncosts[7].

Differentbasicheterogeneouscatalystshavebeeninvestigated, suchasmixedoxidesCaO–CeO2[8],aluminasupportedmetals[9], andespeciallyearthalkalinemetalsoxides,e.g.,CaO[10]andMgO [11],whicharethemostusedforbiodieselsynthesis.Several stud-ieshaveshownthatthebasicityandactivityofbiodieselcatalysts canbeincreasedbytheadditionofdifferentelements,especially potassium.SomeexamplesareK+_{dopedtitanatenanotubes[12]}

∗ Correspondingauthor.Fax:+553134095777.

E-mailaddresses:fballotin@ufmg.br(F.C.Ballotin),tcibaka@gmail.com (T.E.Cibaka),tatianaaribeiros@yahoo.com.br(T.A.Ribeiro-Santos), emsquimica@hotmail.com(E.M.Santos),anapct@ufmg.br(A.P.d.C.Teixeira), rochel@ufmg.br(R.M.Lago).

K+_{supportedonTiO}

2[13],KOH/bentonite[14]andK+/chrysotile [15].

Inthiswork,serpentiniteMg3Si2O5(OH)4wasusedasprecursor toproducea K+ _{basiccatalyst for biodieselsynthesis.} Serpenti-nite,aninexpensivemineralandinsomecasesawaste,isformed byaSioxidetetrahedralsheetboundtoMg(OH)2based octahe-drallayer[16].Serpentinitehasbeenusedinsimpleapplications suchasconstruction,steelmills,magnesiumproduction[17], fer-tilizer[18]andforCO2storage[17,19].Serpentinitecanbeavery versatileprecursortoproduceMgbasedmaterialssinceunder con-trolledthermaltreatment,theMg3Si2O5(OH)4 phasedehydrates toproducedifferentMgcontainingphasessuchasdispersedMgO, Mg2SiO4besidesotherSiO2 basedphases.Asimplifiedequation canbeusedtorepresenttheserpentinitedecomposition:

Mg3Si2O5(OH)4→MgO+Mg2SiO4+SiO2+2H2O (1)

Hereon,itisdescribedthereactionofserpentinitewithKOHto producethephaseK2MgSiO4withbasicproperties,whereK+ions aretrappedinthecavitiesformedbytheSiO4andMgO4structural tetrahedra(Fig.1).

http://dx.doi.org/10.1016/j.molcata.2016.02.006 1381-1169/©2016ElsevierB.V.Allrightsreserved.

Fig.1.SchematicrepresentationofthethermaldecompositionofserpentiniteKOH/Mg3Si2O5(OH)4toformthephaseK2MgSiO4. 0 200 400 600 800 1000 88 90 92 94 96 98 100 20K₉₀₀ We ig ht /% Temperature / °C SER 20K₅₀₀ 20K₇₀₀

Fig.2.TG/DTGcurvesforserpentinite(SER)andserpentinitesamplesimpregnated withK+_{afterthermaltreatmentat500,700and900}◦_{C(20K500,20K700and20K900).}

2. Experimental 2.1. Catalystssynthesis

Pulverizedserpentinitesamples(PedrasCongonhasLtda)were sieved(#200)andimpregnatedwithaqueousKOHinproportions of0,5,10and20wt%ofpotassium.Thematerialsweredriedfor24h andthermallytreatedat500,700or900◦Cfor3hinairatmosphere. Thesecatalystsarenamedhereonas20K700,where20Kindicates 20wt%ofKandthermaltreatmentat700◦C.

2.2. Catalystscharacterization

Themetals (magnesiumand iron) wereanalyzed by atomic absorptionspectrometry(Hitachi-Z8200).TheSEMimageswere obtainedinaFEIQuanta200-FEGequipment.TheXRDstudieswere doneonaShimadzudiffractometer,modelXRD-7000withCuK␣ withascanspeedof4◦min−1.

Fig.3. XRDpatternsofserpentinitesamplesSER,and20K500,20K700and20K900.

Thermogravimetricanalyses(TGA)wereperformedin a Shi-madzu DTG60Hwithairflow (50mLmin−1)withheating rate of 10◦Cmin−1 up to 1000◦C. In order to determine the basic propertiesofthematerial20K700,asimultaneousTGA-MS(mass spectrometry)analysiswasapplied.Thebasepeak(m/z=44)was selectedtobemonitoredinaNETZSCHTG/STAequipmentcoupled withAelosspectrometer,model7.0.Thecatalystswerepreviously treatedinN2atmosphere(50mLmin−1)at500◦Cfor1h,followed bytreatmentat50◦CunderCO2flow(50mLmin−1)for1h.Then, thematerialwasheatedto900◦Cinargonatarateof10◦Cmin−1. Inordertodeterminethetotalbasicsites,0.5gofthesamples weredispersedin50mLofHCl0.1molL−1and20mLofdistilled water.After24hofstirring,25mLofthesolutionwastitratedwith NaOH0.1molL−1andphenolphthaleinasindicator.

The specific surface areas of the samples wereanalyzed by adsorptionofN2 at77KusinganAutosorb1-MPQuantachrome. Thesamplesweredegassedat200◦Cfor24hbeforetheanalyses.

Fig.4.SEMimagesofserpentinitebeforeandafterimpregnationwithKOH(20K)thermallytreatedat500,700and900◦_C.

The absorption spectroscopy measurements for diffuse reflectancein theinfraredregionwithFourier transform(FTIR) wereperformed on a Brukerequipment, Alpha model. Spectra werecollected in therange of 400–4000cm−1 region,with 64 accumulations.

Theinfluenceofcarbonationandhydrationin theactivityof thecatalyst werestudiedduring 30days byexposing the cata-lystto atmosphere. The catalyst wascharacterized by infrared spectroscopy,thermogravimetricanalyses,TPD-MSand transester-ificationreactionsafterthe1,5,10,20and30daysofairexposure.

2.3. Biodieselsynthesis

Biodiesel syntheses were carried out using soybean oil, methanol,and thecatalysts basedon serpentinite impregnated with K (0, 5, 10 and 20wt%) treated at 700◦C/3h. The cat-alyst concentration in the reaction were adjusted to 1, 5 or 10wt%.Thereactionswereconductedat60◦C, inatwonecked glass reactor, under reflux and mechanical stirring for differ-enttimes.Theoil:methanolmolarratiosusedwere1:6,1:9and 1:12.

0 1 2 3 4 5 6 7 8 9 10 11

20K

10K

5K

0K

10K

20K

5K

0K

10K

20K

5K

900

°C

700 °C

500 °C

B

a

si

ci

ty

/

mm

ol OH

g

-1

SER

0K

Fig.5.TotalbasicityofserpentinitesamplesimpregnatedwithKOHbeforeandafterthermaltreatment.

0 200 400 600 0K₇₀₀ 5K₇₀₀ 10K₇₀₀ Carbonate decomposition Medium basic sites Weak basic sites Int en sit y / a. u. Temperature / °C 20K₇₀₀

Fig.6.TPD–MSCO2curves:0K700,5K700,10K700and20K700.

2.4. Catalystreuse

The catalysts reuse reactions were performed under opti-mizedconditionsof5wt%and10wt%ofthecatalyst20K700,with oil:methanolmolarratioof1:6at60◦C/2h.Afterthereaction,the liquidphasewasseparatedfromthecatalysts,andtherecycling experimentsweredoneby:

(i)simplereuseofthecatalystswithoutanytreatment,

(ii)thecatalystwaswashedwith3portionsof3mLofsolvent (acetone,hexaneormethanol),centrifuged,andusedforanew reaction,

(iii)thecatalystwaswashedoncewithmethanolandtreatedat 700◦Cinairfor1h. 0 20 40 60 80 100 10 5 Co v e rs ion / % Catalyst weight / % 1:6 1:9 1:12 1 KOH ho moge neous

Fig.7.Effectofcatalyst20K700concentrationwithdifferentsoybeanoil/methanol

ratiosandKOHhomogeneouscatalystinthebiodieselproduction(60◦_C,180min).

2.5. Biodieselquantification

The biodiesel wasseparated and analyzed by1_{H NMR ona} BrukerDPX200spectrometerfor conventionalreactionsandin BrukerDPX400equipmentforreusereactions.

Inordertocalculatetheyieldofthetransesterificationreaction, 1_{HNMRtechniquewasused[20].Mixturesofpurebiodieseland} soybeanoilwereusedtobuildacalibrationcurve(Supplementary material).

3. Resultsanddiscussion

Theserpentinteusedasprecursorforthecatalystspreparation showedachemicalcompositionofSiO240wt%,MgO30wt%typical forthephaseslizarditeandantigoriteMg3Si2O5(OH)4,associated withtalc(Mg3Si4O11H2O),withasignificantconcentrationofiron (Fe2O310wt%).

Thermogravimetricanalysisofpureserpentinite(SER,Fig.2) showedaweightlossofca.1%until300◦C,relatedtoadsorbed water [21]. In the temperature range 500–800◦C, serpentinite

1ST

USE

2ND USE

3R

D USE

1

:6

(o

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1:

6 (

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:6

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(

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:1

2

(o

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Co

nv

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ion

/ %

1:

12

(

o

il:

m

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an

ol

)

Fig.8.Recyclingreactionsofthecatalysts,using5and10wt%,1:6and1:12oil:methanol(60◦_{C,120min),anddifferentrecoverablemethods.}

decomposes by dehydroxylation with a weight loss of 10% to formnewphases.Based onthisresult,thethermal decomposi-tionofserpentinitewasinvestigatedinmoredetail at500,700 and900◦C.XRDpatternsshowed,forthepureserpentintetreated at500◦C, 0K500 nosignificantchangeinphasecomposition. On theotherhand,thematerialgraduallydehydroxilates(Eq.(2))at 700and900◦CandthenewphasesfosteriteMg2SiO4andolivine (MgFe)2SiO4areformed(Supplementarymaterial).

Mg2Si2O5(OH)4→MgO/Mg2SiO4/SiO2+2H2O (2)

Similarresultshavebeenobservedforthethermal decomposi-tionofchrysotile[15].

InordertopreparetheK2MgSiO4basedcatalysts,serpentinite wasimpregnatedwithKOHat5,10and20wt%potassium,andeach materialwastreatedat500,700and900◦C.Fig.2showstheTG profilesofthecatalystscontaining20%Kafterthermaltreatment (alltheotherTGprofilesareshowninSupplementarymaterial).

Itcan beobserved thattheweight lossof 10%observed for thepureserpentiniteprecursor graduallydecreasedasSERwas impregnatedwithKOH andpretreated at500and 700◦C.After pretreatmentat900◦Cnosignificantweightlosswasobserved.

XRDdata ofthe material20K500 (Fig.3) suggeststhat KOH promotedthe thermal decomposition at 500◦C which was not observedforthepureserpentiniteprecursor.Itwasalsoobserved theformationofthenewmainphaseK2MgSiO4 (JCPDSCardNo. 39-1426)withsmallamountsofforsteriteMg2SiO4(JCPDSCardNo. 4-768)andolivine(MgFe)2SiO4(JCPDSCardNo.2-1326).Similar resultswereobservedforthematerialstreatedat700and900◦C. TheXRDpatternsofthesamples5K700,10K700and 20K700 (see Supplementarymaterial)clearlyshowedthattheformationofthe phaseK2MgSiO4wasstronglyfavoredastheconcentrationofK+ increased.

Itisinterestingtoobservetheabsenceofthediffractionpeak at43◦ relatedtoMgO,whichsuggeststhatifMgOwasformedit islikelyverydisperse.ItcanalsobeenvisagedthattheMgOwas consumedintheformationofthemainphaseK2MgSiO4(Eq.(3)).

K2O+MgO+SiO2→K2MgSiO4 (3)

TheformationofthisphaseK2MgSiO4hasbeenreportedbythe ternarysolidstatereactionofK2O–MgO–SiO2athightemperatures [22].Detailedinvestigationof theK2MgSiO4 structure [23]

sug-gestsanetworkofSiO4tetrahedraconnectedtoMg2+O4tetrahedra wheretheK+_{ionsarelocatedintheinterstitialspacebetweenthe} tetrahedral[23].

SEMimagessuggestthatserpentiniteisabulkycompactsurface, butafterthermaldecomposition,withorwithoutKOH,the mate-rialbecomesmorefragmented(Fig.4).BETsurfaceareaanalyses showed12m2_g−1_{forserpentinite,whichafterthermaltreatment} at700◦Cslightlyincreasedto27m2_g−1_{.Ontheotherhand,the} samplescontainingpotassium(5K700,10K700and20K700)showed adecreaseto7,5and11m2_g−1_{,respectively,suggestingthatthe} KOHhasaneffectonthetextureofthefinalmaterialproduced.

Thedifferentcatalystswereanalyzedforthetotalbasicityby simplereactionwithHCl(Fig.5).

Itcanbeobservedthatpureserpentiniteshowedatotalbasicity of3.3mmolOH−g−1.ThisbasicityislikelyrelatedtotheMg(OH)2 layersolubilizationbyHCl.Thetreatmentofpureserpentiniteat 500◦C(0K500)didnotshowanysignificantincreaseonthe basic-ity.Ontheotherhand,thebasicitystronglyincreasedto7.2mmol OH−g−1aftertreatmentat700◦C,likelyrelatedtothe decompo-sitionofserpentintewiththesegregationofMgO.Itisinteresting toobservethatat900◦Cthebasicitydecreasesprobablybythe reactionofMgOwithSiO2toformtheneutralmagnesiumsilicates. Aspotassiumwasaddedtotheserpentiniteandtreatedat500◦C, thebasicitywentto6.5,7.5and9.9mmolOH−g−1forthesamples 5K,10Kand20K,respectively.Aftertreatmentat700◦Cthe basic-ityslightlyincreasedwhereasat900◦Cadecreasewasobserved. Basedontheseresultsthesamplestreatedat700◦Cwereselected forfurthercharacterizationandusedasheterogeneouscatalystfor biodieselsynthesis.

Thepresenceofsurfacebasicsitesofthecatalystswas inves-tigated by CO2 TPD-MS technique (Fig. 6). The samples were pre-treatedat500◦CinaN2flow,andthenexposedtoCO2at50◦C forsubsequentTPD-MSanalyses.

Itcanbeobservedthatpureserpentinite(SER)andthesample 0K700showednosignificantadsorption/desorptionofCO2,which indicatesverylowconcentrationofsurfacebasicsites.Theseresults suggestthattheMgOproducedduringtheserpentinite decompo-sitionisnotreadilyavailableonthesurfaceforinteractionwith CO2. Thesamples 5K700 and 10K700 showedlow intensity des-orptionpeaks. On theother hand,the TPD-MS for thecatalyst 20K700showedthepresenceof3desorptionpeaks(Fig.6):weak

Fig.9. SEMimagesandEDSanalysesofthecatalysts(20K700,directreuseandacetonewashingreuse)after4uses.

basicsites (upto200◦C), mediumbasic sites(200–500◦C) and peaksabove500◦C.Accordingtoliterature,peaksbelow200◦Care relatedtoCO2weaklyadsorbedonthecatalystsurface, correspond-ingtohydroxylgroupsonthesurface[24]whereasthepeaksnear 350◦CcanbeassignedtoCO2desorbedfrommediumbasicity,for examplemagnesiumoxide[25,26]andMg2+_andO2−_pairs[24]. Above500◦C, theCO2 islikely duetocarbonatedecomposition [27].Studieswiththemineralchrysotileshowedsimilarresults [15].Shenetal.alsodetectedmediumstrengthbasicsites,which wereassignedtoK_xMgO/SiO2[28].

Thebiodieselsynthesiswasinvestigatedwiththedifferent pre-paredcatalysts.The seriesSER,0K500,0K700 and0K900,showed

yieldslowerthan20%indicatingthatthepresenceofpotassium iscriticalforamoreactivecatalyst.Thebestresultswereobtained forthecatalysts20K700and20K900.Moredetailedinvestigation wascarriedoutwiththecatalyst20K700indifferentreaction con-ditions.Theeffectofcatalystconcentrationandoil/methanolratios intheconversionofsoybeanoilispresentedinFig.7.

Itcanbeobservedthatthereactionreachedgoodconversion (>80%)usingonly1wt%ofthecatalystattheoil:alcoholmolarratio of1:9.For5and10wt%,conversionsnear95%wereobtainedfor alloil:methanolratios.

Simpletestinthebiodieselreactionusingtheclassical homoge-neousKOHat0.5%(Fig.7)showedesteryieldsof95±2%.Similar

D0 D1 D-5 D-10 D-20 D-30 0,0 0,5 1,0 1,5 CO₂ basic sites Ba s ici ty / m m o l g -1 Days

CO₂ "bulk carbonate"

Fig.10.QuantificationofgroupsofCO2inthecatalysts.

results were obtained with the heterogeneous 20K700 present inthereactionat 5%.Considering theKcontentin thecatalyst 20K700 is nearly only 20%,similarTON of 30molMe-estermolK−1 wereobtainedforbothcatalysts.TheseresultssuggestthattheK presenceinboth,homogeneousandheterogeneouscatalystshave similaractivities.

Thecatalystsstabilitywasinvestigatedbyconsecutivereuses (Fig.8).Whenreactionswerecarriedoutwith10wt%catalystwith oil:methanolratioof1:12,adecreaseof21%ontheconversionwas observedafterthreeconsecutivereactions.Therecyclabilityofthe catalystwasalsostudiedinamorelimitingcondition(i.e.,5wt% catalystanda1:6oil:methanolratio,at60◦Cfor2h)(Fig.8).

Afterthereaction,thecatalystwascompletelyagglomerated duetotheimpregnationwiththeorganicphase.Theseconduse ofthiscatalystwithoutanytreatmentstillshowedverygood con-version.Also,bywashingthecatalyststoremovetheorganicphase (differentsolventswereused,i.e.,hexane,acetone,methanol)fairly goodconversionswereobtained.Ontheotherhand,thermal treat-mentat700◦Ccausedasignificantdecreaseontheconversion,i.e., from85toca.45%(notshown).Afterthethirduse,theconversion decreasedtolessthan40%especiallyafterthermaltreatmentat 700◦C.Theobtainedresultssuggestthatpotassiumislostafterthe firstreactionandnearly40%waslostafter4reactions.

Thefresh20K700andtheusedcatalysts(directreuseandacetone washingreuse),inthemorelimitingcondition,(after4uses)were analyzedbySEM/EDS(Fig.9)andXRF(X-rayfluorescence).

SimplecalculationoftheEDSsignalratioK/SiandXR fluores-cencemeasurementssuggestedthatafterthefirstuse,thecatalysts lostca.10–15%oftheKcontent.Thecatalystsstabilityisan impor-tantfactortoindustry,inthissense,20K700catalystwasexposed toairandinvestigatedduring30daysbyTG/DTG,FTIRand TPD-MSanalyses.TGanalyses(Supplementarymaterial)showedthat exposedcatalysts(1,5,10,20and30days)presentedsmallweight lossesintwotemperatureranges,upto500◦Crelatedtothelossof wateranddihydroxylationprocesses[29]andbetween600–800◦C likelyrelatedtocarbonatedecomposition.Carbonatationon cata-lystssurfaceshasalsobeenobservedbeforefordifferentmineral oxidesandbasiccatalysts[17,30].Thecarbonatesthermal decom-positionis wellknown totakeplace in thetemperature range 600–800◦C[31].

FTIRspectraforthematerialsexposedtoairupto30days (Sup-plementarymaterial)confirmedagradualhydrationat3300cm−1 referredto OHstretchingandat1659cm−1( OHbending)[32] and carbonation (CO3: 862 (out-of-plane bending)), 1440cm−1

Fig.11.SchematicrepresentationoftheK-trappedintheK2MgSiO4structure

(adaptedfromRef.[21]).

(symmetricbending)and1655cm−1(antisymmetricbending)[33] ofthecatalyst.

TheCO2 desorption wasmeasuredbyTPD-MS aftercatalyst exposurefor1,5,10,20and30days(Fig.10).Thepresenceofthree desorptiongroupswasobserved: surfacebasicsites,carbonates andespeciallyphysisorbedCO2(upto1.7mmolg−1).

Thecatalyst20K700exposedtoairfor1,5,10,20and30dayswas thentestedforbiodieselproductionat5wt%catalyst,methanol:oil ratioof1:6at60◦C.Theobtainedresults(Supplementary mate-rial)showednosignificantdeactivationduetotheexposureofthe catalysttoair.

Although the catalyst nature is not clear, one can consider thatthemaincrystallinephaseK2MgSiO4islikelyinvolvedinthe reaction.Thereisonlyonereportsuggestingthatthisphaseisa basiccatalystforthereactionofhexosestohydroxylmethylfurfural (HMF)[28].Inthisreport,theK2MgSiO4wasproducedfrompure K2CO3,MgOandSiO2atveryhightemperatures.

Thestructureof K2MgSiO4 hasbeendescribedasa “stuffed” crystobalitepolymorphofSiO2 [26],where athree-dimensional arrayofcorner-sharingSiO4tetrahedraformlargeopencavities. SomefractionoftheSiatomscanthenbereplacedwithMg2+_and theK+_{ionsare“stuffed”,i.e.,trappedinsidethecavities[23].Avery} simplerepresentationofthisstructureisshowninFig.11.

ThecompoundK2MgSiO4canalsobeviewedasanegatively chargedsolidmatrix[MgSiO4]2−containingK+_trappedinthe struc-ture.Althoughthenatureofthebasicsitesforthebiodieselreaction isnotclear,itmightberelatedtothesurfaceandstructural oxy-genscontainingthenegativecharges.Moredetailedstudiesare necessarytofurtherinvestigatethenatureofthecatalyticsites.

4. Conclusions

Serpentiniteprovedtobeaveryinterestingprecursorto pro-ducethespecialcatalyticphaseK2MgSiO4whichisapparentlyvery activefor theproduction of biodiesel. Thisnewcatalytic phase seemstobebasedonK+_{ionstrappedinthecavitiesoftheMgSiO}

42−

cat-alyst20K700at10wt%canbereusedforthreeconsecutivetimes withoutsignificantdecrease onthereactionyield. Experiments usingcatalystat5%showedpotassiumleachingandadecreaseon theconversionafterthesecondreaction.

Acknowledgements

TheauthorsacknowledgePedrasCongonhasLTDAforthe sam-ples,theUFMGmicroscopycenterfortheimagesandthesupport ofCNPQ,CAPESandFAPEMIG.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.molcata.2016.02. 006.

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