Catalytic Reduction of CO2 Into Hydrogen Using Micro Reactors

8/8/2019 Catalytic Reduction of CO2 Into Hydrogen Using Micro Reactors

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Malavarayan.S

Ravishankar.MIIndyearChemicalEngineering,SriVenkateswaraCollegeofEngineering,Sriperumbudur

PostalAddress:No:50,NGOColony2ndstreet,Vadapalani,Chennai‐26.

E-mail :[email protected]:+919884314549

CatalyticReductionofCO2IntoHydrogenUsing

Microreactors



A b s t r a c t :

ThestoichiometricreductionofcarbondioxidebysolubleK[Ru[III]

‐(EDTA‐

H)Cl].2H2OcomplexatCO2pressureintherangeof5‐70atmand

temperature40‐80°Cinamicroreactorgaveformicacidandformaldehydeasthereactionproducts.Theratesofformationof

formationofformicacidandformaldehydeareoffirstorder

dependencewithrespecttothecatalystandthedissolvedcarbon

dioxideconcentrations.Theformicacidthisproducedisconvertedinto

hydrogenbywater‐gasshiftmethodusing10%Cu‐Al2O3asacatalystin

aseparatemicroreactoratatemperatureofabout130°Ctogive

hydrogengas.TheEnergyisonlygivenformaintainingtheoverall

experimentalconditions.Thehydrogenthusobtainedcanbeusedfor

variousindustrialpurposesthemostsignificantbeingtheconversionofhydrogenintoElectricalEnergyusingFuelCells.

Introduction:

Carbondioxideandwateraretheendproductsofmanychemicaland

biologicalprocesses.TheUtilizationofCO2tosynthesizevalue‐added

chemicalsis,therefore,ofmuchimportanceinindustry.CO2isusedindustriallyinlargequantitiesforthepreparationofUrea[1]and

inorganiccompoundssuchascarbonatesandbicarbonates.Activationof

CO2bytransitionmetalcomplexesforformadductsisanareaof

investigation[2‐8].Thesynthesisofmethylformatebythereactionof

CO2,H2andCH3OHcatalyzedbyRu(PPh3)Cl2isagoodexampleofthe

reductivefixationofCO2[9].CO2+H2havebeenusedintheplaceofCO

+H2inthesynthesisofhomologousalkenes,alcoholsandaminesinthe

presenceofNH3[10‐12].TherhodiumcomplexRh(diphos)(BPh4)

catalyzestheco‐oligomerizationofCO2andmethylacetylenetogivecycliccompounds.

ThispaperreportsthereductionofCO2toformicacid

andformaldehydebythewatersolubleK[RuIII(EDTA)Cl].2H2Osystem

undermildexperimentalconditions(40‐80°C,5‐70atm).Thesystem

suggeststhepossibilityforacatalyticcycleforthefixationofCO2to

valueaddedproducts.Thereactioniscarriedoutinamicroreactorthat

canprovidethenecessaryexperimentalconditionsviz.thetemperature

andthepressure.Astackcontaining‘n’numberofmicroreactorscan

beemployedwhere‘n’woulddependontheapplicationofuse.



Experimental:

Thecarbondioxidefixationwasperformedina300mlpressurereactor

by[3].Carbondioxidegaswithapuritygreaterthan99.5%andcomplex

K[RuIII(EDTA‐H)Cl).2H2Owereusedinthe300mlreactoralongwithdoubledistilledwater.ThecomplexwaspreparedusingRuCl3.3H2Oand

disodiumsaltofEDTA[15].Inasamplerun,aknownamountofcomplex

1wasdissolvedin100mlofdistilledwaterandplacedinthepressure

reactor.ThebombwaspressurizedbyCO2totherequiredvaluewhen

thedesiredtemperaturewasattained.Theprogressofthereactionwas

monitoredbyanalyzingliquidsampleswithdrawnatdifferentintervals

oftimeforHCOOHandHCHOcontents.TheconcentrationofHCHOand

HCOOHpresentintheliquidsampleswereestimated

spectrophotometricallybymonitoringthepeakat412nmusingNash’s

reagent[16,17].Fromthegraphsoftimevs.amountofHCHOorHCOOH

formed,theratesoftheformationofboththeproductsevaluated.

Gaseoussampleswerealsowithdrawnatsuitableintervalsofreaction

timeandanalyzedforCO2andCOcontentusingaGLC.Thecolumnused

fortheanalysisofgaseswere2.5mlong,100meshwithaTCdetector

(150°C),columntemperature40°C,injectortemperature50°CandH2

carriergaswithaflowrateof30mlmin‐1.Therequiredsolubilitydataof

CO2inwateratdifferenttemperaturesandpressureweretakenfromthereportedvalues[18],andthetotaldissolvedCO2concentration

neededtointerpretthekineticdatawascalculated.

ResultsandDiscussion:

Inordertofixtheconditionsforkineticstudy,theexperimentwas

Conductedwith1mmolofcomplex1,aCO2partialpressureof68.0atm

Andat80°C.TheyieldsofHCOOHandHCHOformedasafunctionof

timeisshowninFig.1.ItisclearfromthegraphthattheconcentrationsOfbothHCHOandHCOOHincreasewithtime,reachingamaximumin

3‐4h;laterboththeproductstendtodissociate,resultinginadecrease

intheirconcentrations.ThereductionintheconcentrationofHCHOis

more pronounced than that of HCOOH. Therefore, the rates of

formationofHCHOandHCOOHwereevaluatedintheintervalbetween

1‐3h,whichensuredalineardependence.ThedecayrateofHCHOwas

evaluatedinthedecliningportionofthecurveafter3.5h.



Fig1.TheyieldsofHCOOHandHCHOinCO2reductionbyK[RuIII(EDTA‐H)‐

Cl].2H2O

KineticsThekineticexperimentsonCO2reductionwereconductedby

varyingtheconcentrationsofCO2andratesofformationofHCHO

andHCOOHwereevaluated.

Effectofcomplexconcentration

Theconcentrationofthecomplex1wasvariedfrom0.5–2×10‐3M

withaconstantdissolvedCO2concentrationof3.9×10‐2mand

temperatureof80°Cforalltheruns.ThedependenceofratesofformationofHCHOandHCOOHonconcentrationofconplex1are

shownFig.2and3,respectively.BothHCHOandHCOOHratesshow

afirstorderdependencewithrespecttocomplexconcentration.

EffectofdissolvedCO2concentration

Figures4&5showtheplotofratesofformationofHCHOand

HCOOHvs.dissolvedcarbondioxideconcentrationtheexperiments

wereperformedkeepingaconstantcomplexconcentrationof1x10‐

3M,80°CandvaryingthedissolvedCO2concentrationfrom1.98X

I 10 1 2 TIM:.(h) L 5 6

Fig. 1. The yields of HCOOH (0) and HCHO (A) in CO2 reduction by K[ Rum(E

C1]*2Hz0.

HCHO and HCOOH were evaluated in the interval between 1 - 3

ensured a linear dependence. The decay rate of HCHO’was evalua

declining portion of the curve after 3.5 h.

Kinetics

The kinetic experiments on CO* reduction were conducted b

the concentrations of COz, and rates of formation of HCHO and

were evaluated.

Effect of complex concentration

The concentration of complex 1 was varied from 0.5 - 2 X lop

a constant dissolved CO2 concentration of 3.9 X lo-* M and tempe

80 “CL!or all the runs. The dependence of rates of formation of H

HCOOH on the concentration of complex 1 are shown in Figs.

respectively. Both HCHO and HCOOH rates show a first order de

with respect to complex 1 concentration.

Effect of dissolved CO2 concentration

Figures 4 and 5 show the plot of rates of formation of H

HCOOH vs. dissolved CO2 concentration. The experiments were p

keeping a constant complex concentration of 1 X 10e3 M, 80 “C an

the dissolved CO2 concentration from 1.98 X lo-* M to 7.73



10‐2Mto7.73X10‐2M.TheratesofformationofbothHCHOand

HCOOHshowalineardependenceofCO2concentration.

Temperature

ThetemperatureoftheexperimentsforreductionofCO2bycomplex1wasvariedbetween40‐80°C,withotherconditionsconstantsuchas

Catalystconcentrationof1X10‐3ManddissolvedCO2concentrationof

3.9X10‐2M.Figure6showsthegraphof‐‐Inratevs.l/T.Fromthe

slopesofboththestraightlines,theactivationenergiesevaluatedfor

HCOOHandHCHOare4.8kcalmol‐1and3.5kcalmol

‐1,respectively

0 0.5 14 1.5

COMPLEX j_CONCENTRATiON,x103 M COMPLEX 1 CONCENTRATION,

Fig. 2. Effect of complex 1 con~entratj~n on the rate of HCHO formation.

Fig, 3. Effect of complex 1 concentration on the rate of HCOOH formation.

~10~ M

Fig. 4. Effect of dissolved CO2 concentration on the rate of HCHO formation.

3.9 X low2 M. Figure 6 shows the graph of --In rate us. l/T. From the slopes

of both the straight lines, the activation energies evaluated for HCOOH and

HCHO are 4.8 kcal mol-* and 3.5 kcd mol-' , respectively.

Mechanism

The reduction of CO2 by complex 1 gave two products, namely HCHO

and HCOOH. The rates of formation of both the products have a first-order

dependence with respect to catalyst and dissolved CO2 concentrations.

0 0.5 14 1.5

COMPLEX j_CONCENTRATiON,x103 M COMPLEX 1 CONCENTRATION,

Fig. 2. Effect of complex 1 con~entratj~n on the rate of HCHO formation.

Fig, 3. Effect of complex 1 concentration on the rate of HCOOH formation.

~10~ M

Fig. 4. Effect of dissolved CO2 concentration on the rate of HCHO formation.

3.9 X low2 M. Figure 6 shows the graph of --In rate us. l/T. From the slopes

of both the straight lines, the activation energies evaluated for HCOOH and

HCHO are 4.8 kcal mol-* and 3.5 kcd mol-' , respectively.

Mechanism

The reduction of CO2 by complex 1 gave two products, namely HCHO



Mechanism:

ThereductionofCO2bycomplex1gavetwoproducts,namelyHCHO

andHCOOH.Theratesofformationofboththeproductshaveafirst‐

orderdependencewithrespecttocatalystanddissolvedCO2concentrations.Basedontheseobservations,themechanismproposed

forthereactionsisshowninScheme1.Thecomplex1undergoesarapid

aquationinpresenceofwatertogiveRu(III)

aquospecies2[19],which

reactswithCO2toformaCO,adduct3.TheformationofsuchCO2

adductswerereportedintheliterature[3–5].Species3activateswater

oxidativelyinstep(2)toformtheRu(V)

species4.TheinsertionofCO2

intotheRu‐Hbondof4takesplaceinstep(3)toformanη1‐formate

species5.Themetalloformatecomplex5undergoesarapid

intermolecularproto‐nationresultingintheformationofformicacid

andRuV‐oxospecies6.Reductiveeliminationofmetalloformate

complexestoformatederivativessuchasmethylformatefrom

methanol,CO2andH2hadbeenreported[9].Theelectronsneededfor

thereductionofCO2toHCOO‐inourcaseareobtainedbythehydride

insertionstep(3).Theformationofruthenium(V)oxospecies6is

characterizedbytheappearanceofabandat390nm.Species6also

givesv(Ru=O)at810cm‐‘.Instep(5),theformicacidformedinstep(3)

iscatalyticallydecomposedtoCOandHz0bycomplex2.

310

K

LRu**~(H~O) + co, s LRu”‘( COz)

2 3

H

fast

LRu’“‘(C02) + Hz0 - L+v(Co,)

OH

4

H

k

L&F&*) -

0

LRuy_-+C”

!3H

k--H ‘OR

1

5

fast

0

IILRuV + HCOOH

6

/o kl H,

LYv-cNc - H’c=O + Hz0 + LRuV

OH

OH- H :::

+I4 6

kzHCOOH e CO + Hz0

LRur”(H20)

(1)

(2)

(3)

(4)

(5)



Ratelaws:

Therateconstantsforthewater–shiftreaction(k3),formicaciddecomposition(k2)andthecarbondioxidereductiontoHCHO(k1)are

1fast

0II

LRuV + HCOOH

6

/o kl H,

LYv-cN- H’c=O + Hz0 + LRuV

OHOH- H ::

:

+I4 6

kzHCOOH e CO + Hz0

LRur”(H20)

k3CO + H,O - COz + Hz

LRuV + H\C=O kq

H’LRu”’ + HCOOH

8

6

Scheme 1. Mechanism for the stoichiometric fixation of C O2 by LFlu”‘(H20).

(4 )

(5 )

(6)

(7 )

The rate of decomp osition of formic acid catalyzed by 1 was stimated in a

separate experimen t. It was confirmed by the an alysis of both liquid as wellas gas samples withdrawn at different time intervals that HCOOH decom-

309

DISSOLVED CO 2 CONCENTRATION, x10 ' M

Fig. 5. Effect of dissolved CO* concentration on the rate of HCOOH formation.

111 I

2 .8 2 .9 3 .0 3 .1 3 .2

' /T ,1O3 K

Fig. 6. Effect of temperature on the rates of formation of HCOOH and HCHO.

Based on these observations, the mechanism proposed for the reactions is

shown in Scheme 1.

The complex 1 undergoes a rapid aquation in presence of water to give

Ru(II1) aquo species 2 [19], which reacts with CO? to form a CO, adduct 3.

The formation of such COz adducts were reported in the literature [3 - 51.

Species 3 activates water oxidatively in step (2) to form the Ru(V) species 4.

The insertion of CO2 into the Ru-H bond of 4 takes place in step (3) to

form an rjl-formate species 5.

The metalloformate complex 5 undergoes a rapid intermolecular proto-

nation resulting in the formation of formic acid and RuV-0x0 species 6.

Reductive elimination of metalloformate complexes to for-mate derivatives

such as methyl formate from methanol, COz and Hz had been reported [9].

The electrons needed for the reduction of CO2 to HCOO- in our case are



givenasinthefollowingprecedence

K3>>K2>>K1>>K>>K4

Fromtheinterceptsandslopesoftherateequationsgivenbelowtherateconstantsareevaluatedas

K=7.46M‐1

K1=0.33min‐1

K3=90.1min‐1.

Bytakinga300mlreactorandmaintainingattheoptimalexperimental

conditionstheoutputofhydrogenobtainedwasfoundtobe0.42m3/s.

ResultsandConclusions:

Thustheabovepaperreportsonproductionof0.42m3/sofhydrogen

with4X10‐2MofHCOOHproductionfromthedissolvedCarbondioxide

ina300mlreactorunderthetemperatureofaround40–70°Cand

pressureofabout70.9bars.TheproposalistheusageofMicroreactorsthatcanmaintaintheoptimumexperimentalconditionsatthesame

timegivethestandardCO2mentionedabove.Theadvantagesofusing

theMicroreactorsare

• Microreactorstypicallyhaveheatexchangecoefficientsofatleast

1megawattpercubicmeterperkelvin,upto500MWm‐³K

‐¹vs.a

fewkilowattsinconventionalglassware(1lflask~10kWm‐³K

‐¹)).

Thus,microreactorscanremoveheatmuchmoreefficientlythan

vesselsandevencriticalreactionssuchasnitrationscanbe

311

poses into CO and HzO. The CO obtained in step (5) is rapidly converted

to CO z by the water-gas shift reaction [20].

The $ -carboxylate species 5 reacts with Hz in step (4) to give HCH O

and Ru(V)-0x0 species 6. Thus, the Ru(V)-0x0 species is an end product

of reduction of COz. In step (7), HCHO is oxidized to HCOOH by LRuV =O

species regenerating species 2.

Rate laws

From the kinetic observations of the reduction of CO2 to HCOO H andHCHO by K[ Ru”‘(EDTA-H)Cl] *2Hz0, the rate equation can be written as:

F l A

kKIRuxl’(EDTA-H)(HaO)]r[COz]

1+ [CO21 kkll([Ruul(EDTA-H)(H20)]*[C02]

F2 =

1+ [CO21 (2)

where rl = rate of formation of HCOOH, r2 = rate of formation of HCHO ,

[ Ru”‘(ED TA-H)(H 20)lT = total complex 1 concentration, [CO,] = dis-

solved CO2 concen tration, K = equilibrium constant, k = rate constant for

the formation of HCOOH, and k, = rate constant of HCHO formation.

Equations (1) and (2) can be rearranged in the following form to evalu-

ate the kinetic constants.

[Ru”‘(EDTA-H)(H20)lT =

Fl

(3)

[Ru”‘(EDTA-H)(H20)IT = 1

i

1 1

‘-2 kk,K x[cozl +kk,(4 )

From the intercepts and slopes of the above equations, th e values of K, k

and k1 evaluated at 80 “C are:

K = 7.46 M-i

k = 0.33 min-’

kl = 3.03 min-’

The water-gas shift reaction occurring between C O and H20 un der

the reaction conditions has a rate constant (k3 = 90.1 min-‘) reported in our

earlier studies [20]. The values of k an d k, , ‘the rate constants for formic

acid and formaldehyde formation, respectively, are much sma ller than k3

and hence during the CO2 reduction the CO generated by the decomposition

of HCO OH p articipates rapidly in the water-gas shift reaction, giving CO2

and Hz. Th e hydrogen produced by the water-gas shift reaction is utilized

in the reduction of HCOO H and HCHO . The value of k2 (rate constant for

HCOO H decomposition), determined independently in the absence of CO2

and in the presence of complex 1 at 80 “C, is 1.52 min-‘. The formic acid

decom position rate constant (k,) is smaller than (k3) (water-gas shift

311

poses into CO and HzO. The CO obtained in step (5) is rapidly converted

to CO z by the water-gas shift reaction [20].

The $-carboxylate species 5 reacts with Hz in step (4) to give HCH O

and Ru(V)-0x0 species 6. Thus, the Ru(V)-0x0 species is an end product

of reduction of COz. In step (7), HCHO is oxidized to HCOOH by LRuV =O

species regenerating species 2.

Rate laws

From the kinetic observations of the reduction of CO2 to HCOOH and

HCHO by K[ Ru”‘(EDTA-H)Cl] *2Hz0, the rate equation can be written as:

Fl A

kKIRuxl’(EDTA-H)(HaO)]r[COz]

1+ [CO21 kkll([Ruul(EDTA-H)(H20)]*[C02]

F2 =

1+ [CO21 (2)

where rl = rate of formation of HCOOH , r2 = rate of formation of HCHO ,

[ Ru”‘(EDT A-H)(H2 0)lT = total complex 1 concentration, [CO,] = dis-

solved CO2 concen tration, K = equilibrium constant, k = rate constant for

the formation of HCOOH, and k, = rate constant of HCHO formation.

Equations (1) and (2) can be rearranged in the following form to evalu-

ate the kinetic constants.

[Ru”‘(EDTA-H)(H20)lT =

Fl

(3)

[Ru”‘(EDTA-H)(H20)IT = 1

i

1 1

‘-2 kk,K x[cozl +kk,(4 )

From the intercepts and slopes of the above equations, the values of K, k

an d k1 evaluated at 80 “C are:

K = 7.46 M-i

k = 0.33 min-’

kl = 3.03 min-’

The water-gas shift reaction o ccurring b etween C O and H20 un der

the reaction conditions has a rate constant(k3 =

90.1 min-‘) reported in ourearlier studies [20]. The values of k an d k, , ‘the rate constants for formic

acid and formaldehyde formation, respectively, are much sm aller than k3

and hence during the CO2 reduction the CO generated by the decomposition

of HCO OH p articipates rapidly in the water-gas shift reaction, giving CO2

and Hz. T he hydrogen produced by the water-gas shift reaction is utilized

in the reduction of HCOO H and HCHO. The value of k2 (rate constant for

HCOO H decomposition), determined independently in the absence of CO2

and in the presence of complex 1 at 80 “C, is 1.52 min-‘. The formic acid

decom position rate constant (k,) is smaller than (k3) (water-gas shift



performedsafelyathightemperatures.Hotspottemperaturesas

wellasthedurationofhightemperatureexpositiondueto

exothermicitydecreasesremarkably.Thus,microreactorsmay

allowbetterkineticinvestigations,becauselocaltemperature

gradientsaffectingreactionratesaremuchsmallerthaninanybatchvessel.Heatingandcoolingamicroreactorisalsomuch

quickerandoperatingtemperaturescanbeaslowas‐100°C.Asa

resultofthesuperiorheattransfer,reactiontemperaturesmaybe

muchhigherthaninconventionalbatch‐reactors.Manylow

temperaturereactionsasorgano‐metalchemistrycanbe

performedinmicroreactorsattemperaturesof‐10°Cratherthan‐

50°Cto‐78°Casinlaboratoryglasswareequipment.

• Microreactorsarenormallyoperatedcontinuously.Thisallowsthe

subsequentprocessingofunstableintermediatesandavoids

typicalbatchworkupdelays.Especiallylowtemperature

chemistrywithreactiontimesinthemillisecondtosecondrange

arenolongerstoredforhoursuntildosingofreagentsisfinished

andthenextreactionstepmaybeperformed.Thisrapidworkup

avoidsdecayofpreciousintermediatesandoftenallowsbetter

selectivities.

• Continuousoperationandmixingcausesaverydifferent

concentrationprofilewhencomparedwithabatchprocess.Inabatch,reagentAisfilledinandreagentBisslowlyadded.Thus,B

encountersinitiallyahighexcessofA.Inamicroreactor,AandB

aremixednearlyinstantlyandBwon'tbeexposedtoalarge

excessofA.Thismaybeanadvantageordisadvantagedepending

onthereactionmechanism‐itisimportanttobeawareofsuch

differentconcentrationprofiles.

• Althoughabench‐topmicroreactorcansynthesizechemicalsonly

insmallquantities,scale‐uptoindustrialvolumesissimplya

processofmultiplyingthenumberofmicrochannels.Incontrast,batchprocessestoooftenperformwellonR&Dbench‐toplevel

butfailatbatchpilotplantlevel

• Pressurisationofmaterialswithinmicroreactors(andassociated

components)isgenerallyeasierthanwithtraditionalbatch

reactors.Thisallowsreactionstobeincreasedinratebyraising

thetemperaturebeyondtheboilingpointofthesolvent.This,

althoughtypicalArrheniusbehaviour,ismoreeasilyfacilitatedin

microreactorsandshouldbeconsideredakeyadvantage.



Pressurisationmayalsoallowdissolutionofreactantgasseswithin

theflowstream.

Thususingstacksofmicroreactorsonecanachieveveryhighpower

yield.Thehydrogenthusproducedcanbeusedintheproductionof

Electricityinfuelcellsaswellinproductionofammoniaandotherorganiccompounds.

References

. . . . . . .considerably more negative than the entropies observed [21] for the conver-

sion of CO to HCOOH and HCHO. This reflects the higher order of rear-

rangements in the coordination sphere of the metal ion in CO2 insertion as

compared to CO insertion. In general, the enthalpies and entropies are more

negative in HCH O formation than HC OO H formation. Since a hydrogen

molecule is needed for the reduction of coordinated q’-format0 or a formyl

group, the transition state requires greater reorganisation in HCHO reaction

as compared to HCOOH, hence a more negative entropy. The negativeenthalpy reflects the higher bond strength in HCHO and HCOOH.

References

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Catalytic Reduction of CO2 Into Hydrogen Using Micro Reactors

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