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8/8/2019 Catalytic Reduction of CO2 Into Hydrogen Using Micro Reactors
http://slidepdf.com/reader/full/catalytic-reduction-of-co2-into-hydrogen-using-micro-reactors 1/10
Malavarayan.S
Ravishankar.MIIndyearChemicalEngineering,SriVenkateswaraCollegeofEngineering,Sriperumbudur
PostalAddress:No:50,NGOColony2ndstreet,Vadapalani,Chennai‐26.
E-mail :[email protected]:+919884314549
CatalyticReductionofCO2IntoHydrogenUsing
Microreactors
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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.
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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.
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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
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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
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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)
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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
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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
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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.
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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.
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