NMR spectroscopy applied to amine–CO2–H2O systems relevant for post-combustion CO2 capture: A review

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IJGGC10811 ContentslistsavailableatScienceDirect

International Journal of Greenhouse Gas Control

jou rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / i j g g c

Highlights

InternationalJournalofGreenhouseGasControlxxx (2013)xxx–xxx NMRspectroscopyappliedtoamine–CO2–H2Osystemsrelevant

forpost-combustionCO2capture:Areview CristinaPerinu,BjørnarArstad,Klaus-JoachimJens^∗

•AsurveyoftheNMRstudiesperformedonamine–CO2–H2Osystemsispresented.

•TechnicalaspectsofNMRexperimentsandmethodsaredepicted.

•ThemainapplicationsandcorrespondingresultsobtainedbyNMRarereported.

•NMRspectroscopyisausefultooltoidentifyandquantifyliquidreactionproducts.

ContentslistsavailableatScienceDirect

International Journal of Greenhouse Gas Control

jo u r n al h om e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / i j g g c

Review

1

NMR spectroscopy applied to amine–CO 2 –H 2 O systems relevant for post-combustion CO ₂ capture: A review

2

3

Cristina Perinu

^a

, Bjørnar Arstad

^b

, Klaus-Joachim Jens

^a,∗

Q1

4

aFacultyofTechnology,TelemarkUniversityCollege,KjølnesRing 56,3901Porsgrunn,Norway 5

bSINTEFMaterialsandChemistry,Forskningsveien1,0314Oslo,Norway 6

7

a r t i c l e i n f o

8 9

Articlehistory:

10

Received16May2013 11

Receivedinrevisedform8October2013 12

Accepted25October2013 13

Availableonlinexxx 14

15

Keywords:

16

NMR 17

CO2capture 18

Quantitative 19

Amine 20

Review 21

a b s t r a c t

NuclearMagneticResonance(NMR)spectroscopyisapowerfulnon-invasiveanalyticaltechniquefor chemicalanalysessincedirectmeasurementsatamolecularlevelcanbeperformed.Inthiswork,asurvey ofNMRspectroscopyappliedforstudiesofCO2absorptioninaqueousaminesolvents(amine–CO2–H2O) relevantforpost-combustionCO2captureispresented.TechnicalaspectsofNMRexperimentsandthe mainapplicationswithcorrespondingresultsareprovided.TheoverviewoftheNMRliteratureinthis fieldsuggeststhatstudiesofamine–CO2–H2Osystemscanbenefitfromafurtherconsiderationofthis spectroscopictechnique.

©2013PublishedbyElsevierLtd.

22

Contents

23

1. Introduction... 00

24

1.1. Background ... 00

25

1.2. Outline... 00

26

2. Overviewofamine–CO2–H2Osystemchemistry... 00

27

3. ExperimentalNMRmethods... 00

28

3.1. NMRExperiments... 00

29

3.2. Quantitativemeasurements:parametersanderroranalysis... 00

30

3.3. Deuteratedandreferencesolvents... 00

31

3.4. Quantificationoffast-exchangingprotonspecies... 00

32

3.5. VariabletemperatureandhighpressureNMRexperiments... 00

33

4. Resultsandapplications... 00

34

4.1. Speciationofamine–CO2–H2Osystemsinabsorptionexperiments... 00

35

4.2. Speciationofblendedamine–CO2–H2Osystemsinabsorptionexperiments... 00

36

4.3. Speciationofsingleandblendedamines–CO2–H2Osystemsinabsorption-desorptionexperiments... 00

37

4.4. Determinationofcarbamatestabilityconstants... 00

38

4.5. Kineticstudies... 00

39

5. Conclusions... 00

40

Acknowledgements... 00

41

AppendixA... 00

42

References... 00

43

∗Correspondingauthorat:FacultyofTechnology,TelemarkUniversityCollege,P.O.Box203,3901Porsgrunn,Norway.Tel.:+4735575193;fax:+4735575001.

E-mailaddress:Klaus.J.Jens@hit.no(K.-J.Jens).

1750-5836/$–seefrontmatter©2013PublishedbyElsevierLtd.

http://dx.doi.org/10.1016/j.ijggc.2013.10.029

Pleasecitethisarticleinpressas:Perinu,C.,etal.,NMRspectroscopyappliedtoamine–CO2–H2Osystemsrelevantforpost-combustion CO₂capture:Areview.Int.J.GreenhouseGasControl(2013),http://dx.doi.org/10.1016/j.ijggc.2013.10.029

Nomenclature Symbols

1D mono-dimensional 2D two-dimensional

K Kelvin(unitoftemperature) Kc carbamatestabilityconstant k rateconstant

M(mol/l) molarconcentration(mole/litre) ppm partpermillion(unit)

T1 spin-lattice(orlongitudinal)relaxationtime S/N signaltonoiseratio

wt.% weightpercent activitycoefficient Acronymsandabbreviations

AEEA 2-(2-aminoethylamino)ethanol AmineH⁺ protonatedamine

AMP 2-amino-2-methyl-1-propanol AMPD 2-amino-2-methyl-1,3-propanediol AP 2-amino-1-propanol

BEA 2-(butylamino)ethanol COSY correlationspectroscopy

DEA diethanolamineor2,2-iminodiethanol

DEPT distortionlessenhancementbypolarizationtransfer DETA diethylenetriamine

DMDEA⁺ dimethyl-diethanolamineion DMMEA 2-(dimethylamino)ethanol

HMBC HeteronuclearMultipleBondCorrelation HSQC HeteronuclearSingleQuantumCorrelation HEA N-(2-hydroxyethyl)acetamide

HEI N-(2-hydroxyethyl)imidazole HEF N-(2-hydroxyethyl)formamide MAPA 3-(methylamino)propylamine

MDEA N-methyldiethanolamine or 2,2- methyliminodiethanol

MEA monoethanolamineor2-aminoethanol MEACOO⁻ monoethanolaminecarbamate MEAH⁺ protonatedmonoethanolamine

MMEA N-methylethanolamineor2-(methylamino)ethanol NMR NuclearMagneticResonance

NOE NuclearOverhauserEffect OZD 2-oxazolidone

PCC post-combustioncapture

PZ piperazineor1,4-diazacyclohexane VLE vapour-liquidequilibrium

SAR sarcosineorN-methylglycine

FurtheracronymsandabbreviationsintheTableA1(Appendix A)

AA aminoacid

AHPD 2-amino-2-hydroxymethyl-1,3-propanediol Abs absorptionexperiments

ALA l-alanineor(S)-2-aminopropionicacid

ATR-FTIR attenuated total reflectance-Fourier transform infraredspectroscopy

BZ benzylamine

Cf/Cd carbamateformation/decomposition CYS cysteine

DAP 1,2-diaminopropane DEAB 4-(diethylamino)-2-butanol DEEA 2-(diethylamino)ethanol Des desorptionexperiments

DGA diglycolamineor2-(2-aminoethoxy)ethanol

DIPA diisopropanolamine EAE 2-(ethylamino)ethanol EDA ethylenediamine f.e.p. fastexchangingproton

GC/LC–MS gas chromatography/liquid

chromatography–massspectrometry GLY glycineoraminoethanoicacid homoPZ homopiperazine

IBA isobutylamine

IPAE 2-(isopropylamino)ethanol MAE 2-(methylamino)ethanol MORP morpholine

MPZ N-methylpiperazine n.a. notavailable

NRTL non-randomtwoliquidmodel 2-PE 2-piperidineethanol

PIPD piperidine

4-PIPDM 4-piperidinemethanol 4-PIPDE 4-piperidineethanol

PRO l-prolineor(S)-pyrrolidine-2-carboxylicacid PYR pyrrolidine

RITE baseamines-additives

SER l-serineor(S)-2-amino-3-hydroxypropanoicacid TAU taurineor2-aminoethanesulphonicacid

TMORP thiomorpholine

1. Introduction ⁴⁴

1.1. Background ⁴⁵

Processesutilizingaqueousaminesolventsforchemicalabsorp- 46

tionofcarbondioxide(amine–CO₂–H₂Osystems)areconsideredto 47

bearobustseparationtechnologytoremovecarbondioxide(CO₂) 48

fromfluegasstreamsandwillprobablybedeployedona large 49

scaleintheyearstocome(Rochelle,2009).However,theenergy 50

demands forCO₂ desorption andamine regeneration,corrosive 51

natureofaminesandtheirdegradationrepresentthemaindraw- 52

backswhichlimittheirapplication(Ahnetal.,2011a).Forrational 53

improvementoftheefficiencyofpost-combustioncapture(PCC) 54

byaminesolventsanddevelopmentsofnewabsorptionsystems, 55

anaccurateunderstandingoftheunderlyingchemicalprocesses 56

(suchasreactionkinetics,equilibriapresentandthermodynam- 57

ics)involvedinthecaptureandreleaseofCO₂isneeded(Conway 58

et al.,2012).In thecontext of PCCdevelopment, vapour-liquid 59

equilibrium(VLE)modelsarecriticaltoolsfordevelopmentand 60

optimizationofthegas–liquidabsorptionprocess(Faramarzietal., 61

2009;KontogeorgisandFolas,2009).However,predictivethermo- 62

dynamicVLEmodelsaredependentonaccuratedeterminationof 63

solventcompositionandsuchdataareofteninferredthroughmea- 64

surementofrelated variablesthatextendthedataavailablefor 65

fitting.Nuclear MagneticResonance(NMR)spectroscopyallows 66

directaccesstothechemicalcompositionofCO₂capturesolvents 67

(speciation)andthereforemoreobjectfunctionsusableforfitting 68

parametersareprovided.Furthermore,bymonitoringspeciation 69

ofthewholecyclicprocessofCO₂capture,itispossibletoobtain 70

informationonreactionmechanisms,kineticsofreaction,factors 71

influencingreactionconditions(suchasthechemicalstructuresof 72

theamines),aminecapacitiesandCO₂solubility(Jakobsenetal., 73

2008;Yangetal.,2009). 74

Beforeenteringmoredetailedissues,afewwordsaboutNMR 75

spectroscopy is useful as background. NMR is a non-invasive 76

analyticaltechniquewhich allowsthestudyof atomsthathave 77

nucleiwithamagneticmoment(i.e.aspinquantumnumberof 78

thenucleusdifferentfromzero).Someexamplesarethefollow-

79

ing isotopes: ¹H, ¹³C, ¹⁵N NMR data are usually presented as

80

spectrashowingpeaks(reportedalong anaxiswithunitsppm,

81

partspermillion)thatcorrespondtothedifferentnucleiinthe

82

molecules.DependingonNMRexperiments,interpretationofthe

83

dataleadstoinformationonchemicalstructureanddynamicsof

84

compounds/materials.Astrongquality oftheNMR techniqueis

85

thatpeak areasare alsodirectlyproportionaltothenumber of

86

nucleicontributingtothesignalsand,withsomecare,quantitative

87

analysesofcompoundsarehencepossible.Anotheradvantageous

88

featureof NMR is that unknowncompounds (e.g.reaction and

89

degradationproducts) can alsobecharacterizedand quantified

90

withouttheuseofanystandardreference(BottingerandHasse,

91

2008).

92

InspiteofthepotentialandpracticalimportanceofNMRspec-

93

troscopy,sofaronlyalimitednumberofstudieshavebeenreported

94

intheliteratureoninvestigationofliquidphase compositionof

95

amine–CO2–H2Osystems.However,interestinNMRspectroscopy

96

isgrowing,asevidencedbytheincreasednumberofpublications

97

inrecentyears(Fig.1).

98

Inthisworkweprovideasurveyofapproximately50articles

99

publisheduptospring2013whereNMRhasbeenappliedforstud-

100

iesofaqueousaminesolventsforCO₂capture.Recently,Shietal.

101

publishedareviewontheuseofNMRfordevelopmentofVLEmod-

102

els(Shietal.,2012a)andwereportedashortoverviewof¹³CNMR

103

studiesonaqueousaminesystemsrelevantforCO₂capture(Perinu

104

etal.,2013).Thepresentreviewgivesanextensivesurveyofall

105

typesofNMRexperimentsandallpossibleapplications,including

106

methodsandmainresults.

107

1.2. Outline

108

Thepresent review is structuredas follows: Section2 gives

109

an overview of the main chemical reactions involved in the

110

amine–CO₂–H₂Osystems,includingthemainissuesof concern,

111

andliststhemainclassofamineabsorbentsstudiedbyNMRspec-

112

troscopy.InSection3,technicalaspectsofNMRexperimentsand

113

methodsusedtoobtainquantitativedataarediscussed.Themain

114

applicationsandresultsobtainedbyNMRarereportedinSection

115

4, whileSection5providessomefinalconsiderationsontherange

116

ofapplicabilityofNMRspectroscopyinthisfield.

117

2. Overviewofamine–CO2–H2Osystemchemistry

118

TheabsorptionofCO₂inaqueousaminesolventsinvolvessev-

119

eralparallelreactionsthatgiverisetoalargenumberofspeciesin

120

theliquidphase(Eqs.(1)–(7)).

121

Reactionratesandequilibriumconstantsforthewaterdissoci-

122

ation(1),thehydrationofCO₂(2),dissociationofcarbonicacid(3)

123

andbicarbonate(4)areknown(Edsall,1969;PockerandBjorkquist,

124

1977),aswellastheprotonationconstantsformanyamines(5)

125

(HamborgandVersteeg,2009;Perrin,1965).

126

H₂O+H₂O H₃O⁺+OH⁻ (1)

127

CO₂+H₂O H₂CO₃ (2)

128

H₂CO₃+H₂OHCO₃⁻+H₃O⁺ (3)

129

HCO3−+H2O CO32−+H3O⁺ (4)

130

R1R2NH+H3O⁺ R1R2NH2++H2O (5)

131

Ontheotherside,thereactionofCO2withaminecompoundsis

132

criticalandstillmeritsfurtherinvestigations.Inparticular,primary

133

andsecondaryamines(withtheexceptionofthosethat areso-

134

calledstericallyhinderedamines)inwaterreactwithCO2toform

135

carbamatewithastoichiometricloadingof0.5molCO₂/molamine, 136

asindicatedinthefollowingcondensedreaction: 137

2R₁R₂NH+CO₂ R₁R₂NCOO⁻+R₁R₂NH₂⁺ (6) 138

In contrast, the reaction between tertiary (and some steri- 139

callyhindered)aminesandCO₂ canbedescribed withthebase 140

catalysisofCO2hydrationasproposedbyDonaldsonandNguyen 141

(DonaldsonandNguyen,1980;Fernandesetal.,2012)andloadings 142

of1.0molCO₂/molamineareachievedinthefollowingway: 143

R1R2R3N+CO2+H2OHCO3−+R1R2R3NH⁺ (7) 144

Reaction(7)ismoreefficientthan6intermsofCO₂absorption 145

capacity,butreactionsofprimaryandsecondaryamineswithCO2 ¹⁴⁶

showfasterkinetics.However,duringtheCO₂desorption/amine 147

regenerationstep,theenergydemandforthereverseof6ishigher 148

than7duetothestabilityofcarbamates(Sartorietal.,1987).Acon- 149

siderablenumberofpublicationsoncarbamateformationarefound 150

inliterature,butonlyafewoftheseproposeempiricalreaction 151

mechanismsthatadequatelymodelthemeasuredgasabsorption 152

data(Caplow,1968;CrooksandDonnellan,1989;Versteegetal., 153

1996).Experimentalevidenceofthechemistryassociatedwithcar- 154

bondioxideandaqueousaminesolvents(mechanismofreaction, 155

equilibrium and ratekineticsconstants) andof thefactorsthat 156

influencecarbamateformationisstillscarce(Arouaetal.,1997; 157

McCannetal.,2009b).Quantummechanical modellinghaspro- 158

videdsomeinsight intothechemistryand possiblemechanistic 159

pathways,butstillneedsexperimentalvalidations(Arstadetal., 160

2007;daSilvaandSvendsen,2004;Guido etal.,2012;Iidaand 161

Sato,2012;Maitietal.,2011). 162

Atthepresent,mostoftheNMR studiesonamine–CO₂–H₂O 163

systems dealwithalkanolamines,which are themostcommon 164

chemical absorbentsfor the removalof acidgases. In addition, 165

ammonia (Ahn etal.,2011a,b;Holmeset al.,1998;Mani etal., 166

2006; Pellegrini etal., 2010; Rowlandet al.,2011), amino acid 167

(Ciftjaetal.,2013;Hartonoetal.,2011;Hook,1997;Xiangetal., 168

2012)andvariousotheraminesystemshavealsobeenexamined 169

byNMR(Conwayetal.,2012;Fernandesetal.,2012;García-Abuín 170

etal.,2012;Hartonoetal.,2006;Kimetal.,2011a;McCannetal., 171

2011b;Mergleretal.,2011;Pauletal.,2008;Yangetal.,2012). 172

Amongthelatter,piperazine(PZ),a cyclicdiamine,is themost 173

commonlystudied(Bishnoiand Rochelle,2000,2002;Böttinger 174

etal.,2008;Conway etal.,2013;Cullinaneand Rochelle,2005; 175

Derksetal.,2010;Ermatchkovetal.,2003;Hartonoetal.,2013; 176

Kimetal.,2011b).TableA1inAppendixAlistsamine–CO₂–H₂O 177

systemsinvestigatedbyNMRcollectedinthepresentmanuscript. 178

3. ExperimentalNMRmethods ¹⁷⁹

3.1. NMRExperiments ¹⁸⁰

Outof variousone- and two-dimensional (1Dand 2D)NMR 181

experiments,singlepulse¹Hand¹³CNMR experimentsarethe 182

mostcommononesforstudiesofamine–CO2–H2Osystems.How- 183

ever,onepaperhasreportedsinglepulse¹⁵NNMRexperiments 184

withtheaimtoevaluatetheelectronreleasingandwithdrawing 185

effects ofsubstituents onthebackgroundof ¹⁵Nchemical shift 186

changes (Yoon, 2003)and Ciftja etal. havereportedtheuseof 187

DEPT(DistortionlessEnhancementbyPolarizationTransfer)exper- 188

imentstoidentifytheprimary,secondaryandtertiary¹³Catoms 189

of thespecies (Ciftjaet al., 2011,2013). Two-dimensional (2D) 190

NMRexperiments,likee.g.COSY(CorrelationSpectroscopy),HSQC 191

(HeteronuclearSingleQuantumCorrelation)andHMBC(Heteronu- 192

clearMultipleBondCorrelation),arequiteoftenreportedtodefine 193

and/or check the chemical structure of the species in solution 194

(Ballardetal.,2011;BishnoiandRochelle,2000,2002;Böttinger 195

Pleasecitethisarticleinpressas:Perinu,C.,etal.,NMRspectroscopyappliedtoamine–CO2–H2Osystemsrelevantforpost-combustion CO₂capture:Areview.Int.J.GreenhouseGasControl(2013),http://dx.doi.org/10.1016/j.ijggc.2013.10.029

Fig.1.Graphofthenumberofarticles,dealingwithNMRspectroscopy,peryear.Thetracedcolumncorrespondingtotheyear2013reportsthepapersuptospring2013.

etal.,2008;BottingerandHasse,2008;Ciftjaetal.,2012,2013;

196

CullinaneandRochelle,2005;Derksetal.,2010;Fanetal.,2009;

197

Hartonoetal.,2006,2011;Jakobsenetal.,2008;Kimetal.,2011a,b;

198

Ma’munetal.,2006;McCannetal.,2009b).

199

Since themain goalof many works is often the estimation

200

oftherelativeamountsofspecies,mostofthepapersdealwith

201

quantitativeNMRstudiesandonlyfewofthemexclusivelyreport

202

qualitativeevaluationfornovelamine–CO2–H2Osystems(García-

203

Abuínetal.,2012;Hartonoetal.,2006,2011;Pauletal.,2008;

204

Rowlandetal.,2011).

205

Forquantitativeanalyses,¹HNMRisfastandreliablebutnot

206

allthespeciesinsolutioncanbeobserved(likee.g.carbonateand

207

bicarbonate).Thus,mostly¹³CNMRexperimentsareusedtostudy

208

thespeciesdistributioninamine–CO2–H2Osystemsbecausedirect

209

informationofallinteractingspecies,withtheexceptionofH₂O,

210

H₃O⁺andOH⁻,canbegathered(Fig.2).

211

Inaddition,¹³CNMRhasagreaterpotentialthan¹HNMRfor

212

thestudyofcomplexorganicsystems,duetothebroadspectral

213

rangeand,usually,anabsenceofinterferencebetweenpeaks.How-

214

ever,¹³CNMRsuffersfrompoorsensitivityduetoarelativelyweak

215

molarreceptivity(0.0159relativeto¹H)andaslowrelaxationofthe

216

13Cspinsbacktothermalequilibriumagain(BhartiandRoy,2012;

217

Claridge,1999).Specifically,inamine–CO₂–H₂Osystems,thereare

218

differentcarbonsofinterestandsomeofthemhaverelativelylong

219

spin-lattice(orlongitudinal)relaxationtime(T1),leadingtolong

220

measuringtimes(e.g.hours).Inparticular,¹³Catomsinthecar-

221

boxylicgroupsofcarbamatesandinHCO3−/CO32−haverelaxation

222

timesmuchlongerthanthoseobservedforcarbonswithbounded

223

hydrogens(i.e.CH,CH₂andCH₃).

224

Quantitative ¹³C NMR dataonamine–CO2–H2O systems are

225

mainlyusedtodevelopand/orsupportVLEthermodynamicmod-

226

els(likee.g.bycomparingexperimentalandpredictedspeciation)

227

(Ahn et al., 2011a,b; Barth et al., 1984; Böttinger et al., 2008;

228

BottingerandHasse,2008;Chakrabortyetal.,1986;Holmesetal.,

229

1998;Jakobsenetal.,2005;Ma’munetal.,2006;Shietal.,2012b)

230

and to obtain information on species distribution for investi-

231

gatingthereactionmechanismsand thefactors influencingthe

232

CO₂ absorption/desorptionprocessesinaqueousaminesolvents

233

(Ballardetal.,2011;Barzaglietal.,2009,2010,2011;Ciftjaetal.,

234

2012;Gotoetal.,2011b; Yamadaetal.,2012;Hook,1997;Jakobsen

235

etal.,2008;Kimetal.,2011b;Manietal.,2006;Parketal.,2003;

236

Rowlandetal.,2011;Yangetal.,2009,2012).

237

Concerning ¹H NMR analyses, quantitative experiments are 238

combinedwithquantitative¹³C datain ordertoconfirmspeci- 239

ation (Ballard et al.,2011; Böttingeret al.,2008; Bottinger and 240

Hasse,2008;Kimet al.,2011a;Ma’mun etal.,2006)and some 241

kineticandequilibriumstudiesonprimary,secondaryandsteri- 242

callyhinderedaminesdealwithspeciationcalculatedonlybyusing 243 1HNMRdata(Conwayetal.,2013,2011,2012;Ermatchkovetal., 244

2003;Fernandesetal.,2012;McCannetal.,2009a,b,2011a,b;Xiang 245

etal.,2012).Aswiththequantitative¹³Cexperiments,quantita- 246

tive¹HNMRdataarealsousedinstudieswherethemaingoalsare 247

thedevelopmentofthermodynamicmodels(BishnoiandRochelle, 248

2000,2002;CullinaneandRochelle,2005;Derksetal.,2010;Fan 249

etal.,2009;Hartonoetal.,2013). 250

3.2. Quantitativemeasurements:parametersanderroranalysis ²⁵¹ ThesensitivityofNMRspectroscopyisconsideredtobewell 252

suitableforengineeringapplications,asreliablequantificationof 253

peakareafractionsdownto0.05%ispossible(Maiwaldetal.,2003). 254

Toachieveaccuracywithanerroroflessthan1.0%itisimportantto 255

setproperNMRparameterssothattheintensitiesofthepeakscan 256

bedirectlylinkedtotheamountofspecies.Somefeaturesofspecific 257

importancearethesettingoftherecycledelaytimebetweeneach 258

pulse,whichmustbeatleast5timesthelongestT₁ofthenuclear 259

spinsincombinationwitha90^◦ pulseangle.Further,toimprove 260

theintegrationaccuracy,asignaltonoise(S/N)ofcirca250:1is 261

recommendedanditcanbeachievedbysettingapropernumber 262

ofscans(alsoknownastransients).Concerningtheselectionof 263

thepulsesequence,thesingle-pulseNMRsequenceiswidelyused 264

toacquirequantitative¹HNMRspectra,whileforquantitative¹³C 265

NMRexperiments,theinversegateddecouplingsequenceisagood 266

choicetoannulthedifferentialNuclearOverhauserEffect(NOE) 267

whichcreatesaninfluencebyneighbouringhydrogenatomson 268

the¹³Csignalintegralandthereforeprecludesquantitativeanaly- 269

ses.Withthenecessaryprecautionstoensurethattheacquireddata 270

reflectstherelativeratiosofthespecieswithinthesample,appro- 271

priateprocessingofthespectracanfurtherenhancetheresults 272

(BhartiandRoy,2012;Claridge,1999). 273

From our survey of NMR studies performed so far on 274

amine–CO₂–H₂Osystems,there isnotmuchinformationonthe 275

parametersthatdecideifaNMRexperimentisquantitativeornot 276

andactuallymostofthepapersdealingwithquantitativeproton 277

Fig.2.¹HNMR(a)and¹³CNMR(b)spectraofanaqueousmonoethanolamine(MEA)solutionloadedwithCO2. experimentsdonotprovideexperimentalparameters.Anexcep-

278

tionisrepresentedbyKimetal.(2011a)where,tocalculatethe

279

relativearearatiosofthepeak’sspecies,arecycledelayof1sec-

280

ondand32scansarereported(Kimetal.,2011a).Furthermore,in

281

Fernandesetal.,2012andinHartonoetal.,2013arecycledelay

282

of3.98secondsand16scans(Fernandesetal.,2012)andarecy-

283

cle delayof 2secondsand 128scans(Hartonoet al.,2013)are

284

documented,respectively.

285

Concerningquantitative¹³CNMRstudies,theyarecarriedout

286

byusingtheinversegateddecouplingsequence,butthebasisfor

287

thesetting ofrecycledelays is oftennotfully reported.T₁ val-

288

uesare indeedpresentedonlybya fewauthors.In1984,Barth

289

etal.reported¹³C NMRstudiesofaqueousalkanolamines-CO₂-

290

H₂Osystems(about0.5Mofstartingamineconcentration)usinga

291

recycledelayof150secondsbasedonestimationoftheT1which

292

waslowerthan30secondsforthe¹³Cofthecarbonate/bicarbonate

293

speciesand thecarboxylicgroupofthecarbamate(Barthetal.,

294

1984).Inthestudiesperformedonvariousaqueousalkanolamine

295

systemsbyYangetal.(2009,2012)andbyYamadaetal.(2012),

296

thelongestT₁wastakenintoaccounttosettherecycledelaytime

297

and,bytheinversionrecoverymethod,itwasestimatedtobeof

298

circa10secondsforthe¹³Cofcarbamateandcarbonates.Recently,

299

Ciftjaetal.reportedanextensivelistof¹³CT₁valuesforCO₂loaded

300

aqueoussolutionsofamines(30wt.%)andblendedamine-amino

301

acidsanditwasobservedthattherelaxationtimesforthe¹³Cnuclei

302

couldvarysignificantlywhenmovingfromsingleaminetomixed

303

aminesystems.Forinstance,ithasbeenreportedthattheT₁values

304

for¹³CofHCO3−/CO32−inthedifferentsystemsareintherangeof

305

3.5–16.3seconds.Theauthorshaveexplainedthistoberelatedto

306

thechangeinchemicalenvironmentsuchaspHandionicstrength

307

(Ciftjaetal.,2013).

308

Usually,inthesestudiesreportedabove,thenumberofscans

309

wasintherangeof32–512,dependingonseveralfactorssuchas

310

thesensitivityofNMRinstruments,concentrationofthespecies,

311

andsoon.

312

Insomeotherpapers,eveniftheT₁ valuesarenotreported,

313

long relaxation timesof the nuclei under studyare taken into

314

consideration. Indeed, Bottinger et al. have reported a recycle

315

delay up to 60seconds and up to 512 scans (Bottinger and

316

Hasse,2008).For2-amino-2-methyl-1-propanol(AMP)-CO₂-H₂O

317

andformonoethanolamine(MEA)-CO₂-H₂O-Fe²⁺,Ciftjaetal.have 318

reportedarecycledelayof150secondsand300scans(Ciftjaetal., 319

2011, 2012), whileKimetal.(2011a,b)reportedarecycledelay 320

of120sandscansupto1024forquantitativeanalysesofPZand 321

homoPZinblendwithK2CO3. 322

In1998Holmesetal.studiedammonia–CO₂–H₂O(about6M 323

ofstartingammoniaconcentration)systemsusingonly5sofrecy- 324

cledelay,eventhoughmeasuredT1valueswereof28.9and29.2s 325

forthecarbonsofHCO₃⁻/CO₃²⁻andNH₂COO⁻,respectively.Actu- 326

ally,in thislaststudyNMRspectrawereusedtodeterminethe 327

ratiosbetweencarbamateandcarbonate/bicarbonatesignalsand, 328

byusingashortrecycledelay,a6%ofvariationinthearearatios 329

betweenthepeakswasfound(Holmesetal.,1998).Similarly,in 330

2011Gotoetal.reportedquantitative¹³CNMRspectraobtained 331

witharecycledelayof30sand400scansbecause,alsointhiscase, 332

onlytheratiosbetweenaminecarbamateandcarbonatessignals 333

wereconsidered(Gotoetal.,2011a,b).Indeed,observationsthat 334

the¹³Cofcarboxylicfunctionalgroupsofaminecarbamateandof 335

bicarbonate/carbonatehavesimilarrelaxationtimesjustifytheuse 336

ofarecycledelayshorterthan5timesthelongestT1sinceonlytheir 337

ratiosareevaluated. 338

Inotherstudies,whenashortrecycledelaywasset,ahigher 339

number of scans was used, like e.g. recycle delay of 30s and 340

2000–3000scans(Barzaglietal.,2009,2010;Parketal.,2003).In 341

particular,inthestudiesbyBarzaglietal.,onlytherelativepeak 342

areasofthe¹³CatomsoftheCH₂ CH₂aminebackboneweretaken 343

intoconsideration,leadingtoquantificationofthecarbamateand 344

aminespecies,buthencenotofthecarbonates. 345

ConcerningtheerrorsanduncertaintyinquantitativeNMRanal- 346

yses,thereislittleinformationinthepapersofthisreview.The 347

uncertaintyinthedeterminationofareasofdifferentpeakswithin 348

thesamemoleculewasfoundtobe1–3%in¹HNMR(Böttinger 349

etal.,2008;Hartonoetal.,2013;McCannetal.,2009b)and2–5% 350

in¹³CNMRspectra(Barzaglietal.,2009;Böttingeretal.,2008). 351

Concerningtheerrorpropagationinthespeciescalculation,this 352

wasfoundtobe±0.1%,whereasthatinthedeterminationofthe 353

areaofpeaksusingthecurvefittingwasfoundtobe±1%(Ciftja 354

et al.,2013).In some otherpapers, theerror in percentof the 355

calculatednumberofmoleswasevaluatedwithrespecttothetotal 356

molesgivenbythesamplepreparation.InJakobsenetal.(2005), 357

Pleasecitethisarticleinpressas:Perinu,C.,etal.,NMRspectroscopyappliedtoamine–CO2–H2Osystemsrelevantforpost-combustion CO₂capture:Areview.Int.J.GreenhouseGasControl(2013),http://dx.doi.org/10.1016/j.ijggc.2013.10.029

theerrorofthecalculatedmolesofalkanolamineswasestimated

358

tobeintherangeof3.8–12.3%fortheNMRexperimentsacquired

359

at293.15Kandof8.8–24.2%forthoseacquiredat313.15K.Simi-

360

larly,inBottingerandHasse(2008)therelativedeviationsfromthe

361

totalnumberofmoleswereestimatedtobelowerthan3%forthe

362

aminesandlowerthan5%forCO₂molefractions(onlyinfewcase

363

10%deviationwasachieved)(Böttingeretal.,2008).Ingeneral,it

364

hasbeenobservedthattheaccuracyindefiningtheareaintegrals

365

isalsodependentontemperatureandelectrolyteconcentration

366

(Böttingeretal.,2008;Jakobsenetal.,2005).

367

3.3. Deuteratedandreferencesolvents

368

InliquidNMRspectroscopy,deuteratedsolventsareneededfor

369

field-frequencystabilization(calledlocking)andforhomogeniza-

370

tionofthemagneticfield(calledshimming)beforetheexperiment

371

isstarted.Inprinciple,deuteratedspeciescanalterthenatureof

372

thestudiedmixture but,generally, it is assumed (andin some

373

casedemonstrated)thatanyhydrogen-deuteriumexchangeonthe

374

aminonitrogendoesnotaffecttheelectronicNMRenvironment

375

ofthe nucleiof theneighbouringatoms, suchas CH,CH2,CH3 376

(CullinaneandRochelle,2005;Ermatchkovetal.,2003).

377

Deuterated water (D₂O) is the most used “lock” compound

378

for NMR studies of amine–CO2–H2O systems. In some works,

379

aminesolutionsarepreparedwith100%ofD2O,resultinginan

380

amine–CO₂–D₂Osystem(BishnoiandRochelle,2000,2002;Choi

381

etal.,2012;Ermatchkovetal.,2003;Fanetal.,2009),butinother

382

workssamplesarepreparedwithonly10%D2O(Ballardetal.,2011;

383

Barzaglietal.,2009,2010,2011;Ciftjaetal.,2013;Cullinaneand

384

Rochelle,2005;Derksetal.,2010;Hartonoetal.,2013;Jakobsen

385

etal.,2005;Manietal.,2006)orwith100%H2O(Ahnetal.,2011a,b;

386

Fernandesetal.,2012;McCannetal.,2009a,b,2011b;Xiangetal.,

387

2012;Yangetal.,2009).When100%H₂Oisused,the“lock”solvent

388

isusuallyinsideacapillarywhichisinsertedintheNMRtube,or

389

theexperimentsareperformedwithoutanydeuteratedsolvent,as

390

foron-lineNMRspectroscopy(Böttingeretal.,2008;Bottingerand

391

Hasse,2008).

392

Selectedcompoundsareusedaschemicalshiftreferencesol-

393

ventsand/orasstandardsforquantitativepurposeand theyare

394

used as internal or external standards. Asinternal standard, a

395

knownamountofareferenceisdissolvedinaknownvolumeof

396

sample,whereasasexternalstandarditisinsertedinasealedcapil-

397

larywhichisplacedintotheNMRtube(Ahnetal.,2011a,b;Ballard

398

etal.,2011;Conwayetal.,2012;Fernandesetal.,2012;McCann

399

etal.,2009a,b,2011b).Thechoiceofanexternalstandardcouldbe

400

motivatedbytheintentofavoidinganysortofinterferenceofthe

401

standardwiththesolution.

402

Inamine–CO₂–H₂Osystems,theNMRexperimentsaremainly

403

performedbyusingthefollowingreferencesolvents:1,4-dioxane

404

(Ballardetal.,2011;Choietal.,2012;Ciftjaetal.,2011,2012,2013;

405

Fanetal.,2009;Hartonoetal.,2006;Holmesetal.,1998;Jakobsen

406

etal.,2005;Kimetal.,2011a;Yangetal.,2012),acetonitrile(Ahn

407

etal.,2011a,b;Barzaglietal.,2009,2010,2011;Manietal.,2006),

408

3-(trimethylsilyl)-propionicacidsodiumsalt(Conwayetal.,2012;

409

Fernandesetal.,2012;McCannetal.,2009a,b,2011b;Xiangetal.,

410

2012)ortetramethylsilane.(Barthetal.,1984;Yamadaetal.,2012;

411

Jakobsenetal.,2008;Yangetal.,2009)

412

3.4. Quantificationoffast-exchangingprotonspecies

413

Awell-knownphenomenainNMRisthatinter-andintramolec-

414

ularexchangingnucleimayleadtomodulationsoftheNMRsignals.

415

Dependingonhowfastthechemicalexchangerateis,thedifferent

416

signalsmaycoalesceandappearatanaveragechemicalshift,which

417

valuealsodependsontherelativeamountofthespeciescompon-

418

ingthesignal.Inthecontextofthepresentwork,intermolecular

419

exchangingprotonspeciesappearwitha commonpeak.Inpar- 420

ticular,it is not possibletodistinguish betweenmolecularand 421

protonatedformsoftheamines,aswellasbetweenbicarbonate 422

andcarbonate,andonlythesumoftheirconcentrationscanbe 423

quantifiedbyNMR(Fig.2).However,toassesstherelativeamount 424

ofeachofthem,variousmethodshavebeenutilized. 425

In1996,SudaandMimuracalculatedthedistributionofallthe 426

chemicalspeciesinamine–CO2–H2Osolutionsbycombiningthe¹H 427

peakareaofcarbamateandamine/protonatedaminetothecharge 428

andmaterialbalances,anddissociationconstantsandionproduct 429

ofwater(SudaandMimura,1996).Later,in1998Holmesetal. 430

performed¹³CNMRanalysesofammonia–CO2–H2Omixturesand, 431

bytakingintoaccountthepeakpositions,thearearatiosandthe 432

carbonbalance,theypresentedamethodtocalculatetheconcen- 433

trationsofcarbonate,bicarbonateandcarbamate(Eqs.(8)–(10)) 434

(Holmesetal.,1998). 435

HCO3−

= 168.09−S

(168.09−160.33)(1+R)[CO2]₀ (8) 436

CO32−

= S−160.33

(168.09−160.33)(1+R)[CO2]₀ (9) 437

NH₂COO⁻

= R

1+R[CO₃]₀ (10) 438

Intheseformulas((8)–(10)),160.33and168.09arethechemical 439

shiftvaluesfor100%bicarbonateand100%carbonate,respectively; 440

Sisthechemicalshiftofthecarbonate/bicarbonatesignalobserved 441

inthe¹³CNMRspectraofCO2loadedaminesolution;Ristheratio 442

oftheareaofcarbamatepeaktotheareaofbicarbonate/carbonate 443

signaland[CO₂]₀isthecarbonbalance. 444

Intheliteraturethismethodwasapplied,notonlyforammonia 445

chemicalsystems(Ahnetal.,2011a;Pellegrinietal.,2010),butalso 446

forthecalculationofcarbonateandbicarbonateconcentrationsin 447

AMP–CO2–H2Osystems(Ciftjaetal.,2011).Inthislastcase,the 448

molecularandprotonatedaminesweredeterminedaccordingto 449

thedissociationconstantoftheamineandtothepHmeasuredfor 450

eachloadedsample,asalsoreportedinsomeotherNMRstudies 451

(Fanetal.,2009). 452

Tocalculatetherelativeamountsoftworapidlyequilibrating 453

componentsshowingacommonsignalintheNMRspectra,cali- 454

brationexperimentshavealsobeencarriedout.Standardaqueous 455

solutionsoffreeamineandprotonatedamine(amineH⁺),aswellas 456

ofHCO₃⁻andCO₃²⁻,arepreparedandmixedindifferentappropri- 457

ateratiosandthevariationsinchemicalshiftsareplottedagainst 458

thevariableparameter.Ingeneral,foraminesitisobservedthat 459

theprotonationhasamajoreffectonatomsdistanttwocovalent 460

bondsfromthesiteofprotonation.Specifically,thesignalsofthe 461

carboninbetaandoftheprotoninalphapositiontothenitrogen 462

ofaminesaremoreinfluencedbyprotonationthanothernuclei, 463

resultinginsignificantchangesinthechemicalshift(Jakobsenetal., 464

2008).Concerningthecarbonatespecies,increasingthebicarbon- 465

ate/carbonateratioleadstoashifttowardslowerppmvaluesof 466

thesignalinthe¹³CNMRspectra.Thefirsttimesuchcalibration 467

experimentswerereporteddatesbackto1982,whereAbbottetal. 468

acquired¹³CNMRspectraofCO₂(aq),HCO₃⁻(aq)andCO₃²⁻(aq) 469

forawiderangeofpHs(Abbottetal.,1982).Later,in2003,Parketal. 470

identifiedprotonatedorganicspeciesbycomparingthe¹³CNMR 471

spectraoftheloadedaminesolutionswiththoseofaqueousamine 472

solutionspreparedbyaddingdifferentaliquotofhydrochloricacid 473

(HCl)(Parketal.,2003).In2005,Jakobsenetal.performedtitra- 474

tioncurvesforaseriesofamine/amineH⁺andforHCO₃⁻/CO₃²⁻, 475

reportingthechangeinchemicalshiftofthe¹³Csignalsofthecal- 476

ibratingsolutionsasafunctionofthepH(Jakobsenetal.,2005).In 477

similarstudies,thechangeinchemicalshifthavebeenplottedas 478

afunctionofthespeciesratios(Barzaglietal.,2009,2010,2011; 479

Manietal.,2006;Shietal.,2012b),sincetheresonancefrequency 480

ofthe¹³Cnucleidependsontheelectronicchemicalenvironment

481

oftheatomsofthefast-exchangingprotonspecies.

482

For quantitative purposes, the calibration experiments per-

483

formedso farare exclusively based on¹³C NMR spectroscopy.

484

However,toinvestigatethe2-(2-aminoethylamino)ethanol(AEEA)

485

absorbent,titratedwithHClorloadedwithCO₂at293.15K,qual-

486

itative¹HNMRexperimentswerealsocarriedout(Jakobsenetal.,

487

2008).

488

3.5. VariabletemperatureandhighpressureNMRexperiments

489

NMR experiments on amine–CO₂–H₂O systems are mostly

490

performedonsamples withdrawnfrom theequilibratedamine

491

solutionsaftertheabsorptionand/ordesorptionprocesses.How-

492

ever,therearereportedNMRstudieswithvariabletemperature

493

and/orpressure(Ahnetal.,2011a;Böttingeretal.,2008; Bottinger

494

and Hasse,2008;Cullinaneand Rochelle,2005;Jakobsen etal.,

495

2005;Parketal.,2003).ThesesortsofNMRstudiescanprovide

496

informationnotonlyonthespeciesdistributionbutalsoonthe

497

CO2solubilityinaqueousamines(Parketal.,2003;Tomizakietal.,

498

2010).

499

Withvaryingtemperatures,theenvironmentofthemagnetic

500

nucleibecomesdifferentand thesechangesareobservedinthe

501

NMR spectrabecauseof theexchange betweensitesand other

502

dynamicprocesses.In2005,Jakobsenetal.investigatedtheliq-

503

uidphasecompositionofMEA-,N-methyldiethanolamine(MDEA)-

504

and2-(butylamino)ethanol(BEA)-CO2-H2Osystems,acquiring¹³C

505

NMRspectraattemperaturesrangingfrom293.15Kto363.15K.

506

Quantitativeanalysiswasperformedonlyforspectraacquiredat

507

293.15Kand313.15K,becausebroadeningofpeakswereobserved

508

athighertemperatures(Jakobsenetal.,2005).Importantly,dur-

509

ing the study of ammonia–CO₂–H₂O systems at temperatures

510

higher than 343.15K, CO2 bubble formations occurred. It was

511

alsoobservedthat,atincreasedtemperatures(293.15–333.15K),

512

the ammonia carbamate peak was not affected by tempera-

513

turechange,whilethecarbonate/bicarbonatesignalsignificantly

514

movedtowardslowerchemicalshiftinthe¹³CNMRspectra,which

515

correspondstoanincreaseofbicarbonatetothedetrimentofcar-

516

bonate(Ahnetal.,2011a).

517

HighpressureNMRexperimentscanbeperformedusingspe-

518

cific high-pressure NMR tubes, which are made of chemical

519

resistantglassandhermeticstoppersandthesamplescanbekept

520

undervacuumorpressureforlongtime(Parketal.,2003).More-

521

over,forstudiesunderprocessconditions,flowNMRprobescan

522

beusedinawiderangeoftemperaturesandpressures.On-line

523

NMRexperimentswerecarriedoutforthestudyofindividually

524

andblendedMEA,diethanolamine(DEA),MDEAandPZ,applying

525

pressuresupto25barand temperaturesbetween293.15K and

526

353.15K(Böttingeretal.,2008;BottingerandHasse,2008).The

527

setupofon-lineNMRexperimentsrequiressystematicstudiesto

528

optimizeflowrates(whichaffectmagnetizationandrelaxation)

529

andtoperformsolventsuppression.Sincedeuteratedsolventsare

530

tooexpensivetobeusedinprocessengineeringapplicationslike

531

this,nofield-frequencystabilization(lock)canbeusedandNMR

532

magnetswithexcellentfieldstabilityisaprerequisiteaswellas

533

accuratehomogeneityofthemagneticfield(Maiwaldetal.,2003).

534

4. Resultsandapplications

535

4.1. Speciationofamine–CO2–H2Osystemsinabsorption

536

experiments

537

Asdescribedabove,speciationsofamine–CO₂–H₂Osystemsare

538

of particularinterest in order to deriveinformation onspecies

539

Fig.3. Stacked¹HNMRspectrafor5.0MMEAsolutionwithvaryingCO2loadingat 295.65K.

ReprintedwithpermissionfromFanetal.(2009).Copyright^©(2009),American ChemicalSociety.

distributionandhypothesisonreactionmechanism,aswellasfor 540

kineticandthermodynamictasks. 541

Typically,inaMEA–CO2–H2Osystem,atincreasingCO2 load- 542

ings, an increment of aminecarbamate is observed, as wellas 543

of bicarbonate and protonatedamines. AtCO₂ loadings higher 544

than0.5molCO₂/molMEA,theprotonatedamineconcentration 545

continuestoriseattheexpenseofcarbamate,andthereleasedcar- 546

bondioxidewillreacttobicarbonate(BottingerandHasse,2008; 547

Jakobsenetal.,2005).Characteristic¹Hand¹³CNMRspectraof 548

MEAatvaryingCO2loadingsareshowninFig.3(Fanetal.,2009) 549

andFig.4(Jakobsenetal.,2005),respectively. 550

AcommonfeatureobservedisthatbyvaryingtheCO₂loading 551

mostoftheNMRsignalschangepositions.Thesignalscorrespond- 552

ingtothenucleiofMEA/MEAH⁺shifttotheleft(highppmvalues) 553

inthe¹HNMRspectraandtotheright(lowppmvalues)in¹³C 554

NMRspectra,duetoanincreaseofprotonatedamines.Ontheother 555

hand,the¹³CsignalcorrespondingtoHCO3−/CO32−movestowards 556

lowerchemicalshifts,duetoanincreaseofbicarbonateconcentra- 557

tion.Concerningthecarbamatespecies,thechangein¹Hand¹³C 558

chemicalshiftsatincreasingCO2loadingsismuchsmallerthanthat 559

observedfortheamines.Inthisregard,bya¹HNMRinvestigation, 560

McCannetal.in2011suggestedthatthe COO⁻groupofcarba- 561

mateamineisprotonated( COOH),ratherthanthenitrogengroup 562

aspreviouslysuggestedwiththezwitterionicmechanism(Caplow, 563

1968;McCannetal.,2011b). 564

Similarly, quantitative and qualitative information on 565

amine–CO₂–H₂O reaction products with different chemical 566

structures, such as secondary, tertiary and sterically hindered 567

amines, can be gathered by the analyses of NMR spectra. For 568

instance,Choietal.in2011performeda¹³CNMRstudyonAMP, 569

aprimarysterichinderedamine,andMDEA,atertiaryamine,and 570

demonstrated that the absorptionreactions involvecompletely 571

different mechanisms. AMP and MDEA have high absorption 572

capacities and follow the same pathway (Eq. (7)) in absorbing 573

CO₂.MDEAshowsahighbicarbonateconcentrationevenatlow 574

loadings,whereasinAMPsolutionsthebicarbonateconcentration 575

increasesatincreasingloadingsasprimaryandsecondaryamines 576

(Choietal.,2012). 577

Ammoniasolutionshavealsobeeninvestigatedandtheanal- 578

ysesoftheNMRspectrahaveledtohypothesisonthereaction 579

mechanism.Inparticular,ithasbeenobservedthatastheabsorbed 580

amountofCO2intheammoniasolutionincreases,thecarbamate 581

peakdecreases,butthatofbicarbonate/carbonateincreases.More- 582

over,bythedropinpH,theamountofbicarbonateincreases,asit 583

isdetectedintheNMRspectrawherethebicarbonate/carbonate 584

Pleasecitethisarticleinpressas:Perinu,C.,etal.,NMRspectroscopyappliedtoamine–CO2–H2Osystemsrelevantforpost-combustion CO₂capture:Areview.Int.J.GreenhouseGasControl(2013),http://dx.doi.org/10.1016/j.ijggc.2013.10.029

Fig. 4.Stacked¹³CNMRspectrafor30wt.%MEAsolutionwithvaryingCO2loading at293.15K.

ReprintedwithpermissionfromJakobsenetal.(2005).Copyright^©(2005),American ChemicalSociety.

signal moves towards lower ppm values (Ahn et al., 2011a,b;

585

Holmesetal.,1998;Manietal.,2006).

586

Recently,Ciftja et al.reported1D and 2DNMR experiments

587

toidentifythemainoxidativedegradationproductsofMEA(5M)

588

loadedwithCO2inthepresenceofFe²⁺(aq).Inparticular,¹³CNMR

589

experimentswereusedtoquantifytheidentifieddegradationprod-

590

ucts,suchas N-(2-hydroxyethyl)imidazole (HEI), 2-oxazolidone

591

(OZD),N-(2-hydroxyethyl)formamide(HEF).TheNMRresultswere

592

ingoodagreementwiththoseobtainedbyGC/LC–MSonthesame

593

solutionsand,althoughN-(2-hydroxyethyl)acetamide(HEA)was

594

notquantified,someadditionalsignalsintheNMRspectrawere

595

detectedbutthecorrespondingcompoundswereneitheridentified

596

norquantifiedinthisstudy(Ciftjaetal.,2012).

597

In2007,Hartonoetal.performed¹³CNMR investigationson

598

aqueousdiethylenetriamine(DETA)toidentifythespeciesreally

599

formedin solution after CO₂ absorption, among the18 poten-

600

tialDETAspecies.AlthoughDETAhasthreenitrogenatomsinthe

601

molecule,whichcanreactwithCO2,notracesoftricarbamatewere

602

observed(Hartonoetal.,2006).

603

In 2008 Jakobsen et al. performed NMR studies and

604

quantum mechanical calculations for the study of 2-(2-

605

aminoethylamino)ethanol (AEEA), a diamine novel absorbent.

606

AEEA,primary carbamateAEEA andsecondary carbamate AEEA

607

werethemajorspeciesidentifiedbyNMR.Inthisstudy,theNMR

608

chemicalshiftswerealsopredictedbyquantummechanicalcal-

609

culationsandcomparedtoexperimentalNMRspectra.Theoverall

610

resultssuggestedthatmostspeciesarepopulatedbyconformers

611

withsome degree of intramolecular hydrogen bondings which 612

couldinfluencetheabsorptionreactions(Jakobsenetal.,2008). 613

4.2. Speciationofblendedamine–CO2–H2Osystemsin ⁶¹⁴

absorptionexperiments ⁶¹⁵

Mixedamineshavebecomeattractivebecausethebestchar- 616

acteristics of the single amines can be combined to improve 617

theefficiencyofCO₂ absorption(likee.g.increasingtherateof 618

absorption).Problemsofdataanalysesmightappear duetothe 619

largercomplexity of thesolutions but, even if the spectracan 620

be quite complex, information from ¹³C NMR experiments on 621

the species distribution in the liquid phase of amine mixtures 622

canbegathered.BlendsofPZandMDEAhavefoundwidespread 623

considerationduetothehighabsorptionrateof PZ(whichrate 624

constantis foundtobeafactor of10higher thanthatofother 625

alkanolamines, suchasMEA) and thelow reaction enthalpyof 626

MDEA(BishnoiandRochelle,2002;Fuetal.,2012).TheNMRstud- 627

ies onthis type of blendsare mainly dealing withthe species 628

distribution for input to thermodynamic models (Bishnoi and 629

Rochelle, 2002; Böttinger et al., 2008; Derks et al., 2010). In 630

theseworksthespecieshavebeenquantifiedby¹H(Bishnoiand 631

Rochelle,2002;Derksetal.,2010)and/or¹³CNMRexperiments 632

(Böttingeretal.,2008)and,ifneeded,thechemicalstructureshave 633

beenidentifiedbyHSQCexperiments.Forinstance,inBottinger 634

etal.thefollowingcomponentswereidentified andquantified: 635

PZ/protonatedPZ,carbamate/protonated carbamatePZ,dicarba- 636

matePZ,MDEA/protonatedMDEA,dimethyl-diethanolamineion 637

(DMDEA⁺),DEA/protonatedDEA,carbonate/bicarbonateandcar- 638

bondioxide.Moreover,productsformedfromDEAandunidentified 639

byproducts were observed but not quantified (Böttinger et al., 640

2008).Thisstudypointsoutthat,althoughthehighpotentialofthis 641

blend,degradationandsecondaryproductscanbeissuesofconcern 642

foritsapplicationinPCCtechnology.However,hypothesisonreac- 643

tionmechanismscouldleadtoarationaldesignofamine/blended 644

aminesystemsaccordingtothedemandsinthisfield. 645

Recently,Ballardetal.havepublishedaNMRstudyonthereac- 646

tionofCO₂withaseriesofmixedaqueousaminesystems(MEA, 647

N-methylethanolamine(MMEA),MEA-PZ,MEA-MDEA,MMEA-PZ). 648 13CNMRexperimentswereperformedtocalculatecarbamateand 649

bicarbonateconcentrations(itwasassumedthatnocarbonatewas 650

formed)atincreasingloadingsandreactiontime.Quantitative¹H 651

NMRexperimentswereusedtoconfirm¹³Cquantitativeresults, 652

whereas HSQC NMR spectra wereperformed todefine or con- 653

firmstructures.Observationoftrendsofthespeciesdistributionin 654

blendedamines–CO2–H2Osystemsledtothefollowinghypothesis 655

onthemechanismofreaction:Forprimaryandsecondaryamines 656

therelativeratesofreactionofeachaminearenotjustameasureof 657

theratecoefficientsforcarbamateformation,butmoreareflection 658

oftherelativethermodynamicstabilitiesofeachaminecarbamate 659

(Ballardetal.,2011). 660

A qualitative ¹³C NMR study on amine aminoacids salts, 661

which are an attractive alternative to alkanolamines, is also 662

found in literature (Hartono et al., 2011). In order to observe 663

the neutralization effect of the aminoacids in loaded samples, 664

sarcosine (SAR)wasblended withorganic and inorganicbases, 665

3-(methylamino)propylamine(MAPA)andpotassium(K⁺),respec- 666

tively.Itwasobservedthat,inthepresenceofCO₂,theMAPA-SAR 667

system showed more species than theSAR-K system and was 668

explainedtobeprobablydue tothepresence ofthecarbamate 669

speciescomingfromthereactionofCO₂withbothSARandMAPA, 670

which limits thecomplete neutralizationof the aminoacids by 671

blendingequinormalamounts. 672

Moreover,¹³CNMRspectroscopyhasalsobeenappliedtoiden- 673

tifyreactionproductspeciesinastudyonthemasstransferofCO2 ⁶⁷⁴