co 2 capture in alkanolamine-rtil blends via carbamate crystallization:...

CO2 Capture in Alkanolamine-RTIL Blends via CarbamateCrystallization: Route to Efficient RegenerationMuhammad Hasib-ur-Rahman and Faïcal Larachi*

Department of Chemical Engineering, Laval University, Quebec QC, G1 V 0A6 Canada

*S Supporting Information

ABSTRACT: One of the major drawbacks of aqueous alkanolamine basedCO2 capture processes is the requirement of significantly higher energy ofregeneration. This weakness can be overcome by separating the CO2-capturedproduct to regenerate the corresponding amine, thus avoiding the consumptionof redundant energy. Replacing aqueous phase with more stable and practicallynonvolatile imidazolium based room-temperature ionic liquid (RTIL) provideda viable approach for carbamate to crystallize out as supernatant solid. In thepresent study, regeneration capabilities of solid carbamates have been investigated.Diethanolamine (DEA) carbamate as well as 2-amino-2-methyl-1-propanol (AMP)carbamate were obtained in crystalline form by bubbling CO2 in alkanolamine-RTIL mixtures. Hydrophobic RTIL, 1-hexyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([hmim][Tf2N]), was used as aqueous phasesubstituent. Thermal behavior of the carbamates was observed by differentialscanning calorimetry and thermogravimetric analysis, while the possible regeneration mechanism has been proposed through13C NMR and FTIR analyses. The results showed that decomposition of DEA-carbamate commenced at lower temperature(∼55 °C), compared to that of AMP-carbamate (∼75 °C); thus promising easy regeneration. The separation of carbamate assolid phase can offer two-way advantage by letting less volume to regenerate as well as by narrowing the gap between CO2capture and amine regeneration temperatures.

■ INTRODUCTIONAnthropogenic industrial activities are causing serious increasein atmospheric concentration of greenhouse gases; and carbondioxide, being the most important of these in perspective of itscontributions toward global warming, is considered as the maincause of environmental problems in this regard.1−4 Major CO2emission sources that offer potential capture convenience com-prise fossil-fuel based power generation installations.5

Various measures are being explored to check CO2 emissionsfrom large point sources into the atmosphere. These includephysical/chemical sorption, membrane separation, and cryo-genic distillation techniques. In industry, the most preferred gasabsorption processes comprise alkanolamine based aqueous sol-vents executing absorber-stripper arrangements, and can principallybe used for postcombustion CO2 capture.

5−7 At temperaturesaround 40 °C aqueous solutions of primary and secondary amines,such as monoethanolamine (MEA), diethanolamine (DEA)respectively, are subjected to absorb CO2 through carbamateformation whereas tertiary amines, such as N-methyldiethanol-amine (MDEA), along with water react with the sour gas to formammonium bicarbonate. In case of primary/secondary amines,predominantly one mole of CO2 reacts with two moles of amineobeying the following mechanism (eqs 1 and 2):8,9

+ ′ ⇌ ′ + −CO RR NH RR NH COO2 (1)

′ + ′ ⇌ ′ + ′+ − + −RR NH COO RR NH RR NH RR NCOO2(2)

However, in presence of water, tertiary amines react with CO2in 1:1 molar ratio, as shown below (eqs 3 and 4):

+ ⇌ ++ −CO H O H HCO2 2 3 (3)

′ ″ + ⇌ ′ ″+ +RR R N H RR R NH (4)

Then the regeneration of these solvents is carried out by heatstripping at temperatures in the range of 100−140 °C.5 In case ofprimary/secondary aqueous alkanolamines, the followingregeneration mechanism (eqs 5 and 6) has been proposed:8,10

′ + ⇌ + ′ +− −RR NCOO H O CO RR NH OH2 2 (5)

′ + ⇌ ′ ++ −RR NH OH RR NH H O2 2 (6)

While regeneration of tertiary amines occurs as follows (eqs 7and 8):

⇌ +− −HCO CO OH3 2 (7)

′ ″ + ⇌ ′ ″ ++ −RR R NH OH RR R N H O2 (8)

Nevertheless, there are many downsides of these CO2 capturesystems like low gas loading, degradation/evaporation of amines,and corrosion of equipment.11−13 Higher regeneration energy

Received: June 21, 2012Revised: August 15, 2012Accepted: September 10, 2012Published: September 10, 2012

Article

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© 2012 American Chemical Society 11443 dx.doi.org/10.1021/es302513j | Environ. Sci. Technol. 2012, 46, 11443−11450

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requirement is one of the major drawbacks of aqueousalkanolamine based state-of-the-art technologies. In a powergeneration plant, up to 40% additional energy is required forcarbon dioxide capture and storage (CCS). Out of this extra bite,roughly 50% is consumed in regeneration step alone.5

Recently, unique room-temperature ionic liquids (RTILs),owing to their tunable physicochemical characteristics, have beenemerging as potential contenders for CO2 capture.

6,14 In thiscontext, thermally stable imidazolium based RTILs are beinginvestigated extensively as prospective alternates.15−19 Pressureswing technique can be used to regenerate such solvents. How-ever, like other physical solvents such as methanol, dimethylethers of polyethylene glycol (currently being used industrially asrectisol/selexol processes), these alone cannot be employedeffectively for separating CO2 from flue gases with low CO2partial pressures.20,21 Neither aqueous alkanolamines nor RTILssolely are proficient enough for economical CO2 separation.In search of an efficient CO2 separation process, various

methodologies are being scrutinized. These include amino func-tionalized solid adsorbants, task specific ionic liquids, as well assupported ionic liquid membranes.14,22 Work has also beeninitiated to combine the advantages of RTILs with those ofprimary/secondary alkanolamines, and in this regardCamper et al.were the first to report MEA-carbamate precipitation in amine-RTIL solution.23−26 In case of alkanolamine solvents, replacingaqueous phase with more stable room-temperature ionic liquid(RTIL) can avoid the corrosion and equilibrium limitation prob-lems particularly arising due to the presence of water. Moresignificantly, the presence of RTIL provides the favorable en-vironment for CO2-captured product to crystallize out, thusmaking it possible to easily separate the solid carbamate from theliquid counterpart in addition to completing the reaction to itsfull stoichiometric potential. As CO2 is about 3 times moresoluble (in terms of moles of CO2 per volume of the solvent) inimidazolium based RTILs than in water,17,27,28 this new approachof CO2 absorption in alkanolamine-RTIL mixtures can ensuregreater mass transfer capacity thus compensating to a certainextent the downside posed by higher viscosity of the ionic liquids.The objectives of this study were to look for an apposite

alkanolamine-hydrophobic RTIL combination that can (a) guar-antee stoichiometric maximum CO2 loading by evading equi-librium constraints; (b) minimize stripping temperatures; (c)manage less volumes to regenerate through separation of CO2-captured product thus letting ensue probable cut down of thegratuitously high regeneration energy to affordable limit. Theoverall concept has been envisaged in Figure 1.The current activity was focused on looking into the regenera-

tion scenario of CO2 absorption process comprising AMP/DEA-RTIL blends. Single crystal X-ray diffraction technique and 13CNMR/FTIR analyses were employed to infer the nature ofCO2-captured products and the regenerated amines. Whereasdecomposition behavior of solid carbamates, obtained by bubbl-ing CO2 through amine-RTIL blends containing either 2-amino-2-methyl-1-propanol (AMP) or diethanolamine (DEA), has beeninvestigated in detail using differential scanning calorimetry (DSC),thermogravimetry (TG), 13C NMR, and FTIR techniques.

■ EXPERIMENTAL SECTIONMaterials. 2-Amino-2-methyl-1-propanol (AMP: purum,

≥97.0%) and Diethanolamine (DEA: ACS reagent, ≥99.0%)were purchased from Sigma-Aldrich, and Triton X-100(t-Octylphenoxypolyethoxyethanol, a nonionic surfactant) wasobtained from EMD Chemicals. IoLiTec Inc. supplied RTIL,

1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([hmim][Tf2N]: 99% purity). While carbon dioxide and nitro-gen gases (≥99% purity) were obtained from Praxair CanadaIncorporation. All the materials were used as received.

Procedures and Techniques. CO2 Capture Studies. Gasabsorption studies were carried out by thermogravimetric anal-yzer (Perkin-Elmer Diamond TG/DTA) under carbon dioxideatmosphere isothermally at 35 °C. For this purpose, 18 (±1) mgsample (amine-RTIL mixture) was loaded in an aluminumpan and placed in the analyzer under N2 atmosphere. Thenthe sample was exposed to pure CO2 to obtain CO2 uptakeprofile. Mass flow meters were used to adjust gas flow rates at100 mL/min.

Figure 2. CO2 absorption isotherm for alkanolamine-[hmim][Tf2N]systems obtained at atmospheric pressure and 35 °C temperature.

Figure 3. Evaporation profiles of amines (in amine-RTIL blends) at35 °C under N2.

Figure 1. The simplified process flow diagram of alkanolamine-RTILbased CO2 capture process.

Environmental Science & Technology Article

dx.doi.org/10.1021/es302513j | Environ. Sci. Technol. 2012, 46, 11443−1145011444

Prior to gas absorption capacity measurements by thermo-gravimetric analyzer, alkanolamine-RTIL samples were preparedusing Omni homogenizer (Omni International, Kennesaw, GA)fitted with rotor-stator generator. Fifteen wt% amine (AMP/DEA) was mixed in [hmim][Tf2N]. Though, in case of DEA/[hmim][Tf2N] blend, Triton X-100 surfactant was added tostabilize the homogeneity of the mixture.In order to get solid carbamates, CO2 was bubbled through

15 wt % amine-RTIL blends (without surfactant) at 35 °C alongwith continuous stirring for two hours. The suspension obtainedas a result of carbamate crystallization was allowed to stand for48 h to help the two phases settle apart. This enabled easyseparation of supernatant crystals that were washed thoroughlywith acetone, dried and stored at room temperature.Carbamate Characterization. To know the nature of the

CO2-captured products (AMP-carbamate, DEA-carbamate), 13CNMR spectra were recorded on a Varian Inova Spectrometer(Palo Alto, CA) at a frequency of 100 MHz with proton de-coupling, after dissolving the crystals in DMSO-d6 solvent (CNDIsotopes, QC, Canada). Whereas a Nicolet Magna 850 spec-trometer (Thermo Scientific, Madison, WI) equipped with hightemperature Golden Gate ATR accessory was used to performFTIR analysis, and Single crystal X-ray diffraction techniqueprovided the detailed information about crystalline structures.Regeneration Behavior. Amine regeneration studies were

carried out using thermogravimetric (TG) analyzer and differ-ential scanning calorimetry (DSC). In case of TG analysis,9 (±1) mg of ground carbamate sample was taken in an aluminumpan and the analysis was conducted using a heating rate of 5 °Cperminute. The regeneration behavior of carbamates was studiedunder two different environments, that is, pure N2, and pureCO2. The onset temperature for carbamate decomposition underN2 atmosphere, at which gas evolution started, was detected byquadrupole mass spectrometer (Thermostar Prisma QMS200,Pfeiffer VacuumGmbH, Asslar, Germany) coupled with thermo-gravimetric analyzer. The gas flow rate was maintained at100mL/min. To ensure the reproducibility, each experiment wasrepeated at least once. Differential scanning calorimetric analyseswere performed using a Mettler-Toledo DSC1 (Columbus, OH)instrument. DSC scans were also managed at a temperature scan

rate of 5 °C per minute. 13C NMR and ATR-FTIR techniqueswere employed to confirm the likely regeneration mechanism.

■ RESULTS AND DISCUSSIONMaximum Gas Capture Capacity. CO2 absorption in

AMP-RTIL and DEA-RTIL blends resulted in crystallization ofthe product. This development enabled the product (carbamate)to move out of the reaction phase and hence helped over-come the equilibrium limitation barrier thus not only allowingmaximum CO2 loading but also enabling easy separation of thesolid product.25 However, due to higher volatility of AMP,29,30

regarding AMP-RTIL combination, it was not possible to main-tain the initial concentration of amine in AMP-RTIL blends. Andso the CO2 capture capacity apparently appeared inferior towhat the theoretical maximum would have been with respect toinitial AMP concentration (Figure 2). The evaporation pheno-menon was quite evident from the mass loss profile of AMP-RTIL blend acquired under N2 atmosphere at 35 °C (Figure 3).In order to verify the CO2 capture capacity in case of AMP-

RTIL blend, the resulting AMP carbamate was titrated against1 M HCl to release captured gas, using Chittick apparatus. Thispractice substantiated the 50 mol % absorption limit of CO2 (wrtAMP) in AMP-RTIL blend. The procedure has been describedin the previous work.26

However, no detectable evaporation loss was observed in caseof emulsified DEA-RTIL mixture under the specified conditions,and CO2 capture resulted in theoretical maximum mass uptake(0.5 mol of CO2 per mole of DEA, in accordance with themechanism proposed by Caplow8).CO2 capture studies at ambient conditions using DEA/[hmim]-

[Tf2N] emulsion has been discussed in our previous study.25

Nature of CO2-Captured Products. Single crystal structuredetermination confirmed the formation of carbamate product,originating from chemical interaction of CO2 with amine;both (AMP-carbamate and DEA-carbamate) possessing mono-clinic crystal system with P21/n and Pn space groups respec-tively (Figure 4; see also Supporting Information (SI)). Ap-pearance of additional 13C NMR signals at 162.59 ppm and162.57 ppm, regarding corresponding CO2-captured products(AMP-carbamate andDEA-carbamate, respectively), also validatedthe CO2 absorption exclusively through carbamate formation.

Figure 4. Packing diagrams: (a) AMP-carbamate; (b) DEA-carbamate (reproduced with permission25).



These outcomes were further complemented by FTIR analysis(Figures 5 and 6).As is observed in case of aqueous AMP based CO2 separation

processes, AMP being a sterically hindered amine favors CO2absorption via bicarbonate formation owing to water involve-ment that can guarantee higher sorption capacity. On the otherhand, in present work, absence of water prohibited the forma-tion of bicarbonate species, limiting the gas capture capacityto 50 mol % of CO2. Thermogravimetric isotherms as well asChittick apparatus measurements also confirmed the same out-come as CO2 capture capacity never exceeded 0.5 CO2/amine

molar ratio. DEA interacts with CO2 preferably through zwit-terion mechanism yielding carbamate product in either case,regarding aqueous DEA or DEA-RTIL blends.The detailed description of crystal structure determination of

AMP-carbamate is provided in the SI file, whereas single crystalX-ray diffraction study of DEA-carbamate has been discussed inthe previous work.25

Regeneration Ability. Regeneration was brought about bythermal decomposition of carbamates at 110 °C that resulted inquick release of CO2 and corresponding alkanolamine (AMP/DEA). 13C NMR as well as ATR-FTIR analyses of fresh and

Figure 5. (a) FTIR spectra, and (b) 13C NMR spectra: AMP (fresh amine), AMPC (AMP-carbamate) and RAMP (regenerated AMP).



regenerated amines demonstrated the excellent regenerationability of both AMP and DEA. Theoretically, the probable mech-anism might comprise the following reactions (eqs 9 and 10)responsible for CO2 liberation during heat treatment.

′ → ′ +− Δ −RR NCOO RR N CO2 (9)

′ + ′ → ′− +RR N RR NH 2RR NH2 (10)

The FTIR as well as 13C NMR spectra of fresh/regeneratedamines and relevant carbamates are shown in Figures 5 and 6.The emergence of respective carbon signals in 13C NMR spectra

at 162.59 ppm and 162.57 ppm (Figures 5b and 6b) confirmedthe CO2 absorption via AMP-carbamate and DEA-carbamateformation. Two series of carbon signals (compared to one seriesfor corresponding fresh amine) in the range of 20−80 ppm, oneoriginating from protonated amine and the other from carbamatemoiety, also complemented the findings. Besides, the identicalnature of NMR spectra of fresh and regenerated amines ruled outany probability of degradation occurrence at least after singleabsorption/desorption cycle. FTIR analysis (Figures 5a, 6a) toorevealed the same outcome.

Amine (AMP/DEA) Regeneration Behavior. Under N2atmosphere, decomposition of AMP-carbamate commenced

Figure 6. (a) FTIR spectra, and (b) 13C NMR spectra of DEA (fresh amine), DEAC (DEA-carbamate) and RDEA (regenerated DEA).



around 75 °Cwith CO2 liberation, accompanied by simultaneousevaporation of amine (Figure 7). Whereas, DEA-carbamate starteddecomposing at much lower temperature (∼55 °C) and thetransition was completed at about 70 °C, as is evident from TG/DSC plots in Figure 8. In case of TG profile of AMP-carbamate,the weight loss can be seen originating much before the de-composition onset temperature. AMP-carbamate, owing to itsunstable nature in humid air,31 most probably underwent hydro-lysis to some extent generating free amine during samplegrinding/mounting process; the evaporation of which resulted inmass loss as appeared in TG plot prior to the commencement ofcarbamate decomposition.To detect CO2 release, QMS was coupled with TG. The QMS

signals showed the evolution of CO2 above 70 °C in case ofAMP-carbamate, and around 55 °C in case of DEA-carbamate(Figure 9); thus complementing the TG/DSC analyses out-comes. The temperature was increased at the rate of 5 °C/minunder N2 (100 mL/min flow rate) and continued until thepositive molecular ion current intensity, originating from CO2

+

(m/z = 44), reached the initial levels.Quite prolonged release of CO2, as appears in ion current

versus time plots (obtained via QMS), might be due to thefoaming buildup as well as slow heat transfer at lower tem-peratures (above decomposition point). Variations in ion cur-rent intensity possibly were fallout of change in foaming make-up with temperature. The foaming phenomenon was alsoobserved during ATR-FTIR analysis while studying regenerationbehavior.Thermal decomposition temperatures of both AMP-carbamate

and DEA-carbamate were also verified through temperature-programmed FTIR analysis, revealing the disappearance of

carbamate absorption peaks above 70 and 50 °C respectively(Figures S4 and S5 in SI).However under 100% CO2 atmosphere, the beginning of de-

composition was delayed significantly (now starting at ∼65 °C)regarding DEA-carbamate (Figure 10). While apparent massloss, observed under N2 atmosphere in case of AMP-carbamatebelow 75 °C (decomposition onset temperature), appears tohave been suppressed under CO2 cover. This trend probablyemerged due to the presence of one of the reactants (CO2) inexcess. Concerning AMP-carbamate, the CO2 atmosphere wouldalso have helped revert some proportion of free amine (stemmedfrom hydrolytic activity during sample preparation) to carbamatethus curtailing the evaporation occurrence.The observations stated above indicate that using RTIL, in

place of water, can act as a suitable medium for carbamate crystalgrowth thus allowing easy recovery of lower density CO2-captured product. This not only can provide feasible opportunityto regenerate solely active species but also can promise milder re-generation conditions. From regeneration capabilities of AMP-/DEA-carbamates, it is quite obvious that DEA-RTIL blends canhelp improve the process efficiency more successfully, regardingregeneration energy penalty in particular. From perspective ofamine evaporation loss, DEA-RTIL recipe is undoubtedly betteroption compared to AMP-RTIL combination.

■ IMPLICATIONSIn case of alkanolamine based gas capture systems; better effi-ciency can be attained by avoiding energy wastage duringregeneration by targeting the active species (responsible for CO2capture) alone; and for this purpose incorporation of thermallystable RTIL can provide with the prospect of CO2-capturedproduct (carbamate) precipitation and thereby easy separation.When compared to aqueous alkanolamine based processes,

Figure 7. DSC/TG profiles of AMP-carbamate: Thermal behaviorobserved under N2 atmosphere at heating rate of 5 °C.

Figure 8. DSC/TG curves of DEA-carbamate: Thermal behavior underN2 atmosphere, using heating rate of 5 °C/min.

Figure 9.QMSmonitoring of carbamates’ decomposition by measuringpositive ion current m/z = 44 (CO2) under N2 atmosphere (100 mL/min flow rate) at 5 °C/min heating rate.



carbamate crystallization in alkanolamine-RTIL systems is notonly meant to lessen the quantity required to regenerate but alsocan help narrow the gap between capture and regenerationtemperatures. Besides, with this strategy we may well overcomethe difficulties being faced regarding gas loading restraints (dueto corrosion/degradation detriments) in current alkanolaminebased industrial processes.13,32

In general, a secondary alkanolamine blended with pertinentRTIL can be a better pick for CO2 capture as is evident fromlower thermal stability of DEA-carbamate compared to that ofAMP-carbamate.Since bringing about regeneration at lower temperature can

help decrease the magnitude of solvent degradation, future workwill be focused on amine degradation studies using alkanolamine-RTIL based CO2 capture processes. Moreover, measures/conditionswill be optimized to minimize foaming as well as evaporationphenomena.

■ ASSOCIATED CONTENT*S Supporting InformationX-ray crystallographic file (CIF format), crystal structure data ofAMP-carbamate and FTIR spectra of respective carbamatesobtained at different temperatures. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: (418) 656-2131 x3566; fax: (418) 656-5993; e-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support from FL Canada Research Chair “Greenprocesses for cleaner and sustainable energy” and the DiscoveryGrant to F. Larachi from the Natural Sciences and EngineeringResearch Council (NSERC) are gratefully acknowledged. Prof.Siaj is acknowledged for AMP single-crystal measurements.

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Figure 10. TG profiles of carbamates: Thermal behavior under CO2 atmosphere, using heating rate of 5 °C/min.



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co 2 capture in alkanolamine-rtil blends via carbamate crystallization:...

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