New Amines for CO2 Capture. I. Mechanisms of Amine Degradation in

Ind. Eng. Chem. Res. 2009, 48, 9061–9067

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APPLIED CHEMISTRY New Amines for CO2 Capture. I. Mechanisms of Amine Degradation in the Presence of CO2 Helene Lepaumier,† Dominique Picq,† and Pierre-Louis Carrette*,‡ LMOPS, UMR 5041 CNRS/UniVersite´ de SaVoie, chemin du canal 69360 Solaize, France, and IFP, ećhangeur de Solaize, BP 3, 69360 Solaize, France

Degradation of 12 different amines with CO2 was evaluated in 100 mL stainless steel batch reactors for 15 days at 140 °C using a 4 mol · kg-1 amine solution and a CO2 pressure of 2 MPa. At the end of the run, most of degradation products were identified by gas chromatography (GC)/mass spectrometry (MS); amounts of starting amine and its degradation products were determined with a quantitative GC method. This work compares the degradation of ethanolamines (including MEA) having one or two hydroxyl groups with the degradation of ethylenediamines. They were chosen to establish relationships between amine structure and stability properties: replacement of one alcohol function by one amine function, effect of amine function nature, impact of steric hindrance and cyclic structure. Significant differences were observed. The main degradation products are described, and some mechanisms are proposed to explain their formation. 1. Introduction One of the main human CO2 emissions sources is flue gases (coal-fired power plants, cement manufacturing plants, refineries) which contribute to the increasing greenhouse effect. CO2 capture in postcombustion is currently the most advanced technology and can be applied to existing plants. In the case of diluted and low-pressure steams, absorption based on chemical reaction with aqueous alkanolamine solutions is the most attractive technology. Among all known solvents, ethanolamine (MEA) is the benchmark molecule because of its properties toward CO2 capture (high absorption capacity, fast kinetics, high water solubility, low price, etc.). However, a major problem associated with chemical absorption is solvent degradation due to the presence of CO2 and O2. Byproducts lead to a significant decrease in efficiency of the process (solvent losses, corrosion, foaming, fouling, and an increase in viscosity). So, development of new amines with higher chemical stability has become essential. More and more attention has been paid to diamines recently; they seem to be good alternative solvents because their second amine function has close properties to alcohols and increases CO2 absorption capacity. Alkanolamines degradation with CO2 has been studied for a while: N-methyldiethanolamine (MDEA) and diethanolamine (DEA) are commonly used in natural gas treatment and MEA is the benchmark solvent for postcombustion application. Contrary to the degradation with CO2, thermal degradation was not studied very much: only Chakma and Meisen1 established the relative stability of MDEA with temperature. Many authors were interested in MEA degradation with CO2 and identified main byproducts such as 2-oxazolidinone, N-(2-hydroxyethyl)imidazolidinone, and N-(2-hydroxyethyl)ethylenediamine (HEEDA).2-4 DEA degradation was first studied by Kennard and Meisen,5-8 followed by Hsu9 and Holub.10 Finally, MDEA degradation was also investigated and some reaction mecha* To whom correspondence should be addressed. E-mail: [email protected]. † Universite´ de Savoie. ‡ IFP, ećhangeur de Solaize.

nisms were suggested by Chakma and Meisen1,11 and Clark.12 These studies permitted to show similarities between MEA, DEA, and MDEA degradation: in fact, the main products are amines, oxazolidinones, and imidazolidinones. On the contrary, polyamines have not yet been described in the literature, and to our knowledge, this work is the first degradation study concerning this family. In order to have a better understanding of degradation phenomenon, this new approach was based on a stability comparison between ethanolamines and ethylenediamines in the presence of CO2. We wanted to identify the impact of one alcohol function replacement by one second amine function. All studied amines were compared with MEA, the benchmark molecule. Other parameters were investigated: the nature of amine function (primary, secondary, and tertiary), steric hindrance (AMP: 2-amino-2methylpropan-1-ol), and cyclic structure (DMP: N,N′-dimethylpiperazine). All studied amines are reported in Table 1. 2. Experimental Section 2.1. Equipment and Chemicals. All amines (reagent grade) were commercially available, and aqueous solutions were prepared by using deionized water. Experiments were performed in 100 mL stainless steel batch reactors equipped with magnetic stirrer. Pressure and temperature were monitored respectively with an electronic captor (accuracy 0.2% of full scale) and a thermocouple (accuracy 0.2%). Reactors were equipped with fittings for introducing gas. Analytical-grade carbon dioxide supplied by Air Liquide (CO2 99.9%, H2O < 3 ppm, O2 < 2 ppm, CnHm < 2 ppm, N2 < 8 ppm, H2 < 0.5 ppm) was used. 2.2. Typical Experimental Run. Experimental conditions were chosen in order to obtain high enough amounts of amine losses to show significant differences between amines. Degradation experiments were conducted using a 4 mol · kg-1 amine concentration. This is a medium value between the different benchmark amine concentrations used for CO2 capture or gas treatment: MEA 30% weight (5 mol kg-1), DEA 40% weight (3.8 mol kg-1), MDEA 50% weight (4.2 mol kg-1). Experimental conditions were close to stripper conditions because

10.1021/ie900472x CCC: $40.75  2009 American Chemical Society Published on Web 09/28/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009

Table 1. Studied Amines Classified by Family (Ethanolamines and Ethylenediamines) and by Amine Nature (Primary I, Secondary II, Tertiary III)

amine degradation due to the presence of CO2 is more favorable in the stripper where CO2 partial pressure and temperature are the highest. So, all aqueous amine solutions were degassed before degradation in order to strip O2. Degassed solutions were then loaded into the reactor (40 mL). Each solution was stirred at 250 rpm and heated at 140 °C. This temperature is used for gas treatment application: it is close to reboiler temperature in the case of an aqueous solution of MDEA 50% weight. To compare the stability of selected amines, we used the same temperature for all of them, even if the stripper temperature would be different from one amine to another because of bubble points differences. When the temperature was stabilized, a 2 MPa CO2 pressure was added to the initial vapor pressure. Higher CO2 pressure than in industrial application was used in order to reduce the length of experiments to 15 days: amine degradation in a postcombustion CO2 capture process is a very slow phenomenon. The use of high CO2 pressure increased degradation rates without changing reaction mechanisms because we noticed the formation of the same degradation products at different CO2 pressures. Throughout the reaction time (15 days), extra gas supply was added as needed to compensate CO2 consumption. Some runs were conducted twice to check the experiments repeatability. 2.3. Analyses. Gas Chromatography. Gas chromatography (GC) was used to determine the remaining amount of starting amine and the amount of identified degradation products. In order to provide the optimal separation of all components, two methods with different capillary columns were carried out: one high polar (CARBOWAX-Amines) and one nonpolar (CPSIL8CB-Amines) column, respectively, on Agilent HP6890 and Varian CP3800 chromatographs.

Two different columns were used because some degradation products were well-separated with one column but not with the other one. For example, MDEA and DEA are not separated with CPSIL8-CB-Amines but with CARBOWAXAmines. On the other hand, CPSIL8-CB-Amines is more suitable than CARBOWAX-Amines for high molar mass degradation products. All liquid samples were analyzed by GC with the conditions given in Table 2. The vapor phase was not analyzed, but some volatile amines (dimethylamine or trimethylamine for example), also present in liquid phase, were identified by GC/MS analysis. At the end of the run, an internal standard, triethylene glycol (TEG), was added to the degraded sample at 1% w/w to perform quantification. GC response factors were determined with commercially available standards or estimated from similar structure compounds. So, the degradation rate τ of starting amine is defined by τ)

C × 100 C0

where C is the remaining concentration of starting amine and C0 is the initial concentration of starting amine. On the other hand, the formation rates τf,i of identified degradation products are defined by τf,i )

Ci × 100 C0

where Ci is the concentration of the degradation product i and C0 is the initial concentration of starting amine.


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Table 2. Specifications of the GC and GC/MS Systems GC

column length internal diameter thickness

GC/MS

method A

method B

EI

CI

CARBOWAX-Amines 15 m 530 µm 1.0 µm

CPSIL8-CB-Amines 25 m 320 µm 1.2 µm

CPSIL8-CB-Amines 30 m 250 µm 1.2 µm

HP5-MS 30 m 250 µm 0.25 µm

35 °C / 3 °C min-1 / / 230 °C 30 min 0.9 mL min-1 275 °C

35 °C / 3 °C min-1 / / 230 °C 30 min 0.9 mL min-1 280 °C

parameters initial temp initial hold time oven ramp (1) intermediate temp oven ramp (2) final temp. final hold time flow rate (constant) Injector temp Detector (FID) temp

60 °C 5 min 5 °C min-1 75 °C 8 °C min-1 200 °C 36.4 min 3.9 mL min-1 280 °C 250 °C

35 °C / 3 °C min-1 / / 230 °C 30 min 1.6 mL min-1 280 °C 300 °C

Mass Spectrometry. Usually, degradation compounds were identified by GC/MS. In some cases, synthesis of standards was necessary. GC/MS analyses were carried out with a Finnigan (TOF) with electronic impact (EI). For chemical ionization (CI), methane was used as a reagent gas and HP5-MS as nonpolar column. This method permitted to obtain the molecular weight of each compound. The highest molecular weight compounds were identified with a high resolution mass spectrometry technique (FT-ICR/MS) with electrospray ionization (ESI). Table 2 provides a summary of GC and GC/MS methods used to identify and to quantify starting amine and its degradation products. 3. Results and Discussion The degradation rates due to temperature (thermal degradation) and CO2 are represented for each amine in Figure 1. Thermal degradation was done in the same conditions without introducing CO2. Thermal degradation was studied to know the contribution of degradation only due to temperature without any gas (CO2 or O2). As it could be noticed, thermal degradation is quite low and could be considered negligible in comparison with

the degradation in the presence of CO2: N,N,N′-triMEDA (N,N,N′-trimethylethylenediamine), MAE (N-methylethanolamine), MEA, and DEA, the four less stable amines, are also very reactive with CO2. So, much more attention was paid to CO2 impact. Large differences have been noticed among amines: DMP is the most stable (only 4.5% degradation), MEA is intermediate (42%), and HEEDA is the least stable (99%). The degradation rates do not establish a significant link between degradation level and amine structure. In fact, identification and quantification of the main degradation products are essential to understand these data. Amine degradation in the presence of CO2 can be summarized into four main reaction types: demethylation/methylation reactions (Scheme 1), oxazolidinones and imidazolidinones formation (Scheme 2), additions (Scheme 3), and ring closures (Scheme 4). The initial step of all degradation reactions is CO2 absorption: primary and secondary amines undergo mostly carbamate formation; tertiary amines lead to carbonate/bicarbonate. These reactions are in equilibrium which means that the aqueous

Figure 1. Degradation rates (τ%) of the different amines with temperature and CO2.

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Scheme 1. Demethylation/Methylation Reactions

Scheme 2. Oxazolidinones and Imidazolidinones Formation

Scheme 3. Addition Products Formation

Scheme 4. Ring Closure

solution is a mixture of electrophilic species (ammonium salts) and nucleophilic species (amine). Their opposite electronic character makes them reactive and initiates degradation reactions. The mechanism for demethylation/methylation reactions is presented in Scheme 1. After carbamate or bicarbonate formation, a nucleophilic substitution involves methyl group migration from ammonium salt to amine: ammonium salts demethylate and amines methylate. Scheme 2 describes oxazolidinones and imidazolidinones formation. This reaction has great importance because it is often observed for both ethanolamines and ethylenediamines. After carbamate formation from primary or secondary amines, the second heteroatom (O for ethanolamines and N for ethylene-

diamines) permits the five-member ring formation. Thus, carbamates ring closure gives oxazolidinones from ethanolamines and imidazolidinones from ethylenediamines. Their synthesis was reported by Bhanage13 with reaction of CO2 on ethanolamines and diamines. It should be noticed that the most important difference between oxazolidinones and imidazolidinones is their stability: whereas oxazolidinones are reactive and will easily give other products as mentioned below, imidazolidinones are stable and will accumulate. Intermolecular nucleophilic substitutions (Scheme 3) lead to addition products (also called dimer, trimer, etc.). Three different mechanisms are possible: oxazolidinone 1,4 ringopening (i), leaving group elimination by another amine (ii) or by neighboring group participation (iii). The first one is only observed with ethanolamines. As mentioned previously, oxazolidinones are very reactive and will be opened by amine (i) to give higher molecular weight compounds (oligomers). This reaction was already described by some authors14,15 to synthesize ethylenediamines. Mechanisms (ii) and (iii) are preferentially observed with diamines: +NR3 elimination is indeed more favorable than OH elimination since +NR3 is a


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a

Table 3. Main Degradation Products of the Ethanolamine-H2O-CO2 Systems

a

R: CH2CH2OH.* denotes the use of estimated response factor.

Scheme 5. General Pathway for Ethanolamines Degradation with CO2

much better leaving group. The last mechanism seems to be highly likely for ethylenediamines but could not be proved because aziridinium salts cannot be isolated: they are very reactive in aqueous and basic medium and will be opened easily to give other products such as dimers. Finally, Scheme 4 illustrates piperazines formation. Cyclic compounds are obtained by intramolecular nucleophilic substitution when molecules have both a leaving group and an amine function. 3.1. Ethanolamines. Structure and formation rates of the main degradation products are summarized in Table 3. Other minor degradation products were identified and quantified. They are reported in the Ph.D. thesis of Lepaumier.16 Secondary amines (MAE, DEA) degrade into addition products (oligomers) and piperazines. MEA and HEEDA, which are primary amines, lead mostly to N-(2-hydroxyethyl)imidazolidinone. HEEDA is a diamine II-I; we will see below why its behavior is closer to a primary amine. Tertiary amines (DMAE, MDEA) do mostly demethylation or dealkylation, and the sterically hindered primary amine (AMP) forms mainly oxazolidinones. Scheme 5 represents the general pathway for ethanolamines degradation in the presence of CO2. It summarizes the previously

seen reactions. As shown, carbamate ring closure leads to oxazolidinones formation 1. Oxazolidinones are very sensitive to nucleophilic reactions and react easily with another amine to give addition products 2 also called dimers. Three different reactions are then in competition: subsequent intermolecular nucleophilic substitutions or addition reactions 3, intramolecular nucleophilic substitutions leading to piperazines9,10 4, and imidazolidinones formation 5 because addition products have an ethylenediamine structure (-NH-CH2-CH2-NH-).4 Secondary amines are less stable because they are usually more nucleophilic than primary and tertiary amines. In this case, R1 is an alkyl group, so the dimer has a nucleophilic secondary amine function and a low nucleophilic tertiary amine function: imidazolidinones 5 formation is not favorable unlike oligomers 3 (polyaddition reactions) or piperazines 4 formation. Differences noticed between MAE and DEA are due to the nature of R1: if it is a hydroxyethyl group, ring closure will be easier than addition because of the presence of a second OH leaving group. Concerning primary amines (MEA or HEEDA), the main degradation product is an imidazolidinone which is a very stable cyclic urea. In this case, R1 is a hydrogen atom, so the addition

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Table 4. Main Degradation Products of Ethylenediamine-H2O-CO2 Systemsa

a

* denotes the use of estimated response factor.

Scheme 6. General Pathway for Ethylenediamine Degradation with CO2

product has a -NH-CH2-CH2-NH- structure. It has two nucleophilic amine functions, one primary and one secondary, which is very favorable to form imidazolidinones. This is the reason why HEEDA, which is the addition product of MEA, is the least stable ethanolamine of the selection. Tertiary amines are more stable because they need a preliminary demethylation or dealkylation reaction, necessary to initiate significant degradation. On the other hand, DMAE formation from MDEA was explained by Chakma and Meisen:1 methylation of MDEA is followed by ethylene oxide elimination. According to this mechanism, DMAE leads to trimethylamine which is not quantified because of its high volatility. Trimethylamine is presumed to be the main degradation product which could explain the higher degradation rate of DMAE in comparison with MDEA, so the equilibrium is displaced which increases degradation. Reactivity of hindered primary amines is generally close to tertiary amines. In this study, lower degradation is observed from AMP but the reason is not the same as for tertiary amines. In fact, steric hindrance of the amine function prevents oxazolidinone ring-opening into an addition product. This explains the lower degradation rate of AMP (22%) in comparison with MEA (42%): the hindered oxazolidinone is stable and so accumulates in the solution.

All these results show that the nature of amine function has absolutely an impact on degradation. Concerning ethanolamines, the chemical stability order is the following: III > hindered I > I > II 3.2. Ethylenediamines. They were studied in order to identify the impact of one alcohol function replacement by one second amine function. Structures and formation rates of main degradation products are reported in Table 4. The same kinds of reactions as for ethanolamines occur: demethylation/methylation reactions, additions, ring closures, and imidazolidinones formation. Scheme 6 represents the general pathway of ethylenediamine degradation with CO2. It is quite similar to that for ethanolamines; the only difference is the addition compound formation which does not come from oxazolidinone 1,4 ringopening. The most likely mechanisms are +NR3 elimination with another amine or with neighboring group participation (Scheme 3ii and iii). From addition product 6, three reactions are in competition: addition 7, ring closure 8, and imidazolidinones formation (9 and 10). It should be noticed that the Chakma and Meisen reaction1 could also happen, as mentioned previously with ethanolamines, but leads to nonquantifiable volatile compounds (methylamine, dimethylamine, and trimethylamine).


Large differences observed among ethylenediamines are explained in the same manner as for ethanolamines. Ethylenediamines III-II (R3 ) alkyle) react as secondary ethanolamines and form mostly addition and cyclic compounds. Ethylenediamines III-I (R3 ) H) lead to imidazolidinones for the same reason that MEA does, i.e., due to the formation of an intermediate product with -NH-CH2-CH2-NH- structure. Both demethylated and addition products can have this structure explaining the two different imidazolidinones (9 and 10) obtained from N,N-dimethylethylenediamine (N,N-diMEDA). By the way, MEA formation (5.4%) is due to nucleophilic substitution of R3N+ with water. Ethylenediamines II-II, which present also the -NH-CH2-CH2-NH- structure, have the highest degradation rate into imidazolidinones. This point confirms that this structure is extremely favorable to cyclic ureas formation. Moreover, if compounds have only tertiary amine functions, demethylation or dealkylation is necessary to promote further degradations, so they are more stable than the others as for ethanolamines III. Finally, comparison of DMP with its homologous linear molecule, N,N,N′,N′-tetramethylethylenediamine (TMEDA), shows significant stability due to cyclic structure: demethylation is indeed the most likely degradation reaction. As for ethanolamines, the nature of the amine function has great importance in understanding the chemical stability of ethylenediamines. The stability order is the following: III-III > III-I > III-II > II-II 4. Conclusions In this work, experimental conditions were close to stripper conditions because amine degradation due to the presence of CO2 is more favorable in the stripper where CO2 partial pressure and temperature are the highest. Higher CO2 pressure than in industrial application was used in order to reduce the length of experiments to 15 days: amine degradation in a postcombustion CO2 capture process is a very slow phenomenon. The use of high CO2 pressure increased degradation rates without changing reaction mechanisms because we noticed the formation of the same degradation products at different CO2 pressures. So, these conditions are suitable to identify degradation mechanisms and also to classify the chemical stability of the different amines in the presence of CO2. We are convinced that even if the conditions are different in the industrial unit, the stability classification is the same at lab and industrial scale, because the differences of amine losses are well-explained by the mechanistic study. On one hand, we have managed to identify some stable structures in the presence of CO2 such as cyclic, hindered primary, and tertiary amines. On the other hand, some structural factors contribute to increase degradation. First, secondary amines are the least stable; second, if a molecule has or can give -NH-CH2-CH2-NH- structure, imidazolidinones will be obtained in large amounts and will decrease absorption

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efficiency; finally, when a compound has both a good leaving group and a nucleophilic function, oligomers and cyclic compound formation is very favorable. So, we have established several structure-properties relationships very helpful to predict comparative chemical stability of ethanolamines and diamines in the presence of CO2 and also the nature of degradation compounds. As we have seen, several amines are more stable than MEA, which is very promising in prospects for replacing this solvent. Acknowledgment This work is a part of Ph.D. thesis of H.L. supported by CNRS and IFP.16 Literature Cited (1) Chakma, A.; Meisen, A. Methyldiethanolamine degradation - Mechanism and kinetics. Can. J. Chem. Eng. 1997, 75, 861. (2) Polderman, L. D.; Dillon, C. P.; Steele, A. B. Why monoethanolamine solution breaks down in gas-treating service. Oil Gas J. 1955, 54, 180. (3) Scheiman, M. A. A review of monoethanolamine chemistry; No. 5746, U. S. Naval Research Laboratory Report, 1962. (4) Strazisar, B. R.; Anderson, R. R.; White, C. M. Degradation pathways for monoethanolamine in a CO2 capture facility. Energy Fuels 2003, 17, 1034. (5) Kennard, M. L.; Meisen, A. Control DEA degradation. Hydrocarbon Process., Int. Ed. 1980, 59, 103. (6) Meisen, A.; Kennard, M. L. DEA degradation mechanism. Hydrocarbon Process., Int. Ed. 1982, 61, 105. (7) Kennard, M. L.; Meisen, A. Gas chromatographic technique for analyzing partially degraded diethanolamine solutions. J. Chromatogr. 1983, 267, 373. (8) Kennard, M. L.; Meisen, A. Mechanisms and kinetics of diethanolamine degradation. Ind. Eng. Chem. Fundam. 1985, 24, 129. (9) Hsu, C. S.; Kim, C. J. Diethanolamine (DEA) degradation under gas treating conditions. Ind. Eng. Chem. Prod. Res. DeV. 1985, 24, 630. (10) Holub, P. E.; Critchfield, J. E.; Su, W.-Y. Amine degradation chemistry in CO2 service. 48th Annual Laurance Reid Gas Conditioning Conference, Norman, Oklahoma, March 1-4, 1998; p 146. (11) Chakma, A.; Meisen, A. Identification of methyldiethanolamine degradation products by gas chromatography and gas chromatography mass spectrometry. J. Chromatogr. 1988, 457, 287. (12) Clark, P. D.; Dowling, N. I.; Davis, P. M. Introduction to sulphur chemistry and sulphur handling. Sulphur 2004 Conference; Barcelona (Spain), October 24, 2004. (13) Bhanage, B. M.; Fujita, S.-I.; Ikushima, Y.; Arai, M. Synthesis of cyclic ureas and urethanes from alkylene diamines and amino alcohols with pressurized carbon dioxide in the absence of catalysts. Green Chem. 2003, 5, 340. (14) Rooney, P. C.; Nutt, M. O. Aminoethylation process for production of substituted ethylenediamines involving oxazolidinone ring opening with secondary amines or alkanolamines. US patent 5,491,263, 1996. (15) Poindexter, G. S.; Owens, D. A.; Dolan, P. L.; Woo, E. The Use of 2-oxazolidinones as latent aziridine equivalents. 2. Aminoethylation of aromatic amines, phenols and thiophenols. J. Org. Chem. 1992, 57, 6257. (16) Lepaumier, H. Etude des mećanismes de de´gradation des amines utiliseés pour le captage du CO2 dans les fumeés. Ph.D. thesis, University of Savoie, France, October 2008.

ReceiVed for reView March 23, 2009 ReVised manuscript receiVed July 17, 2009 Accepted August 30, 2009 IE900472X

New Amines for CO2 Capture. I. Mechanisms of Amine Degradation in

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