Oxidative Degradation of Aqueous 2-Amino-2 ... - ACS Publications

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Oxidative Degradation of Aqueous 2-Amino-2-methyl-1-propanol Solvent for Postcombustion CO2 Capture Tielin Wang†,‡ and Klaus-J. Jens*,†,‡ †

Department of GassTek, Telemark R & D Institute, Kjølnes ring 30, 3918 Porsgrunn, Norway Faculty of Technology, Telemark University College, Kjølnes ring 56, 3901 Porsgrunn, Norway

‡

S Supporting Information *

ABSTRACT: 2-Amino-2-methyl-1-propanol (AMP) and the blends of AMP with other amines appear to be commercially attractive solvents for postcombustion CO2 capture by absorption/stripping. Oxidative degradation experiments involving AMP aqueous solutions were performed in a 200 mL glass batch reactor with initial AMP concentrations of 5 mol·kg−1, oxygen partial pressures of 250−350 kPa, and at temperatures of 100−140 °C in order to elucidate the degradation mechanistic pathways. The amine loss was determined by cation ion chromatography (IC), while the degradation compounds were identified and quantified by gas chromatography−mass spectrometry (GC−MS) and anion IC. The possible chemical pathways of AMP oxidative degradation are proposed on the basis of the identified and quantified products and the context of the current amine degradation schemes. The role of O2 in the proposed pathways is much more explicit than the previously proposed MEA oxidation mechanisms.

1. INTRODUCTION Carbon capture and storage (CCS) is a means of mitigating the contribution of fossil fuel emissions to global warming, based on capturing carbon dioxide (CO2) and storing it away from atmosphere by different means. Among the several technologies proposed for CO2 capture from flue gas streams, aqueous solutions of alkanolamine-based CO2 capture processes1 in postcombustion are currently the most attractive technologies, which are in particular focused for power plant application due to the low CO2 partial pressure. It is known that the challenges of these processes for CO2 capture from flue gas are energyintensive and costly. In this contest, more energy-efficient and cost-effective technologies must be developed. New solvents have been investigated by a number of researchers. Recently, a class of amines, termed sterically hindered amines, have received considerable attention. One hindered amine of interest is 2-amino-2-methyl-1-propanol (AMP), in which the amino group is attached to a tertiary carbon atom. AMP and mixtures of AMP and other amines offer advantages over conventional amines.2,3 As compared with monoethanolamine (MEA), AMP has a higher absorption capacity,4,5 requires less energy for regeneration,6 and is more stable.7 Although AMP has a much higher rate constant for reaction with CO2 than MDEA,8,9 the kinetics of CO2 absorption is still slower than for MEA.10 However, addition of small amounts of other amines to AMP, such as MEA11 and piperazine (PZ),12 result in a significant enhancement of CO2 absorption rates. Therefore, the AMP-based solvents appear to be commercially attractive absorbents for CO2 capture. Solvent degradation is a major problem associated with alkanolamine absorption for CO2 capture from flue gases.13 Solvent degradation has been estimated to account for 10% of the total cost of CO2 capture.14 Oxidative degradation can be significant in flue gas applications typically containing 3−15% O2. Oxidative degradation of alkanolamines has been reported © 2012 American Chemical Society

in the literature, but most of the studies have focused on the degradation of MEA,15−20 the benchmark solvent for CO2 removal, yet the mechanism for the oxidative fragmentation remains unclear, especially the role that O2 plays in the oxidation. Two mechanisms for the oxidation have been proposed: electron abstraction16,21,22 and hydrogen abstraction;17,22,23 i.e., the oxidant could abstract an electron from the nitrogen atom, giving an N-centered radical cation, or the oxidant could abstract a hydrogen atom from the nitrogen, αcarbon, or β-carbon atom (α,β-position with regard to the amino functional group). In the case of the N-centered radical cation the molecule could subsequently split off a proton from the α C−H group. Further abstraction of an electron by the oxidant and a proton will lead to an imine that can be readily hydrolyzed by water. AMP is postulated to be more resistant to oxidative degradation, as it has no α-hydrogen and hence is unable to form an imine, which is assumed to be the first step in the electron abstraction mechanism. Dennis and his co-workers24 reported the oxidative fragmentation of AMP by chlorine dioxide to produce ammonia, formaldehyde, and acetone. But this investigation was not performed under relevant conditions for CO2 capture from flue gas. To compare the new amines for CO2 capture, Lepaumier et al.7 investigated the oxidative degradation of AMP under 2 MPa synthetic air at 140 °C over 15 days. Under the accelerating conditions, N-methyl-AMP and 4,4-dimethyl-2oxazolidinone were reported as the main products. These degradation products were not accounted for in the general summary reaction scheme proposed by the authors. A recent report by Freeman et al.25 measured the apparent first-order Received: Revised: Accepted: Published: 6529

February April 16, April 17, April 17,

8, 2012 2012 2012 2012

dx.doi.org/10.1021/ie300346j | Ind. Eng. Chem. Res. 2012, 51, 6529−6536

Industrial & Engineering Chemistry Research

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Typical Experimental Run. The experimental procedure is similar to that described in reported work.7,15 In a typical experimental run, 140 mL of aqueous amine solution was degassed and loaded into the reactor. The solution was stirred at a speed of 200 rpm (a practical speed in our autoclave reactor) and heated to the desired temperature. The type of main degradation products of AMP was independent of stirring speed as observed in preliminary experiments (see Supporting Information , Figure S4). O2 was then introduced into the vessel up to the desired pressure through the gas inlet valve. To ensure that the desired O2 pressure was maintained throughout the duration of the reaction, the inlet valve of O2 was kept opened at a fixed setting. A check valve was preset in the gas inlet tubing to prevent the gas mixture from flowing backward. A sample (∼2 mL) of the reaction mixture was withdrawn from the reactor through the liquid sampling valve at appropriate predetermined intervals. Between sampling times, the tubing was thoroughly rinsed to prevent carryover contamination of the new sample. To avoid further degradation, each sample was quickly quenched by running cold water over the sampling bottle for several minutes, and then it was stored in a refrigerator. By this procedure, the total gas pressure over the liquid was kept constant and the solvent composition was not changed. In spite of a total amount of approximately 40 mL being withdrawn during the course of the experiment, the conditions for the oxidative degradation of AMP were not changed during the experimental run. The samples were analyzed after each experimental run. Some runs were conducted twice to check the reproducibility of the experiments. Reaction between Acetone, Formaldehyde, Ammonia and Oxygen. Ammonium hydroxide (1.03 g, based on ammonia), acetone (7.18 g), and formaldehyde (1.87 g) in 80 mL of water was treated with 5 M potassium hydroxide to pH 12. The mixture was loaded into a glass reactor with a stainless steel lock and heated to 120 °C. After the temperature increased to the desired value, O2 was introduced into the reactor up to the total pressure of 370 kPa. The reaction was stopped after 24 h. Analyses. Ion Chromatography (IC). All instrument modules and supplies were from Dionex. Chromeleon software on the attached computer analyzed the conductivity output and controlled the entire IC system. Cation Chromatography. The concentration of the AMP aqueous solution and ammonium was determined by cation chromatography. The chromatographic system was a DX_500 ion chromatographic analyzer incorporating an ICS-3000 isocratic pump. A cation exchange analytical column SCS1 (4 × 250 mm) in combination with a guard column SCG1 (4 × 50 mm) was used for the components’s separation. The column temperature was 34 °C. Methanesulfonic acid was used as eluent and acetonitrile was used as an organic modifier at a flow rate of 1.0 mL·min−1. At the end of each degradation run, the quantification was performed after the samples were diluted gravimetrically 2000:1 with Milli-Q water. The error of the cation IC analysis was less than ±2%. Anion Chromatography. Concentrations of carboxylates, nitrate, and nitrite in the degradation samples were determined by anion-exchange chromatography. Analytical columns were an AG15 guard column (4 × 50 mm) with an AS15 analytical column (4 × 250 mm). An EG40 eluent generator was used to generate eluent and a GP50 gradient pump was used to deliver the eluent to the system. The initial concentration was 10 mM

rate constant of AMP for thermal degradation, but no oxidative degradation was observed. Sexton26 reported that AMP was resistant to oxidation at 55 °C when 98% O2 and 2% CO2 were bubbled through the solution. However, any absorbed oxygen that does not react in the absorber is carried over to the cross exchanger and stripper and has the potential to cause an oxidation in higher temperature. Oxidative degradation is an important issue because it can impact the environment and process economics and can decrease equipment life due to corrosion. Our overall goal is degradation prevention, and a good understanding of the fundamentals of degradation chemistry is necessary to propose an effective strategy to inhibit oxidation. This present study was therefore conducted to identify and quantify the oxidative degradation products of aqueous AMP solutions in order to identify the possible degradation pathways. The effects of operating variables such as O2 partial pressure and temperature on AMP degradation rates were also evaluated.

2. EXPERIMENTAL SECTION Equipment and Chemicals. Analytical reagent grade or better chemicals as well as Milli-Q water (18.2 MΩ·cm) were used in the experiments. AMP was purchased from Fluka (Steinheim, Germany). Analytical-grade O2 and N2 were obtained from AGA (Oslo, Norway). Ammonia solution (25 wt %) was supplied by Merck KGaA (Darmstadt, Germany). Formaldehyde (37 wt %) aqueous solution, acetone (99.8%), potassium hydroxide (99.99%), and 2,4-lutidine (99%) were supplied by Sigma-Aldrich (Steinheim, Germany). All the other chemicals were purchased from Fluka or Sigma-Aldrich. Experiments were performed in 200 mL glass batch reactors with stainless steel enclosure (“miniclave steel” type 1, 200 mL, 10 bar, obtained from Büchi Glas Ulster, Switzerland). The reactors were equipped with magnetic stirrers. Pressure and temperature were monitored and controlled respectively with a pressure gauge and a thermocouple. Reactors were equipped with fittings for introducing gas and sampling (Figure 1).

Figure 1. Schematic diagram of the autoclave reactor. 6530



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KOH for 4 min, and then the concentration was stepped to 60 mM at 9 min. Eluent flow rate was 1.0 mL·min−1. Postcolumn eluent suppression was achieved using an anion selfregenerating suppressor (ASRS-300, 4 mm) in external-water mode. At the end of each degradation run, the quantification was performed after the samples were diluted 50:1 to 200:1 with Mill-Q water according to degradation time and conditions. The error of the anion IC analysis was less than ±5%. Gas Chromatography−Mass Spectrometry (GC−MS). Degradation products in the samples were identified using a gas chromatograph−mass spectrometer (model 7890A/ 5975C). All instrument modules and supplies were from Agilent Technologies. A nonpolar column DB-5MS was used to separate the components and sample injection was performed using an autoinjector (model 7683B) at 250 °C. The GC−MS conditions used are summarized as follows. A 10 μL syringe with injection volume of 0.2 μL was used, and a split mode was selected for the inlet with a split ratio of 50:1. The initial temperature of the oven was 110 °C with a hold time of 1 min, while the final temperature was 240 °C with a hold time of 10 min, and the oven was ramped at 5 °C/min. Helium was used as carrier gas and the flow rate was 1.5 mL/min. For the MS parameters, the interface, quadrupole, and source temperatures were 250, 150, and 230 °C, respectively, and the electron multiplier voltage was 1200 V. In order to estimate the relative concentrations, the three main products were quantified in SIM mode. The calibration curves were obtained from commercial standards at different concentrations and performed before analysis of the degraded sample. The error of the GC−MS quantification was less than ±6%. The uncertainty of estimation of 4,4-dimethyl-2oxazolidinone may be higher. This estimation was based on 2-oxazolidinone as the standard, since 4,4-dimethyl-2-oxazolidinone is not commercially available. UV−Vis Spectrophotometry. Formation of 2,4-lutidine was confirmed using an UV spectrophotometer (Shimadzu UV1800). UV−vis spectra were recorded between 200 and 400 nm using a 1-cm path-length quartz cell, and pure water was used as the reference.

Figure 2. Changes in AMP concentration versus time (initial concentration of AMP = 5 mol/kg). Studies were conducted at temperatures ranging from 100 to 140 °C and at oxygen pressures of either 250 or 350 kPa.

experiments with 5 mol/kg AMP and 250 kPa of O2 at 100, 120, and 140 °C. The decline in AMP concentration with time was faster for the studies conducted at a higher temperature. After 168 h, the overall rate of AMP degradation (calculated as δC/δt, where δC is the change in AMP concentration while δt is the change in reaction time from 0 to time t) was 0.0024, 0.0035, and 0.0048 mol/(kg·h) at 100, 120, and 140 °C, respectively. This result indicates that the rate of AMP degradation increases as the temperature rises, as expected. The effect of the O2 partial pressure was evaluated with 5 mol/kg AMP at 120 °C by comparing the results obtained at 250 kPa of O2 pressure vs those at 350 kPa of O2 pressure. The AMP concentration declined over time and was more rapid with higher O2 pressure. After 120 h, the overall rate of AMP degradation with 250 kPa of O2 was 0.0037 mol/(kg·h), while at the same temperature, the rate with 350 kPa of O2 pressure was 0.013 mol/(kg·h). After 216 h, the overall rate of AMP degradation with 250 kPa of O2 was 0.0032 mol/(kg·h), while with 350 kPa of O2 it was 0.0092 mol/(kg·h). This result confirms that oxygen concentration, or oxygen pressure, is a crucial parameter in the degradation rate of AMP. Degradation Products. We employed gas chromatography−mass spectroscopy to identify the major products of degradation in an attempt to understand the degradation mechanism of AMP. At the outset of the study, a single sharp peak corresponding to AMP appeared in the spectrum (Figure 3a). After 384 h, a number of degradation products were observed (Figure 3b), including acetone, 2,4-lutidine, and 4,4dimethyl-2-oxazolidinone, which were identified by MS library match. Computer fitting of the mass spectrum to the mass spectra database (NIST MS search 2.0) to identify the main products was followed by the use of standards (if commercially available) to confirm the identification of the components in the samples. The retention times of the identified compounds were the same as those of the authentic standards. Typical results obtained for acetone indicate that the mass fragmentation pattern of acetone in our samples matched that of the acetone standard as well as that documented in the mass spectrometer database. The confidence that the mass fragmentation pattern of 2,4-lutidine in the degradation samples matched that documented in the mass spectrometer database was 49%. But it should be noted that the confidence that the mass spectrum of authentic 2,4-

3. RESULTS AND DISCUSSION Effects of Temperature and Oxygen Pressure. Elevated temperatures and oxygen partial pressure were used in this study to accelerate the rates of degradation reactions. Thermal degradation of AMP was investigated at two temperatures before the oxidative degradation experiments. The thermal degradation experiments were carried out by heating aqueous solutions of AMP under a blanket of nitrogen at 120 and 140 °C. Thermal degradation of AMP and generation of degradation products were tracked during the course of the experiments. The observed AMP loss was 1.1% at 140 °C and the formation of degradation products was minimal over 4 weeks. Therefore, the thermal degradation of AMP can be disregarded, at temperatures less than 140 °C. The initial concentration of AMP has a slight influence on the degradation rate of AMP;27 thus, the initial concentrations were fixed at 5 mol AMP/kg solution in all the subsequent experiments, and all the experiments were conducted at temperatures ranging from 100 to 140 °C, which are close to normal stripper temperatures. Figure 2 illustrates the changes in the AMP concentration versus reaction time at different temperatures and O2 partial pressures. The effect of temperature was evaluated using 6531



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degradation sample have the same absorption peaks at 258 nm, which is the characteristic absorption of the pyridine ring. These results demonstrate that 2,4-lutidine was produced in the reactor but do not indicate that the secondary product was formed in the GC. After identification, the three main degradation products were quantified by GC−MS. Figure 5 shows the results at 120

Figure 5. Formation of acetone, 2,4-lutidine, and 4,4-dimethyl-2oxazolidinone in the AMP degradation experiment (120 °C, 250 kPa of O2, initial concentration of AMP = 5 mol/kg).

Figure 3. (a) Gas chromatogram of AMP aqueous solution at the beginning of the experiment (0 h) using 5 mol/kg AMP at 120 °C with 250 kPa of O2. (b) Chromatogram of a partially degraded AMP aqueous solution at 384 h using 5 mol/kg AMP at 120 °C with 250 kPa of O2. On the basis of comparison to a library of MS spectra, in addition to water (peak 1) and AMP, the products were identified as acetone (peak 2), 2,4-lutidine (peak 3), trimethyl pyridines (peaks 4 and 5), and 4,4-dimethyl-2-oxazolidinone (peak 6).

°C and 250 kPa of O2. As can be seen in Figure 5, acetone was formed in advance of 2,4-lutidine and 4,4-dimethyl-2oxazolidinone. In addition, the concentration ratio of acetone to 2,4-lutidine decreased with reaction time. We detected the same degradation products under other conditions of temperature and oxygen pressure, except that the relative amounts of the products varied (see Table 1). Cation IC and anion IC were used in this study to analyze positively and negatively charged species in the aqueous solutions of degraded AMP. Ammonium, several carboxylic acids, nitrite, and nitrate were identified and quantified as the oxidative degradation products of AMP, although they were present in only small amounts except for formic acid. Figure 6 shows the typical ion chromatograms of a partially degraded AMP solution. Sodium, which might have come from the glass reactor, was found in all the samples. Degradation Pathways. Once the major degradation products were identified and quantified and the effects of the main operating parameters were established, we turned our efforts toward the identification of possible degradation pathways. Acetone and Ammonia Formation. The formation of a radical on the tertiary carbon in the α-position of the nitrogen atom in AMP is basically impossible; hence, the compound is not able to form an imine, which is assumed to be a first step in the electron abstraction mechanism.16,21,22 Therefore, the oxidative degradation of AMP observed in this work cannot be explained with the proposed electron abstraction mechanism. It is more likely that AMP is degraded via a hydrogen abstraction mechanism as an initial step. Similar to liquid phase autoxidation of higher hydrocarbons we expect initiating radicals to be formed continuously, but seldom. Concentration of thermal vibration energy onto one bond or reaction with oxygen could be the initiating event.28 Hence, formation of initiating radicals is enhanced as temperature and/or oxygen partial pressure is increased. The weakest hydrogen-containing

lutidine standard matched that documented in the database was only 51%. The identified products may have been formed during the GC analytical process due to the rigorous operating conditions of the GC−MS. UV−vis spectroscopy was used to determine if 2,4-lutidine was a degradation compound formed in the reactor. As can be seen in Figure 4, lutidine standard and the AMP

Figure 4. UV−vis spectra of (1) 2,4-lutidine standard and (2) aqueous solution of partially degraded AMP. 6532



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Table 1. Main Degradation Products in Degraded AMP Aqueous Solutions at 216 h under Different Experimental Conditions (initial AMP concentration = 5 mol/kg) concentration (mol/kg)

a

product

% confidence

standard verification

100 °C/250 kPaa

120 °C/250 kPaa

140 °C/250 kPaa

120 °C/350 kPaa

acetone 2,4-lutidine 4,4-dimethyl-2-oxazolidinone

83 49 78

yes yes no

0.21 0.057 0.065

0.23 0.060 0.071

0.33 0.087 0.099

0.67 0.17 0.19

Partial pressure of O2

observed during air oxidation of MEA.32 We propose that the newly formed alkyl radical from AMP is converted into the corresponding peroxyl radical (1) by a fast reaction with O2 (see Figure 7). Then the peroxyl radical decays to further

Figure 7. Scheme for the decay of AMP to primary products. Degradation pathways following the formation of a peroxyl radical (1) to yield an imine (2) or an enamine (3) are illustrated.

Figure 6. (a) Cation chromatogram and (b) anion chromatogram of degraded samples of the AMP−H2O−O2 system at 120 °C using 5 mol/kg AMP with 250 kPa of O2. The identified peaks are ammonium (peak 1), AMP (peak 2), glycolate (peak 3), acetate (peak 4), formate (peak 5), carbonate/bicarbonate (peak 6), nitrite (peak 7), oxalate (peak 8), and nitrate (peak 9).

products. This pathway may explain the dramatic effect of O2 partial pressure on the AMP oxidation rate, as shown in Figure 2. Hydrogen-abstraction reactions by peroxyl radicals are common. These transformations involve intramolecular as well as intermolecular H-transfer.31 The decay of the peroxyl radical is speculated to occur by intramolecular hydrogen abstraction through a six-membered cyclic transition state. This transition state is proposed in analogy to intramolecular decomposition of related peroxyl radicals in aqueous solution at elevated temperature. Figure 7 describes one degradation pathway of AMP. The peroxyl radical (1) abstracts a hydrogen atom from the N−H bond intramolecularly via a six-membered cyclic transition state. After ejection of a •OH radical from this transition state, it decomposes to formic acid and an imine (2). The imine is not expected to remain stable in aqueous solution, and it should readily hydrolyze to ammonia and acetone. At the same time, the peroxyl radical (1) could also decompose to

bond is expected to be readily attacked. The exact bond dissociation energy (BDE) for the hydrogen-containing bonds of AMP are not available. The BDE of the hydrogen-containing bonds in the NH2, OH, and CH2/CH3 groups, which are also present in the AMP molecule, were used for comparison. tertButylamine, ethanol, and ethane were used as reference compounds. According to the literature,29,30 the C−H bond adjacent to the OH group is the weakest hydrogen-containing bond in the AMP molecule, and thus, HOC·HC(CH3)2NH2 would be predominant. Such a step is supported by the literature. Once a radical is created, it will react rapidly with oxygen at a rate close to the rate of a diffusion-controlled process.31 Thus peroxyl radicals are expected to be the primary oxidation products in the presence of O2. Formation of peroxide has been 6533



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formic acid and an enamine (3) via a similar intramolecular hydrogen abstraction from a C−H bond combined with •OH radical formation. The enamine is in equilibrium with an imine and may thus also degrade to acetone and ammonia. NO2− and NO3− Formation. NO2− and NO3− ions were observed during the AMP oxidation process, and NO2− was formed in advance of NO3−. In addition, NO2− and NO3− demonstrated different time profiles in the oxidation of AMP in the presence of O2 (Figure 8). The concentration of NO3−

Figure 9. Formation of carboxylic acids in the AMP degradation experiment (120 °C, 250 kPa of O2, initial concentration of AMP = 5 mol/kg).

the formation of the carboxylic acids in the AMP oxidative degradation process. Formaldehyde, pyruvic, and pyruvaldehyde were detected in acetone degradation,37 but these byproduct were not detected in AMP degradation samples. It may be that these intermediates cannot accumulate to any significant degree in the slow oxidation process of AMP. 4,4-Dimethyl-2-oxazolidinone Formation. 4,4-Dimethyl-2-oxazolidinone (4) was unambiguously identified as a major product by GC−MS. This result indicates that the steric hindrance in AMP molecule does not prevent oxazolidinone formation but makes it less favorable than in the case of MEA. The difference as compared to MEA is that 4,4-dimethyl-2oxazolidinone is stable and will hence accumulate in the solution, contrary to 2-oxazolidinone. 4,4-Dimethyl-2-oxazolidinone (4) was formed under our experimental conditions despite the fact that no CO2 was introduced into the reaction system. 4,4-Dimethyl-2-oxazolidinone has also been identified and quantified by Lepaumier et al.7 In that investigation, 4 mol/ kg AMP aqueous solution was degassed to strip CO2 and then loaded into a stainless steel reactor. Analytical synthetic air (CO + CO2 < 0.5 vpm) was fed into the batch reactor to 2 MPa at 140 °C. It was estimated that the amount of CO2 introduced into the reactor was less than 0.02 μM. In terms of the percentage of formation of the identified product defined by these authors, the percentage of formation of 4,4-dimethyl-2oxazolidinone was estimated at 1.3 × 10−5, even though all the CO2 introduced into the system was converted to 4,4-dimethyl2-oxazolidinone. Surprisingly, the percentage of formation of 4,4-dimethyl-2-oxazolidinone in the investigation was 0.5. These earlier observations suggest that CO2 could be a degradation product of AMP in the AMP−O2−H2O system under these experimental conditions. CO2 reacted further with AMP to form 4,4-dimethyl-2-oxazolidione (4) (Figure 10). The complete mineralization of the carboxylic acids from the decay of acetone was the most probable source of CO2. The Eschweiler−Clarke reaction38 is another possible pathway for CO2 formation, but the occurrence of this pathway is unlikely since the accompanying product, N-methylated AMP, was not found. 2,4-Lutidine Formation. 2,4-Lutidine has never been reported as an oxidative degradation product of AMP, but it was actually the principal product of aqueous AMP degradation in this work. Figure 11 illustrates the proposed mechanism for the formation of 2,4-lutidine (7). Formaldehyde, which could

Figure 8. Formation of nitrite, nitrate, and ammonium in the AMP degradation experiment (120 °C, 250 kPa of O2, initial concentration of AMP = 5 mol/kg).

ascended linearly with reaction time while NO2−concentration at first increased and then decayed with reaction time, indicating that NO2− and NO3− could be produced due to a consecutive oxidation of NH3 by •OH. Once formed, NH3 would be oxidized to •NH2 by the attack of •OH, and then • NH2 would be oxidized to •NHOH. The unstable •NHOH would rapidly convert to NH2O2− and consequently to NO2−. Gradually, NO2− would be oxidized to NO3−. An example of NO2− and NO3− formation similar to this case has appeared previously.33 In that case, •OH radical generated by H2O2 photolysis could oxidize NH3 to NO2− and ultimately to NO3− in aqueous solution. In our case, parallel with NO2−/ NO3− formation, the ammonium ion concentration increased with reaction time. This is probably due to the decreasing pH value of the liquid phase. Moreover, ammonia from the gas phase may redissolve into the liquid phase as liquid phase ammonia is consumed through NO2−/ NO3− formation. Carboxylate Formation. As can be seen in Figure 9, formic, acetic, oxalic, and glycolic acid have been determined as AMP degradation products. The concentration of formic acid is much higher than that of other acids because formic acid is proposed to be a primary product of AMP oxidation, as shown in Figure 7. In fact, carboxylic acids have been detected from oxidation of many ethanolamines, but no general mechanism has been proposed to explain their formation. Rooney et al.34 proposed an explanation for carboxylic acids formation from MEA oxidation. Ye and Zhang35 proposed another route to carboxylic acid formation from MDEA. However, neither of the proposed schemes can explain the formation of acids during AMP oxidation. In the presence of •OH/O2, Stefan et al.36,37 observed formation of carboxylic acids (formic, acetic, oxalic, and glycolic acid) from acetone degradation in aqueous solutions, and their proposed mechanism of acetone decomposition in the presence of •OH/O2 can be applied to explain 6534



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Figure 10. Scheme for formation of 4,4-dimethyl-2-oxazolidinone (4). Figure 12. Gas chromatogram of a model reaction mixture of acetone, formaldehyde, and ammonia in KOH solution at 120 °C with 250 kPa of O2 (24 h). Peak 1 is water, peak 2 is acetone, peak 3 is 2,4-lutidine, and peaks 4 and 5 were identified as trimethylpyridines.

4. CONCLUSION AMP is not stable and does not prevent oxazolidinone formation in the presence of O2. The oxidative degradation rate of AMP was strongly dependent on the O2 concentration under the experimental conditions. The degradation products were identified and quantified by GC−MS and IC. Possible degradation pathways have been proposed on the basis of the identified and quantified products. AMP oxidation is likely to be initiated by hydrogen abstraction. The role of O2 played in AMP degradation was highlighted in the proposed mechanism.

Figure 11. Scheme for formation of 2,4-lutidine (7).

be generated through decomposition of acetone, can be converted by various pathways, such as oxidation to formic acid as was suggested by Stefan et al.37 or through condensation with acetone to form a α,β-unsaturated ketone (5). Analogous to keto−enol tautomerism, the enamine (3) is in equilibrium with the imine (2) because a hydrogen atom can switch its location between the heteroatom (nitrogen) and the second carbon atom. In the proposed mechanistic pathway, the polarized enamine (6) reacts with the α,β-unsaturated ketone (5) to form 2,4-lutidine in the presence of O2. Although the equilibrium of imine−enamine tautomerism is usually poised toward the imine, reaction between the polarized enamine (6) and the α, β-unsaturated ketone (5) may shift the equilibrium in favor of the enamine. The synthesis of alkylpyridines is readily achieved by a liquidphase reaction of aldehyde or ketone mixtures with NH3:39 however, in this synthesis 2,4-lutidine was a minor product. To confirm the possibility of 2,4-lutidine generated via the proposed mechanism shown in Figure 11, the fate of a mixture of acetone, formaldehyde, and NH3 (molar ratio 2:1:1) in alkali condition (pH 12) in the presence of 250 kPa of O2 at 120 °C was investigated. After 1 h, 2,4-lutidine was detected as a major product by GC−MS. The chromatogram at 24 h (Figure 12) shows that a small amount of trimethylpyridines were produced in addition to 2,4-lutidine. Small amounts of trimethylpyridines also were detected in the partially degraded AMP samples. The results provide additional support that 2,4-lutidine can be produced through the proposed pathways. 2,4-Lutidine was also identified as a product of the reaction of acetone, formaldehyde, and ammonia in the presence of O2 at 60 °C, a temperature which may occur in the absorber of a CO2 capture plant. Taking into account that 2,4-lutidine is also formed at different O2 pressures (see Table 1), we believe that 2,4-lutidine can be formed under real conditions if formaldehyde, acetone, and ammonia are present; the last two molecules probably reacting via an imine.

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ASSOCIATED CONTENT

S Supporting Information *

Mass spectra of 2,4-lutidine (Figure S1) and acetone (Figure S2) in degraded AMP samples. The repeatability of AMP degradation rate (Figure S3) of duplicate degradation runs at 120 °C and 250 kPa of O2. Chromatogram of a partially degraded AMP aqueous solution with a stirring speed of 1000 rpm (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +47-35575193. Fax: +47-35575001. E-mail: Klaus.J. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial assistance provided by the Research Council of Norway (Strategic Research Programmes nr. 186944/I30 and 182732/V10) is gratefully acknowledged.

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REFERENCES

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