Article pubs.acs.org/EF
MALDI-MS and HILIC ESI-MS/MS as Versatile Tools for Detection of Monoethanolamine Degradation Products in a Real Postcombustion Carbon Dioxide Capture Plant P. Cotugno,† A. Monopoli,*,† A. Nacci,†,‡ C. G. Zambonin,†,§ and C. D. Calvano*,† †
Dipartimento di Chimica Università degli Studi di Bari “Aldo Moro”, Via Orabona, 4-70126 Bari Italy CNR − ICCOM, Dipartimento di Chimica, Università degli Studi di Bari “Aldo Moro”, Via Orabona, 4-70126 Bari, Italy § Centro Interdipartimentale SMART, Università degli Studi di Bari “Aldo Moro”, Via Orabona, 4-70126 Bari, Italy ‡
S Supporting Information *
ABSTRACT: A simple method based on hydrophilic liquid chromatography (HILIC) and electrospray mass spectrometry (ESIMS) for the detection of monoethanolamine (MEA) degradation products in CO2 postcombustion capture plants has been developed. MEA byproducts determination has traditionally been difficult due to analytical separation problems. Even in recent sophisticated methods, this difficulty remains as the major issue often resulting in time-consuming sample preparations. In this work, we have collected samples directly from a real pilot plant and analyzed them, for the first time, by using matrix assisted laser desorption ionization (MALDI)-MS or ESI-MS without any separation, both in positive and negative ionization modes. Alternatively, a previous liquid chromatography (LC) run was performed before ESI-MS; traditional reverse phase separation and HILIC were compared. Our results indicated that HILIC separation using an amino modified column, coupled to ESI-MS or ESI-MS/MS measurements, is the suitable method for identifying as many degradation products. Moreover, some plausible degradation mechanisms are proposed to explain some peaks in the spectra. The present work is intended as a preliminary study aiming to show the usefulness of these alternative techniques for this kind of investigations.
1. INTRODUCTION The dramatically increased greenhouse effect in the past few years is certainly due to high number of carbon dioxide (CO2) emissions sources (cars, refineries, domestic plants, cement manufacturing plants, etc.). One of the most advanced technologies useful to reduce CO2 emission consists in its capture in postcombustion that can be applied also to existing plants. In particular, when diluted and low-pressure steams are dealt with, the CO2 postcombustion capture process is realized by using solvents mainly composed of aqueous solution of alkanolamines. Among them, monoethanolamine (MEA) is the most employed due to its low cost, large availability, nontoxicity, high efficiency, and fast reaction kinetic for CO2 capture. Unfortunately, a major problem associated with chemical absorption is solvent degradation due to the low chemical stability of different amines exposed to CO2 and/or oxygen.1−3 Moreover, oxidation of MEA leads to volatile compounds (i.e., ammonia) and carboxylic acids that promote corrosion phenomena and also foaming,4 fouling,5 and increased viscosity of the amine solution.6 The usual concentration of MEA in water evaluated as a capture solvent is at 30% wt maximum even if it has been reported that using solvents with an increased MEA concentration as 40% wt would allow higher process performances. However, increasing MEA concentration would especially raise the degradation level whose control becomes a key factor. Oxidative degradation can be potentially solved by introducing oxidation inhibitors7 but the major issue in this case is that the 25−50% of the degradation products remains still unaccounted. Additionally, most of previous studies were © 2014 American Chemical Society
performed with pure gases under laboratory controlled conditions. However, in a real capture pilot plant supplied with flue gas from a coal fired power station, the degradation process is much more complicated and difficult to understand. To date, the major part of identified degradation products8−12 are volatile compounds such as amines (methylamine, ammonia), aldehydes (formaldehyde and acetaldehyde), and carboxylic acids (formic, acetic, glycolic and oxalic acids) that could originate by radical mechanisms as electron transfer3,9 or hydrogen abstraction.13 In other cases, when ethanolamine derivatives are present, such as N-methyldiethanolamine (MDEA), the main degradation compounds are amines, amino-acids derivatives, and carboxylic acids.14,15 Many efforts and different techniques have been devoted to study oxidative degradation of amines in these processes. For example, a powerful tool employed in studying speciation16,17 is Nuclear Magnetic Resonance (NMR) spectroscopy, but actually, only one paper deals with MEA degraded solutions.18 On the other hand, Strazisar et al.8 used combined gas chromatography (GC)-MS and GC-Fourier transform infrared absorption spectrophotometry (FTIR) to identify the volatile organic compounds. Lepaumier et al.10 studied the thermal degradation in presence of CO2 and O2 and identified the low molecular weight products by GC-MS and high molecular weight compounds by a Fourier Transform-Ion Cyclotron Resonance (FT-ICR)-MS with electrospray ionization (ESI). Received: November 19, 2013 Revised: January 20, 2014 Published: January 20, 2014 1295
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Table 1. Chromatographic Separation Conditions for the Tested Columns chromatographic separation conditions SUPELCOSIL LC-NH2
HILIC AMIDE-80
SUPELCOSIL LC-18
SUPELCOSILABZ
250 mm × 2.1 mm, 5 μm
150 mm × 4.6 mm, 3 μm
250 mm × 4.6 mm, 5 μm
PLUS 150 mm × 4.6 mm, 5 μm
mobile phase gradient
isocratic 90% ACN/10% NH4Ac in H2O for 5 min\; from 90 to 10% of ACN in 50 min\; isocratic 10% ACN for 5 min
injection vol. flow run time of analysis
10 μL
isocratic 80% ACN/20% H2O for 5 min; from 80% to 50% of ACN in 20 min; isocratic 50% ACN for 5 min 10 μL
isocratic 2% ACN/98% H2O for 3 min\; from 2 to 80% ACN in 20 min\; isocratic 80% ACN for 5 min 10 μL
isocratic 2% ACN/98% H2O for 3 min\; from 2 to 80% ACN in 20 min\; isocratic 80% ACN for 5 min 10 μL
0.3 mL/min 65 min
0.5 mL/min 40 min
1 mL/min 38 min
1 mL/min 38 min
Here, preliminary results are presented, along with possible mechanisms explaining the formation of some degradation compounds. Further studies are in progress for a certain assignment of the proposed molecular structures.
Moreover, the ionic chromatography was employed to quantify glycolic, formic, acetic, and oxalic acids, nitrite, and nitrate.10 However, direct analysis by GC cannot be considered as the routine technique since the columns tend to degrade rapidly resulting in poor performance;19 the polar nature of MEA determines its adsorption and decomposition on the columns generating tailed elution peaks, ghost peaks, and low detector sensitivity.20,21 In most cases, a particular treatment of the sampling media with acids or derivatization agents is necessary in liquid chromatography by converting MEA to nonvolatile salt compounds20,22 adding time-consuming sample treatment steps. In addition, if reaction conditions are not rigorously maintained, the incomplete derivatization could result in inaccurate estimations of the analytes. In another approach, MEA can be derivatized by Marfey’s reagent (1-fluoro-2,4dinitrophenyl-5-L-alanine amide or FDAA) and analyzed by high-performance liquid chromatography (HPLC) with variable wavelength UV detection (HPLC−UV).23 Then, the main disadvantage of these techniques is that they remain timeconsuming and labor intensive, with limited value for routine use in the screening of MEA byproducts.24 Among new ionization techniques, matrix assisted laser desorption ionization (MALDI) coupled to a time-of-flight (TOF) analyzer has gained great interest, since an unlimited mass range can be examined with many advantages such as tolerance for buffers and contamination, simple spectra (most ions are singly charged), very high absolute sensitivity, rapid analysis, and relatively easy instrumentation. MALDI has also been extensively used for the characterization of low molecular weight (LMW) compounds25−27 but no studies have been addressed to the use of MALDI-MS for the detection of MEA byproducts, until now. Here, we show that MALDI-MS can be very useful for a fast screening of these compounds without any sample pretreatment. Moreover, we propose the use of hydrophilic interaction liquid chromatography (HILIC), coupled to electrospray ionization mass spectrometry (ESI-MS), in tandem mode configuration (MS/MS), for MEA degradation products characterization. It is worth of note that both these techniques have been applied, for the first time, to identify byproducts generated in a large scale CO2 capture facility with a flue gas of a coal fired power plant. This is of a great value since, to the best of our knowledge, only a few papers8,28,29 are present in the scientific literature dealing with postcombustion CO2 capture real plants. In particular, in a very recent paper,28b ESI-MS was used to identify and quantify the main degradation compounds in three MEA campaigns performed in pilot plants succeeding in the detection of volatile and even large organic byproducts.
2. EXPERIMENTAL SECTION 2.1. Experimental Section. Degraded samples of MEA (see Supporting Information Figure S1) are from a real pilot plant (pilot plant, 10.000 N m3/h of flue gas to separate 2.5 t/h CO2) operating on a slipstream of flue gas from a coal fired power station. The plant is composed by a flue gas pretreatment section (able to remove completely the particulate and the SO3 and to reduce SO2 level below 30 mg/Nm3) and by a CO2 separation unit. The design of the plant has been optimized to use conventional 30% wt monoethanolamine (MEA) aqueous solution (solvent flow: 30 m3/h). It operates at a pressure of 1.8 atm with a gas stream approximately composed of (excluding nitrogen): CO2 (14,12% mol), O2 (5,13% mol), SO2 (25 mg/Nm3), and NO2 (10 mg/Nm3). MEA as absorbent was tested for more than 2500 h of piloting (equivalent to 15 weeks) and liquid samples of the solvent were taken after the stripping process (MEA lean) and analyzed. For quantification of MEA total degradation, calibration curves were generated, and the results are given in the Supporting Information (Table S1). LC-MS grade water, methanol, acetonitrile, ammonium acetate, and ammonium formate used as additives for the HILIC mobile phase, N,N,N′,N′-tetramethyl-1,8-diamino naphthalene (DMAN) were all purchased from Sigma-Aldrich (Milan, Italy). The matrix α-cyano-4chloro cinnamic acid (CClCA) was synthesized according to a reported procedure.30,31 2.2. MALDI-MS. MS experiments were performed using a Micromass M@LDI-LR time-of-flight mass spectrometer (Waters MS Technologies, Manchester, U.K.) equipped with a nitrogen UV laser (337 nm wavelength), reflectron optics and a fast dual microchannel plate (MCP) detector. Positive and negative ion spectra were acquired in reflectron mode. The following voltages were applied: pulse, 2610 V; source, 15 000 V; reflectron, 2000 V; MCP, 1900 V; supply, 5000 V (for negative mode). The laser firing rate was 5 Hz, and, unless otherwise specified, 40 laser shots, obtained by a random rastering pattern, were used for each well. The resulting spectra were averaged, background subtracted, and smoothed by a Savitzky−Golay algorithm. Mass calibration was performed using a mixture composed of amino acids. For the analysis, the raw extract was diluted 50 times in MeOH and then, 5 μL were mixed with an equal volume of a CClCA solution (5 mg/mL in MeOH containing 0.1% TFA) or DMAN solution (10 mg/mL in MeOH), for positive and negative analysis, respectively. The sample (1 μL) was directly spotted on the target plate and analyzed by MALDI-MS. 2.3. HPLC-ESI and GC-MS Conditions. The HPLC−MS/MS contained Agilent (Santa Clara, U.S.A.) 1200 HPLC series pump, degasser, autosampler, and MicrOTOF-Q II mass spectrometer from Bruker Daltonics (Bremen, Germany) with electrospray ionization in negative and positive ion modes. Several HPLC−MS and MS/MS runs were performed on each sample. First, only the MS full scan acquisition was run in the m/z range 50−1000 and the following 1296
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Figure 1. ESI (A) and MALDI (B) spectra (positive ion mode) of the sample MEA lean taken at the end of real pilot plant. parameters were optimized: nebulizer gas nitrogen, 2 bar, dry gas nitrogen, 8 L/min at 200 °C, collision gas argon, external calibration using cluster of sodium formate, collision energy 35 eV. The preliminary MS f ull scan acquisitions were necessary to set instrument parameters for the ionization of many m/z ratios potentially related to MEA. Afterward, several LC-MS/MS runs were performed, in data dependent mode, for structural characterization of MEA related products, according to the preliminary elaboration of MS data. For chromatographic separation, different analytical columns were tested: Tosoh Corporation TSK-Gel Amide-80 (15 cm tmex 4.6 mm, particle size 5 μm, Supelco) (Italy); Supelcosil LC-18 (25 cm × 4.6 mm, particle size 5 μm, Supelco); Supelcosil ABZ+PLUS (15 cm × 4.6 mm, particle size 5 μm, Supelco); Supelcosil LC-NH2 (25 cm × 2.1 mm, particle size 5 μm, Supelco). The best chromatographic separation conditions obtained for each tested column are presented in Table 1. The final elution program, based on acetonitrile (solvent A) and 2 mM of ammonium acetate in water (solvent B), was adopted on Supelcosil LC-NH2: 0−5 min, isocratic at 10% of solvent B; 5− 55 min, linear to 90% B; 55−60 min, isocratic; 60−75 min, back to initial composition, followed by 20 min equilibration time with a flow rate of 0.3 mL/min. Unless otherwise specified, the injection volume was always 10 μL. Routinely, GC-MS analyses were run on Agilent 6850/MSD 5975C equipped with a 60 m × 0.32 mm fused silica column coated with 0.25 μm film of 14%-(cyanopropyl-phenyl)-methylpolysiloxane (DB-1701, J&W Scientific). Separations were performed using the following parameters: Tinj = 280 °C; TDET = 250 °C; Ti = 120 °C; ti = 3 min; rate =10 °C/min; Tf = 280 °C; split ratio = 10:1.
intensities due to the ionization process efficiency. Many of these ions can be assigned on the base of known mechanisms of oxidative MEA degradation.11 Table 2 reports their experimental and theoretical m/z values, their probable attribution, and the indication of their occurrence in relevant spectra (from MALDI or ESI). Table 2. Experimental and Theoretical m/z Values, Probable Attribution, and Occurrence in MALDI or ESI Spectra for Ions Observed in Figure 1
3. RESULTS AND DISCUSSION 3.1. MALDI and ESI-MS Experiments. As stated before, preliminary experiments were run without a chromatographic separation on samples taken at the end of the real pilot plant. For MALDI analysis, our objective was to detect compounds in the low mass range (≤800 Da) where some interfering signals from matrix can suppress analyte ions. However, among tested matrices, CClCA was found to be the most useful for obtaining both reliable ionization efficiency and ion abundance information and was therefore used throughout the work. Figure 1 reports the ESI (run in direct injection) and MALDI spectra. As apparent, in both spectra almost the same signals are observed in the range m/z 100−400 even if with different
experimental m/z [M+H]+
theoretical m/z [M+H]+
probable attribution
ESI
MALDI
113.06 129.05 145.07 163.10 177.13 185.12 206.14 244.16 266.18 303.20 331.20 362.24 386.24
113.0709 129.0659 145.0972 163.1077 177.1234 185.1285 206.1499 244.1656 266.1863 303.2027 331.2345 362.2762 386.2762
[C5H9N2O]+ [C5H9N2O2]+ [C6H13N2O2]+ [C6H15N2O3]+ [C7H17N2O3]+ [C9H17N2O2]+ [C8H20N3O3]+ [C11H22N3O3]+ [C14H24N3O2]+ [C13H27N4O4]+ [C15H31N4O4]+ [C16H36N5O4]+ [C18H36N5O4]+
× × × × × × × × × × × × ×
× × × × × × × × × × × × −
It is worth nothing that no evidence of the use of MALDI as a screening technique for MEA byproducts analysis has been reported. Here, we show that MALDI can be a valid alternative to other time-consuming analytical techniques as NMR, HPLCUV-DAD, ESI, or GC-MS offering the great benefit to be rapid and sensitive with direct analysis of the sample without pretreatment (only a solvent dilution) and no need for analyte derivatization, very quick spectra acquisition (one sample can be analyzed in less than one minute), and simultaneous (even often automated) analysis of many samples. In this case, MALDI analysis allowed to obtain very similar results compared to ESI-MS but with no consumption of the sample for instrument set up, as in the latter case. 1297
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Figure 2. Typical TIC (total ion current) chromatograms obtained on separation of the same sample using (A) TSK-Gel Amide-80, (B) Supelcosil LC-18, (C) Supelcosil ABZ PLUS, and (D) Supelcosil LC-NH2.
A clear example of the different results obtained from the selected chromatographic columns on the same extract is shown in Figure 2(A−D). It is clear that on the reversed-phase column (Figure 2B), these compounds are poorly retained and elute early in the chromatogram, together with other nonretained components of the matrix, thus causing decreased ionization efficiency in the ion source. Indeed, ABZ+PLUS and Supelcosil LC-NH2 traces are characterized by more distinct bands, contrary to TSK-Gel Amide-80 traces, where wide bands were observed including several tens of peaks. The improvement in the signal of the polar analytes on HILIC columns is in agreement with studies on other similar compounds.32,33 However, when more complex mixtures are analyzed, a significant lack of chromatographic resolution in ABZ+PLUS column was achieved indicating a possible coelution of isobaric species. Then, we selected Supelcosil LC-NH2 coupled to ESIMS/MS as the most proficient method, which greatly reduces the effect of matrix interferences and significantly improves the analytical characterization of MEA byproducts. To establish the identity of all chromatographic peaks, precursor ions under each HILIC peak were fragmented upon collision induced dissociation (CID) and the obtained ESI-MS/ MS data were interpreted. We need to point out that neither standards are commercially available for each generated compound nor is the structural indication present in the
Although mass spectrometry permitted to detect and identify compounds based on their masses, online HPLC separation can play a crucial role in the analysis of complex matrixes by adding another dimension of purification. It reduces the possibility of analog interferences and signal suppression from matrix components allowing the detection of a higher number of MEA byproducts. 3.2. ESI-MS Analysis: Comparison between Different HPLC Phases. Since many of these compounds are still unknown, it was necessary to evaluate the performances of both reverse and hydrophilic phase liquid chromatography. To this aim, different stationary phases were tested as reported in Table 1. TSK-Gel Amide-80 column was developed for the analysis of active pharmaceutical ingredients and their metabolites. The resulting phase retained polar/hydrophilic compounds that were either only moderately retained, or not at all retained, on reversed phase columns, showing appropriate retention and good separation of sugars. ABZ+Plus with an alkylamide phase as matrix active group allows the analysis of difficult compounds as acids, strongly basic compounds, and zwitterions by using simple mobile phases. Finally, silica-based Supelcosil LC-NH2 columns separate monosaccharides, disaccharides, and trisaccharides, with elution generally in order of increasing molecular weight. 1298
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Figure 3. Proposed structures for the diagnostic ions at 80.0495, 81.0447, 83.0604, 88.0757, 97.076, and 106.0863 m/z.
literature. Then, the structure assignment was carried out on the basis of probable elemental composition estimated from accurate m/z values, a careful analysis of the isotopic patterns and the occurrence of diagnostic ions in MS/MS spectra. Of course, definitive attributions will require authentic standard comparison or more detailed structural investigations. From the inspection of CID fragmentation spectra, it has been noted that different parent ions generate several common ions that are helpful for establishing the correlation of the decomposition products to the family of ethanolamine derivatives. Indeed, diagnostic peak signals were detected at 80.0495, 81.0447, 83.0604, 88.0757, 97.0760, and 106.0863 m/ z and their structures are reported in Figure 3. In particular, the ions at 88.0757 and 106.0863 m/z derive from the diethanolamine (DEA) class, the ions at 81.0447, 83.0604, and 97.0760 m/z belong to the piperazine or imidazole derivatives and the ion at 80.0495 m/z is representative of piridine derivatives. These product ions characterize, then, the basis whereby other products can be explained by matching their m/z values with those expected from the theoretical oxidative degradation pathways for ethanolamine.8,9,34 For instance, the diagnostic ion at 81.0453 m/z is observed in the MS/MS spectrum of the ion at 113.0722 m/z (Figure 4) that can be attributed to 2-(1Himidazol-1-yl)ethanol (other fragments are assigned in the inset of Figure 4) and in the MS/MS spectrum of the ion at 145.0972 m/z (Supporting Information, Figure S2) where all the fragments are in agreement with the proposed structure of 4-(2-hydroxylethyl)-piperazin-2-one already reported by Lepaumier et al.11 As evident from the MS/MS fingerprint spectrum, these two ions are correlated and they share the core structure even if the ion at 145.0972 m/z elutes later in HILIC column (4.5 vs 3.7 min), as expected from its increasing polarity due to an additional carbonyl residue. These findings are confirmed also looking at the ion 129.0660 m/z (peak R in Supporting Information Table S2) attributed to a 1-(2hydroxyethyl)-1H-imidazol-2(3H)-one with the same core at m/z 81.0447 but with more polar substituents lasting the elution time at 20 min. Another very interesting remark can be made for the ion at 145.0972 m/z. In fact, the same m/z ion is observed at higher elution time (5.8 vs 4.5 min) suggesting the
Figure 4. MS and MS/MS spectra for ion at 113.0722 m/z; the structures of fragment ions are indicated in the inset.
presence of an isobaric/isomeric species. However, the possible final identification is achieved using MS/MS measurements; for this latter ion, the main fragment is detected at 88.0757 m/z (Supporting Information Figure S3), which is correlated, as already mentioned, to diethanolamine (DEA) derivative as morpholine ring (vide inf ra). In this specific case, the HPLC was very helpful in solving the assignment of isobaric species occurring in the total crude extract. Another corroboration for this attribution arises from the observation of the ion at 1299
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Figure 5. Total ion chromatogram of the MEA lean sample obtained using the DB-1701 column.
Scheme 1. Plausible Mechanism of Morpholinones Formation
177.1237 m/z eluted at 7.1 min, which correlates to ion 145.0972 m/z, since again the diagnostic fragment at 88.0757 m/z is detected (Supporting Information Figure S4). A different behavior was observed for the ion at 246.1449 m/ z (Figure S5, peak J) where the fragmentation pattern was more complex. However, taking into account that carboxylic acids formed as short byproducts during the CO2 postcombustion capture may react with alcoholic residues to give an ester as better leaving group function,35 the formation of methyl 2-(2-(3-(2-hydroxyethyl)-2-oxoimidazolidin-1-yl)ethylamino)acetate was suggested (vide inf ra), and the daughter ions were attributed. In the same way, considering the diagnostic fragment pattern, the accurate m/z values, and the isotopic distribution, it was possible to clarify the structures of all the eluted peaks and the results are summarized in Supporting Information Table S2. For some ions, together with the protonated molecules the coeluting sodium adducts were present in the spectrum (sodium was probably present as impurities in the solvent and/or in the plant) as for the ion at 211.1070 and 233.0894 m/z, 246.1449 and 268.1269, the ions at 163.1077 and 185.0896 m/z, respectively. From a general survey of these identified molecular formulas, under real conditions and for a long time of MEA use in process capture, degradation products seem to be in agreement with an alkanolamine oxidative degradation pathway instead of a thermal degradation,34 leading to the formation of oxidized compounds such as piperazinone, amides, and esters. Of course,
further experiments such as a comparison with authentic standards or successive structural studies should be done for a certain assignment. In comparison with other papers devoted to the same topic,8,28 GC-MS analyses (see Figure 5) allowed us to detect volatile and well-known compounds such as 2-oxazolidinone (OZD), N-(2-hydroxyethyl)imidazole (HEI), and also two plausible morpholinones at 101 and 115 m/z. These latter compounds are unprecedented in the literature for MEA studies, being usually reported in case of methyldiethanolamine (MDEA) oxidative degradation.36 These products could be explained by the mechanism reported in Scheme 1. In particular, morpholin-3-one can be obtained from dehydration of 2-hydroxy-N-(2-hydroxyethyl)acetamide (HHEA),28,37 whereas N-methylmorpholin-3-one can be obtained speculating a similar mechanism starting from 2(methylamino)ethanol, a MEA degradation product reported by other authors.34,38 In a recent study, da Silva et al.,28b with a similar technique and under MEA degradation conditions close to those reported here, found N-(2-hydroxyethyl)glycine) (HEGLY) and 4-(2hydroxyethyl)piperazin-2-one (HEPO) as the dominant degradation products formed, with also high concentrations of N-(2-hydroxyethyl)acetamide (HEA), HEI, and N-(2hydroxyethyl)-formamide (HEF). As reported in Supporting Information Table S3, in our case the main degradation products are partially in line with da Silva findings, being HEI also formed in relevant amount and 2-(2-hydroxyethylamino)1300
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Scheme 2. Formation Mechanism of Some Degradation Products
2-one (HEIA). This molecule has been already reported by other authors,34 according to a degradation pathway starting from MEA carbonatation (path b), then leading to a Nsubstituted carbamate. Cyclization and successive reaction with another molecule of MEA, can produce N-(2-hydroxyethyl)ethylenediamine (HEEDA), which reacts with CO2 to form a linear carbamate, followed by intramolecular cyclization giving HEIA, then R byproduct. The most abundant detected product F is a substituted imidazole, which, being stable, can accumulate in the solution, but with a formation mechanism still unknown.28 From HEIA, after reaction with MEA, a N-(2aminoethyl)-N′-(2-hydroxyethyl)imidazolidin-2-one (AEHEIA) can be formed. This molecule, which is commonly found in laboratory-scale MEA degradation tests, is not detected in real samples.8,28 Under our conditions, ESI analyses were in line with these findings, revealing the presence of a more complex structure (K), probably originated by condensation of AEHEIA with glycolic acid,34 and subsequent methanol esterification.
N-(2-hydroxyethyl)acetamide (O) closely related to HEGLY, after a subsequent MEA condensation. However, our most abundant MEA byproduct 3-(2-hydroxyethylamino)-N-(2hydroxyethyl)propanamide (M) is not observed by da Silva,28b but it is in agreement with Strazisar et al.8 findings where the relevant gas phase infrared spectrum is also reported. Though, as already reported, HILIC separation permitted the detection of a major number of compounds in comparison with GC-MS analysis. Besides the presented structures (Supporting Information Table S2), the general formation mechanism of some degradation products can be roughly divided in three main pathways, as depicted in Scheme 2. In path a, as already reported by Idem et al.,39 1,3propandiamine can be invoked in the formation of N,N′-bis(2hydroxyethyl)-1,3-propanediamine resulting from the condensation with epoxide, directly derived from MEA deamination under aerobic alkaline conditions.40 A subsequent oxidation can lead to the substituted N-(hydroxyethyl)propanamide (M). Compounds R, F, and K can be presumably obtained from subsequent degradation of the N-(2-hydroxyethyl)imidazolidin1301
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4. CONCLUSIONS The use of MALDI mass spectrometry for a fast screening of the main degradation products formed during CO2 postcombustion capture process in aqueous solution of monoethanolamine is shown here for the first time. This procedure proved to be simple, sensitive and suitable for routine analysis since both sample preparation and instrumental analysis are very fast. Moreover, a full detection of these compounds was accomplished by means of a new analytical method based on HILIC combined with single and tandem ESI-MS. Along with the recognition of the main compounds, a rationalization of their formation mechanism was achieved and compared with theoretical pathways proposed in previous studies. Further studies based on the use of standard compounds or detailed structural analysis are in progress to confirm these preliminary findings.
The presence of glycolic acid is not unexpected and can be explained according to a radical pathway involving MEA degradation, via imine formation.9,39 It should be noted that another degradation pathway for the formation of AEHEIA can be also invoked starting from (HEEDA) that reacts with the oxazolidinone originating a trimer, which, in the presence of CO2, can give the imidazolidinone AEHEIA, after internal cyclizing condensation.28 Lastly, the path c leads to the formation of substituted piperazinones. Strazisar et al.8 in a real postcombustion capture process, found 4-hydroxyethyl-2-piperazinone (I) as the most abundant peak in the GC-MS chromatogram. They proposed a mechanism for its formation starting from a radical reaction between MEA and acetic acid, then leading to 2-hydroxyethylamino-N-hydroxyethyl acetamide (O), which, by internal cyclization, may form the two six-membered ring regioisomers 1-hydroxyethyl-2-piperazinone and 4-hydroxyethyl-2-piperazinone (I). However, we were able to detect only the less hindered isomer (I), both using HILIC and GC separations (see Figure 5). This regioisomer, preferably formed due to sterical reasons, was also reported by Lepaumier et al.,28,11 but a different formation pathway was proposed. Indeed, by this way, it should be possible to explain the formation of the peak L resulting from an internal cyclization of the N-(2-aminoethyl)2-hydroxy-N-(2-hydroxyethyl)acetamide (AEHHEA) (Scheme 3, path b) giving a substituted morpholinone. Therefore,
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ASSOCIATED CONTENT
S Supporting Information *
Additional tables and figures as described in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Phone: +39-080-5442119. E-mail:
[email protected]. *Phone/Fax: +39-080-5442026. E-mail: cosimadamiana.
[email protected].
Scheme 3. Internal Cyclization of AEHHEA
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Dr. A. Rizzuti for his help in carrying out MS analyses. REFERENCES
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contrary to Strazisar findings,8 the ion at m/z 145.0972 (eluting at 5.8 min in LC run) should not be attributed to the regioisomer 1-hydroxyethyl-2-piperazinone but, due to the different pattern fragmentation observed in MS/MS spectra (see Supporting Information Figure S2), to the 4-(2aminoethyl)morpholin-3-one. In our case, the 1-substituted piperazinone could coelute, if formed, with the less hindered isomer I. It is worth of note that many of the attributed ions in Supporting Information Table S2 were already observed in our previous MALDI or ESI-MS analyses without any separation, but as demonstrated, HILIC chromatography allowed to discriminate also isobaric species. 1302
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