Degradation product identification

GC-MS was employed to identify the non-ionic degradation products that were formed during the AMP thermal degradation process. Three of the main degradation products were identified by GC-MS method. A typical GC-MS chromatogram for the thermally degraded AMP sample is shown in Figure 4.5.

Figure 4.5Chromatogram of a 4.75 mol/kg AMP aqueous solution with a loading of 0.3 mol of CO2 per mol AMP held at 135 °C for 5 weeks. In addition to AMP, the products were identified as DMOZD (peak 1), AMPAMP (peak 2), unknwon (peak 3), DMHTBI (peak 4).

Computer fitting of the mass spectrum to the mass spectra database (NIST MS search 2.0) was tried to identify the main products in the thermally degraded AMP samples. The identification was followed by the use of standards (if commercially available) to confirm the identification of the components in the samples.

One of the major products observed in the thermally degraded AMP aqueous solutions was 4, 4-dimethyl-2-oxazolidinone (DMOZD, peak 1). The confidence of the mass fragmentation pattern of DMOZD in the degradation samples matched that documented in the mass spectrometer database was by 78%. The spectrum of the identified DMOZD in the

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degraded sample and the spectrum of DMOZD in the database are shown in Figure 4.6. As can be seen in Figure 4.6, the mass fragmentation pattern of DMOZD in our samples matched that documented in the mass spectrometer database. However, this identification was not confirmed by an authentic standard since DMOZD is not commercially available.

Figure 4.6 (a) Mass spectrum of the compound identified as DMOZD in a 4.75 mol/kg AMP aqueous solution with a loading of 0.3 mol of CO2per mol AMP held at 135 °C for 5 weeks;

(b) Mass spectrum of DMOZD in NIST database.

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The confidence of computer fitting of the mass spectra of peak 2, 3 and 4 (Figure 4.4) to the mass spectra in the database was very low (less than 20%). It is probably due to exclusive of the mass spectra of these three compounds in the database. Thus the mass spectra of these formed products of the current amine thermal degradation with CO2cannot be matched.

The ionization source in mass spectrometer used in this work was an electron impact ionization source (EI). This ionization process often follows cleavage reactions that give rise to fragment ions which, following detection and signal processing, convey structural information about the analyte. But EIMS does not exhibit a molecular ion peak of the analyte, if the analyte is a relatively heavy molecule, a high polar molecule, or a molecule with low thermal stability.

Since the degradation products are amine derivatives, they could be polar molecules and have relatively big molecule mass. We therefore may not find the molecular ions from the mass spectra. However, we can analyze the fragment ions in the spectra to identify the possible products. Figure 4.7 (a) shows the mass spectrum for the peak 2 at 13.4 min in the Figure 4.5. According to the analysis of the fragment ions of the spectrum, the peak at 13.4 min might be 2-[(2-amino-2-methylpropyl) amino]-2-methyl-1-propanol (AMPAMP). This identification was confirmed by authentic standard 2-[(2-amino-2-methylpropyl) amino]-2-methyl-1-propanol. The standard was supplied by Aldrich^® (Milwaukee, USA), and the product number was S347027. This standard was diluted with Milli-Q water and run on GC-MS at the identical conditions as that for the AMP degradation samples. Figure 4.7 (c) shows the gas chromatogram of the standard aqueous AMPAMP. As can be seen from Figure 4.7 (c), there were some impurities in the standard solution or the standard was not very stable in the GC conditions that resulted in some small peaks except for the main peak, however it does not affect the confirmation. The predominant peak at 13.4 min should be AMPAMP, and the corresponding mass spectrum is shown in Figure 4.7 (b). The retention time of the standard was 13.4 min, which is the same as that of the identified peak. On the other hand, the main ion peaks in the mass spectrum of the identified peak are consistent with those of the authentic standard. Therefore, the identification of AMAMP in the degradation samples of AMP is reliable.

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Figure 4.7 (a) Spectrum of the compound identified as AMPAMP in a 4.75 mol/kg AMP aqueous solution with a loading of 0.3 mol of CO2per mol AMP held at 135 °C for 5 weeks.

(b) Spectrum of standard AMPAMP. (c) Gas chromatogram of standard AMPAMP aqueous solution.

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Figure 4.8 shows the mass spectrum of peak 4 at 23.2 min in Figure 4.5. According to the analysis of the fragment ions of the spectrum, the peak at 23.2 min may be 4, 4-dimethyl-1-hydroxytertiobutyl-2-imidazolidinone (DMHTBI, peak 4). Unfortunately, the standard of DMHTBI is not commercially available for confirmation. However, Davis (2009) has found and identified DMHTBI by IC-MS in a thermal degraded AMP solution in the presence of CO2. The protonated imidazolidinone, m/z=187 (see Figure 4.9). This is a strong support for the identification of DMHTBI. And DMHTBI was reported as a major degradation product of AMP in the presence of CO2 (Lepaumier et al., 2009b; Eide-Haugmo et al., 2011). The chemical structures of the identified products are summarized in Figure 4.10. All of these identified products fall within the degradation pathway for carbamate polymerization. Peak 3 in Figure 4.5 remains unknown.

Figure 4.8 Spectrum of the peak 4 (Figure 4.5) in a 4.75 mol/kg AMP aqueous solution with a loading of 0.3 mol of CO2per mol AMP held at 135 °C for 5 weeks.

Figure 4.9 MS spectrum for a solution of 7 m AMP with a loading of 0.4 moles of CO2per mole of amine held at 135^°C for 8 weeks and injected by syringe pump (Davis, 2009).

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H₂N N

H

OH O

O

OH HN

O

HN N

(DMOZD) (DMHTBI)

(AMPAMP)

Figure 4.10 Structure of the identified products.

4.3.3 Degradation pathways

Thermal degradation of MEA in the presence of CO2 occurs in a process termed carbamate polymerization, as presented in Figure 4.1. For AMP thermal degradation, it was postulated that the reactivity of DMOZD is very low due to the steric hindrance. The steric hindrance of the two methyl groups adjacent to the nitrogen atom prevents oxazolidinone ring-opening into an addition product (Lepaumier, 2009b). However, the identification of the products in this work and the reported results (Davis, 2009; Lepaumier et al., 2009b; Eide-Haugmo et al., 2011) demonstrate that AMP also can undergo the similar reactions as MEA does in the presence of CO2.

A possible degradation pathway of AMP with CO2is proposed in Figure 4.11, which is based on the identified products and the literature results (Davis, 2009; Lepaumier et al., 2009b). The difference as compared to MEA is that DMOZD is slightly more stable due to the mild hindrance and will hence accumulate in the solution contrary to 2-oxazolidinone (OZD). AMP trimer and further polymeric products were not observed in AMP degradation samples that could be due to the steric hindrance either. These results indicate that the steric hindrance in AMP molecule can slow down CO2induced degradation of AMP as compared to MEA, but it does not prevent oxazolidinone formation and oxazolidinone ring-opening into further degradation products.

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Figure 4.11 Scheme for degradation of AMP with CO2.

4.3 Conclusions

Aqueous AMP solutions were thermally degraded using closed-batch reactor. AMP is stable at temperatures less than 140°C under nitrogen. However, as with traditional alkanolamines, AMP can be degraded thermally in the presence of CO2. For 4.75 mol/kg AMP solution, the calculated first order rate constant of k1was 13.1×10^-9and 18.6×10^-9s^-1 at 135°C with an initial CO2loading of 0.15 and 0.3 mol CO2/mol AMP, respectively.

A Possible pathway for AMP thermal degradation was proposed and validated via GC-MS analysis. At 135°C, the major products included 4, 4-dimethyl-2-oxazolidinone, 2-[(2-amino-2-methylpropyl) amino]-2-methyl-1-propanol, and 4,4-dimethyl-1-hydroxytertiobutyl-2-imidazolidinone. The presence of the products were identified but not quantified. All of these products fall within the degradation pathway for carbamate polymerization. AMP does form a carbamate and continue to form 4, 4-dimethyl-2-oxazolidinone. The steric hindrance in AMP does not prevent oxazolidinone species formation but make it less favourable than in the case of MEA.

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