Pathways and products of oxidative degradation

Literature review

2.2 Pathways and products of oxidative degradation

Figure 2.1: Chemical structure of typical primary oxidative degradation products of MEA.

Oxidative degradation takes place, when the amine comes in contact with oxidizing species, such as dissolved O2, SOX, or NOX from the flue gas. Oxidation reactions take place after the amine solution absorbs oxidising species from the flue gas in the absorber column. The initiation step of oxidative degradation reaction is assumed to

take place via a radical mechanism, by either electron abstraction, hydrogen abstraction, or less commonly a reaction between water and aminium (Bedell et al., 2011; Hull et al., 1967; Rooney et al., 1998; Smith and Mann, 1969). The main products of these initial reactions are organic acids, mainly formic, acetic, glycolic, and oxalic acid as well as ammonia (NH3), aldehydes, and methylamine (Figure 2.1), especially for MEA, but also many other amines (da Silva et al., 2012). The formation of these acids has proven to be catalysed by dissolved metals (Blachly and Ravner, 1963; Goff, 2005; Sexton and Rochelle, 2009). So far, no experimental studies have identified any of the radical intermediates, although many thorough and likely mechanistic predictions have been made.

Formation of all these acids releases ammonia from the organic molecule. The formation of methylamine was hypothesized to take place via a radical mechanism, simultaneously as the acid formation, first by Rooney et al. (1998) and then in a different mechanism by Lepaumier (2008), as show in Figure 2.2. Likely because of the difficulty in setting up mechanistic studies involving radicals, especially in complex mixtures such as CO2 loaded amine solutions, the exact mechanisms of primary degradation product formation have not been confirmed.

Figure 2.2: Proposed mechanisms of formation of some of the primary degradation products of MEA, by Rooney et al. (1998) and Lepaumier (2008).

Following the formation of the primary degradation compounds; many secondary degradation compounds have been identified. A selection of some of the abundantly studied secondary oxidative degradation compounds can be seen in Table 2.1. Many of these are amides, which may be formed in reactions between the amine and primary

degradation compounds, as shown in Figure 2.3 (da Silva et al., 2012; Lepaumier et al., 2011a; Strazisar et al., 2003). HEA has been shown to form in reaction between MEA and acetic acid, while HEF forms from MEA and formic, or also oxalic acid, (Supap et al., 2011). HHEA is a product from MEA and glycolic acid, while BHEOX is an indirect product formed by reaction of MEA with oxalic acid (Lepaumier et al., 2011a). HEOX is possibly an intermediate, that has only been tentatively identified (Gouedard, 2014; Vevelstad and Svendsen, 2016). HEOX has also been hypothesized to form by hydrolysis of BHEOX (Supap et al., 2011). HEHEAA is suggested to be formed in this manner with either HEA, HEGly or glyoxal, as depicted in Figure 2.4 (da Silva et al., 2012; Gouedard, 2014; Strazisar et al., 2003).

Table 2.1: Names, common abbreviations, CAS number and chemical structure of many of the commonly studied and identified secondary degradation products, mainly of MEA.

Name Abbreviation CAS Structure

One of the big mysteries in oxidative degradation of MEA, is the formation of HEI, being such a dominant degradation product and yet not a direct product of any simple condensation reaction, also being one of few identified aromatic degradation compounds. Patents have suggested that reactions between MEA, glyoxal, formaldehyde and ammonia can produce HEI (Gouedard, 2014; Katsuura and Washio, 2005; Kawasaki et al., 1991), and Vevelstad et al. (2013) proposed a reaction mechanism based on this, shown in Figure 2.4. The fact that the same publication observed that increasing oxygen concentration gives increased HEI production suggests that the formation of HEI is favoured under highly oxidizing conditions, possibly through a radical mechanism.

Figure 2.3: Proposed mechanisms of formation of HEF, HEA, HHEA, HEOX, and BHEOX according to da Silva et al. (2012), Lepaumier et al. (2011a), and Strazisar et al. (2003).

Figure 2.4: Reactions suggested to form HEHEAA, by a) a radical reaction between MEA and HEA, catalysed by ferric (Strazisar et al., 2003), or in condensation reactions between MEA and b) HEGly da Silva et al. (2012), or c) with glyoxal (Gouedard, 2014).

Figure 2.5: Mechanism proposed for the formation of HEI from 2-methyleneamino)ethanol and iminoacetaldehyde, by Vevelstad et al. (2013).

Another unknown is how the dominant degradation product HEGly is formed, which is also present in abundance in degraded MEA, but is not a known condensation product of any two compounds, when tested in laboratory scale. The only mechanisms proposed for HEGly formation were made by Vevelstad et al. (2014) as a condensation reaction between glyoxylic acid and MEA, under dissociation of a CO2 molecule, as given in Figure 2.6, or from HEHEAA as suggested by Gouedard (2014).

Figure 2.6: Formation mechanism of HEGly from MEA and glyoxylic acid, proposed by Vevelstad et al. (2014).

There also seems to be a disagreement between the ratios of oxidative degradation compounds formed on the pilot scale compared to in laboratory scale oxidative degradation studies. HEPO and HEGly are usually the dominant products observed in pilot scale MEA campaigns (da Silva et al., 2012; Morken et al., 2017), whereas in laboratory scale oxidative degradation experiments at simulates absorber conditions, HEF and HEI have been observed in the largest quantities (Vevelstad et al., 2013). A low concentration of O2 has, however, proven to give rise to HEGly formation also in laboratory scale (Vevelstad et al., 2013). HEPO on the other hand, is hypothesized to require higher temperatures than given at absorber conditions, or in the studies of purely oxidative conditions, formed by thermal dehydration of HEHEAA, as shown in Figure 2.6. The same studies also saw and suggested an alternative, analogous mechanism for the formation of the less dominant 1HEPO species. Other mechanisms for the formation of HEPO and 1HEPO were also suggested by Gouedard (2014), which can be viewed in Figure 2.8.

Figure 2.7: Proposed mechanisms of self-condensation of HEHEAA to form HEPO and 1HEPO according to da Silva et al. (2012) and Strazisar et al. (2003).

Figure 2.8: Suggested mechanisms for the formation of HEPO and 1HEPO by Gouedard (2014).

Many researchers have previously studied oxidative degradation of amine in laboratory scale, for nearly a century, a selection of which can be seen in Table 2.2.

The first of these were comparing different amines and their stabilities, as well as looking for inhibitors and catalysts of degradation. In the past two-three decades a lot of studies have aimed to understand the fundamentals of these reactions on a more mechanistic level, at the same time as studying many different amines, and searching for inhibitors of degradation.

Table 2.2: Research contributions towards understanding oxidative amine degradation.