https://doi.org/10.1021/acs.iecr.2c02344
“In postcombustion CO2 capture using chemical absorbents, the principle is based on a reversible chemical reaction between the absorbent and CO2. Therefore, the compounds that can partake in degradation reactions are both the free amines and the products formed in the reactions between amine and CO2, e.g., carbamates. Environmental friendliness and safe work conditions can only be ensured by knowing the structure of the chemical compounds formed during amine degradation. Thus, studying degradation pathways and degradation component identification are crucial tasks during solvent development. Based on previous work, chemistry knowledge, and the structure of the solvent, some predictions can be made about the degradation mechanisms and compounds that can be formed for many amines. There is, however, no single analytical method that can be used to identify, let alone quantify, the degradation products formed in a new, previously unstudied amine. Studying the degradation of new solvents will always require the development of analytical methods and detective work to propose likely degradation compounds, then identify and finally quantify them. Even two amines, where the only difference is one more −CH2– in its alkyl chain, will go through different degradation pathways, and the degradation compounds and their mechanism of formation cannot be entirely predicted based on one of them. To illustrate, let us look at the carbamate polymerization mechanism occurring with amines in the presence of CO2. In the case of MEA and 3-amino-1-propanol (AP), the mechanism leads to two different ring compounds, as illustrated in Scheme 1. The mechanism is the same, but the products are different, needing two different analytical methods for quantification.”
“Scheme 1. : MEA and AP Reacting with CO2 Forming, Respectively, 2-Oxazolidinone (OZD) and 1,3-Oxazinan-2-one (OZN)”
“Despite these challenges, alkanolamines and, to some extent, polyamine studies give an indication of the classes of compounds that can be expected. It is important to note that in these laboratory-scale studies, the solvents were degraded to a high degree, under partly unrealistic conditions, to produce sufficiently high concentrations of degradation products to surpass the detection limits of the analytical instruments. This means that the reaction schemes and degradation compound concentrations are typically higher than what would be seen in a full-scale plant.
Generally, the compounds formed contain a combination of two or more of the following species: N, C, H, and O. The following functional groups and component types are generally observed in degradation studies: ammonia, aldehydes, alkylamines, carboxylic acids, amides, amino acids, ketones (in some cases), urea, cyclic structures such as piperazinone, imidazolidinone, oxazolidinone, and pyrazines. Most of these are not volatile components and will therefore accumulate in the solvent system, giving rise to more operational and cost issues than environmental or health concerns. One might say they could still be a health concern; however, these single components are not present in concentrations believed to cause any danger to operators, especially since personal protective equipment is already used when working with chemical compounds in the lab and industry. As long as the component is not volatile, it would, therefore, be hard to see that it can pose a larger risk than the risks we are surrounded by in our daily life (i.e., detergent, antifreeze, cosmetics).
There are numerous general analyses used for solvent monitoring, due to the variation of functional groups among the degradation compounds. This means that a combination of analytical instrumentation is required to detect them. Buvik et al.
(105) give a thorough overview of analytical methods used in pilot campaigns, including both more general methods and degradation compound-specific methods. Numerous analytical equipment has been used in lab-scale experiments to identify and understand the formation of degradation compounds. This includes instrumentation for both gas and liquid chromatography (GC and LC, respectively), often coupled with mass spectrometry (MS) or other detectors. Common variations of GC and LC are GC−MS,
(12,15,29,46,108−112) GC coupled with nitrogen chemoluminescence detection (GC-NCD)
(56,104) or flame ionization detection (GC-FID),
(113) LC with tandem mass spectrometry (LC–MS/MS),
(27,29,46,54,56,109) high-performance LC (HPLC)
(22,23,26,66,114,115) and HPLC coupled with time-of-flight MS detection (HPLC TOF-MS).
(66,108,116) Other important analytical methods include ion chromatography (IC),
(15,22,23,26,27,46,54,66,108,109,111,115,116) capillary electrophoresis (CE),
(114,117) nuclear magnetic resonance (NMR),
(15,22,109,110,118) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR–MS).
(1,109) A combination of several of these is required for analyzing all degradation compounds; GC-NCD is often used specifically for the analyses of total nitrosamines, while IC is often used for carboxylic acids or other anionic compounds or the amine itself. GC–MS and LC–MS/MS, HPLC techniques are used for various degradation compounds. The choice is often related to the availability of competence and instrumentation. Table S3 in the
Supporting Information gives more details regarding analytical instrumentation and analytical methods for degradation components.
The amount of metals in the solvent is often monitored by inductively coupled plasma mass spectrometry (ICP-MS) and water content by Karl Fisher titration or by an internal standard added to the solution. Titration with a strong acid is used for determining the total amine concentration, and the amount of absorbed CO
2 is analyzed by total inorganic carbon (TIC)/total organic carbon (TOC) analyzer or by titration using e.g., BaCl
2.
(119) Using a total nitrogen (TN) analyzer or total organic nitrogen using the Kjeldahl method
(120) gives the amount of nitrogen in the solutions. Finally, a wet chemistry method based on ion exchange followed by titration
(118) has been used to analyze for the heat-stable salts (HSS, total ionic content in the solution). Density can be used to monitor CO
2 loading of the solvent.”
