NMR Techniques and Prediction Models for the Analysis of Species Formed in CO2 Capture Processes with Amine-Based Sorbents: A Critical Review

Carbon dioxide (CO2) capture by aqueous alkanolamines is among the most mature and efficient technologies to curb the continuous emission of the greenhouse gas CO2 into the atmosphere. However, the widespread use of this technology is limited, mostly due to the energy penalty during CO2 desorption and amine regeneration. A key point to develop more efficient sorbents is the knowledge of the species formed in solution after the reaction of CO2 with the amine. Qualitative and quantitative analysis of ions in solutions can help to understand chemical reaction processes and probe chemical reaction mechanisms to discern important information including the CO2 absorption and desorption rates, the CO2 capture efficiency, the cyclic capacity, and the energy demand for regeneration, which are essential for the commercialization of this technology. Although many researchers have reported the speciation of primary, secondary, and tertiary amines when reacting with CO2 as determined by nuclear magnetic resonance (NMR) and other methods, a few discussed the state-of-the-art research in this area. This paper aims to review and compare NMR spectroscopy, pH + NMR analysis, and model prediction techniques for determining the speciation of CO2 loaded amine solution, to get information for better understanding the fundamental principles and up-to-date progress applied in various amine-CO2 systems. This review illustrates the applications of these three techniques to observe the morphology of CO2 loaded amine solutions including single amines, blended aqueous amines, and nonaqueous amine solutions. Furthermore, the operating principles are described in detail, and the strengths and weaknesses are discussed carefully. Of the three approaches, NMR spectrometry is proven to be more efficient in determining the proportion of ions in simple amine-CO2-H2O systems; however, for more complex systems, the process efficiency varies depending on the situation encountered. In sum, these three analytical techniques can help to design efficient amine materials with high CO2 separation performance and low energy cost.