Unraveling the mechanism and the role of hydrogen bonds in CO2 capture by diluent-free amine sorbents through a combination of experimental and theoretical methods

The utilization of water-lean and non-aqueous amine sorbents is regarded as an appealing approach to reduce the energy costs of CO2 capture via liquid sorbents. However, significant research is still needed to achieve the technological maturity required for industrial-scale implementation. Here, we present a detailed experimental and computational analysis at the molecular level of CO2 capture by dipropylamine (DPA) as a case study to deepen our understanding of the mechanisms governing CO2 absorption by liquid secondary amines that can be used without any additional diluent. CO2 uptake with pure DPA was investigated, and the species produced over time were determined by NMR and FT-IR spectroscopy. In particular, the NMR analysis revealed the formation of carbamic acid at high CO2/DPA ratios. A detailed DFT investigation explained the mechanism of the reaction revealing a dynamic evolution in product distribution as CO2 loading increases. At low CO2 loadings, adducts with at least four DPA molecules are formed, ultimately leading to the carbamate/ammonium ionic pair stabilized through H-bonding interactions with DPA moieties. Conversely, at higher CO2 levels some stabilizing DPA molecules of ionic pair are required for the CO2 activation, resulting in the formation of carbamic acid. A reasonable mechanism for the evolution of product distribution is provided, and the main steps of the mechanistic picture are depicted and commented on. The dependence of carbamate and carbamic acid on the availability of hydrogen bond donors and acceptors in solution is also highlighted.