As the burning of fossil fuels has dramatically grown throughout the industrial revolution, so has the concentration of CO2, increasing from 280 at the start of the industrial revolution to 406 ppm in 2017. There are two approaches being considered for the removal of acid gas pollutants: (1) sequester these gases from the atmosphere or (2) remove these gases directly upon combustion (post-combustion). As such, aqueous and solid-state amines have shown excellent promise for post-combustion removal of CO2, yet the overall mechanisms for these processes as well as the thermodynamics of specific products formation remains elusive. Reaction coordinates for a series of CO2 capture reactions by various alkylamines and H2O were calculated at the G3(MP2) level in the gas phase with aqueous solution calculations performed at the G3(MP2) level with an additional self-consistent reaction field approach at the MP2 level added for the treatment of solution. The G3(MP2) calculations were benchmarked at the CCSD(T)/CBS level for NH3 reactions. In the gas phase, alkylamines are found to reduce the energy barrier more than catalytic waters for both carbamic acid and carbonic acid formation. In aqueous solution, the barriers to form both carbamate and bicarbonate are further reduced, with bicarbonate formation ocurring if only one amine is present and carbamate formation possible only if multiple amines are accessible. Further reactions of carbamic acid to form urea via the Bazarov reaction were also modeled. The barriers for proton transfer in each reaction correlate nicely with amine basicity for alkylammonium carbamate (ΔG‡aq &lt; 15 kcal/mol) and alkylammonium bicarbonate (ΔG‡aq &lt; 30 kcal/mol) formation pathways. These results have implications for CO2 capture by amines in both the gas phase and aqueous solution as well as in the solid state, if enough water is present.

Predicting the mechanism and products of CO2 capture by amines in the presence of H2O

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