CO2Reaction Mechanisms with Hindered Alkanolamines: Control and Promotion of Reaction Pathways

The mechanisms of the reaction of CO2 with hindered alkanolamines are described based on direct reaction monitoring with in situ 13C NMR spectroscopy. Four amines of increasing steric hindrance were studied and compared with unhindered primary, secondary, and tertiary alkanolamines. The number and location of additional methyl (or other) groups on the nitrogen or adjacent carbon atoms create different degrees of steric hindrance. As steric hindrance increases, the Lewis basicity (nucleophilicity), as an affinity of the amine nitrogen to directly attack the electrophilic carbon of CO2 and form a carbamate, decreases. At the same time, methyl (or other) groups present on nitrogen or adjacent carbon atoms do not change the Brønsted basicity of hindered amines. Coupled with lower Lewis basicity, the CO2–amine–water reaction equilibrium favors formation of bicarbonate, with higher CO2 loading capacity and lower thermal stability, both of which are favorable properties for cyclic CO2 capture. Due to lower stability of the hindered N–C bond of the carbamate, hindered amines AMP and MAP in aqueous solution show a phenomenon of steric acceleration: the more rapid transfer of CO2 from the nitrogen of the amine (e.g., carbamate) to form the bicarbonate anion, maintaining relatively high reaction rates. A strong nucleophilic base such as piperazine present in the amine solution at low concentration maintains the high reaction rate by attacking free CO2 and transferring it to the amine as bicarbonate. This described mechanism helps to improve CO2 capture rates with severely hindered amines such as in the secondary amine MAMP, which without a promoter acts as a tertiary amine slowly reacting with CO2. The mechanism of chemical reaction between CO2 and amines dissolved in methanol is similar to that in water—methanol attacks CO2 and forms an O–C bond and methylbicarbonate anion (analogue of bicarbonate) with CO2, which is stabilized by a protonated amine. The effects of amine concentration, CO2 partial pressure, and temperature are discussed.