Characterisation and kinetic study of carbon dioxide absorption by an aqueous diamine solution

Aqueous monoamine solvents have been extensively studied for the purpose of CO2 absorption to reduce emissions from sources such as industrial power stations. However, to improve the economic viability of carbon capture technologies, solvents with higher CO2 absorption capacity and faster kinetics are urgently required. Diamines comprising two amino groups have potentially higher CO2 absorption capacity and rates than monoamine solvents, such as monoethanolamine, and could be superior liquid absorbents for CO2 absorption. In this study, we selected six linear diamines with a structure of NH2(CH2)n-R (n=2 or 3; R=NH2, NHCH3 or N(CH3)2) and four monoamines with a structure of NH2(CH2)n-R (R=OH, CH3 or CH(OH)CH3). We then investigated the effect of diamine molecular structure on absorption kinetics and capacity using a bubble column, and confirmed the observed kinetic behaviours of selected diamines at different concentrations using a wetted-wall column and stopped-flow reactor. Under the conditions studied, all selected diamines had an absorption capacity of more than 0.78moles of CO2 per mole of amine, which far outstrips the capacity of monoamines. The hydroxyl group decreased the rate of CO2 absorption, while the methyl group and longer chain lengths increased CO2 absorption rate and capacity; the tertiary amino group exhibited the lowest kinetic performance. N-methylpropane-1,3-diamine (MAPA) had both the fastest absorption rate and the highest mass transfer coefficient. Using Fourier-transform infrared spectroscopy and 13C nuclear magnetic resonance, we elucidated the mechanism involved in the reaction of MAPA with CO2. Our research provides a method for the future selection and design of new diamines for post-combustion CO2 capture.