Advanced designer amines for CO2 capture: Interrogating speciation and physical properties

Large scale and affordable CO2 capture is currently limited by absorbents with somewhat non-optimised chemical and physical properties. Cyclic designer amines with multiple amine groups and tunable molecular structure are proposed to contribute favorably to improve absorbent performance and robustness for CO2 capture. In this work the impact of cyclic structure on absorbent performance has been investigated using 4-amino-1-methylpiperidine (4-A1MPD) as a model designer amine and is compared with its linear di-amine analogue N,N-dimethyl-1,3-propanediamine (N,N-DM13PDA). The benchmark absorbent monoethanolamine (MEA) was investigated in parallel. To illustrate the significance of the cyclic structural feature, adoption of other structural changes such as methyl and propyl groups, and C-2 to C-3 backbone chain lengths were also investigated for comparison using the cyclic di-amine 4-amino-1-propylpiperidine (4-A1PPD) and linear di-amine N,N-dimethyl-1,2-ethanediamine (N,N-DM12EDA). CO2 absorption and desorption experiments were performed at 40 and 90 degrees C with analysis of the solutions to determine the speciation including carbamate(s), and carbonates, performed using C-13/H-1 NMR spectroscopy. The results in terms of the observed CO2 absorption rate, equilibrium CO2 solubility, CO2 desorption rate and cyclic capacity affirm that the cyclic structure strongly influences the general performance of the absorbents evaluated here. The mild steric hindrance induced by the proximity of the primary amine group to the ring acts to destabilize the carbamate and promote formation of bicarbonate resulting in 70% and 78% more bicarbonate in 4-A1MPD and 4-A1PPD than that in MEA while the values for N,N-DM12EDA and N,N-DM13PDA are 47% and 31%. Furthermore, the cyclic capacity was promoted to 215%, 160%, 81% and 65% for 4-A1PPD, 4-A1MPD, N,N-DM13PDA and N,N-DM12EDA in comparison with MEA solution. Density and viscosity of 2 mol.L-1 di-amines and 4 mol.L-1 MEA solutions were investigated over the temperature range 20-80 degrees C and found to decrease with increasing temperature for all absorbents. The order of the densities can be ranked as: MEA > 4-A1MPD > 4-A1PPD > N,N-DM12EDA > N,N-DM13PDA while the viscosities follow the trend 4-A1PPD > 4-A1MPD > N,N-DM13PDA > N,N-DM12EDA > MEA.