Toward Intelligent CO2 Capture Solvent Design through Experimental Solvent Development and Amine Synthesis

In order to improve toward efficient large scale CO2 capture applications, the largest uncertainties with postcombustion carbon dioxide capture (PCC) still surround the chemical reactivity and reaction rate of the solvent, the large parasitic energy penalty introduced during the regeneration of CO2 from the solvents, and the stability of the amine solvent to resist degradation in the presence of trace impurities present in the flue gas. Heterocyclic amines are a class of molecules that have inherently superior kinetic reactivity with CO2 but, importantly, have demonstrated desirable energy performance and degradation resistance. The current work is focused on further understanding of the chemical behavior of diamine and triamine solvents during CO2 absorption and desorption from laboratory scale measurements. In this study we have proposed and prepared a series of cyclic diamine and triamine derivatives which can potentially offer reductions in solvent related costs associated with the PCC process. Thirty amines were synthesized and their CO2 absorption and cyclic capacities determined between 40 and 90 degrees C using a small reactor with analysis of the solutions performed using quantitative C-13 and H-1 NMR, spectroscopy. Cyclic capacity results indicate the majority of the amines are capable of increases in CO2 uptake and cycle (when expressed as molar or mass ratios) compared to piperazine (PZ, the most commonly used diamine) and monoethanolamine (MEA, the standard amine to which all other amines are compared) over a similar temperature swing. Eight of the amines demonstrated significant improvements with 200% or greater improvement in cyclic capacity over PZ (expressed as moles of CO2/mol of nitrogen), with the largest improvement achieving a 273% increase. The intimate chemical behavior of the amines was examined by considering the relative contributions of specific CO2 species to the cyclic capacity. Nine of the amines investigated showed significant improvements in the amount of the targeted bicarbonate product cycled between 40 and 90 degrees C compared to PZ. Despite the unoptimized and conservative desorption conditions utilized here, the results demonstrate that CO2 can be regenerated from cyclic amines without the requirement for excessive regeneration temperatures as is the case for PZ (similar to 150 degrees C to achieve optimum cyclic capacity). The results here demonstrate the potential for improved amine solvents via amine synthesis and future development pathways through intelligent molecular design.