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 °C using a small reactor with analysis of the solutions performed using quantitative ¹³C and ¹H 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 °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 (∼150 °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.