The effect molecular structural variations has on the CO2 absorption characteristics of heterocyclic amines

Abstract In-situ ATR FT-IR spectroscopy has been used to investigate the reaction between CO 2 and piperidine, as well as commercially available functionalised piperdine derivatives, e.g., those with methyl-, hydroxyl-, and hydroxyalkyl-substituents. The effect of the substituent’s on CO 2 absorption has been assessed in relation to the prevalent IR identifiable ionic reaction products, along with CO 2 absorption capacity and initial absorption rate. The results obtained highlight the enhanced reactivity of cyclic 2° amines compared to conventional 1° and 2° amines, MEA and DEA respectively. Formation of the COO − derivatives of the 3- and 4-hydroxyl and hydroxyalkyl substituted piperidines were found to be kinetically less favourable than that of piperidine and the 3 and 4-methyl substituted piperdines. As the CO 2 loading of piperidine and the 3- and 4-substituted piperidines exceeded 0.5 mol CO 2 /mol amine, hydrolysis of their COO − derivative was observable in the IR spectral profiles. From the subset of amines analysed the 2-alkyl and 2-hydroxyalkyl substituted piperidines were found to favour HCO 3 − formation. Despite forming predominantly HCO 3 − these amines also exhibited initial absorption rates comparable to that of MEA and DEA, 2-MP in particular was found to exhibit a significantly higher initial absorption rate. Computational calculations at the B3LYP/6- 31+G ∗∗ and MP2/6- 31+G ∗∗ level of theory revealed that for the 2-alkyl and hydroxyalkyl substituted piperidines a combination of both the electronic effect exerted by the substituent and a reduction in the exposed area on the nitrogen atom will play a role in destabilising the COO − derivative and increasing its susceptibility to hydrolysis.