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

In-situ ATR FT-IR spectroscopy has been used to investigate the reaction between CO2 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 CO2 absorption has been assessed in relation to the prevalent IR identifiable ionic reaction products, along with CO2 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 CO2 loading of piperidine and the 3- and 4-substituted piperidines exceeded 0.5 mol CO2/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 HCO3− formation. Despite forming predominantly HCO3− 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.