Aqueous amine-based post combustion capture of CO₂ generated from coal-fired power stations has been identified as a crucial part of the global solution to climate change.
Previous studies have shown that a definite relationship exists between the structure of amines and their CO₂ absorption characteristics. Recently, heterocyclic amines such as piperazine have received considerable attention due to their increased reactivity for CO₂. This increased reactivity towards CO₂ has been attributed to piperazine’s cyclic and diamine structure. This prompted us to further characterise the effects of molecular structural variation on CO₂/amine reactivity, particularly with regards to novel heterocyclic amines.
The CO₂ absorption characteristics of a series of synthesised heterocyclic diamines, including hexahydropyrimidine and hexahydropyridazine derivatives were investigated. Similar to piperazine, these amines are a six-membered ring compound containing two nitrogen atoms. However, they offer structural variance with regards to the position of the nitrogen atoms within the six-membered ring. The chemical reactions between CO₂ and the cyclic diamines were followed in-situ using ATR FT-IR spectroscopy. The effect of structural variations on CO₂ absorption was assessed in relation to the ionic reaction products identified (carbamate versus bicarbonate absorbance), CO₂ absorption capacity and initial absorption rate. The hexahydropyrimidine’s were found to exhibit similar reactivity’s towards CO₂ as piperazine, with improved aqueous solubility. Hexahydropyridazine however was found to be relatively non-reactive towards CO₂.
Studying the reaction between CO₂ and piperazine, as well as hexahyhropyrimidine and hexahydropyridazine derivatives, has provided valuable insight into the enhanced activity of cyclic amines compared to conventional amines.