A mechanistic study into the reaction between carbon dioxide and amines

In this study the amine and carbamate properties that confer enhanced CO₂ absorption capacities, rates and carbamate lability has been investigated. This has been achieved using Attenuated Total Reflectance Fourier Transform Infrared (ATR FT-IR) spectroscopy to follow in situ the chemical reactions occurring between CO₂ and a series of heterocyclic monoamines and novel diamines. The monoamines investigated included piperidine and a series of commercially available functionalised piperidine derivatives, e.g. those with methyl-, hydroxyl- and hydroxyalkyl substituents. The diamines investigated included novel hexahyhropyrimidine (HHPY), methyl hexahydropyrimidines (MHHPY and DMHHPY) and hexahydropyridazine (HHPZ); piperazine (PZ), and 2,6- and 2,5- dimethylpiperazines (2,6-DMPZ and 2,5-DMPZ). The effect of structure on CO₂ / amine reactivity was assessed according to a correlation between the infrared active ionic reaction products (carbamate, bicarbonate and protonated amine) and cumulative CO₂ absorption; CO₂ absorption capacity; initial rate of CO₂ absorption; and a correlation between the atomic properties of the amine and carbamate derivative with the infrared spectral data, CO₂ absorption capacity and initial absorption rate. Calculations using B3LYP / 6-31+G** and MP2 / 6-31+G** were performed to investigate the atomic properties of the amines and carbamate derivative. The N-COO- carbamate bond and resonance structure of the carboxylate moiety were analysed. Knowledge of carbamate / bicarbonate speciation during the absorption process enabled the observation of reaction mechanisms as well as the determination of carbamate lability in the studied amine / CO₂ / H₂O systems. We report on the first real-time observation of carbamate hydrolysis in secondary amine absorbent systems.