An FTIR Spectroscopic Study on the Effect of Molecular Structural Variations on the CO2 Absorption Characteristics of Heterocyclic Amines

Abstract

Herein, the reaction between CO(2) and piperidine, as well as commercially available functionalised piperidine derivatives, for example, those with methyl-, hydroxyl- and hydroxyalkyl substituents, has been investigated. The chemical reactions between CO(2) and the functionalised piperidines were followed in situ by using attenuated total reflectance (ATR) FTIR spectroscopy. The effect of structural variations on CO(2) absorption was assessed in relation to the ionic reaction products identifiable by IR spectroscopy, that is, carbamate versus bicarbonate absorbance, CO(2) absorption capacity and the mass-transfer coefficient at zero loading. On absorption of CO(2) , the formation of the carbamate derivatives of the 3- and 4-hydroxyl-, 3- and 4-hydroxymethyl-, and 4-hydroxyethyl-substituted piperidines were found to be kinetically less favourable than the carbamate derivatives of piperidine and the 3- and 4-methyl-substituted piperidines. As the CO(2) loading of piperidine and the 3- and 4-methyl- and hydroxyalkyl-substituted piperidines exceeded 0.5 moles of CO(2) per mole of amine, the hydrolysis of the carbamate derivative of these amines was observed in the IR spectra collected. From the subset of amines analysed, the 2-alkyl- and 2-hydroxyalkyl-substituted piperidines were found to favour bicarbonate formation in the reaction with CO(2) . Based on IR spectral data, the ability of these amines to form the carbamate derivatives was also established. Computational calculations at the B3LYP/6-31+G** and MP2/6-31+G** levels of theory were also performed to investigate the electronic/steric effects of the substituents on the reactivity (CO(2) capture performance) of different amines, as well as their carbamate structures. The theoretical results obtained for the 2-alkyl- and 2-hydroxyalkyl-substituted piperidines suggest that a combination of both the electronic effect exerted by the substituent and a reduction in the exposed area of the nitrogen atom play a role in destabilising the carbamate derivative and increasing its susceptibility to hydrolysis. A theoretical investigation into the structure of the carbamate derivatives of these amines revealed shorter NC bond lengths and a less-delocalised electron distribution in the carboxylate moiety.

An ATR‐FTIR Study on the Effect of Molecular Structural Variations on the CO2 Absorption Characteristics of Heterocyclic Amines, Part II

“…Full details of our experimental approach is given in the Experimental Section and also in our previous work. [ 10 ] Each experiment was conducted until equilibrium was established and a maximum CO 2 loading achieved. This was amine-dependent but typically required 45–90 mins.…”

“…Given the chemical stability of carbamates formed from 1° and 2° amines, hydrolysis does not occur at an industrially relevant rate,11 with the exception of heterocyclic secondary amines such as piperidine, which have been demonstrated to form a labile carbamate that is readily hydrolysed. [ 10 ] …”

“…In particular, the ability of ATR-FTIR to distinguish carbamate from bicarbonate formation accelerates the screening of potential PCC amines. [ 10 , 17 ] …”

“…[ 10 ] Herein we report on the in situ CO 2 absorption characteristics of a series of heterocyclic diamines (Figure 2): piperazine (PZ), 2,6-dimethyl- and 2,5-dimethylpiperazine (2,6-DMPZ and 2,5-DMPZ), hexahydropyrimidine (HHPY), 2-methylhexahydropyrimidine (MHHPY), 2,2-dimethylhexahydropyrimidine (DMHHPY) and hexahydropyridazine (HHPZ).…”

Chemical speciation of CO2 absorption in aqueous monoethanolamine investigated by in situ Raman spectroscopy

“…Antisymmetric and symmetric stretching of COO − typically contribute two bands in the 1650-1540 cm −1 and 1450-1360 cm −1 regions in IR spectrum, depending on the molecular constituents (Robinson et al, 2011). These two bands were too weak to be detected in Raman spectrum.…”