An ATR-FTIR study on the effect of molecular structural variations on the CO2 absorption characteristics of heterocyclic amines, part II.

This paper reports on an ATR-FTIR spectroscopic investigation of the CO(2) absorption characteristics of a series of heterocyclic diamines: hexahydropyrimidine (HHPY), 2-methyl and 2,2-dimethylhexahydropyrimidine (MHHPY and DMHHPY), hexahydropyridazine (HHPZ), piperazine (PZ) and 2,5- and 2,6-dimethylpiperazine (2,6-DMPZ and 2,5-DMPZ). By using in situ ATR-FTIR the structure-activity relationship of the reaction between heterocyclic diamines and CO(2) is probed. PZ forms a hydrolysis-resistant carbamate derivative, while HHPY forms a more labile carbamate species with increased susceptibility to hydrolysis, particularly at higher CO(2) loadings (>0.5 mol CO(2)/mol amine). HHPY exhibits similar reactivity toward CO(2) to PZ, but with improved aqueous solubility. The α-methyl-substituted MHHPY favours HCO(3)(-) formation, but MHHPY exhibits comparable CO(2) absorption capacity to conventional amines MEA and DEA. MHHPY show improved reactivity compared to the conventional α-methyl- substituted primary amine 2-amino-2-methyl-1-propanol. DMHHPY is representative of blended amine systems, and its reactivity highlights the advantages of such systems. HHPZ is relatively unreactive towards CO(2). The CO(2) absorption capacity C(A) (mol CO(2)/mol amine) and initial rates of absorption R(IA) (mol CO(2)/mol amine min(-1)) for each reactive diamine are determined: PZ: C(A)=0.92, R(IA)=0.045; 2,6-DMPZ: C(A)=0.86, R(IA)=0.025; 2,5-DMPZ: C(A)=0.88, R(IA)=0.018; HHPY: C(A)=0.85, R(IA)=0.032; MHHPY: C(A)=0.86, R(IA)=0.018; DMHHPY: C(A)=1.1, R(IA)=0.032; and HHPZ: no reaction. Calculations at the B3LYP/6-31+G** and MP2/6-31+G** calculations show that the substitution patterns of the heterocyclic diamines affect carbamate stability, which influences hydrolysis rates.

An FTIR spectroscopic study on the effect of molecular structural variations on the CO2 absorption characteristics of heterocyclic amines.

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.

N-Nitrosopiperazines form at high pH in post-combustion capture solutions containing piperazine: a low-energy collisional behaviour study.

During the process of exploring aqueous piperazine chemistry under simulated flue-gas scrubbing conditions, positive-ion electrospray ionisation mass spectrometric (ESI-MS) analyses of the resulting reaction mixtures in a triple quadrupole system revealed the presence of peaks at m/z 116 and 145, the putative N-nitroso derivatives of piperazine. Confirmation of the presence of these species in the reaction mixtures was achieved using collision-induced dissociation experiments. A purchased standard, together with in-house synthesised N-nitrosopiperazine standards (including N-nitroso derivatives derived from deuterium-labelled precursor materials), were used for this purpose. Across a small range of collision energies, large fluctuations in the abundance of the two major product ions of protonated N-nitrosopiperazine, m/z 86 and 85, were observed. Using B3LYP/6-311 + +G(d,p) computations, the potential energy surface was determined for loss of NO and [H,N,O]. At an activation energy slightly in excess of 1 eV, intramolecular isomerisation precedes loss of NO (m/z 86) via a 4,1 H-shift, and at activation energies between 2.1-2.3 eV, consecutive loss of NO and atomic hydrogen competes with the direct loss of nitrosyl hydride (m/z 85). It is recommended that any multiple reaction monitoring method for quantifying N-nitrosopiperazines at low collision energies use the sum of both transitions (m/z 116 ← 85, m/z 116 ← 86) to avoid errors that could be introduced by subtle changes in ES source conditions or collision voltages. This approach is adopted in an HPLC/MS/MS method used to monitor the degradation of N-nitrosopiperazine exposed to (i) broad-band UV light and (ii) heat typical of an amine regeneration (stripper) tower. The results reveal that aqueous N-nitrosopiperazine is thermally stable at 150°C but will degrade slowly upon exposure to UV light.