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A mechanistic study into the reaction between carbon dioxide and amines
| Content Provider | Semantic Scholar |
|---|---|
| Author | Robinson, Kelly |
| Copyright Year | 2012 |
| Abstract | The increasing level of atmospheric CO2 has been attributed to anthropogenic emissions from fossil fuel combustion and industrial processes. At present, post combustion CO2 capture (PCC) from coal-fired power stations, using aqueous amine-based absorbents, is considered the most mature technology for reducing CO2 emissions in the short term. The economics and feasibility of this technology is highly dependent on several factors, in particularly the absorbents capacity to absorb CO2, the rate of absorption and the energy required for absorbent regeneration. To reduce the high energy requirements and cost of the process most research and development efforts have focused on the development of more cost effective and superior performing amines. While the CO2 absorption rate and capacity of absorbents are important performance indicators in determining absorbent efficiency, another important aspect of CO2 / amine capture chemistry is the lability of the carbamate species formed and its susceptibility to hydrolysis and subsequent bicarbonate formation. Increased bicarbonate formation is associated with increased absorption capacities and a lower heat of regeneration. Amine structure will determine carbamate lability and thus susceptibility to hydrolysis. The molecular properties that give rise to enhanced CO2 absorption characteristics are still not yet fully understood. 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-, hydroxyland hydroxyalkylsubstituents. The diamines investigated included novel hexahyhropyrimidine (HHPY), methyl hexahydropyrimidines (MHHPY and DMHHPY) and hexahydropyridazine (HHPZ); piperazine (PZ), and 2,6and 2,5dimethylpiperazines (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. For the monoamines analysed the formation of the carbamate derivatives of the 3and 4 hydroxy, 3and 4hydroxymethyl, and 4-hydroxyethyl substituted piperidines were found to be kinetically less favourable than the carbamate derivatives of piperidine and the 3and 4methyl substituted piperidines. As the CO2 loading of piperidine and the 3and 4methyl and hydroxyalkyl substituted piperidines exceeded 0.5 mol CO2 / mol amine, the hydrolysis of the carbamate derivative of these amines was observed. Piperidine and its 3and 4alkyl and hydroxyalkyl substituted analogues were found to display enhanced CO2 absorption capacities and rates compared to that of their acyclic counterpart’s monoethanolamine (MEA) and diethanolamine (DEA). These amines were also found to form a more hydrolytically labile carbamate derivative at high CO2 loadings (≥ 0.5 mol CO2 / mol amine). The 2alkyl and hydroxyalkyl substituted piperidines were found to favour bicarbonate formation in the reaction with CO2. For the first time, the ability of these amines to form a carbamate on absorption of CO2, one that is inherently susceptible to hydrolysis in an aqueous environment was observed. Theoretical analysis for the 2alkyl and hydroxyalkyl substituted piperidines suggest that a combination of both the electronic effect exerted by the substituent and a reduction in the exposed area on the nitrogen atom both play a role in destabilising the carbamate derivative and increasing its susceptibility to hydrolysis. The contribution of each is dependent on the type and size of the substituent present. The structure of the carbamate derivatives of these amines revealed shorter N-COO bond lengths and a less delocalised electron distribution in the carboxylate moiety. For the diamines analysed PZ was found to form a hydrolysis resistant carbamate derivative, while HHPY formed a more labile carbamate species with an increased susceptibility to hydrolysis particularly at higher CO2 loadings (>0.5 moles of CO2 per mole of amine). HHPY exhibited similar reactivity toward CO2 as PZ, but with improved aqueous solubility. The αmethyl substituted MHHPY favoured bicarbonate formation, but MHHPY exhibited a comparable CO2 absorption rate compared with conventional amines MEA and DEA. MHHPY showed improved reactivity compared to the conventional α-methyl substituted primary amine 2-amino-2-methyl-1-propanol (AMP). HHPZ was relatively non-reactive towards CO2. DMHHPY was representative of a blended amine system and its reactivity highlighted the advantages of such a system. 1, 3-dimainopropane (DAP) is the starting material in the synthesis of DMHHPY and was carried through to the final product resulting in a DMHHPY / DAP blended amine system. PZ / AMP and piperidine / AMP blended amine systems were also investigated. The results highlighted the superior performance of PZ as an absorption accelerator in blended amine systems, as opposed to a sole CO2 capture absorbent. The diamine nature of PZ and its hydrolysis resistant carbamate derivative both contribute to this enhanced performance. The B3LYP / 6-31+G** and MP2 / 6-31+G** calculations showed the positions of the heterocyclic diamines affected carbamate stability, which influenced hydrolysis rates. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://ogma.newcastle.edu.au/vital/access/services/Download/uon:12580/ATTACHMENT02 |
| Alternate Webpage(s) | https://ogma.newcastle.edu.au/vital/access/services/Download/uon:12580/ATTACHMENT01 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
