9 research outputs found

    Reactions of CO<sub>2</sub> with Aqueous Piperazine Solutions: Formation and Decomposition of Mono- and Dicarbamic Acids/Carbamates of Piperazine at 25.0 °C

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    Piperazine (PZ) is widely recognized as a promising solvent for postcombustion capture (PCC) of carbon dioxide (CO<sub>2</sub>). In view of the highly conflicting data describing the kinetic reactions of CO<sub>2</sub>(aq) in piperazine solutions, the present study focuses on the identification of the chemical mechanism, specifically the kinetic pathways for CO<sub>2</sub>(aq) in piperazine solutions that form the mono- and dicarbamates, using the analysis of stopped-flow spectrophotometric kinetic measurements and <sup>1</sup>H NMR spectroscopic data at 25.0 °C. The complete set of rate and equilibrium constants for the kinetic pathways, including estimations for the protonation constants of the suite of piperazine carbamates/carbamic acids, is reported here using an extended kinetic model which incorporates all possible reactions for CO<sub>2</sub>(aq) in piperazine solutions. From the kinetic data determined in the present study, the reaction of CO<sub>2</sub>(aq) with free PZ was found to be the dominant reactive pathway. The superior reactivity of piperazine is confirmed in the kinetic rate constant determined for the formation of piperazine monocarbamic acid (<i>k</i><sub>7</sub> = 2.43(3) × 10<sup>4</sup> M<sup>–1</sup> s<sup>–1</sup>), which is within the wide range of published values, making it one of the faster reacting amines. The corresponding equilibrium constant for the formation of the monocarbamic acid, <i>K</i><sub>7</sub>, markedly exceeds that of other monoamines. Kinetic and equilibrium constants for the remaining pathways indicate a minor contribution to the overall kinetics at high pH; however, these pathways may become more significant at higher CO<sub>2</sub> loadings and lower pH values where the concentrations of the reactive species are correspondingly higher

    Insights into the Chemical Mechanism for CO<sub>2</sub>(aq) and H<sup>+</sup> in Aqueous Diamine Solutions - An Experimental Stopped-Flow Kinetic and <sup>1</sup>H/<sup>13</sup>C NMR Study of Aqueous Solutions of <i>N</i>,<i>N</i>‑Dimethylethylenediamine for Postcombustion CO<sub>2</sub> Capture

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    In an effort to advance the understanding of multiamine based CO<sub>2</sub> capture process absorbents, we report here the determination of the kinetic and equilibrium constants for a simple linear diamine <i>N,N</i>-dimethylethylenediamine (DMEDA) via stopped-flow spectrophotometric kinetic measurements and <sup>1</sup>H/<sup>13</sup>C NMR titrations at 25.0 °C. From the kinetic data, the formation of monocarbamic acid (DMEDACOOH) from the reaction of DMEDA with CO<sub>2</sub>(aq) is the dominant reaction at high pH > 9.0 (<i>k</i><sub>7</sub> = 6.99 × 10<sup>3</sup> M<sup>–1</sup>·s<sup>–1</sup>). Below this pH, the formation of protonated monocarbamic acid (DMEDACOOH<sub>2</sub>) via the pathway involving DMEDAH<sup>+</sup> and CO<sub>2</sub>(aq) becomes active and contributes to the kinetics despite the 107-fold decrease in the rate constant between the two pathways. <sup>1</sup>H and <sup>13</sup>C NMR spectra as a function of decreasing pH (increasing HCl concentration) at 25.0 °C have been evaluated here to confirm the protonation events in DMEDA. Calculations of the respective DMEDA nitrogen partial charges have also been undertaken to support the NMR protonation study. A comparison of the DMEDA kinetic constants with the corresponding data for piperazine (PZ) reveals that despite the larger basicity of DMEDA, the enhanced and superior kinetic performance of PZ with CO<sub>2</sub>(aq) above its predicted Bronsted reactivity is not observed in DMEDA

    Toward the Understanding of Chemical Absorption Processes for Post-Combustion Capture of Carbon Dioxide: Electronic and Steric Considerations from the Kinetics of Reactions of CO<sub>2</sub>(aq) with Sterically hindered Amines

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    The present study reports (a) the determination of both the kinetic rate constants and equilibrium constants for the reaction of CO<sub>2</sub>(aq) with sterically hindered amines and (b) an attempt to elucidate a fundamental chemical understanding of the relationship between the amine structure and chemical properties of the amine that are relevant for postcombustion capture of CO<sub>2</sub> (PCC) applications. The reactions of CO<sub>2</sub>(aq) with a series of linear and methyl substituted primary amines and alkanolamines have been investigated using stopped-flow spectrophotometry and <sup>1</sup>H NMR measurements at 25.0 °C. The specific mechanism of absorption for each of the amines, that is CO<sub>2</sub> hydration and/or carbamate formation, is examined and, based on the mechanism, the kinetic and equilibrium constants for the formation of carbamic acid/carbamates, including protonation constants of the carbamate, are reported for amines that follow this pathway. A Brønsted correlation relating the kinetic rate constants and equilibrium constants for the formation of carbamic acid/carbamates with the protonation constant of the amine is reported. Such a relationship facilitates an understanding of the effects of steric and electronic properties of the amine toward its reactivity with CO<sub>2</sub>. Further, such relationships can be used to guide the design of new amines with improved properties relevant to PCC applications

    Toward Intelligent CO<sub>2</sub> Capture Solvent Design through Experimental Solvent Development and Amine Synthesis

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    In order to improve toward efficient large scale CO<sub>2</sub> capture applications, the largest uncertainties with postcombustion carbon dioxide capture (PCC) still surround the chemical reactivity and reaction rate of the solvent, the large parasitic energy penalty introduced during the regeneration of CO<sub>2</sub> from the solvents, and the stability of the amine solvent to resist degradation in the presence of trace impurities present in the flue gas. Heterocyclic amines are a class of molecules that have inherently superior kinetic reactivity with CO<sub>2</sub> but, importantly, have demonstrated desirable energy performance and degradation resistance. The current work is focused on further understanding of the chemical behavior of diamine and triamine solvents during CO<sub>2</sub> absorption and desorption from laboratory scale measurements. In this study we have proposed and prepared a series of cyclic diamine and triamine derivatives which can potentially offer reductions in solvent related costs associated with the PCC process. Thirty amines were synthesized and their CO<sub>2</sub> absorption and cyclic capacities determined between 40 and 90 °C using a small reactor with analysis of the solutions performed using quantitative <sup>13</sup>C and <sup>1</sup>H NMR spectroscopy. Cyclic capacity results indicate the majority of the amines are capable of increases in CO<sub>2</sub> uptake and cycle (when expressed as molar or mass ratios) compared to piperazine (PZ, the most commonly used diamine) and monoethanolamine (MEA, the standard amine to which all other amines are compared) over a similar temperature swing. Eight of the amines demonstrated significant improvements with 200% or greater improvement in cyclic capacity over PZ (expressed as moles of CO<sub>2</sub>/mol of nitrogen), with the largest improvement achieving a 273% increase. The intimate chemical behavior of the amines was examined by considering the relative contributions of specific CO<sub>2</sub> species to the cyclic capacity. Nine of the amines investigated showed significant improvements in the amount of the targeted bicarbonate product cycled between 40 and 90 °C compared to PZ. Despite the unoptimized and conservative desorption conditions utilized here, the results demonstrate that CO<sub>2</sub> can be regenerated from cyclic amines without the requirement for excessive regeneration temperatures as is the case for PZ (∼150 °C to achieve optimum cyclic capacity). The results here demonstrate the potential for improved amine solvents via amine synthesis and future development pathways through intelligent molecular design

    Evaluation and Modeling of Vapor–Liquid Equilibrium and CO<sub>2</sub> Absorption Enthalpies of Aqueous Designer Diamines for Post Combustion Capture Processes

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    Novel absorbents with improved characteristics are required to reduce the existing cost and environmental barriers to deployment of large scale CO<sub>2</sub> capture. Recently, bespoke absorbent molecules have been specifically designed for CO<sub>2</sub> capture applications, and their fundamental properties and suitability for CO<sub>2</sub> capture processes evaluated. From the study, two unique diamine molecules, 4-(2-hydroxyethylamino)­piperidine (A4) and 1-(2-hydroxyethyl)-4-aminopiperidine (C4), were selected for further evaluation including thermodynamic characterization. The solubilities of CO<sub>2</sub> in two diamine solutions with a mass fraction of 15% and 30% were measured at different temperatures (313.15–393.15 K) and CO<sub>2</sub> partial pressures (up to 400 kPa) by thermostatic vapor−liquid equilibrium (VLE) stirred cell. The absorption enthalpies of reactions between diamines and CO<sub>2</sub> were evaluated at different temperatures (313.15 and 333.15 K) using a CPA201 reaction calorimeter. The amine protonation constants and associated protonation enthalpies were determined by potentiometric titration. The interaction of CO<sub>2</sub> with the diamine solutions was summarized and a simple mathematical model established that could make a preliminary but good prediction of the VLE and thermodynamic properties. Based on the analyses in this work, the two designer diamines A4 and C4 showed superior performance compared to amines typically used for CO<sub>2</sub> capture and further research will be completed at larger scale

    Computational Modeling and Simulation of CO<sub>2</sub> Capture by Aqueous Amines

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    We review the literature on the use of computational methods to study the reactions between carbon dioxide and aqueous organic amines used to capture CO<sub>2</sub> prior to storage, reuse, or sequestration. The focus is largely on the use of high level quantum chemical methods to study these reactions, although the review also summarizes research employing hybrid quantum mechanics/molecular mechanics methods and molecular dynamics. We critically review the effects of basis set size, quantum chemical method, solvent models, and other factors on the accuracy of calculations to provide guidance on the most appropriate methods, the expected performance, method limitations, and future needs and trends. The review also discusses experimental studies of amine-CO<sub>2</sub> equilibria, kinetics, measurement and prediction of amine p<i>K</i><sub>a</sub> values, and degradation reactions of aqueous organic amines. Computational simulations of carbon capture reaction mechanisms are also comprehensively described, and the relative merits of the zwitterion, termolecular, carbamic acid, and bicarbonate mechanisms are discussed in the context of computational and experimental studies. Computational methods will become an increasingly valuable and complementary adjunct to experiments for understanding mechanisms of amine-CO<sub>2</sub> reactions and in the design of more efficient carbon capture agents with acceptable cost and toxicities

    CO<sub>2</sub> Absorption into Aqueous Solutions Containing 3‑Piperidinemethanol: CO<sub>2</sub> Mass Transfer, Stopped-Flow Kinetics, <sup>1</sup>H/<sup>13</sup>C NMR, and Vapor–Liquid Equilibrium Investigations

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    Global efforts to reduce carbon dioxide emissions stemming from the combustion of fossil fuels have acknowledged and focused on the implementation of post combustion capture (PCC) technologies utilizing aqueous amine solvents to fulfill this role. The cyclic diamine solvent piperazine has received significant attention for application as a CO<sub>2</sub> capture solvent, predominantly for its rapid reactivity with CO<sub>2</sub>. A thorough investigation of alternative but simpler cyclic amines incorporating a single amine group into the cyclic structure may reveal further insight into the superior kinetic performance of piperazine and the wider applicability of such cyclic solvents for PCC processes. One such example is the cyclic monoamine 3-piperidinemethanol (3-PM). To facilitate the evaluation of 3-PM as a capture solvent requires knowledge of the fundamental chemical parameters describing the kinetic and equilibrium of the reactions occurring in solutions containing CO<sub>2</sub> and 3-PM. Additionally, in parallel with the preceding, experimental measurements of CO<sub>2</sub> absorption into 3-PM solutions, including mass transfer and vapor–liquid equilibrium measurements, can be used to validate the CO<sub>2</sub> absorption performance in 3-PM solutions and compared to that of monoethanolamine (MEA) under similar conditions. The present study is focused in two parts on (a) determination of fundamental kinetic and equilibrium constants via the analysis of stopped-flow kinetic and quantitative equilibrium measurements via <sup>1</sup>H/<sup>13</sup>C nuclear magnetic resonance (NMR) spectroscopy and (b) experimental measurements of CO<sub>2</sub> absorption into 3-PM solutions via wetted wall column kinetic measurements, vapor–liquid equilibrium measurements, and corresponding physical property data including densities and viscosities of the amine solutions over a range of concentrations and CO<sub>2</sub> loadings. Fundamental kinetic rate constants describing the reaction of CO<sub>2</sub> with 3-PM are significantly faster than MEA at similar temperatures (3-PM = 32 × 10<sup>3</sup> M<sup>–1</sup> s<sup>–1</sup>, extrapolated to 40 °C from kinetic data between 15.0 and 35.0 °C; MEA = 13 × 10<sup>3</sup> M<sup>–1</sup> s<sup>–1</sup>, 40 °C). Conversely, the equilibrium constants describing the reaction between bicarbonate and amine, often termed carbamate stability constants, are significantly lower for 3-PM than MEA at similar temperatures. Overall CO<sub>2</sub> absorption rates in 3.0 M solutions of 3-PM and MEA, assessed in overall CO<sub>2</sub> mass transfer coefficients, are lower in the former case over the entire range of CO<sub>2</sub> loadings from 0.0 to 0.4 mol of CO<sub>2</sub> per mol of amine. The reduced absorption rates in the 3-PM solutions can be attributed to higher solution viscosities and thus corresponding reductions in CO<sub>2</sub> diffusion. CO<sub>2</sub> absorption and cyclic capacities in 3.0 M solutions of 3-PM and MEA were found to be significantly higher in the case of 3-PM. The larger CO<sub>2</sub> capacities are attributed to the lower stability 3-PM carbamate and the formation of larger amounts of bicarbonate compared to MEA. Overall, the larger CO<sub>2</sub> absorption capacity, cyclic capacity, and rapid kinetics with CO<sub>2</sub> position 3-PM as an attractive CO<sub>2</sub> capture solvent

    Development and Evaluation of a Novel Method for Determining Absorbent Composition in Aqueous Ammonia-Based CO<sub>2</sub> and SO<sub>3</sub><sup>2–</sup> and SO<sub>4</sub><sup>2–</sup> Loaded Capture Process Solutions via FT-IR Spectroscopy

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    CO<sub>2</sub> capture using aqueous ammonia is a potentially attractive option for emissions reductions from energy production and industrial processes. From an operational perspective, the capture absorbent must be monitored continuously to maintain the maximum efficiency of the capture process. In practice the composition of the absorbent is typically evaluated offline and retrospectively via wet chemistry methods, delaying any necessary variations to the process conditions to maintain maximum efficiency. Online absorbent monitoring methods incorporating spectroscopy via Raman or Fourier transform infrared (FT-IR) are attractive options due to their rapid response times and flexibility of the resulting output to be incorporated directly into process control packages. The present study outlines an evaluation of the FT-IR spectroscopic technique with analysis via partial least squares regression (PLSR) for a range of dilute to concentrated aqueous ammonia absorbents from ∼0.3–6.0 M and over a range of CO<sub>2</sub> loadings from ∼0.0–0.6 mol CO<sub>2</sub>/mol NH<sub>3</sub>. The water concentration in the samples ranges from ∼35.2–55.2 M. The effect of interfering SO<sub><i>x</i></sub> species on the FT-IR method has been evaluated by incorporating dissolved SO<sub>3</sub><sup>2–</sup> and SO<sub>4</sub><sup>2–</sup> components into the solutions from 0.0–1.5 M. The analysis results in accurate concentrations for all analytes. The robustness of the analysis results has been evaluated and discussed. Additionally, FT-IR spectroscopy with PLSR was compared with conventional titration methods for a selected series of mixed NH<sub>3</sub>/CO<sub>2</sub> standard solutions and a series of liquid samples from a bench-scale CO<sub>2</sub> absorption process. At low concentrations where the total NH<sub>3</sub> concentration is less than 4.0 M and the total CO<sub>2</sub> concentration is less than 1.5 M, both the combined PLSR with FT-IR method and the conventional potentiometric titration methods were suitable for the evaluation of the liquid compositions. However, at concentrations out of the low concentration range, the combined PLSR and FT-IR method was proven to have a robustness and accuracy greater than those of the conventional potentiometric titration methods. Therefore, given the simplicity and rapid turnaround of FT-IR spectroscopy in combination with PLSR, we consider this to be a superior and flexible technique for monitoring of CO<sub>2</sub> loaded aqueous ammonia solutions

    Toward Rational Design of Amine Solutions for PCC Applications: The Kinetics of the Reaction of CO<sub>2</sub>(aq) with Cyclic and Secondary Amines in Aqueous Solution

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    The kinetics of the fast reversible carbamate formation reaction of CO<sub>2</sub>(aq) with a series of substituted cyclic secondary amines as well as the noncyclic secondary amine diethanolamine (DEA) has been investigated using the stopped-flow spectrophotometric technique at 25.0 °C. The kinetics of the slow parallel reversible reaction between HCO<sub>3</sub><sup>–</sup> and amine has also been determined for a number of the amines by <sup>1</sup>H NMR spectroscopy at 25.0 °C. The rate of the reversible reactions and the equilibrium constants for the formation of carbamic acid/carbamate from the reactions of CO<sub>2</sub> and HCO<sub>3</sub><sup>–</sup> with the amines are reported. In terms of the forward reaction of CO<sub>2</sub>(aq) with amine, the order with increasing rate constants is as follows: diethanolamine (DEA) < morpholine (MORP) ∼ thiomorpholine (TMORP) < <i>N</i>-methylpiperazine (<i>N</i>-MPIPZ) < 4-piperidinemethanol (4-PIPDM) ∼ piperidine (PIPD) < pyrrolidine (PYR). Both 2-piperidinemethanol (2-PIPDM) and 2-piperidineethanol (2-PIPDE) do not form carbamates. For the carbamate forming amines a Brønsted correlation relating the protonation constant of the amine to the carbamic acid formation rate and equilibrium constants at 25.0 °C has been established. The overall suitability of an amine for PCC in terms of kinetics and energy is discussed
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