Computational Modeling and Simulation of CO2 Capture by Aqueous Amines.

We review the literature on the use of computational methods to study the reactions between carbon dioxide and aqueous organic amines used to capture CO2 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-CO2 equilibria, kinetics, measurement and prediction of amine pKa 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-CO2 reactions and in the design of more efficient carbon capture agents with acceptable cost and toxicities.

Information Derivation from Vapor-Liquid Equilibria Data: A Simple Shortcut to Evaluate the Energy Performance in an Amine-Based Postcombustion CO2 Capture.

Evaluation of amine absorbents is crucial for the development of a technically and economically feasible CO2 capture process. However, the capture performance estimation usually requires a load of experiments, which is time-consuming and labor-intensive. The present study proposed a simple but effective shortcut that employs the fewest experimental data, i.e., vapor-liquid equilibria (VLE) data only, to estimate the CO2 capture performance by developing a validated chemical VLE model and a simple shortcut approach. The reliability of the proposed method was validated by the excellent agreement with the results from the laboratory and pilot plant experiments, and rigorous rate-based MEA model in Aspen Plus. We demonstrated that this approach can reliably predict the important capture performance indicators, such as CO2 solubility, heat of CO2 reaction, lean/rich CO2 loadings and heat requirement of absorbent regeneration. Moreover, this shortcut approach can provide guidance for process modification to achieve the minimum regeneration energy. The extended application of this approach to other amines, i.e., piperazine (PZ), 2-amino-2-methyl-1-propanol (AMP), and blended PZ and AMP (PZ/AMP), also showed the good consistency with the published experimental and simulation results, further indicating the reliability of the shortcut approach to estimate the energy performance of amine processes. It is anticipated that the proposed method would simplify the evaluation of CO2 capture performance using VLE data only, providing an efficient and effective shortcut for screening and evaluating amine-based CO2 capture.

Insights into the Chemical Mechanism for CO2(aq) and H+ in Aqueous Diamine Solutions - An Experimental Stopped-Flow Kinetic and 1H/13C NMR Study of Aqueous Solutions of N,N-Dimethylethylenediamine for Postcombustion CO2 Capture.

In an effort to advance the understanding of multiamine based CO2 capture process absorbents, we report here the determination of the kinetic and equilibrium constants for a simple linear diamine N,N-dimethylethylenediamine (DMEDA) via stopped-flow spectrophotometric kinetic measurements and 1H/13C NMR titrations at 25.0 °C. From the kinetic data, the formation of monocarbamic acid (DMEDACOOH) from the reaction of DMEDA with CO2(aq) is the dominant reaction at high pH > 9.0 (k7 = 6.99 × 103 M-1·s-1). Below this pH, the formation of protonated monocarbamic acid (DMEDACOOH2) via the pathway involving DMEDAH+ and CO2(aq) becomes active and contributes to the kinetics despite the 107-fold decrease in the rate constant between the two pathways. 1H and 13C 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 CO2(aq) above its predicted Bronsted reactivity is not observed in DMEDA.

Mechanism Investigation of Advanced Metal-Ion-Mediated Amine Regeneration: A Novel Pathway to Reducing CO2 Reaction Enthalpy in Amine-Based CO2 Capture.

The high energy consumption of CO2 and absorbent regeneration is one of the most critical challenges facing commercial application of amine-based postcombustion CO2 capture. Here, we report a novel approach of metal-ion-mediated amine regeneration (MMAR) to advance the process of amine regeneration. MMAR uses the dual ability of amine to reversibly react with CO2 and reversibly complex with metal ions to reduce the enthalpy of the CO2 reaction, thus decrease the overall heat requirement for amine regeneration. To elucidate the mechanistic effects behind MMAR's ability to reduce CO2 reaction enthalpy, we developed a comprehensive chemical model describing the chemistry of Me(II)-monoethanolamine(MEA)-CO2-H2O system. The model was then validated using experimentally determined CO2 partial pressures via vapor-liquid equilibrium (VLE) measurements. We used the validated chemical model to gain insight into VLE behavior and solution chemistry, and to identify the specific changes in CO2 reaction enthalpy with and without metal ions. Two metals and five amines were evaluated in detail, which revealed that metal-ions with high complexation enthalpy and amines with large carbamate stability constant are preferred in MMAR, owing to their large reduction in reaction enthalpy and regeneration duty. We anticipate that MMAR could provide an alternative pathway to reducing the energy consumption of absorbent regeneration, ultimately making amine-based processes more technically and economically viable.

Evaluation and Modeling of Vapor-Liquid Equilibrium and CO2 Absorption Enthalpies of Aqueous Designer Diamines for Post Combustion Capture Processes.

Novel absorbents with improved characteristics are required to reduce the existing cost and environmental barriers to deployment of large scale CO2 capture. Recently, bespoke absorbent molecules have been specifically designed for CO2 capture applications, and their fundamental properties and suitability for CO2 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 CO2 in two diamine solutions with a mass fraction of 15% and 30% were measured at different temperatures (313.15-393.15 K) and CO2 partial pressures (up to 400 kPa) by thermostatic vapor-liquid equilibrium (VLE) stirred cell. The absorption enthalpies of reactions between diamines and CO2 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 CO2 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 CO2 capture and further research will be completed at larger scale.

Catalysis of CO2 absorption in aqueous solution by inorganic oxoanions and their application to post combustion capture.

To reduce CO2 emission into the atmosphere, particularly from coal-fired power stations, post combustion capture (PCC) using amine-based solvents to chemically absorb CO2 has been extensively developed. From an infrastructure viewpoint, the faster the absorption of CO2, the smaller the absorber required. The use of catalysts for this process has been broadly studied. In this manuscript, a study of the catalytic efficiencies of inorganic oxoanions such as arsenite, arsenate, phosphite, phosphate, and borate is described. The kinetics of the accelerated CO2 absorption at 25 °C was investigated using stopped-flow spectrophotometry. The catalytic rate constants of these anions for the reaction of CO2 with H2O were determined to be 137.7(3), 30.3(7), 69(2), 32.7(9), and 13.66(7) M(-1)s(-1), respectively. A new mechanism for the catalytic reaction of oxoanions with CO2 has also been proposed. The applicability of these catalysts to PCC was further studied by simulation of the absorption process under PCC conditions using their experimental catalytic rate constants. Arsenite and phosphite were confirmed to be the best catalysts for CO2 capture. However, considering the toxicological effect of arsenic and the oxidative instability of phosphite, phosphate would be the most promising inorganic catalyst for PCC process from the series of inorganic oxoanions studied.

Toward the understanding of chemical absorption processes for post-combustion capture of carbon dioxide: electronic and steric considerations from the kinetics of reactions of CO2(aq) with sterically hindered amines.

The present study reports (a) the determination of both the kinetic rate constants and equilibrium constants for the reaction of CO(2)(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(2) (PCC) applications. The reactions of CO(2)(aq) with a series of linear and methyl substituted primary amines and alkanolamines have been investigated using stopped-flow spectrophotometry and (1)H NMR measurements at 25.0 °C. The specific mechanism of absorption for each of the amines, that is CO(2) 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(2). Further, such relationships can be used to guide the design of new amines with improved properties relevant to PCC applications.

Reactions of CO2 with aqueous piperazine solutions: formation and decomposition of mono- and dicarbamic acids/carbamates of piperazine at 25.0 °C.

Piperazine (PZ) is widely recognized as a promising solvent for postcombustion capture (PCC) of carbon dioxide (CO(2)). In view of the highly conflicting data describing the kinetic reactions of CO(2)(aq) in piperazine solutions, the present study focuses on the identification of the chemical mechanism, specifically the kinetic pathways for CO(2)(aq) in piperazine solutions that form the mono- and dicarbamates, using the analysis of stopped-flow spectrophotometric kinetic measurements and (1)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(2)(aq) in piperazine solutions. From the kinetic data determined in the present study, the reaction of CO(2)(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 (k(7) = 2.43(3) × 10(4) M(-1) s(-1)), 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, K(7), 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(2) loadings and lower pH values where the concentrations of the reactive species are correspondingly higher.

Toward rational design of amine solutions for PCC applications: the kinetics of the reaction of CO2(aq) with cyclic and secondary amines in aqueous solution.

The kinetics of the fast reversible carbamate formation reaction of CO(2)(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(3)(-) and amine has also been determined for a number of the amines by (1)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(2) and HCO(3)(-) with the amines are reported. In terms of the forward reaction of CO(2)(aq) with amine, the order with increasing rate constants is as follows: diethanolamine (DEA) < morpholine (MORP) ~ thiomorpholine (TMORP) < N-methylpiperazine (N-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.

Modeling CO2 mass transfer in amine mixtures: PZ-AMP and PZ-MDEA.

The most common method of carbon dioxide (CO(2)) capture is the absorption of CO(2) into a falling thin film of an aqueous amine solution. Modeling of mass transfer during CO(2) absorption is an important way to gain insight and understanding about the underlying processes that are occurring. In this work a new software tool has been used to model CO(2) absorption into aqueous piperazine (PZ) and binary mixtures of PZ with 2-amino-2-methyl-1-propanol (AMP) or methyldiethanolamine (MDEA). The tool solves partial differential and simultaneous equations describing diffusion and chemical reaction automatically derived from reactions written using chemical notation. It has been demonstrated that by using reactions that are chemically plausible the mass transfer in binary mixtures can be fully described by combining the chemical reactions and their associated parameters determined for single amines. The observed enhanced mass transfer in binary mixtures can be explained through chemical interactions occurring in the mixture without need to resort to using additional reactions or unusual transport phenomena such as the "shuttle mechanism".

Kinetics of the reversible reaction of CO2(aq) with ammonia in aqueous solution.

The kinetics of the interactions of aqueous ammonia with aqueous carbon dioxide/carbonate species has been investigated using stopped-flow techniques by monitoring the pH changes via indicators. The reactions include the reversible formation of ammonium carbamate/carbamic acid. A complete reaction mechanism has been established, and the temperature dependence of all rate and equilibrium constants including the protonation constant of the amine between 15 and 45 °C are reported and analyzed in terms of Arrhenius, Eyring, and van't Hoff relationships.

Comprehensive kinetic and thermodynamic study of the reactions of CO2(aq) and HCO3(-) with monoethanolamine (MEA) in aqueous solution.

The temperature dependence of the reversible reaction between CO(2)(aq) and monoethanolamine (MEA) has been investigated using stopped-flow spectrophotometry by following the pH changes during the reactions with colored acid-base indicators. Multivariate global analysis of both the forward and backward kinetic measurements for the reaction of CO(2)(aq) with MEA yielded the rate and equilibrium constants, including the protonation constant of MEA carbamate, for the temperature range of 15-45 °C. Analysis of the rate and equilibrium constants in terms of the Arrhenius, Eyring, and van't Hoff relationships gave the relevant thermodynamic parameters. In addition, the rate and equilibrium constants for the slow, reversible reaction of bicarbonate with MEA are reported at 25.0 °C. At high pH, reactions of the amine with CO(2) and with bicarbonate are significant.

CdS-Enhanced Ethanol Selectivity in Electrocatalytic CO2 Reduction at Sulfide-Derived Cu-Cd.

The development of Cu-based catalysts for the electrochemical CO2 reduction reaction (eCO2 RR) is of major interest for generating commercially important C2 liquid products such as ethanol. Cu is exclusive among the eCO2 RR metallic catalysts in that it facilitates the formation of a range of highly reduced C2 products, with a reasonable total faradaic efficiency but poor product selectivity. Here, a series of new sulfide-derived copper-cadmium catalysts (SD-Cux Cdy ) was developed. An excellent faradaic efficiency of around 32 % but with a relatively low current density of 0.6 mA cm-2 for ethanol was obtained using the SD-CuCd2 catalyst at the relatively low overpotential of 0.89 V in a CO2 -saturated aqueous 0.10 m KHCO3 solution with an H-cell. The current density increased by an order of magnitude under similar conditions using a flow cell where the mass transport rate for CO2 was greatly enhanced. Ex situ spectroscopic and microscopic, and voltammetric investigations pointed to the role of abundant phase boundaries between CdS and Cu+ /Cu sites in the SD-CuCd2 catalyst in enhancing the selectivity and efficiency of ethanol formation at low potentials.

Kinetics and mechanism of carbamate formation from CO2(aq), carbonate species, and monoethanolamine in aqueous solution.

Removal of carbon dioxide from fossil-based power generation is a potentially useful technique for the reduction of greenhouse gas emissions. Reversible interaction with aqueous amine solutions is most promising. In this process, the formation of carbamates is an important reaction of carbon dioxide. In this contribution, a detailed molecular reaction mechanism for the carbamate formation between MEA (monoethanolamine) and dissolved CO(2) as well as carbonate species in aqueous solution is presented. There are three parallel, reversible reactions of the free amine with CO(2), carbonic acid, and the bicarbonate ion; the relative importance of the three paths is strongly pH dependent. Kinetic and equilibrium measurements are based on (1)H NMR and stopped-flow measurements with rate constants, equilibrium constants, and protonation constants being reported.

Activity-based analysis of potentiometric pH titrations.

The discrepancy between concentrations and activities is a predicament well known to the analytical chemist. Because of the difficulty of determining activity coefficients, the standard technique for quantitative equilibrium studies is to work under a particular 'constant ionic strength' by adding an excess of an inert salt. Under such conditions, activity coefficients are approximately constant and can be taken into the equilibrium constants which are defined for the chosen ionic strength (I). Here we propose a fundamentally different approach. Throughout the numerical analysis of the titration data, activity coefficients for all individual species are approximated by well-known equations based on the work of Debye-Hückel. The computational analysis of the measurements strictly obeys the law of mass conservation and obeys the law of mass action only approximately. The main novelty is that now the addition of inert salts is no longer required and measurements are done at minimal I. Consequently, the thermodynamic equilibrium constants are now determined much more robustly based on experiments taken at low I. The approach has been tested and validated with the two very well investigated 3-protic phosphoric and citric acids. In summary: the technique of artificially keeping ionic strength constant has been replaced by improved computational analysis.

Equilibrium modeling of mixtures of methanol and water.

An understanding of the species that form in mixtures of alcohol and water is important for their use in liquid chromatography applications. In reverse-phase liquid chromatography the retention of solutes on a chromatography column is influenced by the composition of the mobile phase, and in the case of alcohol and water mobile phases, the amount of free alcohol and water present. Previous and similar modeling studies of methanol (MeOH) and water mixtures by near-infrared (NIR) spectroscopy have found up to four species present including free MeOH and water and MeOH and water complexes formed by hydrogen bonding associations. In this work an equilibrium model has been applied to NIR measurements of MeOH and water mixtures. A high-performance liquid chromatography (HPLC) pump was coupled to an NIR flow cell to produce a gradual change in mixture composition. This resulted in a greater mixture resolution than has been achieved previously by manual mixture preparation. It was determined that five species contributed to the data. An equilibria model consisting of MeOH, H2O, MeOH(H2O) (log K(MeOH)H2O = 0.10+/-0.03), MeOH(H2O)4 (log K(MeOH)4H2O = -2.14+/-0.08), and MeOH(H2O)9 (log K(MeOH)9H2O = -8.6+/-0.1) was successfully fitted to the data. The model supports the results of previous work and highlights the progressive formation of MeOH and water complexes that occur with changing mixture composition. The model also supports that mixtures of MeOH and water are not simple binary mixtures and that this is responsible for observed deviations from expected elution behavior.

A speciation study of sulfur(iv) in aqueous solution.

In the current work ultraviolet spectrophotometric titrations at different S(iv) concentrations have been globally analysed using the entire spectral dataset to determine the complete speciation of S(iv) in aqueous solution over a large pH range (from 9.6 to 1). As a result, the dimerisation constant for the formation of disulfite from hydrogen sulfite has been accurately determined. Further, protonated disulfite has been identified and quantified for the first time. In addition, the molar absorptivities of all S(iv) species are also reported over the studied wavelength range.

Carbon dioxide postcombustion capture: a novel screening study of the carbon dioxide absorption performance of 76 amines.

The significant and rapid reduction of greenhouse gas emissions is recognized as necessary to mitigate the potential climate effects from global warming. The postcombustion capture (PCC) and storage of carbon dioxide (CO2) produced from the use of fossil fuels for electricity generation is a key technology needed to achieve these reductions. The most mature technology for CO2 capture is reversible chemical absorption into an aqueous amine solution. In this study the results from measurements of the CO2 absorption capacity of aqueous amine solutions for 76 different amines are presented. Measurements were made using both a novel isothermal gravimetric analysis (IGA) method and a traditional absorption apparatus. Seven amines, consisting of one primary, three secondary, and three tertiary amines, were identified as exhibiting outstanding absorption capacities. Most have a number of structural features in common including steric hindrance and hydroxyl functionality 2 or 3 carbons from the nitrogen. Initial CO2 absorption rate data from the IGA measurements was also used to indicate relative absorption rates. Most of the outstanding performers in terms of capacity also showed initial absorption rates comparable to the industry standard monoethanolamine (MEA). This indicates, in terms of both absorption capacity and kinetics, that they are promising candidates for further investigation.

A hybrid genetic algorithm for estimating the equilibrium potential of an ion-selective electrode.

Non-linear equations can be used to model the measured potential of ion-selective electrodes (ISEs) as a function of time. This can be done by using non-linear least squares regression to fit parameters of non-linear equations to an ISE response curve. In iterative non-linear least squares regression (which can be considered as local optimisers), the determination of starting parameter estimates that yield convergence to the global optimum can be difficult. Starting values away from the global optimum can lead to either abortive divergence or convergence to a local optimum. To address this issue, a global optimisation technique was used to find initial parameter estimates near the global optimum for subsequent further refinement to the absolute optimum. A genetic algorithm has been applied to two non-linear equations relating the measured potential from selected ISEs to time. The parameter estimates found from the genetic algorithm were used as starting values for non-linear least squares regression, and subsequent refinement to the absolute optimum. This approach was successfully used for both expressions with measured data from three different ISEs; namely, calcium, chloride and lead ISEs.

A comparative kinetic study of modified Pt(dppf)Cl2 complexes and their interactions with L-cys and L-met.

With the success of cisplatin (cis-diamminedichloroplatinum(II)), strong interest has developed in the application of inorganic metal complexes to the treatment of cancer. Research has focused on platinum(II) complexes with a variety of spectator ligands that provide novel physicochemical properties. In this paper we report a kinetic study of 1',1'-bis(diphenylphosphino)ferrocenedichloroplatinum(II) and two related compounds with either an acetate or amide ester substituent attached to the cyclopentadienyl ring. For all compounds the reactivity towards L-cysteine and L-methionine in aqueous solution has been investigated (25 degrees C, I= 0.010 M and pseudo-first-order conditions). For the reactions with l-cysteine and l-methionine the reactions proceeded via a steady-state aquated intermediate to form mono (0.92(2)-3.25(4)) x 10(-3) s(-1)) and bis adducts (0.97(2)-3.67(4)) x 10(-4) s(-1)). For reactions with l-cysteine, direct reactions with the starting complex also contributed (mono adduct: 0.36(2)-1.41(4) M(-1) s(-1), bis adduct: 0.080(1)-0.96(1) M(-1) s(-1)). The attached substituents were found to have a significant effect upon the reaction kinetics, with the substituted complexes found to have increased reactivity. It is proposed that the increased reactivity stems from hydrogen bonding between the substituent and the entering ligand and subsequent outer-sphere complex stabilisation. Evidence in support of this theory was obtained form measurements in dichloromethane with 1-propanethiol as the entering ligand. The reactivity of the dppf containing complexes was also compared to that of cisplatin (mono adduct: (0.170(1)-0.175(1)) x 10(-3) s(-1), bis adduct: (0.183(1)-0.397(1)) x 10(-4) s(-1)) and found to be significantly enhanced.