Carbamate Stability Research Articles

Structure–Activity Correlation Analyses of MEA + 3A1P/MAE Isomers with a Coordinative Effect Study

The “coordinative effect”, which enhances absorption and desorption performance of single RR′NH at once, was verified repeatedly within MEA + BEA, MEA + DEA, and MEA + BEA + AMP. This work studied the “coordinative effect” within specific MEA + 3-amino-1-propanol 3A1P(RNH2) and MEA + 2-(methylamine)ethanol MAE(RR′NH). The amine 3A1P and MAE were isomers with the same formula but different molecular structures. Both CO2 absorption and desorption tests were conducted in bi-solvents MEA + 3A1P and MEA + MAE separately, with the help of CO2 absorption–desorption parameters. Experimental results demonstrated that the “coordinative effect” existed within MEA + 3A1P and MEA + MAE. Both CO2 absorption/desorption performances of MEA + 3A1P/MAE were superior to those of 3A1P/MAE single amine simultaneously. The coordinative effect reached an optimum ratio for amine blends at 0.2–0.3 + 2 mol/L of MEA + 3A1P and 0.3 + 2 mol/L of MEA + MAE. Based on scientific analysis, carbamate stability is the dominant factor contributing to strong or negligible coordinative effects of MEA + AMP and 3A1P. The synthesized MEA + RR′NH will be blended with other R3N/AMP to generate tri-solvents with a large concentration and cyclic capacities for industrial applications.

Study of “coordinative effect” within bi‐blended amine MEA + AMP and MEA + BEA at 0.1 + 2–0.5 + 2 mol/L with absorption–desorption parameter analyses

AbstractThe “coordinative effect” was discovered in MEA + DEA bi‐solvents already, and it enhances both CO2 absorption and desorption within MEA + RR'NH simultaneously. A recent study verified strong coordinative effects within MEA + BEA (RR'NH) + AMP tri‐solvents, but whether the coordination was 100% attributed to interaction of MEA + BEA needs to be confirmed. This study investigated the “coordinative effect” separately within MEA + AMP (hindered amine) and MEA + BEA (RR'NH). The CO2 absorption and desorption processes were performed onto MEA + AMP and MEA + BEA (0.1 + 2–0.5 + 2 mol/L) separately. The CO2 absorption–desorption parameters were calculated to evaluate the coordinative effects on a consistent level. Results indicated negligible “coordinative effect” within MEA + AMP but strong coordination within MEA + BEA. Both CO2 absorption and desorption performance of MEA + BEA (0.2 + 2 mol/L) was better than 2.0 mol/L BEA simultaneously. The coordinative effect was optimum within MEA + BEA bi‐solvents at 0.2 + 2 mol/L close to 0.3 + 2 + 2 mol/L of MEA + BEA + AMP tri‐solvents. Based on analysis, carbamate stability and pKa were two main factors that determine negligible–strong coordinative effects between AMP and BEA; different pKa reflected the optimized blending ratios of MEA + DEA (3/7) and MEA + BEA (2/20).

Modeling temperature dependent and absolute carbamate stability constants of amines for CO2 capture

Thermodynamic properties and carbamate stability constants (Kc) for a dataset of 25 amines and alkanolamines, with desirable post combustion CO2 capture (PCC) solvent properties, have been studied extensively employing various gaseous phase calculations and solvation models. A comprehensive study of gaseous phase free energy and enthalpy is carried out using density functional methods [B3LYP/6-311++G(d,p)], and composite methods (G3MP2B3, G3MP2, G4MP2 and CBS-QB3). Implicit solvation models (PCM, SM8T and DivCon) and the Explicit Solvation Shell Model (ESS) were used to study solvation free energy of various neutral and ionic species present in the amine-carbamate formation reaction. Temperature dependent carbamate stability constants are calculated for the carbamate formation reaction of amines and alkanolamines to better understand the effect of temperature on temperature swing absorption-desorption PCC processes. The temperature dependency of Kc was compared against available experimental data.

Understanding Carbamate Formation Reaction Thermochemistry of Amino Acids as Solvents for Postcombustion CO2 Capture.

The carbamate stability constant for a data set of 10 amino acids, having potential for being postcombustion CO2 capture (PCC) solvents, has been calculated using various implicit and explicit solvation shell models. This work also includes an extensive study of gas-phase free energy and enthalpy for the amino acid carbamate formation reaction with the Hartree Fock method, density functional methods [B3LYP/6-311++G(d,p)], and composite methods (G3MP2B3, G3MP2, CBS-QB3, and G4MP2). Ideal PCC solvent properties require finding a profitable tradeoff between various thermodynamic and system optimization parameters. Benchmark gaseous-phase and solution-phase thermodynamic properties given in this work can help in making informed decisions when choosing promising PCC solvents. The temperature dependency of the carbamate stability constant of amino acids is predicted using PCM and SM8T implicit solvation models. PCC is a temperature swing absorption-desorption process, and the high-temperature sensitivity of the ln KcAmCOO- value is of vital importance in attaining cost-efficient processes.

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.

Cyclic CO2 absorption capacity of aqueous single and blended amine solvents

The cyclic CO2 absorption capacity (cyclic capacity) of aqueous single and blended amines was systematically analyzed for imaginary as well as real amines using thermodynamic models. The Kent–Eisenberg model and Henry’s law were employed to design the single imaginary amines. The protonation constant (pKa) and carbamate stability constant (Kc) were varied over a wide range to cover the main characteristics of existing monoamines applicable for CO2 capture, and the effects of the pKa, Kc, and amine concentration on the cyclic capacity were carefully investigated. Binary imaginary amine blends were designed by mixing an amine with low carbamate preference with one able to generate carbamates. In addition, an analogous investigation was conducted by considering the effect of the blending ratio of binary blends of four real monoamines (methyldiethanolamine, monoethanolamine, diethanolamine, and aminomethylpropanol) to demonstrate the practicality of this approach. Finally, the cyclic capacity of piperazine (diamine) and its binary blends with the four real monoamines was investigated.

Amine regeneration tests on MEA, DEA, and MMEA with respect to cabamate stability analyses

AbstractThe CO2 desorption analyses of several amines were performed to reveal the behaviour of the amine regeneration process. A typical primary amine (MEA) and two other secondary amines (MMEA and DEA) were selected, in preparation for amine solutions under different concentrations, from 1–7 mol/L. The regeneration curves were generated and plotted to describe the process. It was discovered that the specific CO2 loadings (mol/mol) that distinguish the amine regeneration curves into different regions were the same for the specific amine, despite different concentrations. These points were defined as “turning points” on regeneration curves. The turning points of MEA, MMEA, and DEA are located at CO2 loadings of 0.40 mol/mol, 0.38 mol/mol, and 0.28 mol/mol, respectively. The carbamate stabilities of the three amines were compared with each other based on the slope of regeneration curves at slow region. The amine regeneration tests compared the relative heat duty at the first 2 h: MMEA > MEA > > DEA, which is the same order as carbamate stability.

The pKa values of amine based solvents for CO2 capture and its temperature dependence—An analysis by density functional theory

A computational chemistry study has been employed to calculate the pKa values of alkylamines (primary, secondary and tertiary) used in the removal of carbon dioxide. A DFT-B3LYP level of theory with 6-311++G (d, p) basis set is used for the computational analysis. The pKa values of amines are of great importance as it has a major role in carbon dioxide capturing mechanism. The temperature dependence of pKa values has also been evaluated and it has been observed that the pKa values of alkylamines shows an inverse relation with the temperature. The Secondary and tertiary amines show maximum and minimum pKa values respectively. The gas phase proton affinity (PA) and gas the phase basicity (GB), which is difficult to measure experimentally, have been computed easily and accurately by this method. It is observed that the PA and GB values increase from primary to tertiary alkylamines. The linear relationship between the pKa values and the free energy of protonation has been computationally verified; the results are in good agreement with the experimental results and the method is simpler and less expensive. The work can be extended to study the carbamate stability and carbon dioxide capture mechanism and there by screening the candidates for the Carbon dioxide Capture and Storage.

Understanding the corrosion of CO2-loaded 2-amino-2-methyl-1-propanol solutions assisted by thermodynamic modeling

Corrosion of A106 carbon steel in a naturally aerated 30wt.% 2-amino-2-methyl-1-propanol-based solution (AMP, a sterically hindered primary amine) with 0.43molCO2/molAMP was evaluated at 80°C. Substantial decrease in corrosion rate, i.e., over two orders of magnitude, was observed over the initial 70h, which is the result of formation of a protective FeCO3 layer followed by passivation of the A106 surface. Mechanisms for formation of these protective layers are discussed with comparison to corrosion in a 30wt.% monoethanolamine solution as well as with the help of thermodynamic modeling of the AMP-H2O-CO2 system. Experimental solubility data from literature were employed to extract a thermodynamic model for aqueous solutions of AMP with concentrations ranging from 17.8 to 36.6wt.% at various CO2 loadings. Liquid phase speciation was determined by employing an electrolyte-NRTL model. The AMP carbamate stability constant, molecule-ion pair, and molecule–molecule interaction parameters in the studied concentrations were obtained. The determined CO2 equilibrium properties are in agreement with previously reported experimental data.

Semicontinuum Solvation Modeling Improves Predictions of Carbamate Stability in the CO2 + Aqueous Amine Reaction.

Quantum chemistry computations with a semicontinuum (cluster + continuum) solvation model have been used to cure long-standing misprediction of aqueous carbamate anion energies in the industrially important CO2 + aqueous amine reaction. Previous errors of over 10 kcal mol(-1) are revealed. Activation energies were also estimated with semicontinuum modeling, and a refined discussion of the competing hypothetical mechanisms for CO2 + monoethanolamine (MEA) is presented. Further results are also presented to demonstrate that the basicity of an amine (aqueous proton affinity) correlates only with CO2 affinity within an amine class: secondary amines have an extra CO2 affinity that primary amines do not have.

Thermokinetic properties and performance evaluation of benzylamine-based solvents for CO2 capture

Carbon dioxide (CO2) post combustion capture and storage is the most mature technology option for the mitigation of CO2 emissions from fossil fuel electricity generation. Typically CO2 separation at low pressure is achieved by reactive chemical absorption using aqueous amines. In this work benzylamine (BZA) has been assessed in terms of the chemical and physical properties relevant for its application as an aqueous amine CO2 absorbent. BZA was found to have similar reaction kinetics with CO2 to monoethanolamine (MEA) (kcarb=7600M−1s−1 at 35°C and Ea=38kJmol−1) and similar carbamate stability but with a ∼40% larger enthalpy of protonation. It was also found to be less corrosive and have lower viscosity and heat capacity. Significant performance gains relative to MEA 30wt% were predicted by using BZA in a formulation with either MEA or 2-amino-2-methyl-1-proponal (AMP) with predicted reductions in reboiler duty up to 13%, improvements in mass transfer up to 20% and low corrosion potential.

CO2 Absorption into Aqueous Solutions Containing 3-Piperidinemethanol: CO2 Mass Transfer, Stopped-Flow Kinetics, 1H/13C NMR, and Vapor–Liquid Equilibrium Investigations

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 CO2 capture solvent, predominantly for its rapid reactivity with CO2. 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 CO2 and 3-PM. Additionally, in parallel with the preceding, experimental measurements of CO2 absorption into 3-PM solutions, including mass transfer and vapor–liquid equilibrium measurements, can be used to validate the CO2 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 1H/13C nuclear magnetic resonance (NMR) spectroscopy and (b) experimental measurements of CO2 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 CO2 loadings. Fundamental kinetic rate constants describing the reaction of CO2 with 3-PM are significantly faster than MEA at similar temperatures (3-PM = 32 × 103 M–1 s–1, extrapolated to 40 °C from kinetic data between 15.0 and 35.0 °C; MEA = 13 × 103 M–1 s–1, 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 CO2 absorption rates in 3.0 M solutions of 3-PM and MEA, assessed in overall CO2 mass transfer coefficients, are lower in the former case over the entire range of CO2 loadings from 0.0 to 0.4 mol of CO2 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 CO2 diffusion. CO2 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 CO2 capacities are attributed to the lower stability 3-PM carbamate and the formation of larger amounts of bicarbonate compared to MEA. Overall, the larger CO2 absorption capacity, cyclic capacity, and rapid kinetics with CO2 position 3-PM as an attractive CO2 capture solvent.

Quantum chemical studies on solvents for post-combustion carbon dioxide capture: calculation of pKa and carbamate stability of disubstituted piperazines.

Piperazine is a widely studied solvent for post-combustion carbon dioxide capture. To investigate the possibilities of further improving this process, the electronic and steric effects of -CH(3), -CH(2)F, -CH(2)OH, -CH(2)NH(2), -COCH3 , and -CN groups of 2,5-disubstituted piperazines on the pKa and carbamate stability towards hydrolysis are investigated by quantum chemical methods. For the calculations, B3LYP, M11L, and spin-component-scaled MP2 (SCS-MP2) methods are used and coupled with the SMD solvation model. The experimental pK(a) values of piperazine, 2-methylpiperazine, and 2,5-dimethylpiperazine agree well with the calculated values. The present study indicates that substitution of -CH(3), -CH(2) NH(2), and -CH(2) OH groups on the 2- and 5-positions of piperazine has a positive impact on the CO(2) absorption capacity by reducing the carbamate stability towards hydrolysis. Furthermore, their higher boiling points, relative to piperazine itself, will lead to a reduction of volatility-related losses.

Comparison of Equilibrium Constants of Various Reactions Involved in Amines and Amino Acid Solvents for CO2 Absorption

The ionization constants and carbamate stability constants for post combustion CO2 capture solvents are in this work studied with the help of molecular modeling. These two equilibrium constants are very important in understanding the thermodynamics of any solvent for post combustion carbon capture. Temperature effects on these two equilibrium constants are also studied for pKa of a data set of 10 amines and 10 amino acids. Temperature dependency of carbamate stability constants of MEA and DEA along with experimental data is also presented.

Carbamate Stability Measurements in Amine/CO2/Water Systems with Nuclear Magnetic Resonance (NMR) Spectroscopy

Abstract NMR experiments were carried out for 30 mass % aqueous solutions of monoethanolmine (MEA), ethylenediamine (EDA) and piperazine (PZ) at 25 °C. To distinguish between the amine/protonated amine and bicarbonate/carbonate pairs, an earlier method developed for primary amine (2-amino-2-methyl-1-propanol) was implemented. The amine protonation constant, the dissociation constant of carbonic acid from literature, and pH measurements for different ionic strengths, were all employed and the data were correlated with ionic strength.

Application of 15N-NMR Spectroscopy to Analysis of Amine Based CO2 Capture Solvents

Abstract 15 N NMR spectroscopy is a useful tool for amine reactivity characterization since it can provide information about the availability of the lone pair of electrons on nitrogen through the measured chemical shift values, which depend on molecular structure and medium effects. Although the amino nitrogen is the focal nucleus of the carbon dioxide-amine reaction, 15 N NMR measurements have so far received little attention in the field of amine-based chemical absorption of carbon dioxide. In this study, from one hand, through 15 N NMR chemical shifts measurements, the effect of solvent on the electron density of the nitrogen of 2-methyl-2-amino-1-propanol (AMP) in solvent blends was investigated; from the other hand, 15 N NMR chemical shifts of aqueous primary non-hindered and hindered alkyl amines were related to the corresponding carbamate forming and carbamate stability equilibrium constants as well as carbamate forming kinetic constants, showing linear relationships. Such correlations could be useful for predictive estimation of carbamate-related equilibrium and kinetic constants.

Experimental study on carbamate formation in the AMP–CO2–H2O system at different temperatures

Carbamate formation in the 30wt% of 2-amino-2-methyl-1-propanol (AMP) system at different CO2 loadings and temperatures was studied via nuclear magnetic resonance (NMR) spectroscopy. The results indicate that the main species in this system are AMP/AMPH+, AMPCO2− and HCO3−/CO32−. The carbamate was also observed at low loadings and the apparent carbamate stability constant was estimated based on the experimental concentrations of the species from NMR analysis. Carbamate formation was found to have weak temperature dependence in the range tested (25–45°C). To distinguish within the AMP/AMPH+ and HCO3−/CO32− pairs, the amine protonation constant, the dissociation constant of carbonic acid from literature, and pH measurements for different ionic strengths were all employed. All the data were correlated with ionic strength and temperature.The accuracy of the carbamate stability constant determined from the concentration measurements will depend on pKa, ionic strength (I) used to calculate the speciation and on the uncertainty in the species concentration determinations from NMR, particularly for the carbamate species at low CO2 loadings and high temperature.

Carbamate Stabilities of Sterically Hindered Amines from Quantum Chemical Methods: Relevance for CO2 Capture

The influence of electronic and steric effects on the stabilities of carbamates formed from the reaction of CO2 with a wide range of alkanolamines was investigated by quantum chemical methods. For the calculations, B3LYP, M11-L, MP2, and spin-component-scaled MP2 (SCS-MP2) methods were used, coupled with SMD and SM8 solvation models. A reduction in carbamate stability leads to an increased CO2 absorption capacity of the amine and a reduction of the energy required for solvent regeneration. Important factors for the reduction of the carbamate stability were an increase in steric hindrance around the nitrogen atom, charge on the N atom and intramolecular hydrogen bond strength. The present study indicates that secondary ethanolamines with sterically hindering groups near the N atom show significant potential as candidates for industrial CO2-capture solvents.

Promoting CO2 absorption in aqueous amines with benzylamine

Post-combustion capture of CO2 is the most mature technique for reducing greenhouse gas emissions from coal fired power stations. In this study, aqueous benzylamine (BA) was investigated as a potential solvent for CO2 capture. First, CO2 loading capacities in 30% wt/wt aqueous benzylamine were determined by vapour liquid equilibrium (VLE) experiment in a stirred vessel between 40°C and 80°C, over a pressure range up to 900kPa. At 15kPa and 40°C, 0.45mol CO2/mol BA was absorbed. Protonation and carbamate thermodynamic equilibrium constants as well as the standard enthalpy of protonation were determined. Benzylamine showed a similar pKa (8.89 at 40°C) but a larger pKa variation with temperature than MEA. The carbamate stability constant changed little with temperature. Benzylamine was then investigated as a CO2 absorption rate promoter in aqueous MDEA in a stirred vessel with a plane horizontal gas-liquid interface. Benzylamine performed better than MEA and DEA as a rate promoting agent, a larger amount of benzylamine in aqueous MDEA resulting in a higher enhancement of CO2 absorption rate.

Improving the Capture of CO2by Substituted Monoethanolamines: Electronic Effects of Fluorine and Methyl Substituents

The influence of electronic and steric effects on the reaction between CO(2) and monoethanolamine (MEA) absorbents is investigated using computational methods. The pK(a) of the alkanolamine, the reaction enthalpy for carbamate formation, and the hydrolytic carbamate stability are important factors for the efficiency of CO(2) capture. The steric and electronic effects of CH(3), CH(2)F, CHF(2), CF(3), F, dimethyl, difluoro, and bis(2-trifluoromethyl) substituents at the α carbon of MEA on this reaction are investigated. Density functional theory (DFT) (B3LYP, M06-2X, M08-HX and M11-L) and ab initio methods [spin component-scaled second-order Møller-Plesset theory (SCS-MP2), G3], each coupled with solvent models [conductor-like polarizable continuum model (CPCM) and universal solvation models (SM8 and SMD)], are shown to yield accurately calculated pK(a) values of the substituted MEAs. Specifically, G3, SCS-MP2, and M11-L methods coupled with the SMD and SM8 solvation models perform well with a mean unsigned error (MUE) of only 0.15, 0.24 and 0.25 pK(a) units, respectively. SCS-MP2 is used to calculate the reaction enthalpy for carbamate formation and the carbamate stability towards hydrolysis. With the introduction of β-fluoro substituents (especially the CH(2) F moiety) the reaction enthalpy for the formation of carbamates can be fine-tuned to be less exothermic than that using the unsubstituted MEA. This implies a reduced energy requirement for the solvent-regeneration step in the post-combustion carbon-capture method, which is currently the energy-limiting step in efficient CO(2) capture. β-Fluoro-substituted MEAs are also shown to form less stable carbamates than MEA. Thus, β-fluoro-substituted MEAs display a great potential for the use in the post-combustion carbon-capture process. Finally, a clear correlation is observed between the gas-phase basicity and the tendency to form carbamates. This allows for the rapid prediction of which species will be formed experimentally, and thus the CO(2)-absorbing capacities of alkanolamines can be estimated.