Zwitterion Mechanism Research Articles

A study on reaction mechanism and kinetics of CO2 and MEA/DEA-tertiary amines in non-aqueous and water-lean solutions

The kinetic behavior of the reaction of CO2 with MEA-TREA in ethanol solution was studied using the stopped-flow technique. The results indicate that the zwitterion mechanism successfully correlates the experimentally measured pseudo first-order reaction constant (k0) and that the TREA participated in the deprotonation step of MEA-zwitterion as an alkaline substance after the formation of MEA-zwitterion. Then, the applicability of this proposed mechanism was investigated by using the reaction kinetics of CO2 with MEA-DMEA, MEA-DEEA, MEA-MDEA, DEA-TREA, DEA-DMEA blended amines in non-aqueous ethanol solution, as well as MEA-TREA, MEA-DMEA, DEA-TREA, DEA-DMEA blended amines in water-lean solutions. It was found that the mechanism of the reaction of CO2 with these mixed amines can be expressed by:k0=KA-Az[A]2+KA-Bz[A][B]+K1z[A]. The ethanol and water molecules which contribute to K1z can be used as proton transfer channels to accelerate proton transfer, but they do not directly participate in the reaction. In addition, the alkalinity and the steric hindrance effect controlled by the molecular structure of tertiary amines affected the reaction rate at the same time. The activation energies in MEA-EtOH and MEA-TREA-EtOH system were estimated by fitting the second-order rate constants to the Arrhenius expression, indicating that TREA can reduce the reaction activation energy and make the reaction more kinetically favorable.

Spontaneous alternating copolymerization of aziridines with tosyl isocyanate toward polyureas

We report a new synthetic approach toward cyclic polyureas through alternating copolymerization of aziridines with tosyl isocyanate. Aziridines (ethyleneimine, and (S)-2-benzylaziridine) react with tosyl isocyanate spontaneously in the absence of any catalyst to produce polyureas. The obtained polyureas exhibit an alternating structure as determined by NMR and MALDI-TOF MS analysis. MALDI-TOF analysis indicates that the synthesized polyureas are cyclic. A spontaneous zwitterionic copolymerization mechanism is proposed. DFT calculations reveal that the copolymerization of tosyl isocyanate with ethyleneimine is more kinetically and thermodynamically favored than 2-benzylaziridine.

CFD modeling of MEA-based CO2 spray scrubbing with computational-effective interphase mass transfer description

Great concerns for global warming and stringent regulations have spurred the development of feasible technologies to reduce CO2 emissions from the power generation sector. In thermal power plants, spray scrubbing is one of the most used methods for separating gaseous pollutants, which inspires the idea of applying spray scrubbing in CO2 capture. However, this new idea still necessitates investigation, particularly in developing a novel modeling tool. This motivates us to present the CFD modeling of CO2 spray scrubbing using MEA solution in this work. CO2 spray scrubbing is a complex process. Herein, the established model combines the multiphase flow with interphase transfer and chemical reactions in the Euler-Lagrange framework. To efficiently describe the interphase mass transfer, we proposed a new method based on the thermodynamic consistency between the CO2 equilibrium pressure and the enthalpy of CO2 absorption. Validated by the experimental data, this method can achieve an acceptable balance between accuracy and computational cost. The zwitterion mechanism was employed to describe the chemical reactions. Kinetic analysis shows that E = (1 +Ha2)0.5 can reasonably evaluate the enhancement factor value in a wide range of MEA concentrations. Additionally, the effects of operating parameters on CO2 removal were numerically studied, focusing on mass transfer characteristics. Therefore, the proposed numerical model will be beneficial for facilitating the deployment of CO2 spray scrubbing in the power sector, and the numerical results provide helpful information for design and optimization.

An experimental and theoretical study on the effects of amine chain length on CO2 absorption performance

AbstractTo explore the effect of amine chain length on CO2 absorption performance, the reaction kinetics of CO2 absorption in aqueous 1‐dimethylamino‐2‐propanol (DMA2P), 1‐diethylamino‐2‐propanol (DEA2P), 2‐(methylamino)ethanol (MAE), and 2‐(ethylamino)ethanol (EAE) solutions with different concentrations were explored using the stopped‐flow apparatus. Additionally, Density Functional Theory (DFT) calculations were conducted to examine the reaction mechanism and the free energy barrier of the elementary reactions underlying CO2 absorption in these four aqueous amine solutions. Kinetic models for CO2 absorption in tertiary amines and secondary amines were established, based on the base‐catalyzed hydration mechanism and the zwitterion mechanism, respectively, both of which perform well in predicting the relationship between k0 and the amine concentration. The free energy barrier obtained by DFT is consistent with the activation energy barrier trend obtained by experiment. In addition, the effect of chain length on the free energy barrier was investigated through the chemical bond and weak interaction analysis.

Unveiling the zwitterionic and stepwise mechanism for the domino reactions of amidine with 1,2,4,5‐ and 1,2,3,5‐tetrazines

AbstractThe mechanisms of the domino reactions between 4,6‐diphenyl‐1,2,3,5‐tetrazine (TTZ 1) and 3,6‐diphenyl‐1,2,4,5‐tetrazine (TTZ 2) with amidine 3, have been studied at the M06‐2X/6‐311G(d,p) level of theory. The domino reactions of TTZ1 with amidine comprise three consecutive reactions, an aza Diels‐Alder (ADA) reaction with a stepwise mechanism, a retro Diels‐Alder (rDA) reaction, and an amine elimination process, which leads to the resulting aromatic triazine ring. However, the most favorable pathway for domino reaction of the amidine with TTZ2 consists of nucleophilic addition, N2 elimination, and cyclization steps which is deal with the formation of a neutral intermediate. The N2 loss step in both mechanisms is spontaneously, with extremely exergonic formation of the corresponding intermediates. Finally, NH3 is removed through an irreversible intramolecular elimination. The global electron density transfer (GEDT) calculations unleash the polar natures of the first step of the reactions.

Polyvinyl alcohol membrane incorporated with amine-modified silica nanoparticles and ionic liquid for facilitated transport of CO2

The application of polymeric membranes in post-combustion carbon capture is limited by solution-diffusion. In this work, polyvinyl alcohol (PVA) membranes with facilitated transport of carbon dioxide (CO2) were improved by incorporating the fixed carrier, amine-modified silica nanoparticles (SiO2) and the mobile carrier [bmim][Tf2N] ionic liquid. The BET surface area of diethanolamine (DEA)-modified SiO2 nanoparticles decreased from 150.14 m2/g to 22.68 m2/g, but they improved the surface hydrophilicity of PVA membranes more significantly than the unmodified SiO2 nanoparticles due to the existence of secondary amine groups. Fourier transform infrared spectra confirmed the incorporation of secondary amine and anion of IL that can facilitate CO2 permeation. The thermogravimetric analysis also revealed that the thermal stability of PVA membranes was improved by incorporating inorganic nanoparticles and ionic liquid. The thickness of the dense PVA layer coated on polyethersulfone support increased by incorporating SiO2 and DEA-modified SiO2 nanoparticles, as shown by scanning electron microscope images. The positive effects of SiO2 nanoparticles on the CO2 separation performance of PVA membranes were enhanced by reducing the particle size and increasing particle loading. DEA elevated the enhancement through the Zwitterion mechanism. [bmim][Tf2N] IL also improved the CO2 separation performance of PVA membranes. Hence, the PVA membrane containing 2 wt% of DEA-modified SiO2 nanoparticles (15-20 nm) and 0.4 M of [bmim][Tf2N] IL in ethanol attained a CO2 permeance of 3016 GPU and CO2/N2 selectivity of 62.08.

Anti-Fouling, Adhesive Polyzwitterionic Hydrogel Electrodes Toughened Using a Tannic Acid Nanoflower

Conductive polyzwitterionic hydrogels with good adhesion properties show potential prospect in implantable electrodes and electronic devices. Adhesive property of polyzwitterionic hydrogels in humid environments can be improved by the introduction of catechol groups. However, common catechol modifiers can usually quench free radicals, resulting in a contradiction between long-term tissue adhesion and hydrogel toughness. By adding tannic acid (TA) to the dispersion of clay nanosheets and nanofibers, we designed TA-coated nanoflowers and nanofibers as the reinforcing phase to prepare polyzwitterionic hydrogels with adhesion properties. The hydrogel combines the mussel-like and zwitterionic co-adhesive mechanism to maintain long-term adhesion in underwater environments. In particular, the noncovalent cross-linking provided by the nanoflower structure effectively compensates for the defects caused by free-radical quenching so that the hydrogel obtained a high stretchability of over 2900% and a toughness of 1.16 J/m3. The hydrogel also has excellent anti-biofouling property and shows resistance to bacteria and cells. In addition, the hydrogel possesses a low modulus (<10 kPa) and ionic conductivity (0.25 S/m), making it an ideal material for the preparation of implantable electrodes.

Study on the mechanism and kinetics of amine with steric hindrance absorbing CO2 in non-aqueous/aqueous solution

Several alkanolamines (1A2P, 2A1P, AMP, MAE, EAE, and IPAE) were selected to study the steric hindrance effect on reaction kinetics between amine and CO2 in ethanol and aqueous solutions. Pseudo first-order reaction rate constant k0 was measured by the stopped-flow apparatus. Zwitterion mechanism was applied to describe the reaction and the k0 was well fitted by the corresponding kinetic model with average absolute relative deviation (AARD) in a range of 1.08%-3.89%. Apparent second-order reaction rate constant KA revealed that the reaction rate is simultaneously controlled by steric hindrance and pKa of amines. Additional alkyl groups increase the steric hindrance of amines and accelerate the decomposition of zwitterions. As a result, the reaction rate decreases with stronger steric hindrance effect. The larger pKa is conducive to a better proton receiving of amine. This leads to an acceleration of the deprotonation reaction of zwitterion intermediate. In addition, the relative influence of steric hindrance and pKa on the reaction kinetics is also related to the solvent environment.

Kinetics of CO2 absorption into ethanolamine + water + ethanol system—mechanism, role of water, and kinetic model

The kinetics of CO2 absorption in ethanolamine (MEA) + H2O + ethanol systems was investigated using stopped-flow apparatus. The first-order reaction rate constant (k0) values were tested with varying H2O and MEA concentrations. Transiting from the aqueous to non-aqueous solvents, the k0 decreases and the reaction order for MEA shifts from 1.05 to 1.88. The zwitterion mechanism with the rate-limiting step of deprotonation was used to explore the role of water and its effect. Results showed that water could impact reactions through direct involvement and changing solvation environment. An improved kinetic model based on the zwitterion mechanism was obtained to indicate the quantitative influence of H2O concentration on the reaction kinetics of MEA + H2O + ethanol systems. The predicted values from the model well agreed to the experimental data with the average absolute relative difference of 6.73%. This study highlights the important effects of water on the reaction kinetics of CO2 absorption into water-lean amine solutions.

An experimental/computational study of steric hindrance effects on CO2 absorption in (non)aqueous amine solutions

AbstractThe reaction kinetics and molecular mechanisms of CO2 absorption using nonaqueous and aqueous monoethanolamine (MEA)/methyldiethanolamine (MDEA)/2‐amino‐2‐methy‐1‐propanol (AMP) solutions were analyzed by the stopped‐flow technique and ab initio molecular dynamics (AIMD) simulations. Pseudo first‐order rate constants (k0) of reactions between CO2 and amines were measured. A kinetic model was proposed to correlate the k0 to the amine concentration, and was proved to perform well for predicting the relationship between k0 and the amine concentration. The experimental results showed that AMP/MDEA only took part in the deprotonation of MEA‐zwitterion in nonaqueous MEA + AMP/MEA + MDEA solutions. In aqueous solutions, AMP can also react with CO2 through base‐catalyzed hydration mechanism beside the zwitterion mechanism. Molecular mechanisms of CO2 absorption were also explored by AIMD simulations coupled with metadynamics sampling. The predicted free‐energy barriers of key elementary reactions verified the kinetic model and demonstrated the different molecular mechanisms for the reaction between CO2 and AMP.

New method of kinetic modeling for CO2 absorption into blended amine systems: A case of MEA/EAE/3DEA1P trisolvent blends

AbstractIn order to establish an accurate kinetic model for the aqueous amine blends, monoethanolamine (MEA), 2‐(ethylamino) ethanol (EAE), and 3‐(diethylamino)‐1‐propanol (3DEA1P) have been chosen as a typical CO2 absorption trisolvents. The reaction kinetics of aqueous amine blends with carbon dioxide have been investigated first combining experiments and molecular simulations. The stopped‐flow technology has been used to obtain the observed reaction rate constant of the overall reactions over the temperature range of 293 to 313 K and at different amine concentrations. A theoretical kinetic model, based on the first‐principles quantum‐mechanical simulations, has been put forward to interpret the reactions between CO2 and the aqueous trisolvent amine blends systems. The proposed model, based on the zwitterion mechanism and the base‐catalyzed mechanism, shows good prediction with an acceptable absolute average deviation (AAD) of 6.32%, and has been found to be satisfactory in determining the kinetics of the involved complicated reactions.

Innovative cycling reaction mechanisms of CO2 absorption in amino acid salt solvents

Amino acid salt (AAS) solvents with equimolar base (used to obtain the salt) to amino acid (AA) have been well studied for CO2 absorption, however, very little is known about the reactions when more base is applied. This study determined the reaction mechanisms of AAS solvents with base to AA ratio over equimolar and the potential benefits of such solvents for CO2 absorption. The CO2 loading capacity was found to be dictated by the molar concentration of the base and was approximately half of the base applied. The reaction mechanisms were investigated based on the compositions of carbamate, carbonate/bicarbonate, and AA using 13C-quantitative nuclear magnetic resonance (13C qNMR). An innovative cycling reaction mechanism of CO2 absorption was proposed and experimentally confirmed for AAS solvents with base/AA > 1. Besides all steps of the current widely accepted Zwitterion mechanism, this reaction pathway also contains two cycles: 1) The extra base (i.e., OH−) may react with the protonated AA generated during CO2 absorption to form deprotonated AA which further absorbs CO2 to form more carbamate. 2) The carbamate, as a product of reaction cycle 1, undergoes hydrolysis to yield bicarbonate and deprotonated AA, which further absorbs CO2. As a result, high CO2 loadings were achieved. Compared to the use of alkaline solutions like KOH alone for CO2 absorption, AAS solvents with base to AA ratio over equimolar resulted in lower pH and temperature while maintaining high CO2 loading. These innovative cyclic mechanisms will enable advanced CO2 management approaches using AAS solvents with base/AA > 1.

A novel solid–liquid ‘phase controllable’ biphasic amine absorbent for CO2 capture

The main issues in the application of biphasic amine absorbent include the inconvenient separation of the liquid–liquid biphase and the clogging that is associated with the solid–liquid mixture. A novel solid–liquid ‘phase controllable’ biphasic amine absorbent was therefore developed, in which the phase separation occurred only when the absorbent was near saturation. The absorbent, composed of triethylene tetramine (TETA), 2-amino-2-methyl-1-propanol (AMP), and N-methylformamide (NMF), turned into solid–liquid biphase under CO2 loading of 0.85 mol mol−1, which is close to saturation (0.92 mol mol−1). The solid phase comprised white powder that accounted for only 42 % of the total volume while absorbing 91 % of the total load. Turbidity and particle size tests showed that the phase separation was self-aggregating. The mechanisms of absorption and regulation were obtained using 13C NMR and molecular simulation. During the absorption, CO2 first combined with TETA to generate TETAH+CO2–/TETACO2– through the zwitterion mechanism, and then combined with AMP to generate AMPCO2– because of the lower reaction activity between CO2 and AMP. The TETA-carbamate was found to greatly weakened the strength of the hydrogen bonds and the van der Waals forces between AMPH+ and AMPCO2–, increasing the solubility of the intermediate products, and therefore achieved the purpose of controlling the phases separation inherent in the reaction.

Enhancement of kinetic rates of CO2 in aqueous solutions of piperazine and methyldiethanolamine with addition of sulfolane

AbstractMeasurements of kinetics rates of CO2 in aqueous solutions of methyldiethanolamine (MDEA), piperazine (PZ), and mixtures of (MDEA + PZ), (PZ + sulfolane) and (MDEA + sulfolane) were carried out using the stopped flow technique, and reported in terms of pseudo‐first‐order rate constants (k0). When possible, the second‐order reaction rate constants (k2) were regressed from the data. Experiments were performed over new concentration ranges of (10–60), (200–800), (200–800, 10–40), (10–40, 10–200), and (200–800, 10–200) mol/m3 for the above‐mentioned five systems, respectively, and at temperatures varying from (298.15–313.15 K). When sulfolane was added to the amine solution, pseudo‐first‐order rate constants in the mixed solvents were higher than in aqueous MDEA and PZ solutions at all temperatures. The kinetic rates were highest at 298.15 K and decreased at higher temperatures for aqueous (MDEA + sulfolane) solutions but increased with temperature for aqueous (PZ + sulfolane) systems. Reaction orders for both PZ and MDEA were practically one at all sulfolane concentrations and temperatures. The base catalysis mechanism was used to regress very well data for aqueous MDEA and (MDEA + sulfolane + water) and the termolecular mechanism was used for (PZ + sulfolane + water) system. Both the zwitterion and termolecular models were able to fit the experimental data for the aqueous PZ system well. Finally, the termolecular and a hybrid model based on the combination of the Zwitterion and base catalysis mechanisms were able to successfully correlate the experimental data for the mixed aqueous (MDEA + PZ) systems.

Amine-functionalized metal-organic framework ZIF-8 toward colorimetric CO2 sensing in indoor air environment

Carbon dioxide (CO2) has been shown to contribute to human health consequences indoors, such as shortness of breath, nasal and optic irritation, dizziness, and nausea. In this work, we explore the potential of metal–organic frameworks (MOFs) as highly-porous, crystalline sorbents for sensitive colorimetric CO2 detection. In particular, the zeolitic imidazolate framework (ZIF-8) is chosen as the sorptive material due to its chemical stability and tunable CO2 affinity. The colorimetric gas sensor is developed in methanol with three components: (i) MOF ZIF-8 as a high surface area adsorbent; (ii) ethylenediamine (ED) as the CO2-affinitive basic function; and (iii) phenolsulfonpthalein (PSP) as the pH indicator. Colorimetric assays and ratiometric analysis confirm a colorimetric response to variable CO2 concentrations of relevance to indoor air quality. The color response is attributed to a zwitterion mechanism whereby ED reacts with CO2 to form a zwitterionic intermediate. This intermediate is then deprotonated by the pH indicator, shifting the pH and inducing a color change. Given its simple fabrication, rapid and obvious response, and stability in ambient environment, the ZIF-8-based colorimetric sensor provides a promising route for an improved indoor air quality monitoring.

Absorption performance and reaction mechanism study on a novel anhydrous phase change absorbent for CO2 capture

• A novel phase change absorbent was proposed for CO 2 capture with low energy consumption. • Ethanol promoted phase change behavior and increased CO 2 loading. • Reaction of CO 2 capture with [N 1111 ][Gly]/ethanol was a three-step reaction. • Intramolecular hydrogen bonds affected products’ solubility, causing phase change. Carbon capture and utilization technologies are urged more than ever due to the excessive emissions of CO 2 that could lead to serious environmental problems. Phase change absorbents are quite competitive in reducing the energy consumption of carbon capture. In this work, tetramethylammonium glycinate ([N 1111 ][Gly]), an efficient absorbent, was dissolved in ethanol and 1-propanol, respectively, to absorb CO 2 . After CO 2 absorption, the saturated solution separated into two phases. CO 2 was enriched in the lower phase, so that only the CO 2 -rich phase needed to be regenerated and the energy consumption of regeneration could be thus reduced. Under the optimal conditions, the CO 2 loading of fresh absorbents could reach 0.85 mol CO 2 /mol IL. On the basis of the 13 C NMR analysis and quantum chemical calculation, a three-step reaction mechanism of CO 2 capture with [N 1111 ][Gly]/ethanol was proposed. CO 2 reacted with [N 1111 ][Gly] to form carbamate, which was similar to the two-step Zwitterionic mechanism of MEA with CO 2 . Then the formed carbamate could react with ethanol to form ethyl carbonate and [N 1111 ][Gly], which resulted in higher CO 2 loading. Moreover, hydrogen bond characteristics were analysed to clarify the phase change mechanism. The number and bonding energy of intramolecular hydrogen bonds were increased after capturing CO 2 , leading to the formation of precipitation.

Well-Defined Poly(Ester Amide)-Based Homo- and Block Copolymers by One-Pot Organocatalytic Anionic Ring-Opening Copolymerization of N-Sulfonyl Aziridines and Cyclic Anhydrides.

We report a new synthetic methodology for poly(ester amide)s by anionic ring‐opening copolymerization of N‐sulfonyl aziridines and cyclic anhydrides. Phosphazenes organocatalysts have been found to promote a highly‐active, controlled, and selective alternating copolymerization in the absence of any competitive side reaction (zwitterionic mechanism and exchange transacylations). Mechanistic studies have shown first‐order dependence of the copolymerization rate in N‐sulfonyl aziridines and phosphazenes, and zero‐order in cyclic anhydrides. This one‐pot methodology leads not only to homopolymers but also to poly(ester amide)‐based block copolymers. Two catalytic cycles involving ring‐opening alternating copolymerization of N‐sulfonyl aziridines with cyclic anhydrides and ring‐opening polymerization of N‐sulfonyl aziridines have been proposed to explain the one pot synthesis of poly(ester amide)‐based homo‐ and block copolymers.

Well‐Defined Poly(Ester Amide)‐Based Homo‐ and Block Copolymers by One‐Pot Organocatalytic Anionic Ring‐Opening Copolymerization of N‐Sulfonyl Aziridines and Cyclic Anhydrides

AbstractWe report a new synthetic methodology for poly(ester amide)s by anionic ring‐opening copolymerization of N‐sulfonyl aziridines and cyclic anhydrides. Phosphazenes organocatalysts have been found to promote a highly‐active, controlled, and selective alternating copolymerization in the absence of any competitive side reaction (zwitterionic mechanism and exchange transacylations). Mechanistic studies have shown first‐order dependence of the copolymerization rate in N‐sulfonyl aziridines and phosphazenes, and zero‐order in cyclic anhydrides. This one‐pot methodology leads not only to homopolymers but also to poly(ester amide)‐based block copolymers. Two catalytic cycles involving ring‐opening alternating copolymerization of N‐sulfonyl aziridines with cyclic anhydrides and ring‐opening polymerization of N‐sulfonyl aziridines have been proposed to explain the one pot synthesis of poly(ester amide)‐based homo‐ and block copolymers.

Analysis of zwitterionic membrane fouling mechanism caused by HPAM in the presence of electrolytes.

Membrane fouling has always been a tough issue that is urgent to solve. Electrolytes which are prevalent in wastewater have a major influence on membrane fouling. Therefore, it is of great significance to understand the role and fouling mechanism of electrolytes in the membrane fouling process. In this work, the zwitterionic membrane is used to process hydrolyzed poly(acrylamide) (HPAM) with the addition of electrolytes (CaCl2, NaCl). Meanwhile, the effect of different electrolytes on the zwitterionic membrane fouling process by hydrolyzed poly(acrylamide) (HPAM) is systematically investigated. It was found that the flux recovery ratio (FRR) of the zwitterionic membrane is nearly 100% after treating HPAM with the addition of electrolytes. Therefore, molecular dynamics (MD) simulations were applied to illustrate the impact of electrolytes on the change of foulant structures and confirm the consequent effect of electrolytes on membrane fouling. According to the experiment and MD simulation results, it is found that the positive ion layer which exists between the HPAM and zwitterionic surface results in the excellent fouling resistance performance of the zwitterionic membrane. The zwitterionic membrane fouling mechanism is analyzed, which is helpful to the understanding of zwitterionic membrane fouling in high salinity wastewater.

A comparison of zwitterionic and anionic mechanisms in the dual-catalytic polymerization of lactide

Two different polymerization mechanisms for lactide are selectivity addressed to illuminate the respective role of organobase and Lewis acid component.