Computational Modeling and Simulation of CO2 Capture by Aqueous Amines

Semiempirical Peak Fitting Guided by ab Initio Calculations of X-ray Photoelectron Spectroscopy Narrow Scans of Chemisorbed, Fluorinated Silanes

Here, we address the issue of finding correct CF₂/CF₃ area ratios from X-ray photoelectron spectroscopy (XPS) C 1s narrow scans of materials containing -CH₂CH₂(CF₂)_nCF₃ (n = 0, 1, 2, ...) moieties. For this work, we modified silicon wafers with four different fluorosilanes. The smallest had a trifluoropropyl (n = 0) moiety, followed by nonafluorohexyl (n = 3), tridecafluoro (n = 5), and finally, heptadecafluoro (n = 7) moieties. Monolayer deposition of the fluorosilanes was confirmed by spectroscopic ellipsometry, wetting, and XPS. Analysis of the trifluoropropyl (n = 0) surface and a sample of polytetrafluoroethylene provided pure-component XPS spectra for -CF₃ and -(CF₂)_n- moieties, respectively. Initial XPS C 1s peak fitting, which follows the literature precedent, was not entirely adequate. To address this issue, six different fitting approaches with increasing complexity and/or input from the Hartree-Fock theory (HF) were considered. Ultimately, we show that by combining HF results with empirical analyses, we obtain more accurate CF₂/CF₃ area ratios while maintaining high-quality fits.

Reaction Mechanism and Free Energy Barriers for the Chemisorption of CO2 by Ionic Entities

Ionic liquids, a class of alternative solvents, are known for their ability to capture carbon dioxide (CO₂). The understanding of the role of the individual ionic entity of the ionic liquid (IL) and the involved mechanism is essential to design a better solvent for the capture process. In the present study, we employed density functional theory based electronic structure calculations and metadynamics method based first-principles molecular dynamics (FPMD) simulations to investigate the roles of the cation and anion of the IL by analyzing the energetics and free energy profile of the involved chemical reactions. The mechanism of chemisorption of CO₂ by the aprotic N-heterocyclic and phenolate anions paired with tetrapropyl phosphonium cation [P₃₃₃₃] were studied to understand the reaction mechanism of the initial capture process. The process of uptaking of CO₂ by the [P₃₃₃₃][1,2,4-Triz] was studied by the first-principles calculations. The transition states in the reaction pathways were computed by the synchronous transit-guided quasi-Newton method and confirmed by the intrinsic reaction coordinate calculations using first-principles simulations. The dynamics of the energetics of the chemisorption process were studied by constructing the free energy surface using metadynamics-based FPMD simulations. First, the nucleophilic center was generated at the α-carbon of the cation by transferring a proton to the anion with the formation of the phosphorus ylide. The formed cation ylide chemisorbs CO₂ through the formation of a bond between the α-carbon of ylide and the carbon of CO₂. The direct addition of CO₂ to the anion of the ionic pair was studied as the second pathway. We find that the chemisorption of CO₂ by the anion is more favorable than that by the cation. By comparing the chemisorption of CO₂ by the ions, we observe that the deprotonation of the alkyl chain is the more deciding factor, which depends on the basicity of anion and the length of the alkyl chain. We computed the free energy landscapes for the ionic pairs by varying another four anions like cyclohexanolate, 2,4,6-trifluorophenolate, imidazolate, and benzotriazolide paired with tetrapropyl phosphonium cation. The effect of the alkyl chain on the proton transfer was studied by tetrabutyl and tetrapentyl phosphonium cations paired with 1,2,4-triazolide anion. The carbonated product, formed from the anion, is thermodynamically controlled, while the carboxylated product (formed from cation) is kinetically controlled. We hope that our findings will enhance the knowledge of the selectivity of ionic entities for designing IL-based solvents for the capture process of CO₂.

Enthalpic and entropic contributions to the basicity of cycloalkylamines

Large-ring cycloalkylamines are slightly less basic than other cycloalkylamines such as cyclohexylamine, even though all have tetrahedral carbons and are strain-free. To understand why, enthalpy and entropy for protonation of a series of cycloalkylamines were accurately determined by isothermal titration calorimetry in 3 : 1 methanol–water. The study required resolving a discrepancy between these measurements and those in pure water. The data show that the lower basicity of large-ring cycloalkylamines is not due to enthalpy but to a more negative entropy of protonation. Computations show that this can be attributed in part to an entropy of conformational mixing, but the dominant contribution is steric hindrance to solvation, also corroborated by computation.

Large-ring cycloalkylamines are slightly less basic than other cycloalkylamines such as cyclohexylamine, even though all have tetrahedral carbons and are strain-free.

High-performance carbon dioxide capture and storage by multi-functional sphingosine kinase inhibitors through a CO2-philic membrane

Carbon dioxide (CO₂) capture using environmentally friendly sphingosine-based materials was theoretically studied.

The effect of CO2 loading on alkanolamine absorbents in aqueous solutions

Post-combustion carbon capture by amine scrubbing is the most frequently used process to remove CO2 from pulverized coal-fired power plants and also biogas flue gas streams. The quest for novel absorbents for CO2 capture with improved properties requires insight into the properties of the CO2-loaded mixed solutions. A comparative molecular dynamics study of the product state solutions, with chemically-bound CO2 of standard monoethanolamine (MEA) and the new alternative 4-diethylamino-2-butanol (DEAB) at various CO2-loadings yields solvent properties in good agreement with experimental data. The concentration of all post-reaction species in solution was based on experimental equilibria distributions. The data generated provide detailed insight into the properties of reactive mixed alkanolamine solutions. The liquid structure of aqueous MEA solutions undergoes only minor changes when absorbing CO2. The diffusion coefficients of all molecular species, however, decrease significantly with increasing CO2-loadings. The large hydrophobic clusters formed in the reactant state by DEAB molecules in water prior to CO2 binding significantly decrease in size and structure upon CO2 absorption. The diffusion coefficients of all components decrease with increasing CO2-loading, whereas the pre-reaction alkanolamine DEAB shows an increase in diffusion coefficient. This structural and kinetic information supports the molecular design and further development of novel compounds and provides data for a global process simulation and optimization.

Role of Water in the Reaction Mechanism and endo/exo Selectivity of 1,3-Dipolar Cycloadditions Elucidated by Quantum Chemistry and Machine Learning

Asymmetric 1,3-dipolar cycloadditions of azomethine ylides with activated olefins are among the most important and versatile methods for the synthesis of enantioenriched pyrroline and pyrrolidine derivatives. Despite both theoretical and practical importance, the role of water molecules in the reactivity and endo/exo selectivity remains unclear. To explore how water accelerates the reactions and improves the endo/exo selectivity of the cycloadditions of 1,3-dipole phthalazinium-2-dicyanomethanide (1) and two dipolarophiles, an ab initio-quality neural network potential that overcomes the computational bottleneck of explicitly considering water molecules was used. It is demonstrated that not only the nature of both the dipolarophile and the 1,3-dipole, but also the solvent medium, can perturb or even alter the reaction mechanism. An extreme case was found for the reaction of 1,3-dipole 1 with methyl vinyl ketone, in which the reaction mechanism changes from a concerted to a stepwise mode on going from MeCN to H₂ O as solvent, with formation of a zwitterionic intermediate that is a very shallow minimum on the energy surface. Thus, high stereocontrol can still be expected despite the stepwise nature of the mechanism. The results indicate that water can induce global polarization along the reaction coordinate and highlight the role of microsolvation effects and bulk-phase effects in reproducing the experimentally observed aqueous acceleration and enhanced endo/exo selectivity.

Enhancing CO2 Adsorption and Separation Properties of Aluminophosphate Zeolites by Isomorphous Heteroatom Substitutions

Mg, Co-substituted aluminophosphate zeolites with ERI framework topology (denoted as MgAPO-ERI and CoAPO-ERI) have been synthesized under hydrothermal conditions by using N, N, N', N'-tetramethyl-1,6-hexanediamine as organic template. Their CO₂ adsorption properties are investigated in comparison to those of the pure aluminophosphate counterpart AlPO-ERI. CoAPO-ERI shows the highest CO₂ uptake of 57.3 cm³ g^-1 (273 K and 1 bar) and the highest isosteric heat of 39.0 kJ mol^-1 among the three samples. Importantly, the incorporation of Mg²⁺ and Co²⁺ ions in the framework of AlPO-ERI can greatly improve the adsorption selectivities of CO₂ over CH₄ and N₂. Whereafter, transient breakthrough simulations were investigated and further proved the advantages of heteroatoms for separations. These results demonstrate that isomorphous heteroatom substitutions in aluminophosphate zeolites play a key role in enhancing CO₂ adsorption and separation abilities.

Catalyst-TiO(OH)2 could drastically reduce the energy consumption of CO2 capture

Implementing Paris Climate Accord is inhibited by the high energy consumption of the state-of-the-art CO₂ capture technologies due to the notoriously slow kinetics in CO₂ desorption step of CO₂ capture. To address the challenge, here we report that nanostructured TiO(OH)₂ as a catalyst is capable of drastically increasing the rates of CO₂ desorption from spent monoethanolamine (MEA) by over 4500%. This discovery makes CO₂ capture successful at much lower temperatures, which not only dramatically reduces energy consumption but also amine losses and prevents emission of carcinogenic amine-decomposition byproducts. The catalytic effect of TiO(OH)₂ is observed with Raman characterization. The stabilities of the catalyst and MEA are confirmed with 50 cyclic CO₂ sorption and sorption. A possible mechanism is proposed for the TiO(OH)₂-catalyzed CO₂ capture. TiO(OH)₂ could be a key to the future success of Paris Climat e Accord.

The notoriously slow kinetics in CO₂ desorption hinders the development of efficient CO₂ capture technologies. Here, the authors discover that nanostructured TiO(OH)₂ as a catalyst is capable of dramatically increasing the rates of CO₂ desorption from spent monoethanolamine.

Molecular Dynamics Study of the Solution Structure, Clustering, and Diffusion of Four Aqueous Alkanolamines

CO₂ sequestration from anthropogenic resources is a challenge to the design of environmental processes at a large scale. Reversible chemical absorption by amine-based solvents is one of the most efficient methods of CO₂ removal. Molecular simulation techniques are very useful tools to investigate CO₂ binding by aqueous alkanolamine molecules for further technological application. In the present work, we have performed detailed atomistic molecular dynamics simulations of aqueous solutions of three prototype amines: monoethanolamine (MEA) as a standard, 3-aminopropanol (MPA), 2-methylaminoethanol (MMEA), and 4-diethylamino-2-butanol (DEAB) as potential novel CO₂ absorptive solvents. Solvent densities, radial distribution functions, cluster size distributions, hydrogen-bonding statistics, and diffusion coefficients for a full range of mixture compositions have been obtained. The solvent densities and diffusion coefficients from simulations are in good agreement with those in the experiment. In aqueous solution, MEA, MPA, and MMEA molecules prefer to be fully solvated by water molecules, whereas DEAB molecules tend to self-aggregate. In a range from 30/70-50/50 (w/w) alkanolamine/water mixtures, they form a bicontinuous phase (both alkanolamine and water are organized in two mutually percolating clusters). Among the studied aqueous alkanolamine solutions, the diffusion coefficients decrease in the following order MEA > MPA = MMEA > DEAB. With an increase of water content, the diffusion coefficients increase for all studied alkanolamines. The presented results are a first step for process-scale simulation and provide important qualitative and quantitative information for the design and engineering of efficient new CO₂ removal processes.

Enhanced carbon dioxide capture in an indole-based microporous organic polymer via synergistic effects of indoles and their adjacent carbonyl groups

A carbonyl-functionalized indole-based microporous organic polymer (PKIN) was designed and synthesized in the presence of the FeCl₃ catalyst by a facile direct oxidative coupling reaction.

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

CO₂ conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO₂ utilization as a measure for global CO₂ mitigation. Whereas the latter focuses on reducing the end-of-pipe problem "CO₂ emissions" from todays' industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO₂-based chemistry does not depend primarily on the absolute amount of CO₂ emissions that can be remediated by a single technology. Rather, CO₂-based chemistry is stimulated by the significance of the relative improvement in carbon balance and other critical factors defining the environmental impact of chemical production in all relevant sectors in accord with the principles of green chemistry.