Computational Modeling and Simulation of CO2 Capture by Aqueous Amines

Cerium-MOF-Derived Composite Hierarchical Catalyst Enables Energy-Efficient and Green Amine Regeneration for CO2 Capture

The excessive energy consumed restricts the application of traditional postcombustion CO2 capture technology and limits the achievement of carbon-neutrality goals. Catalytic-rich CO2 amine regeneration has the potential to accelerate proton transfer and increase the energy efficiency in the CO2 separation process. Herein, we reported a Ce-metal-organic framework (MOF)-derived composite catalyst named HZ-Ni@UiO-66 with a hierarchical structure, which can increase the CO2 desorbed amount by 57.7% and decrease the relative heat duty by 36.5% in comparison with the noncatalytic monoethanolamine (MEA) regeneration process. The composite catalyst of the CeO2 coating from the UiO-66 precursor on the HZ-Ni carrier shows excellent stability with a long lifespan. The HZ-Ni@UiO-66 catalyst also shows a universal catalytic effect in typical blended amine systems with a large cyclic capacity. The HZ-Ni@UiO-66 catalyst effectively decreases the energy barrier of the CO2 desorption reaction to reduce the time required to reach thermodynamics, consequently saving the energy consumption generated by water evaporation. This research provides a new avenue for advancing amine regeneration with less heat duty at low temperatures.

Triethanolamine-modified layered double oxide for efficient CO2 capture with low regeneration energy

Various adsorbents for CO2 capture have been developed to mitigate the greenhouse effect. In this work, a novel CO2 adsorbent was fabricated by depositing triethanolamine (TEOA) onto the surface of nickel-cobalt-aluminum layered double oxide (NiCoAl-LDO) via the impregnation method. The CO2 capacity of the TEOA-LDO composite reached 1.27 mmol/g at 0 °C and 100 kPa, which was twice that of unmodified NiCoAl-LDO. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that the hydroxyl groups (-OH) on the surface of NiCoAl-LDO played a significant role in facilitating CO2 adsorption, similar to CO2 adsorption in the presence of H2O, where CO2 is not converted to carbamates but to bicarbonates through base-catalyzed hydration. This bicarbonate pathway doubles the theoretical amine efficiency, increases the CO2 capacity, and reduces the energy consumption during CO2 desorption. The work provides valuable insights into the development of CO2 adsorbents with high capacity, excellent cycling stability, and low regeneration energy.

Carbon capture, utilization and sequestration systems design and operation optimization: Assessment and perspectives of artificial intelligence opportunities

Carbon capture, utilization, and sequestration (CCUS) is a promising solution to decarbonize the energy and industrial sectors to mitigate climate change. An integrated assessment of technological options is required for the effective deployment of CCUS large-scale infrastructure between CO2 production and utilization/sequestration nodes. However, developing cost-effective strategies from engineering and operation perspectives to implement CCUS is challenging. This is due to the diversity of upstream emitting processes located in different geographical areas, available downstream utilization technologies, storage sites capacity/location, and current/future energy/emissions/economic conditions. This paper identifies the need to achieve a robust hybrid assessment tool for CCUS modeling, simulation, and optimization based mainly on artificial intelligence (AI) combined with mechanistic methods. Thus, a critical literature review is conducted to assess CCUS technologies and their related process modeling/simulation/optimization techniques, while evaluating the needs for improvements or new developments to reduce overall CCUS systems design and operation costs. These techniques include first principles- based and data-driven ones, i.e. AI and related machine learning (ML) methods. Besides, the paper gives an overview on the role of life cycle assessment (LCA) to evaluate CCUS systems where the combined LCA-AI approach is assessed. Other advanced methods based on the AI/ML capabilities/algorithms can be developed to optimize the whole CCUS value chain. Interpretable ML combined with explainable AI can accelerate optimum materials selection by giving strong rules which accelerates the design of capture/utilization plants afterwards. Besides, deep reinforcement learning (DRL) coupled with process simulations will accelerate process design/operation optimization through considering simultaneous optimization of equipment sizing and operating conditions. Moreover, generative deep learning (GDL) is a key solution to optimum capture/utilization materials design/discovery. The developed AI methods can be generalizable where the extracted knowledge can be transferred to future works to help cutting the costs of CCUS value chain.

Reactivity of Monoethanolamine at the Air-Water Interface and Implications for CO2 Capture

The development of CO2-capture technologies is key to mitigating climate change due to anthropogenic greenhouse gas emissions. These cover a number of technologies designed to reduce the level of CO2 emitted into the atmosphere or to eliminate CO2 from ambient air. In this context, amine-based sorbents in aqueous solutions are broadly used in most advanced separation techniques currently implemented in industrial applications. It has been reported that the gas/liquid interface plays an important role in the early stages of the capture process, but how the interface influences the chemistry is still a matter of debate. With the help of first-principles molecular dynamics simulations, we show that monoethanolamine (MEA), a prototypical sorbent molecule, has a weak affinity for the air-water interface, where in addition it exhibits a lower nucleophilicity compared to bulk solution. The change in reactivity is due to the combination of structural and electronic factors, namely, the shift of the conformational equilibrium and the stabilization of the N-atom lone pair. Based on these results, strategies for improving the efficiency of alkanolamine sorbents are proposed.

CO2 Capture and Release in Amine Solutions: To What Extent Can Molecular Simulations Help Understand the Trends?

Absorption in amine solutions is a well-established advanced technology for CO2 capture. However, the fundamental aspects of the chemical reactions occurring in solution still appear to be unclear. Our previous investigation of aqueous monoethanolamine (MEA) and 2-amino-2-methyl-1,3-propanediol (AMPD), based on ab initio molecular dynamics simulations aided with metadynamics, provided new insights into the reaction mechanisms leading to CO2 capture and release with carbamate formation and dissociation. In particular, the role of water-strongly underestimated in previous computational studies-was established as essential in determining the development of all relevant reactions. In this article, we apply the same simulation protocol to other relevant primary amines, namely, a sterically hindered amine (2-amino-2-methyl-1-propanol (AMP)) and an aromatic amine (benzylamine (BZA)). We also discuss the case of CO2 capture with the formation of bicarbonate. New information is thus obtained that extends our understanding. However, quantitative predictions obtained using molecular simulations suffer from several methodological problems, and comparison among different chemical species is especially demanding. We clarify these problems further with a discussion of previous attempts to explain the different behaviors of AMP and MEA using other types of models and computations.

Machine Learning in the Development of Adsorbents for Clean Energy Application and Greenhouse Gas Capture

Addressing climate change challenges by reducing greenhouse gas levels requires innovative adsorbent materials for clean energy applications. Recent progress in machine learning has stimulated technological breakthroughs in the discovery, design, and deployment of materials with potential for high-performance and low-cost clean energy applications. This review summarizes basic machine learning methods-data collection, featurization, model generation, and model evaluation-and reviews their use in the development of robust adsorbent materials. Key case studies are provided where these methods are used to accelerate adsorbent materials design and discovery, optimize synthesis conditions, and understand complex feature-property relationships. The review provides a concise resource for researchers wishing to use machine learning methods to rapidly develop effective adsorbent materials with a positive impact on the environment.

Molecular design of redox carriers for electrochemical CO2 capture and concentration

Developing improved methods for CO₂ capture and concentration (CCC) is essential to mitigating the impact of our current emissions and can lead to carbon net negative technologies. Electrochemical approaches for CCC can achieve much higher theoretical efficiencies compared to the thermal methods that have been more commonly pursued. The use of redox carriers, or molecular species that can bind and release CO₂ depending on their oxidation state, is an increasingly popular approach as carrier properties can be tailored for different applications. The key requirements for stable and efficient redox carriers are discussed in the context of chemical scaling relationships and operational conditions. Computational and experimental approaches towards developing redox carriers with optimal properties are also described.

Recent advances in CO2 capture and reduction

Given the continuous and excessive CO₂ emission into the atmosphere from anthropomorphic activities, there is now a growing demand for negative carbon emission technologies, which requires efficient capture and conversion of CO₂ to value-added chemicals. This review highlights recent advances in CO₂ capture and conversion chemistry and processes. It first summarizes various adsorbent materials that have been developed for CO₂ capture, including hydroxide-, amine-, and metal organic framework-based adsorbents. It then reviews recent efforts devoted to two types of CO₂ conversion reaction: thermochemical CO₂ hydrogenation and electrochemical CO₂ reduction. While thermal hydrogenation reactions are often accomplished in the presence of H₂, electrochemical reactions are realized by direct use of electricity that can be renewably generated from solar and wind power. The key to the success of these reactions is to develop efficient catalysts and to rationally engineer the catalyst-electrolyte interfaces. The review further covers recent studies in integrating CO₂ capture and conversion processes so that energy efficiency for the overall CO₂ capture and conversion can be optimized. Lastly, the review briefs some new approaches and future directions of coupling direct air capture and CO₂ conversion technologies as solutions to negative carbon emission and energy sustainability.

Emerging Trends in Sustainable CO2 -Management Materials

With the rising level of atmospheric CO₂ worsening climate change, a promising global movement toward carbon neutrality is forming. Sustainable CO₂ management based on carbon capture and utilization (CCU) has garnered considerable interest due to its critical role in resolving emission-control and energy-supply challenges. Here, a comprehensive review is presented that summarizes the state-of-the-art progress in developing promising materials for sustainable CO₂ management in terms of not only capture, catalytic conversion (thermochemistry, electrochemistry, photochemistry, and possible combinations), and direct utilization, but also emerging integrated capture and in situ conversion as well as artificial-intelligence-driven smart material study. In particular, insights that span multiple scopes of material research are offered, ranging from mechanistic comprehension of reactions, rational design and precise manipulation of key materials (e.g., carbon nanomaterials, metal-organic frameworks, covalent organic frameworks, zeolites, ionic liquids), to industrial implementation. This review concludes with a summary and new perspectives, especially from multiple aspects of society, which summarizes major difficulties and future potential for implementing advanced materials and technologies in sustainable CO₂ management. This work may serve as a guideline and road map for developing CCU material systems, benefiting both scientists and engineers working in this growing and potentially game-changing area.

Inverse molecular design of alkoxides and phenoxides for aqueous direct air capture of CO2

Aqueous direct air capture (DAC) is a key technology toward a carbon negative infrastructure. Developing sorbent molecules with water and oxygen tolerance and high CO₂ binding capacity is therefore highly desired. We analyze the CO₂ absorption chemistries on amines, alkoxides, and phenoxides with density functional theory calculations, and perform inverse molecular design of the optimal sorbent. The alkoxides and phenoxides are found to be more suitable for aqueous DAC than amines thanks to their water tolerance (lower pK_a prevents protonation by water) and capture stoichiometry of 1:1 (2:1 for amines). All three molecular systems are found to generally obey the same linear scaling relationship (LSR) between [Formula: see text] and [Formula: see text], since both CO₂ and proton are bonded to the nucleophilic (alkoxy or amine) binding site through a majorly [Formula: see text] bonding orbital. Several high-performance alkoxides are proposed from the computational screening. Phenoxides have comparatively poorer correlation between [Formula: see text] and [Formula: see text], showing promise for optimization. We apply a genetic algorithm to search the chemical space of substituted phenoxides for the optimal sorbent. Several promising off-LSR candidates are discovered. The most promising one features bulky ortho substituents forcing the CO₂ adduct into a perpendicular configuration with respect to the aromatic ring. In this configuration, the phenoxide binds CO₂ and a proton using different molecular orbitals, thereby decoupling the [Formula: see text] and [Formula: see text]. The [Formula: see text] trend and off-LSR behaviors are then confirmed by experiments, validating the inverse molecular design framework. This work not only extensively studies the chemistry of the aqueous DAC, but also presents a transferrable computational workflow for understanding and optimization of other functional molecules.

Why does 2-(2-aminoethylamino)ethanol have superior CO2 separation performance to monoethanolamine? A computational study

Our computational reaction analysis shows that 2-(2-aminoethylamino)ethanol (AEEA) has superior performance to monoethanolamine for CO₂ separation, in terms of its ability to sorb CO₂ by its primary amine and desorb CO₂ by its secondary amine.

CO2 Absorption from Biogas Using Piperazine-Promoted 2-Amino-2-methyl-1-propanol: Process Performance in a Packed Column

In this work, CO2 absorption from simulated biogas is investigated using different blends of a PZ + AMP solution in an absorption system at CO2 partial pressures ranging between 20 and 110 kPa. The collected data were presented as CO2 removal profiles along the packed column and were evaluated in terms of CO2 removal efficiency (%) and average overall volumetric mass transfer coefficient in the gas phase (KGav¯). An increased PZ concentration in the AMP solution was found to significantly increase the CO2 removal efficiency and KGav¯ values. It was observed that, when conducted at different CO2 partial pressures, gas and liquid flow rates, and chemical concentrations, the Lamine/GCO2 ratio strongly influenced the process behaviour in the packed column. Additionally, the optimal inlet liquid temperature was observed to be 35 ± 2 °C in this study.

Porous Liquids: Computational Design for Targeted Gas Adsorption

In this Perspective, we present the unique gas adsorption capabilities of porous liquids (PLs) and the value of complex computational methods in the design of PL compositions. Traditionally, liquids only contain transient pore space between molecules that limit long-term gas capture. However, PLs are stable fluids that that contain permanent porosity due to the combination of a rigid porous host structure and a solvent. PLs exhibit remarkable adsorption and separation properties, including increased solubility and selectivity. The unique gas adsorption properties of PLs are based on their structure, which exhibits multiple gas binding sites in the pore and on the cage surface, varying binding mechanisms including hydrogen-bonding and π-π interactions, and selective diffusion in the solvent. Tunable PL compositions will require fundamental investigations of competitive gas binding mechanisms, thermal effects on binding site stability, and the role of nanoconfinement on gas and solvent diffusion that can be accelerated through molecular modeling. With these new insights PLs promise to be an exceptional material class with tunable properties for targeted gas adsorption.

Advanced Theory and Simulation to Guide the Development of CO2 Capture Solvents

Increasing atmospheric concentrations of greenhouse gases due to industrial activity have led to concerning levels of global warming. Reducing carbon dioxide (CO₂) emissions, one of the main contributors to the greenhouse effect, is key to mitigating further warming and its negative effects on the planet. CO₂ capture solvent systems are currently the only available technology deployable at scales commensurate with industrial processes. Nonetheless, designing these solvents for a given application is a daunting task requiring the optimization of both thermodynamic and transport properties. Here, we discuss the use of atomic scale modeling for computing reaction energetics and transport properties of these chemically complex solvents. Theoretical studies have shown that in many cases, one is dealing with a rich ensemble of chemical species in a coupled equilibrium that is often difficult to characterize and quantify by experiment alone. As a result, solvent design is a balancing act between multiple parameters which have optimal zones of effectiveness depending on the operating conditions of the application. Simulation of reaction mechanisms has shown that CO₂ binding and proton transfer reactions create chemical equilibrium between multiple species and that the agglomeration of resulting ions and zwitterions can have profound effects on bulk solvent properties such as viscosity. This is balanced against the solvent systems needing to perform different functions (e.g., CO₂ uptake and release) depending on the thermodynamic conditions (e.g., temperature and pressure swings). The latter constraint imposes a "Goldilocks" range of effective parameters, such as binding enthalpy and pK _a, which need to be tuned at the molecular level. The resulting picture is that solvent development requires an integrated approach where theory and simulation can provide the necessary ingredients to balance competing factors.

Oxygen Isotope Tracing Study to Directly Reveal the Role of O2 and H2O in the Photocatalytic Oxidation Mechanism of Gaseous Monoaromatics

O₂ and H₂O influence the photocatalytic oxidation mechanism of gaseous monoaromatics, but still in an unclear manner, due to the lack of direct evidence. Tracing an oxygen atom from ¹⁶O₂ and H₂¹⁸O to intermediates can clarify their roles. The low H₂¹⁸O content suppressed the formation of benzenedicarboxaldehydes during the oxidation of xylenes and ¹⁶O₂ greatly affected the yield of total intermediates, while neither of them altered the percentage order of the products. Methylbenzaldehydes, methylbenzyl alcohols, and benzenedicarboxaldehydes possessed greater ¹⁶O percentage (≥69.49%), while higher ¹⁸O distribution was observed in methylbenzoic acids and phthalide (≥59.51%). Together with the interconversion results of the products revealed, ¹⁶O₂ determined the transformation of xylenes initially to methylbenzaldehydes and then to methylbenzyl alcohols or benzenedicarboxaldehydes, while H₂¹⁸O mainly contributed to conversion of methylbenzaldehydes to methylbenzoic acids or phthalide. Further interaction sites of xylene and its products with H₂O and O₂ were confirmed by molecular dynamics calculations. The same roles of ¹⁶O₂ and H₂¹⁸O in the degradation of toluene, ethylbenzene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene were also verified. This is the first report that provides direct evidence for the roles of O₂ and H₂O in the photocatalytic oxidation mechanism of gaseous monoaromatics. These findings are helpful to achieve controllable product formation from the oxidation of monoaromatics and predict their migration process in the atmospheric environment.

Perspective and challenges in electrochemical approaches for reactive CO2 separations

The desire toward decarbonization and renewable energy has sparked research interests in reactive CO2 separations, such as direct air capture that utilize electricity as opposed to conventional thermal and pressure swing processes, which are energy-intensive, cost-prohibitive, and fossil-fuel dependent. Although the electrochemical approaches in CO2 capture that support negative emissions technologies are promising in terms of modularity, smaller footprint, mild reaction conditions, and possibility to integrate into conversion processes, their practice depends on the wider availability of renewable electricity. This perspective discusses key advances made in electrolytes and electrodes with redox-active moieties that reversibly capture CO2 or facilitate its transport from a CO2-rich side to a CO2-lean side within the last decade. In support of the discovery of new heterogeneous electrode materials and electrolytes with redox carriers, the role of computational chemistry is also discussed.

The Kinetics Investigation of CO2 Absorption into TEA and DEEA Amine Solutions Containing Carbonic Anhydrase

Tertiary amines have been used as alternative absorbents for traditional primary and secondary amines in the process of carbon capture. However, the carbon dioxide (CO2) absorption rates in these kinds of amine are relatively slow, which implies greater investment and construction costs and limits the large-scale application of carbon capture. Carbonic anhydrase (CA) is considered to be an ideal homogeneous catalyst for accelerating the rate of CO2 into aqueous amine solution. In this work, CO2 absorption combining CA with two single aqueous tertiary amines, namely triethanolamine (TEA) and 2-(diethylamino)ethanol (DEEA), was studied by use of the stopped-flow apparatus over temperature ranging from 293 to 313 K. The concentrations of selected aqueous amine solution and CA used in the experiment were ranging among 0.1–0.5 kmol/m3 and 0–50 g/m3 , respectively. Compared to the solution without the addition of CA, the pseudo first-order reaction rate in the presence of CA (k0,withCA) is significantly increased. The values of k0,withCA have been calculated by a new kinetics model. The results of experimental and calculated k0,amine and k0,withCA in CO2-amine-H2O solutions were also investigated,respectively.

Computational Investigation of the Thermochemistry of the CO2 Capture Reaction by Ethylamine, Propylamine, and Butylamine in Aqueous Solution Considering the Full Conformational Space via Boltzmann Statistics

Rubisco is the enzyme responsible for CO₂ fixation in nature, and it is activated by CO₂ addition to the amine group of its lysine 201 side chain. We are designing rubisco-based biomimetic systems for reversible CO₂ capture from ambient air. The oligopeptide biomimetic capture systems are employed in aqueous solution. To provide a solid foundation for the experimental solution-phase studies of the CO₂ capture reaction, we report here the results of computational studies of the thermodynamics of CO₂ capture by small alkylamines in aqueous solution. We studied CO₂ addition to methyl-, ethyl-, propyl-, and butylamine with the consideration of the full conformational space for the amine and the corresponding carbamic acids and with the application of an accurate solvation model for the potential energy surface analyses. The reaction energies of the carbamylation reactions were determined based on just the most stable structures (MSS) and based on the ensemble energies computed with the Boltzmann distribution (BD), and it is found that ΔG_BD ≈ ΔG_MSS. The effect of the proper accounting for the molecular translational entropies in solution with the Wertz approach are much more significant, and the free energy of the capture reactions Δ^WG_BD is more negative by 2.9 kcal/mol. Further accounting for volume effects in solution results in our best estimates for the reaction energies of the carbamylation reactions of Δ^WA_BD = -5.4 kcal/mol. The overall difference is ΔG_BD - Δ^WA_BD = 2.4 kcal/mol for butylamine carbamylation. The full conformational space analyses inform about the conformational isomerizations of carbamic acids, and we determined the relevant rotational profiles and their transition-state structures. Our detailed studies emphasize that, more generally, solution-phase reaction energies should be evaluated with the Helmholtz free energy and can be affected substantially by solution effects on translational entropies.

Stability of a Monoethanolamine-CO2 Zwitterion at the Vapor/Liquid Water Interface: Implications for Low Partial Pressure Carbon Capture Technologies

The need to chemically convert CO₂ at the interface of aqueous amine solutions has become particularly relevant for the development and the broad distribution of cost-effective and near-future devices for direct air capture working at low (e.g., ambient) partial pressure. Here, we have determined the stability of a CO₂-monoethanolamine zwitterion and its chemical conversion into carbamate at the vapor/liquid water interface by first-principles molecular dynamics simulations coupled with a recently introduced enhanced sampling technique. Contrary to the bulk water case, our results show that both the zwitterion and carbamate ions are poorly stable at the vapor/amine aqueous interface, further stating the differences between the homogeneous and heterogeneous CO₂ chemical conversion. The design of novel and cost-effective capture systems, such as those offered by amine-based scrubbing solutions, working at low (e.g., ambient) CO₂ partial pressure should explore the use of novel solvents, different from aqueous mixtures, to overcome the limits of the current absorbents.

Quantitative Kinetic Model of CO2 Absorption in Aqueous Tertiary Amine Solvents

Aqueous tertiary amine solutions are increasingly used in industrial CO₂ capture operations because they are more energy-efficient than primary or secondary amines and demonstrate higher CO₂ absorption capacity. Yet, tertiary amine solutions have a significant drawback in that they tend to have lower CO₂ absorption rates. To identify tertiary amines that absorb CO₂ faster, it would be efficacious to have a quantitative and predictive model of the rate-controlling processes. Despite numerous attempts to date, this goal has been elusive. The present computational approach achieves this goal by focusing on the reaction of CO₂ with OH^- forming HCO₃^-. The performance of the resulting model is demonstrated for a consistent experimental data set of the absorption rates of CO₂ for 24 different aqueous tertiary amine solvents. The key to the new model's success is the manner in which the free energy barrier for the reaction of CO₂ with OH^- is evaluated from the differences among the solvation free energies of CO₂, OH^-, and HCO₃^-, while the pK_a of the amines controls the concentration of OH^-. These solvation energies are obtained from molecular dynamics simulations. The experimental value of the free energy of reaction of CO₂ with pure water is combined with information about measured rates of absorption of CO₂ in an aqueous amine solvent in order to calibrate the absorption rate model. This model achieves a relative accuracy better than 0.1 kJ mol^-1 for the free energies of activation for CO₂ absorption in aqueous amine solutions and 0.07 g L^-1 min^-1 for the absorption rate of CO₂. Such high accuracies are necessary to predict the correct experimental ranking of CO₂ absorption rates, thus providing a quantitative approach of practical interest.

Distribution and Transport of CO2 in Hydrated Hyperbranched Poly(ethylenimine) Membranes: A Molecular Dynamics Simulation Approach

Hyperbranched poly(ethylenimine) (HB-PEI) has been distinguished as a promising candidate for carbon dioxide (CO2) capture. In this study, we investigate the distribution and transport of CO2 molecules in a HB-PEI membrane at various hydration levels using molecular dynamics (MD) simulations. For this, model structures consisting of amorphous HB-PEI membranes with CO2 molecules are equilibrated at various hydration levels. Under dry conditions, the primary and secondary amines are highly associated with CO2, indicating that they would participate in CO2 capture via the carbamate formation mechanism. Under hydrated conditions, the pair correlations of CO2 with the primary and secondary amines are reduced. This result suggests that the carbamate formation mechanism is less prevalent compared to dry conditions, which is also supported by CO2 residence time analysis. However, in the presence of water molecules, it is found that the CO2 molecules can be associated with both amine groups and water molecules, which would enable the tertiary amine as well as the primary and secondary amines to capture CO2 molecules via the bicarbonate formation mechanism. Through our MD simulation results, the feasibilities of different CO2 capture pathways in HB-PEI membranes are demonstrated at the molecular level.

Utility of Core-Shell Nanomaterials in the Catalytic Transformations of Renewable Substrates

In recent years, core-shell nano-catalysts have received increasing attention due to their tunable properties and broad applications in catalysis. Control of the two components of these materials allows their catalytic properties to be tuned to various sustainable processes in synthetic and energy-related applications. This Concept article describes recent state-of-the-art core-shell materials and their application as heterogeneous catalysts for a range of sustainable catalytic transformations, focusing on two important classes of renewable substrates, CO₂ and biomass. In the discussion, emphasis is directed to the role of the constituent parts of the core-shell structure and how they can be manipulated to enhance activity.

Tuning CO2 Capture at the Gas/Amine Solution Interface by Changing the Solvent Polarity

Carbon dioxide scrubbing by aqueous amine solution is considered as a promising technology for post-combustion CO₂ capture, while mitigating climate change. The lack of physicochemical details for this process, especially at the interface between the gas and the condensed phase, limits our capability in designing novel and more cost-effective scrubbing systems. Here, we present classical and first-principles molecular dynamics results on CO₂ capture at the gas/amine solution interfaces using solvents of different polarities. Even if it is apolar, carbon dioxide is absorbed at the gas/monoethanolamine (MEA) aqueous solution interface, forming stable and interfacial [CO₂·MEA] complexes, which are the first reaction intermediate toward the chemical conversion of CO₂ to carbamate ions. We report that the stability of the interfacial [CO₂·MEA] precomplex depends on the nature and polarity of the solution, as well as on the conformer population of MEA. By changing the polarity of the solvent, using chloroform, we observed a shift in the interfacial MEA population toward conformers that form more stable [CO₂·MEA] complexes and, at the same time, a further stabilization of the complex induced by the solvent environment. Thus, while lowering the polarity of the solvent could decrease the solubility of MEA, at the same time, it favors conformers that are more prone to CO₂ capture and mineralization. The results presented here offer a theoretical framework that helps in designing novel and more cost-effective solvents for CO₂ scrubbing systems, while shedding further light on the intrinsic reaction mechanisms of interfacial environments in general.

Role of hydrogen bond capacity of solvents in reactions of amines with CO2: A computational study

Various computational methods were employed to investigate the zwitterion formation, a critical step for the reaction of monoethanolamine with CO₂, in five solvents (water, monoethanolamine, propylamine, methanol and chloroform) to probe the effect of hydrogen bond capacity of solvents on the reaction of amine with CO₂ occurring in the amine-based CO₂ capture process. The results indicate that the zwitterion can be formed in all considered solvents except chloroform. For two pairs of solvents (methanol and monoethanolamine, propylamine and chloroform) with similar dielectric constant but different hydrogen bond capacity, the solvents with higher hydrogen bond capacity (monoethanolamine and propylamine) facilitate the zwitterion formation. More importantly, kinetics parameters such as activation free energy for the zwitterion formation are more relevant to the hydrogen bond capacity than to dielectric constant of the considered solvents, clarifying the hydrogen bond capacity could be more important than dielectric constant in determining the kinetics of monoethanolamine with CO₂.

Modeling and Simulation of the Simultaneous Absorption/Stripping of CO2 with Potassium Glycinate Solution in Membrane Contactor

Global warming is an environmental problem caused mainly by one of the most serious greenhouse gas, CO₂ emissions. Subsequently, the capture of CO₂ from flue gas and natural gas is essential. Aqueous potassium glycinate (PG) is a promising novelty solvent used in the CO₂ capture compared to traditional solvents; simultaneous solvent regeneration is associated with the absorption step. In present work, a 2D mathematical model where radial and axial diffusion are considered is developed for the simultaneous absorption/stripping process. The model describes the CO₂/PG absorption/stripping process in a solvent–gas membrane absorption process. Regeneration data of rich potassium glycinate solvent using a varied range of acid gas loading (mol CO₂ per mol PG) were used to predict the reversible reaction rate constant. A comparison of simulation results and experimental data validated the accuracy of the model predictions. The stripping reaction rate constant of rich potassium glycinate was determined experimentally and found to be a function of temperature and PG concentration. Model predictions were in good agreement with the experimental data. The results reveal that the percent removal of CO₂ is directly proportional to CO₂ loading and solvent stripping temperature.

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.