Mechanism Of CO2 Capture Research Articles

Mechanism of CO2 capture in nanostructured sodium amide encapsulated in porous silica

Nanostructured sodium amide encapsulated in a porous silica gel matrix (“NaNH2-SG”) was investigated for CO2 capture and storage by in-situ gravimetric gas sorption. Exposure of NaNH2-SG to CO2 at 25 °C and 1 bar pressure resulted in ~3.6 wt% CO2 uptake over eight sorption/desorption cycles. Over 90% of the CO2 uptake was non-reversible due to reaction between CO2 and NaNH2 to form sodium carbamate, as confirmed by 13C and 23Na solid-state NMR. Electronic structure calculations suggest a two-stage reaction process involving initial formation and subsequent rearrangement of the carbamate product. This research confirms the feasibility of sequential reactions of nanoparticles in a porous substrate (Na-SG to NaNH2-SG to Na-carbamate-SG), and of CO2 capture by NaNH2-SG nanoparticles stabilised by encapsulation within the porous substrate. This encapsulation method could allow further hygroscopic or reactive starting reagents or compounds to be explored for CO2 capture and long-term storage.

CO2 capture performance of calcium-based synthetic sorbent with hollow core-shell structure under calcium looping conditions

A novel calcium-based synthetic CO2 sorbent with hollow core-shell structure was prepared by a carbon microsphere template route where carbide slag, alumina cement and glucose were employed as the low-cost calcium precursor, support and carbon source, respectively. The effects of the alumina cement addition, the pre-calcination temperature during the preparation process, the carbon template addition and calcination conditions on CO2 capture performances of the calcium-based synthetic sorbents were studied during calcium looping cycles. The synthetic sorbent containing 5 wt.% alumina cement possesses the highest CO2 capture capacity during calcium looping cycles, which is mainly composed of CaO and Ca12Al14O33. The CO2 capture capacities of the synthetic sorbent under mild and severe calcination conditions can retain 0.37 and 0.29 g/g after 20 cycles, which are 57% and 99% higher than those of carbide slag under the same conditions, respectively. The synthetic sorbent possesses a hollow micro-sphere morphology with a nano-structured shell and meso-porous structure, which decreases the diffusion resistance of CO2. Periodic density functional theory (DFT) calculations are used to explain why Ca12Al14O33 can effectively retard both agglomeration and sintering of the synthetic sorbent. The hollow core-shell model is proposed to explain the CO2 capture mechanism of the synthetic sorbent. For the same CO2 capture efficiency, the energy consumption in the calciner using the synthetic sorbent is much lower than those using carbide slag and natural limestone. This work designs a good method to prepare the hollow sphere-structured synthetic sorbents with high CO2 capture capacity and provides a promising way to integrate efficient CO2 capture with the utilization of industrial waste.

A novel biphasic solvent of amino-functionalized ionic liquid for CO2 capture: High efficiency and regenerability

Due to its great potential of energy consumption saving, the biphasic solvent has attracted more and more attention. In the present work, an efficient absorbent of amino-functionalized ionic liquids (AFILs) was dissolved in ethanol-water solvent to form a novel biphasic solvent for CO2 capture. When the concentration of triethylenetetramine l-lysine ([TETAH][Lys]) was 0.5 mol/L and the volume ratio of ethanol-water solvent was 4:6, the [TETAH][Lys] solution separated into two phases after CO2 absorption. Under the optimal conditions, the volume of the CO2-rich phase was about one third of the total volume of the mixture, which meant that only one third of the solution needed to be regenerated and the energy consumption could be greatly reduced. Meanwhile, the loading of the CO2-rich phase contributed about 93% of the total loading of the solution, and its viscosity was only 27.95 mPa s. The regeneration efficiency of the solvent after the 5th absorption-desorption cycle still kept at 91.3%. Based on the experimental and 13C NMR analysis results, the reaction and phase change mechanisms of CO2 capture into the [TETAH][Lys]-ethanol-water solution were clarified. CO2 absorption into [TETAH][Lys] firstly formed carbamate, which would further hydrolyze to HCO3−/CO32− and react with ethanol to form the ethyl carbonate. Due to the limited solubility of these absorption products in ethanol, as well as the good solubility in H2O, the products transferred to the lower phase while the ethanol stayed in the upper phase in the biphasic system.

Oxazolidine Transient Bases as Molecular Platforms for Testing Dynamic CO2 Capture in Biochemical Systems.

Understanding the dynamic processes of CO2 capture in biosystems is important because of the great effect CO2 has on the carbon cycle, human health, the global climate, and living environments. After years of multidisciplinary studies, researchers have gained only basic mechanistic knowledge about how enzymes or protein-aggregates capture and deliver CO2, a process involving reversible bonding of CO2 with basic amino acid residues. However, vital mechanistic details of how the activated basic residues within these enzymes or protein-aggregates are initially formed, a crucial step for CO2 capture, are still lacking. Herein, we designed specific molecules, i.e., oxazolidines, which are able to reversibly change their alkalinity via ultrafast isomerizations. Serving as so-called transient bases, these oxazolidines mimic the activated/deactivated states of enzymes or protein-aggregates responsible for dynamic CO2 capture/release. A detailed mechanism for CO2 capture, which involves dynamic covalent bonding and multimolecular cooperative interactions among functional groups that occur with the help of a polyhydroxyl environment, is demonstrated by UV−vis and multiple NMR spectroscopies as well as theoretical calculations. Using suitable oxazolidine transient bases, applications for visual CO2 detection under different detection limit requirements were also developed. Insights for further understanding the process of dynamic CO2 capture in biosystems are also discussed. This oxazolidine-inspired biomimetic CO2 capture serves as a platform for the future development of additional biomimicking systems, as well as offers unique perspectives for other complicated life processes.

A catalytic role of surface silanol groups in CO2 capture on the amine-anchored silica support.

A new mechanism of CO2 capture on the amine-functionalized silica support is demonstrated using density functional theory calculations, in which the silica surface not only acts as a support to anchor amines, but also can actively participate in the CO2 capture process through a facile proton transfer reaction with the amine groups. The surface-mediated proton transfer mechanism in forming a carbamate-ammonium product has lower kinetic barrier (8.1 kcal mol-1) than the generally accepted intermolecular mechanism (12.7 kcal mol-1) under dry conditions, and comparable to that of the water-assisted intermolecular mechanism (6.0 kcal mol-1) under humid conditions. These findings suggest that the CO2 adsorption on the amine-anchored silica surface would mostly occur via the rate-determining proton transfer step that is catalyzed by the surface silanol groups.

Humidity-swing mechanism for CO2 capture from ambient air.

A humidity-swing polymeric sorbent captures CO2 from ambient air at room temperature simply by changing the humidity level. To date there has been no direct experimental evidence to characterize the chemical mechanism for this process. In this report we describe the use of solid-state NMR to study the humidity-swing CO2 absorption/desorption cycle directly. We find that at low humidity levels CO2 is absorbed as HCO3-. At high humidity levels, HCO3- is replaced by hydrated OH- and the absorbed CO2 is released.

Controlled synthesis of MgO with diverse basic sites and its CO2 capture mechanism under different adsorption conditions

Mesoporous MgO adsorbents with diverse basic sites were prepared via a urea hydrolysis synthesis method for CO2 capture. With elevated hydrolysis temperatures, the phase transition process made great contributions to the morphological changes of the precursor architectures. In situ DRIFTS analysis demonstrated that various carbonate surface species including bicarbonate, bidentate, and unidentate carbonates were formed on the obtained MgO during interaction with CO2. Furthermore, the main component of adsorbed CO2 surface species swings from bicarbonate to bidentate and unidentate carbonates with increase of adsorption temperature. The highest CO2 uptake of 1.22–1.99 mmol g−1 was attained for MgO sample at the lowest calcination temperature in a wide temperature range of 60–300 °C. High specific surface area (372.0 m2 g−1), large pore volume (0.38 cm3 g−1) as well as diverse basic sites of the synthesized MgO make it an eligible candidate for CO2 capture, with a nearly 20-fold enhancement of the commercialized light MgO. Additionally, the results of CO2 uptake studied under diluted and wet (H2O containing) CO2 conditions for the as-prepared MgO adsorbent also suggested good prospect in practical applications.

Assessment of the Use of Switchable Polarity Solvents for Delivery of CO2 for Electrochemical Reduction

Biomass conversion is a rising alternative to substitute fuels, chemicals and other products currently obtained from fossil sources. However, biomass conversion technologies such as pyrolysis suffer from low carbon-to-fuel efficiency (38-47%), with the remainder of the carbon ultimately ending up forming CO2 and resulting in biogenic green-house gas (GHG) emissions [1]. An alternative to decrease the CO2 emission from biomass conversion, and contribute to the sustainability of the biomass processing, is the capture and conversion of CO2 to beneficial products such as chemicals and fuels via renewable electricity. Aqueous electrochemical reduction of CO2 has been of growing interest as a solution for GHG mitigation and due to the high diversity of products that can be selectively obtained depending on the electrocatalyst used (e.g. alcohols, hydrocarbons, CO etc.) [2]. However, three main technical problems have been identified for the process feasibility: 1) low solubility of CO2 in aqueous media hinders mass transport, 2) high reaction overpotentials, and 3) obtaining a quality concentrated CO2 feedstock [3,4] In this work, switchable polarity solvents (SPS) are used to capture and increase the CO2 concentration in aqueous media. SPS are emerging as an extremely versatile class of materials which shift polarity and solubility upon being exposed to a chemical agent, namely CO2 with water [5]. Insoluble tertiary amines react with CO2 to form soluble ions [HNR3 +] and bicarbonate [6]. Hence, an ion exchange membrane assembly can be used to generate a localized acidic environment on the surface of the cathode electro-catalyst releasing CO2 for reduction. Results of electrochemical analysis performed on metal supported catalysts will be presented References [1] A.P. Borole, Sustainable and efficient pathways for bioenergy recovery from low-value process streams via bioelectrochemical systems in biorefineries, Sustainability, 7 (2015) 11713-11726. [2] M. Gattrell, N. Gupta, A. Co, A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper, Journal of Electroanalytical Chemistry, 594 (2006) 1-19. [3] S. Ma, P.J. Kenis, Electrochemical conversion of CO2 to useful chemicals: current status, remaining challenges, and future opportunities, Current Opinion in Chemical Engineering, 2 (2013) 191-199. [4] J. Durst, A. Rudnev, A. Dutta, Y. Fu, J. Herranz, V. Kaliginedi, A. Kuzume, A.A. Permyakova, Y. Paratcha, P. Broekmann, T.J. Schmidt, Electrochemical CO2 Reduction – A Critical View on Fundamentals, Materials and Applications, CHIMIA International Journal for Chemistry, 69 (2015) 769-776. [5] P.G. Jessop, S.M. Mercer, D.J. Heldebrant, CO2-triggered switchable solvents, surfactants, and other materials, Energy & Environmental Science, 5 (2012) 7240-7253. [6] B. Lv, B. Guo, Z. Zhou, G. Jing, Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading during the Absorption/Desorption Processes, Environ. Sci. Technol., 49 (2015) 10728-10735.

Determination and analysis of CO2 capture kinetics and mechanisms on the novel graphene-based adsorbents

Some graphene-based adsorbents such as mesoporous graphene oxide/TiO2 nanocomposite, thermally treated graphene nanosheets, holey graphene frameworks and 3-D graphene-based porous adsorbent have recently been introduced as promising adsorbents for post-combustion CO2 capture while, their CO2 adsorption kinetics and mechanisms have not sufficiently been reported by the scholars. To begin, in this study, CO2 capture kinetics of these adsorbents are investigated by different adsorption kinetic models including pseudo-first order model, pseudo-second order model, Avrami model, and the fractal-like exponential kinetic model. Subsequently, the Eyring and Arrhenius equations are used to determine the activation parameters related to the CO2 adsorption on these adsorbents. Furthermore, a mechanism is proposed to describe the negative activation energy assessed for CO2 adsorption on the studied adsorbents. Eventually, the rate-limiting kinetic models including interparticle diffusion model, intraparticle diffusion model and Boyd’s film diffusion model are applied to determine CO2 capture mechanisms on the mentioned graphene-based adsorbents.

Exploring the General Characteristics of Amino-Acid-Functionalized Ionic Liquids through Experimental and Quantum Chemical Calculations

Amino-acid-functionalized ionic liquids (AAILs) show significant potential for energy-efficient post-combustion CO2 capture. In this work, the interaction energy calculation was performed with the Gaussian 09 package, and the activation barriers and energy change calculation were performed with the Material Studio 7.0 package. The experimental and quantum chemical calculation results showed that both the CO2 absorption capacity and the viscosity of AAILs were negatively correlated to their interaction energy. The results of 13C nuclear magnetic resonance analysis and activation barriers indicated that, under the same conditions, the effect of the AAIL cation on the regeneration ability was more significant than that of the anion. In our previous work, CO2 absorption into AAIL solution was proven to be divided into two periods. In the first period, carbamate was produced after the CO2 absorption reaction, and in the second period, the production was carbonate. Research on thermodynamic properties in this work made the mechanism for CO2 capture into AAILs more clear. The activation barrier, energy change, and enthalpy change during the two periods revealed that the AAIL–CO2 absorption reaction should be a neutralizing and exothermic reaction in the first period and a hydrolysis and endothermic reaction in the second period.

Simulation and parametric analysis of CO2 capture from natural gas using diglycolamine

Conventional natural gas sweetening processes are mainly focused on the H2S removal with the bulk removal of CO2. However, the placement of natural gas processes within the carbon capture and storage (CCS) framework provides an opportunity to achieve the synergies between the two technologies. The purpose of this paper is to investigate the CO2 capture process from the natural gas processing plants. In this study, a steady-state model has been developed that can reasonably predict the CO2 capture mechanism from the natural gas using diglycolamine (DGA) solvent. The developed model is first validated against a commercial operating plant using the required data and operating conditions. The results show that the developed model is in good agreement with the operational data. The model is then tested for all the possible key parameters that can affect the performance of the process. Since, achieving the product specification with minimum possible energy requirement is the goal of any process plant, hence, the effect of operational parameters is investigated in terms of CO2 content in the sweet gas and reboiler duty.

Experimental study on CO2 capture mechanisms using Na2ZrO3 sorbents synthesized by soft chemistry method

The CO2 capture performances were demonstrated for Na2ZrO3 sorbents prepared by soft chemistry using varied sodium precursors and drying methods. It was found that ‘Na2C2O4 precursor+heated drying’ enabled superior sorption capacity (e.g. 22.77wt% CO2 uptake within 10min at 800°C in a 15vol% CO2 stream balanced by N2) to the other precursor-drying combinations. The as-identified strongest sorbent was further investigated at a series of carbonation temperatures (400–800°C) in N2-balanced purging streams with 15vol% and 50vol% CO2, corresponding post- and pre-combustion CO2 capture scenarios, respectively. The analysis of the resulting sorption capacity, stability and kinetics suggests that sodium diffusion is a more efficient driver than CO2 diffusion for the capture of CO2. Sintering and the subsequent densification inhibits CO2 diffusion but enhances sodium diffusion, which leads to a positive overall effect on the sorption performance. Hence, unlike other high-temperature solid sorbents such as CaO, sintering resistance during cyclic sorption is not a necessity for Na2ZrO3. In addition, higher CO2 fractions in the feed gas improve the sorption kinetics and stability by promoting sodium diffusion as well as CO2 diffusion.

Enhanced CO2 capture on graphene via N, S dual-doping

N, S doped graphene-based materials have been recently recognized as promising adsorbents for CO2 capture, but understanding of the adsorption mechanism at the atomic level is still limited. Herein, the local structures and promotion mechanism of CO2 capture by N, S doped graphene were investigated by combining density functional theory and ab initio thermodynamics. A single vacancy defected graphene involving N, S dual-doping was found to be a superior adsorbent for CO2 capture under mild conditions (<100°C, 1atm). The enhanced CO2 adsorption performance should be ascribed to a charge delivery channel along the S→N→CO2 path, leading to extra charge transfer from graphene to CO2. It is worth mentioning that the extra charge transfer was stimulated by the unique sp2 hybridization of pyridine N and further enhanced by S in N, S dual-doped graphene. A possible mechanism has been proposed to explain the high adsorption performance of CO2 by N, S dual-doped graphene, which offers insights for the design of new graphene-based adsorbents.

Simultaneous measurement of CO2 sorption and swelling of phosphate-based ionic liquid

The development of alternative CO2 capture solvents such as ionic liquids (ILs) and nanoparticle organic hybrid materials (NOHMs) have provided interesting options for CO2 capture. In this study, CO2 interactions with 1,3-dimethylimidazolium dimethylphosphate ([MMIM]DMP), 1-ethyl-3-methylimidazolium dimethylphosphate ([EMIM]DMP) and 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM]DEP) that contain inorganic ester groups based on phosphate, were investigated using ATR FT-IR spectroscopy. CO2-induced swelling, CO2 diffusivity and CO2 capture capacity were simultaneously measured to identify CO2 capture mechanisms, kinetics and diffusion behaviors as a function of the alkyl chain length of the cation. Henry's law constants of CO2 were found to be in the range of 4–11 MPa, which is in agreement with those reported in other studies.

Mechanism of CO2 Capture in 1-Alkyl-3-Methylimidazolium Acetate: Is N-Heterocyclic Carbene Involved?

Ionic liquids (ILs) offer a promising alternative absorbent for CO2 capture because of their unique properties, such as negligible vapor pressures, high thermal stabilities, excellent CO2 solubilities and tunability. More importantly, ILs consisting of 1-alkyl-3-alkyl’-imidazolium cations paired with acetate anions can react with CO2 and form carboxylate compounds. N-heterocyclic carbene that can be formed via proton transfer from an imidazolium cation to an acetate anion is often invoked in the mechanistic explanation of these CO2–IL reactions. However, the detection of the carbene species has been elusive experimentally. In this talk, we present our results for classical and ab initio molecular dynamics simulations as well as for density functional theory calculations to gain molecular insight into CO2 capture by 1-ethyl-3-methylimidazolium acetate. Our analysis indicates that N-heterocyclic carbene is not formed and thus not involved in CO2 capture at room temperature. Based on our computational results, a novel concerted mechanism is proposed.

Generalized two-dimensional correlation infrared spectroscopy to reveal mechanisms of CO2 capture in nitrogen enriched biochar

In order to reveal mechanisms of CO2 adsorption–desorption on the nitrogen enriched biochar, the in situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS) was used to monitor theinterfacialreaction between CO2 and nitrogen enriched biochar, and all spectral data sets of in situ DRIFTS was parsed by two-dimensional (2D) perturbation correlation. The results show that when the adsorption temperature is between 30 and 60°C, the hydroxyl, primary amide, amines, azo compound NN, secondary amide groups and aliphatic CN/CO are all effective active sites to adsorb CO2; while the temperature rises to 120°C, the CO2 adsorption capacity of secondary amide group shows a downward trend. After the adsorption of CO2, these active sites are mainly converted to CO groups (such as NCOOH, NCOO− and aldehyde groups), nitrogen atom-containing heterocyclic groups (pyridine-like groups), and NO groups (nitrate). Moreover, at 200°C, the majority of hydroxyl and secondary amide groups as well as partof primary amide, amines and azo compound NN can be regenerated, while the aliphatic CN/CO groups are difficult to be regenerated.

New Insights into CO2 Absorption Mechanisms with Amino‐Acid Ionic Liquids

AbstractInvited for this month′s cover is the group of Huabin Xing at Zhejiang University. The image depicts the dominance of a 1:1 reaction mechanism for CO2 capture using a proline‐based ionic liquid. The Full Paper itself is available at 10.1002/cssc.201501691.

Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading during the Absorption/Desorption Processes.

Though the mechanism of MEA-CO2 system has been widely studied, there is few literature on the detailed mechanism of CO2 capture into MEA solution with different CO2 loading during absorption/desorption processes. To get a clear picture of the process mechanism, (13)C nuclear magnetic resonance (NMR) was used to analyze the reaction intermediates under different CO2 loadings and detailed mechanism on CO2 absorption and desorption in MEA was evaluated in this work. The results demonstrated that the CO2 absorption in MEA started with the formation of carbamate according to the zwitterion mechanism, followed by the hydration of CO2 to form HCO3(-)/CO3(2-), and accompanied by the hydrolysis of carbamate. It is interesting to find that the existence of carbamate will be influenced by CO2 loading and that it is rather unstable at high CO2 loading. At low CO2 loading, carbamate is formed fast by the reaction between CO2 and MEA. At high CO2 loading, it is formed by the reaction of CO3(-)/CO3(2-) with MEA, and the formed carbamate can be easily hydrolyzed by H(+). Moreover, CO2 desorption from the CO2-saturated MEA solution was proved to be a reverse process of absorption. Initially, some HCO3(-) were heated to release CO2 and other HCO3(-) were reacted with carbamic acid (MEAH(+)) to form carbamate, and the carbamate was then decomposed to MEA and CO2.

Absorption/desorption mechanism of carbon dioxide capture into amino acid ionic liquid aqueous solution

Amino acid ionic liquids (AAILs) are considered to be an efficient absorbent for CO2 capture with low viscosity and high absorption capacity in the aqueous solution. However, there is no literature reported regarding to the mechanism of CO2 capture into AAILs solution. To explain the reaction process, some researchers just referenced the zwitterion mechanism, which was proposed to depict the reaction among CO2 and alkanolamines. To disclose the reaction mechanism, the CO2 absorption/desorption process of tetramethylammonium glycinate ([N1111][Gly]) aqueous solution was investigated by using 13C Nuclear Magnetic Resonance. The results showed that the absorption/ desorption process of CO2 into [N1111][Gly] aqueous solution were very complicated. In the solution, [N1111][Gly] was completely dissociated to [N1111]+ and [Gly]-, and CO2 was initially captured in a carbamate form by [Gly]-. As CO2 continuously flowed into the reactor, the pH value of the solution decreased and the carbamate was hydrolyzed to glycine and HCO3-. At this stage, CO2 was mainly captured by hydration reaction. Finally, CO2 remaining in the solution shifted to bicarbonate and trace amount of carbamate. The regeneration of CO2-saturated [N1111][Gly] aqueous solution was the reverse process of absorption. Initially, the regeneration process was the thermolysis of HCO3- to release CO2, and part of HCO3- reacted with glycine to form carbamate, then the carbamate decomposed to [Gly]- and CO2.

The chemistry of resorcinol carboxylation and its possible application to the CO2 removal from exhaust gases

The carbon dioxide uptake by resorcinol solutions has been investigated under different operational procedures and experimental conditions. Batch experiments have been carried out in aqueous and glycerol–water solutions of KOH, KHCO3 or K2CO3, and the yield of resorcinol carbonatation has been investigated as a function of the CO2 partial pressure as well as reaction temperature and time. The β-resorcylic acid (2,4-dihydroxy benzoic acid) has been isolated in the solid state and identified on the basis of its IR spectrum. The 13C NMR analysis has been applied to identify and quantify the carbonated species in solution upon CO2 uptake and after thermal resorcinol regeneration. The maximum yield of resorcinol conversion into β-resorcylic acid was 60% with 1.0bar of CO2 and 120min reaction time at 110°C. A 34% yield of resorcinol conversion was also obtained with the resorcinol/K2CO3 solution in the absence of CO2. A relationship between the CO2 absorption capacity and the possible mechanism of CO2 capture has been proposed. The CO2 absorption efficiency from a simulated flue gases (15% CO2 in air) has been measured in continuous cycles of CO2 absorption–desorption carried out in packed columns and with a glycerol–water solution of resorcinol/KOH/KHCO3. The maximum absorption efficiency was 82% at absorption-desorption temperatures of 70°C and, respectively, 170°C. Oxidative conditions and the presence of H2S did not affect the yield of β-resorcylic acid and the resorcinol capacity of CO2 capture. Finally, a simple method for the selective separation of H2S from CO2 has been reported.