Understanding CO2 capture kinetics and energetics by ionic liquids with molecular dynamics simulation

Room temperature ionic liquids (ILs) are recognized to be potential media for CO₂ capture, but the interaction nature is less documented which hinders the future development of ILs with high CO₂ solvation capability. Here, through all atom molecular dynamics (MD) simulations, the solvation process of CO₂ with four representative ILs, [EMIM][BF₄], [BMIM][BF₄], [EMIM]CL and [BMIM]CL was systematically studied. Our data clearly reflect the fact that hydrophobic components from both cations and anions dominate CO₂ solvation because IL–CO₂ attraction is mainly driven by the van der Waals attractions. Consequently, cations with longer alkyl chain (for instance, [BMIM]⁺ than [EMIM]⁺) and anions with higher hydrophobicity (for instance, [BF₄]⁻ than CL⁻) effectively enhance CO₂ solvation. For all the ILs under study, addition of water into ILs abates CO₂ solvation through regulating the anion–CO₂ interactions. Moreover, it is worth mentioning that ILs with a hydrophobic anion ([BF₄]⁻ in this study) are more resistant to the existence of water to capture CO₂ than ILs with a hydrophilic anion (Cl⁻ in this study). Free energy decomposition analyses were conducted which support the above findings consistently. Generally, it is predicted that cations with long alkyl chain, anions with high hydrophobicity, and ILs with smaller surface tension are potentially effective CO₂ capturing media. Our present study helps the deep understanding of the CO₂ capturing mechanism by ILs and is expected to facilitate the future design and fabrication of a novel IL medium for gas capture and storage.

The interactions between ionic liquids (ILs) and CO₂ were studied by molecular dynamics simulations. Several key characters, including the volumes of cations and anions, the length of the alkyl chain have been assessed on CO₂ capture capability.     

Microporous organic hydroxyl-functionalized polybenzotriazole for encouraging CO2 capture and separation

We report a mild, hydroxyl functionalized and thermal stable benzotriazole-based aerogel (HO-PBTA), which is inspired by phenolic resin chemistry. Taking advantage of the synergistic adsorption interactions between hydroxy-benzotriazole and CO₂, and the phobic effect between benzotriazole and nitrogen (N₂), the CO₂ uptake capacity of the HO-PBTA reaches an encouraging level (6.41 mmol g⁻¹ at 1.0 bar and 273 K) with high selectivity (CO₂/N₂ = 76 at 273 K).

We report a mild, hydroxyl functionalized and thermal stable benzotriazole-based aerogel (HO-PBTA), which is inspired by phenolic resin chemistry.

Nowadays, global warming caused by increased concentrations of carbon dioxide (CO₂) in the atmosphere is one of the most serious environmental problems.^1–3 The development of novel functional materials and new technologies for CO₂ capture and storage has gained great attention. Microporous organic polymers (MOPs) with intrinsic properties including large specific surface area, narrow pore size distribution, good chemical stability, and low skeleton density have exhibited potential applications in gas storage and separation.^4–7 In addition, microporous porous materials with excellent intrinsic properties also made significant breakthroughs in liquid separation.^8,9 The structure and CO₂ adsorption of the MOPs have complicated relationships. Therefore, the design of high performance CO₂ capture materials often involves sophisticated molecular design and careful adjustments of the ingredient ratio. It has been shown that the incorporation of N-containing groups into the pore wall of MOPs has a profound impact on both CO₂ uptake and selectivity by enhancing their physisorption interactions,^10–15 however, it still remains a great challenge to make a facile synthesis of functional MOP materials that capture CO₂ efficiently and selectively.In the application of CO₂ capture, the nitrogen-containing MOPs act as capable storage media due to physisorption that involves an electron donor–acceptor mechanism between a heteroatom nitrogen and CO₂ on the inner surface of the networks.^16–18 The first principles study indicated that nitrogen-containing heteroaromatic groups can form strong physical interactions with CO₂via “dispersive π–π stacking” and electrostatic “in-plane” mechanisms.¹⁹ This theoretical calculation guides us how to design the functional materials that capture CO₂ efficiently and selectively. In previous work, we described a new strategy for CO₂ capture based on the synergistic effect of electrostatic in-plane and dispersive π–π stacking interactions of two functional groups with CO₂, and the proposed synergistic effect can be considered as a new rationale for the design and fabrication of CO₂ capture materials.²⁰ In addition, it has been demonstrated that azo-functionalized MOPs exhibit the N₂ phobicity due to the entropic loss of N₂ gas molecules upon binding, which endows the networks with the unprecedented CO₂ selectivity.²¹ Inspired by these reported studies, we hypothesized that both the CO₂ adsorption capacity and CO₂/N₂ selectivity can be improved greatly by involving multiple, more than two, functional groups in MOP networks where multiple mechanisms work for CO₂ capture and separation. In this work, the synergistic adsorption interactions between the HO-PBTA polymer and CO₂ molecules, the N₂-phobic effect between the HO-PBTA polymer and N₂ have been investigated in details. It is expected that the comprehensive effects of the CO₂-philic and N₂-phobic behaviors will provide new design principles for the development of next-generation functional porous polymers with high CO₂ adsorption capacity and selectivity.To achieve this objective, a benzotriazole-based microporous aerogel (HO-PBTA), with azo, hydroxyl and imino groups in the polymer chains, was fabricated via sol–gel technology involving phenolic resin-inspired chemistry^22–24 and followed by CO₂ supercritical drying (Fig. 1A). The material preparation and characterization are detailed in the ESI.^† The as-prepared HO-PBTA is a dark gray, porous ultralight material, as shown in Fig. 1B. HO-PBTA aerogel was characterized by Fourier transform infrared and ¹³C CP/MAS NMR, and the results were in good agreement with the proposed structures (Fig. 2). The FTIR spectrum of the HO-PBTA is shown in Fig. 2a, in which the absorption peaks at about 3190 cm⁻¹ and 3402 cm⁻¹ correspond to the structure of NH and the OH groups. The peak at 1618 cm⁻¹ is attributed to the vibration of the aromatic ring skeleton. And the absorption at 1536 cm⁻¹ corresponds to the structure of C N in the network. As shown in Fig. 2B, the broad peaks at 150–110 ppm are ascribed to the benzotriazole group carbons, and the peaks at 75–25 ppm corresponds to methylene carbons.Open in a separate windowFig. 1Synthesis of HO-PBTA aerogel. (A) Dark gray aerogel of HO-PBTA was obtained by reacting 4-hydroxy-1H-benzotriazole with formaldehyde, in water at 80 °C in the presence of potassium hydroxide under air conditions; (B) the photograph of the HO-PBTA aerogel.Open in a separate windowFig. 2Chemical structure characterization of the HO-PBTA polymer material. (A) FTIR spectrum, recorded as KBr pellet; (B) ¹³C CP/MAS NMR spectrum.The porosity of the HO-PBTA aerogel was quantified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and sorption analysis. A SEM image shows that the HO-PBTA aerogel consists of aggregated particles with submicrometer sizes (Fig. 3A). The TEM image (Fig. 3B) reveals the micropore structure, which is an essential requirement for CO₂ capture. The porosity of aerogel was further quantified by sorption analysis using nitrogen (N₂) as the sorbate molecule, and the HO-PBTA aerogel is microporous and exhibits a combination of type I and II N₂ sorption isotherms according to the IUPAC classification (Fig. 4A).²⁵ The increase in the nitrogen sorption at a high relative pressure above 0.9 may arise in part from interparticulate porosity associated with the meso- and macrostructures of the samples and interparticular void.²⁶ According to ref. 27, the specific surface areas calculated in the relative pressure (P/P₀) range from 0.01 to 0.1 shows that the Brunauer–Emmett–Teller (BET) specific surface area of HO-PBTA is up to 2160 m² g⁻¹ (Fig. S3^†). The pore size distribution (PSD) of the network calculated from the adsorption branch of the isotherms with the nonlocal density functional theory (NLDFT) approach indicates that HO-PBTA aerogel exhibits a dominant pore diameter centered at about 0.60 nm (inset in Fig. 4A). Thermal property of HO-PBTA aerogel was evaluated via thermogravimetric analysis (TGA) in nitrogen and air conditions, and the typical TGA curves are shown in Fig. S4.^† HO-PBTA exhibits great thermal stability with high decomposition temperature (T_d, 5% = ∼500 °C) at N₂ atmosphere. The TGA curve indicated that the HO-PBTA aerogel still exhibited good thermal stability at air condition.Open in a separate windowFig. 3Microstructural characterization of HO-PBTA aerogel. (A) Scanning electron microscopy (SEM) image of HO-PBTA aerogel; (B) transmission electron microscopy (TEM) image of HO-PBTA aerogel.Open in a separate windowFig. 4(A) Nitrogen adsorption–desorption isotherms and the pore size distribution calculated by the nonlocal density functional theory (inset) of HO-PBTA aerogel. (B) Gas adsorption isotherms of HO-PBTA aerogel at 273 K.The CO₂ adsorption capacity and selectivity (CO₂/N₂) in HO-PBTA aerogel were evaluated by adsorption isotherm measurements. As shown in Fig. 4B, the CO₂ capture exhibits an increase with the increasing of the pressure. The CO₂ adsorption capacity of the HO-PBTA aerogel is as high as 6.41 mmol g⁻¹ at 1.0 bar and 273 K, while the adsorption capacity of N₂ is only 0.09 mmol g⁻¹ at the same conditions (inset in Fig. 4B). The CO₂ adsorption capacity of the HO-PBTA aerogel is up to 2.3 mmol g⁻¹ at 1 bar and 323 K, and 0.02 mmol g⁻¹ for N₂ at the same conditions (Fig. S5^†). Equilibrium CO₂ adsorption capacity is found to decrease with an increase in temperature due to the exothermic nature of the adsorption process, as expected for physical adsorbents. We found that the CO₂ adsorption capacity of the HO-PBTA aerogel is higher than most of the CO₂ capture MOP materials, and close to some of the MOF materials (Table S1^†).^28–30 This high affinity is a consequence of the favourable interactions of the polarizable CO₂ molecules through multiple adsorption interactions with the framework, and also the inherent microporosity of HO-PBTA aerogel. Additionally, HO-PBTA aerogel also has great CO₂ adsorption capacities and selectivities at 298 and 323 K (Fig. S5^†), and these values are still comparable to the high surface area MOP networks.^31–33For the further CO₂ adsorption, CO₂ was also adsorbed preferentially over N₂ at high temperatures. The selectivities were calculated using the Ideal Adsorbed Solution Theory (IAST) for CO₂/N₂ mixture in the ratio of 0.15 : 0.85. At 1 bar, the IAST CO₂/N₂ selectivities of the HO-PBTA were 76 at 273 K, 97 at 298 K and become 110 at 323 K, which close to the highest one reported to date under the same conditions. Additionally, the HO-PBTA aerogel have the high selectivity of CO₂/N₂ at high temperature than other porous materials (Table S2^†). It is worth noting that the CO₂/N₂ selectivities increased with the temperature increasing, which are essential requirements for high temperature post-combustion CO₂ adsorption. N₂ uptake drops ∼70% at a temperature increase from 273 to 298 K, compared with a ∼60% drop for CO₂. This phenomenon is in line with the conventional CO₂ affinity and the concept of N₂ phobic in nitrogen-rich porous polymers where nitrogen-rich groups (–N N–) will reject N₂ gas selectively.²¹ To further akin to capture from post-combustion gas streams, CO₂ adsorption capacity of HO-PBTA aerogel at higher temperature have been characterized (Fig. S6^†). The HO-PBTA shown the good CO₂ adsorption capacity of 1.8 mmol g⁻¹ at 333 K and 1.5 mmol g⁻¹ 343 K. The N₂ uptakes of HO-PBTA aerogel under the same conditions were 0.016 mmol g⁻¹ and 0.015 mmol g⁻¹, resulting in selectivity of 112 and 101, respectively.The isosteric heat of adsorption (Q_st) for HO-PBTA aerogel was calculated using the virial equations.³⁴ As shown in Fig. S7,^† HO-PBTA has a Q_st value of 33.9 kJ mol⁻¹, and this value can be considered as the optimum for gas adsorption and separation because of a balance between the reversibility and selectivity. The Q_st values is highest among reported values for organic porous polymers and comparable to some MOFs compounds.^21,35,36 The impressive Q_st of 33.9 kJ mol⁻¹ further indicates the strong interaction of HO-PBTA polymers with CO₂ guest molecules. It should be noted that the higher and more optimized Q_st value of the HO-PBTA aerogel can be ascribed to the synergistic effect of azo (–N N–), hydroxyl (–OH) and imino (–NH) units arising from different interaction mechanisms.To illustrate the synergistic adsorption mechanism, we used density functional theory (DFT)^{19,20,37–39} to investigate the interaction of HO-PBTA with CO₂ and to track the CO₂ capture process. The calculation is detailed in the ESI. Fig. 5 shows a series of snapshots for CO₂ capture by 4-hydroxybenzotriazole, as the model compound, where benzotriazole and hydroxyl work synergistically to adsorb multiple CO₂ molecules. The minimum energy structure of the CO₂–azo complex is obtained when CO₂ lies on the azo at a bond distance of 3.06 Å to form the π–π stacking conformation (Fig. 5B). The electrostatic “in-plane” equilibrium conformation of CO₂–HO-PBTA involves two sites: one is the electron deficient central carbon atom of CO₂ to the lone pair of electrons on a nitrogen atom of azo group via dipole–quadrupole interaction; the other is lone pairs of oxygen on CO₂ to a hydrogen atom on the imino group (or a hydrogen atom on the hydroxyl) via hydrogen bonding (Fig. 5C and F). Either dipole–quadrupole interaction or hydrogen bonding cannot stabilize the CO₂–HO-PBTA complex because of their low binding energies.¹⁹ However, previous works and our calculation indicated that simultaneous formation of dipole–quadrupole interaction and hydrogen bonding at both sites cause a much more stable in-plane conformation of CO₂–HO-PBTA complex (Fig. S8^†).^19,20 However, the capture of flowing CO₂ by electrostatic “in-plane” interactions is difficult due to a small binding area by only two atomistic sites. On the other hand, CO₂ can be rapidly adsorbed on the azo group because of its relatively large binding area (Fig. 5A). To our knowledge, the desorption occurs frequently driven by thermal fluctuation. Once CO₂ desorption, the starting speed should be much slower than the bulk speed, resulting in a high probability to be captured by an adjacent hydroxyl and imino groups. The in-plane conformation of CO₂–HO-PBTA complex is, therefore, formed much easily and efficiently with help of the functional azo while retaining the high selectivity of CO₂ over other gas molecules (Fig. 5H). Final coordinates of DFT geometry optimization was shown in Table S3.^†Open in a separate windowFig. 5DFT results to track the full CO₂ capture process. (A) A CO₂ molecule is adsorbed on the face of an electron-rich azo group via the dispersive π–π stacking interaction. (B and C) The desorbed CO₂ molecule can be captured by an adjacent hydroxyl, a stable “electrostatic in-plane” conformation including dipole–quadrupole and hydrogen bond interactions is formed. (D) The second CO₂ molecule comes close to the azo group, (E) and the CO₂ molecule is adsorbed on the azo group. (F) The desorbed CO₂ molecule can be captured by an adjacent imino group. (G) The third CO₂ molecule comes close to the azo group, (H) and the CO₂ molecule is adsorbed on the azo group. The gray, white, blue and red spheres represent C, H, N, and O atoms, respectively.     

Waste wool derived nitrogen-doped hierarchical porous carbon for selective CO2 capture

The goal of this research is to develop a low-cost porous carbon adsorbent for selective CO₂ capture. To obtain advanced adsorbents, it is critical to understand the synergetic effect of textural characteristics and surface functionality of the adsorbents for CO₂ capture performance. Herein, we report a sustainable and scalable bio-inspired fabrication of nitrogen-doped hierarchical porous carbon by employing KOH chemical activation of waste wool. The optimal sample possesses a large surface area and a hierarchical porous structure, and exhibits good CO₂ adsorption capacities of 2.78 mmol g⁻¹ and 3.72 mmol g⁻¹ at 25 °C and 0 °C, respectively, under 1 bar. Additionally, this sample also displays a moderate CO₂/N₂ selectivity, an appropriate CO₂ isosteric heat of adsorption and a stable cyclic ability. These multiple advantages combined with the low-cost of the raw material demonstrate that this sample is an excellent candidate as an adsorbent for CO₂ capture.

In this work, N-doped hierarchical porous carbon has been successfully fabricated by KOH activation of waste wool. The optimal sample exhibits good CO₂ adsorption capacity under atmospheric pressure (1 bar), as well as excellent CO₂/N₂ selectivity.     

Biowaste-derived 3D honeycomb-like N and S dual-doped hierarchically porous carbons for high-efficient CO2 capture

Considering the characteristics of abundant narrow micropores of <1 nm, appropriate proportion of mesopores/macropores and suitable surface functionalization for a highly-efficient carbon-based CO₂ adsorbent, we proposed a facile and cost-effective strategy to prepare N and S dual-doped carbons with well-interconnected hierarchical pores. Benefiting from the unique structural features, the resultant optimal material showed a prominent CO₂ uptake of up to 7.76 and 5.19 mmol g⁻¹ at 273 and 298 K under 1 bar, and importantly, a superb CO₂ uptake of 1.51 mmol g⁻¹ at 298 K and 0.15 bar was achieved, which was greatly significant for CO₂ capture from the post-combustion flue gases in practical application. A systematic study demonstrated that the synergetic effect of ultramicroporosity and surface functionalization determined the CO₂ capture properties of porous carbons, and the synergistic influence mechanism of nitrogen/sulfur dual-doping on CO₂ capture performance was also investigated in detail. Importantly, such as-prepared carbon-based CO₂ adsorbents also showed an outstanding recyclability and CO₂/N₂ selectivity. In view of cost-effective fabrication, the excellent adsorption capacity, high selectivity and simple regeneration, our developed strategy was valid and convenient to design a novel and highly-efficient carbonaceous adsorbent for large-scale CO₂ capture and separation from post-combustion flue gases.

We proposed a facile and cost-effective strategy to prepare N/S dual-doped carbons with abundant micropores of <1 nm, appropriate proportion of meso/macropores and suitable surface functionalization for highly efficient CO₂ capture.     

Absorption and thermodynamic properties of CO2 by amido-containing anion-functionalized ionic liquids

In this contribution, two kinds of amido-containing anion-functionalized ionic liquids (ILs) were designed and synthesized, where the anions of these ILs were selected from deprotonated succinimide (H-Suc) and o-phthalimide (Ph-Suc). Then, these functionalized ILs were used to capture CO₂. Towards to this end, solubility of CO₂ in the ILs was determined at different temperatures and different CO₂ partial pressures. Based on these data, chemical equilibrium constants of CO₂ with the ILs were derived at different temperatures from the “deactivated IL” model. The other thermodynamic properties such as reaction Gibbs energy, reaction enthalpy, and reaction entropy in the absorption were also calculated from the corresponding equilibrium constant data at different temperatures. It was shown that these anion-functionalized ILs exhibited high CO₂ solubility (up to 0.95 mol CO₂ mol⁻¹ IL) and low energy desorption, and enthalpy change was the main driving force for CO₂ capture by using such ILs as absorbents. In addition, the interactions of CO₂ with the ILs were also investigated by ¹H NMR, ¹³C NMR, and FT-IR spectroscopy.

Amido-containing anion-functionalized ionic liquids exhibit high CO₂ absorption capacity and low desorption energy.     

Nitrogen-decorated,porous carbons derived from waste cow manure as efficient catalysts for the selective capture and conversion of CO2

The catalytic conversion of CO₂ is a promising solution to the greenhouse effect and simultaneously recycles the carbon sources to produce high value-added chemicals. Herein, we demonstrated a class of nanoporous carbons, which were synthesized by the direct carbonization of bio-waste cow manure, followed by activation with KOH and NaNH₂. Various characterizations indicate that the resultant nanoporous carbons have abundant nanopores and nitrogen sites. As a result, their performances for the capture and catalytic conversion of CO₂ were investigated. The synthesized nanoporous carbons exhibited superior properties for the selective capture and catalytic cycloaddition of CO₂ to propylene oxide as compared to various solid materials.

Nitrogen-doped, hierarchically porous carbons were prepared by the activation of waste cow manure at 600 °C, which acted as efficient catalysts for the highly selective capture and conversion of CO₂ into valuable chemicals.     

A depth-suitable and water-stable trap for CO2 capture

In terms of CO₂ capture and storage (CCS), it is highly desired to substitute of high efficiency process for the applied one which is far from the ideal one. Physical processes cannot capture CO₂ effectively, meanwhile CO₂ desorption is energy-intensive in chemical processes. Herein, a depth-suitable and water-stable trap for CO₂ capture was discovered. Carboxylates can react with polybasic acid roots by forming united hydrogen bonds. Carboxylate ionic liquid (IL) aqueous solutions can absorb one equimolar CO₂ chemically under ambient pressure, and its CO₂ desorption is easy, similar to that in physical absorption/desorption processes. When used as aqueous solutions, carboxylate ILs can replace alkanolamines directly in the applied CCS process, and the efficiency is enhanced significantly due to the low regenerating temperature. CO₂ (or polybasic acids) can be used as a polarity switch for ILs and surfactants. A new method for producing carboxylate ILs is also proposed.

Carboxylates can react with carbonic acid and form two united hydrogen bonds, which is depth-suitable for CO₂ capture.

Being the main greenhouse gas, CO₂ capture and storage (CCS) is indispensable for achieving carbon neutrality, and it has been a hot topic for many years.¹ Besides CO₂ generated from the energy demand of human being''s daily activities, energy requirement in CCS also produces more CO₂, which comes from fuel consumption, and this will lead to an increase in electricity prices.^2,3 So, the enhancement of energy efficiency in CCS is highly desired. Alkanolamines are the main absorbents applied for CCS in industry, and their regeneration processes are energy intensive, as shown in Scheme 1 (chemical trap). There are more inherent shortages, such as amine degradation and volatilize, and hence, the CCS process applied in this study is far from the ideal one.Open in a separate windowScheme 1Schematic diagram of a depth-suitable trap for CO₂ capture.Having many unique features, such as extremely low vapor pressure, ionic liquids (ILs) have aroused the interest of many scientists,^4–6 including their ability to absorb CO₂ physically.⁷ Physical absorption is conducted under high pressure and anhydrous conditions. There are several shortcomings as follows: the sorption capacity is limited at low pressure, water is invariably present in almost all ILs,⁸ CO₂ and water are the two main exhaust products in fossil fuel burning. It is difficult to get satisfactory results in physical absorption, as shown in Scheme 1. Functional ILs have also been tested for absorbing CO₂ (ref. 9 and ¹⁰) and SO₂ (ref. 11 and ¹²) chemically, and some authors claimed that the products obtained are carbamic acids or amidates while absorbing CO₂. Recently, Dupont et al. considered that carbonates/bicarbonates are generated because water is inevitable in most cases,⁸ and this is similar to the alkanolamine aqueous solution in the applied CCS process. The regeneration of those ILs is energy intensive, and the stronger the functional ILs used for absorbing CO₂, the more energy is needed in the regeneration process. The high viscosity of ILs is also unfavourable for capturing CO₂ effectively. Finding a depth-suitable trap (as shown in Scheme 1) with low viscosity and water-stability for CO₂ capture is highly desirable.Various chemical reactions are going on at this moment in nature and our bodies, and many of them have been discovered and explained clearly.¹³ Most of them form new chemical bonds, including covalent bonds, ionic bonds, and metal bonds.¹⁴ In comparison, hydrogen bonding is a weak interaction that has been known for a century¹⁵ and has been redefined recently.¹⁶ Although weak, hydrogen bonds are vital for water keeping its state as we know generally¹⁷ and for life passing on its genetic code.^18,19Herein, a depth-suitable and water-stable trap for CO₂ capture was discovered. Carboxylate roots can react with polybasic acid roots by forming united hydrogen bonds, and this lower energy of the products make them more stable even in water. Carboxylate IL aqueous solutions can replace alkanolamine aqueous solutions directly in the applied CCS process, and the efficiency is enhanced significantly, which comes from the low regenerating temperature. CO₂ (or polybasic acids) can be used as a polarity switch for ILs and surfactants. A new way for producing carboxylate ILs is also proposed.     

A detailed atomistic molecular simulation study on adsorption-based separation of CO2 using a porous coordination polymer

Emission of CO₂ is considered as one of the sources of global warming. Besides its currently inevitable production via several processes such as fuel consumption, it also exists in some other gaseous mixtures like biogas. Separation of carbon dioxide using solid adsorbents, for example porous coordination polymers and metal–organic frameworks, is an interesting active area of separation science. In particular, we performed detailed molecular simulations to investigate the response of a recently reported cobalt-based, pillared-layer, porous polymer on the CO₂ separation from biogas, natural gas, and flue gas. The effect of the coordinated water molecules to the open metal sites on the corresponding properties was studied and revealed enhanced results even in comparison with HKUST-1. Additionally, our results provide insights on the role of –NO₂ groups on the applications examined herein. Overall this study offers valuable insights about secondary building units of the examined materials which we expect to prove useful in the enhancement of carbon dioxide separation and capture.

We examined a new pillared-layered Co-nitroimidazolate dicarboxylate porous coordination polymer for potential use in adsorption-based CO₂ capture     

Modeling study of the heat of absorption and solid precipitation for CO2 capture by chilled ammonia

The contribution of individual reactions to the overall heat of CO₂ absorption, as well as conditions for solid NH₄HCO₃(s) formation in a chilled ammonia process (CAP) were studied using Aspen Plus at temperatures between 2 and 40 °C. The overall heat of absorption in the CAP first decreased and then increased with increasing CO₂ loading. The increase in overall heat of absorption at high CO₂ loading was found to be caused mostly by the prominent heat release from the formation of NH₄HCO₃(s). It was found that NH₄HCO₃(s) precipitation was promoted for conditions of CO₂ loading above 0.7 mol CO₂/mol NH₃ and temperatures less than 20 °C, which at the same time can dramatically increase the heat of CO₂ absorption. As such, the CO₂ loading is recommended to be around 0.6–0.7 mol CO₂/mol NH₃ at temperatures below 20 °C, so that the overall absorption heat is at a low state (less than 60 kJ mol⁻¹ CO₂). It was also found that the overall heat of CO₂ absorption did not change much with temperature when CO₂ loading was less than 0.5 mol CO₂/mol NH₃, while, when the CO₂ loading exceeded 0.7 mol CO₂/mol NH₃, the heat of absorption increased with decreasing temperature.

Prediction of (a) solution speciation change and (b) heat of CO₂ absorption in the chilled ammonia process (CAP).     

Cooperative CO2 absorption by amino acid-based ionic liquids with balanced dual sites

In this study, a variety of functionalized ILs with dual sites including amino acid group (AA) and basic anion (R) were synthesized to investigate the suppression and cooperation between the sites in CO₂ absorption. The basic anions selected in this study with different basicity include sulfonate (Su), carboxylate (Ac), imidazolium (Im), and indolium (Ind). These ILs ([P₆₆₆₁₄]₂[AA–R]) were applied to CO₂ absorption. The results present that CO₂ capacity increases first and then decreases later with the continuous increase in the activity of the anion site. Combined with CO₂ absorption experiments, IR and NMR spectroscopic analyses and DFT calculation demonstrate that the ability of one site to capture CO₂ would be suppressed when the activity of another site is much stronger. Thus, the cooperation of dual site-functionalized ILs and high CO₂ capacity might be achieved through balancing the two sites to be equivalent. Based on this point, [P₆₆₆₁₄]₂[5Am–iPA] was further synthesized by taking the advantage of the conjugated benzene ring. As expected, [P₆₆₆₁₄]₂[5Am–iPA] showed capacity as high as 2.38 mol CO₂ per mol IL at 30 °C and 1 bar without capacity decrease even after 10 times recycling performance of CO₂ absorption and desorption.

Cooperative CO₂ absorption by anion functionalized ILs with dual sites including amino acid group (AA) and basic anion (R) could be achieved through regulating the relative activation of two sites.     

Spherical amine grafted silica aerogels for CO2 capture

The objective of this research was to develop a novel spherical amine grafted silica aerogel for CO₂ capture. A spherical silica gel was synthesized by dropping a sodium silicate based silica sol into an oil bath. Amine grafting was achieved by bonding 3-aminopropyltriethoxysilane onto the framework of the silica gel. The spherical amine grafted silica gels were dried using vacuum drying to prepare the spherical amine grafted silica aerogels (SASAs). The synthetic mechanism of the SASAs was proposed. The structures and the CO₂ adsorption performances of SASAs were researched. The amine loading of the SASAs increased with the grafting time, however, the specific surface area and pore volume sharply decreased owing to the blockage of the pore space. Excess amine loading led to the decrease of the CO₂ adsorption capacity. The optimal CO₂ adsorption capacity was 1.56 mmol g⁻¹ with dry 1% CO₂ and at 35 °C. This work provides a low-cost and environmentally friendly way to design a capable and regenerable adsorbent material.

The objective of this research was to develop a novel spherical amine grafted silica aerogel for CO₂ capture.     

Novel porous organocatalysts for cycloaddition of CO2 and epoxides

Three classes of organosilicas (DMO, OMOs and PMOs) containing immobilized multi-hydroxyl bis-(quaternary ammonium) iodide salts were prepared and tested in the cycloaddition of CO₂ and epoxides. Owing to its higher surface area, pore volume and optimum nucleophilicity of the iodide ion, OMO-2 with two hydroxyl groups was found to be the most active catalyst. For substrates that are easy to activate such as propylene oxide, 1,2-epoxybutane and epichlorohydrin, excellent yields and selectivities were obtained under mild reaction conditions (0.5 MPa CO₂, 50 °C and 10–15 h). Moreover, OMO-2 showed very good catalytic properties (yield ≥ 93% and selectivity ≥ 98%), and excellent chemical and textural stability in the synthesis of 1,2-butylene carbonate over 5 cycles.

Three classes of organosilicas (DMO, OMOs and PMOs) containing multi-hydroxyl bis-quaternary ammonium iodide were tested in the cycloaddition of CO₂ and epoxides. OMO with two hydroxyl groups was the most active, with good stable and reusability.     

Polysulfone metal-activated carbon magnetic nanocomposites with enhanced CO2 capture

In the present study, polysulfone (PSF)-activated carbon nanocomposites were synthesized by a melt mixing technique. Here, 2 wt% activated carbon (CA, CA–Ni, and CA–Co) was used as filler, and effects on thermal, mechanical, magnetic, morphological, and carbon dioxide capture properties were studied. The pyrolysis of wood sawdust produced carbon materials activated by Co and/or Ni salt. The thermal degradation and the amount of metal in the carbon materials were investigated by thermogravimetric analysis. The maximum degradation temperature showed an improvement of up to 3 °C, while the initial degradation temperature decreased up to 4 °C with the addition of metal-activated carbons. The values of T_g estimated by differential scanning calorimetry appear to be practically identical for pure PSF and its nanocomposites. The elasticity modulus of the nanocomposite shows an enhancement of 17% concerning the neat PSF. The water contact angle showed a decrease with the incorporation of the fillers, indicating the hydrophilic nature of the composite. The carbon dioxide sorption capacity of the nanocomposite showed an enhancement of almost 10% in contrast to neat PSF. Ferromagnetic behavior of the thermoplastic nanocomposite was observed with the introduction of 2.0 wt% metal-carbonized filler. The exceptional magnetic properties, for a thermoplastic material such as polysulfone, make it promising for various industrial applications.

In the present study, polysulfone (PSF)-activated carbon nanocomposites were synthesized by a melt mixing technique.     

Confined toluene within InOF-1: CO2 capture enhancement

The toluene adsorption properties of InOF-1 are studied along with the confinement of small amounts of this non-polar molecule revealing a 1.38-fold increase in CO₂ capture, from 5.26 wt% under anhydrous conditions to 7.28 wt% with a 1.5 wt% of pre-confined toluene at 298 K. The InOF-1 affinity towards toluene was experimentally quantified by ΔH_ads (−46.81 kJ mol⁻¹). InOF-1 is shown to be a promising material for CO₂ capture under industrial conditions. Computational calculations (DFT and QTAIM) and DRIFTs in situ experiments provided a possible explanation for the experimental CO₂ capture enhancement by showing how the toluene molecule is confined within InOF-1, which constructed a “bottleneck effect”.

The confinement of small amounts of toluene demonstrated an enhanced CO₂ capture for InOF-1 as a result of a bottleneck effect and synergistic interactions.     

A diamino-functionalized silsesquioxane pillared graphene oxide for CO2 capture

In the race for viable solutions that could slow down carbon emissions and help in meeting the climate change targets a lot of effort is being made towards the development of suitable CO₂ adsorbents with high surface area, tunable pore size and surface functionalities that could enhance selective adsorption. Here, we explored the use of silsesquioxane pillared graphene oxide for CO₂ capture; we modified silsesquioxane loading and processing parameters in order to obtain pillared structures with nanopores of the tailored size and surface properties to maximize the CO₂ sorption capacity. Powder X-ray diffraction, XPS and FTIR spectroscopies, thermal analysis (DTA/TGA), surface area measurements and CO₂ adsorption measurements were employed to characterize the materials and evaluate their performance. Through this optimisation process, materials with good CO₂ storage capacities of up to 1.7/1.5 mmol g⁻¹ at 273 K/298 K in atmospheric pressure, were achieved.

Study of the CO₂ uptake performance of silsesquioxane pillared graphene oxide prepared with different pillar loading and way of drying.     

Reversible CO2 storage and efficient separation using Ca decorated porphyrin-like porous C24N24 fullerene: a DFT study

The search for novel materials for effective storage and separation of CO₂ molecules is a critical issue for eliminating or lowering this harmful greenhouse gas. In this paper, we investigate the potential application of a porphyrin-like porous fullerene (C₂₄N₂₄) as a promising material for CO₂ storage and separation using thorough density functional theory calculations. The results show that CO₂ is physisorbed on bare C₂₄N₂₄, implying that this material cannot be used for efficient CO₂ storage. Coating C₂₄N₂₄ with Ca atoms, on the other hand, can greatly improve the adsorption strength of CO₂ molecules due to polarization and charge-transfer effects. Furthermore, the average adsorption energy for each of the maximum 24 absorbed CO₂ molecules on the fully decorated Ca₆C₂₄N₂₄ fullerene is −0.40 eV, which fulfills the requirement needed for efficient CO₂ storage (−0.40 to −0.80 eV). The Ca coated C₂₄N₂₄ fullerene also have a strong potential for CO₂ separation from CO₂/H₂, CO₂/CH₄, and CO₂/N₂ mixtures.

Using dispersion-corrected DFT calculations, the potential application of a porphyrin-like porous fullerene (C₂₄N₂₄) as an efficient material for CO₂ storage and separation was investigated.     

Measurement of CO2 diffusion coefficients in both bulk liquids and carven filling porous media of fractured-vuggy carbonate reservoirs at 50 MPa and 393 K

Diffusion coefficients are necessary to describe the mass transfer and adsorption rate of CO₂ in formation fluids. However, data is scarcely reported for actual reservoir conditions of high pressure and temperature, which are normal in most scenarios of the CO₂-enhanced oil recovery process in China''s fractured-vuggy reservoirs and carbon storage process. Accordingly, this work employed the pressure decay method (PD) and relevant mathematical models to determine the CO₂ diffusion coefficient in both liquids and cavern filling porous media at 50 MPa and 393 K. The effects of the type of reservoir fluids, the properties of carven filling porous media, and water saturation on CO₂ diffusion coefficients were investigated. Results in bulk reservoir liquids showed that the CO₂ diffusion coefficient in the oil sample was 4.1243 × 10⁻⁸ m² s⁻¹, much higher than those in the pure alkane phase, pure water and brine sample from reservoirs. Results of CO₂ diffusion in carven filling porous media saturated with oil demonstrated a significant dependence on properties such as porosity and permeability, and a correlation in the CO₂ diffusion coefficients between the bulk oil phase and cavern filling porous media in the form of touristy was documented. CO₂ diffusion in the fractured cavern porous media was much higher than that without fracture. An increase in water saturation reduced CO₂ diffusion coefficients in the carven filling porous medium studied, herein. Thus, the CO₂ diffusion coefficient is essentially related to the type of liquid and properties of the filling media.

Herein, the pressure decay method was improved to obtain the CO₂ diffusion coefficient in fractured-vuggy carbonate reservoirs at 393 K and 50 MPa and obtained good correlation results between bulk and porous media by porosity and tortuosity.     

The development of activated carbon from corncob for CO2 capture

The accumulation and incineration of crop waste pollutes the environment and releases a large amount of CO₂. In this study, corncob crop waste was directly activated using solid KOH in an inert atmosphere to prepare porous activated carbon (AC) to capture CO₂, and to introduce N-containing functional groups that favour CO₂ adsorption, urea was mixed with corncob and KOH to prepare N-doped AC. The physical and chemical properties of the AC were characterized, and the effects of the mass ratio of KOH and urea to corncob, the activation temperature and time as well as regeneration were investigated to explore the optimal preparation process. The pores in the AC are mainly micropores, with the specific surface area and pore volume reaching 926.07 m² g⁻¹ and 0.40 cm³ g⁻¹ for KOH-activated corncob and 1096.70 m² g⁻¹ and 0.48 cm³ g⁻¹ after N-doping; the C–O plus O–H ratio and the –NH– ratio, which favour CO₂ adsorption in N-doped AC were 6.04 and 1.92%, respectively. The maximum adsorption capacities for KOH-activated corncob before and after N-doping were 3.49 and 4.58 mmol g⁻¹, respectively, at 20 °C and remained at 3.44 and 4.52 mmol g⁻¹ after ten regenerations. The prepared corncob-based AC showed good application prospects for CO₂ capture.

The accumulation and incineration of crop waste pollutes the environment and releases a large amount of CO₂.     

Yttrium and europium separation by solvent extraction with undiluted thiocyanate ionic liquids

An yttrium/europium oxide obtained by the processing of fluorescent lamp waste powder was separated into its individual elements by solvent extraction with two undiluted ionic liquids, trihexyl(tetradecyl)phosphonium thiocyanate, [C101][SCN], and tricaprylmethylammonium thiocyanate, [A336][SCN]. The best extraction performances were observed for [C101][SCN], by using an organic-to-aqueous volume ratio of 1/10 and four counter-current extraction stages. The loaded organic phase was afterwards subjected to scrubbing with a solution of 3 mol L⁻¹ CaCl₂ + 0.8 mol L⁻¹ NH₄SCN to remove the co-extracted europium. Yttrium was quantitatively stripped from the scrubbed organic phase by deionized water. Yttrium and europium were finally recovered as hydroxides by precipitation with ammonia and then calcined to the corresponding oxides. The conditions thus defined for an efficient yttrium/europium separation from synthetic chloride solutions were afterwards tested on a leachate obtained from the dissolution of a real mixed oxide. The purity of Y₂O₃ with respect to the rare-earth content was 98.2%; the purity of Eu₂O₃ with respect to calcium was 98.7%.

Yttrium and europium are separated from a mixed oxide through solvent extraction with undiluted thiocyanate ionic liquids.