Adsorption of CO2 from dry gases on MCM-41 silica at ambient temperature and high pressure. 2: Adsorption of CO2/N2, CO2/CH4 and CO2/H2 binary mixtures

Innovative Strategy for Truly Reversible Capture of Polluting Gases-Application to Carbon Dioxide

This paper consists of a deep analysis and data comparison of the main strategies undertaken for achieving truly reversible capture of carbon dioxide involving optimized gas uptakes while affording weakest retention strength. So far, most strategies failed because the estimated amount of CO2 produced by equivalent energy was higher than that captured. A more viable and sustainable approach in the present context of a persistent fossil fuel-dependent economy should be based on a judicious compromise between effective CO2 capture with lowest energy for adsorbent regeneration. The most relevant example is that of so-called promising technologies based on amino adsorbents which unavoidably require thermal regeneration. In contrast, OH-functionalized adsorbents barely reach satisfactory CO2 uptakes but act as breathing surfaces affording easy gas release even under ambient conditions or in CO2-free atmospheres. Between these two opposite approaches, there should exist smart approaches to tailor CO2 retention strength even at the expense of the gas uptake. Among these, incorporation of zero-valent metal and/or OH-enriched amines or amine-enriched polyol species are probably the most promising. The main findings provided by the literature are herein deeply and systematically analysed for highlighting the main criteria that allow for designing ideal CO2 adsorbent properties.

Valorization of spent disposable wooden chopstick as the CO2 adsorbent for a CO2/H2 mixed gas purification

A series of activated carbons (ACs) derived from spent disposable wooden chopsticks was prepared via steam activation and used to separate carbon dioxide (CO₂) from a CO₂/hydrogen (H₂) mixed gas at atmospheric pressure. A factorial design was employed to investigate the effects of the activation temperature and time as well as their interactions on the production yield of ACs and their CO₂ adsorption capacity. The activation temperature exhibited a much higher impact on both the production yield and the CO₂ adsorption capacity of ACs than the activation time. The interaction of both parameters did not significantly affect the yield of ACs, but did affect the CO₂ adsorption capacity. The optimal preparation condition provided ACs with a desirable yield of around 23.18% and a CO₂ adsorption capacity of 85.19 mg/g at 25 °C and 1 atm and consumed the total energy of 225.28 MJ/kg AC or 116.4 MJ/g-mol CO₂. A H₂ purity of greater than 96.8 mol% was achieved from a mixed gas with low CO₂ concentration (< 20 mol%) during the first 3 min of adsorption and likewise around 90 mol% from a mixed gas with a high CO₂ concentration (> 30 mol%) during the first 2 min. The CO₂ adsorption on the as-prepared ACs proceeded dominantly via multilayer physical adsorption and was affected by both the surface area and micropore volume of the ACs. The adsorption capacity was diminished by around 18% after six adsorption/desorption cycles. The regeneration of the as-prepared chopstick-derived ACs can be easily performed via heating at a low temperature and ambient pressure, suggesting their potential application in the temperature swing adsorption process.

Adsorption of simple gases into the porous glass MCM-41

The porous glass MCM-41 is an important adsorbent to study the process of adsorption of gases onto a cylindrical surface. In this work, we study the adsorption of oxygen, nitrogen, deuterium, and deuteriated methane gases into MCM-41 using a combination of neutron diffraction analysis and atomistic computer modeling to interpret the measured data. Adsorption is achieved by immersing a sample of MCM-41 in a bath of the relevant gas, keeping the gas pressure constant (0.1 MPa), and lowering the temperature in steps toward the corresponding bulk liquid boiling point. All four gases have closely analogous behaviors, with an initial layering of liquid on the inside surface of the pores, followed by a relatively sharp capillary condensation (CC) when the pore becomes filled with dense fluid, signaled by a sharp decrease in the intensity of (100) Bragg diffraction reflection. At the temperature of CC, there is a marked distortion of the hexagonal lattice of pores, as others have seen, which relaxes close to the original structure after CC, and this appears to be accompanied by notable excess heterogeneity along the pore compared to when CC is complete. In none of the four gases studied does the final density of fluid in the pore fully attain the value of the bulk liquid at its boiling point at this pressure, although it does approach that limit closely near the center of the pore, and in all cases, the pronounced layering near the silica interface seen in previous studies is observed here as well.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

An automated multi-component gas adsorption system (MC GAS)

The knowledge gap on adsorption of complex mixtures in the literature relative to single component data represents a persistent obstacle to developing accurate process models for adsorption separations. The collection of mixed gas adsorption data is an imminent need for improved understanding of the behavior of adsorbent systems in these diverse adsorption applications. Current approaches to understanding mixture adsorption using predictive theories based on pure component adsorption experiments often fail to capture the behavior of more complex, non-ideal systems. In this work, we present an automated volumetric instrument for the measurement of mixed gas adsorption isotherms. This instrument was validated by comparison to other in-house instruments and data available in the literature, and the binary adsorption measurements were found to be thermodynamically consistent. The automation of this instrument allows for rapid collection of high-quality mixture adsorption data.

The potential of hydrogen hydrate as a future hydrogen storage medium

Hydrogen is recognized as the "future fuel" and the most promising alternative of fossil fuels due to its remarkable properties including exceptionally high energy content per unit mass (142 M J / k g ), low mass density, and massive environmental and economical upsides. A wide spectrum of methods in H 2 production, especially carbon-free approaches, H 2 purification, and H 2 storage have been investigated to bring this energy source closer to the technological deployment. Hydrogen hydrates are among the most intriguing material paradigms for H 2 storage due to their appealing properties such as low energy consumption for charge and discharge, safety, cost-effectiveness, and favorable environmental features. Here, we comprehensively discuss the progress in understanding of hydrogen clathrate hydrates with an emphasis on charging/discharging rate of H 2 (i.e. hydrate formation and dissociation rates) and the storage capacity. A thorough understanding on phase equilibrium of the hydrates and its variation through different materials is provided. The path toward ambient temperature and pressure hydrogen batteries with high storage capacity is elucidated. We suggest that the charging rate of H 2 in this storage medium and long cyclic performance are more immediate challenges than storage capacity for technological translation of this storage medium. This review and provided outlook establish a groundwork for further innovation on hydrogen hydrate systems for promising future of hydrogen fuel.

Adsorption selectivity of CO2 over CH4, N2 and H2 in melamine–resorcinol–formaldehyde xerogels

Sorptive selectivity of melamine–resorcinol–formaldehyde xerogels, towards CO2, CH4, N2 and H2, is reported, where all systems demonstrate potential for selective adsorption of CO2 from corresponding binary gas mixtures. Selected gas mixtures represent important gas separation applications found in industry, i.e. CO2 removal from power plant flue gases (CO2–N2), sour gas sweetening (CO2–CH4), and separation of species in the water–gas shift reaction (CO2–H2). All materials tested exhibit microporous character, enhancing adsorption of small molecules, however, it is the inclusion of a nitrogen-rich material into the gel matrix that results in enhanced selectivities for these systems. Despite the porous character of the gels, under the test conditions used to simulate industrial parameters, all three balance gases, i.e. H2, N2 and CH4, showed low affinities for the xerogels, while CO2 adsorption was notably higher and increased with the inclusion and increased concentration of melamine. Ideal adsorbed solution theory was used to demonstrate significant differences in adsorption uptake, especially for CO2–CH4, and high selectivities for CO2 over N2. In all cases, selected xerogels exhibited industrially relevant adsorption timescales for CO2 over competitor gases, demonstrating the potential of these materials for the selective adsorption of CO2 from process streams.

A Strong Test of Atomically Detailed Models of Molecular Adsorption in Zeolites Using Multilaboratory Experimental Data for CO2 Adsorption in Ammonium ZSM-5

A recent international interlaboratory study led by the U.S. National Institute of Standards (NIST) reported CO₂ adsorption isotherms measured independently by 11 groups on reference material RM 8852, an ammonium ZSM-5 zeolite. Good reproducibility and high reliability of this experimental data provide a strong test for the ability of atomically detailed models to predict adsorption of CO₂ in zeolites. We developed force fields for CO₂ in ammonium zeolites based on first-principles calculations and also independently performed experiments with RM 8852 by microcalorimetry. At low pressures good agreement was obtained between predictions and experiments. At high pressures, however, deviations were observed. We show that the charge-balancing cations in the experimental material are the predominant source of the discrepancy between simulation and experiment at high pressures; the experimental sample treatment causes deammoniation. In addition, accounting for a small amount of noncrystalline mesoporosity in the zeolite brings predictions into much better agreement with experiments.

Synergetic promotion by oxygen doping and Ca decoration on graphene for CO2 selective adsorption

The selective adsorption of CO2 by alkali earth metal (AEM)-decorated double vacancy graphene (DVG) was investigated with the first principles method. It is found that Be, Ca, Sr and Ba can be anchored stably on the DVG (whereas Mg cannot), and the Ca-decorated sample (Ca_DVG) possesses the strongest CO2 adsorption with a heat release of -0.45 eV per CO2. Furthermore, the doping of oxygen atoms on Ca_DVG (denoted as Ca_PyODVG) can remarkably increase the adsorption energy to -0.74 eV per CO2. This considerable promotion is ascribed to a synergetic effect of Ca decoration and O doping, which boosts extra electrons to transfer from the Ca_PyODVG substrate to the adsorbed CO2 molecule via the Ca 3p-O 2s hybridization. Notably, the obtained Ca_PyODVG is demonstrated to have a more practical CO2 desorption temperature, as well as a broader window for the selective adsorption of CO2 over CH4 and H2. Our theoretical results imply that Ca_PyODVG should be a promising candidate for CO2 capture. Additionally, the adsorption energy of CO2 is linearly correlated to the work function of a substrate, which may be used to accelerate the experimental screening of promising adsorbents.

Enhanced N-doped Porous Carbon Derived from KOH-Activated Waste Wool: A Promising Material for Selective Adsorption of CO₂/CH₄ and CH₄/N₂

Separation of impurities (CO₂ and N₂) from CH₄ is an important issue for natural gas alternatives (such as coalbed gas, biogas, and landfill gas) upgrading. It is notably challenging to synthesize high N-doped porous carbon with an appropriate porous structure. In this work, high N content (14.48 wt %) porous carbon with micropore size of 0.52 and 1.2 nm and specific surface area of 862 m² g-1 has been synthesized from potassium hydroxide (KOH) activated waste wool upon the urea modification. Pure component adsorption isotherms of CO₂, CH₄, and N₂ are systematically measured on this enhanced N-doped porous carbon at 0 and 25 °C, up to 1 bar, to evaluate the gases adsorption capability, and correlated with the Langmuir model. These data are used to estimate the separation selectivities for binary mixtures of CO₂/CH₄ and CH₄/N₂ at different mixing ratios according to the ideal adsorbed solution theory (IAST) model. At an ambient condition of 25 °C and 1 bar, the predicted selectivities for equimolar CO₂/CH₄ and CH₄/N₂ are 3.19 and 7.62, respectively, and the adsorption capacities for CO₂, CH₄, and N₂ are 2.91, 1.01, and 0.13 mmol g-1, respectively. This report introduces a simple pathway to obtain enhanced N-doped porous carbon with large adsorption capacities for gas separation of CO₂/CH₄ and CH₄/N₂.

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

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

In situ Formation of a Monodispersed Spherical Mesoporous Nanosilica-Torlon Hollow-Fiber Composite for Carbon Dioxide Capture

We describe a new template-free method for the in situ formation of a monodispersed spherical mesoporous nanosilica-Torlon hollow-fiber composite. A thin layer of Torlon hollow fiber that comprises silica nanoparticles was created by the in situ extrusion of a tetraethyl orthosilicate/N-methyl-2-pyrrolidone solution in a sheath layer and a Torlon polymer dope in a core support layer. This new method can be integrated easily into current hollow-fiber composite fabrication processes. The hollow-fiber composites were then functionalized with 3-aminopropyltrimethoxy silane (APS) and evaluated for their CO2 -capture performance. The resulting APS-functionalized mesoporous silica nanoparticles/Torlon hollow fibers exhibited a high CO2 equilibrium capacity of 1.5 and 1.9 mmol g(-1) at 35 and 60 °C, respectively, which is significantly higher than values for fiber sorbents without nanoparticles reported previously.

Polyfuran-Derived Microporous Carbons for Enhanced Adsorption of CO₂ and CH₄

Oxygen-doped microporous carbons were synthesized by chemical activation of polyfuran with KOH or ZnCl2 at 600 and 800 °C. It was found that KOH preserves and ZnCl2 eliminates the O-C functional groups in the activation process. The O-doped carbon activated with KOH at 800 °C exhibited a high CO2 capacity (4.96 mmol g(-1), 273 K, 1 bar) and CH4 adsorption capacity (2.27 mmol g(-1), 273 K, 1 bar). At 298 K and 1 bar, a very high selectivity for separating CO2/N2 (41.7) and CO2/CH4 (6.8) gas mixture pairs was obtained on the O-doped carbon activated with KOH at 600 °C. The excellent separation ability of the O-doped carbons was demonstrated in transient breakthrough simulations of CO2/CH4/N2 mixtures in a fixed bed adsorber. The isosteric adsorption heats of the O-doped carbons were also significantly lower than those of MOF-74 and NaX zeolite. The O-doped microporous carbon adsorbents appear to be a very promising adsorbent for CO2 capture from flue gas, biogas upgrading, and CH4 storage.

Hydroquinone and Quinone-Grafted Porous Carbons for Highly Selective CO2 Capture from Flue Gases and Natural Gas Upgrading

Hydroquinone and quinone functional groups were grafted onto a hierarchical porous carbon framework via the Friedel-Crafts reaction to develop more efficient adsorbents for the selective capture and removal of carbon dioxide from flue gases and natural gas. The oxygen-doped porous carbons were characterized with scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. CO2, CH4, and N2 adsorption isotherms were measured and correlated with the Langmuir model. An ideal adsorbed solution theory (IAST) selectivity for the CO2/N2 separation of 26.5 (298 K, 1 atm) was obtained on the hydroquinone-grafted carbon, which is 58.7% higher than that of the pristine porous carbon, and a CO2/CH4 selectivity value of 4.6 (298 K, 1 atm) was obtained on the quinone-grafted carbon (OAC-2), which represents a 28.4% improvement over the pristine porous carbon. The highest CO2 adsorption capacity on the oxygen-doped carbon adsorbents is 3.46 mmol g(-1) at 298 K and 1 atm. In addition, transient breakthrough simulations for CO2/CH4/N2 mixture separation were conducted to demonstrate the good separation performance of the oxygen-doped carbons in fixed bed adsorbers. Combining excellent adsorption separation properties and low heats of adsorption, the oxygen-doped carbons developed in this work appear to be very promising for flue gas treatment and natural gas upgrading.

CO2 adsorption on diatomaceous earth modified with cetyltrimethylammonium bromide and functionalized with tetraethylenepentamine: Optimization and kinetics

The carbon dioxide (CO2) adsorbent diatomaceous earth (DE) was modified with cetyltrimethylammonium bromide (CTAB) and functionalized with varying levels of tetraethylenepentamine (TEPA). The CO2 absorption at atmospheric pressure was optimized by varying the TEPA-loading level (0-40% (w/w)), operating temperature (40-80 °C) and water vapor concentration (0-16% (v/v)) in a 10% (v/v) CO2 feed stream in helium balance using a full 2(3) factorial design. The TEPA/CTAB-DE adsorbents were characterized by X-ray diffractometry, Fourier transform infrared spectrometry and thermogravimetric analyses. The CO2 adsorption capacity increased as each of these three factors increased. The TEPA loading level-water concentration interaction had a positive influence on the CO2 adsorption while the operating temperature-water concentration interaction was antagonistic. The optimal condition for CO2 adsorption on 40%TEPA/CTAB-DE, evaluated via a factorial design response surface method (RSM), was a temperature of 58-68 °C and a water vapor concentration of 9.5-14% (v/v), with a maximum CO2 adsorption capacity of 149.4 mg g(-1) at 63.5 °C and 12% (v/v) water vapor concentration in the feed. Validation and sensitivity tests revealed that the estimated CO2 adsorption capacity was within ±4% of the experimental values, suggesting that the RSM model was satisfied and acceptable. From three kinetic models (pseudo-first-order, pseudo-second-order model and Avrami's equation), assessed using an error function (Err) and the coefficient of determination (R(2)), Avrami's equation was the most appropriate to describe the kinetics of CO2 adsorption on the 40%TEPA/CTAB-DE adsorbent and suggested that more than one reaction pathway occurred in the CO2 adsorption.

A computational study of carbon dioxide adsorption on solid boron

The study demonstrates these “electron deficient” boron solids can capture CO₂ on their basic sites due to Lewis acid–base interactions.

Adsorption of CO₂, CH₄, and N₂ on ordered mesoporous carbon: approach for greenhouse gases capture and biogas upgrading

Separation of CO₂ and N₂ from CH₄ is significantly important in natural gas upgrading, and capture/removal of CO₂, CH₄ from air (N₂) is essential to greenhouse gas emission control. Adsorption equilibrium and kinetics of CO₂, CH₄, and N₂ on an ordered mesoporous carbon (OMC) sample were systematically investigated to evaluate its capability in the above two applications. The OMC was synthesized and characterized with TEM, TGA, small-angle XRD, and nitrogen adsorption/desorption measurements. Pure component adsorption isotherms of CO₂, CH₄, and N₂ were measured at 278, 298, and 318 K and pressures up to 100 kPa, and correlated with the Langmuir model. These data were used to estimate the separation selectivities for CO₂/CH₄, CH₄/N₂, and CO₂/N₂ binary mixtures at different compositions and pressures according to the ideal adsorbed solution theory (IAST) model. At 278 K and 100 kPa, the predicted selectivities for equimolar CO₂/CH₄, CH4/N₂, and CO₂/N₂ are 3.4, 3.7, and 12.8, respectively; and the adsorption capacities for CH₄ and CO₂ are 1.3 and 3.0 mmol/g, respectively. This is the first report of a versatile mesoporous material that displays both high selectivities and large adsorption capacities for separating CO₂/CH₄, CH₄/N₂, and CO₂/N₂ mixtures.

Computational study of adsorption and separation of CO2, CH4, and N2 by an rht-type metal-organic framework

In this work, a computational study is performed to evaluate the adsorption-based separation of CO(2) from flue gas (mixtures of CO(2) and N(2)) and natural gas (mixtures of CO(2) and CH(4)) using microporous metal organic framework Cu-TDPAT as a sorbent material. The results show that electrostatic interactions can greatly enhance the separation efficiency of this MOF for gas mixtures of different components. Furthermore, the study also suggests that Cu-TDPAT is a promising material for the separation of CO(2) from N(2) and CH(4), and its macroscopic separation behavior can be elucidated on a molecular level to give insight into the underlying mechanisms. On the basis of the single-component CO(2), N(2), and CH(4) isotherms, binary mixture adsorption (CO(2)/N(2) and CO(2)/CH(4)) and ternary mixture adsorption (CO(2)/N(2)/CH(4)) were predicted using the ideal adsorbed solution theory (IAST). The effect of H(2)O vapor on the CO(2) adsorption selectivity and capacity was also examined. The applicability of IAST to this system was validated by performing GCMC simulations for both single-component and mixture adsorption processes.