High CO2 Uptake Research Articles

Valorization of shrimp shell biowaste for environmental remediation: Efficient contender for CO2 adsorption and separation

Over the years, single heteroatom-doped biowaste-derived activated carbons were studied for effective CO2 adsorption. However, binary or ternary heteroatoms-doping is equally important and could significantly affect the CO2 adsorption and flue gas (i.e., CO2/N2) separation. Herein, for the first time, shrimp shell-derived chitosan was used to design a series of ternary (N, S, O)-doped hierarchically porous carbons. The resultant carbons exhibit a large specific surface area (up to 2095 m2/g), micropore volume (up to 1.2647 cm3/g), and high heteroatoms content i.e., N up to 4.1 at. %, S up to 4.6 at. %, and O up to 13.4 at. %. Consequently, high CO2 uptake of 236.80 mg/g at 273 K/1 bar and an excellent CO2/N2 gas selectivity (84.3) was observed, attributed to the synergistic role of narrow micropores (<1 nm) and optimum heteroatom content. Furthermore, the stable CO2 adsorption-desorption cyclic behavior under flue gas conditions i.e., 15% CO2/85% N2 reveals the physisorption mechanism of CO2 adsorption and appears to be an energy-efficient regeneration process. Concluding, our work demonstrates a facile route of valorization of biowaste for environmental remediation to combat biowaste accumulation and mitigating atmospheric CO2 levels, simultaneously.

Pyrolysis of sulfonic acid substituted benzenes and investigation of CO2 capture capability of resulting carbons

Aromatic organic compounds5-sulfosalicylic acid (SSA) and p-toluenesulfonic acid (TsOH) were used to prepare sulfur doped ultramicropore (pores smaller than 0.7 ​nm) containing carbons by pyrolysis under Argon atmosphere. SSA derived carbon processed at 1000 ​°C showed the best CO2 (carbon dioxide) adsorption performance. CO2 sorption capacity of SSA1000 is 3.91 ​mmol g-1at 1 ​bar at 273 ​K and 2.88 ​mmol g-1at 1 ​bar at 298 ​K. Remarkably, at typical flue gas conditions (0.15 ​bar@298 ​K) SSA1000 takes up 0.95 ​mmol ​g-1 CO2. SSA1000 possessed good gas sorption properties, recyclability for regeneration, and Ideal Adsorbed Solution Theory (IAST)-based selectivity of 14.7 (1 ​bar, CO2/N2:0.15/0.85). This work shows the possibility of production of ultramicropore containing sulfur and oxygen doped carbons exhibiting high CO2 uptake capability via simple pyrolysis method from molecular precursors.

Preparation and characterization of templated porous carbons from sucrose by one-pot method and application as a CO2 adsorbent.

The templated porous carbons were prepared from sucrose by one-pot method. In this method in which the pre-synthesis of the hard template is eliminated, the porous carbons were produced by organic-inorganic self-assembly of sucrose, tetraethyl ortosilicate (TEOS), Pluronic P123 and n-butanol in an acidic medium, and subsequent carbonization. The synthesis parameters such as sucrose amount, TEOS molar ratio and carbonization temperature were evaluated for describing their effects on the pore structures of the synthesized carbons. The prepared porous carbons were characterized by N2 adsorption, thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) techniques. The carbon dioxide adsorption uptakes of the obtained porous carbons were determined at 1 bar and 273 K. The templated carbon obtained with the lowest TEOS molar ratio exhibited the highest BET surface area of 1289 m2/g and micropore volume of 0.467 cm3/g, and showed the highest CO2 uptake of 2.28 mmol/g.

Influence of Hydrothermal Pretreatment Temperature on the Hydration Properties and Direct Carbonation Efficiency of Al-Rich Ladle Furnace Refining Slag

The influence of hydrothermal pretreatment temperature on the hydration products and carbonation efficiency of Al-rich LF slag was investigated. The results showed that the carbonation efficiency was strongly dependent on the morphology of hydration products and the hydration extent of the raw slag. Hydrothermal pretreatment at 20 °C or 80 °C favored the formation of flake-shaped products with a higher specific surface area and therefore resulted in a higher CO2 uptake of 20 °C and 80 °C-pretreated slags (13.66 wt% and 10.82 wt%, respectively). However, hydrothermal pretreatment at 40 °C, 60 °C or 100 °C led to the rhombohedral-shaped calcite layer surrounding the unreacted core of the raw slag and the formation of fewer flake-shaped products, resulting in a lower CO2 uptake of 40 °C, 60 °C and 100 °C-pretreated slags (9.21 wt%, 9.83 wt%, and 6.84 wt%, respectively).

The CO2 sequestration by supercritical carbonation of electric arc furnace slag

The current study puts the emphasis on developing a supercritical carbonation process of electric arc furnace slag for the CO2 sequestration. In this process, the feed is mixed with water and processed with supercritical CO2 at 40−80 °C and 80–120 bar for 48 h. This process offers several advantages including higher reactivity, faster kinetics, less waste generation, better economic feasibility, and less pretreatment compared with aqueous carbonation. The empirical model established by using a systematic design of experiments methodology is demonstrated to be suitable for calculating the carbonation efficiency as a function of operating parameters and performing the process optimization. With fundamental investigations into carbonation mechanisms, the slag particle size is determined to have the most significant positive impact on carbonation efficiency. The sample with a smaller particle size has a higher total carbonated volume, resulting in higher CO2 uptake. Fundamental investigations into the carbonation process indicate that the diffusion barrier to the carbonation process is a product calcium carbonate layer formed on the outer shell of the slag particle and the thickness of this layer is constant regardless of the slag particle size. Furthermore, it was revealed that the reaction mechanism of the carbonation process is mainly driven by the hydrogen ion which reacts with dicalcium silicate in EAF slag and produces dissolved calcium ions. In this study, the reaction pathway for supercritical carbonation of EAF slag is elucidated and the results of mechanistic investigations shed light on the diffusion barrier to the carbonation process and the rate-determining step. We expect the results of this study help enable the carbonation of industrial byproducts, in particular, steelmaking slag.

Channeling carbon dioxide

Gas Separation The separation of gas molecules with physisorbents can be challenging because there is often a tradeoff between capacity and selectivity. Zhou et al. report a template-free hydrothermal synthesis of the one-dimensional channel zeolite mordenite, in which some silicon was replaced by iron. Rather than forming a powder that requires further shaping, this mechanically stable material self-assembled into monoliths. Iron atoms bound in tetrahedral zeolite sites narrowed the channels and enabled the size-exclusion separation of carbon dioxide (CO2) over nitrogen (N2) and methane. High CO2 uptake and highly efficient CO2–N2 separation was demonstrated for both dry and humid conditions. Science , aax5776, this issue p. [315][1] [1]: /lookup/doi/10.1126/science.aax5776

Methane fluxes but not respiratory carbon dioxide fluxes altered under Si amendment during drying – rewetting cycles in fen peat mesocosms

Peatlands are globally important sinks and sources for greenhouse gasses. Their carbon (C) balances are dependent on input of biomass, its quality and decomposition. Silicon (Si) is known to influence the C quality, especially of graminoids, and it influences decomposition under oxic conditions. Drying – rewetting events (D/W), which may influence C mineralization and change nutrient availability, have been predicted to increase in frequency. This raises the question if effects of Si on decomposition are modified by such D/W events.To elucidate this question, we conducted a study with mesocosms extracted from a fen peatland.We hypothesized, that (I, II) there would be higher gas and solute concentrations after Si addition compared to the control after D/W. Further we expected (III) that higher respiratory fluxes of CO2 and CH4 would occur after Si addition and D/W and that (IV) there would be a higher gross primary production (GPP) under Si addition due to more viable biomass that is less prone to drought effects. Finally we hypothesized that (V) the chemical composition of Carex rostrata Stokes leaves grown under Si fertilization after a D/W cycle would be less recalcitrant compared to of C. rostrata leaves from the controls. To test these hypotheses, a control and a Si treatment group of mesocosms were subjected to a controlled D/W cycle. We measured soil gas (DIC and CH4), and solute concentrations (P, Si, Fe and TOC) and gas fluxes of CO2 and CH4. Furthermore, we characterized the graminoid biomass for major element contents and by infrared spectroscopy (FTIR) for organic matter quality.Higher CH4 fluxes were observed in the Si fertilized mesocosms. There was no difference between the treatments in the ER CO2 fluxes, whereas the NEE and GPP fluxes indicated higher CO2 uptake in the Si treated mesocosms. Solute concentrations showed no differences, neither for dissolved gasses , nor for solutes. C. rostrata leaves had lower shares of lignin, waxes, fats, and phenols in plants from the Si fertilized mesocosms.We concluded, that during D/W, plants from Si fertilized mesocosms accumulated more Si than plants from the control mesocosms. These plants were more viable, produced potentially more exudates and had more readily decomposable litter than controls, due to their lower share in recalcitrant moieties. This may have stimulated CH4 production and emission of CH4 from the soil into the atmosphere through abundant aerenchyma roots, bypassing oxic, methanotrophic peat layers.

Self-assembled iron-containing mordenite monolith for carbon dioxide sieving.

The development of low-cost, efficient physisorbents is essential for gas adsorption and separation; however, the intrinsic tradeoff between capacity and selectivity, as well as the unavoidable shaping procedures of conventional powder sorbents, greatly limits their practical separation efficiency. Herein, an exceedingly stable iron-containing mordenite zeolite monolith with a pore system of precisely narrowed microchannels was self-assembled using a one-pot template- and binder-free process. Iron-containing mordenite monoliths that could be used directly for industrial application afforded record-high volumetric carbon dioxide uptakes (293 and 219 cubic centimeters of carbon dioxide per cubic centimeter of material at 273 and 298 K, respectively, at 1 bar pressure); excellent size-exclusive molecular sieving of carbon dioxide over argon, nitrogen, and methane; stable recyclability; and good moisture resistance capability. Column breakthrough experiments and process simulation further visualized the high separation efficiency.

The feasibility of CO2 emission reduction by adsorptive storage on Polish hard coals in the Upper Silesia Coal Basin: An experimental and modeling study of equilibrium, kinetics and thermodynamics

Carbon dioxide storage in unmineable coal seams is advantageous in the highly industrialized areas, such as the Upper Silesia Coal Basin (USCB), Poland, where heavy industry constitutes the source of huge CO2 emissions and coal mines will be closed in the future, due to unprofitability. The paper presents the results of experimental and theoretical research of CO2 capture on medium rank C and B bituminous coals coming from three mines located in the USCB. The porous texture of the investigated adsorbents was analyzed using SEM images and the N2 and CO2 isotherms at −196 °C and 0 °C, respectively. Qualitative studies using DRIFT spectroscopy showed that band intensity attributed to the functional groups of coals changed after CO2 adsorption. The analyses encompassed the equilibrium, kinetics and thermodynamics of CO2 adsorption on coals at 25, 50 and 75 °C (up to 2000 kPa). The adsorption isotherms were obtained by the static gravimetric method and described by means of the Langmuir, Freundlich, Dubinin-Radushkevich and Dubinin-Astakhov models. The highest CO2 uptakes were obtained for medium rank C bituminous coals at 25 °C; the values were 1.600 mol/kg and 1.274 mol/kg. The adsorption kinetics was better characterized by the Avrami fractional-order model rather than by the pseudo-first and pseudo-second order models. The results reveal that the adsorption process is the fastest for medium rank C bituminous coals. The isosteric heats of adsorption were calculated in the following two ways: based on the multi-temperature Toth isotherm and the Clausius-Clapeyron equations. Depending on degree of coal metamorphism, the heat of adsorption ranged from 18 to 26 kJ/mol. The estimated maximum temperature increase due to heat accumulation in the insulated coalbed during CO2 adsorption was 6 °C and did not reach the self-ignition temperature in any of the tested adsorption systems.

Synthesis of High-Quality Mg-MOF-74 Thin Films via Vapor-Assisted Crystallization.

The unique features of metal-organic frameworks (MOFs), such as their large surface areas and diversity of structures, make them suitable for a broad range of applications including storage, separation, and sensing of gases. Among all the MOFs, Mg-MOF-74 with the highest CO2 uptake at 1 bar and 25 °C would be particularly beneficial for CO2-related applications. One of the most critical enabling technologies for implementing Mg-MOF-74 is the preparation of dense and continuous films that would maximize the sorption behaviors. However, Mg-MOF-74 thin films present significant challenges in demonstrating large-scale coatings. Herein, we demonstrate for the first time high-quality Mg-MOF-74 films synthesized via a vapor-assisted crystallization (VAC) process. The VAC process described herein provides dense and highly crystalline layers of the Mg-MOF-74 thin film with a low coefficient of variation of film thickness below 7%. By minimizing the solvent use, the VAC process is also more environmentally friendly than conventional techniques. In this work, we first optimized a precursor solution for the VAC process and then investigated the effects of synthesis temperature, time, and droplet volume on the growth, crystallinity, and thickness of VAC Mg-MOF-74 films. The porosity of the MOF film was assessed by measuring the CO2 uptake at room temperature and 1 bar. The obtained VAC Mg-MOF-74 films possess a well-defined microporosity, as deduced from CO2 adsorption studies via quartz crystal microbalance (QCM) and comparison with bulk Mg-MOF-74 reference data. Furthermore, CO2 cyclic adsorption-desorption experiments on the VAC Mg-MOF-74 films showed scaled uptakes to a wide range of CO2 concentration without showing significant variations in the baseline. We specifically demonstrate how the film's quality of the MOF affects adsorption behavior of CO2 on VAC Mg-MOF-74 and drop-cast Mg-MOF-74 films.

Effect of Calcination Temperature and Chemical Composition of PAN-Derived Carbon Microfibers on N2, CO2, and CH4 Adsorption.

This work investigates the interplay of carbonization temperature and the chemical composition of carbon microfibers (CMFs), and their impact on the equilibration time and adsorption of three molecules (N2, CO2, and CH4). PAN derived CMFs were synthesized by electrospinning and calcined at three distinct temperatures (600, 700 and 800 °C), which led to samples with different textural and chemical properties assessed by FTIR, TGA/DTA, XRD, Raman, TEM, XPS, and N2 adsorption. We examine why samples calcined at low/moderate temperatures (600 and 700 °C) show an open hysteresis loop in nitrogen adsorption/desorption isotherms at −196.15 °C. The equilibrium time in adsorption measurements is nearly the same for these samples, despite their distinct chemical compositions. Increasing the equilibrium time did not allow for the closure of the hysteresis loop, but by rising the analysis temperature this was achieved. By means of the isosteric enthalpy of adsorption measurements and ab initio calculations, adsorbent/adsorbate interactions for CO2, CH4 and N2 were found to be inversely proportional to the temperature of carbonization of the samples (CMF-600 > CMF-700 > CMF-800). The enhancement of adsorbent/adsorbate interaction at lower carbonization temperatures is directly related to the presence of nitrogen and oxygen functional groups on the surface of CMFs. Nonetheless, a higher concentration of heteroatoms also causes: (i) a reduction in the adsorption capacity of CO2 and CH4 and (ii) open hysteresis loops in N2 adsorption at cryogenic temperatures. Therefore, the calcination of PAN derived microfibers at temperatures above 800 °C is recommended, which results in materials with suitable micropore volume and a low content of surface heteroatoms, leading to high CO2 uptake while keeping acceptable selectivity with regards to CH4 and moderate adsorption enthalpies.

Application of Metal-Organic Frameworks (MOFs) for Capturing CO2: Advancement and Challenges

The development of suitable solutions for capturing CO2 is one of the leading technical scenarios, among others, to mitigate greenhouse effect. Among all splitting techniques that could potentially address this important challenge, membrane and swing adsorption technologies are recognized to be of great promise for a variety of CO2 containing gas streams in industry. Nevertheless, the deployment of such technologies requires advanced materials with excellent/suitable thermodynamic and kinetics features that are further married to an optimally engineered design. The ultimate purpose is to achieve the desired high CO2 capturing capacity and efficiency. The targeted materials should possess adequate affinity toward CO2, in addition to high CO2 uptake and excellent chemical stability toward impurities such as SOx and NOx. Metal-Organic Frameworks (MOFs), solid-sate materials consisting of metal ions or clusters coordinated to organic ligands, showed technically interesting capabilities for gas splitting in general and CO2 capture in particular. This review presents an overview about the perspective of applying MOFs for capturing CO2 from different sources. The authors offer multidisciplinary discussion about the different aspects that would be key elements in progressing or cutting the path of research, development and innovation to final deployment of MOF as adsorbent for capturing CO2. An overview about the main MOFs with reported studies on CO2 capture from different sources will be proposed. Some general direction on how to design MOFs in order to address the trade-off of capacity vs. selectivity, which is highly desirable for large-scale CO2 emitting industries, will also, be given.

Molten Salt Templated Synthesis of Covalent Isocyanurate Frameworks with Tunable Morphology and High CO2 Uptake Capacity.

The use of reactive molten salts, i.e., ZnCl2, as a soft template and a catalyst has been actively investigated in the preparation of covalent triazine frameworks (CTFs). Although the soft templating effect of the salt melt is more prominent at low temperatures, close to the melting point of ZnCl2, leading to the formation of abundant micropores, a significant mesopore formation is observed that is due to the partial carbonization and other side reactions at higher temperatures (>400 °C). Evidently, high-temperature synthesis of CTFs in various eutectic salt mixtures of ZnCl2 with alkali metal chloride salts also leads to mesopore formation. We reasoned that using the isocyanate moieties instead of cyano groups in the monomer, 1,4-phenylene isocyanate, could enable efficient interactions between carbonyl moieties and alkali metal ions to realize efficient salt templating to form covalent isocyanurate frameworks (CICFs). In this direction, the trimerization of 1,4-phenylene diisocyanate was carried out under ionothermal conditions at different reaction temperatures using ZnCl2 (CICF) and the eutectic salt mixture of KCl/NaCl/ZnCl2 (CICF-KCl/NaCl) as the reactive solvents. We observed notable differences in the morphologies of the two polymers, whereas CICF showed irregular-shaped micrometer-sized particles, the CICF-KCl/NaCl exhibited a filmlike morphology. Moreover, favorable ion-dipole interactions between alkali metal cations and oxygen atoms of the monomer facilitated two-dimensional growth and the formation of a purely microporous framework in the case of CICF-KCl/NaCl along with a near theoretical retention of the nitrogen content at 500 °C. The CICF-KCl/NaCl showed a BET surface area of 590 m2 g-1 along with a CO2 uptake capacity of 5.9 mmol g-1 at 273 K and 1.1 bar because of its high microporosity and nitrogen content. On the contrary, in the absence of alkali metal ions, CICF showed high mesopore content and a moderate CO2 uptake capacity. This study underscores the importance of the strength of the interactions between the salts and the monomer in the ionothermal synthesis to control the morphology, porosity, and gas uptake properties of the porous organic polymers.

Synthesis of SAPO-56 using N,N,N’,N’-tetramethyl-1,6-hexanediamine and co-templates based on primary, secondary, and tertiary amines

Biomethane is a renewable fuel with a small environmental footprint. In its production, the removal of CO2 from the fermentation gas is critical. Pressure and vacuum swing adsorption (PSA and VSA) processes have certain advantages over other processes for the removal. Silicoaluminophosphate-56 (SAPO-56) has promising properties as an adsorbent for PSA- or VSA-based upgrading of raw biogas. It is typically synthesized by using N,N,N’,N’-tetramethyl-1,6-hexanediamine (TMHD) as a structure directing agent (SDA). In this study, TMHD was partly replaced with three different low-cost templates: isopropylamine (IPA), dibutylamine, and tripropylamine. SAPO-56 was co-crystallized with mixtures of templating amines with up to a ratio of 30%:70% of TMHD:IPA. With using TMHD and IPA, small and defined crystals of SAPO-56 plus SAPO-47 formed instead of the large aggregates of SAPO-56 that formed when only TMHD was used. Solid-state 13C NMR spectroscopy was used to show that the IPA and TMHD had not been decomposed and that both molecules were included within the as-synthesized crystals of SAPO-56. Synthetic composition diagrams were drawn with respect to the P2O5, SiO2, and Al2O3 compositions of the reaction mixtures and the formed crystalline SAPOs. In relation to these diagrams, the domains for stability of SAPO-56 were contrasted with those of SAPO-11, -17, -20, and -47. In particular, it was observed that SAPO-47 co-crystallized with SAPO-56 when a very large fraction of IPA was used under otherwise optimized conditions. As consistent with other studies, the SAPO-56 synthesized with dual SDAs had a very high uptake of CO2 at conditions relevant for PSA- or VSA-driven upgrading of raw biogas into methane.

Frontispiece: Host–Guest Interaction Modulation in Porous Coordination Polymers for Inverse Selective CO2/C2H2 Separation

Porous Materials In their Communication on page 11688, Fengting Li, Susumu Kitagawa et al. report two new microporous coordination polymers exhibiting high CO2 uptake and strong separation performance for C2H2 purification at room temperature.

Truxene/triazatruxene-based conjugated microporous polymers with flexible@rigid mutualistic symbiosis for efficient CO2 storage

Conjugated microporous polymers (CMPs), with rigid π‒conjugated skeletons and tunable microporous structure, have shown great potential as gas storage and sequestration materials, however, the inherent poor solubility of initial building block in the polymerization process always leads to low crosslinking polymers and limited application. Here, in this work, we have designed the flexible@rigid mutualistic symbiosis in two series of CMPs (2D extend truxene and triazatruxene cores, P‒C(‒Et) and P‒N(‒Et). The results demonstrated that rational flexible groups with additional rigid skeletons structure could change the 2D C3 symmetric building core into 3D steric configuration, which increased the target polymers' BET surface areas more than 3 times and narrowed the microporous width. The optimized molecule structure delivers a high CO2 uptake ability, 5.8–11.5 wt% for truxene, and 7.8–17.6 wt% for triazatruxene at 273 K and 1.0 bar. Besides, the optimized structures also have superior selectivity of CO2 when the N2 or CH4 still exists in the system. In conclusion, this work provides a fundamental understanding for the design of CMPs for high CO2 adsorption and separation performances.

In-situ amino-functionalization of zeolitic imidazolate frameworks for high-efficiency capture of low-concentration CO2 from flue gas

The capture of low-concentration CO2 from flue gas is an on-going challenge. In this work, a novel series of amino-functionalized zeolitic imidazolate framework-8 materials (ZIF-DIA) was facilely synthesized via in-situ incorporating an large-size amino-functional imidazolium ligand during the synthesis process under atmospheric conditions, and its adsorption performance of CO2 from a simulated flue gas (15 vol% CO2) was investigated. With the introduction of amino-functional imidazolium ligand, the regular hexahedron structure of pure ZIF-8 is gradually converted into cluster-like morphology. The amino-functionalized ZIF materials exhibit high specific surface areas (712–941 m2·g−1), rapid CO2 adsorption rate (≤80 s), excellent CO2 adsorption capacities of up to 6.17 mmol·g−1 under wide temperature range (35–110 °C), overwhelming that of pure ZIF-8 (≤1.96 mmol·g−1) and other reported metal–organic frameworks (MOFs). DFT calculation verified that π-π stacking and hydrogen bonding between imidazole/aromatic rings on ligands and CO2 play crucial role in high CO2 uptake. Excellent recyclability in 10 runs under both low and high temperatures (55 °C, 110 °C) is exhibited, endowing this kind of materials with great potential for practical applications.

Construction of a 3D porous Zn(II) compound with (3,4)‐connected topology: luminescent and gas sorption properties

AbstractA new 3D porous Zn(II) compound [Zn2(L)2(C2O4)]n ⋅ 2(DMA) (1, HL=4‐(4‐carboxyphenyl)‐2,2′ : 4′,4′′‐terpyridine, DMA=N, N’‐dimethylacetamide) was successfully synthesized with carboxyphenyl‐terpyridine ligand. X‐ray structural analysis revealed that compound 1 features a 2‐fold interpenetrated framework with (3,4)‐connected network topology. Despite interpenetration, compound 1 still contains high solvent‐accessible free volume owing to the existence of large 1D nanotube channels. Gas adsorption properties investigation of the activated 1 a confirms its high BET surface area, high CO2 uptake capacity and excellent adsorption selectivity of CO2 over CH4. Luminescent properties investigation indicate that compound 1 displays intense luminescence at room temperature.

Design of Graphene/Ionic Liquid Composites for Carbon Capture.

Pore size is a crucial factor impacting gas separation in porous separation materials, but how to control the pore size to optimize the separation performance remains a challenge. Here, we propose a design of graphene/ionic liquid composites with tunable slit pore sizes, where cations and anions of ionic liquids are intercalated between graphene layers. By varying the sizes of the ions, we show from first-principles density functional theory calculations that the accessible pore size can be tuned from 3.4 to 6.0 Å. Grand canonical Monte Carlo simulations of gas sorption find that the composite materials possess high CO2 uptake at room temperature and 1 bar (up to ∼8.5 mmol/g). Further simulations of the sorption of gas mixtures reveal that high CO2/N2 and CO2/CH4 adsorption selectivities can be obtained when the accessible pore size is <5 Å. This work suggests a new strategy to achieve tunable pore sizes via the graphene/IL composites for highly selective CO2/N2 and CO2/CH4 adsorption.

Biogas Upgrading via Cyclic CO2 Adsorption: Application of Highly Regenerable PEI@nano-Al2O3 Adsorbents with Anti-Urea Properties.

Solid amine adsorbents are among the most promising CO2 adsorption technologies for biogas upgrading due to their high selectivity toward CO2, low energy consumption, and easy regeneration. However, in most cases, these adsorbents undergo severe chemical inactivation due to urea formation when regenerated under a realistic CO2 atmosphere. Herein, we demonstrated a facile and efficient synthesis route, involving the synthesis of nano-Al2O3 support derived from coal fly ash with a CO2 flow as the precipitant and the preparation of polyethylenimine (PEI)-impregnated Al2O3-supported adsorbent. The optimal 55%PEI@2%Al2O3 adsorbent showed a high CO2 uptake of 139 mg·g-1 owing to the superior pore structure of synthesized nano-Al2O3 support and exhibited stable cyclic stability with a mere 0.29% decay per cycle even under the realistic regenerated CO2 atmosphere. The stabilizing mechanism of PEI@nano-Al2O3 adsorbent was systematically demonstrated, namely, the cross-linking reaction between the amidogen of a PEI molecule and nano-Al2O3 support, owing to the abundant Lewis acid sites of nano-Al2O3. This cross-linking process promoted the conversion of primary amines into secondary amines in the PEI molecule and thus significantly enhanced the cyclic stability of PEI@nano-Al2O3 adsorbents by markedly inhibiting the formation of urea compounds. Therefore, this facile and efficient strategy for PEI@nano-Al2O3 adsorbents with anti-urea properties, which can avoid active amine content dilution from PEI chemical modification, is promising for practical biogas upgrading and various CO2 separation processes.