CH4 Adsorption Research Articles

Investigation on the Nanopore Heterogeneity in Coals and its Influence on Methane Adsorption: A Multifractal Theory Study.

Coalbed methane (CBM) reservoirs constitute a distinct class of dense organic rocks characterized by extremely low porosity and permeability. Conducting an in-depth investigation into pore heterogeneity assumes paramount importance for the exploration and development of CBM. This study focuses on the multifractal analysis of the pores with diameters below 300 nm in six coal samples sourced from the Junggar Basin and the Qinshui Basin in China. The analysis is based on a series of experiments involving CO2 adsorption, low-temperature N2 adsorption/desorption, and CH4 isothermal adsorption. This work delves into the influence of pore heterogeneity on gas adsorption capacity by linking the structural parameters to CH4 adsorption properties. The results indicate that both the micropores, as assessed through CO2 adsorption, and mesopores to macropores, measured via N2 adsorption, exhibit multifractal behavior. In contrast to micropores, the mesopores and macropores display stronger heterogeneity and lower connectivity. Generally, uniform and well-connected nanopores are anticipated to positively contribute to gas adsorption. However, there is a positive correlation between the Langmuir volume and the heterogeneity degree of micropores. This phenomenon is ascribed to the fact that the greater surface complexity in micropores involves a larger specific surface area and a higher abundance of adsorption sites. This research contributes to a more profound and precise comprehension of the heterogeneous pore structure within CBM reservoirs, thereby establishing a theoretical foundation for the sustainable exploitation of CBM.

Competitive adsorption of CO2/CH4 on coal: Insights from thermodynamics

The main reason for CO2 enhanced coalbed methane recovery is the competitive adsorption of CO2 and CH4 on coal. Therefore, it is necessary to study the equilibrium adsorption of CO2, CH4 and their mixed gases on coal over a wide range of pressures. This work investigates the mechanism adsorption of CO2/CH4 on coal by thermodynamics. The findings indicate that CO2 demonstrates a greater increase in Gibbs free energy and a larger decrease in entropy loss related to CH4, indicating that the adsorption process of CO2 is more to occur spontaneously. In CO2/CH4 mixed gas adsorption system, the amount of gas adsorbed for different gas ratios (yCO2:yCH4 = 1:3; yCO2:yCH4 = 2:2; yCO2:yCH4 = 3:1) falls between the amounts adsorbed for pure CO2 and CH4 components. As CO2 concentration rises, the amount of mixed gases adsorbed similarly increases. Additionally, the amount of CH4 adsorbed in any combination is lower than that of CO2, suggesting that CO2 has a predominant influence regarding the adsorption process. Furthermore, isosteric heat of adsorption for CO2 in the mixed gases exceeds that of CH4, suggesting a stronger connection between CO2 and the surface of the coal matrix during the competitive adsorption stage.

Synergistic effect of Ni and CeOx in NiCeOx/SSZ-13 catalyst for boosting activity and stability during dry reforming of methane

The stability of industrial catalysts for dry reforming of methane (DRM) reaction is still an urgent challenge. The multifunctional nanocatalyst of SSZ-13 supported Ni/CeOx bimetallic catalysts was successfully fabricated by the impregnation method. The Ni/CeOx species was dispersed highly in the microporous structure of SSZ-13, and the Ni active sites can activate the C-H bond dissociation of CH4, while the oxygen vacancy could promote the adsorption and activation of CO2. The NiCeOx/SSZ-13 catalyst showed excellent catalytic performance for dry reforming of methane, i.e., its conversion of CH4 and CO2 were 91 and 94%, and the ratio of H2/CO was about 0.88 at 750 °C after 50 h. Based on the results of CH4&CO2-TPSR and NH3-TPD, the synergy effect of NiCeOx species can promote the adsorption and activation of CH4 and CO2, and the coke on the surface during DRM is readily converted to CO by reacting with the surface oxygen from CeOx species, which effectively boost the elimination of carbon deposits. It highlights a new strategy for the fabrication of high-efficient zeolite catalysts for DRM to enhance the utilization of CH4 and CO2.

High-pressure adsorption of H2S, CO2 and CH4 on porous aromatic framework (PAF-30) at different temperatures

High-pressure adsorption of H2S, CO2 and CH4 on porous aromatic framework (PAF-30) at different temperatures

Fine-tuning the pore environment of ultramicroporous three-dimensional covalent organic frameworks for efficient one-step ethylene purification

The construction of functional three-dimensional covalent organic frameworks (3D COFs) for gas separation, specifically for the efficient removal of ethane (C2H6) from ethylene (C2H4), is significant but challenging due to their similar physicochemical properties. In this study, we demonstrate fine-tuning the pore environment of ultramicroporous 3D COFs to achieve efficient one-step C2H4 purification. By choosing our previously reported 3D-TPB-COF-H as a reference material, we rationally design and synthesize an isostructural 3D COF (3D-TPP-COF) containing pyridine units. Impressively, compared with 3D-TPB-COF-H, 3D-TPP-COF exhibits both high C2H6 adsorption capacity (110.4 cm3 g−1 at 293 K and 1 bar) and good C2H6/C2H4 selectivity (1.8), due to the formation of additional C-H···N interactions between pyridine groups and C2H6. To our knowledge, this performance surpasses all other reported COFs and is even comparable to some benchmark porous materials. In addition, dynamic breakthrough experiments reveal that 3D-TPP-COF can be used as a robust absorbent to produce high-purity C2H4 directly from a C2H6/C2H4 mixture. This study provides important guidance for the rational design of 3D COFs for efficient gas separation.

Dissociative Adsorption of Hydrogen Molecules at Al2O3 Inclusions in Steels and Its Implications for Gaseous Hydrogen Embrittlement of Pipelines

Hydrogen embrittlement (HE) of steel pipelines in high-pressure gaseous environments is a potential threat to the pipeline integrity. The occurrence of gaseous HE is subjected to associative adsorption of hydrogen molecules (H2) at specific “active sites”, such as grain boundaries and dislocations on the steel surface, to generate hydrogen atoms (H). Non-metallic inclusions are another type of metallurgical defect potentially serving as “active sites” to cause the dissociative adsorption of H2. Al2O3 is a common inclusion contained in pipeline steels. In this work, the dissociative adsorption of hydrogen at the α-Al2O3(0001)/α-Fe(111) interface on the Fe011¯ plane was studied by density functional theory calculations. The impact of gas components of O2 and CH4 on the dissociative adsorption of hydrogen was determined. The occurrence of dissociative adsorption of hydrogen at the Al2O3 inclusion/Fe interface is favored under conditions relevant to pipeline operation. Thermodynamic feasibility was observed for Fe and O atoms, but not for Al atoms. H atoms can form more stable adsorption configurations on the Fe side of the interface, while it is less likely for H atoms to adsorb on the Al2O3 side. There is a greater tendency for the occurrence of dissociative adsorption of O2 and CH4 than of H2, due to the more favorable energetics of the former. In particular, the dissociative adsorption of O2 is preferential over that of CH4. The Al-terminated interface exhibits a higher H binding energy compared to the O-terminated interface, indicating a preference for hydrogen accumulation at the Al-terminated interface.

Enhancing carbon capture efficiency in biogas upgrading: A comprehensive review on adsorbents and adsorption isotherms

Biogas results from the anaerobic digestion of organic materials, a reliable and sustainable process that simultaneously manages organic waste and generates renewable energy. However, the presence of secondary impurities, such as carbon dioxide (CO2) and other gases, in raw biogas diminishes its efficacy, significantly lowering its energy content and restricting its utility across industry sectors. Moreover, these impurities contribute to various health and environmental concerns, including their role in exacerbating climate change and global warming. Consequently, efficient separation of CO2 is essential for upgrading biogas. The interest in utilizing biogas as a transportation fuel or as a substitute for natural gas has spurred the advancement of biogas upgrading technologies. While various methods exist for biogas upgrading, those relying on carbon dioxide absorption stand out as particularly significant. Carbon capture efficiency in biogas upgrading pertains to the ability of a method to effectively capture and separate CO2 from biogas, typically composed of methane (CH4) and other gases. This process is crucial for producing high-quality biogas with minimal carbon emissions, thus promoting environmental sustainability. Enhancing the carbon capture efficiency of the biogas upgrading process is essential for reducing greenhouse gas emissions and promoting cleaner energy production. The efficacy of CO2 separation relies on adsorbents and adsorption isotherms, which are integral components of this process. Improving these elements is vital for enhancing biogas purity, ensuring its suitability for various applications, and mitigating its environmental footprint. Traditional methods enhance the carbon capture efficiency by employing adsorbents, such as zeolites and activated carbon, as well as by optimizing adsorption isotherms. Surface modifications and adjustments to process parameters have also led to improved CO2 selectivity over other gases. Traditional methods still have drawbacks, such poor selectivity, difficulties with regeneration, and scalability. These limitations draw attention to the necessity of ongoing optimization, investigating substitute materials, and gaining a thorough grasp of how capacities, kinetics, and selectivity interact. Adsorbents and adsorption isotherms are the main topics of this study’s thorough analysis, which examines the state of the art in increasing carbon capture efficiency in biogas upgrading. It discusses conventional methods, their drawbacks, and suggests alternate materials, customized adjustments, and optimization techniques as a means of achieving ongoing progress. It is suggested that customized changes, ongoing optimization, and investigation of substitute materials be used to increase the effectiveness of carbon capture. To guarantee consistency, the study suggested specific rules for the procurement, preparation, and calcining of materials such as eggshells. In addition, to balancing CO2 and CH4 adsorption, improving adsorbent composition and addressing scalability, long-term stability, and practical implementation challenges are critical. The results of this study direct future studies toward a more sustainable and efficient energy landscape by adding to our understanding of carbon capture in biogas upgrading.

Competitive adsorption of CO2, N2, and CH4 in coal-derived asphaltenes, a computational study

Greenhouse gases are major contributors to global warming, and their concentration is increasing due to the widespread use of fossil fuels. Coal bed methane (CBM) offers a potential solution to this issue. However, the gas adsorption mechanisms of CBM, particularly in the context of coal-derived asphaltenes, are not fully understood. This study provides a comprehensive theoretical investigation of the competitive adsorption of carbon dioxide (CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}), methane (CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document}), and nitrogen (N2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}) in the processes of CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}- and N2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}-enhanced coalbed methane recovery, with a focus on coal-derived asphaltenes functionalized with CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document}, NH, O, and S groups. Using the Grand Canonical Monte Carlo (GCMC) simulation method and performing Molecular Dynamics (MD) simulations, we studied the adsorption process. To investigate the electronic effects and nature of the interactions, we performed density functional theory (DFT) calculations. The adsorption energy values and non-covalent interactions (NCI) for the adsorption of gases signify the physical adsorption (van der Waals interaction), with CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} exhibiting the highest (absolute) adsorption energy. The Monte Carlo results indicated that elevated temperatures led to a reduction in adsorption capacity. Coal-derived asphaltenes demonstrated greater selectivity for CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} compared to CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document} and N2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} in competitive adsorption, especially at elevated temperatures. Our findings highlight the significant potential of our asphaltene model, not only in mitigating CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} greenhouse gas emissions but also in recovering CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document}, which is a valuable resource.

Molecular dynamics insights into gas-water interfacial tension: Optimizing hydrogen storage in subsurface conditions

This study presents a comprehensive analysis of interfacial tension (IFT) between pore water and hydrogen-cushion gas mixtures in subsurface porous media, a key variable in the distribution of gas mixtures for hydrogen storage. Employing molecular dynamics simulations, we developed two IFT models at 20 MPa and 373 K: one for an H2–CO2–H2O system and another for an H2–CH4–H2O system. The models reveal that increasing cushion gas concentration reduces gas-water IFT. Notably, for H2–CO2–H2O mixtures, the IFT substantially decreases with CO2 concentration up to 40%, after which the reduction rate diminishes. In H2–CH4–H2O mixtures, the IFT decreases linearly with CH4 concentration. The study attributes these variations to local density distributions and molecular orientation effects. Specifically, CO2 adsorption at the interface up to 40% concentration significantly lowers IFT, while CH4 adsorption proportionally decreases IFT. The influence of CO2 on water molecule orientation at the interface, in contrast to the non-effect of CH4, is also critical in enhancing IFT reduction. Our results also highlight the reduction of H2 self-diffusion at the interface caused by CO2 and CH4, an essential factor in optimizing subsurface hydrogen storage operations.

Molecular modeling of CO2 affecting competitive adsorption within anthracite coal

This study aimed to investigate the adsorption properties of CO2, CH4, and N2 on anthracite. A molecular structural model of anthracite (C208H162O12N4) was established. Simulations were performed for the adsorption properties of single-component and multi-component gases at various temperatures, pressures, and gas ratios. The grand canonical ensemble Monte Carlo approach based on molecular mechanics and dynamics theories was used to perform the simulations. The results showed that the isotherms for the adsorption of single-component CO2, CH4, and N2 followed the Langmuir formula, and the CO2 adsorption isotherm growth gradient was negatively correlated with pressure but positively correlated with temperature. When the CO2 injection in the gas mixture was increased from 1 to 3% for the multi-component gas adsorption, the proportion of CO2 adsorption rose from 1/3 to 2/3, indicating that CO2 has a competing-adsorption advantage. The CO2 adsorption decreased faster with increasing temperature, indicating that the sensitivity of CO2 to temperature is stronger than that of CH4 and N2. The adsorbent potential energies of CO2, CH4, and N2 diminished with rising temperature in the following order: CO2 < CH4 < N2.

Efficient C2H2/CO2 separation in two 2-fold interpenetrating metal-organic frameworks with ultrahigh C2H2 packing density

Adsorptive separation of C2H2/CO2 is industrially important and feasible but the design of highly efficient adsorbents remains very challenging owing to the almost identical physicochemical properties of the two gases. Herein, we constructed two new microporous metal–organic frameworks (MOFs), termed CTGU-39/40 (CTGU = China Three Gorges University), with 2-fold interpenetrating frameworks and unique pore geometry/chemistry for C2H2/CO2 separation. Both the activated MOFs (CTGU-39/40-a) can preferentially adsorb C2H2 over CO2 with excellent selectivity of 8.4 and 3.3 at 298 K and 100 kPa, respectively, as verified by static adsorption measurements. More importantly, attributed to the suitable pore aperture size (∼5 Å) and abundant adsorption sites, CTGU-39-a combines exceptionally high packing density of C2H2 (0.41 g mL−1) and moderate isosteric heat of adsorption (Qst) for C2H2 (26.8 kJ mol−1), outperforming most previously reported adsorbents. Theoretical calculations reveal that the preferential adsorption of C2H2 within two MOFs is driven by cooperative adsorption behavior through multiple H-bonding and Coulomb interactions. Experimental breakthrough results further confirmed their practical separation ability and recyclability toward an equimolar C2H2/CO2 mixture at different feeding rates. This work may offer new insights into the design of advanced porous materials for C2H2 purification and potentially other gas separations.

Adsorption behavior of CuO doped GeS monolayer on the thermal runaway gas evolution in lithium battery energy storage systems

Considering that the thermal runaway failure of lithium battery energy storage system will cause serious safety accidents, and lithium battery thermal runaway will precipitate various detectable gases through the safety gas valve. Effective monitoring of the precipitated gas can ensure the safe operation of the energy storage system. In this study, the adsorption behavior of CuO-GeS towards the main precipitated gases CO, CO2, CH4, and C2H4 in Lithium battery before thermal runaway is systematically studied based on DFT. The adsorption structure, DOS, DCD, and ELF are analyzed to explore the interactions between CuO-GeS monolayer and the gases. The results show that CuO-GeS monolayer achieves strong adsorption of CO and C2H4 and highly sensitive monitoring of CO2 and CH4. Furthermore, the relative molecular mass ratios of CxHy and CxHy-1F (x = 1, 2 y=2, 4, 6) gases to doped CuO can satisfy Smoothing Spline with adsorption energy and charge transfer, which can be used to estimate the adsorption properties of the homologous gases. The potential application of CuO-GeS monolayer in lithium battery energy storage systems lays a theoretical research foundation for the adsorption and detection of thermal runaway gas in lithium battery energy storage systems.

Significance of fluid properties, pore structure, and surface chemistry in CMK series mesoporous carbon materials for N2/CH4/CO2 separation

We employed a combination of macroscopic approaches (Ideal Adsorption Solution Theory, IAST, and Two-Dimensional Equation of State, 2D-EOS) and microscopic molecular simulations to explore the adsorption and separation of N2, CH4, and CO2 in mesoporous materials CMK-1, CMK-3, and CMK-5. Our findings reveal that these behaviors are intricately linked to the material's structure, surface properties, and the adsorbed fluid type. For nonpolar N2/CH4 supercritical fluid, both macroscopic theories align remarkably well with molecular simulations. For gas mixtures with a supercritical gas (N2 or CH4) and a subcritical gas (CO2), the impact of material structure on adsorption and separation varies. While both adsorption theories perform well for CMK-3, with excellent agreement to simulations, only IAST accurately predicts CMK-1. This discrepancy indicates that 2D-EOS has a less satisfactory predictive accuracy for CMK-1 compared to CMK-3 due to structural differences. The hexagonal arrangement of nanorods in CMK-3 leads to a more uniform adsorption site, while CMK-1's diverse adsorption sites, including nanorod surfaces and inter-rod grooves, contribute to a more complex adsorption behavior. In CMK-5, a unique jump in the CO2 adsorption selectivity is observed, attributed to a phase transition within the pores. Interestingly, this transition-induced jump defies accurate prediction by common theoretical models, except for IAST, which correctly predicts trends in selectivity and adsorption amounts. This phenomenon is specific to CMK-5's dual-pore structure, where most CO2 adsorption occurs inside the pipes until saturation, followed by adsorption on the external surface. Examining the impact of –COOH and –OH functional groups on adsorption behavior, we find minimal influence on separation in nonpolar N2–CH4 fluid, independent of the material type. However, in CO2-containing systems, the –COOH functional group has a substantial impact, highlighting its significance in influencing adsorption behavior.

Efficient crystal structure materials as reactive sorbent for the CO2 and CH4 adsorption and storage

The efficient dirubidium cobalt bis(dihydrogendiphosphate) dihydrate compound is successfully synthesized in a solution and used as a reactive sorbent for the CO2 and CH4 gases adsorption and storage. A crystal of this Rb2Co(H2P2O7)2·2H2O compound has been isolated and characterized by single X-ray diffraction analysis and was found to crystallize in the triclinic system (P1¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$P\\overline{1}$$\\end{document}) with the cell parameters (Å): 6.980(1), 7.370(1), 7.816(1), 81.74(1), 70.35(1), 86.34(1); V = 374.68(9) Å3, Z = 2. The crystal-packing consists of a three-dimensional framework made upon corners and edges sharing of [RbO7], [H2P2O7] and [CoO6] entities, furthermore linked by a network of H-bonds. The UV–Vis spectroscopy revealed usual transitions between the ground state 4T1g and the upper levels 4T2g, 4A2g and 4T1g (P). Moreover, the CO2 and CH4 gases sorption measurements were successfully performed at two different temperatures (25 and 45 °C) and various pressures ranging from vacuum to 50 bar. Our results show that rate of CO2 and CH4 capturing was 3.10 mmol/g and 2.35 mmol/g at temperature 25 °C and pressure 50 bar, respectively. This compound showed a clear potential for CO2/CH4 adsorption and storage thereby paving the way towards its exploration and adaptation for capturing and collecting carbon dioxide and greenhouse gases from the air, and their conversion into hydrocarbon fuels using existing mature technologies. We have also conducted density functional theory calculations to study the CO2 and CH4 adsorption properties of Rb2Co(H2P2O7)2·2H2O. The simulation results show enhanced adsorption of both types of molecules on the surface of the material.

Supramolecular Interactions Induce Dynamics in Metal-Organic Layers to Selectively Separate Acetylene from Carbon Dioxide.

We report the synthesis and structural characterization of a 2D metal-organic framework with AB-packing layers, [Co2(pybz)2(CH3COO)2]·DMF (Co2, pybz= 4-(4-pyridyl)benzoate), containing a stable (4,4)-grid network fabricated by paddle-wheel nodes, ditopic pybz, and acetate ligands. After removal of the guest, the layer structure is retained but reorganized into an ABCD packing mode in the activated phase (Co2a). Consequently, the intralayer square windows (7.2 × 5.0 Å2) close, while the interlayer separation is decreased slightly from 3.69 to 3.45 Å, leaving a narrow gap. Importantly, the dangling methyl group of the acetate with H-bonds to the adjacent layers and also the well-distributed π-π interactions between the aromatic rings of neighboring layers facilitate the structural stability. These weak supramolecular interactions further allow for favorable dynamic exfoliation of the layers, which promotes efficient adsorption of C2H2 (41.6 cm3 g-1) over CO2 with an adsorption ratio of 6.3 (0.5 bar, 298 K). The effective separation performance of equimolar C2H2/CO2 was verified by cycling breakthrough experiments and was even tolerable to moisture (R.H = 52%). DFT calculations, in situ PXRD, and PDF characterization reveal that the favorable retention of C2H2 rather than that of CO2 is due to its H-bond formation with the paddle-wheel oxygen atoms that triggers the increase in interlayer separation during C2H2 adsorption.

Phase engineering of Fe2O3 nanocrystals for the direct oxidation of CH4 to HCOOH

The direct oxidation of methane (CH4) under mild conditions is a highly desirable technology to achieve the low-carbon and atom-economic production of high-value-added C1 oxygenates. However, the difficult activation of the CH bond of CH4 and the overoxidation of C1 oxygenates to carbon dioxide (CO2) are still the major bottlenecks. Herein, we systematically investigate the phase engineering of Fe2O3 in dependence on the performance of the direct oxidation of CH4. The cubic phase of Fe2O3 (Fe2O3-c) nanocrystals enables the direct oxidation of CH4 to HCOOH (DOMF) under mild aqueous conditions, and the HCOOH yield and selectivity could reach 6.315 mmol gcat−1 h−1 and 89.4%, which are 69.3 and 1.34 times higher than those of the hexagonal phase of Fe2O3 (Fe2O3-h) nanocrystals. Mechanism experiments indicate that Fe2O3-c nanocrystals can optimal CH4 adsorption behavior, favoring to the occurrence of the DOMF. In addition, in situ DRIFTS demonstrates that the DOMF is the radical reaction process that the activation of CH bond is caused by the ·OH radical. Subsequently, the unstable CH3OOH and stable CH3OH intermediates undergo further oxidation to form HCOOH. This work offers the phase engineering strategy to enhance the catalytic performance and reveal the reaction mechanism for the direct oxidation of CH4.

Differential Analysis of Various Moisture Phases on N2 Displacement of CH4 in Coal.

The aim of the study was to elucidate the impact of different moisture phases during gas injection on coalbed methane displacement. The coal samples were treated with two methods: water vapor adsorption and liquid water stirring. The differences in the coal samples treated with various moisture phases during gas injection for coalbed methane displacement were investigated by using the isothermal adsorption curves of CH4, N2, and H2O in coal and N2 displacement of CH4 in coal. The results indicate that variations in the gas adsorption capacity of coal are treated with different moisture phases. The gas adsorption capacities and displacement capacities of the coal samples treated with the water vapor adsorption methods are better than those treated with the stirring methods. In the isothermal adsorption experiment, for the coal samples treated with different moisture phases, at a moisture content of 2.75%, the saturated adsorption capacities of CH4/N2 are 0.204/0.189 (cm3/g), and at a moisture content of 5.63%, the saturated adsorption capacities of CH4/N2 are 0.151/0.139 (cm3/g). In addition, in the displacement experiment, for the coal samples treated with different moisture phases, at a moisture content of 2.75%, the difference in the total gas adsorption capacities is 0.62 cm3/g and the difference in the CH4 adsorption capacities is 0.473 cm3/g, and at a moisture content of 5.63%, the difference in the total gas adsorption capacities is 0.3 cm3/g and the difference in CH4 adsorption capacities is 0.22 cm3/g. For the coal samples treated with various moisture phases, the differences in the CH4/N2 adsorption and displacement capacities are greater at a moisture content of 2.75% than at 5.63%. Notably, the moisture phase has only a marginal influence on the CH4 desorption capacity and desorption rate. The study is important to understand the interactions between coal and moisture.

High-Pressure adsorption of supercritical methane and carbon dioxide on Coal: Analysis of adsorbed phase density

The occurrence of supercritical CH4 and CO2 in coal is significant for both coalbed methane recovery and CO2 storage. Adsorption isotherms of CH4 and CO2 on two coals have been measured at three temperatures up to pressures of 25 MPa. Three modified supercritical adsorption models (SL-V, SBET-V, and SDR-V) were used to describe the excess adsorption behavior, with the pressure-independent adsorbed phase volume being considered. The adsorbed phase density (APD) and absolute adsorption amount were calculated based on the modified models and compared with those estimated by previous models. The results indicate that SDR-V is the most suitable for CH4 adsorption due to the dominant role of filling of micropores by CH4, while SBET-V is the most appropriate model for CO2 adsorption because of multilayer adsorption of CO2 in both micropores and meso-macropores. The APD increases with pressure until reaching a maximum value but decreases as temperature increases following a Langmuir-style trend. The previous models, which use a fixed APD, seriously underestimate the absolute adsorption, the extent of which increases with increasing pressure and temperature, leading to significant errors in deep formations with high temperature and pressure conditions. Furthermore, as burial depth increases, the adsorption process can be divided into three stages based on changes in APD and free phase density (FPD). These findings contribute to a comprehensive understanding of the behavior and occurrence of CH4 and CO2 within deep coal seams, enhancing the applicability of CO2-ECBM (CO2 Geological Storage Enhanced Coalbed Methane Recovery).

Transition-metal-doped Hf2CO2 as a promising sensor for methane: a DFT study

Density functional theory (DFT) methods were used to study the atomic and electronic structures of methane molecules adsorbed on pristine, vacancy-defective, and transition-metal (TMs: Mn, Fe, Y, Mo, Ta, W)-doped Hf2CO2. The interaction distance between the methane molecules and original substrate was large, and the adsorption energy was small, suggesting that the a physisorption adsorption. After introducing O-vacancy, the adsorption of CH4 changed into chemisorption. Through the doping with TM dopants, the methane molecule adsorbed onto the Ta-doped Hf2CO2 exhibited the highest stability and the largest adsorption energy. In addition, the W-doped Hf2CO2 changed from being a semiconductor to being a conductor after the adsorption of methane molecules onto the material; this phenomenon suggests that the change can be used as a signal for sensors in detecting methane gas. These results are expected to provide new insights into the design of gas sensors based on MXenes materials.

Simulation study on functional group-modified Ni-MOF-74 for CH4/N2 adsorption separation.

This study employs grand canonical Monte Carlo (GCMC) simulations to investigate the impact of functional group modifications (CH3, OH, NH2, and OLi) on the adsorption performance of CH4/N2 on Ni-MOF-74. The results revealed that functional group modifications significantly increased the adsorption capacity of Ni-MOF-74 for both CH4 and N2. The packed methyl groups in CH3-Ni-MOF-74 create an environment conducive to CH4, leading to the highest CH4 adsorption capacity. The electrostatic potential distribution indicates that the strong electron-donating effect introduced by the alkali metal Li results in the highest electrostatic potential gradient in Li-O-Ni-MOF-74, leading to the strongest adsorption of N2, this is unfavorable for CH4/N2 separation. At 1500 kPa the selectivity order of adsorbents for mixed gases was as follows: CH3-Ni-MOF-74 > NH2-Ni-MOF-74 > OH-Ni-MOF-74 > Ni-MOF-74 > Li-O-Ni-MOF-74. This study highlights that CH3-Ni-MOF-74 possesses optimal CH4 selectivity and adsorption performance. Given the current lack of research on functionalized MOF-74 for the separation of CH4 and N2, the findings of this study will serve as a theoretical guide and provide references for the applications of CH4 adsorption and CH4/N2 separation.