Canonical Monte Carlo Simulations Research Articles

Isoelectric Substitution Strategy to Construct a Partially Charged MOF for C2H2 Capture and Selective Dye Adsorption

The charged metal–organic framework (MOF) (anionic and cationic MOF) material plays an important role in gas adsorption and separation and ecological environment management. Herein, we reported a partially charged Mg-based MOF (FJU-129) with dimethylamine counterions, which can efficiently separate C2H2 from CO2 and rapidly adsorb the cationic dye methylene blue (MB). FJU-129a showed a high uptake capacity of up to 133.8 cm3/g for C2H2 at 273 K, 1 bar, and low isosteric heat of adsorption of C2H2 (24.78 kJ/mol). The C2H2/CO2 separation is validated by a dynamic breakthrough experiment at 296 K, 1.5 bar, and the Grand Canonical Monte Carlo simulation revealed that the partially remaining charge on the magnesium sites has an electrostatic force on the −C≡C– bond of C2H2 molecules, and C–H···π interactions play an important role in the separation of C2H2/CO2 selective mixtures. In addition, owing to the pore size and electrostatic effect, FJU-129 selectively absorbs MB and excludes other cationic dyes and anionic dyes (MB, CV, RB, R6G, MO, and SD-III).

Prediction of Adsorption and Diffusion of Shale Gas in Composite Pores Consisting of Kaolinite and Kerogen using Molecular Simulation.

Natural gas production from shale formations is one of the most recent and fast growing developments in the oil and gas industry. The accurate prediction of the adsorption and transport of shale gas is essential for estimating shale gas production capacity and improving existing extractions. To realistically represent heterogeneous shale formations, a composite pore model was built from a kaolinite slit mesopore hosting a kerogen matrix. Moreover, empty slabs (2, 3, and 4 nm) were added between the kerogen matrix and siloxane surface of kaolinite. Using Grand-Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, the adsorption and diffusion of pure methane, pure ethane, and a shale gas mixture were computed at various high pressures (100, 150, and 250 atm) and temperature of 298.15 K. The addition of an inner slit pore was found to significantly increase the excess adsorption of methane, as a pure component and in the shale gas mixture. The saturation of the composite pore with methane was observed to be at a higher pressure compared to ethane. The excess adsorption of carbon dioxide was not largely affected by pressure, and the local number density profile showed its strong affinity to kerogen micropores and the hydroxylated gibbsite surface under all conditions and pore widths. Lateral diffusion coefficients were found to increase with increasing the width of the empty slab inside the composite pore. Statistical errors of diffusion coefficients were found to be large for the case of shale gas components present at low composition. A larger composite pore configuration was created to investigate the diffusion of methane in different regions of the composite pore. The calculated diffusion coefficients and mean residence times were found to be indicative of the different adsorption mechanisms occurring inside the pore.

Vapor-Liquid phase transition of associating fluids in a functionalized cylindrical pore: a Monte Carlo study

In this study, we investigate the phase behaviour of associating fluids with two interacting sites inside a functionalised cylindrical pore using Grand Canonical Monte Carlo simulations. The structural behaviour of the coexisting liquid and vapor phases is characterised by the radial density, orientation, and bond formation inside cylindrical pores of different radii. The critical properties of the phase diagrams of the confined fluids were investigated as functions of the associating strengths at different pore sizes. The impact of site strength on the phase diagram is characterised by the critical and structural properties of the coexisting phases inside the cylindrical pore. The organisation of the associating fluids near the pore surface strongly depends on the surface – site interaction with the interfacial fluid molecules that propagate to the bulk regime of the system. Thus, the surface site density is a crucial parameter for manipulating the properties of coexisting phases and critical properties of the phase diagram.

Adsorption characteristics of shale gas in organic–inorganic slit pores

With the gradual advancement of shale gas exploration and development technology, shale gas has replaced conventional oil and gas as the primary energy source. As the main occurrence form of natural gas in shale reservoirs, the adsorption state is always an important research topic for evaluating the gas-bearing property of shale reservoirs and improving shale gas recovery. However, at present, there are few studies on control factors and control mechanisms of adsorbed gas in organic–inorganic slit pores, with several unexplained control factors and mechanisms. Taking Songliao Basin shale as an example, molecular simulations are employed to investigate shale gas adsorption in organic–inorganic slit pores, the graphene and kaolinite rock matrix model is employed, and molecular dynamics simulations, particularly the grand canonical Monte Carlo simulation is studied for pore sizes of 2, 4, and 6 nm, The adsorption behavior of methane on heterogeneous shale surfaces at temperatures of 298, 323, 348, 373, and 398 K and pressures of 0.1, 5, 10, 15, and 20 MPa reveals the microscopic mechanism of shale gas adsorption in organic–inorganic slit pores at a molecular scale. The results show that as temperature and pressure increase simultaneously, methane is more strongly adsorbed on the surface of graphene than kaolinite, the number of methane molecules also gradually decreases, the root mean square of the displacement of methane and different surface forces between atoms exhibit certain regularity, and the adsorption capacity of the system is proportional to the changes in diameter within the system. Van der Waals energy and electrostatic energy in the system also increase gradually with changes in temperature and pressure, and electrostatic energy has a greater influence on the adsorption of the system than van der Waal energy. These findings provide ideas for the adsorption capacity of shale gas in organic–inorganic slit pores and are critical for the accurate evaluation of shale reservoir gas content and improvement of shale gas recovery.

Computational investigation of hysteresis and phase equilibria of n-alkanes in a metal-organic framework with both micropores and mesopores

Adsorption hysteresis is a phenomenon related to phase transitions that can impact applications such as gas storage and separations in porous materials. Computational approaches can greatly facilitate the understanding of phase transitions and phase equilibria in porous materials. In this work, adsorption isotherms for methane, ethane, propane, and n-hexane were calculated from atomistic grand canonical Monte Carlo (GCMC) simulations in a metal-organic framework having both micropores and mesopores to better understand hysteresis and phase equilibria between connected pores of different size and the external bulk fluid. At low temperatures, the calculated isotherms exhibit sharp steps accompanied by hysteresis. As a complementary simulation method, canonical (NVT) ensemble simulations with Widom test particle insertions are demonstrated to provide additional information about these systems. The NVT+Widom simulations provide the full van der Waals loop associated with the sharp steps and hysteresis, including the locations of the spinodal points and points within the metastable and unstable regions that are inaccessible to GCMC simulations. The simulations provide molecular-level insight into pore filling and equilibria between high- and low-density states within individual pores. The effect of framework flexibility on adsorption hysteresis is also investigated for methane in IRMOF-1.

Ultramicroporous Metal-Organic Framework with Inert Pore Surfaces for Inversed Separation of Ethylene from C2 Hydrocarbons Mixtures.

The achievement of direct C2H4 separation from C2 hydrocarbons is very challenging in the petrochemical industry due to their similar molecular sizes, boiling points, and physicochemical properties. In this work, a nonpolar/inert ultramicroporous metal-organic framework (MOF), [Co3(μ3-OH)(tipa)(bpy)1.5]·3DMF·6H2O (1), with stand-alone one-dimensional square tubular channels was successfully constructed, its pore enriched with plenty of aromatic rings causing nonpolar/inert pore surfaces. The MOF shows preferential adsorption of C2H6 compared to C2H4 and C2H2 in the low-pressure region, which is further verified by adsorption heats and selectivities. The C2H4 separation potential in one step for binary C2H6/C2H4 (50/50 and 10/90) and ternary C2H4/C2H6/C2H2 (89/10/1) is also examined by transient breakthrough simulations. Moreover, grand canonical Monte Carlo simulations demonstrate that the unique reversed adsorption mechanism is due to the shortest and most number of C-H···π interactions between C2H6 and the framework.

Efficient Xe/Kr separation in fluorinated pillar-caged metal-organic frameworks

The separation of xenon/krypton (Xe/Kr) mixture is of great significance in industry, yet challenging due to their close physical properties. Traditional porous materials typically fail to realize a balance between adsorption capacity and selectivity. Herein, we report a series of water stable fluorinated anion pillared metal-organic frameworks (TIFSIX-Cu-TPA, GeFSIX-Cu-TPA and NbOFFIVE-Cu-TPA) with suitable cages for efficient capture/separation of Xe from Kr. The adsorption capacities of TIFSIX-Cu-TPA, GeFSIX-Cu-TPA and NbOFFIVE-Cu-TPA for Xe at 298 K and 1.0 bar are 64.5, 61.2 and 59.0 STP cm3 g−1, respectively, while those for Kr are only 16.9, 15.8 and 14.9 STP cm3 g−1. Correspondingly, the IAST selectivities for these MOFs are respectively 5.4, 5.3 and 5.1 at 298 K and 1 bar for the Xe/Kr (20/80) mixtures, superior to those of most popular porous materials in the context of Xe/Kr separation. The adsorption heats for Xe are as modest as 22.3, 21.7 and 21.5 kJ/mol for TIFSIX-Cu-TPA, GeFSIX-Cu-TPA and NbOFFIVE-Cu-TPA respectively, indicating the facile conditions for regeneration of the materials. The Xe/Kr binding sites were further studied by Grand Canonical Monte Carlo (GCMC) simulations and density functional theory (DFT) calculations, indicating the preferred binding with Xe over Kr in the large cages decorated with fluorine atoms, corroborating the experimental results. The practical performance of dynamic Xe/Kr separation is further demonstrated by transient breakthrough simulations and experiments.

High-Throughput Computational Screening of Two-Dimensional Covalent Organic Frameworks (2D COFs) for Capturing Radon in Moist Air.

Radon (Rn) and its decay products are the primary sources of natural ionizing radiation exposure for the public, posing significant health risks, including being a leading cause of lung cancer. Porous material-based adsorbents offer a feasible and efficient solution for controlling Rn concentrations in various scenes to achieve safe levels. However, due to competitive adsorption between Rn and water, finding candidates with a higher affinity and capacity for capturing Rn in humid air remains a significant challenge. Here, we conducted high-throughput computational screening of 8641 two-dimensional covalent organic frameworks (2D COFs) in moist air using grand canonical Monte Carlo simulations. We identified the top five candidates and revealed the structure-performance relationship. Our findings suggest that a well-defined cavity with an approximate spherical inner space, with a diameter matching that of Rn, is the structural basis for a proper Rn capturing site. This is because the excellent steric match between the cavity and Rn maximizes their van der Waals dispersion interactions. Additionally, the significant polarization electrostatic potential surface of the cavity can regulate the adsorption energy of water and ultimately impact Rn selectivity. Our study offers a potential route for Rn management using 2D COFs in moist air and provides a scientific basis for further experimentation.

An XGBoost Algorithm Based on Molecular Structure and Molecular Specificity Parameters for Predicting Gas Adsorption

In this paper, an improved Extreme Gradient Boosting (XGBoost) algorithm based on the Graph Isomorphic Network (GIN) for predicting the adsorption performance of metal-organic frameworks (MOFs) is developed. It is shown that the graph isomorphic layer of this algorithm can directly learn the feature representation of materials from the connection of atoms in MOFs. Then, XGBoost can be used to predict the adsorption performance of MOFs based on feature representation. In this sense, it is not only possible to achieve end-to-end prediction directly from the structure of MOFs to adsorption performance but also to ensure the accuracy of prediction. The comparison between Grand Canonical Monte Carlo (GCMC) simulation and prediction supports the performance and effectiveness of the proposed algorithm.

Mechanism exploration of surface functional groups and pore sizes on CO2 adsorptive separation by GCMC and DFT simulations

The purpose of this study is to analyze the mechanistic effect of surface functional groups and pore sizes of graphene on the CO2 adsorption and separation behaviors by grand canonical Monte Carlo (GCMC) simulation and density functional theory (DFT) calculation. For the surface functional groups, the M−doped (M = N, P, S, O) functionalized graphene materials (G-Rs) exhibit a higher CO2/N2, CO2/H2 and CO2/CH4 adsorption selectivity than pristine graphene (G-None) in the entire pressure range. Especially, due to a stronger electrostatic interaction, the P-doped functionalized graphene materials (P-G-Rs) present the best CO2 separation performance. For the pore size, the influence of pore size on the CO2 adsorptive separation process depends on pressure ranges. At low pressures, 0.8 nm pore shows the highest CO2 adsorption selectivity. However, as for high pressure, the actual pore volume plays the dominant role. Furthermore, by analyzing the isosurface maps of electrostatic potential and van der Waals (vdW) potential, we conclude that the electrostatic effect determines the optimal adsorption sites for polar gas molecules (CO2) rather than non-polar molecules (N2, H2 and CH4), while the vdW interaction plays a secondary role in the intermolecular interaction. This study provides a novel theoretical guidance in designing and preparing modified graphene for CO2/other gas molecules mixture separation.

Molecular simulations for carbon dioxide capture in silica slit pores

In present work, we have performed the Grand Canonical Monte Carlo (GCMC) simulations to quantify CO2 capture inside porous silica at high operating temperatures of 673.15 K and 873.15 K; and over a operating pressure range of 500 kPa – 4000 kPa that are methane steam reforming process parameters. Related chemical potential values at these thermodynamic conditions are obtained from the bulk phase simulations in the Canonical ensemble in conjunction with Widom’s insertion technique, where the CO2 has been accurately represented by TraPPE force field. Present structure of the porous silica is a single slit pore geometry of various heights (H = 20 Å, 31.6 Å, 63.2 Å and 126.5 Å), dimensions in which possible vapour-liquid equilibria for generic square well fluids has been reported in literature. Estimation of the pore-fluid interactions show a higher interaction between silica pore and adsorbed CO2 compared to the reported pore-fluid interactions between homogeneous carbon slit pore and adsorbed CO2; thus resulting in an enhancement of adsorption inside silica pores of H = 20 Å and H = 126.5 Å, which are respectively 3.5 times and 1.5 times higher than that in homogeneous carbon slit pores of same dimensions and at 673.15 K and 500 kPa. Estimated local density plots indicate the presence of structured layers due to more molecular packing, which confirms possible liquid-like and vapour-like phase coexistence of the supercritical bulk phase CO2 under confinement.

Hydrogen physisorption in earth-minerals: Insights for hydrogen subsurface storage

Physisorption can contribute to hydrogen subsurface storage. However, hydrogen physisorption at the surface of geological samples is mostly unknown. We are unaware of previous studies that investigate H2 Physisorption in wide range of pure inorganic and organic earth minerals. Knowledge about H2 Physisorption is required to evaluate the suitability of underground H2 storage. We thus evaluated H2 physisorption on common earth minerals, including illite, kaolinite, quartz, calcite, and kerogen via lab experiments and Grand Canonical Monte Carlo (GCMC) simulations at temperature ranging between 273 and 373 K and pressure ranging between 5 MPa to 40 MPa. The simulation results, including densities and H2 adsorption uptakes, are in agreement with experimental results. The quantity of adsorbed hydrogen ranked as: illite > kerogen > kaolinite > quartz > calcite; and Gibbs energy change, enthalpy change, and isosteric heat energy indicated that H2 adsorption spontaneity increases as illite > kerogen > kaolinite > calcite > quartz. We conclude that H2 adsorption is high on the high-surface-area materials (clays or kerogen), and that H2 adsorption on earth minerals could improve the effectiveness of subsurface H2 storage.

One Atom Can Make All the Difference: Gas-Induced Phase Transformations in Bisimidazole-Linked Diamondoid Coordination Networks.

Coordination networks (CNs) that undergo gas-induced transformation from closed (nonporous) to open (porous) structures are of potential utility in gas storage applications, but their development is hindered by limited control over their switching mechanisms and pressures. In this work, we report two CNs, [Co(bimpy)(bdc)]n (X-dia-4-Co) and [Co(bimbz)(bdc)]n (X-dia-5-Co) (H2bdc = 1,4-benzendicarboxylic acid; bimpy = 2,5-bis(1H-imidazole-1-yl)pyridine; bimbz = 1,4-bis(1H-imidazole-1-yl)benzene), that both undergo transformation from closed to isostructural open phases involving at least a 27% increase in cell volume. Although X-dia-4-Co and X-dia-5-Co only differ from one another by one atom in their N-donor linkers (bimpy = pyridine, and bimbz = benzene), this results in different pore chemistry and switching mechanisms. Specifically, X-dia-4-Co exhibited a gradual phase transformation with a steady increase in the uptake when exposed to CO2, whereas X-dia-5-Co exhibited a sharp step (type F-IV isotherm) at P/P0 0.008 or P 3 bar (195 or 298 K, respectively). Single-crystal X-ray diffraction, in situ powder XRD, in situ IR, and modeling (density functional theory calculations, and canonical Monte Carlo simulations) studies provide insights into the nature of the switching mechanisms and enable attribution of pronounced differences in sorption properties to the changed pore chemistry.

A stable microporous framework with multiple accessible adsorption sites for high capacity adsorption and efficient separation of light hydrocarbons

Metal-organic frameworks (MOFs), a type of designable porous materials, are effectively employed to realize light hydrocarbons storage and separation in favor of energy conservation and environmental protection, superior to traditional separation technology. Significantly, a novel MOF, Ni2(L)2(HCOO)2·4H2O (1) has been constructed from a bifunctional group ligand of 3-hydroxy-4-(4H-1,2,4-triazol-4-yl) benzoic acid ligand (HL). MOF 1 possesses stable porous structure with narrow channels functionalized with multiple accessible adsorption sites of hydroxy group, carboxyl oxygen atoms from the L− and HCOO−, exhibiting a well trade-off between good capacity and selectivity for light hydrocarbons. The single-component sorption isotherm results of Ni2+ based MOF not only shows good gas sorption capacity for light hydrocarbons of C2H2, C2H6, C2H4, C3H8, and greenhouse gas CO2, especially remarkable uptake for C2H2 (122.0 cm3 g−1 at ambient condition), but also favorable selectivity for binary mixtures of C2H2/CH4, C2H6/CH4, C3H8/CH4, C2H4/CH4, C2H2/C2H4 as well as C3H8/C2H6/CH4 ternary mixture, as confirmed by ideal adsorbed solution theory (IAST) calculations, and dynamic breakthrough experiment. Grand canonical Monte Carlo (GCMC) simulation reveals the multiple synergism of accessible adsorption sites including functional OH groups, carboxylate groups, HCOO− and π···π stacking interaction between guest molecules and host-pore are crucial to contribute selective gas adsorption of light hydrocarbons.

GPU-specific algorithms for improved solute sampling in grand canonical Monte Carlo simulations.

The Grand Canonical Monte Carlo (GCMC) ensemble defined by the excess chemical potential, μex , volume, and temperature, in the context of molecular simulations allows for variations in the number of particles in the system. In practice, GCMC simulations have been widely applied for the sampling of rare gasses and water, but limited in the context of larger molecules. To overcome this limitation, the oscillating μex GCMC method was introduced and shown to be of utility for sampling small solutes, such as formamide, propane, and benzene, as well as for ionic species such as monocations, acetate, and methylammonium. However, the acceptance of GCMC insertions is low, and the method is computationally demanding. In the present study, we improved the sampling efficiency of the GCMC method using known cavity-bias and configurational-bias algorithms in the context of GPU architecture. Specifically, for GCMC simulations of aqueous solution systems, the configurational-bias algorithm was extended by applying system partitioning in conjunction with a random interval extraction algorithm, thereby improving the efficiency in a highly parallel computing environment. The method is parallelized on the GPU using CUDA and OpenCL, allowing for the code to run on both Nvidia and AMD GPUs, respectively. Notably, the method is particularly well suited for GPU computing as the large number of threads allows for simultaneous sampling of a large number of configurations during insertion attempts without additional computational overhead. In addition, the partitioning scheme allows for simultaneous insertion attempts for large systems, offering considerable efficiency. Calculations on the BK Channel, a transporter, including a lipid bilayer with over 760,000 atoms, show a speed up of ~53-fold through the use of system partitioning. The improved algorithm is then combined with an enhanced μex oscillation protocol and shown to be of utility in the context of the site-identification by ligand competitive saturation (SILCS) co-solvent sampling approach as illustrated through application to the protein CDK2.

IRMOF-3 nanosheet-filled glass fiber membranes for efficient separation of hydrogen and carbon dioxide

Metal-organic framework (MOF) nanosheets have been identified as promising membrane materials for various separation applications. However, the development of MOF nanosheet membranes for practical applications is impeded by the difficulties in preparing ultrathin nanosheets and the limitations of conventional membrane production methods. For the first time, conveniently prepared and purified isoreticular MOF-3 (IRMOF-3) nanosheets are filled into macroporous glass fiber (GF) filters to prepare IRMOF-3 nanosheet-filled GF (IRMOF-3@GFX, X = filling amount expressed in mg) membranes. The IRMOF-3@GF2.0 membrane exhibits an impressively high H2/CO2 selectivity of 1341 ± 134, and the H2 permeance reaches 974 ± 41 gas permeance units (GPU). In addition, the IRMOF-3@GF2.0 membrane shows excellent durability with no performance degradation during a 150-h test. Grand Canonical Monte Carlo simulations are performed to investigate the H2/CO2 separation mechanism and the results suggest a synergistic separation mode based on selective adsorption and size exclusion. The innovative filled-membrane fabrication route and the presented gas separation mechanism offer a new strategy to design and prepare MOF nanosheet-based gas separation membranes.

An investigation for H2/N2 adsorptive separation in SIFSIX-2-Cu-i

Hydrogen recovery from ammonia purge gas is a desirable method to relieve the pressure of increasing demand for hydrogen. Metal-organic frameworks (MOFs) are promising adsorbents for the separation of the H2–N2 mixture. However, the separation of the H2–N2 mixture using MOFs remains challenging due to the lack of adsorptive mechanism. Herein, the adsorption mechanism of the H2–N2 mixture in SIFSIX-2-Cu-i has been systematically investigated with the method of a combination of excess adsorption experiments, molecular simulations, and DFT calculations. In the Grand Canonical Monte Carlo (GCMC) simulations, the polarizable force field was first applied to the adsorption of pure H2 or N2 and H2–N2 mixture in SIFSIX-2-Cu-i. Ab Initio Molecular Dynamics (AIMD) was performed to investigate the distribution of the guest molecules in the pores of SIFSIX-2-Cu-i. Finally, Density functional theory (DFT) calculations were performed to predict the transfer of charge between MOF and guest molecules. Results revealed two binding sites of H2 or N2 in SIFSIX-2-Cu-i. A relatively high thermodynamic adsorption selectivity for N2 over H2 has been observed in SIFSIX-2-Cu-i. Furthermore, SIFSIX-2-Cu-i also exhibits a greater affinity with N2 than with H2, which is probably because of the stronger induction interactions between F2 atoms and N2 molecules. These findings are of significance to guide the design of the new highly selective adsorbents for the separation of H2/N2.

Evaluation of the BET and GAB models for interpretation of soil water isotherms: A molecular simulation study

Mathematical characterization of soil water isotherm (SWI) plays a significant role in modeling various processes in soils and estimating the soil properties, and the Brunauer-Emmet-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models are most widely used. Each model uses the amount of adsorbed gas in the monolayer on the surface for area estimates. Although these two models are widely used with soils, studies on the applicabilities of the SWIs estimated with the two models and the reliabilities of the derived monolayer contents are noticeably lacking. This work presents detailed evaluations of the two models and the resulting SWIs and determines whether these provide meaningful monolayer contents. The SWIs for four representative soils were generated with Grand Canonical Monte Carlo (GCMC) simulations, and the data were evaluated along with a wealth of information on the physical states of the adsorbed water molecules. The results showed that the GAB model was much superior to the BET model in terms of fitting the SWIs. A series of analyses validated the soundness of the monolayer contents determined, including analyses of the layer thickness, the area occupied by the water monolayer, and the orientations of the water molecules. The molecular simulation also revealed the very different effects of the surface hydroxyls and cations in adsorption by considering the densities, space locations, and orientations of the adsorbed water. This work provides crucial information for rational application of the BET and GAB models to SWIs.

A stable metal-organic framework with oxygen site for efficiently trapping acetylene from acetylene-containing mixtures

Highly selective separation of acetylene from various other gases (e.g., carbon dioxide and methane) is of great importance in the petrochemical industry but remains a challenge due to their similar sizes and physical properties. Herein, we reported a microporous metal − organic framework Zn4O(NTB)2 (H3NTB = 4,4′,4″-nitrilotribenzoicacid acid), features oxygen binding sites explored for C2H2 capture and separation. A high C2H2 uptake of 2.62 mmol g−1 combined with excellent selectivity of C2H2/CO2 (4.8) and C2H2/CH4 (21.6) under ambient conditions (i.e., 50:50 mixture at 298 K and 1 bar) was realized. Moreover, the grand canonical Monte Carlo (GCMC) simulations and first-principles density functional theory (DFT) calculations have identified that the high C2H2 capture and selectivity ascribe to oxygen sites on the pores (C–H…O interaction). Dynamic breakthrough experiments comprehensively demonstrate that Zn4O(NTB)2 is a promising adsorbent for practical C2H2/CO2 and C2H2/CH4 separation.

Molecular simulation on CO2/H2S co-adsorption in organic and inorganic shale nanopores

Geological CO2 sequestration (GCS) in depleted shale reservoirs is recognized as one of promising techniques to mitigate CO2 emissions, while the high cost of CO2 purification greatly hinders its deployment. Co-injection of CO2/H2S mixtures has been proposed as an economical alternative to further promote GCS application. In this work, we used Grand Canonical Monte Carlo (GCMC) and molecular dynamic (MD) simulations to investigate CO2/H2S mixture adsorption in organic and inorganic shale nanopores (at 333.15 K and pressure up to 50 MPa). CO2 and H2S present distinct adsorption behaviors on the different surfaces. H2S adopts unordered structures on kerogen surfaces, but a perpendicular orientation on illite due to H-bonding, which also results in higher adsorption, while CO2 is preferentially distributed parallel along the surfaces. Furthermore, CO2 and H2S accumulate in the concave parts of kerogen surfaces, while they concentrate around the tetrahedral aluminum (AT) atoms and K+ cations on illite surfaces. The adsorption selectivity of H2S over CO2 is always greater than 1, indicating the stronger adsorption propensity of H2S. The preferential adsorption of H2S on illite surface is facilitated by the stronger dipole–dipole interactions. PMF results show that the binding interaction between CO2 or H2S and illite is higher than on kerogen, and H2S always shows a stronger interaction than CO2. Overall, the total CO2 and H2S adsorption is higher in illite nanopores, implying a new promising storage method. This study provides important insights into the competitive adsorption mechanisms of CO2 and H2S in depleted shale reservoirs and sheds light on how sour gas geological sequestration capacity can be further improved.