The effect of gme topology on multicomponent adsorption in zeolitic imidazolate frameworks

Adsorptive separation of CH4, H2, CO2, and N2 using fullerene pillared graphene nanocomposites: Insights from molecular simulations

CONTEXT

The adsorptive separation performances of fullerene pillared graphene nanocomposites (FPGNs) with tunable micro and meso porous morphology are investigated for the binary mixtures of CH4, H2, CO2 and N2 by using grand canonical Monte Carlo (GCMC) simulations. Different fullerene types are considered in designs as pillar to investigate the effects of porosity on the gas separation performances of FPGNs, and the GCMC simulations are performed for an equimolar binary mixture of CO2/H2, CO2/CH4, CO2/N2 and CH4/H2 inspired by industrial gas mixtures. It is found that CO2/N2, CO2/H2 and CH4/H2 selectivity of FPGNs are about 72, 410 and 145 at 298 K and 1 bar, which are higher than those for several adsorbent materials reported.

METHODS

Five different FPGN models which contain covalently bonded periodical fullerene and graphene units were constructed using C60, C180, C320, C540 and C720 fullerenes, followed by geometry optimization using Open Babel. All GCMC simulations of adsorption were performed in the RASPA. The adsorption isotherms of FPGNs for pure gases are comparatively examined, and their performances are discussed based on the pore structure and isosteric heat of adsorption. Then, the separation factors of FPGNs for equimolar binary mixtures of these gases are elucidated from the difference in the heat of adsorption and the adsorption selectivity.

Simulation Study on Molecular Adsorption of Coal in Chicheng Coal Mine

To study the importance of the adsorption mechanism of methane (CH₄) and carbon dioxide (CO₂) in coal for coalbed methane development, we aimed to reveal the influence mechanism of adsorption pressure, temperature, gas properties, water content, and other factors on gas molecular adsorption behavior from the molecular level. In this study, we selected the nonsticky coal in Chicheng Coal Mine as the research object. Based on the coal macromolecular model, we used the molecular dynamics (MD) and Monte Carlo (GCMC) methods to simulate and analyze the conditions of different pressure, temperature, and water content. The change rule and microscopic mechanism of the adsorption amount, equal adsorption heat, and interaction energy of CO₂ and CH₄ gas molecules in the coal macromolecular structure model establish a theoretical foundation for revealing the adsorption characteristics of coalbed methane in coal and provide technical support for further improving coalbed methane extraction.

Influence of Substitutional Defects in ZIF-8 Membranes on Reverse Osmosis Desalination: A Molecular Dynamics Study

In this study, molecular dynamics simulation is used to investigate the effects of water-based substitutional defects in zeolitic imidazolate frameworks (ZIF)-8 membranes on their reverse osmosis (RO) desalination performance. ZIF-8 unit cells containing up to three defect sites are used to construct the membranes. These substitutional defects can either be Zn defects or linker defects. The RO desalination performance of the membranes is assessed in terms of the water flux and ion rejection rate. The effects of defects on the interactions between the ZIF-8 membranes and NaCl are investigated and explained with respect to the radial distribution function (RDF) and ion density distribution. The results show that ion adsorption on the membranes occurs at either the nitrogen atoms or the defect sites. Complete NaCl rejection can be achieved by introducing defects to change the size of the pores. It has also been discovered that the presence of linker defects increases membrane hydrophilicity. Overall, molecular dynamics simulations have been used in this study to show that water-based substitutional defects in a ZIF-8 structure reduce the water flux and influence its hydrophilicity and ion adsorption performance, which is useful in predicting the type and number of defect sites per unit cell required for RO applications. Of the seven ZIF-8 structures tested, pristine ZIF-8 exhibits the best RO desalination performance.

Enhanced toluene adsorption/desorption dynamic performances over modified USY zeolites after an aqueous ammonia treatment

The dynamic adsorption/desorption performances of modified hierarchical USY zeolites treated with an ammonia solution (NH₄OH) at different concentrations were investigated using gas-phase toluene as an indicator. The characterization results indicated that the ammonia treatment could result in the expansion of microporous channels and the formation of a mesoporous structure without evident decrease in crystallinity. The experiment results regarding dynamic adsorption/desorption performances revealed that the mass transfer resistance of modified USY adsorbents were greatly reduced treating with NH₄OH. Among the modified samples, the 0.1 mol L⁻¹ NH₄OH treated USY adsorbent exhibited large adsorptive capacity and highest desorption rate, which show good cyclic performance that could preserve its adsorbent capacity after 20 cycles. In contrast, pristine USY samples had lost around 28% of the initial adsorption capacity after 20 cycles. Moreover, the NaOH-treated sample showed great crystallinity decline compared to the NH₄OH-treated samples due to excessive silicon atom leaching from the USY framework, and had lower adsorption capacity under humid conditions. Therefore, NH₄OH-modified USY zeolites could be promising adsorbents for the adsorption/desorption process of volatile organic compounds (VOCs).

USY zeolites treated by aqueous ammonia (a simple method) shows enhanced and stabilized toluene adsorption/desorption dynamic performances.

Calculating adsorption isotherms using Lennard Jones particle density distributions

The computation of particle density distributions in pore channels is a fundamental post process practice in molecular adsorption simulations. The distributions, although not appropriate for direct experimental interrogation, when expressed in variable temperature, may be used to evaluate thermodynamic properties. As with molecular simulations, we can spotlight any frame or region of interest inside the computational cell, the distributions and subsequently the thermodynamic evaluations can be pore or site specific. This allows us to establish correlations for the adsorption capacity, on different pore partitions and surface textures and compare strengths of explicit interactions. We confirm this assumption using adsorption simulations of Lennard Jones particles such as argon in the dual cylindrical cavity of ZIF-69. We initiate the computations from the density distribution functions and reproduce full curves of adsorption isotherms which correlate well with the direct simulations. The computations are fast and implemented using linear van' t Hoff plots for the surface coverage at a sequence of pressures.