Search

A microscopic insight into hybrid CH 4 physisorption-hydrate formation in halloysite nanotubes (HNTs) is vital for understanding the solidification storage of natural gas in the HNTs and developing energy storage technology. Herein, large-scale microsecond classical molecular dynamics simulations are conducted to investigate CH 4 storage in the HNTs via the adsorption-hydration hybrid (AHH) method to reveal the effect of gas-water ratio. The simulation results indicate that the HNTs are excellent nanomaterials for CH 4 storage via the adsorption-hydration hybrid method. The CH 4 physisorption and hydrate formation inside and outside of the HNTs are profoundly influenced by the surface properties of the HNTs and the kinetic characteristics of CH 4 /H 2 O molecules. The outer surfaces of the HNTs exhibit relative hydrophobicity and adsorb CH 4 molecules to form nanobubbles. Moreover, CH 4 molecules adsorbed on the outer surface are tightly trapped between the hydrate solids and the outer surface, inhibiting their kinetic behavior and favoring CH 4 storage via physisorption. The inner surface of the HNTs exhibits extreme hydrophilicity and strongly adsorbs H 2 O molecules; thus, CH 4 hydrate can form inside of the HNTs. It is more difficult for CH 4 and H 2 O molecules inside of the HNTs to convert into hydrates than for those outside of the HNTs. A moderate gas-water ratio is advantageous for CH 4 physisorption and hydrate formation, whereas excessively high or low gas-water ratios are unfavorable for efficient CH 4 storage. These insights can help to develop an efficient CH 4 solidification storage technology.

Hydrogen is a clean-burning fuel that can be converted to other forms. of energy without generating any greenhouse gases. Currently, hydrogen is stored either by compression to high pressure (>700 bar) or cryogenic cooling to liquid form (<23 K). Therefore, it is essential to develop safe, reliable, and energy-efficient storage technology that can store hydrogen at lower pressures and temperatures. In this work, we systematically designed 2902 Mg-alkoxide-functionalized covalent-organic frameworks (COFs) and performed high-throughput (HT) computational screening for hydrogen storage applications at 111, 231, and 296 K. To accurately model the interaction between Mg-alkoxide sites and molecular hydrogen, we performed MP2 calculations to compute the hydrogen binding energy for different types of functionalized models, and the data were subsequently used to fit modified-Morse force field (FF) parameters. Using the developed FF models, we conducted HT grand canonical Monte Carlo (GCMC) simulations to compute hydrogen uptakes for both original and functionalized COFs. The generated data were subsequently used to evaluate the materials' gravimetric and volumetric storage performance at various temperatures (111, 231, and 296 K). Finally, we developed machine learning (ML) models to predict the hydrogen storage performance of functionalized structures based on the features of the original structures. The developed model showed excellent performance with a mean absolute error (MAE) of 0.061 wt % and 0.456 g/L for predicting the gravimetric and volumetric deliverable capacities, enabling a quick evaluation of structures in a hypothetical COF database. The screening results demonstrated that the Mg-alkoxide functionalization yields greater improvements in volumetric H 2 storage capacities for COFs with smaller pores compared to those with larger (mesoporous) pores.

The confinement effect of porous materials on the thermodynamical equilibrium of the CO 2 hydrogenation reaction presents a cost-effective alternative to transition metal catalysts. In metal-organic frameworks, the type of metal center has a greater impact on the enhancement of formic acid production than the scale of confinement resulting from the pore size. The M-MOF-74 series enables a comprehensive study of how different metal centers affect HCOOH production, minimizing the effect of pore size. In this work, molecular simulations were used to analyze the adsorption of HCOOH and the CO 2 hydrogenation reaction in M-MOF-74, where M = Ni, Cu, Co, Fe, Mn, Zn. We combine classical simulations and density functional theory calculations to gain insights into the mechanisms that govern the low coverage adsorption of HCOOH in the surrounding of the metal centers of M-MOF-74. The impact of metal centers on the HCOOH yield was assessed by Monte Carlo simulations in the grand-canonical ensemble, using gas-phase compositions of CO 2 , H 2 , and HCOOH at chemical equilibrium at 298.15-800 K, 1-60 bar. The performance of M-MOF-74 in HCOOH production follows the same order as the uptake and the heat of HCOOH adsorption: Ni > Co > Fe > Mn > Zn > Cu. Ni-MOF-74 increases the mole fraction of HCOOH by ca. 10 5 times compared to the gas phase at 298.15 K, 60 bar. Ni-MOF-74 has the potential to be more economically attractive for CO 2 conversion than transition metal catalysts, achieving HCOOH production at concentrations comparable to the highest formate levels reported for transition metal catalysts and offering a more valuable molecular form of the product.

Two-dimensional (2D) boron-based materials are receiving much attention as H 2 storage media due to the low atomic mass of boron and the stability of decorating alkali metals on the surface, which enhance interactions with H 2 . This work investigates the suitability of Li, Na, and K decorations on 2D honeycomb borophene oxide (B 2 O) for H 2 storage, using dispersion corrected density functional theory (DFT-D2). A high theoretical gravimetric density of 8.3 wt % H 2 is achieved for the Li-decorated B 2 O structure. At saturation, each Li binds to two H 2 with an average binding energy of -0.24 eV/H 2 . Born-Oppenheimer molecular dynamics simulations at temperatures of 100, 300, and 500 K demonstrate the stability of the Li-decorated structure and the H 2 desorption behavior at different temperatures. Our findings indicate that Li-decorated 2D B 2 O is a promising material for reversible H 2 storage and recommend experimental investigation of 2D B 2 O as a potential H 2 storage medium.

According to the ISO 14687-2:2019 standard, the water content of H 2 fuel for transportation and stationary applications should not exceed 5 ppm (molar). To achieve this water content, zeolites can be used as a selective adsorbent for water. In this work, a computational screening study is carried out for the first time to identify potential zeolite frameworks for the drying of high-pressure H 2 gas using Monte Carlo (MC) simulations. We show that the Si/Al ratio and adsorption selectivity have a negative correlation. 218 zeolites available in the database of the International Zeolite Association are considered in the screening. We computed the adsorption selectivity of each zeolite for water from the high-pressure H 2 gas having water content relevant to vehicular applications and near saturation. It is shown that due to the formation of water clusters, the water content in the H 2 gas has a significant effect on the selectivity of zeolites with a helium void fraction larger than 0.1. Under each operating condition, five most promising zeolites are identified based on the adsorption selectivity, the pore limiting diameter, and the volume of H 2 gas that can be dried by 1 dm 3 of zeolite. It is shown that at 12.3 ppm (molar) water content, structures with helium void fractions smaller than 0.07 are preferred. The structures identified for 478 ppm (molar) water content have helium void fractions larger than 0.26. The proposed zeolites can be used to dry 400-8000 times their own volume of H 2 gas depending on the operating conditions. Our findings strongly indicate that zeolites are potential candidates for the drying of high-pressure H 2 gas.

Water ice and gas hydrates can coexist in the permafrost and polar regions on Earth and in the universe. However, the role of ice in the mechanical response of ice-contained methane hydrates is still unclear. Here, we conduct direct million-atom molecular simulations of ice-contained polycrystalline methane hydrates and identify a crossover in the tensile strength and average compressive flow stress due to the presence of ice. The average mechanical shear strengths of hydrate-hydrate bicrystals are about three times as large as those of hydrate-ice bicrystals. The ice content, especially below 70%, shows a significant effect on the mechanical strengths of the polycrystals, which is mainly governed by the proportions of the hydrate-hydrate grain boundaries (HHGBs), the hydrate-ice grain boundaries (HIGBs), and the ice-ice grain boundaries (IIGBs). Quantitative analysis of the microstructure of the water cages in the polycrystals reveals the dissociation and reformation of various water cages due to mechanical deformation. These findings provide molecular insights into the mechanical behavior and microscopic deformation mechanisms of ice-contained methane hydrate systems on Earth and in the universe.

A computational screening of 2930 experimentally synthesized metal-organic frameworks (MOFs) is carried out to find the best-performing structures for adsorption-driven cooling (AC) applications with methanol and ethanol as working fluids. The screening methodology consists of four subsequent screening steps for each adsorbate. At the end of each step, the most promising MOFs for AC application are selected for further investigation. In the first step, the structures are selected on the basis of physical properties (pore limiting diameter). In each following step, points of the adsorption isotherms of the selected structures are calculated from Monte Carlo simulations in the grand-canonical ensemble. The most promising MOFs are selected on the basis of the working capacity of the structures and the location of the adsorption step (if present), which can be related to the applicable operational conditions in AC. Because of the possibility of reversible pore condensation (first-order phase transition), the mid-density scheme is used to efficiently and accurately determine the location of the adsorption step. At the end of the screening procedure, six MOFs with high deliverable working capacities (∼0.6 mL working fluid in 1 mL structure) and diverse adsorption step locations are selected for both adsorbates from the original 2930 structures. Because the highest experimentally measured deliverable working capacity to date for MOFs with methanol is ca. 0.45 mL mL -1 , the selected six structures show the potential to improve the efficiency of ACs.

The separation and purification of light hydrocarbons is challenging in the industry. Recently, a ZJNU-30 metal-organic framework (MOF) has been found to have the potential for adsorption-based separation of olefins and diolefins with four carbon atoms [H. M. Liu et al. Chem.-Eur. J. 2016, 22, 14988-14997]. Our study corroborates this finding but reveals Fe-MOF-74 as a more efficient candidate for the separation because of the open metal sites. We performed adsorption-based separation, transient breakthrough curves, and density functional theory calculations. This combination of techniques provides an extensive understanding of the studied system. Using this MOF, we propose a separation scheme to obtain a high-purity product.