Grand Canonical Molecular Dynamics Research Articles

Adsorption and Diffusion of Shale Gas Reservoirs in Modeled Clay Minerals at Different Geological Depths

The rising worldwide energy demands and the difficulty in developing novel clean energies have greatly stimulated the exploitation of shale gas. Understanding adsorption and diffusion of shale gas under different geological depths is an important issue. In this work, we use grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations to investigate adsorption and diffusion behavior of shale gas (main component is methane) in a modeled shale in different burial depths up to 6 km. To examine the diffusion of shale gas, the equilibrium configuration of GCMC simulation is used as initial inputs for further MD simulations. The results indicate that the capacity of shale gas increases slightly with the depth, while the diffusion coefficient of shale gas in the shale matrix decreases with the increase of the pressure. Interestingly, a maximum diffusion coefficient of methane appears in a burial depth of 5 km. By cooperatively considering adsorption and diffusion results, we propose that the optimum operating condition is under a depth of 3–5 km. Moreover, we find that, when the basal spacing increases to 100 Å, the diffusion coefficients obtain an improvement of 80 times compared to the case with basal spacing of 8 Å, which provides useful guidance for exploitation of shale gas.

Effects of electrostatic interactions on gas adsorption and permeability of MOF membranes

We examined the effects of electrostatic interactions on gas adsorption and permeability of metal organic framework (MOF) membranes and MOF-filled mixed matrix membranes (MMMs) for CO2/CH4, CO2/N2 and H2/CO2 separations using molecular simulations. Adsorption isotherms and diffusion rates of CO2, CH4, N2 and H2 in several MOFs were computed using grand canonical Monte Carlo and equilibrium molecular dynamics simulations, respectively. Gas permeability and selectivity of pure MOF membranes and MOF-filled MMMs were then evaluated using theoretical permeation models. The accuracy of our molecular simulations was validated by comparing theoretical predictions for gas permeability and selectivity of MOF-filled MMMs with the available experimental data. We then used the same modelling approach to predict the performance of new MOF-filled MMMs in CO2/CH4, CO2/N2 and H2/CO2 separations. Promising MOF/polymer combinations which offer high selectivity and permeability relative to pure polymer membranes were identified. Our results showed that including electrostatic interactions between adsorbate molecules and MOF atoms is crucial for modelling pure MOF membranes but has less significance for modelling MOF-filled MMMs whenever the MOF volume fraction, ≤ 0.30. This result suggests that preliminary screening studies of MOF-filled MMMs can be carried out without assigning partial charges to MOF atoms.

Molecular Modelling of Cellulose Dissolution

In this work we present computational studies that shed light on the molecular mechanism of the initial stages of cellulose dissolution in saturated steam, which is an important pretreatment step in the conversion of lignocellulose to biofuel. The COMPASS, Dreiding and Universal molecular mechanics force fields and the B3LYP density functional with 6-311G, 6-311++G(d, p) and 6-311++G(2d, 2p) basis sets were used to study systems containing glucose, cellobiose and water. These molecular systems were studied since they are sufficiently small to perform the density functional theory calculations in a tractable time, while also being relevant to the dissolution of cellulose in saturated steam. Comparison of the energies and structures obtained from the three force fields with those obtained from the first principles method showed that the COMPASS force field is preferred to the other two and that this force field gives similar structures obtained from the first principles method. This supports the validity of the COMPASS force field for studying cellulose dissolution in saturated steam, and preliminary simulations were performed using grand canonical Monte Carlo and molecular dynamics simulations of cellulose dissolution in saturated steam at 100 degrees C and 1 bar, 160 degrees C and 6.2 bar, and 250 degrees C and 39.7 bar. The results show that the cellulose crystal dissolves in saturated steam at the higher temperatures and pressures.

Mean Residence Time of CO2 Molecules in Flexible ZIF-8 Cages Explored by Molecular Dynamics Simulations

The adsorption sites and diffusion mechanism of CO2 molecules in the flexible Zn(MeIM)2 (MeIM=2-methylimidazole) (ZIF-8) have been investigated by grand canonical Monte Carlo and molecular dynamics simulations. A reasonable time correlation function is for the first time constructed to explore the mean residence time of CO2 molecules in the ZIF-8 cages, suggesting that CO2 molecules can remain in the same cage for up to several tens of picoseconds. Furthermore, we find that the mean residence time almost linearly increases with the increasing pressure (or loading) at 273 and 298 K.

A comparative study by the grand canonical Monte Carlo and molecular dynamics simulations on the squeezing behavior of nanometers confined liquid films

Grand canonical Monte Carlo (GCMC) and liquid-vapor molecular dynamics (LVMD) simulations are performed to investigate the squeezing and phase transition of a simple liquid argon film confined between two solid surfaces. Simulation results show that the LVMD simulation is capable of capturing the major thermodynamic equilibrium states of the confined film, as predicted by the GCMC simulations. Moreover, the LVMD simulations reveal the non-equilibrium squeeze out dynamics of the confined film. The study shows that the solvation force hysteresis, observed in many surface force experiments, is attributed to two major effects. The first is related to the unstable jumps during the laying transitions of the confined film, in which the gradient of force profile is larger than the driving spring constant. The second effect is related to the squeeze out dynamics of the confined film even though the first effect is absent. In general, these two dynamic processes are non-equilibrium in nature and involve significant energy dissipations, resulting in the force hysteresis.

Molecular Simulations of Supercritical Fluid Permeation through Disordered Microporous Carbons

Fluid transport through microporous carbon-based materials is inherent in numerous applications, ranging from gas separation by carbon molecular sieves to natural gas production from coal seams and gas shales. The present study investigates the steady-state permeation of supercritical methane in response to a constant cross-membrane pressure drop. We performed dual control volume grand canonical molecular dynamics (DCV-GCMD) simulations to mimic the conditions of actual permeation experiments. To overcome arbitrary assumptions regarding the investigated porous structures, the membranes were modeled after the CS1000a and CS1000 molecular models, which are representative of real microporous carbon materials. When adsorption-induced molecular trapping (AIMT) mechanisms are negligible, we show that the permeability of the microporous material, although not significantly sensitive to the pressure gradient, monotonically decreases with temperature and reservoir pressures, consistent with diffusion theory. However, when AIMT occurs, the permeability increases with temperature in agreement with experimental data found in the literature.

A Multiscale Study of MOFs as Adsorbents in H2 PSA Purification

In this multiscale study, four robust zirconium oxide based metal–organic frameworks (MOFs) were examined using powerful molecular simulation tools as well as indispensable full-scale PSA system modeling to determine their potential for H2 purification. Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were employed to evaluate the MOF working capacities, binary-mixture selectivities, and micropore transport diffusivities for each of the components of a steam methane reformer offgas (SMROG) stream: H2, CO, CH4, N2, and CO2. The small, functionalized pores of UiO-66(Zr)-Br were found to result in high N2 and CO selectivities and working capacities, whereas the slightly larger pore volume of UiO-66(Zr) favored higher CO2 and CH4 working capacities. The collective impact of impurity uptakes and selectivities on the purification of H2 from five-component steam methane reformer offgas mixtures was investigated through PSA column modeling. The breakthrough behavior of SMROG mixtures in ...

Competitive I2 Sorption by Cu-BTC from Humid Gas Streams

Competitive sorption of molecular iodine gas from a mixed stream containing iodine and water vapor is identified and characterized for the hydrophilic Cu-BTC metal–organic framework. By combining simulation (Grand Canonical Monte Carlo and molecular dynamics simulations) with crystallography (high-energy synchrotron-based powder X-ray diffraction data and pair distribution function analyses), we show that I2 substantially adsorbs, in preference to water vapor, into two principal areas. First, it adsorbs in the smallest cage close to the copper paddle wheel. Second, it adsorbs within the main pore with close interactions with the benzene tricarboxylate organic linker. Analysis suggests that I2 forms an effective hydrophobic barrier to minimize water sorption. The finding is relevant to mixed gas streams in nuclear energy industrial processes and accident remediation. This also represents the highest reported I2 sorption by a metal–organic framework (175 wt % I2 or 3 I/Cu).

Relation Between Flow Enhancement Factor and Structure for Core-Softened Fluids Inside Nanotubes

The relationship between enhancement flow and structure of core-softened fluids confined inside nanotubes has been studied using nonequilibrium molecular dynamics simulation. The fluid was modeled with different types of attractive and purely repulsive two length scale potentials. Such potentials reproduce in bulk the anomalous behavior observed for liquid water. The dual control volume grand canonical molecular dynamics method was employed to create a pressure gradient between two reservoirs connected by a nanotube. We show how the nanotube radius affects the flow enhancement factor for each one of the interaction potentials. The connection between structural and dynamical properties of the confined fluid is discussed, and we show how attractive and purely repulsive fluids exhibit distinct behaviors. A continuum to subcontinuum flow transition was found for small nanotube radius. The behavior obtained for the core-softened fluids is similar to what was recently observed in all-atom molecular dynamics simulations for classical models of water and also in experimental studies. Our results are explained in the framework of the two length scale potentials.

Computational design of tetrahedral silsesquioxane-based porous frameworks with diamond-like structure as hydrogen storage materials

A novel type of three-dimensional (3D) tetrahedral silsesquioxane-based porous frameworks (TSFs) with diamond-like structure was computationally designed using the density functional theory (DFT) and classical molecular mechanics (MM) calculations. The hydrogen adsorption and diffusion properties of these TSFs were evaluated by the methods of grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The results reveal that all designed materials possess extremely high porosity (87–93 %) and large H2 accessible surface areas (5,268–6,544 m2 g−1). Impressively, the GCMC simulation results demonstrate that at 77 K and 100 bar, TSF-2 has the highest gravimetric H2 capacity of 29.80 wt%, while TSF-1 has the highest volumetric H2 uptake of 65.32 g L−1. At the same time, the gravimetric H2 uptake of TSF-2 can reach up to 4.28 wt% at the room temperature. The extraordinary performances of these TSF materials in hydrogen storage made them enter the rank of the top hydrogen storage materials so far.

Molecular Simulation of Oxygen Sorption and Diffusion in the Poly (lactic acid)

Grand canonical Monte Carlo and molecular dynamics simulation methods are used to simulate oxygen sorption and diffusion in amorphous poly(lactic acid) (PLA). The simulated solubility coefficient of oxygen is close to experimental data obtained from the quartz crystal microbalance but much higher than those from the time-lag method. This discrepancy is explained by using the dual-mode sorption model. It is found that oxygen sorption in PLA is predominantly Langmuir type controlled, i.e., through the process of filling holes. The time-lag method only takes into account oxygen molecules that participate the diffusion process whereas a large proportion of oxygen molecules trapped in the void have little chance to execute hopping due to the glassy nature of PLA at room temperature. The simulated diffusion coefficient of oxygen is reasonably close to the data obtained from the time-lag method. The solubility coefficient of oxygen decreases linearly with increasing relative humidity while its diffusion coefficient firstly decreases and then increases as a function of relative humidity.

Predicting Noble Gas Separation Performance of Metal Organic Frameworks Using Theoretical Correlations

In this work, we examined the accuracy of theoretical correlations that predict the performance of metal organic frameworks (MOFs) in separation of noble gas mixtures using only the single-component adsorption and diffusion data. Single component adsorption isotherms and self-diffusivities of Xe, Kr, and Ar in several MOFs were computed by grand canonical Monte Carlo and equilibrium molecular dynamics simulations. These pure component data were then used to apply Ideal Adsorbed Solution Theory (IAST) and Krishna–Paschek (KP) correlation for estimating the adsorption isotherms and self-diffusivities of Xe/Kr and Xe/Ar mixtures at various compositions in several representative MOFs. Separation properties of MOFs such as adsorption selectivity, working capacity, diffusion selectivity, permeation selectivity, and gas permeability were evaluated using the predictions of theoretical correlations and compared with the data obtained from computationally demanding molecular simulations. Results showed that theoret...

Methane adsorption and diffusion in a model nanoporous carbon: an atomistic simulation study

A constricted slit model was introduced to improve, one step further, the performance of the simple slit model in prediction of the adsorption and diffusion behavior of simple molecules in the nanoporous carbons (NPCs). The grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations are performed to study the adsorption and diffusion behavior of methane within the constricted slit models. The models are called slit-1, 2, and 3 with constriction heights 5, 7, and 9 A respectively. For comparison, we used the slit-0 name for the simple slit without constriction. Adsorption results show that at low pressures, the constriction increases the adsorbed amount irrespective of its height. Slit-2 with a constriction height as a molecular diameter has the greatest heat of adsorption and has highest loading at pressures up to 3,000 kPa. At high pressures, when all pores are filled, the adsorption trend is in line with the pore volumes of slits where slit-0 with higher pore volume is dominant. The density profiles in the models were calculated and examined. The spatial distribution of adsorbed methane molecules was examined by various radial distribution functions calculated by MD. Also, MD simulation results show that the diffusion coefficient of methane decreases in constricted slits. The calculated diffusion coefficients in slit-2 in the direction of the constriction are one order of magnitude smaller than the calculated one in the simple slit model but it is far from the experimental values in the NPCs.

Effects of elevated temperature on the structure and properties of calcium–silicate–hydrate gels: the role of confined water

The binding phase of cementitious materials, calcium–silicate–hydrates, can be described as nanogranular and as an inorganic hydrogel. Similar to other hydrated “soft matter,” the water confined within the nano- to microscale pores of such cementitious materials plays a crucial role in the structure and properties of cement pastes. When compared to organic hydrogels, non-stoichiometric calcium–silicate–hydrates (C–S–H) are relatively robust against changes in humidity and temperature. However, under extreme physical environments, changes in the amount, and location, and physical state of water can limit damage tolerance and sustainability of otherwise stiff and strong cementitious macrostructures. Here, we employed Grand Canonical Monte-Carlo and Molecular Dynamics simulations to investigate the effect of temperature on the water content within and between C–S–H grains constituting the cement microstructure, and on the associated physical and mechanical properties of this material. We found water content within grains decreased with increasing relative temperature up to T/T* = 2 (where T* is the transition temperature at which the bulk liquid and gas are in equilibrium for a given pressure), and that C–S–H grains densified with attendant increases in heat capacity, stiffness, and hardness. Although intragranular cohesion increased monotonically with increasing relative temperature over this range, intergranular cohesion increased up to a relative temperature of T/T* = 1.1 and then decreased at higher relative temperatures. This finding suggests a rationale for the decreased mechanical performance of cement paste and concrete at high relative temperatures, and supports previous claims of peak hardness in C–S–H at an intermediate relative temperature between 1 and 2.61. Further, these atomistic simulations underscore the important role of confined water in modulating the structure and properties of calcium–silicate–hydrates upon exposure to extreme environments.

Gas adsorption and diffusion in a highly CO2 selective metal–organic framework: molecular simulations

Grand canonical Monte Carlo and equilibrium molecular dynamics simulations were used to assess the performance of an rht-type metal–organic framework (MOF), Cu-TDPAT, in adsorption-based and membrane-based separation of CH4/H2, CO2/CH4 and CO2/H2 mixtures. Adsorption isotherms and self-diffusivities of pure gases and binary gas mixtures in Cu-TDPAT were computed using detailed molecular simulations. Several properties of Cu-TDPAT such as adsorption selectivity, working capacity, diffusion selectivity, gas permeability and permeation selectivity were computed and compared with well-known zeolites and MOFs. Results showed that Cu-TDPAT is a very promising adsorbent and membrane material especially for separation of CO2 and it can outperform traditional zeolites and MOFs such as DDR, MFI, CuBTC, IRMOF-1 in adsorption-based CO2/CH4 and CO2/H2 separations.

Confined fluid and the fluid-solid transition: Evidence from absolute free energy calculations

The debate on whether an organic fluid nanoconfined by mica sheets will undergo a fluid-to-solid transition as the fluid film thickness is reduced below a critical value has lasted over two decades. Extensive experimental and simulation investigations have thus far left this question only partially addressed. In this work, we adapt and apply absolute free energy calculations to analyze the phase behavior of a simple model for nanoconfined fluids, consisting of spherical Lennard-Jones (LJ) molecules confined between LJ solid walls, which we use in combination with grand-canonical molecular dynamics simulations. Absolute Helmholtz free energy calculations of the simulated nanoconfined systems directly support the existence of order-disorder phase transition as a function of decreasing wall separation, providing results in close agreement with previous experiments and detailed atomistic simulations.

Solvated calcium ions in charged silica nanopores

Hydroxyl surface density in porous silica drops down to nearly zero when the pH of the confined aqueous solution is greater than 10.5. To study such extreme conditions, we developed a model of slit silica nanopores where all the hydrogen atoms of the hydroxylated surface are removed and the negative charge of the resulting oxygen dangling bonds is compensated by Ca(2+) counterions. We employed grand canonical Monte Carlo and molecular dynamics simulations to address how the Ca(2+) counterions affect the thermodynamics, structure, and dynamics of confined water. While most of the Ca(2+) counterions arrange themselves according to the so-called "Stern layer," no diffuse layer is observed. The presence of Ca(2+) counterions affects the pore filling for strong confinement where the surface effects are large. At full loading, no significant changes are observed in the layering of the first two adsorbed water layers compared to nanopores with fully hydroxylated surfaces. However, the water structure and water orientational ordering with respect to the surface is much more disturbed. Due to the super hydrophilicity of the Ca(2+)-silica nanopores, water dynamics is slowed down and vicinal water molecules stick to the pore surface over longer times than in the case of hydroxylated silica surfaces. These findings, which suggest the breakdown of the linear Poisson-Boltzmann theory, provide important information about the properties of nanoconfined electrolytes upon extreme conditions where the surface charge and ion concentration are large.

Adsorption of Ethanol Vapor on Mica Surface under Different Relative Humidities: A Molecular Simulation Study

The adsorption of ethanol vapor on a mica surface at 298 K and different relative humidities (RHs) are studied using grand canonical Monte Carlo and molecular dynamics simulations. The simulations show that the adsorbed ethanol molecules form a monolayer on the mica surface, sharply contrasting the behavior of water, which forms multiple adsorption layers on the mica surface. Simulations of an ethanol and water mixture reveal that the adsorbed molecules are segregated into a water-rich domain near the mica surface and an ethanol-rich domain on top of the water-rich domain. The water-rich domain exhibits multilayers unless the RH is extremely low (<1%), whereas the ethanol-rich domain exhibits a monolayer. These findings are supported by calculations of the isosteric heats of adsorption and analyses of configurations, concentrations, and diffusivities of molecules in different layers.

Accelerating Applications of Metal–Organic Frameworks for Gas Adsorption and Separation by Computational Screening of Materials

The selection of metal-organic frameworks (MOFs) for gas adsorption and separation has become a significant challenge over the past decade because of the large number of new structures reported every year. We applied a multiscale computational approach to screen existing MOFs for CO(2)/N(2) separation. Pore characteristics of 1163 MOFs were analyzed by the method developed by Haldoupis, Nair, and Sholl (Haldoupis, E.; Nair, S.; Sholl, D. S. J. Am. Chem. Soc.2010, 132, 7528) using a simple steric model. On the basis of the pore size analysis, 359 MOFs were examined by classical molecular simulations. Adsorption and diffusion properties were computed using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, respectively. These molecular simulations were used to assess which materials hold the greatest promise as membrane materials for CO(2)/N(2) separations. Finally, density functional theory (DFT) calculations were performed to provide preliminary information on the dynamic framework motion of selected MOFs.

Atomically Detailed Modeling of Metal Organic Frameworks for Adsorption, Diffusion, and Separation of Noble Gas Mixtures

Atomically detailed simulations have been widely used to assess gas storage and gas separation properties of metal organic frameworks (MOFs). We used molecular simulations to examine adsorption, diffusion, and separation of noble gas mixtures in MOFs. Adsorption isotherms and self-diffusivities of Xe/Kr and Xe/Ar mixtures at various compositions in ten representative MOFs were computed using grand canonical Monte Carlo and equilibrium molecular dynamics simulations. Several properties of MOFs such as adsorption selectivity, working capacity, diffusion selectivity, permeation selectivity, and gas permeability were evaluated and compared with those of traditional nanoporous materials. Results showed that MOFs are promising candidates for Xe/Kr and Xe/Ar separations due to their high Xe selectivity and permeability.