Structure and dynamics of ethane confined in silica nanopores in the presence of CO2 (original) (raw)
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CO2–C4H10 Mixtures Simulated in Silica Slit Pores: Relation between Structure and Dynamics
The Journal of Physical Chemistry C, 2015
Equilibrium molecular dynamics simulations were conducted for pure n-butane and for mixtures containing n-butane and carbon dioxide confined in 2 nm wide slit-shaped pores carved out of cristobalite silica. A range of thermodynamic conditions was explored, including temperatures ranging from sub-critical to super-critical, and various densities. Preferential adsorption of carbon dioxide near the-OH groups on the surface was observed, where the adsorbed CO 2 molecules tend to interact simultaneously with more than one-OH group. Analysis of the simulation results suggests that the preferential CO 2 adsorption to the pore walls weakens the adsorption of n-butane, lowers the activation energy for n-butane diffusivity, and consequently enhances n-butane mobility. The diffusion results obtained for pure CO 2 are consistent with strong adsorption on the pore walls, as the CO 2 self-diffusion coefficient is low at low densities, increases with loading, and exhibits a maximum as the density is increased further because of hindrance effects. As the temperature increases, the maximum in self-diffusion coefficient is narrower, steeper and shifted to lower loading. The simulation results are also quantified in terms of molecular density profiles for both butane and CO 2 and in terms of residence time of the various molecules near the solid substrate. Our results could be useful for designing separation devices and also for better understanding the behavior of fluids in sub-surface environments.
Molecular Simulation of CO2 and H2 Encapsulation in a Nanoscale Porous Liquid
Nanomaterials
In this study we analyse from a theoretical perspective the encapsulation of both gaseous H2 and CO2 at different conditions of pressure and temperature in a Type II porous liquid, composed by nanometric scale cryptophane-111 molecules dispersed in dichloromethane, using atomistic molecular dynamics. Gaseous H2 tends to occupy cryptophane–111’s cavities in the early stages of the simulation; however, a remarkably greater selectivity of CO2 adsorption can be seen in the course of the simulation. Calculations were performed at ambient conditions first, and then varying temperature and pressure, obtaining some insight about the different adsorption found in each case. An evaluation of the host molecule cavities accessible volume was also performed, based on the guest that occupies the pore. Finally, a discussion between the different intermolecular host–guest interactions is presented, justifying the different selectivity obtained in the molecular simulation calculations. From the resu...
Molecular Simulation, 2015
Equilibrium molecular dynamics simulations were conducted to study the competitive adsorption and diffusion of mixtures containing n-octane and carbon dioxide confined in slitshaped silica pores of width 1.9 nm. Atomic density profiles substantiate strong interactions between CO2 molecules and the protonated pore walls. Non-monotonic change in n-octane self-diffusion coefficients as a function of CO2 loading was observed. CO2 preferential adsorption to the pore surface is likely to attenuate the surface adsorption of n-octane, lower the activation energy for n-octane diffusivity, and consequently enhance n-octane mobility at low CO2 loading. This observation was confirmed by conducting test simulations for pure noctane confined in narrower pores. At high CO2 loading, n-octane diffusivity is hindered by molecular crowding. Thus, n-octane diffusivity displays a maximum. In contrast, within the concentration range considered here, the self-diffusion coefficient predicted for CO2 exhibits a monotonic increase with loading, which is attributed to a combination of effects including the saturation of the adsorption capacity of the silica surface. Test simulations suggest that the results are strongly dependent on the pore morphology, and in particular on the presence of edges that can preferentially adsorb CO2 molecules and therefore affect the distribution of these molecules equally on the pore surface, which is required to provide the effective enhancement of n-octane diffusivity.
Langmuir, 2017
Despite the multiple length and time scales over which fluid-mineral interactions occur, interfacial phenomena control the exchange of matter and impact the nature of multiphase flow, as well as the reactivity of C−O−H fluids in geologic systems. In general, the properties of confined fluids, and their influence on porous geologic phenomena are much less well understood compared to those of bulk fluids. We used equilibrium molecular dynamics simulations to study fluid systems composed of propane and water, at different compositions, confined within cylindrical pores of diameter ∼16 Å carved out of amorphous silica. The simulations are conducted within a single cylindrical pore. In the simulated system all the dangling silicon and oxygen atoms were saturated with hydroxyl groups and hydrogen atoms, respectively, yielding a total surface density of 3.8 −OH/nm 2. Simulations were performed at 300 K, at different bulk propane pressures, and varying the composition of the system. The structure of the confined fluids was quantified in terms of the molecular distribution of the various molecules within the pore as well as their orientation. This allowed us to quantify the hydrogen bond network and to observe the segregation of propane near the pore center. Transport properties were quantified in terms of the mean square displacement in the direction parallel to the pore axis, which allows us to extract self-diffusion coefficients. The diffusivity of propane in the cylindrical pore was found to depend on pressure, as well as on the amount of water present. It was found that the propane self-diffusion coefficient decreases with increasing water loading because of the formation of water bridges across the silica pores, at sufficiently high water content, which hinder propane transport. The rotational diffusion, the lifespan of hydrogen bonds, and the residence time of water molecules at contact with the silica substrate were quantified from the simulated trajectories using the appropriate autocorrelation functions. The simulations contribute to a better understanding of the molecular phenomena relevant to the behavior of fluids in the subsurface.
Energy & Fuels, 2018
One of the main mechanisms contributing to enhanced oil recovery processes using compressed (supercritical) carbon dioxide (sc-CO 2) is alterations in the oil−water interfacial properties. However, it has been a challenge to experimentally investigate such effects. In our investigation presented here, we performed molecular dynamics simulations to explore these changes. We studied the role of sc-CO 2 in changing the interfacial and transport properties of systems composed of water and pure hydrocarbons, namely, hexane, octane, benzene, and xylene. The simulations were performed at 100 bar and 350 K. It was observed that sc-CO 2 accumulates at the interface, which leads to a reduction in the interfacial tension (IFT) of water−oil systems. Our further analysis of such accumulation showed that the ratio of sc-CO 2 density at the interface to sc-CO 2 bulk density decreases as the sc-CO 2 mole fraction increases. This interesting behavior is owed to the difference in the interaction between CO 2 and water and between CO 2 and hydrocarbon, which diverges as the CO 2 mole fraction increases in the system. Moreover, our investigation indicated that sc-CO 2 forms a film between the two phases, which displaces oil molecules away from the interface. This film was stabilized by hydrogen bonds between water and CO 2. We also found that, as the CO 2 content increases, the interfacial width increases, which contributes negatively to the IFT. Furthermore, it was found that, as the sc-CO 2 mole fraction increases, the hydrocarbon diffusion coefficients increase. The diffusivity response to CO 2 addition was determined by the molecular weight and polarity of the hydrocarbon.
Membranes, 2021
The behavior of fluids under nano-confinement varies from that in bulk due to an interplay of several factors including pore connectivity. In this work, we use molecular dynamics simulations to study the behavior of two fluids—ethane and CO2 confined in ZSM-22, a zeolite with channel-like pores of diameter 0.55 nm isolated from each other. By comparing the behavior of the two fluids in ZSM-22 with that reported earlier in ZSM-5, a zeolite with pores of similar shape and size connected to each other via sinusoidal pores running perpendicular to them, we reveal the important role of pore connectivity. Further, by artificially imposing pore connectivity in ZSM-22 via inserting a 2-dimensional slab-like inter-crystalline space of thickness 0.5 nm, we also studied the effect of the dimensionality and geometry of pore connectivity. While the translational motion of both ethane and CO2 in ZSM-22 is suppressed as a result of connecting the pores by perpendicular quasi-one-dimensional pores ...
Energy & Fules Journal, 2018
One of the main mechanisms contributing to enhanced oil recovery processes using compressed (supercritical) carbon dioxide (sc-CO 2) is alterations in the oil−water interfacial properties. However, it has been a challenge to experimentally investigate such effects. In our investigation presented here, we performed molecular dynamics simulations to explore these changes. We studied the role of sc-CO 2 in changing the interfacial and transport properties of systems composed of water and pure hydrocarbons, namely, hexane, octane, benzene, and xylene. The simulations were performed at 100 bar and 350 K. It was observed that sc-CO 2 accumulates at the interface, which leads to a reduction in the interfacial tension (IFT) of water−oil systems. Our further analysis of such accumulation showed that the ratio of sc-CO 2 density at the interface to sc-CO 2 bulk density decreases as the sc-CO 2 mole fraction increases. This interesting behavior is owed to the difference in the interaction between CO 2 and water and between CO 2 and hydrocarbon, which diverges as the CO 2 mole fraction increases in the system. Moreover, our investigation indicated that sc-CO 2 forms a film between the two phases, which displaces oil molecules away from the interface. This film was stabilized by hydrogen bonds between water and CO 2. We also found that, as the CO 2 content increases, the interfacial width increases, which contributes negatively to the IFT. Furthermore, it was found that, as the sc-CO 2 mole fraction increases, the hydrocarbon diffusion coefficients increase. The diffusivity response to CO 2 addition was determined by the molecular weight and polarity of the hydrocarbon.
Extensive nonequilibrium molecular dynamics simulations were carried out to study the transport and separation of binary mixtures of CO 2 and n-alkane chains in a molecular pore network model of nanoporous carbon molecular-sieve membrane. Separation of both sub-and supercritical mixtures was studied. The membrane was generated atomistically by the Voronoi tessellation of the space, using tens of thousands of atoms. The simulations indicate that there is an optimal pressure drop for separation of mixtures of CO 2 and n-alkanes, when the mixtures enter the membrane under supercritical conditions. In addition, separation of CO 2 from n-alkanes becomes increasingly more efficient as the size of the alkane increases.
When water molecules are confined to nanoscale spacings, such as in the nanometer size pores of activated carbon fiber (ACF), their freezing point gets suppressed down to very low temperatures (∼ 150 K), leading to a metastable liquid state with remarkable physical properties. We have investigated the ambient pressure diffusive dynamics of water in microporous Kynol TM ACF-10 (average pore size ∼11.6Å, with primarily slit-like pores) from temperature T = 280 K in its stable liquid state down to T = 230 K into the metastable supercooled phase. The observed characteristic relaxation times and diffusion coefficients are found to be respectively higher and lower than those in bulk water, indicating a slowing down of the water mobility with decreasing temperature. The observed temperature-dependent average relaxation time τ when compared to previous findings indicate that it is the size of the confining pores -not their shape -that primarily affects the dynamics of water for pore sizes larger than 10Å. The experimental observations are compared to complementary molecular dynamics simulations of a model system, in which we studied the diffusion of water within the 11.6Å gap of two parallel graphene sheets. We find generally a reasonable agreement between the observed and calculated relaxation times at the low momentum transfer Q (Q ≤ 0.9Å −1 ). At high Q however, where localized dynamics becomes relevant, this ideal system does not satisfactorily reproduce the measurements. Consequently, the simulations are compared to the experiments at low Q, where the two can be best reconciled. The best agreement is obtained for the diffusion parameter D associated with the hydrogen-site when a representative stretched exponential function, rather than the standard bi-modal exponential model, is used to parameterize the self-correlation function I(Q, t) .
Physical Chemistry Chemical Physics, 2010
To model the equilibrium and transport properties of carbonaceous molecular sieves (CMS) (i.e., carbon membranes, coals, activated carbons with ink-bottle pore geometry, etc.) the new microscopic turbostratic carbon pore model (TCPM) is developed. Analysis of experimental Gibbs excess of methane adsorption on Shirasagi CMS 3K-161 at 298 K indicates that investigated CMS is structurally a heterogonous material (i.e., it is composed of slit-shaped and turbostratic carbon nanopores of different sizes). The predicted absolute methane isotherm, total pore volume of 0.22 cm 3 g À1 , enthalpy of methane adsorption of 17.5-18.6 kJ mol À1 on Shirasagi CMS 3K-161 at 298 K are in good agreement with existing experimental and theoretical data. Applying TCPM, we model the equilibrium and kinetic separation of hydrogen and methane mixtures adsorbed in CMS turbostratic carbon nanopores at infinite dilution and 194.7, 293.2, 313.2, 423.2, and 573.2 K. We found that near ambient temperatures one can reach equilibrium selectivity of methane over hydrogen (CH 4 /H 2 ) of 10 2 in the turbostratic carbon nanopores having effective cage sizes of E5 Å . Lowering an operating temperature down to the dry ice one increases the equilibrium CH 4 /H 2 selectivity in these nanopores up to 10 3 . The kinetic selectivity of hydrogen over several investigated fluids, including: methane, argon, xenon, nitrogen, and carbon dioxide at studied operating conditions does not depend on the size of the carbon nanopore cage. This simply means that the kinetic separation factor is controlled by the size of the carbon nanopore constriction. Taking this into account, we predicted the effective size of the carbon nanopore constriction of real CMS from the experimentally measured kinetic H 2 /CH 4 selectivities at infinite dilution. The high kinetic H 2 /CH 4 selectivity of 10 2 -10 3 corresponds to the effective size of the carbon nanopore constriction of r2.958 Å (i.e., lower or equal to the collision diameter of hydrogen molecule). However, decreasing/ increasing of the effective size of the carbon nanopore constriction by E0.1-0.2 Å exponentially increases/decreases kinetic H 2 /CH 4 separation factor. Finally, we showed that the efficiency of kinetic separation at 298 K and infinite dilution depends on the s H 2 /s X and not only on s H 2 (where s denotes the collision diameter of hydrogen and the mentioned above fluids, respectively).