Interconnected Cylindrical Pores Research Articles

Insights into the Solubility of Carbon Dioxide in Grafted Mesoporous Silica for the Catalytic Synthesis of Cyclic Carbonates by Nanoconfinement.

Gas solubility can go beyond classical bulk-liquid Henry's law saturation under the nanoconfinement of a liquid phase. This concept establishes the foundation of the current study for developing a novel catalytic system for transformation of carbon dioxide to cyclic carbonates at mild conditions with major emphasis on application for CO2 capture and utilization. A series of mesoporous silica-based supports of various pore sizes and shapes grafted with a quaternary ammonium salt is synthesized and characterized. CO2 sorption in styrene oxide, either in bulk or nanoconfined state, as well as catalytic reactivity for CO2 transformation into styrene carbonate, are experimentally evaluated. The family of mesoporous catalysts with aligned cylindrical pores (MCM-41 and SBA-15) with pore sizes ranging from 3.5 to 9 nm exhibit enhanced sorption of CO2 in nanoconfined styrene oxide with maximum sorption capacity taking place in MCM-41 with the smallest pore size. The catalysts with interconnected cylindrical pores (KIT-6) with pore sizes ranging from 4.5 to 8.7 nm showed CO2 solubilities almost equal to the bulk solubility of styrene oxide. Monte Carlo simulations revealed that the oversolubility in styrene oxide confined complex is directly related to the density of adsorbed solvent in the nanopore, which is less than its bulk density. Catalytic reactivities correlate with CO2 sorption enhancement, showing higher turnover frequencies for catalysts having higher CO2 sorption capacity. The turnover frequency is increased by a factor of 7.5 for grafted MCM-41 with the smallest pore size with nanoconfined styrene oxide in comparison to the homogeneous reaction implemented in bulk.

Pore Network Drying Model for Particle Aggregates: Assessment by X-Ray Microtomography

Convective drying of disordered glass bead packings has been investigated both experimentally and numerically. X-ray microtomography (XMT) and image analysis techniques have been used to determine the three-dimensional spatial distribution of the liquid and solid phases at the pore scale within the wet particle aggregates. The evolution of the liquid distribution in the aggregate has been tracked during the drying process. Particle center coordinates and radii have been extracted from the X-ray images using binarization and segmentation techniques. Based on this geometric data for a real aggregate, a pore network approximation of the pore space has been generated from a Voronoi tessellation about particle centers by designating Voronoi edges as interconnected cylindrical pores with radii computed from the distance between neighboring particles. This three-dimensional irregular pore network takes into account both the geometrical and topological characteristics (pore size distribution and connectivity) of the actual pore space. Drying simulations have been carried out for the pore networks obtained from the XMT and results are presented as phase distributions and moisture profiles. The simulated liquid phase distributions are in qualitative agreement with the experimental result, which indicates that pore network models are suited to describe the drying of dense particle aggregates at the pore scale.

An irregular pore network model for convective drying and resulting damage of particle aggregates

An irregular pore network model for convective drying of random aggregates composed of mono-sized primary particles is introduced in this study. The network structure is generated by Voronoi tessellation of the aggregate and by designating the Voronoi edges as interconnected cylindrical pores; pore radii are computed from the distances to the neighboring particles. The obtained irregular structure captures many essential features of the pore space of real aggregates such as random node locations, distributed coordination number of nodes, randomly established pore connections as well as random spatial correlations of pore radius. In a next step, an isothermal pore network drying model, which uses invasion percolation concepts, is generalized to irregular structures, and drying simulations are carried out. Results are given as phase distributions, moisture profiles and drying rate curves. From this information, local capillary forces are approximated. These are seen as the reason for micro-cracks in the network of bonded particles. In a one-way coupling approach, the capillary forces are applied to the aggregate and the mechanical behavior is simulated by Discrete Element Method (DEM) to investigate inter-particle bond responses for different drying conditions.

High “C” rate Li-S cathodes: sulfur imbibed bimodal porous carbons

A highly ordered mesoporous carbon with a bimodal pore structure which exhibits a high specific area and large pore volume, was synthesized by a triblock-copolymer-templating approach. This optimized framework served as the scaffold for the preparation of carbon/sulfur (C/S) nanocomposites that serve as novel cathodes for Li-S batteries. They exhibit high discharge capacities and good cycling stability at very high current rates of 1675 mA g−1 (1 C), which can be attributed to the unique bimodal porous structure of the carbon. The small mesopores contain the majority of the sulfur mass and aid in suppressing the diffusion of polysulfide species into the electrolyte, whilst the large interconnected cylindrical pores favour rapid transport of solvated Li+ on charge/discharge. Additional doping with hydrophilic nanoporous silica also aids in capacity retention on cycling.

Monte Carlo simulation and experimental heat and mass transfer in direct contact membrane distillation

A Monte Carlo (MC) simulation model is developed to study heat and mass transfer through hydrophobic membranes applying direct contact membrane distillation (DCMD) process. In this study, the membrane pore space is described by a three-dimensional network model of inter-connected cylindrical pores with distributive pore size. Vapor flux through membrane pores is described by gas transport mechanism(s) based on the kinetic theory of gases. The present MC model can take into consideration the influence of temperature polarization phenomenon, membrane physical properties including pores interconnectivity and the DCMD fluid dynamic conditions. The developed model can simultaneously predict the DCMD process vapor flux and membrane surface temperatures, contrary to other models in which one of them has to be given in order to solve the other. The model is comprehensive in its approach and does not involve any adjustable parameter. The simulated results were compared with the experimental ones of different membranes and the comparisons were found to be in excellent qualitative and quantitative agreement.

Stability of Cubic Ice in Mesopores

We examined the effect of pore geometry on the stability of cubic ice in mesopores by measuring the change of the X-ray diffraction pattern for ice confined in cylindrical, interconnected cylindrical, interconnected spherical, and interconnected irregular pores of silica during a heating process. The results obtained clearly indicated that the stability of cubic ice in the mesopores depends on the pore geometry. Cubic ice confined to the interconnected spherical cavities of KIT-5 is metastable with respect to hexagonal ice, similar to bulk cubic ice. On the other hand, cubic ice confined to the cylindrical and interconnected cylindrical pores of SBA-15 and KIT-6 is stable up to the melting point of the ice, in accord with previous studies.

A Monte Carlo simulation model for vacuum membrane distillation process

Vacuum membrane distillation (VMD) process received a great deal of attention by many investigators because of its promising applications in many separation areas. Theoretical models published in literature describe vapor flux through membrane pores ignoring membrane pore space interconnectivity. Those models, in most cases are restricted to Knudsen type of flow and in a less extent to the combined Knudsen-viscous mechanism also described by the dusty gas model. This paper is one of a series of papers in which a Monte Carlo simulation model is developed to describe the vapor flux across the membrane in association with membrane distillation process. Unlike earlier paper in the series, this paper deals with VMD for the first time. The membrane pore space is described by a three-dimensional network of interconnected cylindrical pores, having a pore size distribution. The model can take into account all related mass transport mechanisms based on the kinetic theory of gases. When this model was applied to VMD process, it was assumed that water vapor is the only permeant, the VMD process is in an isothermal condition and there is no surface effect. The effects of the bulk feed temperature, downstream pressure and membrane pore size on the simulated VMD flux were discussed. The Monte Carlo model predictions were found to be in qualitative agreement with available experimental data when Knudsen type of flow was considered. When viscous type of flow is considered, the simulation results indicate that the higher feed solution temperature and higher pore size do not necessarily increase the vapor flux.

Effects of pore size distribution and pore-architecture assembly on drying characteristics of pore networks

Simulation of isothermal drying using two-dimensional networks comprised of interconnected cylindrical pores is presented. Transport of moisture inside pore segments was described by Fick’s law. The results have shown that the shielding of large pores by the smaller pores in the stochastic pore network, which is supposed to be representative of real porous medium, causes the lower drying rate and hence lower effective diffusion coefficient as compared to those predicted from the idealized network of pores with a single size. The strength of shielding is found to vary with the characteristics of pore size distribution as interpreted by the moisture concentration experienced by the pores, which is remarkably different amongst the pore size distributions. The inefficient transport of moisture through the stochastic pore network can be improved or even better with the suitable architecturally assembled structure. The minimum shielding archetype network, appearing very high porous at particle surface, is predicted to enhance greatly the drying rate. On the other hand, the maximum shielding network, which is small pores allocated onto the network exterior, exhibits the slowest drying rate.

Shirasu Porous Glass membrane emulsification: Characterisation of membrane structure by high-resolution X-ray microtomography and microscopic observation of droplet formation in real time

The microstructure of Shirasu Porous Glass (SPG) membrane has been investigated using metallographic microscope and high-resolution X-ray microtomography (XMT) and the obtained results are compared with Hg-porosimetry data. The porosity in 600 cross sections analysed by high-resolution XMT was found to vary in a narrow range between 52.5 and 57.4% with a mean value of 55.1%. The membrane microstructure looks similar on SEM and XMT images with tortuous, interconnected cylindrical pores extending in all directions within the membrane. The formation of O/W and W/O/W emulsion droplets have been observed in real time using SPG or Microporous Glass (MPG) membrane disks with a mean pore size between 10.2 and 16.2 μm. The quality of video recordings was much better when membrane surface was finely polished with diamond paste, although it did not affect the droplet formation behaviour. The droplets formed at the same pore were highly monodispersed and detached in regular time intervals, but this time interval showed significant variations for different pores. The SDS-stabilised droplets were detached from the pores as soon as they were formed, due to strong electrostatic repulsions between anionic droplets and negatively charged SPG surface. The Tween 80-stabilised droplets were kept attached to the membrane surface after formation, before being pushed away by the next droplet formed at the same pore. Under the same conditions the SDS-stabilised droplets were smaller than the droplets stabilised by Tween 80.

Simulation of heat and mass transfer in direct contact membrane distillation (MD): The effect of membrane physical properties

A Monte Carlo simulation model is developed to study vapor flux through hydrophobic membranes in association with direct contact membrane distillation (DCMD) process. The porous membrane is represented by a three-dimensional network model of inter-connected cylindrical pores with distributive effective pore sizes. Vapor flux through membrane pores is described by gas transport mechanisms founded on the kinetic theory of gases for a single cylindrical pore. This model can take into consideration the effects of temperature polarization phenomenon, porous membrane physical properties, including membrane pores inter-connectivity (topology), process dynamics, and apply them into the prediction of process vapor flux (permeability) and the description of MD transporting process. The model can simultaneously solve for the MD processes vapor flux and membrane effective surface temperatures, contrary to other models in which one of them has to be given in order to solve for the other. We argue that this Monte Carlo simulation model can adequately describe MD processes transport phenomenon and predicts process vapor flux without resorting to any adjustable parameters. It is comprehensive in its approach; and it can be applied to all configurations of MD processes published in the literature. The results obtained, in general, show an excellent qualitative agreement with available experimental data.

A Monte Carlo simulation model for membrane distillation processes: direct contact (MD)

Direct contact membrane distillation (DCMD) has a great potential for applications in many areas of separation processes. Usually, in these processes, water constitutes a major component in fluid permeation, which is caused by the temperature difference between the feed and permeate. The transport models that are found in the literature in association with this process are often based on the Dusty-Gas model (DGM) for gas transport through porous media. These models’ predictions are restricted to those cases in which only molecular diffusion and/or viscous flow contribute to the transport process. In this paper, a Monte Carlo simulation model is developed to study vapor permeation through membrane pores in association with DCMD. A three-dimensional network of interconnected cylindrical pores with a pore size distribution represents the porous membrane. The model takes also into account all molecular transport mechanisms based on the kinetic gas theory for a single cylindrical tube. The mechanisms are further dictated by the mean free path of permeating vapor molecules, by the size of the pores and by the operating conditions. Therefore, this model is comprehensive in its approach and can be applied to all forms of membrane distillation (MD) processes without resorting to any adjustable parameters. Model prediction(s) are in good qualitative agreement with available experimental data.

A Monte Carlo simulation model for membrane distillation processes: direct contact (MD)

Direct contact membrane distillation (DCMD) has a great potential for applications in many areas of separation processes. Usually, in these processes, water constitutes a major component in fluid permeation, which is caused by the temperature difference between the feed and permeate. The transport models that are found in the literature in association with this process are often based on the Dusty-Gas model (DGM) for gas transport through porous media. These models’ predictions are restricted to those cases in which only molecular diffusion and/or viscous flow contribute to the transport process. In this paper, a Monte Carlo simulation model is developed to study vapor permeation through membrane pores in association with DCMD. A three-dimensional network of interconnected cylindrical pores with a pore size distribution represents the porous membrane. The model takes also into account all molecular transport mechanisms based on the kinetic gas theory for a single cylindrical tube. The mechanisms are further dictated by the mean free path of permeating vapor molecules, by the size of the pores and by the operating conditions. Therefore, this model is comprehensive in its approach and can be applied to all forms of membrane distillation (MD) processes without resorting to any adjustable parameters. Model prediction(s) are in good qualitative agreement with available experimental data.

Diffusive relaxation processes and low-frequency dynamical properties in bulk and confined ethylene glycol by neutron spectroscopy

In this work, we report on a detailed study of the diffusional and vibrational dynamics of liquid ethylene glycol (EG: HO–CH2CH2–OH) in the bulk state and confined in a matrix of sol-gel porous glass with 26 Å interconnected cylindrical pores. The investigation is performed, by incoherent quasielastic and inelastic neutron scattering (IQENS and IINS), as a function of temperature. IQENS data have been definitively interpreted in terms of the α-relaxation dynamics predicted by mode coupling theory for supercooled liquids. We verified that a description by means of an Havriliak–Negami profile is consistent with the measured spectral line shape. Furthermore, IINS spectra show a clear different evolution versus T of the vibrational density of states, Z(ω), passing from the bulk samples to the confined ones, revealing, in particular, a thermal stabilization of the disordered longitudinal acoustic mode induced by interaction of EG molecules with the active silanol (Si–OH) groups of the surface.

Vibrational dynamics of ethylene glycol in mesoscopic confinement by incoherent inelastic neutron scattering (IINS) investigation

In this paper, we go deeply into an analysis performed, as a function of T, by means of incoherent inelastic neutron scattering (IINS), of vibrational dynamics of liquid ethylene glycol (EG: HO–CH 2CH 2–OH) in the bulk state and confined into 26 Å interconnected cylindrical pores of a sol-gel porous glass. IINS spectra revealed, as far as the vibrational density of states, Z( ω) is concerned, a different trend in temperature passing from bulk to confined samples. In particular, we observed a thermal stabilization of the disordered longitudinal acoustic mode as a consequence of the interaction between the hydroxyl groups (OH) of the EG molecules and the active silanol groups (Si–OH) on the surface of the porous glass.

Diffusional and vibrational dynamics of confined ethylene glycol and homologous systems: a light and neutron scattering investigation

The understanding of new phenomena arising in confined condensed matter is important for the physics of interfaces and finite systems. This has stimulated intensive research on solids, liquid crystals and liquids confined in different geometries. Experimental techniques and theoretical approaches have shown that a liquid confined to a region that approaches molecular dimension exhibits dynamic and thermodynamic properties strongly changed relatively to the bulk with increasing geometrical confinement. This was interpreted by geometric as well as absorption effects: the first ones are related to the tortuosity of the interconnected channel for diffusion, the second ones take into account the degree of adsorption of molecules on the active inner surface sites. We present a detailed study of the reorientational, vibrational and diffusional dynamics of ethylene glycol (EG: OH–CH 2CH 2OH), ethylene glycol monomethyl ether (EGmE: CH 3–OCH 2CH 2OH) and ethylene glycol dimethyl ether (EGdE: CH 3–OCH 2CH 2OCH 3), in the bulk state and confined in a matrix of sol–gel porous glass with 26 Å interconnected cylindrical pores. This investigation has been performed by means of Rayleigh wing (as far as reorientational dynamics is concerned), Raman (as far as vibrational dynamics is concerned) and Incoherent Quasi Elastic Neutron Scattering (as far as diffusional dynamics is concerned). The clear influence of the confinement on the mobility of the studied liquids, with a dramatic frozen-in effect with respect to the bulk state, is evidenced.

Diffusive Relaxations and Vibrational Properties of Water and H-bonded Systems in Confined State by Neutrons and Light Scattering: State of the Art

A detailed spectroscopic analysis is presented in order to study the reorientational and vibrational dynamics of water and other fundamental H-bonded systems: ethylene glycol (HO−CH2−CH2−OH) and its homologous species, such as ethylene glycol monomethyl ether (CH3O−CH2−CH2−OH) and ethylene glycol dimethyl ether (CH3O−CH2−CH2−OCH3), propylene glycol (HO−CH(CH3)−CH2−OH) and its oligomers, that are confined in a matrix of sol−gel porous glasses with 26 A interconnected cylindrical pores. The use of different spectroscopic techniques, light scattering (Rayleigh wing and Raman scattering), FT-IR absorption, neutron scattering (incoherent quasi elastic and inelastic neutron scattering, IQENS and IINS, respectively) allow the marking of different dynamical parameters, with different probes being used to investigate H-bonded systems. The clear influence of the confinement on the mobility of the studied liquids, with a dramatic frozen-in observed effect respect to the bulk state, is evidenced and compared with li...

Experimental and theoretical study of oxygen diffusion within packed beds of carbon black

Abstract Modelling the performance of regenerative soot traps for Diesel exhausts requires a knowledge of oxygen diffusivity within beds of Diesel soot. For convenience carbon black, a commercial product with similar properties to Diesel soot, is often used for combustion experiments. Both of these carbonaceous materials are composed of spherules of approximately 20 nm in diameter assembled within aggregates. The oxygen diffusivity within beds of carbon black with different densities was measured at 22°C. An increase in the porosity of the bed from 0.746 to 0.820 increased the diffusion coefficient of oxygen from 0.7×10 −6 to 2.0×10 −6 m 2 s −1 . Two simulations of molecular movement in aggregates of spheres or ‘cannonball’ solids were performed. The kinetic theory (KT) model assumes that the carbon spherules are regularly placed at the summits of cubes and a mean travel distance replaces the mean free path. The Monte Carlo (MC) model is based on a random walk among spherules placed randomly but homogeneously. The diffusivity values returned by the two models are strongly dependent on the tortuosity of the bed. The KT model returned oxygen diffusivities that were similar to those measured while the MC model gave values which were higher. When the MC values were modified to allow for non-homogeneous packing, i.e., uneven distribution of density, the results were improved. The concept of tortuosity, which is based on interconnected cylindrical pores, seems unrealistic in the case of beds of highly porous cannonball materials. Beds of Diesel soot are so open that Fickian diffusion of oxygen should occur, whereas combined Fickian and Knudsen diffusion seems to operate within the beds of carbon black. The mechanisms are uncertain since Knudsen diffusion within aggregates of carbon black or Diesel soot probably controls the overall diffusion process.

Multicomponent Diffusion and Reaction in Three-Dimensional Networks: General Kinetics

Over the last 10 years the design of catalyst particles and porous structures has made considerable progress. Due to the complicated interaction of diffusion and reaction in catalysts, more detailed models of porous structures are needed. We have based our model on a three-dimensional network of interconnected cylindrical pores as pore model, although the treatment is applicable to alternative pore geometries, e.g., slit pores. The network assumed has predefined distributions of pore radii, connectivity, and porosity. Mass transport in the individual pores of the network is described by the dusty-gas model. In contrast to previous publications, the present network model can be applied to any common reaction kinetics. This becomes quite inevitable in order to make three-dimensional network models applicable to practical problems in industry. To solve the mass balances within the entire network, the mass balances for individual pores have to be solved simultaneously, since these mass balances are coupled by...

Transport of macromolecules in porous media

Three models of flow and diffusion of macromolecules in a porous medium are developed. The first model is fully deterministic and is, for given pore space and molecular configurations, exact. It can take into account the effect of both the convection and diffusion of the macromolecules, is valid for both steady and transient transport, and can yield all the important quantities characterizing the transport process. The second model is stochastic in which the macromolecules execute a random walk in the pore space, the transition probability of which takes into account the effect of molecular and pore sizes, and the flow field in the medium. The third model is based on an effective-medium approximation and can be used for both steady and transient diffusion of the molecules in the pore space. The porous medium is represented by a two- or three-dimensional network of interconnected cylindrical pores, in which the effective pore sizes are selected from an experimentally measured pore size distribution. The macromolecules are represented by hard spheres of a given hydrodynamic radius. Detailed calculations are carried out with the three models, and the advantages and disadvantages of each model are discussed. The predictions are compared with the experimental data on polymer diffusion in a porous catalyst, and in silica glasses. Good agreement between the predictions and the data is found in both cases. The extension of the models to more complex molecular and pore shapes is also discussed.

Computer simulation of particle transport processes in flow through porous media

In this paper we develop a Monte Carlo computer simulation model for a class of particle transport processes in flow through a porous medium. This class of problems includes transport of macromolecules in porous media, fines migration, flow of stable emulsion, deep-bed filtration and size-exclusion chromatography. The porous medium is represented by a three-dimensional network of interconnected cylindrical pores with nonuniform (possibly fractal) surfaces. The effective radii of the pores are distributed according to an experimentally-measured pore size distribution. The paths of the particles throughout the pore space are determined rigorously, taking into account the effect of various forces that contribute to the interaction of the particles with the pore space. The model can also take into account the effect of possible pore plugging, particle deposition and macromolecular adsorption on the surface of the pores, in which case such phenomena are percolation processes and are characterized by a percolation threshold which is the volume fraction of the open pores below which the medium loses its macroscopic connectivity. When the model is applied to the problem of fines migration in flow through a porous medium, the predictions are in quantitative agreement with the available experimental data.