Pore Intersections Research Articles

Effects of Mixing at Pore Intersections on Large‐Scale Dissolution Patterns and Solute Transport

AbstractThe flow‐induced dissolution of porous rocks governs many important subsurface processes and applications. Solute mixing, which determines pore‐scale concentration fields, is a key process that affects dissolution. Despite its importance, the effects of pore‐scale mixing on large‐scale dissolution patterns have not been investigated. Here, we use a pore network model to elucidate the mixing effects on macroscopic dissolution patterns and solute transport. We consider two mixing rules at pore intersections that represent two end members in terms of the mixing intensity. We observe that the mixing effect on dissolution is the strongest at moderate Damköhler number, when the reactive and advective time scales are comparable. This is the regime where wormholes spontaneously appear. Incomplete mixing is shown to enhance flow focusing at the tips of the dissolution channels, which results in thinner wormholes and shorter breakthrough times. These effects on passive solute transport are evident independent of initial network heterogeneity.

Effects of mixing at pore intersections on large-scale dissolution patterns

Effects of mixing at pore intersections on large-scale dissolution patterns

Rapid and low-cost synthesis of ZSM-5 zeolite nanosheet assemblies for conversion of methanol to gasoline

Nanoscale zeolites with short diffusion paths show significantly enhanced mass transportation and easier accessibility for active sites in catalysis compared to microsized zeolites. However, the high cost and complex process involved in existing synthetic methods limit their application, so the development of a rapid and low-cost synthesis technology is of great significance. Herein, ZSM-5 zeolite nanosheet assemblies (Z5-NS-t) are rapidly synthesized by a microwave irradiation (MW) assisted seed-induced method using low-cost polyhexamethylene biguanide hydrochloride (PHMB) as a crystal growth inhibitor of the b-axis. The crystallization kinetics of the resultant materials and the formation mechanism of ZSM-5 zeolite nanosheets, as well as the influence of physico-chemical properties on their catalytic performance for the methanol-to-gasoline (MTG) reaction, are systematically investigated. Compared with the ZSM-5 nanoparticle assemblies (Z5-NP-60), which are synthesized by using the same method except for the addition of PHMB, Z5-NS-180 nanosheets with an approximate density of strong Brønsted acid sites exhibit excellent catalytic performance in the MTG reaction, giving rise to a high gasoline selectivity of 73.2% at a methanol conversion of 99.9% and a large amount of external carbon deposits. This is attributed to the synergy of accessibility of acid sites located in the pore intersections and the shorter diffusion paths for coke precursors out of the zeolite channel. This study provides an effective strategy for the synthesis of ZSM-5 nanosheets as catalysts for the MTG reaction, and it is expected to be applied for the synthesis of other zeolite nanosheets for important catalytic processes.

Dramatic change of methylenedianiline activity and selectivity in different pore geometry of zeolites

The activity and isomers selectivity of the synthesis of methylenedianiline (MDA) from aminal on the various zeolites with different topology structure have been investigated. The poor activity in H-ZSM-5 and H-mordenite contributed to the narrow porous and scarcity of pore intersections or supercages in the MOR structure, respectively. H-beta zeolite shown high activity for the rearrangement reaction can take place in the pore intersections of zeolite BEA. For the dealuminated USY catalysts, the highest activity and selectivity to 4,4′-MDA is attributed to the rearrangement of aminal in the supercages and mesoporous, which the mass transport limitation can be overcome.

Study of Structural Features of Porous TINI-Based Materials Produced by SHS and Sintering

Structural properties of porous TiNi-based materials produced by SHS method and sintering have been investigated. The material having different pore wall surface topography, porosity and pore size distribution was shown to be produced depending on the powder metallurgy method for porous TiNi-based alloy. All the materials having porosity of 55-70%, mean pore size 90-150 μm, as well as normal pore size distribution are most preferable. Ultimate strength and breaking point were determined to depend on porosity, pore size distribution, pore intersections and phase chemical composition of the material. Strength properties of the sintered alloy are twice as much compared to the SHS-produced ones due to homogeneity of its macrostructure, low chemical heterogeneity and TiNi3 precipitations strengthening the TiNi matrix.

Mass transport in porous media from first principles: An experimental and theoretical study

Abstract In the present work, the mass transport of helium through zeolite is experimentally determined by measuring the flow of helium through a zeolite membrane. By using a mathematical model, the mass transport through defects was accounted for to arrive at mass transport through zeolite pores. For the first time, we could thereby experimentally show that the mass transport of helium in zeolite pores is strongly controlled by the amount and location of hydrocarbons in the zeolite pores and varies several orders of magnitude. The mass transport of helium in ZSM-5 zeolite pores is first reduced gradually more than one order of magnitude when the loading of n-hexane is increased from 0 to 47% of saturation. As the loading of n-hexane is further increased to 54% of saturation, the mass transport of helium in the zeolite pores is further reduced abruptly by more than two orders of magnitude. This gradual decrease followed by an abrupt decrease of mass transport is caused by adsorption of n-hexane in the zeolite pores. In a similar yet different fashion, the mass transport of helium in the zeolite pores is reduced abruptly by almost two orders of magnitude when the loading of benzene is increased from 0 to 19% of saturation due to adsorption of benzene in the pore intersections. Effective medium approximation percolation models with parameters estimated using density functional theory employing the local density approximation, i.e. models with no adjustable parameters and the most sophisticated theory yet applied to this system, can adequately describe the experimental observations.

Mixing within fracture intersections during colloidal suspension flow

[1] The effects of mixing within pore or fracture intersections on dispersion in porous media are investigated through the development and use of different simulation techniques that play particular attention to the flow field and mass transport details inside the intersection region. The flow field in an orthogonal intersection is computed using a lattice Boltzmann technique. Particle trajectories are computed taking into account both convection and molecular diffusion and allow the calculation of the transverse and longitudinal dispersion coefficients over a wide range of Peclet number values. The mixing ratio in the intersection is also calculated and is used in two distinct ways to yield the transverse dispersion coefficient. It is found that a random walk method that is introduced here reproduces the dispersion coefficient with excellent accuracy in the low and moderate Peclet number range, thanks to the fact that it accounts for upstream motion of solute particles and for different travel times within each fracture. Both slip and no-slip conditions at the solid walls are used, and a direct comparison shows that the former, which is known to develop in the presence of a wetting film or a porous wall, for instance, reduces the longitudinal dispersion considerably but affects the transverse dispersion only slightly.

Experimental and modelling studies of the kinetics of mercury retraction from highly confined geometries during porosimetry in the transport and the quasi-equilibrium regimes

Mercury porosimetry is a widely used method for pore structural characterisation but, often, half of the data obtained in any given experiment, in the form of the retraction curve, is simply not used. This may be because mercury retraction is significantly less well understood than intrusion, and thus it is more difficult to extract reliable information. In this work, detailed studies have been made of the retraction process from mesoporous materials, with non-random heterogeneities in their void structure, to obtain fundamental knowledge that could form the basis of methods to reliably extract accurate pore space descriptors from the retraction curve. In one material the initiation of retraction was found to be independent of the experimental time-scale because it was governed by the rapid equilibration of the narrowing of mercury necks, and subsequent snap-off of mercury connections at a small number of pore intersections. This scenario has been validated by simulations of quasi-equilibrium mercury retraction on structural models obtained from magnetic resonance images. In another material, the slow relaxation of entrapped mercury was followed by integrated gas sorption experiments and SAXS. This process was also simulated using Monte-Carlo methods. It was found that the temporal evolution of the remaining pore size distribution was sensitive to the initial configuration of entrapped mercury ganglia, and could thus be used to validate specific models of mercury entrapment for particular amorphous materials.

Low‐Temperature Adsorption/Storage of Hydrogen on FAU, MFI, and MOR Zeolites with Various Si/Al Ratios: Effect of Electrostatic Fields and Pore Structures

Several zeolites, such as faujasite, mordenite, and ZSM-5, with various aluminum contents have been used to analyze the effect of aluminum or cation concentration (strength of electrostatic field) on hydrogen adsorption at low temperature. Irrespective of the zeolite structure, the adsorption capacity, isosteric heat of adsorption (-DeltaHads), surface coverage, and micropore occupancy increase with increasing aluminum content of a zeolite. Zeolites with a higher amount of aluminum favorably adsorb hydrogen at relatively low pressures. For zeolites with similar aluminum contents, the adsorption capacity, isosteric heat of adsorption, surface coverage, and micropore occupancy are in the order of mordenite>ZSM-5>faujasite, probably due to differing pore sizes and the presence or absence of pore intersections. This work demonstrates that zeolites with strong electrostatic fields and narrow pores without intersections are beneficial for high hydrogen uptake.

Capillary Condensation of Nitrogen in Ordered Mesoporous Silica with Bicontinuous Gyroid Structure

To know the accurate relationship between pore size and the pressure of capillary condensation or evaporation and also to elucidate the pore-connectivity effects on adsorption hysteresis and pore criticality for the ordered mesoporous silicas (MCM-48 and KIT-6) with bicontinuous gyroid structure, we carried out X-ray structural study of MCM-48 and KIT-6 and measurements of the adsorption−desorption isotherms of nitrogen in comparison with MCM-41 and SBA-15 with unconnected cylindrical pores. The results clearly show that the hydraulic diameter describes incorrectly the mean pore size of these materials expressing the strength of confinement under the assumption of cylindrical pores with no networking effects. In addition, the average pore diameter at the middle point of pore intersections still underestimates the strength of confinement compared to the unconnected cylindrical pores of MCM-41 and SBA-15. Despite a large difference in porous structure, the shape and thermal behavior of the adsorption hysteresis for KIT-6, as well as the sorption scanning behavior, were indistinguishable from those for SBA-15, indicating that interconnections among pores of almost the same size do not have a significant effect on the adsorption hysteresis and pore criticality.

Mechanism of hierarchical porosity formation in silica thin films using cellulose nitrate

Ordered mesoporous silica thin films have potential applications as, e.g., sorbents, catalysts, and analytical instruments. In addition to high internal surface areas, these applications also require high permeability, which can be obtained by engineering hierarchical porosity into the film. This paper presents a novel approach using cellulose nitrate to generate a system of larger pores (15–30 nm diameter) connected by the smaller mesopores (2–10 nm diameter) generated using traditional organic surfactants. The unimolecular reaction for cellulose nitrate deflagration avoids the usual diffusion limitations of traditional combustion synthesis. A family of hierarchically porous films was produced using a range of surfactant mixtures, and exhibited a range of surface areas along with relatively nonporous surface phases (‘skins’) on the film free boundaries. These skins were formed by an extrusion mechanism due to pore expansion coupled with the lateral (in-plane) symmetry constraints unique to thin films. The skin thickness depends primarily on the ratio of the large pore spacing ( l 0) over its diameter ( d). The specific surface area (SA, m 2/g) of the film increases monotonically as l 0/ d decreases, reaching a maximum at about l 0/ d ∼ 0.67. Precipitous reductions in SA for smaller values of l 0/ d are caused by pore intersections and the associated excluded surface areas. At high pore intersection (low l 0/ d), the film structure evolves from ‘a solid with pores’ to ‘pores surrounded by a cage-like silica matrix.’ The cage-like film structure is less susceptable to thin film cracking behavior, but is probably in a more metastable state that is susceptable to thermal or hydrothermal exposure.

On the evaluation of dispersion coefficients from visualization experiments in artificial porous media

High-resolution single source–solute transport experiments in glass-etched pore networks presented in a previous publication are used for quantifying the hydrodynamic dispersion as a function of Peclet number ( Pe). This type of artificial porous media is ideal for understanding the effects of pore-scale phenomena on solute dispersion thanks to its well characterized topology and pore geometry. The hydrodynamic dispersion coefficients are estimated by matching the spatial and temporal distributions of the solute concentration over various regions of the network with the numerical solution of the advection–dispersion equation and using a parameter space analysis to ensure well conditioning of the parameters. The source release function and assumptions on boundary conditions are found to affect significantly the reliability of the dispersion coefficient estimates. The estimated longitudinal dispersion coefficients are in close quantitative agreement with literature data for dispersion in porous media. The extracted transverse dispersivity ( α T) values indicate an apparent decrease of α T with increasing flow velocity, which is at variance of constant dispersivity approximation adopted commonly in soil hydrology models and is attributed to incomplete diffusive mixing at the pore intersections. Calculated values of an effective mixing ratio at pore junctions are in agreement with published theoretical estimates and reveal a transition from partial mixing to streamline routing as Pe increases within the range investigated here.

Blueprint for wafer-scale three-dimensional photonic band-gap synthesis by photoelectrochemical etching.

We present an architecture for three-dimensional photonic band-gap (PBG) material synthesis by oblique angle photoelectrochemical pore etching. This technique provides high aspect ratio pores, in which the pore diameter can be modulated by changing light intensity during the etch process. The naturally occurring "Kielovite" structure is a stretched version of the face-centered-cubic lattice of crisscrossing pores and exhibits a PBG to center frequency ratio of 8% in a background dielectric constant of 11.9 (silicon). We demonstrate that by modulating the pore radius in between pore intersections, the PBG can be doubled in size to nearly 16%. The enlarged PBG is robust against a number of structural perturbations.

Self-diffusion of charged colloidal tracer spheres in transparent porous glass media: Effect of ionic strength and pore size

The influence of charge on diffusion in porous media was studied for fluorescent colloidal silica spheres diffusing in a porous glass medium. The bicontinuous porous silica glasses were optically matched with an organic solvent mixture in which both glass and tracers are negatively charged. Using fluorescence recovery after photobleaching, the long-time self-diffusion coefficient DSL of the confined silica particles was monitored in situ as a function of the ionic strength and particle to pore size ratio. At high salt concentration DSL reaches a relatively high plateau value, which depends on the particle to pore size ratio. This plateau value is unexpectedly higher than the value found for uncharged silica spheres in these porous glasses, but still significantly smaller than the free particle bulk diffusion coefficient of the silica spheres. At low salt concentration DSL reduces markedly, up to the point where colloids are nearly immobilized. This peculiar retardation probably originates from potential traps and barriers at pore intersections due to deviations from cylinder symmetry in the double layer interactions between tracers and pore walls. This indicates that diffusion of charged particles in tortuous porous media may be very different from transport in long capillaries without such intersections.

Hysteresis and Avalanche Behavior of Helium in Anopore and Nuclepore

Superfluid4He exhibits hysteretic behavior in the porous material Nuclepore due to capillary condensation, and one observes avalanches in the draining for a narrow range of chemical potential. We have demonstrated that the avalanche behavior involves pores distributed at a low density across the entire sample, and that avalanches are enabled by the superfluid film. We present results which compare the behavior of superfluid4He in Nuclepore to its behavior in Anopore, a porous membrane having no interconnected pores, and draw conclusions concerning the role of the superfluid film and the pore intersections.

Memory, congruence, and avalanche events in hysteretic capillary condensation.

We report measurements of the hysteretic capillary condensation of helium in the porous material Nuclepore which display a hierarchy of return point memory erects and near congruence of closed internal hysteresis loops. Deviations from predictions based on the Preisach model of independent elementary hysteresis domains, originally developed to understand magnetic phenomena, are observed and are attributed to the appearance of additional domains on draining induced by the internal structure of the porous material. Pore intersections result in weak interactions among the pores which lead to avalanches

The length of intersection lines and the number of cusps in assemblies of interpenetrating spheres

A previously derived expression for the specific surface area of a dispersion of mutually penetrable spheres or pores is generalized for the calculation of the length of sphere–sphere intersections (lines common to two pores) and the number of cusps (points common to three pores). The results are in the form of alternating series involving correlation functions of successively higher order. Closed-form expressions are obtained for the case of randomly centered spheres. For a porous material with number density ρ of spherical pores of radius a, the length of pore–pore intersections per unit volume is 2π3a4ρ2 exp(−4πa3ρ/3) and the number of cusps per unit volume is (4)/(3) π4a6ρ3 exp(−4πa3ρ/3). The analysis is also extended to the general case of dispersions of hyperspheres in d dimensions.

Three-dimensional trajectory analysis and network simulation of deep bed filtration

A three-dimensional simulator of deep bed filtration in granular porous media is developed. The void space of the porous medium is modeled as a network of unit cells of the constricted-tube type with mouth and constriction diameters distributed according to prescribed size distributions. The rate of deposition in each cell is determined using the particle trajectory analysis which requires determination of the flow field in the unit cell and of all the forces and torques acting on the particle. The overall rate of deposition, expressed in terms of the filter coefficient, is determined through a detailed numerical procedure that involves simultaneous solution of the local deposition rate equations, the mass balance equation at the nodes of the network, and the equations of perfect mixing of outgoing streams at pore intersections (nodes). Numerical results agree with the experimental observation that deposition in oblique flow channels contributes substantially to the overall rate of deposition. It is found that substitution of even a few relatively large unit cells (weak collectors) into a network of otherwise uniformly sized unit cells increases the flow rate of lateral streams and leads to higher filter coefficients thanks to high impacted fractions in transverse pores.

ChemInform Abstract: Isomerization and Formation of Xylenes Over ZSM‐22 and ZSM‐23 Zeolites.

Abstract The shape selective catalytic properties of HZSM-22 and HZSM-23 in the isomerization of meta -xylene and methylation of toluene are compared with those of HZSM-5. Their activities in both these reactions decrease in the order ZSM-5 > ZSM-23 > ZSM-22. Their para selectivities, however, follow the reverse order. Even though all three zeolites contain pores constituted by 10 membered rings, differences in the shape of the pores as well as the presence of pore intersections (in ZSM-5) lead to significant differences in their shape selective catalytic properties. While loss of xylenes in the xylene isomerization process occurs due to transalkylation reactions over ZSM-5 zeolites, dealkylation is the main cause over ZSM-22. ZSM-23 occupies an intermediate position. ZSM-22 and ZSM-23 are shown to have superior shape selectivity vis-a-vis ZSM-5 even at high equivalent conversion levels.

Shape selectivity of zeolite EU-1 in reactions of aromatic hydrocarbons

The shape-selective properties of H-EU-1 in the isomerization of m-xylene and the methylation and disproportionation of toluene were studied and compared with those of H-ZSM-48. The presence of an one-dimensional ten-ring channel system together with deep side pockets off the channels in EU-1 leads to peculiar shape selectivity effects. The concentration of the para isomer in the isomerization products decreases in the order ZSM-48>EU-1>ZSM-5. The presence of large side pockets, in which bimolecular reactions such as xylene disproportionation can occur, leads to the following selectivity for isomerization versus disproportionation: ZSM-48>ZSM-5>EU-1. It is concluded that apart from the shape and size of the channel system, the presence or absence of pore intersections and cavities can also significantly affect the shape selectivity of medium-pore zeolites in catalytic reactions. It is suggested that shape selectivity effects in the isomerization of m-xylene can be used as diagnostic tools to reveal not only the size of the pores but also the presence or absence of cavities and the dimensionality of the pores in zeolites of unknown structure.