Pore Topology Research Articles

Direct N2O decomposition over iron-substituted small-pore zeolites with different pore topologies

Four cage-based, small-pore zeolites with similar framework Fe contents (0.7-0.9 wt%) but different framework structures (i.e., Fe-PST-7, Fe-LTA, Fe-SSZ-13, and Fe-RTH) have been synthesized and employed as direct N2O decomposition catalysts. When steam-activated, the light-off temperatures (770 K) for Fe-PST-7 and Fe-LTA were found to be about 40 K lower under wet feed conditions than that (810 K) for Fe-ZSM-5, the most widely studied catalyst for this reaction. Furthermore, their light-off temperatures were found to be even lower than the temperature (780 K) for Fe-FER, the best direct N2O decomposition catalyst. The overall characterization results of our study strongly suggest that the presence of strong Brønsted acid sites in zeolites, as well as the zeolite framework topology, has a profound effect on the type and distribution of extraframework Fe species, and therefore the N2O decomposition activity of supported Fe catalysts.

Investigating the effects of pore types on the pore geometry, topology and electrical resistivity in fractured and vuggy samples using pore-scale models

ABSTRACTThe interpretation of electrical resistivity for assessing the saturation in rocks with complicated pore structures and various pore types requires the intrinsic parameters quantifying pore structures incorporated in modified resistivity models. Therefore, the objectives are to investigate pore types effect on the pore morphology and electrical resistivity and to determine the key pore morphological parameters for each pore type. The maximal ball method was applied to extract the pore networks of the reconstructed pore-scale models using a process-based method, and the pore morphology was characterized using a pore-network analysis and a random walk simulation. The resistivity was simulated using a finite element method. The key parameters determined by the Morris screening method for the fractured, isolated and connected vuggy models were the Euler number, the cross-sectional pore-to-throat area ratio (PTAR) and the pore radius, respectively. The qualitative relationships between the Euler number, the PTAR and the porosity exponent for the fractured and vuggy models were determined and validated using three Coquinas carbonates. This investigation will help to elucidate the variations of the porosity exponent and to further derive a new electrical resistivity model that correlates the pore morphology to the resistivity in fractured and vuggy carbonate reservoirs.

Effects of pore clusters on the permeability, formation factor and porosity exponent of porous media

ABSTRACTMicropore and pore evolution both severely influence pore structure and transport properties. The micropore connects isolated pores to predominate connected pore cluster while the pore evolution generates isolated pores. Therefore, investigating separate impacts of predominate and remaining pore clusters is the first step to modify the existing electrical resistivity and permeability models with consideration of the micropore and pore evolution.The main pore clusters in six porous media including sandstone and carbonate were first extracted. Then, the pore geometry, topology, electrical resistivity and permeability from the main pore cluster compared with them from all the pore clusters in the same porous medium. The calculations of pore morphological and the petrophysical properties were all based on the pore networks extracted from the porous media by maximal ball method.The predominant pore cluster mainly determined the pore geometry, electrical resistivity and permeability of the corresponded porous medium. However, the isolated pore clusters from pore evolution undermined the pore connectivity. The developments of the remaining pore clusters slightly affected the electrical resistivity and permeability while seriously impacted the porosity exponents. Therefore, the micropore could increase the pore connectivity, and then slightly increase the permeability and reduce the electrical resistivity.

Architected cellular piezoelectric metamaterials: Thermo-electro-mechanical properties

Advances in additive manufacturing have recently made possible the manufacturing of smart materials with arbitrary microarchitectures, which leads to developing lightweight smart metamaterials with unprecedented multifunctional properties. In this paper, asymptotic homogenization (AH) method is developed for predicting the effective thermo-electro-mechanical properties of architected cellular piezoelectric metamaterials. The effect of pore microarchitecture (relative density and cell topology) and polarization direction on elastic, dielectric, piezoelectric, pyroelectric and thermal properties of periodic cellular piezoelectric metamaterials is explored. The pore topology is determined by Fourier series expansion. Alternative pore microarchitectures are considered by tailoring shape parameters, scaling factor, and rotation angle of the constitutive pore. Smart cellular metamaterials made of both single-phase (BaTiO3) and bi-phase (BaTiO3-expoy) piezoelectric materials are considered. Apart from effective thermo-electro-mechanical properties, a series of figures of merit for the cellular piezoelectric metamaterials, i.e. piezoelectric coupling constant, acoustic impedance, piezoelectric charge coefficient, hydrostatic figure of merit, current responsivity, voltage responsivity and pyroelectric energy harvesting figures of merit, are presented and the reason for difference between the figures of merit of different types of piezoelectric metamaterials is discussed. The figures of merit shed lights on the effect of microarchitecture on optimizing the multifunctional performance of smart cellular metamaterials for applications as structurally efficient multifunctional energy harvesters. It is found that the piezoelectric and pyroelectric figures of merit of cellular piezoelectric metamaterials can be significantly improved compared to the commonly used honeycomb cellular materials and composite materials with solid circular inclusion if an appropriate microarchitecture is selected for the pore. For example, piezoelectric charge coefficient (dh) for a transversely polarized single-phase cellular piezoelectric metamaterial with a solid volume fraction of 0.4 can be 350% higher than the corresponding figures of merit of honeycomb piezoelectric material; voltage responsivity of transversely polarized bi-phase cellular piezoelectric metamaterials with an inclusion volume fraction of 0.3 can be also 249% higher than the corresponding value of composite materials with a solid circular inclusion.

Role of Pore Chemistry and Topology in the CO2 Capture Capabilities of MOFs: From Molecular Simulation to Machine Learning

Open framework materials (OFMs) such as metal–organic frameworks (MOFs) can provide structurally and chemically tailorable nanopores. This exceptional tunability has allowed for careful positioning of optimal adsorption sites within MOF pores to enable selective CO2 physisorption, making these materials promising for energy-efficient CO2 capture. However, given the multitude of features that can be simultaneously altered within the thousands of MOFs synthesized to date, it can be daunting to elucidate the most critical features for boosting CO2 capture capabilities. Here we use a multiscale approach—density functional theory (DFT), grand canonical Monte Carlo (GCMC), and machine learning (ML)—to investigate the role of various pore chemical and topological features in the enhancement of CO2 capture metrics of MOFs. To enable a thorough “sweep” of a target region of MOF structure-space, we used computational synthesis methods to create sets of MOFs encompassing all possible combinations of 16 topologies an...

Thermal behaviour of clay ceramics obtained by Spark Plasma Sintering: Is fractal geometry a new possible road to design porous structures?

Porous clay ceramics represent an important class of construction materials. Their properties are strongly conditioned by microstructure features. In particular, a special attention has to be paid to porosity (pore fraction, pore shape, pore size, pore size distribution and topology distribution). In this work, thermal behaviour of porous ceramics has been studied comparing different modelling procedure. Fractal modelling, proposed in this paper, proves to be most reliable in predicting thermal behaviour of porous ceramics. For this reason, a design procedure for obtaining porous structures with specific thermal properties has been illustrated.

Multiphysics modeling of water transport in high-intensity lignite drying process on pore scale

ABSTRACTThe understanding of the moisture transfer process in the pore network is quite important to improve the lignite drying efficiency. Scanning electronic microscopy image was used for construction of pore topology closely approximating the true topology of the real lignite for the heat and mass transfer processes on pore scale by COMSOL simulation. Considering the gas‒liquid phase coexistence of water, “Laminar Two-Phase Flow, Phase Field” module and “Liquid Heat Transfer” module were used. The pore size had significant effects on the flow velocity and the larger pores acted as the main pathway for the moisture transport, therefore affected the maximum drying rate. On the other hand, the connection of pores and the throats distribution in the pathway also had a significant effect on the flow velocity, and the moisture between the throats was hard to transfer as a flow, maybe by vapor diffusion. In high-intensity lignite drying process, the moisture vaporization quickly when heated up and vapor pressure was beneficial to keep the pore size and ensure the smooth of moisture flow pathway, thus improving the efficiency of the drying process.

Ice Nanoribbons Confined in Uniaxially Distorted Carbon Nanotubes

Water confined inside nanopores exhibits unusual static and dynamic properties that depend on the pore size, pore topology, and hydrophobicity and roughness of the pore walls. The properties also d...

Femtosecond laser-induced porosity on poly(ethylene) surfaces—A crystallographic and rheological study

In recent years, femtosecond (fs) laser irradiation of polymer surfaces has been shown to produce novel porous topologies that present a multi-scale roughness. In our study, grazing incidence x-ray diffractometry results showed the appearance of a monoclinic crystalline phase on fs laser machined high density polyethylene (HDPE) which is a deformation of the orthorhombic phase typically observed in pristine HDPE. This was accompanied by a local decrease in crystallinity. These findings confirmed that the induced porosity was a consequence of the rapid quenching of a superheated melt layer undergoing phase explosion. In addition, several poly(ethylene) samples with different average molecular weights were machined under the same conditions. Scanning electron micrography, along with small angle oscillatory shear tests, indicated that the final pore size decreases with increasing average molecular weight or increasing melt viscosity since these parameters act against bubble growth during phase explosion. In addition, through computed tomography, the internal structure of the porous layer was investigated. Through this technique, the average pore size and pore size distribution can be computed by chord length analysis. In conclusion, in this study, the mechanisms behind the formation of femtosecond laser-induced porous structures and the parameters that control their dimensions have been established.

Carbide-derived carbons for dense and tunable 3D graphene networks

The mechanical properties of carbide-derived carbons (CDCs) are computed using molecular dynamics simulations, spanning the experimental density range and synthesis temperatures. The structures consist of nanoporous networks with continuous graphene walls enclosing the pores. Calculation of elastic constants and simulation of tensile strain reveal a direct relationship between the microstructure and elasticity, with the density and temperature inducing significant changes in the pore topology and medium-range order. CDCs have a high elastic moduli and high ultimate tensile strengths while showing resistance to brittle fracture. This suggests that CDCs are a promising route to achieve dense 3D graphene networks with tunable mechanical properties.

Pushing the Limits on Metal-Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for o-Xylene Isomerization and Disproportionation.

Acid-catalyzed skeletal C-C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal-organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WO x clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WO x-ZrO2, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal-organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization.

The Biocompatibility of Nanoporous Acupuncture Needles

We investigate the biocompatibility of a new class of acupuncture needles that possess a hierarchical nano/microscale porous surface topology, referred to as porous acupuncture needles (PAN). The PAN is synthesized via a facile electrochemical anodization technique by which a surface area approximately 20 times greater than a conventional acupuncture needle, of approximately the same diameter, is obtained. PAN biocompatibility is evaluated using a variety of standard tests, with results indicating that the PAN can safely be used within therapeutic practice.

Green Synthesis of Cyclodextrin-Based Metal-Organic Frameworks through the Seed-Mediated Method for the Encapsulation of Hydrophobic Molecules.

Metal-organic frameworks (MOFs) are attracting considerable attention as a result of their unique structural properties, such as a high surface area, highly porous topology, and tunable size and shape, which enable them to have potential applications as a new class of carriers for functional agent or drug delivery. However, most of the MOFs and the polymers used are not pharmaceutically acceptable. For the first time, this study successfully conducted the rapid synthesis of cyclodextrin metal-organic frameworks (CD-MOFs) through a facile and green seed-mediated method. The size control, crystal structure, and thermal properties of CD-MOFs with and without seeds were investigated. When 1 mg/mL seed was added, the size of γ-CD-MOF crystals decreased from 6.2 ± 0.8 to 1.8 ± 0.4 μm. The CD-MOFs synthesized though the seed-mediated method had higher crystallinity and thermal stability than those that were not. Furthermore, the CD-MOFs could encapsulate hydrophobic molecules, such as Nile red (NR), which was chosen as a model, and the interaction mechanism between γ-CD-MOFs and NR was investigated. Results showed the formation of a 1:1 complex between NR and CD-MOFs, demonstrating the potential of these polymers as carriers for hydrophobic drug delivery applications.

Metal-Organic Frameworks for Cultural Heritage Preservation: The Case of Acetic Acid Removal.

The removal of low concentrations of acetic acid from indoor air at museums poses serious preservation problems that the current adsorbents cannot easily address owing to their poor affinity for acetic acid and/or their low adsorption selectivity versus water. In this context, a series of topical water-stable metal-organic frameworks (MOFs) with different pore sizes, topologies, hydrophobic characters, and functional groups was explored through a joint experimental-computational exploration. We demonstrate how a subtle combination of sufficient hydrophobicity and optimized host-guest interactions allows one to overcome the challenge of capturing traces of this very polar volatile organic compound in the presence of humidity. The optimal capture of acetic acid was accomplished with MOFs that do not show polar groups in the inorganic node or have lipophilic but polar (e.g., perfluoro) groups functionalized to the organic linkers, that is, the best candidates from the list of explored MOFs are MIL-140B and UiO-66-2CF3. These two MOFs present the appropriate pore size to favor a high degree of confinement, together with organic spacers that allow an enhancement of the van der Waals interactions with the acetic acid. We establish in this work that MOFs can be a viable solution to this highly challenging problem in cultural heritage protection, which is a new field of application for this type of hybrid materials.

Tuning thermal conductivity of porous graphene by pore topology engineering: Comparison of non-equilibrium molecular dynamics and finite element study

Tuning thermal conductivity of porous graphenes has attracted much interest in the thermal management of nanoelectronics devices due to the promising multifunctional properties of engineered nanomaterials. To explore the potential of tuning thermal properties of monolayer porous graphenes in multiple scales, non-equilibrium molecular dynamics (NEMD) and finite element method (FEM) are implemented to manipulate their thermal conductivity and temperature distribution by the engineering of pore topology. Results indicate that the thermal conductivity of porous graphenes can be significantly lower than a pristine graphene. The thermal conductivity reduction is attributed to phonon scattering at the boundaries of defects described by the phonon density of states analysis. It is found that the thermal conductivity and the temperature distribution of a porous graphene can be desirably tuned by the simultaneous engineering of relative density, pore topology, and pore orientation. Then, the effect of unit cell periodicity on the thermal conductivity of periodic porous graphenes, called phononic graphene or graphene metamaterial, is explored. Finally, comparing the results of continuum mechanics approach through the implementation of FEM and NEMD simulation presents the advantages of NEMD for predicting the thermal conductivity of engineered porous graphenes with characteristic length of lower than 50 nm.

Topology Optimization of Lightweight Lattice Structural Composites Inspired by Cuttlefish Bone

Lattice structural composites are of great interest to various industries where lightweight multifunctionality is important, especially aerospace. However, strong coupling among the composition, microstructure, porous topology, and fabrication of such materials impedes conventional trial-and-error experimental development. In this work, a discontinuous carbon fiber reinforced polymer matrix composite was adopted for structural design. A reliable and robust design approach for developing lightweight multifunctional lattice structural composites was proposed, inspired by biomimetics and based on topology optimization. Three-dimensional periodic lattice blocks were initially designed, inspired by the cuttlefish bone microstructure. The topologies of the three-dimensional periodic blocks were further optimized by computer modeling, and the mechanical properties of the topology optimized lightweight lattice structures were characterized by computer modeling. The lattice structures with optimal performance were identified.

Mineralogy and pore topology analysis during matrix acidizing of tight sandstone and dolomite formations using chelating agents

Hydrochloric acid (HCl) is commonly used during acidizing but certain problems are associated with its application such as corrosion of pipes, environment hazards, precipitation of fluosilicates or calcium fluorides and incompatibility of HCl with clay minerals. In order to mitigate these problems, few studies have proposed the use of chelating agents (the chemical compound that reacts with metal ions to form stable, water-soluble metal complexes) as an alternative. In this study, three different chelating agents, Ethylenediaminetetraacetic Acid (EDTA), N-(2-Hydroxyethyl) ethylenediamine-N,N′,N′-triacetic Acid (HEDTA) and N-Acetyl-l-glutamic Acid (GLDA) were used to stimulate Colton tight sandstone and Guelph dolomite samples. The pH value of these chelates ranged from 1.7 to 3 and was measured before and after the core flooding to observe the physicochemical changes in rock/fluid mixtures. Core flood experiments under 180 °F temperature were conducted on core samples at slow injection rate which increased the contact time between the fluid and the rock and increased the amount of dissolved ions.Porosity, permeability, Inductively Coupled Plasma (ICP), and Tescan Integrated Mineral Analysis (TIMA) were employed to measure the changes in the formation properties. The effluent samples were analyzed for calcium, magnesium, aluminium, sodium, potassium, silicon and iron using the ICP to assess the ability of these chelates on the complexation of these ions.HEDTA showed a strong ability in chelating calcium, iron, magnesium, sodium and it chelated small amounts of aluminium ions from the sandstone cores. Porosity distribution analysis showed that the HEDTA was more effective in creating fresh pore spaces in sandstone formation while GLDA introduced a large amount of pore spaces in the dolomite. A large number of solid particles were dissolved using HEDTA in sandstone and GLDA in dolomite formations. Panorama of each sample shows that new wormholes had been created by all chelates.The research introduced the chelates application in sandstone and dolomite formations and they showed good results in terms of matrix acidizing. Moreover, the Tescan Integrated Mineral Analysis (TIMA) has not been applied before to test the change in pore structure and mineralogy during the acidizing.

2D COS analysis of m-xylene transformation over medium-pore zeolites

m-Xylene isomerisation, as well as secondary disproportionation reactions, were examined over the 10-ring zeolites with different pore topology. As recognised in Py-sorption IR studies, all the 3D zeolites tested, i.e. IM-5, TNU-9, and ZSM-5 delivered the same protonic functionality, both in the manner of the amount and strength of the Si(OH)Al groups. Only zeolite TNU-10 provided lower number of Brønsted acid sites due to the limited accessibility of the sites hidden in the 8-ring system. Two-dimensional correlation spectroscopy (2D COS) analysis was applied to investigate the dataset obtained from IR spectroscopic studies. Both diffusion characteristics, as well as the different size of internal cavities, were found as crucial parameters determining the catalytic effectiveness. The presence of intermediate species, whose the origin is related to the distortion of the substrate molecule geometry during the adsorption process was recognised for all the studied structures. Such species are believed to be represented by the 1485 cm−1 band and assigned to the intermediate arenium ions, e.g. methylbenzenium ions, CH3–C6H5+–CH3. Both m-xylene and intermediate were consumed for coke precursors in the most spacious zeolite TNU-9. The production of p-xylene was detected only in the first period of the reaction, then coke formation was proved to be the privileged process. The simultaneous loss of m-xylene and methylbenzenium ions was also detected for the zeolite IM-5. Both species underwent isomerisation to p-xylene. No formation of coke species was observed for less spacious structures, i.e. ZSM-5 and TNU-10. The generation of isomerization products was found to be the most rapid in TNU-10. The formation of the intermediate species correlates with other products in the first minutes of the reaction (1–6 min), then the consumption to other reaction products distort the correlation.

Reconstruction of 3D porous media using multiple-point statistics based on a 3D training image

To date many methods of constructing porous media have been proposed. Among them, the multiple-point statistics (MPS) method has a unique advantage in reconstructing 3D pore space because it can reproduce pore space of long-range connectivity. The Single Normal Equation Simulation (SNESIM) is one of most commonly used algorithms of MPS. In the SNESIM algorithm, the selection of training image is vital because it contains the basic pore structure patterns. In the previous reconstructions of 3D porous media using SNESIM, a 2D slice was usually employed as the training image. However, it is difficult for a 2D slice to contain complex 3D pore space geometry and topology patterns. In this paper, a 3D training image is used in order to provide more realistic 3D pore structure features. Besides, a multi-grid search template is applied for the purpose of capturing the pore structures of different scales and speeding up the reconstruction process. Two sandstone cores are taken as test examples and the 3D porous media are reconstructed. The two-point correlation function, pore network structure parameters and absolute permeability are applied as the evaluation indexes to validate the accuracy of the reconstructed models. The comparison result shows that the reconstructed models are in good agreement with the real model obtained by X-ray computed tomography scanning in the pore throat geometry and topology and transport property, which justifies the reliability of the proposed method.

Transition metal-promoted hierarchical ETS-10 solid base for glycerol transesterification.

A transesterification reaction has been carried out over a transition metal modified hierarchical ETS-10 (METS-10) catalyst to synthesize glycerol carbonate (GC) from glycerol. The inherent Lewis basicity of ETS-10 favors oriented conversion of glycerol, and the hierarchical structure benefits exposure of more active sites, shortens the molecular diffusion path, suppresses the formation of coke in the micropores, and then enhances the catalytic reactivity and stability. Furthermore, the influence of transition metals (Fe, Co, Ni, Cu, Zn and Mn) on the basic sites of supports has been investigated in detail. It is found that basicity change of the catalyst depends on not only the cation size, nature and composition of the transition metal but also the zeolite structure and pore topology. Besides, the presence of a certain amount of Ni0 species from catalyst reduction has proved to play a critical role in strengthening the interaction of Lewis basic sites (TiO62−) with active glycerol hydroxyl groups. Finally, a 97.7% glycerol conversion and 97.1% GC yield have been obtained over Ni/METS-10, of which the high catalytic performance can be maintained after 8 runs.