Pore Topology Research Articles

The role of Ir4 cluster in enhancing the adsorption of CO2 on selected zeolites – GCMC simulations

We have investigated the adsorption of CO2 molecules inside the EMT, SAO, SBS, SBT and IWS zeolites with respect to the influence of the Ir4 clusters on the adsorption capabilities of these materials. We have determined that the capabilities of CO2 adsorption depend on the combined effect of the framework topology and the position of the Ir4 cluster. Adsorption intensifies despite the fact that a fraction of the pore volume is occupied by the Ir4 cluster, and thus, the adsorption is more intense than that on empty zeolite. The pore topology however is also playing a crucial role in the effect, as in certain cases it allows the CO2 molecules to order in such a way they fill the most pore space.

On the performance of FAU and MFI zeolites for the adsorptive removal of a series of volatile organic compounds from air using molecular simulation.

Volatile organic compound (VOC) emissions can cause serious risk to human health and the environment. In this work, we used Monte Carlo simulations to assess the performance of industrially important zeolites for the adsorption-based removal of a number of common air pollutants, particularly small saturated and unsaturated hydrocarbons: propane, butane, propene, and 1-butene. We focused on the cage-like FAU and channel-like MFI zeolites. The adsorption isotherms of the multicomponent N2/O2/Ar/VOC mixtures at real concentrations and room temperature reveal a considerable influence of the host topology and pore dimensions. While the adsorption of the VOCs from the mixture in FAU is almost negligible, it is remarkable in MFI. The adsorption selectivity of each VOC over the air compounds exhibits a maximum at about 10(6)-10(7) Pa, and then decreases to virtually zero due to entropic effects. This behaviour for selectivity is maintained regardless of the chain length and the presence of double bonds in the VOC, but the values are indeed affected. Also, we examined the selectivity at 10(7) Pa for a number of other widely used zeolites, with pore features ensuring the diffusion of the adsorbates. Apart from MFI, we also found the channel-like MEL and MTW zeolite candidates for the targeted air decontamination.

Metal-organic frameworks for radionuclide adsorption

With more and more radionuclides were used, radionuclide wastes could also cause a threat to the environment and human health. Metal-organic frameworks (MOFs) as a new material have widely used in gas storage, catalysis, sensing, nonlinear optics, separation and biomedical research because of their large surface area, high adsorption affinity, diverse structures and pore topologies, accessible functionalization of tunnels. MOFs for radionuclides adsorption began to interest radiochemistry and environmental scientists. In this review, recent advances in MOFs for radionuclides adsorption are highlighted, including synthetic methods and properties of MOFs, and the application of adsorption and separation for iodine, xenon, uranium and palladium. Furthermore, the research trends and future prospects are briefly discussed.

Template-free ordered mesoporous silicas by binary nanoparticle assembly.

Evaporation-induced convective binary assembly of large (A) and small (B) silica nanoparticles is demonstrated as a template-free route to three-dimensionally ordered mesoporous silicas (OMSs), the pore topology of which derives from the interconnected interstices of the resulting ordered nanoparticulate structures. Even without explicit solvent index matching or stabilization (e.g., charge or steric) beyond intrinsic properties of the amino acid nanoparticle synthesis solution, assembly of binary mixtures of silica nanoparticles of ca. 10-50 nm in diameter primarily obeys hard-sphere phase behavior despite differences in electrostatic character of the particles. Specifically, the particle size ratio, γ, governs symmetry of the assemblies among AB2 and AB13 phases and enables access of the AB phase. Small-angle X-ray scattering (SAXS) reveals the high yield of ordered binary assemblies and confirms, in combination with transmission electron microscopy, the AlB2, NaZn13, and NaCl crystalline isostructures. Interstitial solid solutions result for the smallest γ considered (γ ≤ 0.3), wherein cubic crystallization of the large particles is templated by interstitially mobile small particles. New mechanistic insight into factors influencing the yield of ordered binary structures includes the degree to which the smaller particles (ca. 15-24 nm) within the mixture undergo unary crystallization, as influenced by lysine or other basic amino acids used in the nanoparticle synthesis, as well as matching of the time scales for convective nanoparticle assembly and crystallization. Ultimately, the demonstrated robustness of the binary nanoparticle assembly and the control over silica particle size translate to a facile, template-free approach to OMSs with independently tunable pore topology and pore size.

PACKING ISSUE IN CEMENT BLENDING FOR SUSTAINABILITY DEVELOPMENTS - APPROACH BY DISCRETE ELEMENT METHOD

Common cement blending materials for concrete like fly ashes, blast furnace slag, silica fume, metakaolin and rice husk ash have been investigated experimentally as to their impact on concrete’s mechanical, physical and sustainability capabilities. Such efforts offer but case-related information on involved materials and used procedures. Generalizations will be difficult because also contradicting information is provided: a weak basis to guide further developments. Published research data on blends with inert components (carbon black) confirm fineness to be the leading parameter governing their mechanics. Experimental efforts revealing blending effects on durability issues are more scarce because of their complicated, laborious and time-consuming nature. Hence, generalization capabilities are even more restricted. An approach is therefore presented for studying cementitious materials in the virtual reality, employing the concurrent algorithm-based dynamic discrete element method (DEM), HADES. Hydration simulation of the simulated (blended) Portland cement grain mixture is thereupon accomplished with an extended version of the vector approach. Robotics-based pore delineation provides topological information, while geometric characterization of the pore networks is accomplished by star volume measurements. Topological and geometric parameters can be combined in a hydraulic model for durability estimation. This approach allows covering a wide range of parameters. This would render possible more systematic and economic development of such materials for sustainability purposes, among other things. The production stages (DEM and hydration simulation) and the analysis stages (pore delineation and topology and geometry assessment) are briefly outlined. Application of the methodology is illustrated on finer and coarser grained cements blended with rice husk ash produced with different fineness. The effects of the gap in grading on the pore network characteristics relevant for transport-based durability items are outlined.

Gas Diffusion in a Porous Organic Cage: Analysis of Dynamic Pore Connectivity Using Molecular Dynamics Simulations

Molecular dynamics simulations were used to investigate the diffusion of six small gas molecules in a crystalline porous organic cage, CC3. A flexible host model was used to simulate transient channel formation, the effects of which are reflected in the calculated diffusion coefficients for the six gases of 5.64 × 10–8, 5.94 × 10–9, 2.60 × 10–9, 9.60 × 10–9, 2.40 × 10–9, and 1.83 × 10–10 m2 s–1, respectively, for H2, N2, CO2, CH4, Kr, and Xe. By contrast, a larger gas molecule, SF6, was predicted to be unable to diffuse in the pores of this material. We introduce a new method—a void space histogram—to analyze dynamic pore topologies and to graphically illustrate the structural factors determining guest diffusion.

Evolution of unburnt hydrocarbons under “cold-start” conditions from adsorption/desorption to conversion: On the screening of zeolitic materials

The general purpose of this work is to examine the relative ability of some well-selected zeolitic materials for the reduction of HC emissions generated within the Diesel “cold-start” period, i.e. when the work temperature of the Diesel Oxidation Catalyst (DOC) has not been reached. More peculiarly, this study is focused on the chemical, textural and structural parameters of zeolites influent on the elimination, namely by adsorption, of unburnt HC (propene, toluene and decane) in presence of potential inhibitors (H2O, CO, NO). Simulated “cold-start” conditions consisted in the rapid heating of the pre-treated zeolite sorbent/catalyst under the whole gas mixture from 35 to 530°C. The quantity of trapped HC and those converted to COx by oxidation were measured in function of the temperature, as well as the amount of NOx converted by the HC-SCR reaction. The interpretation of the HC emission profiles in close relation with the porous and acidic (through FTIR of adsorbed pyridine) properties of the corresponding zeolites allowed to gain insight onto the relative contributions of the pore topology, the pore size and the acid strength. For some selected zeolites, several consecutive cold-start cycles were performed in order to assess their stability.

The application of sorption hysteresis in nano-petrophysics using multiscale multiphysics network models

Scanning electron microscopy (SEM) images of organic-rich mudrock (shale) samples show a wide distribution of pore sizes (commonly between 1nm and 1μm) and complex pore spatial configurations (Loucks et al., 2012). Pore size and pore connectivity are important parameters in that they have first order impact on macroscopic flow properties of a porous medium. However, given the significant difficulty in capturing multiscale pores within a single three-dimensional image, and the possible uncertainties in the existence or absence of original throats in an acquired image, it is imperative to explore indirect methods to quantify the pore structure. In this paper, we simulate sorption in heterogeneous pore network models and study sorption and permeability hysteresis analyses as indirect methods for rock characterization.Three network types are introduced to represent the multiscale pore topology of shale rocks; specifically: regular (type 1), series (type 2) and parallel (type 3). We conclude that, in appropriate size ranges, sorption hysteresis can distinguish the three types whereas permeability hysteresis can only separate parallel from series and regular. Furthermore, the simulations show that sorption hysteresis is sensitive to compaction/cementation (closing of throats) in all network types whereas permeability hysteresis is sensitive to the diagenesis in parallel networks only.

Zeolite screening for the separation of gas mixtures containing SO2, CO2and CO

We used a combination of experiments and molecular simulations to investigate at the molecular level the effects of zeolite structure on the adsorption and diffusion of sulfur dioxide, carbon dioxide and carbon monoxide as well as separation processes of their mixtures. Our study involved different zeolite topologies and revealed numerous structure-property trends depending on the temperature and pressure conditions. Sulfur dioxide, which has the strongest interactions with zeolites due to its size and polarity, showed the largest adsorption across investigated temperatures and pressures. Our results indicate that structures with channel-type pore topology and low pore volume are the most promising for selective adsorption of sulfur dioxide over carbon dioxide and carbon monoxide under room conditions, while structures with higher pore volume exhibit better storage capacity at higher pressure. Our results emphasize the need for considering both adsorption and diffusion processes in the selection of the optimal structure for a given separation process. Our findings help to identify the best materials for effective separation processes under realistic operating conditions.

Modeling and optimization of atomic layer deposition processes on vertically aligned carbon nanotubes.

Many energy conversion and storage devices exploit structured ceramics with large interfacial surface areas. Vertically aligned carbon nanotube (VACNT) arrays have emerged as possible scaffolds to support large surface area ceramic layers. However, obtaining conformal and uniform coatings of ceramics on structures with high aspect ratio morphologies is non-trivial, even with atomic layer deposition (ALD). Here we implement a diffusion model to investigate the effect of the ALD parameters on coating kinetics and use it to develop a guideline for achieving conformal and uniform thickness coatings throughout the depth of ultra-high aspect ratio structures. We validate the model predictions with experimental data from ALD coatings of VACNT arrays. However, the approach can be applied to predict film conformality as a function of depth for any porous topology, including nanopores and nanowire arrays.

Free energy analysis of conductivity and charge selectivity of M2GlyR-derived synthetic channels

Significant progresses have been made in the design, synthesis, modeling and in vitro testing of channel-forming peptides derived from the second transmembrane domain of the α-subunit of the glycine receptor (GlyR). The latest designs, including p22 (KKKKP ARVGL GITTV LTMTT QS), are highly soluble in water with minimal aggregation propensity and insert efficiently into cell membranes to form highly conductive ion channels. The last obstacle to a potential lead sequence for channel replacement treatment of CF patients is achieving adequate chloride selectivity. We have performed free energy simulation to analyze the conductance and charge selectivity of M2GlyR-derived synthetic channels. The results reveal that the pentameric p22 pore is non-selective. Moderate barriers for permeation of both K+ and Cl− are dominated by the desolvation cost. Despite previous evidence suggesting a potential role of threonine side chains in anion selectivity, the hydroxyl group is not a good surrogate of water for coordinating these ions. We have also tested initial ideas of introducing additional rings of positive changes to various positions along the pore to increase anion selectivity. The results support the feasibility of achieving anion selectivity by modifying the electrostatic properties of the pore, but at the same time suggest that the peptide assembly and pore topology may also be dramatically modified, which could abolish the effects of modified electrostatics on anion selectivity. This was confirmed by subsequent two-electrode voltage clamp measurements showing that none of the tested mono-, di- and tri-Dap substituted sequences was selective. The current study thus highlights the importance of controlling channel topology besides modifying pore electrostatics for achieving anion selectivity. Several strategies are now being explored in our continued efforts to design an anion selective peptide channel with suitable biophysical, physiological and pharmacological properties as a potential treatment modality for channel replacement therapy. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Porous Metal–Organic Frameworks for Heterogeneous Biomimetic Catalysis

Metalloporphyrins are the active sites in monooxygenases that oxidize a variety of substrates efficiently and under mild conditions. Researchers have developed artificial metalloporphyrins, but these structures have had limited catalytic applications. Homogeneous artificial metalloporphyrins can undergo catalytic deactivation via suicidal self-oxidation, which lowers their catalytic activity and sustainability relative to their counterparts in Nature. Heme molecules in protein scaffolds can maintain high efficiency over numerous catalytic cycles. Therefore, we wondered if immobilizing metalloporphyrin moieties within porous metal-organic frameworks (MOFs) could stabilize these structures and facilitate the molecular recognition of substrates and produce highly efficient biomimetic catalysis. In this Account, we describe our research to develop multifunctional porphyrinic frameworks as highly efficient heterogeneous biomimetic catalysts. Our studies indicate that porous porphyrinic frameworks provide an excellent platform for mimicking the activity of biocatalysts and developing new heterogeneous catalysts that effect new chemical transformations under mild conditions. The porous structures and framework topologies of the porphyrinic frameworks depend on the configurations, coordination donors, and porphyrin metal ions of the metalloporphyrin moieties. To improve the activity of porous porphyrinic frameworks, we have developed a two-step synthesis that introduces the functional polyoxometalates (POMs) into POM-porphyrin hybrid materials. To tune the pore structures and the catalytic properties of porphyrinic frameworks, we have designed metalloporphyrin M-H8OCPP ligands with four m-benzenedicarboxylate moieties, and introduced the secondary auxiliary ligands. The porphyrin metal ions and the secondary functional moieties that are incorporated into porous metal-organic frameworks greatly influence the catalytic properties and activities of porphyrinic frameworks in different reactions, such as the oxidation of alkylbenzenes, olefins, and hexane and the photo-oxygenation of 1,5-dihydroxynaphthalene and sulfides. The porphyrin metal ions and the secondary auxiliary sites in the pores can work together synergistically to enhance the catalytic activities of porphyrinic frameworks. Compared with their homogeneous counterparts, the activities and stabilities of the heterogeneous porphyrinic frameworks are remarkable: the immobilization of metalloporphyrins onto the pore surfaces of MOFs not only prevents their suicidal self-oxidation but also allows them to activate inert substrate molecules, such as cyclohexane. Moreover, because the bulky molecules cannot easily access the active sites inside the pores of porphyrinic frameworks, these porous materials demonstrate interesting size-selective catalytic properties toward substrates.

Petrophysical rock classification in the Cotton Valley tight-gas sandstone reservoir with a clustering pore-system orthogonality matrix

Petrophysical rock classification is an important component of the interpretation of core data and well logs acquired in complex reservoirs. Tight-gas sandstones exhibit large variability in all petrophysical properties due to complex pore topology resulting from diagenesis. Conventional methods that rely dominantly on hydraulic radius to classify and rank reservoir rocks are prone to rock misclassification at the low-porosity and low-permeability end of the spectrum. We introduce a bimodal Gaussian density function to quantify complex pore systems in terms of pore volume, major pore-throat radii, and pore-throat radius uniformity. We define petrophysical dissimilarity (referred to as orthogonality) between two different pore systems by invoking the classic “bundle of capillary tubes” model and subsequently classify rocks by clustering an orthogonality matrix constructed with all available mercury injection capillary pressure data. The new method combines several rock textural attributes including porosity, pore-throat radius, and tortuosity for ranking reservoir rock quality in terms of flow capacity. We verify the new rock classification method with field data acquired in the Cotton Valley tight-gas sandstone reservoir located in the East Texas basin. The field case shows that the new method consistently identifies and ranks rock classes in various petrophysical data domains, including porosity-permeability trends, pore-size distribution, mercury injection capillary pressure, and NMR transverse relaxation time ([Formula: see text]) spectra. Relative permeability curves, which are difficult to measure in the laboratory for tight rocks, are quantified with Corey-Burdine’s model using the bimodal Gaussian pore-size distribution and are validated with core data.

Diffusion of hydrocarbon in zeolite and effect due to pore topology: Neutron scattering and MD simulation studies

Here we report detailed dynamical landscape of propylene adsorbed in ZSM5 and Na–Y zeolites as studied by neutron scattering and molecular dynamics (MD) simulation. Separation of propylene molecule from propane in the petrochemical industry is an important issue because it is one of the most demanding energetic separation processes due to their very close relative volatilities and molecular sizes. Aim here is to investigate the correlation of the host topology towards the dynamics of guest molecules. ZSM5 zeolite is typified by a network of intersecting channels while Na–Y has a network of spherical supercages interconnected by windows. Both neutron scattering and MD simulation studies indicated that translational diffusion of propylene is more restricted in ZSM5 compared to Na–Y zeolite. Fully atomistic MD simulation studies showed that the translation involves three different time scales and rotational motion of the propylene is much faster than translation. The observed dynamics in QENS spectrometer (ΔE∼200μeV) corresponds to one of the three translational components indicated in the MD simulation. The faster rotational motion is observed in a wider energy window spectrometer (ΔE∼3meV). MD simulation results also show some interesting features like, non-isotropic rotation in ZSM5 while it has been isotropic in Na–Y zeolite. It is also found that propylene molecules prefer to orient along channels of ZSM5 zeolite.

The stability and defluoridation performance of MOFs in fluoride solutions

The stability of metal–organic frameworks (MOFs) in fluoride solutions was studied based on 11 water-stable MOFs (MIL-53(Fe, Cr, Al), MIL-68(Al), CAU-1, CAU-6, UiO-66(Zr, Hf) and ZIFs-7, -8, -9), and several factors were found to have large influence, such as the central metal activity, pore topology and coordination number. Furthermore, the defluoridation performance of UiO-66(Zr) was investigated comprehensively, which shows an adsorption capacity of 41.36mgg−1 that is higher than most of the reported adsorbents. The initial pH of fluoride solution has little effect in neutral range, while shows a negative effect when pH >9.0; the adsorption data of UiO-66(Zr) fit well with the Langmuir model, and pseudo-second-order kinetic model was found to be suitable for depicting the F− adsorption process, and finally, it was observed that the extent of the effect of co-existing ions is largely ion-dependent. On the basis of the systemic study, this work suggests that increasing the number of –OH groups is an efficient strategy to improve the defluoridation performance of MOFs, and MOFs are promising adsorbents for defluoridation in waste water.

A porous C/LiFePO4/multiwalled carbon nanotubes cathode material for Lithium ion batteries

Three dimensional (3D) porous C/LiFePO4/MWCNTs was synthesized by a hybrid of in situ sol gel strategy and a facile electro-polymerization polyaniline technique and a simultaneous sintering progress. In combined with the 3D hierarchical pore topologies and high electronic conduction facilitating the kinetics of both electron transport and lithium ion diffusion within the particles, the optimized electrodes exhibit an ultrahigh rate capacity, stable charge/discharge cycle ability, and a comparative volume capacity. The synthesized LiFePO4 composite offers a discharge capacity of 169.6mAhg−1 (nearly to its the theoretical capability 170mAhg−1) at the C/10 rate and delivers a good rate performance with a capacity of 141.9mAh g−1 at a high rate of 20C, and stable charge/discharge cycle ability (>95% capacity retention after 200 charge/discharge cycles).This non-organic facile synthesize avenue can be high desirable to prepare high-power electrode materials

Stability of the Reaction Intermediates of Ethylbenzene Disproportionation over Medium-Pore Zeolites with Different Framework Topologies: A Theoretical Investigation

The strain energies of the main reaction intermediates (i.e., monoethylated diphenylethane (mEDPE) and diethylated diphenylethane (dEDPE) derivatives), which can be formed during ethylbenzene (EB) disproportionation over six 10-ring zeolites with different framework topologies, as well as over the large-pore zeolite Y, were determined by the density functional theory calculations in order to more precisely investigate the effects of the pore structure of medium-pore zeolites on their formations. It was found that while the strain energies of mEDPE and dEDPE intermediates in zeolite Y, MCM-22 and TNU-9, were always lower than 19.6 kJ mol(-1), some of them were characterized by considerably higher energies (>32.8 kJ mol(-1)) when positioned in the intersection channels of ZSM-5 and ZSM-57. As expected, in addition, all the mEDPE and dEDPE derivatives embedded in TNU-10 and ZSM-22 with narrower 10-ring channels were strongly distorted, giving them much higher strain energies (>37.7 kJ mol(-1)), which were in excellent agreements with our recently reported experimental results (J. Phys. Chem. C 2010, 115, 16124). This led us to conclude that the size and shape of void spaces in the medium-pore zeolites play a crucial role in governing the type of mEDPE and dEDPE formations during the EB disproportionation. Our work also shows that the strain energies of various reaction intermediates confined within zeolites with different pore topologies could be regarded as a useful quantitative means in better understanding the shape-selective nature of this important class of microporous crystalline catalysts.

Effect of mesoporous silica topology on the formation of active sites in copper supported catalysts for methanol decomposition

Modified incipient wetness impregnation procedure was applied for deposition of finely dispersed CuO or metallic copper particles into ordered mesoporous silica host matrix (KIT-6 and SBA-15). This modified procedure consists of number of consecutive impregnation steps with copper nitrate solution with changeable concentration and soft drying under vacuum procedures. Nitrogen physisorption, XRD, UV–vis, TEM–EDXS, FTIR of probe molecules (CO, NO and N2) and TPR were applied for characterization of copper state in the matrix. The obtained materials were tested as catalysts in methanol decomposition. Selected catalytic experiments were carried out under very low coverage of the surface with 11C-radiolabelling methanol. In situ FTIR study of methanol decomposition was also performed. It was found that the catalytic behaviour of copper, supported on mesoporous materials, depends in a complex way on the dispersion and accessibility of copper species within the host matrix. They can be tuned by copper loading and pore topology of the support. The optimal catalytic activity in methanol decomposition is achieved at about 8 and 15wt.% copper content for the SBA-15 and KIT-6 mesoporous supports, respectively.

Biomass‐Derived Chemicals: Synthesis of Biodegradable Surfactant Ether Molecules from Hydroxymethylfurfural

A new class of biodegradable anionic surfactants with structures based on 5-alkoxymethylfuroate was prepared starting from 5-hydroxymethylfurfural (HMF), through a one-pot-two-steps process which involves the selective etherification of HMF with fatty alcohols using heterogeneous solid acid, followed by a highly selective oxidation of the formyl group with a gold catalyst. The etherification step was optimized using aluminosilicates as acid catalysts with different pore topologies (H-Beta, HY, Mordenite, ZSM-5, ITQ-2, and MCM-41), different active sites (Bronsted or Lewis) and different adsorption properties. It was shown that highly hydrophobic defect-free H-Beta zeolites with Si/Al ratios higher than 25 are excellent acid catalysts to perform the selective etherification of HMF with fatty alcohols, avoiding the competitive self-etherification of HMF. Moreover, the 5-alkoxymethylfurfural derivatives obtained can be selectively oxidized to the corresponding furoic salts in excellent yield using Au/CeO2 as catalyst and air as oxidant, at moderated temperatures. Both H-Beta zeolite and Au/CeO2 could be reused several times without loss of activity.

Strong Reactivity Enhancement through Molecular Traffic Control in Zeolites

AbstractIn the past, the investigation of catalytic reactivity enhancement through molecular traffic control (MTC) by means of dynamical Monte Carlo simulations of catalysis was initiated in simple, idealized zeolite channel networks. These results are reviewed here and, emphasizing more recent work, the conditions of reactivity enhancement through MTC are examined for a realistic channel topology based on the pore structure of the zeolite TNU‐9. For a wide range of reaction rates a very strong MTC effect can be found that increases with grain size. This effect is argued to be a generic feature of a certain class of zeolite pore topologies.