Trimodal Pore Size Distribution Research Articles

Hierarchically mesostructured MIL-101 metal–organic frameworks: supramolecular template-directed synthesis and accelerated adsorption kinetics for dye removal

Hierarchically mesostructured MIL-101 metal–organic frameworks (MOFs) were successfully synthesized under solvothermal synthesis conditions by using the cationic surfactant cetyltrimethylammonium bromide as a supramolecular template. The mesostructured MIL-101 MOFs were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption–desorption isotherms at 77 K. The results suggest that the mesostructured MIL-101 MOFs obtained are composed of numerous MOF nanocrystals. Pore size distribution analyses of the as-synthesized MOF samples reveal that such mesostructured MIL-101 MOFs have well-defined trimodal pore size distributions showing simultaneous existence of meso- and macropore channel systems. Significantly, such hierarchically mesostructured MIL-101 exhibits remarkably accelerated adsorption kinetics for dye removal in comparison with the bulk MIL-101 crystals, which is due to unique hierarchically meso- and macropores created in the solid.

Fabrication porous Si3N4 ceramics via starch consolidation–freeze drying process

Si3N4 ceramics with porosity of ~72% were prepared via starch consolidation–freeze drying process. Pre-gelled method was used to reduce the granular size of starch which acted as consolidator/binder and pore forming agent. Polyvinyl alcohol (PVA) was added to control ice crystal growth. The results indicated that the porosities of Si3N4 ceramics were mainly controlled by slurry concentrations, starch/PVA contents had slight influence on the porosities. With higher PVA content, large pores around 35μm were decreased, the microstructure turned more uniform. Typical trimodal pore size distributions were obtained with 0 and 2.5% PVA addition, the sample with 5% PVA only showed bimodal pore size distribution without large pores. The flexural strength was increased from 6.7 to 42.3MPa with increasing PVA content.

MIL-53(Al) mesostructured metal-organic frameworks

Hierarchically mesostructured metal-organic frameworks (designated here as L-MOFs) were designed and synthesized using block copolymers (P123 and F127) as structure directing agents under solvothermal synthesis conditions. Porous mesostructure of L-MOF materials was established from nitrogen adsorption–desorption isotherms (BET) at 77 K, high-resolution transmission electron micrograps (HRTEM), and small-angle X-ray diffraction (SAXD) patterns. MOF crystal structures were characterized by wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR). Results demonstrated that L-MOF materials have well-defined trimodal pore size distributions showing micro-, and simultaneous existence of up to two mesopore channel systems. L-MOF materials have two kinds of mesoporous phases: one has disordered wormhole-like structure with pore size diameter around 4 nm, and the other one, only appearing in L-MOF(1) and L-MOF(2) (corresponding to P123 and F127), has a needle-like shape with BJH pore sizes are of 7.6 and 5.4 nm, respectively.

Mesoporous ethanesilica materials with bimodal and trimodal pore-size distributions synthesised in the presence of cobalt ions

Mesoporous organosilica materials containing ethane groups in their framework were formed with two and three pore sizes (i.e. bimodal and trimodal pores) when synthesised by the sol-gel method in the presence of cobalt ions. The compounds 1,2-bistrimethoxysilylethane and tetraethylorthosilicate were used as silicon sources and the reactions were done in the presence of a surfactant, which served as a template. Diffuse reflectance infrared Fourier transform spectroscopy revealed that organic functional groups were incorporated into the ethanesilica. Powder X-ray diffraction and nitrogen adsorption data indicated that the mesophase and textural properties (surface area, pore volume, pore diameter) of the materials were dependent on the ageing temperature, the amount/ratio of silica precursors and cobalt ion incorporation. Secondary mesopores were drastically reduced by changing the ratio of silicon precursors.

Hierarchical pore ZSM-5 zeolite structures: From micro- to macro-engineering of structured catalysts

Zeolite crystals coated on mesoporous glass materials with hierarchical tri-modal pore size distributions have been successfully prepared and characterized. Starting glass monoliths, having regular meso- and macro-pores, were partly re-constructed into zeolite material, thus introducing additional microporosity via the growth of these zeolite crystals. Hierarchical 3D porous glass scaffolds were prepared by a combination of sol–gel, double polymer templating, and rapid prototyping (RP) techniques. Structured zeolitic catalytic materials were produced by conventional self-assembly of template cations and silica species on the glass surface. Zeolite crystals are therefore in an intimate contact with the glass support, featuring micropores in addition to the mesopores. A higher catalytic performance and selectivity toward low alkenes were achieved in n-hexane cracking reaction over these zeolite/glass composites in comparison with purely microporous zeolite materials.

Processing and microstructure of particle stabilized silica foams

Silica foams containing ~ 85% porosity and with different shapes and sizes have been prepared by air entrainment in suspensions of hydrophobized silica + alumina (5 wt.%) powder mixture in aqueous solution of isopropanol and binders, followed by casting, drying and sintering. The silica powder with surfaces modified by presence of long chain amphiphilic molecules has been used successfully for stabilization of air bubbles in suspensions, so that their disproportionation and collapse could not be observed even after 4 weeks. Microstructural examination using optical and scanning electron microscopy as well as measurements by mercury porosimetry has shown trimodal pore size distribution with fine (4–10 μm), medium (50–100 μm) and coarse (~ 680 μm) pores.

Mesoporous Zirconium Titanium Oxides. Part 3. Synthesis and Adsorption Properties of Unfunctionalized and Phosphonate-Functionalized Hierarchical Polyacrylonitrile-F-127-Templated Beads

A method is presented for the preparation of zirconium titanate mixed oxides in bead form having hierarchical pore structure. This method entailed the use of both preformed polyacrylonitrile (PAN) polymer beads and surfactants as templates. The templates were removed by calcination at temperatures below about 500 degrees C, resulting in mixed oxide beads with trimodal pore size distributions and interconnected pores. The pore size distributions as determined using nitrogen adsorption-desorption showed clear maxima at 4.5 and 45 nm length scales and also clear evidence of microporosity. The macroporous framework morphology was a replica of the PAN beads with radial structure. The mesoporous framework possessed wormhole-like pores with pore size of about 6 nm that was consistent with the F-127 triblock copolymer template used. The mixed oxide beads exhibited surface areas of 215 and 185 m2/g after calcination at 500 and 600 degrees C. Thermal stability up to 650 degrees C is unprecedented for bulk systems. The adsorption properties were characterized using uranyl as the target cation and the mass transport in the beads with the present hierarchical architectures has been shown to be exceptional. The beads were functionalized with 4-amino,1-hydroxy,1,1-bis-phosphonic acid (HABDP) and amino-tris-methylene phosphonic acid (ATMP) and the adsorption properties for the extraction of uranyl sulfate complexes from acidic solution examined. Of the two molecules investigated, ATMP functionalization resulted in the best extraction efficiency with equilibrium uptake of about 90% of uranium available in solution between pH 1 and 2. The beads could potentially be utilized as catalysts, catalyst supports, adsorbents, and separation materials.

Design of MFI Zeolite-Based Composites with Hierarchical Pore Structure: A New Generation of Structured Catalysts

Zeolite/glass composite materials with hierarchical trimodal pore size distributions have been successfully prepared and subsequently characterized. Starting from glass monoliths having meso- and macropores, additional microporosity was introduced while allowing the growth of zeolite crystals via partial recrystallization of the glass support into ZSM-5 zeolite phase. Structured zeolitic catalytic materials were produced using a combination of supramolecular templating methods to produce the glass support, followed by conventional self-assembly of template cations and silica species. Zeolite crystals are therefore in intimate contact with the glass support, featuring micropores in addition to the mesopores. Hence, these zeolite/glass composites exhibit enhanced diffusional properties in comparison with purely microporous zeolite materials. These enhanced mass transport properties allowed improvement of the selectivity toward light olefins in the important n-hexane cracking reaction.

Nano/macroporous monolithic scaffolds prepared by the sol–gel method

Development of optimal scaffolds for bone tissue engineering and regeneration is still a challenge, since many materials and structures have been proposed but few have reached clinical expectations. This work reports on the preparation and characterization of SiO2-CaO and SiO2-CaO-P2O5 sol–gel derived monoliths, with potential application as glass scaffolds for bone regeneration, exhibiting a nano/macro trimodal pore size distribution, including pores of ~100’s of micrometers (μm), several microns and just a few nanometers (nm) in size. Interconnected macropores (~20–200 μm) have been obtained in the present work by polymerization-induced spinodal phase separation along with the sol–gel transition, when a water soluble polymer [poly(ethylene oxide)] was added to the sol–gel solution; the several-micron pores are spherical and isolated and might be the result of secondary phase separation by nucleation-growth mechanism; the interconnected nanopore (~5–25 nm) structure of the macroporous gel skeleton, on the other hand, was tailored by solvent exchange procedures. The morphological and textural characterization of these materials was performed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray ultra microscopy (XuM), nitrogen adsorption and mercury intrusion porosimetry. The factors affecting the porosity exhibited by the scaffolds, such as glass composition and solvent exchange conditions, have been assessed.

Templating Mesoporous Zeolites

The application of templating methods to produce zeolite materials with hierarchical bi- or trimodal pore size distributions is reviewed with emphasis on mesoporous materials. Hierarchical zeolite materials are categorized into three distinctly different types of materials: hierarchical zeolite crystals, nanosized zeolite crystals, and supported zeolite crystals. For the pure zeolite materials in the first two categories, the additional meso- or macroporosity can be classified as being either intracrystalline or intercrystalline, whereas for supported zeolite materials, the additional porosity originates almost exclusively from the support material. The methods for introducing mesopores into zeolite materials are discussed and categorized. In general, mesopores can be templated in zeolite materials by use of solid templating, supramolecular templating, or indirect templating. In this categorization of templating methods, the nature of the interface between the zeolite crystal and the mesopore exactly when the mesopore starts to form is emphasized. In solid templating, the zeolite crystal is in intimate contact with a solid material that is being removed to produce the mesoporosity. Similarly, in supramolecular templating, the zeolite crystal is in direct contact with a supramolecular assembly of organized surfactants, which is removed to generate the mesopores. On the other hand, in the indirect templating method, the interface is between the zeolite crystal and solvent molecules, or possibly a gas phase. It is shown that the available templating approaches are quite versatile, and accordingly, it is possible to produce a very wide range of hierarchical zeolite materials. The resulting zeolite materials, featuring noncrystallographic mesopores in addition to the crystallographic micropores, exhibit significantly enhanced diffusional properties in comparison with purely microporous zeolite materials. These enhanced mass transport properties have been shown in several cases to result in significantly improved catalytic properties in a range of important reactions.

Hydrothermal Preparation and Photocatalytic Activity of Hierarchically Sponge-like Macro-/Mesoporous Titania

Trimodally sponge-like macro-/mesoporous titania was prepared by hydrothermal treatment of precipitates of tetrabutyl titanate (Ti(OC4H9)4) in pure water. Effects of hydrothermal time on the phase composition, porosity, and photocatalytic activity of hierarchically porous titania were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) and N2 adsorption−desorption measurements. All TiO2 powders prepared at 180 °C showed trimodal pore-size distributions in the macro-/mesoporous region: fine intraparticle mesopores with peak pore diameters of ca. 3.7−6.9 nm, larger interparticle mesopores with peak pore diameters of ca. 23−39 nm, and macropore with pore diameter of ca. 0.5−3 μm. With increasing hydrothermal time, crystallinity, and average anatase crystallite size, pore size and pore volume increased, while specific surface area decreased. The hierarchically porous titania prepared ...

Templating mesoporous hierarchies in silica thin films using the thermal degradation of cellulose nitrate

Materials containing a hierarchical pore structure (i.e. large pores leading to small pores) are highly desirable because they combine the advantages of high surface area with the rapid mass transport. Mesoporous SiO2 with hierarchical pore structure was prepared by a novel dual templating approach using a combination of cellulose nitrate and surfactants as the templates. Both ionic and non-ionic surfactants, or mixtures of surfactants, in conjunction with cellulose nitrate were used as the pore templates. Low angle XRD patterns show well-defined pore structures and BET shows surface areas from 500 to over 800m2/g, with tunable bimodal or trimodal pore-size distributions from 18Å to 0.3μm. The hierarchical pore structure can be controlled by manipulating the template composition.

Preparation of three-dimensional ordered macroporous SiCN ceramic using sacrificing template method

Three-dimensional (3D) long range well ordered macroporous SiCN ceramics were prepared by infiltrating sacrificial colloidal silica templates with the low molecular weight preceramic polymer, polysilazane. This was followed by a thermal curing step, pyrolysis at 1250 °C in a N 2 atmosphere, and finally the removal of the templates by etching with dilute HF. The produced macroporous SiCN ceramics showed high BET surface areas (pore volume) in the range 455 m 2/g (0.31 cm 3/g)–250 m 2/g (0.16 cm 3/g) with the pore sizes of 98–578 nm, which could be tailored by controlling the sizes of the sacrificial silica spheres in the range 112–650 nm. The sphere-inversed macropores were interconnected by 50 ± 30 nm windows and 3–5 nm mesopores embedded in the porous SiCN ceramic frameworks, which resulted in a trimodal pore size distribution. The surface of the achieved porous SiCN ceramic was then modified by Pt–Ru nanoparticle depositing under mild chemical conditions.

Removal of sulfates and other inorganics from potable water by nanofiltration membranes of characterized porosity

In search for the optimal water treatment of a sulfate-containing water for drinking purposes from the location Prud (Croatia) nanofiltration experiments were undertaken to get data needed for the membrane treatment process considerations. The results of the nanofiltration experiments have shown very high retentions of sulfates by the examined nanofiltration membranes. The membranes’ porosity characterization showed bimodal and trimodal pore size distributions (PSDs) with a large fraction of medium-size pores of approximately 0.85 nm in the skin of both examined NF membrane types. The effective number of pores in two types of the NF membranes differed by an order of magnitude. Although the retention of inorganic salts was primarily affected by the membranes’ fixed charge, the sieving effect is also important especially for the retention of salts with the common anion.

Synthesis of mesoporous silica foams with hierarchical trimodal pore structures

A new type of mesoporous silica foam having extremely low bulk density (<0.01 g cm−3), high mesopore volume (>2 cm3 g−1) and specific surface area (>1000 m2 g−1) was prepared by the spraying of aqueous siliceous solutions containing alkyltrimethylammonium surfactants under high pH conditions. These remarkable properties were ascribed to macrovoids and two kinds of mesopores ranging around 3 and 40 nm. The macrocellular structure produced with air bubbles was stabilized through a rapid condensation reaction during drying. The framework of the macrovoids contained mesopores with a bimodal distribution derived from the surfactant micelles and the interparticle spaces of silica nanoparticles. The hierarchical self-assembly of surfactant micelles, silica nanoparticles and air bubbles provided the specific architecture with a trimodal pore size distribution.

Quantitative image analysis of unirradiated fast breeder reactor fuel

Three variants of fast breeder reactor fuel, sintered at different temperatures, are examined with the following methods of image analysis: • lateral porosity distribution, • pore size and grain size distribution, • • dependence of the shape on the size of pores and grains. These methods show that, in principle, two types of fuel exist: Type A: sintering temperatures 1610 and 1710°C, porosity distribution relatively homogeneous, bimodal pore size distribution, monomodal grain size distribution. TypeB: sintering temperature 1510°C, porosity distribution more inhomogeneous, trimodal pore size distribution, bimodal grain size distribution. These methods are briefly described and the quantitative results displayed. Using these methods these types of nuclear fuel will be re-examined after in-pile irradiation. The results, which are shown here, can then be correlated with those obtained after irradiation in order to recognize changes in structure dependent on temperature, time and burnup.

Evolution of pore structure during gas-solid reactions: Discrete models

Discrete numerical models were developed to describe pore structure transformations in gas-solid reacting systems. Computational grids with up to 16 million cells are employed to simulate random phenomena. Regions of various phases are defined and allowed to grow or interact according to rules modeling gas-solid reactions. This approach accurately tracks the loss of surface area due to pore overlap and transient profiles of porosities and surface areas can be obtained as functions of conversion. These models were used to simulate the noncatalytic gasification of coal chars containing ash and having unimodal, bimodal and trimodal pore size distributions. Simulation results of diffusional limitations in the micropores were used to interpret experimentally observed behavior of microporous chars under different reactive atmospheres. Model predictions for combustion with strong diffusional limitations indicate the macropore structure of some chars can lead to particle fragmentation at low conversions and enhancement of the gasification rate.