Controlled Pore Size Distribution Research Articles

Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption

Cryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future trans- portation needs. Inexpensive carbide-derived carbons (CDCs), produced by chlorination of metal carbides, have up to 80% open-pore volume with tunable pore size and specific surface area (SSA). Tuning the carbon structure and pore size with high sensitivity by using different starting carbides and chlorination temperatures allows rational design of carbon materials with en- hanced C-H2 interaction and thus increased H2 storage capacity. A systematic experimental investigation of a large number of CDCs with controlled pore size distributions and SSAs shows how smaller pores increase both the heat of adsorption and the total volume of adsorbed H2. It has been demonstrated that increasing the average heat of H2 adsorption above 6.6 kJmol -1 substantially enhances H2 uptake at 1 atm (1 atm=101325 Pa) and -196°C. The heats of adsorption up to 11 kJmol -1 exceed values reported for metal-organic framework compounds and carbon nanotubes.

Low-Temperature Sol-Gel Synthesis of Single-Phase ZrTiO4 Nanoparticles

Ceramic ZrTiO4 powders were prepared by a sol-gel method using zirconium oxychloride and titanium tetraisopropoxide. In situ high temperature X-ray diffraction results show that crystallization of the amorphous gel starts at 400 ∘C. Single-phase ZrTiO4 nanoparticles were obtained after heat treatment at 450 ∘C for one hour. An average particle size of 46 nm has been determined by nitrogen adsorption analysis. After pressing these sinteractive powders, pellets with controlled pore size distribution were obtained by sintering at temperatures as low as 400 ∘C. The analysis of pores by mercury porosimetry gives an average porosity of 45%. The electrical resistivity, determined by impedance spectroscopy measurements at 24 ∘C under different humidity environments, shows the ability of these pellets to adsorb water vapor in the porous surfaces. Pellets fabricated with the nanosized powders prepared by the sol-gel technique are proposed as good candidates to be used in humidity sensing devices.

Microporous carbon derived from boron carbide

Materials with high specific surface area (SSA) and controlled pore size distribution are of great importance because of their potential applications in molecular sieves, gas storage, catalyses, absorbents, battery electrodes, supercapacitors, water/air filters and medical devices. Carbide derived carbons (CDCs) represent a new class of microporous carbons produced by selective thermo-chemical etching of the carbide-forming element from metal carbide in which control over the pore size distribution is possible. The synthesis of microporous carbons done by chlorination of boron carbide at various temperatures (400–1200 °C) is presented in this work. Analysis of carbon powders which were characterized by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows that the structure obtained can be controlled by varying the chlorination temperature. The resultant carbon demonstrated high specific surface area close to 2000 m 2/g (for argon adsorption) at the moderately low chlorination temperature of 800–900 °C.

Synthesis of ordered nanoporous carbons of tunable mesopore size by templating SBA-15 silica materials

Ordered mesoporous carbons with controllable pore size distributions were synthesised by using mesostructured SBA-15 silica samples as templates. By varying the synthesis temperature of SBA-15 between 90 and 150 °C, it is possible to tune the size of silica mesopores between 7.2 and 11.3 nm. As the synthesis temperature of SBA-15 silica increases over this range, the size of the mesopores of templated carbons can be modified between 3.0 and 5.2 nm. Modification of the synthesis temperature of silica does not affect its structural order, which is that characteristic of SBA-15. Independent of the type of SBA-15 used as template, the prepared carbons are highly ordered. The porosity of the carbons synthesised from SBA-15 silica in one impregnation–carbonisation (N 2) cycle is made up of two types of mesopores, i.e. structural mesopores (derived from the silica skeleton) and mesopores confined within the carbon rods. The structure of a carbon with these characteristics is described by the CMK-3/5 model, which is presented here for the first time. When the impregnation–carbonisation cycle is repeated twice, the carbon rod confined mesopores disappear and the structure of the resulting carbon is well described by the CMK-3 model.

Controllable porosity hydroxyapatite ceramics as spine cage: fabrication and properties evaluation.

A procedure was designed to prepare porosity-graded hydroxyapatite (HA) ceramics simulating the bimodal structure of natural bone, which could be used to build a cage that would promote the reconstruction of the anterior column after vertebrectomy or corpectomy in tumor and trauma surgery. HA ceramics with controllable pore size distribution and porosity were developed by using chitosan and Poly(vinyl alcohol) (PVA) as the pore-forming agents. HA ceramics with worthwhile properties such as a wide range of volume porosity (10-50%) and pore size (nanometer to 400 microm) can be obtained from this method, which allows the fabrication of HA ceramics with desirable porous characteristics simulating the bimodal natural bone architecture expected to provide advantages for bony fusion in the intervertebral foramina. When coated with chitosan-gelatin network, the bending strength of the porous HA ceramics significantly improved. The polymer network coated porous HA have potential application in the construction of cages for spinal operations.

Control of mesoporous structure of carbons synthesised using a mesostructured silica as template

Mesoporous skeleton carbons with controllable pore size distributions (PSDs) were synthesised by modifying the extent of which the carbon precursor infiltrated the pores of the mesostructured silica, which was used as a template. When the silica porosity was fully infiltrated, carbons with a unimodal narrow PSD (∼3 nm) were obtained. This porosity was an inverse replica of the silica framework. If the silica pores were not completely filled, carbons with bimodal PSDs were synthesised. The carbons exhibited two kinds of mesopores: pores derived from the silica walls (∼3 nm) and larger pores, with sizes of up to 18 nm, formed from the coalescence of unfilled silica pores when the silica walls were dissolved. These results show that the synthesis of mesoporous carbons can be accurately controlled to obtain carbon materials with different PSDs using only one type of silica template.

Preparation of porous carbon derived from mixtures of furfuryl resin and glycol with controlled pore size distribution

This paper describes the preparation and properties of porous carbon by a technique which consists of mixing a carbon precursor (furfuryl resin and furfural alcohol), a pore-forming agent and a solvent (glycol), polymerizing the resin mixture, and pyrolyzing the hybrid of resin and glycol. The properties of porous carbons have been systematically investigated as a function of composition and heat treatment, with emphasis on understanding and controlling their morphology and pore size distribution. The results seem to indicate that by varying the ratios of the constituents in the polymer system, porous carbons with a wide variation in pore size distribution and morphology can be obtained. Three types of morphologies were observed: interconnected carbon with secondary spherical pores, discrete carbon particulates, and a crosslinked carbon network. Porous carbons with a very narrow pore size distribution have been obtained and the average pore size was controlled between 5 and 0.008 μm. The microstructure of porous carbon formed as a result of phase separation of resin-rich phase and glycol-rich phase, rather than a result of the pyrolysis process. Heat treatment had little effect on the properties of the porous carbons.

Preparation and properties of phenolic resin-based activated carbon spheres with controlled pore size distribution

Three kinds of phenolic resin-based activated carbon spheres (P-ACS) with different pore size distribution were prepared successfully by adding pore-forming agents to novolac-type phenolic resin. Polyethylene glycol and polyvinyl butyral, serving as pore-forming agents, evaporated during pyrolysis and left a small amount of carbon residue in the matrix of the phenolic resin-based carbon, thus changing the carbonization and activation behavior of the resin. Mesopores between 3 and 5 nm were created in the P-ACS, which possessed excellent adsorption properties for creatinine. Ferrocene has little effect on the carbonization process of the phenolic resin, but has a great impact on the activation process. Mesopores and macropores with a range from 3–5 to 10–90 nm were produced in the P-ACS, which exhibited large adsorption properties for VB 12, a larger molecule than creatinine. P-ACS without pore-forming agents exhibited a small specific surface area and mainly micropores, which resulted in a very small amount of creatinine and VB 12 adsorbed.

Ultrasound as a tool to synthesize nano-sized silica-alumina catalysts with controlled mesoporous distribution by a novel sol-gel process

A novel sol–gel process is used to synthesize mesoporous silica–alumina catalysts with controlled pore size distribution without using templates or pore-regulating agents. The nitrogen sorption analysis shows that the synthesized materials have high specific surface area in the range 587–692 m2/g and similar mesoporous distribution within 2–11 nm. Ultrasound is applied to prepare precursor silica–alumina sol with narrow particle diameter distribution. By calculation, it is found that ultrasonic treatment is able to provide 22.94 kJ/mol energy, which is just in accordance with the weak bond's bonding energy. Based on the calculation result, it is proposed that the role of ultrasonic treatment followed by acid peptization is to provide energy to break down the weak bonds. After gelation, the regular and close packing of similar-sized sol particles is capable of formation of controlled-sized interstices which are the precursors of mesopores. Solid-state 27Al MAS NMR and TEM are used to characterize the samples' structure and particle morphology. They show that most of the aluminum is located in the tetrahedral position in the present materials. According to TEM results, it is shown that the synthesized materials have spherical particles with size of about 15 nm.

Synthesis of hierarchical porous silicas with a controlled pore size distribution at various length scales

Hierarchically structured porous materials are of great interest to catalysis, where an accurately controlled pore texture at different length scales can help to reduce or otherwise control transport limitations. A method is presented to synthesize bimodal structured silicas, with an independently controlled small meso- and large meso-to macroporosity. Small primary MCM-41 particles assemble around micelles formed by a tri-block copolymer surfactant that is added as a low-concentration ethanolic solution to the particles, while these still form a flexible gel. Cross-linking of the particles in an autoclave, followed by drying and calcination, leads to bimodal materials with the controlled small mesopores of MCM-41, and a larger meso- to macropore size distribution that depends on the micelle shape and size. The latter is a function of the conditions in the second step, such as the amount and composition of the surfactant, the aging time, the temperature, the pH and the type of solvent. Fine tuning of this procedure, application to other primary structured particles, and combination with other structuring methods, should enable to construct multi-structured hierarchical materials with a desired texture at all scales.

Preparation of Porous Poly(styrene-co-divinylbenzene) Monoliths with Controlled Pore Size Distributions Initiated by Stable Free Radicals and Their Pore Surface Functionalization by Grafting

A stable free radical (SFR) mediated preparation of porous poly(styrene-co-divinylbenzene) monoliths using new types of SFRs and a novel binary porogenic solvent consisting of poly(ethylene glycol) ...

Polymer substrate topography actively regulates the multicellular organization and liver-specific functions of cultured hepatocytes.

This study examines the role of topography of porous synthetic polymer substrates in regulating the tissue-specific morphogenesis of cultured hepatocytes. Porous foams of amorphous 50/50 poly(D,L glycolic-co-lactic acid) (PGLA) with a wide range of controlled pore-size distributions ( approximately 1 to 100 microm) were used as culture model surfaces. We found that the induction of microporosity in PGLA substrates significantly improved cell attachment and viability in comparison to those observed on non-porous PGLA films. A detailed evaluation of cellular morphogenesis on the microporous matrices showed that hepatocellular organization was sensitively dependent on the topographical feature size of the foam surfaces. Foams with subcellular size voids ( approximately 3 microm) induced kinetics of two-dimensional hepatocyte reorganization, yet limited the extent of three-dimensional aggregation. In contrast, foams with supercellular size voids ( approximately 67-microm) restricted hepatocyte motility, thereby promoting the kinetics of 3D aggregation. At intermediate void sizes ( approximately 17 microm), both 2D and 3D reorganization kinetics were promoted. Albumin secretory kinetics progressively increased on all void size configurations, the most rapid and sustained kinetics observed in supercellular sized voids, which may serve to minimize cell-polymer contacts and maximize cell-cell contacts in 3D. Overall, these studies demonstrate that void topography of porous polymer substrates is a critical textural feature to induce short-term cell adhesion and viability, and to also selectively regulate the kinetics and extent of multicellular spreading versus 3D aggregation. By virtue of its effects on cell adhesion and morphogenesis, the void topography of nonphysiological polymer scaffolds also is a powerful variable to microengineer hepatospecific activity of tissue analogs.

Characterization of Natural Organic Matter Using High Performance Size Exclusion Chromatography

High performance size exclusion chromatography (HPSEC) was used to obtain the molecular weight distributions of natural organic matter (NOM) from two South Australian drinking water sources. The NOM was separated into five nominal molecular weight fractions ( 30K) using ultrafiltration membranes prior to HPSEC analysis. The use of HPSEC as a tool for NOM characterization was compared with an independent method, flow field−flow fractionation (FlFFF), which separates molecules via a different mechanism. Unlike HPSEC, which uses a porous gel with a controlled pore size distribution to separate molecules, FlFFF uses hydrodynamic and molecular diffusion principles to separate molecules on the basis of molecular size, in the absence of a porous gel. The comparison was made using the following parameters: weight-average molecular weight (Mw), number-average molecular weight (Mn), peak molecular weight (Mp), polydispersivity (Mw/Mn), and molecular weight range (80% confidence l...

Phase Development in Sol Derived Microporous Membranes for Gas Permselectivity

A sol-gel route has been used to produce a nanoporous membrane for the separation of “greenhouse” gases from power plants at elevated temperatures (700–900 K). The membrane has a controlled pore size distribution and is multiphasic, in order to prevent grain growth and pore coarsening. The alumina/titania diphasic ceramic was obtained from alkoxide precursors with additional magnesium or lanthanum doping to stabilize low temperature phases and to provide possible chemisorption of CO2. It was found that both Mg and La stabilized the γ-alumina phase to higher temperature by as much as 200 K, although Mg is less effective.

97/03144 A kind of porous ceramic with controlled poresize distribution

97/03144 A kind of porous ceramic with controlled poresize distribution

Esterification Activity in Organic Medium of Lipase Immobilized on Silicas with Differently Controlled Pore Size Distribution.

Lipase was covalently immobilized on silicas with differently controlled pore size distribution and utilized in benzene to catalyze esterification. When silicas were converted to alkylamino derivatives in preparation for immobilization, their physical properties, such as average pore size, changed remarkably, but were relatively unchanged during the succeeding immobilization. Thus the derivatives determined immobilized quantity of the enzyme with their physical properties such that they generally immobilized more enzyme as their pores became larger in size and in volume. The activity of the immobilized enzyme depended on the immobilized quantity of the enzyme, water content, resistance of intra-mass transfer affected by pore size distribution, chemical properties of the silica surface and an over-crowding effect possibly emerging among immobilized lipase molecules. Among these factors, the first two were principal ones and activity was highest at the water content where their pores were filled, being generally higher with an increase in the immobilized quantity of the enzyme.

Porous Ca–P–O bio-glassceramics by loose-powder-sintering

Porous resorbable Ca–P–O glassceramic tooth roots based on calcium biphosphate were prepared by loose-powder-sintering then crystallization-annealing. The glass composition is, by weight, CaO/(CaO + P2O5) = 0.33. The sintering behavior of the glass can be described by a conventional viscous flow model. The resultant glassceramic has a pore size of 6 to 36 μm, depending on starting particle size and sintering temperature, while the pore size distribution is independent of sintering time. The flexural strength is 33 to 150 MPa depending mostly on crystallization-annealing treatment. The crystalline phases are β−Ca2P2O7 and CaP2O6. After being implanted into rabbits, these porous implants show remarkable biocompatibility and induce the ingrowth of new bone within 30 days. They are partly resorbed after 90 days and replaced by new bone. In spite of the original small porosity, the ingrowth of blood vessels and bone cells is abundantly seen after 90 days, due to enlarged pores produced by progressive resorption. This glassceramic is a good candidate for resorbable tooth and bone implants. The loose-powder-sintering technique resulting in intricate bulk shapes with controllable pore size distribution and good machinability as well as adjustable mechanical strength hence is a powerful technique for the preparation of porous glassceramics.

Early Stages of Pore Formation on Si During Etching in HF Solutions

AbstractThe fabrication of porous silicon (PS) with controlled pore size distributions is of importance for reproducible manufacturing as well as it may help to elucidate the origin of light emission from this material. The pore formation on p‐doped Si(111) surfaces is studied. For this purpose hydrogen passivated surfaces are prepared by wet chemical treatment. After introduction into UHV system a 1 × 1 LEED pattern is readily obtained without any further treatment. These surfaces are etched at the open circuit potential (OCP) in HF solutions with pH‐values ranging from 1 to 10. Atomic force microscopy is used to characterize the dependence of pore formation on time and acidity. It is found that narrow pore size distributions can be obtained when HF solutions with high pH‐values are used.

Preparation of monodisperse carbonaceous particles with micro‐, meso‐, and macroporous structures

AbstractSpherical, highly porous synthetic carbon particles have been prepared from formulations of either poly(furfuryl alcohol) or poly(vinyl acetate) mixed with a solvent and poreforming agents. Production was carried out by atomizing these mixtures inside a thermal reactor and drying the resulting aerosol. The size of the collected polymeric particles was controlled by preadjusting the parameters of the atomization process. Fine‐tuning of the process resulted in the generation of batches of monosized particles. Subsequent thermal degradation at 600°C resulted in porous chars with controlled pore‐size distributions. Several distinct pore structures were superimposed using this technique with pore widths spanning the region from a few nanometers to a few tens of micrometers. Spherical and equal‐sized macropores were also generated with a special technique described herein. All chars were produced in batches of spherical and monodisperse particles, with diameters in the range of 20–150 μm. BET surface areas were measured to vary from 1 m2/g to over 150 m2/g prior to any oxidative treatment. Porosities varied from 10 to 80%.

Influence of Hipping Pressure on the Strength and the Porosity of Porous Copper

ABSTRACTOpen porous copper metals, which have high strength, high open porosity and well controlled pore size distribution, were produced by a hot isostatic press (HIP) process. They were sintered at different temperatures from 973 to 1273K under various HIPping pressures up to 200MPa. Pore size distribution and Young's modulus of the sintered samples were analyzed. The HIPped products have greater strength and higher open porosity than those of the normally sintered ones. The internal structural parameters such as pore size distribution were controlled by changing the HIPping pressure.