Showing papers on "Mesoporous material published in 1997"

Storage of hydrogen in single-walled carbon nanotubes

Abstract: Pores of molecular dimensions can adsorb large quantities of gases owing to the enhanced density of the adsorbed material inside the pores1, a consequence of the attractive potential of the pore walls. Pederson and Broughton have suggested2 that carbon nanotubes, which have diameters of typically a few nanometres, should be able to draw up liquids by capillarity, and this effect has been seen for low-surface-tension liquids in large-diameter, multi-walled nanotubes3. Here we show that a gas can condense to high density inside narrow, single-walled nanotubes (SWNTs). Temperature-programmed desorption spectrosocopy shows that hydrogen will condense inside SWNTs under conditions that do not induce adsorption within a standard mesoporous activated carbon. The very high hydrogen uptake in these materials suggests that they might be effective as a hydrogen-storage material for fuel-cell electric vehicles.

Functionalized Monolayers on Ordered Mesoporous Supports

Abstract: Mesoporous silica materials containing functionalized organic monolayers have been synthesized. Solid-state nuclear magnetic resonance suggests that a cross-linked monolayer of mercaptopropylsilane was covalently bound to mesoporous silica and closely packed on the surface. The relative surface coverage of the monolayers can be systematically varied up to 76 percent. These materials are extremely efficient in removing mercury and other heavy metals from both aqueous and nonaqueous waste streams, with distribution coefficients up to 340,000. The stability of these materials and the potential to regenerate and reuse them have also been demonstrated. The surface modification scheme reported here enables rational design of the surface properties of tailored porous materials and may lead to the synthesis of more sophisticated functionalized composites for environmental and industrial applications.

Continuous formation of supported cubic and hexagonal mesoporous films by sol–gel dip-coating

Abstract: Thin films of surfactant-templated mesoporous materials1,2 could find applications in membrane-based separations, selective catalysis and sensors. Above the critical micelle concentration of a bulk silica–surfactant solution, films of mesophases with hexagonally packed one-dimensional channels can be formed at solid–liquid and liquid–vapour interfaces3,4,5. But this process is slow and the supported films3,5 are granular and with the pore channels oriented parallel to the substrate surface, so that transport across the films is not facilitated by the pores. Ogawa6,7 has reported a rapid spin-coating procedure for making transparent mesoporous films, but their formation mechanism, microstructure and pore accessibility have not been elucidated. Here we report a sol–gel-based dip-coating method for the rapid synthesis of continuous mesoporous thin films on a solid substrate. The influence of the substrate generates film mesostructures that have no bulk counterparts, such as composites with incipient liquid-crystalline order of the surfactant–silica phase. We are also able to form mesoporous films of the cubic phase, in which the pores are connected in a three-dimensional network that guarantees their accessibility from the film surface. We demonstrate and quantify this accessibility using a surface-acoustic-wave nitrogen-adsorption technique. We use fluorescence depolarization to monitor the evolution of the mesophase in situ, and see a progression through a sequence of lamellar to cubic to hexagonal structures that has not previously been reported.

Mesoporous Platinum Films from Lyotropic Liquid Crystalline Phases

Abstract: The lyotropic liquid crystalline phases of surfactants exhibit a rich polymorphism of structures that have long-range periodicities and whose characteristic repeat distances range from 2 to 15 nanometers. The electrochemical reduction of platinum salts confined to the aqueous environments of these phases leads to the deposition of platinum films that have a well-defined long-ranged porous nanostructure and high specific surface areas. These results suggest that the use of liquid crystalline plating solutions could be a versatile way to create mesoporous electrodes for batteries, fuel cells, electrochemical capacitors, and sensors.

Pore Structure Control of Silica Gels Based on Phase Separation

Abstract: In the alkoxy-derived sol-gel system, various macroporous morphologies can be obtained by inducing the phase separation parallel to the sol-gel transition. This principle of macroporous morphology control can be best applied to pure silica and silica-based multicomponent oxide systems. The earlier the phase separation takes place than the sol-gel transition, the larger the characteristic sizes of pores and gel skeletons become. The time resolved light scattering measurements revealed that the morphology formation process exhibits the features of spinodal decomposition and that the final gel morphology is determined by the competitive kinetics between the domain coarsening and the structure freezing by sol-gel transition. The mesopore structure of such macroporous gel skeletons could be easily tailored by the solvent exchange procedures. Silica gels with controlled macropores and mesopores were successfully applied as a material for the continuous rod type column for high performance liquid chromatography.

Manganese oxide mesoporous structures: Mixed-valent semiconducting catalysts

Abstract: Hexagonal and cubic phases of manganese oxide mesoporous structures (MOMS) have been prepared by means of the oxidation of Mn(OH)2. The hexagonal MOMS materials form a hexagonal array of pores with an open porous structure, thick walls (1.7 nanometers), and exceptional thermal stability (1000°C). The walls of the mesopores are composed of microcrystallites of dense phases of Mn2O3 and Mn3O4, with MnO6 octahedra as the primary building blocks. The calcined hexagonal MOMS have an electrical conductivity of 8.13 × 10−6 per ohm·centimeter, an average manganese oxidation state of 3.55, and a band gap of 2.46 electron volts. Catalytic oxidations of cyclohexane and n-hexane in aqueous solutions in a batch reactor show conversions of ∼10 and ∼8 percent, respectively. Characterization and catalytic data suggest that MOMS systems show significant enhancement in thermal stability with respect to octahedral molecular sieve materials.

Bacterial templating of ordered macrostructures in silica and silica-surfactant mesophases

Abstract: The synthesis of inorganic frameworks with specified and organized pore networks is of potential importance in catalysis1,2, separation technology3 and biomaterials engineering4,5. Ordered arrangements of porous channels have been produced in silica-based materials by post-synthetic removal of surfactant templates from inorganic–organic mesostructures6,7. The resulting pore sizes are commensurate with the packing dimensions of the organic molecules, and are currently limited to length scales of up to 10nm. Here we show how a bacterial superstructure, consisting of a thread of coaligned multicellular filaments of Bacillus subtilis8,9, can be used to extend the length scale of inorganic materials patterning. We produce ordered macroporous fibres of either amorphous silica or ordered mesoporous silica6,7 (MCM-41) by template-directed mineralization of the interfilament spaces followed by removal of organic material by heating to 600°C. The inorganic macrostructures consist of a macroporous framework of 0.5-μm-wide channels with curved walls of either silica or mesoporous silica, 50 to 200 nm in thickness. The formation of ordered pores in the MCM-41 replica on both the mesoscopic and macroscopic length scales illustrates how supramolecular and supercellular templates might be combined for the fabrication of inorganic materials with structural hierarchy.

Mesoporous Silica Synthesized by Solvent Evaporation: Spun Fibers and Spray-Dried Hollow Spheres

Abstract: Simple synthesis methods for mesoporous fibers and powders by rapid evaporation of hydrolyzed silicon alkoxide−surfactant solutions are described. Mesoporous fibers are prepared by dry spinning, and mesoporous powders by spray drying of alkoxide−surfactant solutions. Precursor solutions, containing fully hydrolyzed tetraethoxysilane, cetyltrimethylammonium chloride surfactant, and, in the case of the fibers, a fiber-forming polymer, in an acidic alcohol/water mixture, are drawn into continuous filaments or atomized into droplets in a heated air stream. During solvent drying, silica and surfactant self-assemble to form the hexagonally ordered mesophase structure, and all of the nonvolatile components (silica, polymer, and surfactant) are incorporated into the mesophase. The pore diameter and surface area of calcined fibers were 20 A and 1100 m2/g, respectively. For the powders, pore sizes of 25 A and surface areas as high as 1770 m2/g were measured. Spray-dried powders consisting of hollow spherical partic...

Magnetic Field Alignment of Ordered Silicate-Surfactant Composites and Mesoporous Silica

Abstract: Macroscopic orientational ordering of the pores of condensed hexagonal mesostructured silica (MCM-41) was achieved through alignment of an unpolymerized, hexagonal, lyotropic silicate-surfactant liquid crystal in a high magnetic field. This alignment was preserved after polymerization of the silicate species by acid treatment. Subsequent calcination to remove the surfactant yielded a mesoporous silica solid that retained both macroscopic pore alignment and mesoscale periodicity. Potential applications of such liquid crystal processing strategies range from the formation of anisotropic silica-based bulk ceramics to the production of oriented mesoporous thin films for chemical sensors, separations, catalysis, or host-guest applications.

Stabilization of Mesoporous Molecular-Sieves by Trimethylsilylation

Abstract: The stability of Si-MCM-48 to moisture and compression can be improved by trimethylsilylation, as a result of the increase in hydrophobicity. Moreover, the stability is retained after calcination, although the calcined material is no longer hydrophobic. This stabilizing effect was also observed for Si-MCM-41 and Ti-MCM-41.

Photoactivity of Titanium Dioxide Supported on MCM41, Zeolite X, and Zeolite Y

Abstract: Titanium dioxide supported on microporous zeolites of type X and Y and on mesoporous molecular sieves of the MCM41 type was studied for the photocatalytic degradation of acetophenone in an aqueous medium. The photoactivity of the supported catalyst is strongly influenced by the method of titania loading, but less affected by the temperature at which the sample was calcined. The highest photoactivity among the supported catalysts is observed for a support that has a lower Si/Al ratio in the framework and relatively large pore size. The measured apparent activation energies for photocatalyzed consumption of acetophenone for a commercial TiO2 (P-25) and 10 wt % TiO2-loaded samples of zeolite A, X, Y, and Al-MCM41 are 4.2−6.7, 16.9, 13.5, 32.2 and 3.6 kJ/mol, respectively. Selective doping by Fe, Mn, and V into the framework of MCM41 suppresses the photoactivity of the supported titanium dioxide. The physical state of the titanium dioxide on the supports is characterized by XRD, adsorption and pore size analy...

Capillary Critical Point of Argon, Nitrogen, Oxygen, Ethylene, and Carbon Dioxide in MCM-41

Abstract: The effect of temperature on adsorption isotherms of Ar, N2, O2, C2H4, and CO2 on mesoporous MCM-41 molecular sieves with different pore sizes (pore radius rp = 1.2, 1.4, 1.8, 2.1 nm) has been measured. The occurrence of adsorption hysteresis upon capillary condensation of these molecules in mesopores depends on their capillary critical temperatures (Tcc), which are far below the corresponding bulk critical temperatures (Tc). In accord with theoretical predictions of confined geometry effects on the criticality of fluids in narrow pores, a plot of (Tc − Tcc)/Tc against d/rp for Ar, N2, O2, and C2H4 seems to form a single straight line passing through the origin, where d is the molecular diameter. The data for CO2 deviates from this relationship. These results are compared with critical point shifts of fluids in conventional mesoporous adsorbents with an interconnected network of pores available in the literature.

Solution-phase grafting of titanium dioxide onto the pore surface of mesoporous silicates: Synthesis and structural characterization

Abstract: Titanium dioxide, a large-bandgap semiconductor and versatile photocatalyst, has been grafted onto the pore surface of MCM-41 and FSM-16 (a mesoporous material derived from kanemite) by reacting TiCl4 in hexanes with the as-synthesized mesostructured silicate. The products have been extensively characterized by powder XRD, TEM, SEM, EDS, XPS, N2 adsorption, SANS contrast matching, solid-state 1H MAS NMR, IR, and UV−vis spectroscopies. It was found that titania forms well-dispersed isolated (TiO2)n clusters (n ∼ 30−70) within the channel structure. These are attached to the silicate walls via Si−O−Ti bonds. A minor second phase consisting of anatase crystallites ca. 100−250 A in diameter on the external surface of the mesoporous silicate crystals was sometimes obtained. It is concluded that an organic moiety, such as the surfactant present in the pores, or a physical constraint, such as the pore walls, is necessary to prevent the creation of large TiO2 agglomerates and enable the formation of nanosized TiO...

Synthesis of MCM-41 with Different Pore Diameters without Addition of Auxiliary Organics

Abstract: Swollen and highly ordered MCM-41 type materials with varied pores sizes, but with similar wall thicknesses, have been directly synthesized in the absence of auxilliary organics by controlling the preparation conditions. The most important parameter in the synthesis in order to control the pore opening is the cetyltrimethylammonium (CTMA+) concentration in the synthesis gel. Our results suggest that the mechanism of swelling is related with the replacement of some CTMA+ species by others cations in the interface formed between the liquid crystal and the silica surfaces. This approach could open new insights into the mechanism of formation of MCM-41 type materials as well as having in addition an important impact on the preparation of large mesoporous materials.

Synthesis and Characterization of the Mesoporous Silicate Molecular Sieve MCM-48

Abstract: We have used powder X-ray diffraction, diffuse reflectance Fourier-transform infrared, 29Si MAS NMR, N2 adsorption, and transmission electron microscopy to examine the role of the concentration of silica, the surfactant, and the reaction time in the synthesis of high-quality silica mesoporous molecular sieve MCM-48. The XRD patterns of the products are consistent with the hexagonal → cubic → lamellar phase transformation.

Synthesis of microporous transition-metal-oxide molecular sieves by a supramolecular templating mechanism

Abstract: Mesoporous bulk1,2,3,4 and thin-film5,6,7 silicates with pore sizes of 20–100 A can be synthesized by using micellar aggregates of long-chain organic surfactant molecules as templates to direct the structure of the silicate network. Because of the potential applications of these molecular-sieve materials as catalysts, separation membranes and components of sensors, it is desirable to extend the range of accessible pore sizes and material compositions. Mesoporous oxides in which transition metals partially8 and fully9,10,11,12,13 substitute for silicon have been made by similar means, in the latter case by ensuring strong interactions between the surfactants and the transition-metal alkoxide precursors. Templating with organic molecules has also been long used for the synthesis of microporous materials—synthetic zeolites—which have smaller pore sizes (4–15 A), but here the organic molecules are shorter-chain amphiphiles which are too small to be considered true surfactants and so act as discrete entities around which the framework crystallizes14,15,16. Here we show that even such short-chain molecules can aggregate into supramolecular templates when they form bonds with transition-metal (niobium) alkoxides, and that in this way they can direct the formation of transition-metal oxides with pore sizes of less than 20 A. These pore sizes, which result from the smaller diameter of micellar structures of the short-chain amines relative to the longer-chain surfactants used for the synthesis of mesoporous materials, qualify the resulting molecular sieves as microporous, even though the supramolecular templating mechanism is similar to that used to make the mesoporous materials. Thus our approach extends the supramolecular templating method to afford microporous transition-metal oxides.

Characterisation of porous solids IV

Abstract: The Kelvin Equation and Adsorption Hysteresis Micropore Size Distribution in Carbon Molecular Sieves by Immersion Calorimetry Magic Angle Spinning NMR Studies of Adsorption onto Activated Carbons Analysis of HRADS Adsorption Neutron Scattering Investigations of Adsorption in Porous Solids A Pore Width Sensitive Ordered Structure of Water Molecules in a Carbon Micropore Computer Simulation Study of Sorption in Cylindrical Pores with Varying Pore Wall Heterogeneity Transport Diffusion of Adsorbates in Micropores: Simulation in Model Systems Simulation Studies of Pore Blocking Phenomena in Model Porous Networks Characterisation of Microporous Carbons by Using Molecular Simulation to Analyse the Adsorption of Molecules of Different Sizes Ordered Mesoporous MCM-41 Adsorbents: Novel Routes in Synthesis, Product Characterisation and Specification Characterisation of Zirconia-Based Ordered Mesoporous Materials Physisorption of Gases by MCM-41 Characterisation of Porosity During Thermal Decomposition of Nickel Hydroxide A Thermodynamic Investigation of Physisorbed Phases Within the Model Mesoporous Material: MCM-41 Helium Adsorption on Mesoporous Solids at 4.2K - Density of Adsorbed Helium in Porous Systems Estimating the Dimensions of Ultramicropores Using Molecular Probes Off-lattice Reconstructed Biphasic Media Using Gaussian Random Fields: Evaluation for Different Disordered Porous Solids Micropore Size Distributions From Experimental Isotherm Data and Grand Canonical Monte Carlo Simulations Measuring and Modelling the Pore-level Properties of Meso- and Macro-porous Media - An Overview Pore Formation in Carbons Derived from Phenol Resin and their Gas Adsorption Characterisation of Porous Solids: Adsorption Potential Distribution and its Thermodynamic Implications Within the Hysteresis: Insight into the Bi-modal Pore Size Distribution of Close-packed Spheres Properties of the Nanoengineered Two-dimensional Silica-based Porous Networks The Presence of "Shadowed" Macropores in the Porous Network of an Activated Alumina Nuclear Magnetic Relaxation Study of Water Adsorbed in Mesoporous Activated Carbons Analysis of the Topological and Geometrical Characteristics of the Pore Space of Permeable Solids Using Serial Tomography, Mercury Porosimetry and Theoretical Simulation Morphology of Biphasic Random Media: Chord Distributions and Two-point Correlation Functions Interparticle Capillary Condensation and the Pore Size Distribution of Several Adsorbents Dual Site Bond Models in the Presence of Correlations Determination of the Micropore Size in Carbons by Means of Experimental and Calculated Isotherm Derivatives Study of Activated Physical Adsorption by ESR Method Synthesis and Characterisation of a Zeolite A with Detergent Characteristics Gravimetric and Volumetric Measurements of Helium Adsorption Equilibria on Different Porous Solids Multiscaling Texture Characterisation of Freeze-dried Resorcinol-formaldehyde Aerogels Comparison of t-plots for Carbons, Silicas and Aluminas Fractal Analysis of Aerogels and Carbon Blacks Using Nitrogen Adsorption Data: Comparative Study of Two Methods Certification of Reference Materials for Pore Analysis by the Gas Adsorption Method Adsorption and Displacement of Formic Acid and Acetic Acid on Microporous Phenolic Resin Carbons Characterisation of Sulphated Oxides of Titanium: Infra-red, Adsorption and Related Studies Analysis of Pore Properties from Low Pressure Hysteresis in Nitrogen Adsorption Isotherms Adsorption Characterisation of H-ZSM-5 Surface Functional Groups and Texture Characterisation of Catalytic Supports Based on Activated Carbon Modelling Stratified Porous Media Modelling of Pore Level Properties Sorptive Properties of Cloverite Comparison of the Translational Mobility Dt of Hydrogen, Methane and Neo-pentane Molecules Sorbed on AIPO4-5 Characterisation of the Solid Texture of Porous Particles by Application of a Non-Catalytic Gas

Synthesis and Characterization of Cobalt-Complex Functionalized MCM-41

Abstract: Silanation of the internal pore surface of the mesoporous molecular sieve MCM-41 (40 A) has yielded ethylenediamine (ED), diethylenetriamine (DET), and ethylenediaminetriacetic acid salt (EDT) functionalized sites. These grafted ligands were used for the preparation of cobalt(II) complexes covalently bound to the MCM-41 support. These materials were characterized by SEM, UV−vis, FTIR, ESR, and cyclic voltammetry as well as elemental analysis. Preliminary studies suggest the formation of a reversible cobalt−oxygen adduct with MCM-41 grafted ED and DET. The nature and reactivity of the MCM-41 bound complexes were found to differ significantly from functionalized amorphous silica.

Optimal parameters for the synthesis of the mesoporous molecular sieve si-mcm-41

Abstract: Highly crystalline MCM-41 with a very narrow pore-size distribution (FWHM=1.5 A), high surface area (1185 m2 g -1 ), large grain size and thick channel walls (ca. 17 A) was prepared in alkali-free media. The properties of the product depend on the source and concentration of the reactants, the gel aging time, the temperature and the duration of the synthesis. There is no induction period in the course of the synthesis and Ostwald's rule of successive transformations applies. The initially produced hexagonal phase is transformed into the lamellar phase and then into an amorphous phase. In the 150°C synthesis the most stable product is amorphous silica. The course of the synthesis is conveniently monitored by pH measurement. Gel aging, during which a spatial distribution of silicate polyanions and micellar cations is established, is essential for preparing high-quality MCM-41. Surfactants with the same cationic organic group but different counter-anions alter the course of the synthesis. The degree of polymerization of silica is also important. Highly basic gels favour the lamellar product; when the gel is weakly basic the quality of MCM-41 is lower as insufficient TMAOH is available to dissolve the silica. The result of excess silica is similar but even more pronounced. Purely lamellar products are made at low SiO 2 concentrations, when the gel is more strongly basic. The best quality MCM-41 is prepared from a gel of molar composition SiO 2 :0.19 TMAOH:0.27 CTABr:40 H 2 O (with CTABr/SiO 2 =0.27, similar to the ratio in the solid product) aged at 20°C for 24 h and synthesis lasting for 48 h.