Showing papers on "Mesoporous material published in 1999"

Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation

Abstract: Ordered carbon molecular sieves exhibiting Bragg diffraction of X-ray lines have been synthesized for the first time, using mesoporous silica molecular sieves as the template. Sucrose was converted to carbon inside the mesopores of the silica molecular sieves through a mild carbonization process using a sulfuric acid catalyst. The carbon molecular sieves were obtained after the removal of the silica framework using an aqueous solution of sodium hydroxide. The X-ray diffraction, transmission electron microscopy, and pore size analysis showed that the structure of the carbon molecular sieves consisted of a three-dimensional regular array of uniform mesopores 3 nm in diameter. The structure was not simply a negative replica of the used silica template, but the synthesis mechanism involved the unique transformation into a new ordered array that was triggered by the removal of the silica frameworks. The highly ordered mesoporous texture suggested its scientific and technological importance as a new shape-selec...

Synthesis and applications of supramolecular-templated mesoporous materials

Abstract: The unique structure of MCM-41 silicates (shown in the picture) has provided for extremely attractive properties—uniform pore sizes greater than 20 A, surface areas exceeding 1000 m2 g−1, and long-range ordering of the pores. Recent research in supramolecular-templated mesoporous materials has led to a wide range of compositions, to uses in a variety of catalytic reactions, and to a better control of bulk morphologies.

Periodic mesoporous organosilicas with organic groups inside the channel walls

Abstract: Surfactant-mediated synthesis methods have attracted much interest for the production of inorganic mesoporous materials, which can, on removal of the surfactant template, incorporate polymeric, organic, inorganic and organometallic guests' in their pores 1,2 . These materials-initially made of silica 3-5 , but now also available in the form of other oxides 6-9 , sulphides 10,11 , phosphates 12 and metals 13 -could find application in fields ranging from catalysis, adsorption and sensing technology to nanoelectronics. The extension of surfactant-mediated synthesis to produce inorganic-organic hybrid material (that is, materials that contain organic groups as an integral part of their framework structure) promises access to an even wider range of application possibilities. Such hybrid materials have been produced in the form of amorphous silicates (xerogels) that indeed display unique properties different to those of the individual components 14-20 , but their random networks with broad pore-size distributions severely limit the shape and size selectivity of these materials. Mesoporous hybrid materials with periodic frameworks have been synthesized, but the organic groups are all terminally bonded to the pore surface, rather than incorporated into the pore walls 21-26 . Here we describe a periodic mesoporous organosilica containing bridge-bonded ethene groups directly integrated into the silica framework. We are able to solvent-extract and ion-exchange the surfactant templates to create a stable and periodic mesoporous ethenesilica with high surface area and ethene groups that are readily accessible for chemical reaction. Recent syntheses of similar periodic mesoporous organosilicas 27,28 and the ability to incorporate a variety of bridging organic and organometallic species raise the prospect of being able to fuse organic synthesis and inorganic materials chemistry to generate new materials with interesting chemical, mechanical electronic, optical and magnetic properties.

Novel Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks

Abstract: Novel organic−inorganic hybrid mesoporous materials have been synthesized, in which organic and inorganic oxide moieties are distributed homogeneously at the molecular level in the framework, forming a covalently bonded network. They are highly ordered at the mesoscale, with two- and three-dimensional hexagonal symmetries and well-defined external morphologies. Nitrogen adsorption measurements show a uniform pore-size distribution with pore diameters of 31 and 27 A, and high surface areas of 750 and 1170 m2/g. The synthetic procedure to polymerize the organosilane monomer containing two trialkoxysilyl groups in the presence of surfactant can be applied to the synthesis of a variety of highly ordered organic−inorganic hybrid mesoporous materials.

Mesoporous sieves with unified hybrid inorganic/organic frameworks

Abstract: Mesoporous materials have been synthesized that are composed of hybrid frameworks in which inorganic and organic components have a fixed stoichiometry and are covalently bonded. The creation of UOFMN (unified organically functionalized mesoporous networks) materials incorporates concepts employed in the synthesis of MCM-41 mesoporous silicates, making use of a quaternary ammonium cationic surfactant and a double trialkoxysilyl precursor such as bis(triethoxysilyl)ethane (BTSE) or bis(triethoxysilyl)ethylene (BTSEY). The cetyltrimethylammonium (CTA+) surfactant is removed by extraction with acid, resulting in a high surface area porous organosilicate framework in which Si atoms are bridged by ethane (from BTSE) or ethylene (BTSEY) groups. The channels are wormlike and uniform in diameter. UOFMN materials are more hydrothermally stable than MCM-41 prepared under similar conditions and have thicker pore walls. Ethylene groups in products made with BTSEY can be brominated, the brominated product itself being ...

Block Copolymer Templating Syntheses of Mesoporous Metal Oxides with Large Ordering Lengths and Semicrystalline Framework

Abstract: A simple and general procedure has been developed for the syntheses of ordered largepore (up to 14 nm) mesoporous metal oxides, including TiO2, ZrO2 ,N b 2O5 ,T a 2O5 ,A l 2O3, SiO2, SnO2 ,W O 3, HfO2, and mixed oxides SiAlOy ,A l 2TiOy, ZrTiOy, SiTiOy, ZrW2Oy. Amphiphilic poly(alkylene oxide) block copolymers were used as structure-directing agents in nonaqueous solutions for organizing the network-forming metal oxide species. Inorganic salts, rather than alkoxides or organic metal complexes, were used as soluble and hydrolyzable precursors to the polymerized metal oxide framework. These thermally stable mesoporous oxides have robust inorganic frameworks and thick channel walls, within which high densities of nanocrystallites can be nucleated. These novel mesoporous metal oxides are believed to be formed through a mechanism that combines block copolymer self-assembly with alkylene oxide complexation of the inorganic metal species.

Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes

Abstract: The synthesis of bulk amounts of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chemical compositions and textural properties of the catalyst material used in the chemical vapor deposition (CVD) of methane A series of catalysts are derived by systematically varying the catalytic metal compounds and support materials The optimized catalysts consist of Fe/Mo bimetallic species supported on a novel silica−alumina multicomponent material The high SWNT yielding catalyst exhibits high surface-area and large mesopore volume at elevated temperatures Gram quantities of SWNT materials have been synthesized in ∼05 h using the optimized catalyst material The nanotube material consists of individual and bundled SWNTs that are free of defects and amorphous carbon coating This work represents a step forward toward obtaining kilogram scale perfect SWNT materials via simple CVD routes

Comparative Studies of Grafting and Direct Syntheses of Inorganic−Organic Hybrid Mesoporous Materials

Abstract: Vinyl-functionalized MCM-41 samples were prepared by either a postsynthesis grafting (PSG) process or a direct co-condensation synthesis. The structures, stabilities, and reactivities of products from both methods were compared. The mesoscopic order of the hexagonal pore structure of vinyl-grafted MCM-41 (v-gr-MCM-41) resembled that of the MCM-41 host. On the basis of powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and bromination kinetics data, the vinyl groups appeared to be nonuniformly distributed in v-gr-MCM-41 prepared by the present PSG process, with a large proportion of vinyl groups on the external surface of the crystallites or inside channels but near the channel openings. The mesoscopic order of products from the direct synthesis (v-MCM-41) depended on the type of alkoxysilane precursor used and on the ratio of vinylsiloxane to alkoxysilane in the reaction mixture. The vinyl groups appeared to be more uniformly distributed in v-MCM-41. Vinyl-grafted MCM-41 exhibited gr...

Silicatein filaments and subunits from a marine sponge direct the polymerization of silica and silicones in vitro

Abstract: Nanoscale control of the polymerization of silicon and oxygen determines the structures and properties of a wide range of siloxane-based materials, including glasses, ceramics, mesoporous molecular sieves and catalysts, elastomers, resins, insulators, optical coatings, and photoluminescent polymers. In contrast to anthropogenic and geological syntheses of these materials that require extremes of temperature, pressure, or pH, living systems produce a remarkable diversity of nanostructured silicates at ambient temperatures and pressures and at near-neutral pH. We show here that the protein filaments and their constituent subunits comprising the axial cores of silica spicules in a marine sponge chemically and spatially direct the polymerization of silica and silicone polymer networks from the corresponding alkoxide substrates in vitro, under conditions in which such syntheses otherwise require either an acid or base catalyst. Homology of the principal protein to the well known enzyme cathepsin L points to a possible reaction mechanism that is supported by recent site-directed mutagenesis experiments. The catalytic activity of the “silicatein” (silica protein) molecule suggests new routes to the synthesis of silicon-based materials.

Mesocellular siliceous foams with uniformly sized cells and windows

Abstract: Molecular sieves with uniform large pores are desirable for chemical reactions and for use in separations involving large molecules.1 Periodic cubic and hexagonal mesoporous silica phases with uniform large pores have been synthesized by using nonionic triblock and star diblock copolymers as templates.2 Control over the pore size is achieved by adjusting the hydrophobic volumes of the self-assembled aggregates.2,3 In this paper, we describe how adding a sufficiently large amount of an organic cosolvent induces a phase transformation from the highly ordered p6mm mesostructure of SBA-15-type mesoporous silicas to remarkable mesostructured cellular foams (mesocellular foams, MCFs) composed of uniformly sized, large spherical cells that are interconnected by uniform windows to create a continuous 3-D pore system. The interconnected nature of the large uniform pores makes these new mesostructured silicas promising candidates for supports for catalysts and in separations involving large molecules, and they may be of interest in low-dielectric applications. The MCFs have been synthesized in aqueous acid by using dilute Pluronic P123 solutions in the presence of 1,3,5-trimethylbenzene (TMB) as organic cosolvent.4 X-ray diffraction (XRD) experiments5 reveal well-resolved peaks at small angles, as shown in Figure 1 for a sample with a cell diameter of 33 nm. Careful analyses of the scattering data for MCFs show that the higher order peaks cannot be indexed to any plane or space group (e.g., p6mm) or to a lamellar diffraction pattern. In fact, after subtraction of the background,6 the X-ray data are in good agreement with simulated scattering7 due to monodisperse spheres (cells) of diameter D (see Table 1), while attempts to fit the X-ray data to

Ordered Mesoporous Polymers of Tunable Pore Size from Colloidal Silica Templates

Abstract: Ordered mesoporous polymers have been prepared by replication of colloidal crystals made from silica spheres 35 nanometers in diameter. The pores in the colloidal crystals were filled with divinylbenzene (DVB), ethyleneglycol dimethacrylate (EDMA), or a mixture of the two. Polymerization and subsequent dissolution of the silica template leaves a polycrystalline network of interconnected pores. When mixtures of DVB and EDMA are used, the pore size of the polymer replicas can be varied continuously between 35 and 15 nanometers because the polymer shrinks when the silica template is removed.

Evaluating Pore Sizes in Mesoporous Materials: A Simplified Standard Adsorption Method and a Simplified Broekhoff−de Boer Method

Abstract: The diameters of primary mesopores in materials with well-defined cylindrical pores can be accurately determined by the 4V/A method when the volume and surface area are determined by a standard adsorption method and 13.5 A2 is used as the molecular area of adsorbed nitrogen. A simple method for determining standard adsorption using the statistical thickness of the adsorbed gas layer defined by Frenkel−Halsey−Hill (FHH) theory is described. For materials in which the pores are not cylindrical or well-defined, a simplified Broekhoff−de Boer method can be used to determine pore dimensions. The use of Hill's approximation for the thickness of the adsorbed gas layer in the Broekhoff−de Boer method simplifies its use. The results of these methods on small pore MCM-41 materials, large pore SBA-15 materials, and spherical pore mesocellular foams are reported.

Alumination and Ion Exchange of Mesoporous SBA-15 Molecular Sieves

Abstract: Mesoporous silica SBA-15 molecular sieve has been synthesized and incorporated with aluminum via three different postsynthesis procedures by reacting SBA-15 with AlCl3 in dry ethanol (route 1), wit...

Preparation of Macroporous Metal Films from Colloidal Crystals

Abstract: Template-directed syntheses have been broadly applied to the creation of both mesoand macroporous ceramics1-4 and polymers5 The extension of such methods to metals is of particular interest Monolithic metals containing large internal surface areas could be used in numerous applications, ranging from electrochemical sensors6 to catalytic converters7 Moreover, recent work on lithographically prepared metallodielectric structures suggests metals with well-ordered porous networks would exhibit interesting photonic properties8 The adaptation of template methods to the formation of porous metals has been relatively limited9-12 Lyotropic liquid crystalline12 and anodic alumina10 have been used as templates to produce mesoporous (voids 50 nm) and high void volume porous films have not yet been reported In this work we describe a strategy for extending the general methodology of template-directed synthesis to the formation of porous metals These samples are three-dimensional highly ordered, free-standing macroporous metal films with interconnected spherical voids (200-400 nm) We use as templates single-crystal colloidal multilayers made by a vertical deposition technique13 The uniform deposition of metals into colloidal arrays is challenging for several reasons Existing examples of metal deposition into lyotropic liquid crystal12 and anodic alumina10 templates exploit flat and conductive surfaces to catalyze metal formation Such surfaces are not available in the interstitial regions of colloidal crystals which are narrow and relatively inaccessible Also, these templates as made are relatively fragile and easily disrupted by agitation or gas evolution during deposition To overcome these obstacles, a metal nanocrystal catalyzed electroless deposition technique14,15 has been developed First, colloidal silica is coated with a coupling agent,16 3-mercaptopropyltrimethoxysilane (3-MPTMS), which leaves a

Silica Sol as a Nanoglue: Flexible Synthesis of Composite Aerogels

Abstract: Low-density nanoscale mesoporous composites may be readily synthesized by adding a colloidal or dispersed solid to an about-to-gel silica sol. The silica sol can "glue" a range of chemically and physically diverse particles into the three-dimensional silica network formed upon gelation. If the composite gel is supercritically dried so as to maintain the high porosity of the wet gel, a composite aerogel is formed in which the nanoscopic surface and bulk properties of each component are retained in the solid composite. The volume fraction of the second solid can be varied above or below a percolation threshold to tune the transport properties of the composite aerogel and thereby design nanoscale materials for chemical, electronic, and optical applications.

Freezing and melting of water in a single cylindrical pore: The pore-size dependence of freezing and melting behavior

Abstract: In order to clarify the origin of the hysteresis between freezing and melting of pore water, we performed x-ray diffraction measurements of water confined inside the cylindrical pores of seven kinds of siliceous MCM-41 (a member of ordered mesoporous materials denoted by Mobil Oil researchers) with different pore radii (1.2–2.9 nm) and the interconnected pores of Vycor glass as a function of temperature. The hysteresis effect depends markedly on the size of the cylindrical pores: the hysteresis is negligibly small in smaller pores and becomes remarkable in larger pores. This strongly suggests that the hysteresis is arisen from size-dependent supercooling of water confined to the mesopores. For the water confined to the mesopores with pore radius of 1.2 nm, a continuous transition between a liquid and a solid precedes the first-order freezing transition of the pore water which would occur by the same mechanism as in bulk water.

Imprint coating : a novel synthesis of selective functionalized ordered mesoporous sorbents

Abstract: Molecular imprinting of surfaces of mesoporous sorbents is a novel method for introducing template-selective recognition sites. This method makes use of the unique surface environment of hexagonally packed mesopore surfaces of selected pore sizes (see the schematic representation) and coats such surfaces with functional ligands by binding to a metal ion template.

Preparation and characterization of Fischer-Tropsch active Co/SiO2 catalysts

Abstract: Silica supported cobalt catalysts have been prepared by a new method combining the precipitation of the metal precursor (cobalt nitrate) by oxalic acid with the hydrolysis and condensation of the silicium precursor (tetraethoxysilane). Depending on the pH during preparation, the textural properties (BET specific surface area, porosity) of the Co/SiO 2 catalysts can be modified. In an acid medium (pathway A), the resulting silica is constituted by a polymeric net with few branchings, the catalysts are microporous. In basic medium (pathway B) silica is composed of more branched polymers leading to mesoporous catalytic systems. After calcination, the only crystallized phase detected by XRD is the Co 3 O 4 spinel. At 773 K, the surface degrees of reduction of the catalysts at cobalt isocontent (25 wt.% Co) as determined by XPS is of 81% and 69% for systems prepared by pathways A and B, respectively. The presence of small unreduced Co II suggests the existence of non-crystallized cobalt silicate formed during the reduction by reaction of CoO with silica. The activity for the CO + H 2 reaction for the 25 wt.% catalysts prepared by pathway A increased with the specific surface area which can be controlled by the preparation parameters. The reduction degree has a direct influence on the selectivity for the Co/SiO 2 catalysts. The presence of a part of unreduced cobalt (in interaction with the support) results in a better selectivity to the C 5 –C 13 fraction (gasoline), whereas a higher reduction degree of cobalt favors the production of higher molecular weight hydrocarbons (waxes) (C 22 + selectivity > 40%).

Surfactant‐Assisted Synthesis of Mesoporous Alumina Showing Continuously Adjustable Pore Sizes

Abstract: Porous materials displaying tailor-made pore sizes and shapes are particularly interesting in a great variety of real and potential applications where molecular recognition is needed, such as shape-selective catalysis, molecular sieving, and selective adsorption. Classically, apart from silica, materials most commonly used for catalysis and catalyst supports have been those based on high surface aluminas, owing to their thermal, chemical, and mechanical stability and their low cost. Earlier aluminas with high surface areas (~500 m/g) had been prepared using structure-directing agents. However, they were X-ray amorphous materials and their porosity was purely textural, characterized by wide pore size distributions. More recently, the discovery by researchers at Mobil of the M41S family of mesoporous silicas synthesized by using micellar aggregates as templates, has promoted considerable development in the synthesis of materials with uniform pores in the mesoporous range. However, in the case of mesoporous aluminum oxide, the usual strategies used in the synthesis of mesoporous silica have not always yielded satisfactory results and only a few papers have reported on surfactant-assisted synthesis of mesoporous alumina. Davis and co-workers have reported the preparation of aluminas with narrow pore size distributions by the use of anionic surfactants but their solids always have an approximately constant pore size (ca. 20 Š) that cannot be tailored by changing the surfactant length. Conversely, Pinnavaia and co-workers report the use of neutral polyethylene oxides as directing agents for the synthesis of mesoporous solids for which both the d spacing and the pore diameters increase as the surfactant size does. In both cases, the synthetic pathway is based on typical procedures originally used for mesoporous silicas: the variation of the micelle diameter is achieved by increasing the surfactant chain length and/or addition of hydrophobic organic molecules. However, the scarcity and diversity of the reported results suggest that there is still a long way to go to obtain real control of the synthetic procedures for the preparation of mesoporous aluminas. In this context, we show that self-assembling processes leading to the formation of mesoporous aluminas can be controlled by adequately balancing such processes and the hydrolysis and condensation reactions occurring at the inorganic phase. This method has allowed us to isolate for the first time mesoporous aluminum oxides using cationic surfactants and, what is more important, to tune their pore size by the sole adjustment of the molar ratio of the reactants. Thermally stable aluminas with different pore diameters, henceforth denoted as ICMUV-1, were synthesized using CTABr (cetyltrimethylammonium bromide) as surfactantdirecting agent in a water/TEA (triethanolamine) medium. A constant 2/1 Al/CTABr molar ratio was always used, and the pore size adjustment was achieved by changing the Al (or surfactant)/water/TEA molar ratio. A typical synthetic procedure is as follows: 1) A solution containing 0.4 g (10 mmol) of NaOH in 2 mL of water was added to 40 mL of TEA and heated at 120 C for 5 min to evaporate the water. Over this solution, 10.9 mL (40 mmol) of Al sec-butoxide was slowly added with stirring. The resulting clear solution was then heated at 150 C for 10 min (solution I). 2) 7.28 g (20 mmol) of CTABr was dissolved in 40 mL of water at 60 C (solution II). 3) Solution I was slowly added, with vigorous stirring, to solution II at 60 C, and the mixture was allowed to age for 72 h to give a white suspension. The aged solid was then filtered, washed with ethanol, and dried at 30 C. Finally, the assynthesized mesostructured composite was calcined for 5 h at 500 C under air atmosphere to favor surfactant decomposition, and a mesoporous solid with a pore size of 33 Š was obtained. Solids with different pore sizes have been obtained by adjusting the Al (or surfactant)/water/TEA molar ratio in the range from 2/111/15 to 2/195/4 (see Table 1). In all cases, the samples have high thermal stability, as measured by the permanence of their X-ray diffraction peak after heating at 900 C.

Design and synthesis of selective mesoporous anion traps

Abstract: Arsenic contamination of groundwater has recently commanded widespread public attention. Under many conditions, arsenic, and certain other environmentally relevant toxic metals such as chromium, exist in nature as oxyanions. Selective binding of anions is one of the most challenging problems in chemistry, biology, and materials and environmental science. In this paper we report the synthesis and use of metal-chelated ligands immobilized on mesoporous silica as novel anion binding materials. Nearly complete removal of arsenate and chromate has been achieved in the presence of competing anions for solutions containing more than 100 mg/L (ppm) toxic metal anions under a variety of conditions. Anion loading of more than 120 mg (anion)/g of adsorption materials is observed. A binding mechanism is also proposed on the basis of computer modeling. First, Cu(II) ions are bonded to ethylenediamine (EDA) ligands to form octahedral complexes on the surface of the mesoporous silica. This gives rise to positively charg...