Mesoporous Arrays Research Articles

Cracking behavior of asphaltene in the presence of iron catalysts supported on mesoporous molecular sieve with different pore diameters

Cracking of a mixture of petroleum asphaltene and 10 wt% Fe catalyst supported on mesoporous molecular sieve (SBA-15) possessing a hexagonal array of uniform mesopores has been studied with a fixed bed reactor at 573 K under atmospheric He. When average pore diameter of Fe/SBA-15 is varied between 4.5 and 15 nm, asphaltene conversion increases almost linearly with increasing pore diameter up to 12 nm and reaches 65%, though the increment is small beyond this value. On the other hand, yield of maltene formed is almost independent of the diameter and less than 15% but greatly improved by using pressurized H 2 in place of He. Although pore volumes of all Fe/SBA-15 catalysts decrease by mixing with feed asphaltene, the extent of the decrease is larger for the catalyst with a larger pore diameter, which shows that higher asphaltene conversion may arise from the presence of larger amounts of asphaltene molecules held inside the larger mesopores. The N 2 adsorption measurements reveal that pore structures of Fe/SBA-15 catalysts are almost unchanged after cracking and subsequent re-calcination to remove deposited coke. The X-ray diffraction analysis and temperature programmed oxidation after cracking suggest that Fe species are highly dispersed inside the mesopores and present as the sulfided phases at the outermost layer.

Nonionic Block Copolymer Synthesis of Large-Pore Cubic Mesoporous Single Crystals by Use of Inorganic Salts

Large-pore cubic (Im3m) mesoporous silica single crystals have been synthesized by using commercial nonionic block copolymers as templates and inorganic salts as additives. These single crystals possess exclusively uniform rhombdodecahedron shapes ( approximately 1 mum) with approximately 100% yield. The mesopore lattice array in each crystal face is solved by TEM, further confirming that these crystals are perfect single crystals.

An ordered mesoporous organosilica hybrid material with a crystal-like wall structure.

Surfactant-mediated synthesis strategies are widely used to fabricate ordered mesoporous solids in the form of metal oxides, metals, carbon and hybrid organosilicas. These materials have amorphous pore walls, which could limit their practical utility. In the case of mesoporous metal oxides, efforts to crystallize the framework structure by thermal and hydrothermal treatments have resulted in crystallization of only a fraction of the pore walls. Here we report the surfactant-mediated synthesis of an ordered benzene-silica hybrid material; this material has an hexagonal array of mesopores with a lattice constant of 52.5 A, and crystal-like pore walls that exhibit structural periodicity with a spacing of 7.6 A along the channel direction. The periodic pore surface structure results from alternating hydrophilic and hydrophobic layers, composed of silica and benzene, respectively. We believe that this material is formed as a result of structure-directing interactions between the benzene-silica precursor molecules, and between the precursor molecules and the surfactants. We expect that other organosilicas and organo-metal oxides can be produced in a similar fashion, to yield a range of hierarchically ordered mesoporous solids with molecular-scale pore surface periodicity.

Synthesis and Characterization of Mesostructured Titanium(IV) Fluorophosphates with a Semicrystalline Inorganic Framework

Hexagonal and lamellar mesostructured titanium(IV) fluorophosphates [CH3(CH2)nNR3]xTi2OF2H2-x(PO4)2·2H2O (x = 1, 2; n = 9−17; R = CH3, H) have been synthesized using aqueous solutions of titanium fluorophosphates and surfactants such as alkyltrimethylammoniumbromide or alkylamine. X-ray powder diffraction and HREM show that the hexagonal phase consists of arrays of hexagonal mesopores filled with surfactant. An extensive study of the synthesis conditions of the hexagonal CTAB phase was conducted with parameters such as the nature of the surfactant, concentrations of precursors, molar ratios, presence or absence of stirring, aging times, and temperature. Well-crystallized compounds appear only at low concentrations of surfactant and titanium, and the formation of the hexagonal phase occurs through the competitive complexation of titanium by fluorine and phosphates. It appears that the mesostructures form almost immediately at room temperature, whereas crystallization of the inorganic walls occurs only during hydrothermal aging. HREM, XRD, and solid-state NMR experiments confirm the existence of a semicrystalline order inside the inorganic framework. Removal of the surfactant by calcination of the hexagonal phase under nitrogen atmosphere leads to a mesoporous solid with semicrystalline pore walls exhibiting a surface area of over 400 m2/g. A mechanism of formation is proposed on the basis of the TEM results, followed by a brief discussion concerning the mesostructured metallophosphates synthesized to date.

Synthesis, characterization and catalytic activity of actinide Th-MCM-41 and U-MCM-41 hexagonal packed mesoporous molecular sieves

The synthesis and characterization of Th-MCM-41 and U-MCM-41 new members of a growing family of hexagonal packed metallo-silicates molecular sieves are described. The synthesis of these materials was achieved by a “liquid crystal templating” mechanism (LCT), with a C 16 length chain for the cationic amine surfactant, when the additional metal salt was added to the synthesis mixture, forming large agglomerates imbedded within the silica matrix. For uranium, the synthesis was performed using UO 2(NO 3) 2·6H 2O as a precursor whereas for the thorium, the reaction was carried out using Th(NO 3) 4·5H 2O (thorium nitrate pentahydrate) as the metal source. The incorporation of the uranium into the mesoporous material was performed via a complementary basic synthesis, whereas for the latter thorium, an acidic route was utilized. The Th-MCM-41 and U-MCM-41 were characterized by small angle X-ray scattering (SAXS), specific surface areas (Brunauer–Emmett–Teller (BET)), scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer and X-ray photon spectroscopy (XPS). SAXS study revealed the mass fractal scattering characteristics of these materials with fractal dimension in the range of 2.78–2.95 Å. These materials also exhibited a hexagonal array of cylindrical mesopores from 27 to 30 Å in size, surface areas of over 850 m 2/g, and high thermal and hydrothermal stability. The incorporation of actinide metals into the MCM-41 matrixes was 3.23 (2) at.% of Th and 11.22 at.% for U, respectively. SEM study revealed the particle morphologies with large monolithic aggregates whereas XPS indicates the geometric configuration and coordination of the metal in the silicate. We have found that in U-MCM-41, most of the uranium metal in encounter in an octahedral geometry arrangement whereas in the Th-MCM-41, the corresponding thorium metal is assemble in tetrahedral environment In addition, the catalytic reactivity of these materials was measured for the hydroxylation of phenol and the alkylation of 2,4-di-tert-butylphenol with cinnamyl alcohol.

Existence of a Microporous Corona around the Mesopores of Silica-Based SBA-15 Materials Templated by Triblock Copolymers

Materials showing long-range two-dimensional hexagonal order (called SBA-15 in the literature) were produced by templating a silica precursor (TEOS) with two Pluronic copolymers, EOxPOyEOx, of nearly the same x/y (0.3) ratio but different y values (x = 18, y = 60 and x = 20, y = 70, respectively). These materials were hydrothermally treated to increase the condensation of silicate species around the Pluronic aggregates and calcined to liberate the hexagonal array of mesopores. All materials, i.e., before and after hydrothermal treatment and calcination, were investigated by X-ray diffraction (XRD), and all calcined samples were further characterized by transmission electron microscopy and N2 sorption experiments. The large number and narrow width of the XRD powder diffraction lines demonstrate the good crystallographic quality of the materials. This allows us to quantitatively exploit the XRD reflection intensities and to show that simple structural models of the silica lattice cannot account for them. Th...

Synthesis of Highly Ordered Carbon Molecular Sieves via Template-Mediated Structural Transformation

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...

Interface Charge Density Matching as Driving Force for New Mesostructured Oxovanadium Phosphates with Hexagonal Structure, [CTA]xVOPO4·zH2O

Hexagonal mesostructured mixed-valence oxovanadium phosphates [CTA]xVOPO4·zH2O, in short ICMUV-2, have been synthesized through a S+I- cooperative mechanism using cationic surfactant (cetyltrimethylammonium, CTAB) rodlike micelles as a template. On the lines of the hypothesis that the driving force leading to the formation of mesostructured solids is the charge density matching at the interface between the supramolecular−organic and supramolecular−inorganic moieties, the self-assembling process between CTA+ micelles and VOPO4q- planar anions can be thought of as consequence of the adequate adjustment of the metal mean oxidation state. X-ray powder diffraction and TEM techniques show that the solids consist of hexagonal arrays of mesopores filled with surfactant, whereas spectroscopic results allow us to propose an oxovanadium phosphate bond topology similar to that observed in the NaxVOPO4·nH2O layered derivatives. The thermal behavior of the mesostructured materials has also been investigated, given that...

Characterization and catalytic activity of the AlMCM-41 prepared by a method of gel equilibrium adjustment

Aluminum-containing MCM-41 (AlMCM-41) was prepared by a method of gel equilibrium adjustment, in which the pH of the gel was adjusted to 11 or 10 using acetic acid during the synthesis in order to shift the equilibrium into the formation of mesoporous materials, and its structure, solid acidity, and catalytic activity were measured. The material thus obtained had a clear hexagonal array of mesopores with a Si/Al 2 molar ratio of 17 to 16; the maximum concentration of aluminum. More than 70% of aluminum atoms had a tetrahedral configuration. Test reaction for the acidity, TPD of ammonia and IR of adsorbed pyridine showed that it had an acidity almost similar to usual silica-alumina. The AlMCM-41 showed a high and durable activity in the isopropylation of 2-isopropylnaphthalene, which was not observed in the HY zeolite and the usual silica-alumina, but no shape-selectivity for the product formation was observed.

96/04966 Synthesis of mesoporous molecular sieves and their application for catalytic conversion of polycyclic aromatic hydrocarbons

Molecular sieves such as Y and ZSM-5 are widely used catalysts in acid-catalyzed reactions for the production of fuels, petrochemicals, and fine chemicals. Despite their enormous use as environmentally safe catalysts, they are limited to convert relatively small molecules as their pore size is restricted to micropore size range (usually 1.4 nm). However, with the growing demand of technologies for treating heavier feeds, as well as for synthesizing large molecules for producing commodities and fine chemicals, it is necessary to develop catalysts with wider pores. Recently, Mobil workers have reported a new series of mesoporous molecular sieves: MCM-41 is one of the members of this extensive family of mesoporous series possessing a hexagonal array of uniform mesopores. Many reports have since appeared on synthesis and characterization of these new materials. However, information on their catalytic activity is still very limited. The pore dimensions of these materials can be tailored (in the range of 1.5-10.0 nm or more) through the choice of surfactant and auxiliary chemicals as templates and the crystallization conditions in the synthesis procedure. The BET surface area of these materials is more than 1000 m{sup 2}/g with high sorption capacities of 0.7 cc/g and greater. Moreover, these materialsmore » can be synthesized in a large range of framework Si/Al ratios and therefore can develop acid sites of different strength. Hence, these new mesoporous aluminosilicate molecular sieves, Al-MCM-41, might open new possibilities in developing catalysts for processing large molecules.« less

Nucleation Events during the Synthesis of Mesoporous Materials Using Liquid Crystalline Templating

A novel family of silica-based mesoporous molecular sieveM41Swas synthesized by one of the Mobil research groups. Surfactant molecules were mixed with a silica solution, resulting in a regular array of uniform mesopores, 2−10 nm in diameter, capable of acting as a potential catalyst. The final long-range ordering of the silica−surfactant biphase composite material was found to be, depending on the reaction conditions, hexagonal, cubic, or lamellar. In this paper, we present an insight into the intermediate structures present in the solution during the synthesis of the hexagonal derivativeMCM41before long-range ordering is obtained. Cryo-transmission electron microscopy provides direct information on the primary building unitsclusters of elongated micelleswhich appear at the very early stage of the hexagonal mesophase formation, while small angle X-ray scattering is used for accurate structural analysis.

Small Angle Neutron Scattering Study of the Structure and Formation of Ordered Mesopores in Silica

AbstractThe nanoscale structure and synthesis mechanisms of a recently developed class of inorganic mesoporous materials with ordered arrays of uniform mesopores have been investigated by small angle neutron scattering (SANS). SANS measurements with solvents imbibed into the pores to vary the scattering contrast demonstrate that the low angle diffraction peaks from these materials are entirely due to the pore structure and that the pores are fully accessible to both aqueous and organic solvents. SANS measurements on the concentrated cationic surfactant and silicate precursor solutions typically used in the synthesis of the mesopore materials indicate that the existence of preassembled supramolecular arrays that mimic the final pore structure is not essential for the synthesis of these materials.

Kinetics of liquid phase hydrochlorination of acetylene in presence of cuprous chloride

In this study, we propose gold-intercalated ordered mesoporous carbon nanosphere catalysts in which gold nanoparticles are reduced by high-temperature carbonization of the carbonaceous matrix to catalyze the selective hydrogenation of aromatic nitro compounds. A spherical morphology of particles with size approximately 90 nm and ordered mesoporous arrays was clearly observed on the surface in the high-resolution scanning electron microscopy images. The X-ray photoelectron spectra and transmission electron microscopy images showed that the dispersed metallic gold nanoparticles (2.8 nm in diameter) were intercalated into a carbon framework. The IR spectra for CO chemisorption suggested the involvement of neutral gold atoms in a low-coordination state in clusters or a stepped surface. The gold nanocatalysts intercalated into the nanospherical mesoporous carbons exhibited high activity and selectivity for the hydrogenation of nitroarenes to the corresponding amines, using H2 as a reduction agent. The initial reaction rate reached 12.7 and 6.5 min−1 in the hydrogenation of p-chloronitrobenzene and 4-nitrophenol, respectively. This catalytic performance was retained for more than five catalytic runs with no obvious activity loss or gold leaching, indicative of high stability. The trapping test using a mercapto-functionalized SBA-15 solid revealed undetected soluble gold species in the reaction solution. The high activity and stability of the ordered mesoporous carbon nanosphere-supported gold catalysts was strongly correlated to the particle morphology change by thermal reduction, which can generate new low-coordinated gold atoms for intercalated gold NPs in the matrix.