Small angle neutron scattering study of the structure and formation of MCM-41 mesoporous molecular sieves
TL;DR: The nanoscale structure and synthesis mechanisms of the MCM-41 class of inorganic mesoporous materials have been investigated by small angle neutron scattering (SANS) measurements with solvents imbibed in the pores to vary the scattering contrast as discussed by the authors.
Abstract: The nanoscale structure and synthesis mechanisms of the MCM-41 class of inorganic mesoporous materials have been investigated by small angle neutron scattering (SANS). SANS measurements with solvents imbibed in 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. Static and shear flow 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.
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TL;DR: In this paper, the mesoscopic order of the hexagonal pore structure of a vinyl-grafted MCM-41 (v-gr-MCM41) was compared with a direct co-condensation synthesis.
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...
785 citations
TL;DR: In this paper, the mesostructured cellular foams (MCFs) with well-defined ultralarge mesopores and hydrothermally robust frameworks are described, and the pore size can be controlled by adjusting the amount of the organic swelling agent that is added and by varying the aging temperature.
Abstract: Siliceous mesostructured cellular foams (MCFs) with well-defined ultralarge mesopores and hydrothermally robust frameworks are described. The MCFs are templated by oil-in-water microemulsions and are characterized by small-angle X-ray scattering, nitrogen sorption, transmission electron microscopy, scanning electron microscopy, thermogravimetry, and differential thermal analysis. The MCFs consist of uniform spherical cells measuring 24−42 nm in diameter, possess BET surface areas up to 1000 m2/g and porosities of 80−84%, and give, because of their pores with small size distributions, higher-order scattering peaks even in the absence of long-range order. Windows with diameters of 9−22 nm and narrow size distribution interconnect the cells. The pore size can be controlled by adjusting the amount of the organic swelling agent that is added and by varying the aging temperature. Adding ammonium fluoride selectively enlarges the windows by 50−80%. In addition, the windows can be enlarged by postsynthesis treatm...
562 citations
TL;DR: In this article, an iron(III) oxide nanoparticles were synthesized within mesoporous MCM-48 silica phases by using multiple cycles of wet impregnation, drying, and calcination procedures.
Abstract: Iron(III) oxide nanoparticles were synthesized within mesoporous MCM-48 silica phases. By using multiple cycles of wet impregnation, drying, and calcination procedures, it was possible to form haem...
221 citations
TL;DR: Titanium dioxide has been grafted onto the pore surface of MCM-41 and FSM-16 by reacting TiCl4 in hexanes with the as-synthesized mesostructured silicate as mentioned in this paper.
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...
219 citations
TL;DR: In this article, a periodic wall model has been developed to calculate the structure factor of hexagonal MCM-41 molecular sieves, which is a discrete lattice version of the continuum periodic cylindrical shell model of Oster and Riley.
140 citations
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TL;DR: In this paper, the synthesis of mesoporous inorganic solids from calcination of aluminosilicate gels in the presence of surfactants is described, in which the silicate material forms inorganic walls between ordered surfactant micelles.
Abstract: MICROPOROUS and mesoporous inorganic solids (with pore diameters of ≤20 A and ∼20–500 A respectively)1 have found great utility as catalysts and sorption media because of their large internal surface area. Typical microporous materials are the crystalline framework solids, such as zeolites2, but the largest pore dimensions found so far are ∼10–12 A for some metallophosphates3–5 and ∼14 A for the mineral cacoxenite6. Examples of mesoporous solids include silicas7 and modified layered materials8–11, but these are invariably amorphous or paracrystalline, with pores that are irregularly spaced and broadly distributed in size8,12. Pore size can be controlled by intercalation of layered silicates with a surfactant species9,13, but the final product retains, in part, the layered nature of the precursor material. Here we report the synthesis of mesoporous solids from the calcination of aluminosilicate gels in the presence of surfactants. The material14,15 possesses regular arrays of uniform channels, the dimensions of which can be tailored (in the range 16 A to 100 A or more) through the choice of surfactant, auxiliary chemicals and reaction conditions. We propose that the formation of these materials takes place by means of a liquid-crystal 'templating' mechanism, in which the silicate material forms inorganic walls between ordered surfactant micelles.
15,125 citations
TL;DR: Model Q230 proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material and suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.
Abstract: A model is presented to explain the formation and morphologies of surfactant-silicate mesostructures. Three processes are identified: multidentate binding of silicate oligomers to the cationic surfactant, preferential silicate polymerization in the interface region, and charge density matching between the surfactant and the silicate. The model explains present experimental data, including the transformation between lamellar and hexagonal mesophases, and provides a guide for predicting conditions that favor the formation of lamellar, hexagonal, or cubic mesostructures. Model Q(230) proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material. This model suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.
1,431 citations
TL;DR: In this article, the organization of cationic or anionic organic and inorganic molecular species to produce three-dimensional periodic biphase arrays is described, which uses cooperative nucleation of molecular inorganic solution species with surfactant molecules and their assembly a t low temperatures into liquid-crystal-like arrays.
Abstract: The organization of cationic or anionic organic and inorganic molecular species to produce three-dimensional periodic biphase arrays is described. The approach uses cooperative nucleation of molecular inorganic solution species with surfactant molecules and their assembly a t low temperatures into liquid-crystal-like arrays. The organic/inorganic interface chemistry makes use of four synthesis routes with @+I-), @-I+), (S+X-I+), and (S-M+I-) direct and mediated combinations of surfactant (cationic S+, anionic S-) and soluble inorganic (cationic I+, anionic I-) molecular species. The concepts can be widely applied to generate inorganic oxide, phosphate or sulfide framework compositions. Distinct lamellar, cubic silica mesophases were synthesized in a concentrated acidic medium (S+X-I+), with the hexagonal and the cubic phases showing good thermal stability. For the hexagonal mesostructured silica materials high BET surface areas (>lo00 m2/g) are found. Hexagonal tungsten(V1) oxide materials were prepared in the presence of quaternary ammonium surfactants in the pH range 4-8. Cubic (Iu3d) and hexagonal antimony(V) oxides were obtained by acidifying (pH = 6-7) homogeneous solutions of soluble Sb(V) anions and quaternary ammonium surfactants a t room temperature @+I-). Using anionic surfactants, hexagonal and lamellar lead oxide mesostructures were found (S-I+). Crystalline zinc phosphate lamellar phases were obtained a t low synthesis temperatures (4 \"C) and lamellar sulfide phases could be also readily generated a t room temperature. The synthesis procedure presented is relevant to the coorganization of organic and inorganic phases in biomineralization processes, and some of the biomimetic implications are discussed.
1,379 citations
TL;DR: In this paper, the ability to plan zeolite and molecular sieve syntheses is discussed and a strategy for synthesizing chiral zeolites is used to demonstrate the current limitations in "designing" new molecular sieves.
Abstract: Zeolite and molecular sieve syntheses are reviewed. The synthesis of aluminum-rich zeolites, high-silica
zeolites, and phosphate-based molecular sieves are evaluated. Unresolved mechanistic issues are outlined
and areas for exploration suggested. The ability to plan zeolite and molecular sieve syntheses is discussed
and a strategy for synthesizing a chiral zeolite is used to demonstrate the current limitations in "designing"
new molecular sieves.
1,285 citations
TL;DR: The basic constructional processes of biomineralization (supramolecular pre-organization, interfacial molecular recognition (templating) and cellular processing) can provide useful archetypes for molecular-scale building, or "molecular tectonics" in inorganic materials chemistry.
Abstract: The systematic fabrication of advanced materials will require the construction of architectures over scales ranging from the molecular to the macroscopic. The basic constructional processes of biomineralization—supramolecular pre-organization, interfacial molecular recognition (templating) and cellular processing—can provide useful archetypes for molecular-scale building, or ‘molecular tectonics’, in inorganic materials chemistry.
1,083 citations