M41S Family Research Articles

Novel synthesis of a porous stainless steel-supported Knudsen membrane with remarkably high permeability

A stainless steel-supported Knudsen membrane (SKM) with remarkably high permeability was successfully synthesized using only 100 nm-sized colloidal silica sol by means of a dipping–rolling–freezing–fast drying (DRFF) and soaking–rolling–freezing–fast drying (SRFF) method. Hydrogen and nitrogen permeances of the SKM were (6.7–8.2) × 10 −6 and (1.8–2.3) × 10 −6 mol m −2 s −1 Pa −1 with a H 2/N 2 permselectivity of 3.5–3.7, which approaches to a theoretical H 2/N 2 selectivity for the Knudsen diffusion mechanism (3.74). In comparison with a typical mesoporous γ-alumina membrane supported on a porous stainless steel or α-alumina substrate, the SKM had 20 and 5.5 times permeance with almost same H 2/N 2 permselectivity, respectively. Generally, mesoporous materials such as γ-alumina and the M41S family with several nm-sized pores are employed to obtain the Knudsen-dominated permeation characteristics. In this case, a decrease in the gas permeance is inevitable due to deposition of a mesoporous skin layer. However, in the case of the SKM, the Knudsen-dominated permselectivity was extraordinarily obtained through modification of porous stainless steel substrates with the colloidal silica particles having relatively large particle size, because a well-densified layer of the 100 nm-sized colloidal silica could be obtained without formation of defects via the freezing procedure. In addition, the large porosity of the 100 nm-sized colloidal silica layer gave rise to the remarkably high gas permeance.

Synthesis and Mechanism Study of Mesoporous SnO<SUB>2</SUB>/SiO<SUB>2</SUB> Composites

Mesoporous SnO2/SiO2 composite particles (Si/Sn < or = 0.25) sustainable to calcination up to 600 degrees C have been fabricated using a stepwise sol-gel technique on nonionic surfactant template (tetradecylamine, TDA). The newly designed preparation method involved the pre-formation of SnO2 sol solution from SnCl4. Subsequently, SnO2 nanocrystals were covered by the silicate species (from the hydrolysis of tetraethylorthosilicate, TEOS) in a pH controlled colloidal solution. Upon mixing with the surfactant solution, mesophase composite was obtained. After the removal of templates at various temperatures (400 to 600 degrees C), worm-like mesoporous SnO2/SiO2 with large specific surface area and pore volume as high as 362 m2/g and 0.33 cc/g were obtained, respectively. High thermal stability is mainly due to the effective inhibition of SnO2 crystal growth (mean crystallite size <30 A) by the amorphous SiO2 species at the grain boundaries. Formation of mesoporous silicate skeleton such as M41S family material was prevented. The obtained materials maintain the relatively narrow pore size distribution typically in the range of 30 to 70 A. Relations between material properties and key synthesis parameters (i.e. TDA/Si/Sn molar ratio and calcination temperature) were investigated by TGA, wide/small-angle X-ray scattering, (HR)TEM, BET, and FTIR techniques. Mechanisms on the mesostructure formation and crystal growth inhibition were also proposed with detailed discussion.

Preparation of M41S family mesoporous silica thin films on porous oxides

The preparation of thin films of silicate MCM-41 and silicate MCM-48 on porous oxides have been studied using spin-coating, hydrothermal deposition, X-ray diffraction (XRD), scanning electron microscopy (SEM), N 2 adsorption–desorption and thermogravimetry (TG). The work has included the template removal conducted on both supported thin films and powder samples via supercritical fluid extraction (SFE) at 85 °C and calcination at 540 °C. XRD and SEM observations show that films of both mesoporous molecular sieves are synthesized on the substrates with cetyltrimethylammonium bromide (CTMABr)–tetraethyl orthosilicate (TEOS) gels and beneficially formed on the substrates with pore sizes of 0.15 μm upwards. XRD and N 2 adsorption–desorption studies reveal that removing the template via calcination leads to mesoporous structural distortion of supported MCM-41 and large pore contraction of supported MCM-48 thin films, whereas removing the template via SFE results in stable supported thin films with uniform mesopores. TG analysis indicates that the template SFE efficiency attains to 78% upwards. The behavior of supported thin films and powder samples in synthesis and in template removal are compared. The effects of the substrate on the formation and structural stability of thin films are observed.

The effect of the hydrophobicity of aromatic swelling agents on pore size and shape of mesoporous silicas

Benzene and four trisubstituted aromatic compounds were used as swelling agents for mixed hexadecyl- and dodecyltrimethylammonium micelles. These micelles served as templates for the synthesis of mesoporous M41S materials. By varying the amount and the nature of the aromatic swelling agent, M41S materials with any pore diameter between 3 and 11 nm could be synthesized. All materials exhibited specific surface areas around 1000 m 2/g and large specific pore volumes as it is characteristic for the M41S family. Phases with layered, rod-like, and globular building units were obtained by varying the nature and/or the amount of aromatic swelling agent. A detailed analysis of the trends showed that the hydrophobicity of the swelling agents determines the size and shape of the pores: with the more hydrophobic swelling agents smaller pores with higher curvature are obtained.

Structural Transformation of Template-Synthesized Mesoporous Silica with Addition of Chloroform

M41S family mesoporous silica materials were synthesized via the hydrolysis and condensation of tetraethylorthosilicate (TEOS) using cetyltrimethylammonium bromide (CTAB) as a template and sodium hydroxide as a catalyst. MCM-48 and MCM-41 were prepared in the absence and in the presence of chloroform, respectively. The addition of chloroform likely changed both aggregation and swelling behavior of CTAB micelles and consequently hexagonal mesoporous structures were formed.

Improvement of the Hydrothermal Stability of Siliceous MCM-48 by Fluorination

Abstract The siliceous MCM-48 molecular sieve, a member of the M41s family, is prepared for the first time in the presence of fluoride ions, and the hydrothermal stability of fluorinated MCM-48 framework is found to be remarkably improved as compared with those synthesized in other media.

Synthesis, characterization and potential applications of new materials in the mesoporous range

M41S is the designation of a new type of mesoporous structures. The characteristics of these materials are large surface areas and very narrow pore size distributions, with pore diameters tunable from 15 to 100 Å. The M41S family consists of MCM-48, which has a cubic ordered pore structure, MCM-41, which has a hexagonally ordered pore structure and MCM-50, which has an unstable lamellar structure. In this review we cover the synthesis, modification, characterization and potential applications of these materials.

Siliceous mesoporous molecular sieves derived from crown ether surfactants

New surfactants derived from crown ethers were synthesized and used as templates in the preparation of inorganic mesostructures. These surfactants having a crown ether group as hydrophilic head yielded silica mesoporous molecular sieves of the M41S family which were transformed with subsequent thermal treatment into a material of particular unprecedented morphology. The crown ether derived surfactants were characterized by 1H NMR and mass spectroscopy and the mesoporous materials by X-ray diffraction (XRD), nitrogen physisorption, transmission electron microscopy (TEM) and solid state 29Si NMR spectroscopy. The TEM technique was used to assess the different phases present in the calcined samples. The morphology of these materials is quite different from that of MCM-41 and shows lamellar stackings of silica sheets pillared by minor amounts of hexagonal phase. Using molecular simulations of the surfactants and their cation complexed forms, some detailed conclusions could be drawn regarding the effect of various cations on pore size and shape.

Base-functionalized MCM-41 as catalysts for the synthesis of monoglycerides

The mesoporous molecular sieves of the M41S family have little intrinsic catalytic activity. However, it is possible to functionalize the material by grafting organic side chains on the walls. These organic–inorganic hybrid materials maintain the advantages of the inorganic support, notably a high surface area and structural stability at elevated temperature and pressure. Catalysts based on MCM-41 functionalized with either hindered amine bases or free primary amino groups have been evaluated for the facile synthesis of monoglycerides from fatty acids and glycidol. Yields as high as 95% in 8 h have been achieved. The catalysts can be reused several times with little loss of activity.

Novel synthesis of spherical MCM-48

A novel synthesis route was developed for the cubic member of the M41S family, MCM-48, with a three-dimensional pore system allowing the formation of submicrometre- to micrometre-sized beads with a narrow pore-size distribution. The synthesis is based on the modified Stöber method applying tetraethoxysilane, ethanol, water, ammonia and n-hexadecyltrimethylammonium bromide as template. The specific surface area, the specific pore volume and the average pore diameter were varied in the following ranges: 900–1600 m2 g−1, 0.5–0.9 cm3 g−1 and 2–3 nm. Aluminium-, chromium-, gallium-, niobium- and vanadium-MCM-48 were also synthesized following this procedure.

Ordered and disordered pNA molecules in mesoporous MCM-41

Para-nitroaniline (pNA) has been incorporated into the one-dimensional channels of mesoporous MCM-41, the hexagonal member of the M41S family of ordered mesoporous silicate materials. The composite material shows a number of remarkable features, depending on the sorption procedure for the organic component, the after treatment, and the annealing time of the composite. In X-ray diffraction experiments different states of one-dimensional ordering of the sorbate molecules in the channel pores are observed. In agreement with the molecular dipole, chain-like coherent domains with the long axis of the sorbate molecule parallel as well as perpendicular to the host channel axis show up. In aged samples the periodic ordering of the pNA molecules is lost, and nonlinear optical properties of the composite have been measured in second harmonic generation experiments indicating the presence of a polar direction in the sample.

The syntheses of mesoporous molecular sieves in fluoride medium

The MCM-41 molecular sieve, a member of the M41S family, is prepared for the first time in the presence of fluoride ions; the mesophase obtained from surfactant : silicon molar ratios in the range 0.2–2:1 is always hexagonal.

Surface Properties and Catalytic Performance of Novel Mesostructured Oxides

AbstractMesoporous oxides of the M41S family, discovered by scientists of the Mobil Oil Corporation in 1991, are an exciting class of porous materials. Their unique pore system with the extremely sharp pore size distribution, which is adjustable in the range between about 2 and 10 nm at will, and the extremely high surface areas up to 1500 m2/g open very promising prospects with respect to the development of new catalysts. The oxides were initially prepared based on silicon and aluminum, but later other elements could be incorporated. Moreover, totally non siliceous oxides could be synthesized using surfactants, similar to the synthesis of the M41S materials. First examples for the use of these oxides as catalysts have been reported. In addition, also the properties of the materials as supports for noble metals have been investigated recently.

High-resolution electron microscopy and X-ray diffraction studies of MCM-48

A cubic member of the M41S family with a three-dimensional pore system, denoted as MCM-48, was synthesised. By combining X-ray powder diffraction (XRD) with high-resolution electron microscopy (HREM), a cubic symmetry with an Ia 3 d space group was determined for the as-synthesised and the calcined MCM-48 material. Upon calcination the unit cell decreased from around 10.09 nm to about 8.1 nm. The overall pore structure was found to be quite unaffected by local variations in the structure and the decrease in the unit cell size upon calcination.

TEM investigations of self-assembly through physical synthesis

A novel way of enlarging the realm of surfactant solution microstructure is the "physical synthesis" of new microstructures and surfactant-templated materials by mixing simple surfactants or surfactant and inorganic and ionic species. Mixing can often produce properties not possessed by any one species alone. As we have recently shown, mixtures of single-tailed cationic and anionic surfactants associate in solution to form a quasi-double-tailed catanionic surfactant that forms bilayers rather than the simple micelles each species would form on its own. Mixed surfactant systems apparently produce new microstructures by altering the intermolecular and interaggregate forces in ways impossible for single component systems. In addition to spontaneous vesicles, we have found other new microstructures including dilute lamellar and L3 phases depending on the details of the surfactant mixture. Surfactant interactions with ionic species also play an important role in the in synthesis of a new M41S family of mesoporous silicas, and are likely to be important in the templating of biominerals at interfaces. By understanding the molecular and chemical basis of these interactions, we can begin to tailor new microstructured and/or biomimetic materials by controlling surfactant aggregation and phase behavior through physical synthesis rather than through much more elaborate chemical synthesis.