M41S mesoporous silica

As one member of the M41S mesoporous silica materials, MCM-48 is the target for many research projects. MCM-41 can be made easily under different synthesis conditions. The synthesis of MCM-48 is believed to be difficult. Low-quality MCM-48 samples were obtained in most cases, even for repeated application of well known recipes. 

Since this initial work there has been a plethora of literature on mesoporous molecular sieves. In addition to the silica and aluminosilicate frameworks similar mesoporous structures of metal oxides now include the oxides of Fe, Ti, V, Sb, Zr, Mn, W and others. Templates have been expanded to include nonionic, neutral surfactants and block copolymers. Pore sizes have broadened to the macroscopic size, in excess of 40 nm in diameter. A recent detailed review of the mesoporous molecular sieves is given in ref [73]. Vartuli and Degnan have reported a Mobil M41S mesoporous-based catalyst in commercial use, but to date the application has not been publicly identified.[74]. 

More recently, attention has shifted to the preparation of hybrid organic-inorganic ordered mesoporous silicas, e.g. of the M41S type (see earlier), bearing pendant alkylsulfonic acid moieties [18, 91-96]. These materials combine a high... 

Pinnavaia etal. used neutral alkylamines as templates to form disordered mesoporous silica, named hexagonal molecular sieves (HMS). The S°I° formation mechanism was proposed between neutral amine micelles (S°) and neutral inorganic precursors (1°). The interactions between S° and 1° were assumed to be hydrogen bonding. The resulting HMS has a worm-like pore structure, with thicker framework walls and smaller X-ray scattering domain sizes compared to M41S. 

For larger pores with a narrow diameter distribution, mesoporous silica of the M41S family can be used. Since their discovery by scientists... 

As for the acidity of Al containing MCM-41, ammonia TPD data indicate that it is comparable to that of amorphous silica-alumina, and much lower than the acidity of zeolites such as USY or H-mordenite [115,120,134]. This is consistent with Raman, FTlK and Si NMR data which indicate that despite their long range order, M41S mesoporous silicates and aluminosilicates exhibit essentially amorphous walls [48,49,115]. 

The pore sizes of M41S family mesoporous materials are about 3 1 nm. 2 31 The pore sizes of mesoporous silica are extended to 30 nm by SBA-15 and FDU-12, etc. material. The classical method to adjust pore size is to vary the chain length of the surfactant or to add swelling agent. Now, there are many strategies available for changing pore size.[163] The basic principle is the same change the size and volume of micelles. Most methods are listed as follows ... 

The three M41S family silica phases obtained with cationic surfactants in a basic medium represent the primary set of principal mesoporous structures. The observed phase transitions are attributed to changing surface curvature resulting from decreasing packing factor, g, which is related to the surfactant molecule dimensions and defined as [35] ... 

Basic quantitative relationships and trends governing the formation of the M41S mesoporous molecular sieves MCM-41 and MCM-48 are presented. The syntheses with hydroxide/surfactant molar ratio close to unity afforded high quality MCM-41 and proved particularly suitable for quantifying the effects of synthesis mixture composition and synthesis conditions upon the nature and properties of the mesoporous products. Increasing temperature and duration of crystallization resulted in unit cell expansion and a more robust silica framework. Syntheses with silica to surfactant molar ratio equal to 5 1 and higher gave MCM-41 while MCM-48 was observed at lower (3.5 1) ratios. 

Synthesis of M41S mesoporous materials was attempted using MFI type zeolite as sources of silica and aluminum, that is, dissolution of MFI zeolite in an alkaline solution and successive precipitation of dissolved aluminosilicate species with a surfactant, cethyltrimethylammonium bromide (CTAB). Pure phase of M41S was obtained when the filtrate of alkali-treated slurry was mixed with CTAB and crystallized at 293K. The M41S materials obtained in this method showed a catalytic activity originated from Bronsted acid site of the parent MFI zeolite. This method enables us to obtain a new type of mesoporous materials, which have both characteristics of zeolitic and mesoporous materials a strong Bronsted acidity and mesopores with a uniform size. 

Ordered Mesoporous Molecular Sieves (M41S Materials), 283 Synthesis Pathways for Ordered Mesoporous Materials, 284 Ordered Mesoporous Silica, 288 Fimetionalization of Mesoporous Materials, 296 Nonsdiceous Mesostructured and Mesoporous Materials, 302 Morphology, 309... 

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