Micelle-templated materials

The time from discovery to process for synthetic zeolites has been about ten years. This lag has already elapsed since the discovery of the ordered mesoporous materials and times seem ripe for their industrial development. The main obstacle towards viable applications is the presence on the market of much cheaper amorphous alternatives, mainly based on silica gels. Micelle-templated materials has to compete for new applications, to obtain results that can be achieved only thanks to their narrow pore size distribution. 

The onset of the pore closure depends on the synthesis eonditions for the formation of micelle-templated materials and on their framework type. The main factors that were identified as important for the successful pore-elosing process include ... 

Most examples discussed so far made use of amorphous inorganic supports or sol-gel processed hybrid polymers. Highly disperse materials have recently become accessible via standard processes and, as a result, materials with various controlled particle size, pore diameter are now available. Micelle-templated synthesis of inorganic materials leads to mesoporous materials such as MCM-41, MCM-48, MSU, and these have been extensively used as solid supports for catalysis [52]. Modifications of the polarity of the material can increase the reactivity of the embedded centre, or can decrease its susceptibility to deactivation. In rare cases, enhanced stereo- or even... 

Recently, mesoporous aluminosilicates with strong acidity and high hydrothermal stability have been synthesized via self-assembly of aluminosilicate nanoclusters with templating micelles. The materials were found to contain both micro- and mesopores, and the pore walls consist of primary and secondary building units, which might be responsible for the acidity and stability (181). These materials were tested in isobutane/n-butene alkylation at 298 K, showing a similar time-on-stream behavior to that of zeolite BEA. No details of the product distribution were given. 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

Fig. 9 Schematic representation of three approaches to generate nanoporous and meso-porous materials with block copolymers, a Block copolymer micelle templating for mesoporous inorganic materials. Block copolymer micelles form a hexagonal array. Silicate species then occupy the spaces between the cylinders. The final removal of micelle template leaves hollow cylinders, b Block copolymer matrix for nanoporous materials. Block copolymers form hexagonal cylinder phase in bulk or thin film state. Subsequent crosslinking fixes the matrix hollow channels are generated by removing the minor phase, c Rod-coil block copolymer for microporous materials. Solution-cast micellar films consisted of multilayers of hexagonally ordered arrays of spherical holes. (Adapted from [33])...

The structure of M41S-type materials is built up of pores with amorphous walls that are formed around micelles of templating material (surfactants). One of the extreme structures of M41S-type materials (MCM-41) is a hexagonal ordering of the pores, an other extreme is a worm-hole disordered type of arrangement of the pores. A lamellar layered structure is another form in which these type of materials often (partially) appear, but this phase collapses to amorphous material upon removal of the surfactant (eg by calcination). A cubic ordering of the pores is also encountered. This form has been named MCM-48 and will not be discussed in the current paper. 

The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed. 

Since the disclosure by Mobil of Micelle-Templated Silicate structures called MCM-41 (hexagonal symmetry) or MCM-48 (cubic symmetry) [1,2] many other structures have been synthesized using different surfactants and different synthesis conditions. All of these Micelle-Templated Silicas (MTS) have attracted much interest in fields as diverse as catalysis, adsorption, waste treatment and nanotechnology. MTS materials possess a high surface area ( 1000 m2/g), high pore volume ( 1 mL/g), tunable pore size (18-150 A), narrow pore size distribution, adjustable wall thickness (5-20 A). The silica walls can be doped with different metals for catalytic applications, like Al orTi, for acidic or oxydation reactions, respectively. 

Hence, in this work, we report the heterogeneization of this new chiral macrocycle onto micelle-templated silicate (MTS) surface by substitution of chlorine atom of previously grafted 3-chloropropyl chain. After A-alkylation of the tetraazamacrocycle with propylene oxide and metalation with Mn(lI)Cl2, the catalytic performance of the corresponding hybrid materials was evaluated in the heterogeneous enantioselective olefin epoxidation. 

Even though there is great interest in such systems based on polymers as supporting materials, this chapter essentially focuses on the use of minerals as support. The use of silica has been historically investigated, mainly for its large surface area. As mentioned later, micelle-templated silica (MTS) has recently been disclosed. 

Zr-containing mesoporous silicas offer some potential for the oxidation of aniline with H2O2 as the oxidant. The Zr is introduced as Zr(OiPr)4 during the micelle-templated hydrothermal synthesis of the MCM-41-type structure (206). The materials are calcined prior to use in catalytic experiments. Azobenzene and azoxybenzene are formed ... 

Some degree of success in supported enantioselective catalysis was accomplished by using functionalisation of mineral support. Due to their unique textural and surface properties, mesoporous micelle-templated silicas are able to bring new interesting properties for the preparation of optically active solids. Many successfully examples have been reported for enantioselective hydrogenation, epoxidation and alkylation. However, the stability of the immobilised catalysts still deserves efforts to allow industrial development of such attractive materials. 

In this work, the synthesis of high surface densities of chlororopropyl groups covalently grafted on mesoporous micelle templated aluminosilicates (Al-MTS) of various initial pore diameters is presented. The hybrid chiral materials resulting from halogen substitution are applied in the enantioselective addition of diethylzinc to benzaldehyde. 

Hybrid silica materials were prepared via a sol-gel pathway at pH 9. The influence of anionic surfactant (SDS) was studied by comparing templated materials (TbSn series) with hybrid materials obtained without surfactant (Tbn series). Two mechanisms of mesostructure formation can be considered as represented on Fig. 2. The pka of aminopropyl chain is about 10.6 in the reaction mixture propyl-amines are partially protonated. Electrostatic interactions between dodecylsulfate anion and NH and sodium cation neutralization may then occur, resulting in the condensation of the silica structure around surfactant micelles and aminopropyl groups at the surface of the pores. The other mechanism is SDS chains complex-ation by P-CD cavity, which wonld result in P-CD gronps located at the surface of the pores and aminopropyl less accessible, due to steric hindrance caused by P-CD bulky groups. A complete characterization of the prodncts and adsorption capacities will help nnderstanding the formation mechanism of mesoporons hybrid silica. 

Aqueous molecular assemblies such as micelles and bilayer membranes are formed by the self-assembly of amphiphihc compounds (Figure 11.la, b) [10]. Aqueous micelles have been utihzed for a variety of apphcations in surfactant industry, including emulsification, washing, and extraction processes [11]. BUayer membranes are basic structural components of biomembranes, and their structures are maintained even in dilute aqueous media. This is in contrast to micelles that show dynamic equihbrium between aggregates and monomeric species. Thus bilayers are more stable and sophisticated self-assemblies, and they require suitable molecular design of the constituent amphiphiles. BUayer membranes and vesicles have wide-ranging applications, as exemphfied by drug dehvery [12], sensors [13], and bilayer-templated material synthesis [14]. 

The present report focuses on the intrusion-extrusion characteristics of grafted silica-based materials conventional chromatography adsorbents and Micelle-Templated-Silica materials (MCM-41) [3]. Finally, the mechanisms of energy dissipation will be analyzed in terms of general thermodynamic considerations. 

In conclusion, the study of the influence of grafted chain length on the hydrophobicity of silica gel allowed us to choose a convenient surface treatment, which was successfully transposed to MCM-41 materials. These former materials were demonstrated to be particularly suitable for the considered new field of applications, and to present advantages compared to silica gels. Finally, Micelle-Templated-Silica materials represent a class of useful models for a comprehensive study of the mechanisms of energy dissipation during forced intrusion and more generally of the physical chemistry of surfaces. 

J. Aguado, D. P. Serrano, R. van Grieken, J. M. Escola, and E. Garagorri, Catalytic properties of micelle templated microporous and mesoporous materials for the conversion of low-density polyethylene. Stud. Surf Sci. Catal, 135, 3915-3922 (2001). 

Figure 37. Nitrogen adsorption-desorption isotherms of parent mesoporous material MTS (solid line), Cl-MTS (dashed line) and tSalpr-MTS (dotted line). MTS = micelle templated silica, Cl-MTS = MTS modified by 3-chloropropylsilane and tSalpr-MTS = MTS modified with Mn(III) iV,iV -bis[3-(3,5-di-ieri-butylsalicylidenamino)propyl]amine.

In this respect, the recently discovered family of materials synthetized by silicate condensation around surfactant micelles (Micelle-Templated Silica, MTS) provides unique inorganic support [11,12] due to their regular arrays of uniformly sized channels in the mesopore range of 20-100 A. This solid was recently impregnated with cesium oxides with the aim to obtain superbase catalysts [13]. Nevertheless, the leaching of the basic particles cannot be excluded. In view to avoid such possible phenomena, we have studied the covalent attachment of basic functions such as amino groups on the MTS surface... 

The surface of the materials obtained after calcination of the micelle-templated silicates (MTS) has been functionalized by covalent linkage of organic moieties this has been achieved by silylation of the surface with organotrialkoxysilane (Figure 1) [17]. 

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