Mesostructure stability

Grosso, D. Soler-Illia, G. Crepaldi, E. L. Cagnol, F. Sinturel, C. Bourgeois, A. Brunet-Bruneau, A. Amenitsch, H. Albouy, P. A. Sanchez, C. 2003. Highly porous Ti02 anatase optical thin films with cubic mesostructure stabilized at 700 degrees C. Chem. Mater. 15 4562 570. 

Most of the supports so far studied are conventional in the field of catalysis. Some new kinds of support have emerged including mesostructured materials,193 dendrimers, organic-inorganic hybrids, and natural polymers such as polysaccharides or polyaminoacids. Nevertheless, at the moment, supported catalysts still suffer from relatively poor stability when compared to classical heterogeneous catalysts, and from limited activity when compared to homogeneous catalysts. The driving force for this research is thus to make up some of these deficits. 

Another way to complete condensation of an inorganic matrix is a treatment of the deposited films at different humidity in combination with slightly elevated temperatures. This approach, called delayed humidity treatment (DHT), enables controlled mesoorganization and condensation of the metal oxides (e.g., Sn02) with a low condensation rate.53,54 The thermal stability of such mesostructures is, however, low. So, in most cases, just semicrystalline metal oxide films can be obtained. 

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. 

We attempted to improve the eatalytic performanee, including stability, of the silica-immobilized Co-POM catalysts by using hydrothermally stable supports, specifically, the mesostructured silicates SBA-15 and MCF, both modified with amino groups by grafting 3-aminopropyltrietoxysilane [97], The physico-chemical properties of three representative NH2-X (X = xerogel, SBA-15 and MSF) supported Co-POM catalysts are given in Table 1. The textural properties of the initial, POM-free supports are shown for comparison. 

To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po 0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). 

Formation of Integrated MCM-41 Mesostructure in Fluoride Medium An Improvement of Hydrothermal Stability... 

MCM-41 samples with increased hydrothermal stability could be successfully synthesized by adding additional salts like tetraalkylammonium bromide or sodium bromide to the synthesis gel. The increased stability is related to the increased condensation of the silanol groups during the formation of the mesostructure. Hydrothermally stable MCM-41 structures with different pore diameter can also be synthesized by this method using surfactants with varied chain length. 

However, solvent extraction has proven to be more effective. When a sample of 1 g MTS-W was dispersed in 50 mL of ethanol and refluxed for 12 hours. Elemental analysis showed that ca. 50% of the organic templates could be removed without any apparent effect on the stability of the mesostrucutre. We have also found that the mesostructured compounds are stable in concentrated HC1 solution. For example, when MTS-W was stirred in a 12M HC1 solution at room temperature for 12 hours, ca. 30% of the organic templates could be removed without collapsing the mesostructure or decomposing the inorganic walls. 

Mesostructured molecular sieves such as MCM-41 ( 1,2 ), because of their easily accessible mesoporosity, acidity, thermal stability to 800°C, and pore diameter in the 2 to 10 nm range could prove particularly useful in the preparation of novel catalysts and... 

The effect of PEG on thermal stability of the mesostructured Ce02 powders has been observed by comparing the difference of properties between the samples CPN-2 and CN. Figure 10 shows the small angle XRD patterns of the samples CPN-2 and CN. After calcination at 673K, the low-angle diffraction peak in the XRD pattern of CN disappeared, but still existed in that of CPN-2. This may be attributed to the thermal stability improvement deriving from non-ionic surfactant PEG. 

In a detailed study, mesostructured zirconia has been prepared by using various amphiphilic surfactants with different headgroups (anionic and nonionic) and different tail lengths (1-18 carbons) as templates. Removal of snrfactants leads to the loss of structural order and a decrease ofthe siuface area. However, the presence of phosphates and snlfates in the walls may improve the stability. 

This paper presents an overview of the pore structural properties of different mesostructured materials prepared at room temperature, namely MCM-41, MCM-48 and MMS. The stability towards water vapour of some samples is inferred by XRD and nitrogen adsorption after and prior to water adsorption. It is shown that MMS materials are more stable than MCM-41. A new method for the stabilisation of MCM-41 using tetraethoxysilane (TEOS) in hexane as silicification agent is proposed and it is shown to be effective in stabilising the pore structure and to increase significantly the surface hydrophobicity. 

In this work we report a comparison of the structural properties of different mesostructured materials prepared at room temperature and present a study of the stability of MMS materials. We also describe a new and temative route to stabilisation of MCM-41 materials, using TEOS (in hexane) as a post-synthesis silicification agent. 

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