Ordered mesoporous oxides

Ordered mesoporous oxides, obtained for example by block copolymer-... 

In comparison to zeolites, ordered mesoporous materials overcome the pore-size constraint of zeolites and allow easier diffusion of bulky substrates. Unrestricted diffusion of reactants and products for mesoporous materials was observed even after the incorporation of large catalytically active sites in the mesopore system. Ordered mesoporous oxides have been used as supports for metals and metal oxides, and as a host material for anchoring stereo- and enantio-selective species. In reactions that require milder acidity and also involve bulky reactants and products, such as mild hydrocracking reactions, mesoporous materials exhibit great potential.[40,296]... 

M. Grun, I. Lauer, and K.K. Unger, The Synthesis of Micrometer- and Submicrometer-size Spheres of Ordered Mesoporous Oxide MCM-41. Adv. Mater., 1997, 9, 254—257. 

Research in ordered mesoporous oxides has increased dramatically in intensity over the last years. Developments not foreseen at the time when MCM-41 and FSM-16 were discovered have taken place, and in some cases applications of these materials are on the horizon. This paper will cover the discoveries which have expanded the scope of research in this field and try to give an outlook where new developments could take place. 

When one searches for the key words (MCM-41 or FSM-16 or SBA-15 or ordered mesoporous oxides or ordered mesoporous materials) in the Web of Science,... 

Fig. 1 Development of publications on ordered mesoporous oxides since 1990.

The first ordered mesoporous materials reported were silicas or 2duminumsilicates. The majority of all published investigations is still based on such compositions. In addition, many different elements, recently listed in a comprehensive review [10], have been introduced into the framework of ordered mesoporous oxides, just as in zeolitic frameworks. Such compositions could... 

Iron modified zeolites and ordered mesoporous oxides have been studied as catalysts for the sulfur dioxide oxidation in sulfur rich gases. Both zeolitic materials and mesoporous oxides show very good activity in this reaction. Other than solid state or incipient wetness loaded MCM-41 materials, the zeolites do not show an initial loss of activity. However, they loose activity upon prolonged exposure to reaction conditions around 700°C. The zeolitic samples were analyzed via X-ray absorption spectroscopy, and the deactivation could be related to removal of iron from framework sites to result in the formation of hematite-like species. If the iron can be stabilized in the framework, these materials could be an interesting alternative to other iron based catalysts for the commercial application in sulfur rich gases. 

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

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