Reactions Using Heteroatom Substituted Zeolites

Following the discovery of TS-1 [125], a titanium-substituted MFl, the use of zeolitic materials for oxidation increased significantly. The presence of the Ti atom in the framework of a zeolite structure provides a site-isolated Ti center, a situation not possible with other Ti-containing materials while also allowing shape-selective oxidations. The combination of the two effects gives highly active and selective oxidation reactions [126]. 

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Alongside the classic Si- and Al-containing zeolites, the design and catalytic use of zeolites and other microporous materials with Lewis acidic heteroatoms has been reported, with a focus on substituted heteroatoms in zeolite frameworks. In 2009, Taaming et al. used Sn-Beta zeolites and reported LA and lactate yields of 90% and 99%, respectively, for the complete conversion of DHA in water and methanol, respectively, at either 100°C or 80°C with an Si Sn ratio of 125 [65]. The reaction was near to completion after 6 h. The initial turnover frequency was calculated to be 45 mol molsn h In comparison, Hayashi s soluble Sn Cl4.5H20 salt only reached about 4.2 mol molsn h. Since then, numerous reports have surfaced in the literature studying (among other catalysts) the use of Sn-MCM-41 [113], Sn-SBA-15 [122], Sn-MFI [123], Sn-montmorillonite [124], Sn-MWW [125],... 

In recent years, a large body of work emphasized the use of zeolites for production of fine chemicals (refs.1-4). The interests stand in replacement of liquid acids to lower corrosion of equipment and pollution, and to reach specific selectivities. However, the hopes raised up in a rapid development of processes seems restrained nowadays. Many patents claimed zeolites as catalysts but very few have received industrial applications. Actually, basic research on the stability, the origin of deactivation, the regenerability of the catalysts have to be developed. Moreover, fundamental aspects of the mechanism of this new kind of reactions are lacking, in particular, the possibility of radical mechanisms, which are rather scarce with hydrocarbons, but can likely occur when heteroatoms are involved in the reactant. Those were our objectives in the study of the isomerisation of substituted halobenzenes on zeolites (refs.5-7). Indeed this reaction was claimed to occur readily on zeolites (refs.8-9), but it is supposed that no industrial development has followed. 

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